Introduction to Aluminum Alloys

## **Chapter 1**

## Introductory Chapter: Introduction to Aluminum Alloys

*Emre Altaş, Shashanka Rajendrachari and Vutukuru Mahesh*

## **1. Introduction**

With the advancement of technology and scientific studies, new areas of use have been created for existing materials. In this way, the interest in new materials with high quality that can meet current needs has increased [1]. The developing technology and scientific studies to date are in search of improving the desired properties of existing materials and the emergence of new materials that can be alternatives. This search continues with the emergence of materials with new qualities [2]. Compatibility of physical and characteristic properties is very important in material selection. For this reason, many methods have been developed and continue to be developed today. Particularly in the aviation, space, gas turbines, automotive, and maritime industries, materials that are high-performance, light, and durable, and can combine features such as stability at high temperatures are needed [3–5].

Aluminum and its alloys, which are widely used especially in the aviation industry, can be preferred and used in long-lasting applications due to their features such as low density, high specific strength, and resistance to oxidation [6, 7]. Among the reasons why aluminum metal and its alloys have different usage areas in different sectors are that they can be easily produced, shaped, and processed, have high corrosion resistance, are lightweight, and have good strength properties. Additionally, aluminum is extremely suitable for recycling [8, 9]. Aluminum alloy is almost infinitely recyclable, and the recycling process requires only 5% of the energy of primary aluminum production. High-purity aluminum is a soft material with an ultimate strength of approximately 10 MPa, which limits its usability in industrial applications. To compete with other building materials, the strength of Al-based materials needs to be significantly increased. There are several ways to increase the strength of metallic materials: alloying with sufficient elements, adding appropriate strengthening particles, plastic deformation, or grain size reduction [10].

Aluminum alloys are the most used metallic engineering materials after steel today. Aluminum and its alloys, which are widely used especially in the aviation industry, can be preferred and used in long-lasting applications due to their features such as low density, high specific strength, and resistance to oxidation. Among the reasons why aluminum metal and its alloys have different usage areas in different sectors are that they can be easily produced, shaped, and processed, have high corrosion resistance, are lightweight, and have good strength properties. In addition, aluminum alloys provide significant advantages over other engineering materials due to their features such as high thermal conductivity, nonflammability, and being completely recyclable and weldable [11, 12].

During the production phase, material scientists constantly improve the properties of the materials they have obtained by moving them to macro dimensions under more minimized conditions. The purpose of this is that today's technology is competitive in the market and the features of the materials needed are no longer affordable. Above all else, one of the most distinctive features required from materials is to ensure continuity [13–15]. In other words, the material produced gives the same physical results and reactions when used at different times but under the same conditions. Therefore, it is important that the material has a homogeneous microstructure and an equal stress distribution on the material [16–18]. In recent years, different production methods have been developed in addition to traditional powder metallurgy methods in the production of some critical parts. Mechanical alloying is one of these methods. This method is used to alloy metals without exposure to any chemical or heat treatment [19–21]. With mechanical alloying, metals can be alloyed without the need for melting or heat treatment. The mechanical alloying process involves cold welding of solid powder particles to each other and breaking them after deformation hardening. At the same time, particle-reinforced composites are produced using the mechanical alloying method, in which the best microscopic or macroscopic combinations of different materials are provided in order to obtain fine microstructures in powders [22–25].

## **2. Aluminum and aluminum alloys**

Aluminum is found in the form of aluminum silicate in rocks, feldspars, feldspathotites, and micas in clay soils formed by the disintegration of bronze, bauxite, and iron-rich laterite. Bauxite, the most important aluminum ore, contains 52% aluminum oxide [26, 27].

Aluminum is the most abundant metallic element in the earth's crust after iron, and it became economically produced in engineering applications toward the end of the nineteenth century. With the emergence of the first internal combustion engine vehicles, the engineering value of aluminum as an automotive material began to increase. Aluminum, which meets the need for a light and conductive material for the transmission of electricity over long distances, has also taken its place in the new industry that was born on strong, light, and break-resistant parts, along with the work of the Wright brothers on aviation. Nowadays, aluminum is used to obtain value-added products in many transportation sectors such as automotive, defense industry, aviation, rail systems, and maritime. Aluminum production is generally made from ore and recycled scrap. Today, aluminum production from ore is approximately twice that of aluminum production from scrap [28, 29]. Aluminum is the third most abundant element after oxygen and silicon, with a content of 8% in the earth's crust. Even though there are so many, it was discovered after minerals such as iron, copper, tin, lead, gold, and silver. Aluminum is found in nature as a compound. The existence of this abundant element was detected only in 1808 by the British Sir Humphry Davy [30].

The most distinctive feature of aluminum, which is used in all areas of human life and especially in engineering applications, is its lightness. It is the lightest metal after magnesium and beryllium. Due to the superior properties provided by aluminum and its alloys, their consumption is increasing rapidly and new areas of use are opening up every day. Although pure aluminum is a very active metal in the galvanic series, the protective oxide layer that easily forms on its surface makes it widely used.

## *Introductory Chapter: Introduction to Aluminum Alloys DOI: http://dx.doi.org/10.5772/intechopen.113373*

This impermeable, hard, and protective oxide layer consisting of aluminum oxide (Al2O3) significantly increases the corrosion resistance of aluminum. Accordingly, as aluminum is purified, its corrosion resistance and conductivity increase. For this reason, aluminum alloys, which are very sensitive to corrosion, are now protected from corrosion by cladding in pure aluminum [31]. On the other hand, the very low strength of pure aluminum can be increased by cold working. Today, aluminum and its alloys have, due to its properties, become one of the most important construction and engineering materials used in the industry. While it has properties, such as high thermal and electrical conductivity and corrosion resistance in its pure form, these properties have spread to a much wider spectrum with alloying and have become widely used. Today, more than 100 aluminum alloys are widely used in industry. The most important features are summarized below [26]:


## **2.1 Mechanical properties of aluminum**

The modulus of elasticity of aluminum is approximately one-third of that of steel. In other words, the amount of elastic deformation of aluminum under the same load is three times that of steel. This feature is of great importance in design calculations. The low modulus of elasticity is considered an advantage when exposed to impact loads since the resistance of aluminum is higher than that of steel. It allows the absorption of large amounts of energy [32]. One of the main features of aluminum is

its ease of shaping and processing according to conventional manufacturing methods. Since pure metal is soft and has the ability to become wire, it can be rolled, drawn, and shaped by applying various cold processes such as drawing, bending, pressing, and molding [33].

#### **2.2 Chemical properties of aluminum**

Aluminum is not available in pure form due to its high chemical activity. Therefore, its production is made from bauxite ore consisting of iron oxide and aluminum silicate. Due to a fixed oxide layer formed on the aluminum surface in contact with air, the metal and its alloys generally show great resistance to the corrosive effects of the atmosphere. Aluminum reduces the oxides of other metals due to its affinity for oxygen. Due to this feature, powdered aluminum is used in the production of metal oxides such as chromium, vanadium, barium, and lithium by reducing them [10]. Since aluminum is nontoxic, it is widely used in many areas, especially in the production of equipment in the food industry. Again, due to this feature, it is widely used in the packaging of food and medicines, cigarettes, and tea [34].

#### **2.3 Physical properties of aluminum**

Low density, one of the physical properties of aluminum, comes to the fore in many applications. The density of aluminum in the commercial group is approximately 2.70 g/cm3 . When equal volumes are compared, aluminum has approximately one-third the weight of iron, copper, and zinc. In some applications, it is not enough to focus on the lightness advantage of metal alone. For example, a material that does not have sufficient strength but has low density is not very useful for the elements that make up the structure of an aircraft. In this case, while pure aluminum is not suitable for use, aluminum alloys are used, where strength and lightness are desired together [5].

Aluminum and its alloys also need different criteria in comparison with the traditional materials and manufacturing methods with which they have to compete. When expressed in terms of concepts such as specific strength, specific stiffness, and discontinuous yielding during forming, aluminum alloys exhibit equivalent and sometimes superior performance compared to traditional materials [2].

These properties of aluminum make it preferred for the automotive and manufacturing sectors. Saving fuel and reducing costs due to its lightness in the transportation sector and regulations on the emission number of vehicles on national and international platforms have made aluminum the best alternative material for the transportation sector [35].

#### **2.4 Usage areas of aluminum alloys**

Increasingly complex production methods, differentiating and diversifying consumer demands, increasing population, and the awareness that the limits of natural energy resources are rapidly approaching over time with production have necessitated radical changes in the production methods and raw materials that have been customary so far in many sectors. Although their industrial usage date is recent, aluminum alloys have rapidly taken part in production [28].

Aluminum is used extensively in the automotive industry because it is a light metal, and its use is constantly increasing. Aluminum is used in the production of *Introductory Chapter: Introduction to Aluminum Alloys DOI: http://dx.doi.org/10.5772/intechopen.113373*

#### **Figure 1.**

radiators, engine parts, body sheets, and structural parts in the automobile industry. Aluminum is used in the construction of cargo transportation and passenger compartments in airplanes train transportation systems and in the production of ship hulls and propellers in the ship industry. Considering that energy will become more valuable in the future as a new area of use, aluminum batteries will find a wide range of applications (**Figure 1**). Aluminum-sulfur batteries are the first examples of these applications. With these batteries, it is possible to reach an efficiency of 250 Wh/kg. Another example is aluminum air-fuel cells [37].

Esthetic applications of aluminum alloys in the construction industry have a longer history than manufacturing and other strategic applications. In the construction industry, the needs could be met without the need for high technology, but according to the strength and corrosion properties of aluminum and in most applications, where both are required together, the aluminum industry had to carry out basic studies in technology and production methods, which resulted in the development of alloys and different production methods [38].

*Recent Advancements in Aluminum Alloys*

## **Author details**

Emre Altaş1 , Shashanka Rajendrachari<sup>2</sup> \* and Vutukuru Mahesh3

1 Department of Mechanical Engineering, Faculty of Engineering, Architecture and Design, Bartın University, Bartın, Turkey

2 Department of Metallurgical and Materials Engineering, Bartin University, Bartin, Turkey

3 Mechanical Engineering, SR University, Warangal, Telangana, India

\*Address all correspondence to: shashankaic@gmail.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Introductory Chapter: Introduction to Aluminum Alloys DOI: http://dx.doi.org/10.5772/intechopen.113373*

## **References**

[1] Santos MC, Machado AR, Sales WF, Barrozo MA, Ezugwu EO. Machining of aluminum alloys: A review. The International Journal of Advanced Manufacturing Technology. 2016;**86**:3067-3080

[2] Ma Z, Feng A, Chen D, Shen J. Recent advances in friction stir welding/ processing of aluminum alloys: Microstructural evolution and mechanical properties. Critical Reviews in Solid State and Materials Sciences. 2018;**43**:269-333

[3] Abd El-Aty A, Xu Y, Guo X, Zhang S-H, Ma Y, Chen D. Strengthening mechanisms, deformation behavior, and anisotropic mechanical properties of Al-Li alloys: A review. Journal of Advanced Research. 2018;**10**:49-67

[4] StJohn D, Easton M, Qian M, Taylor J. Grain refinement of magnesium alloys: A review of recent research, theoretical developments, and their application. Metallurgical and Materials Transactions A. 2013;**44**:2935-2949

[5] Canakci A, Varol T. Microstructure and properties of AA7075/Al–SiC composites fabricated using powder metallurgy and hot pressing. Powder Technology. 2014;**268**:72-79

[6] Bhagat RB. Advanced aluminum powder metallurgy alloys and composites. In: ASM Handbook, Volume 7: Powder Metal Technologies and Applications. OH, USA: ASM Digital Library; 2013. pp. 840-858

[7] Liao J, Tan M-J. Mixing of carbon nanotubes (CNTs) and aluminum powder for powder metallurgy use. Powder Technology. 2011;**208**:42-48

[8] Vaidya M, Muralikrishna GM, Murty BS. High-entropy alloys by mechanical alloying: A review. Journal of Materials Research. 2019;**34**:664-686

[9] Suryanarayana C, Al-Aqeeli N. Mechanically alloyed nanocomposites. Progress in Materials Science. 2013;**58**:383-502

[10] Aboulkhair NT, Simonelli M, Parry L, Ashcroft I, Tuck C, Hague R. 3D printing of aluminium alloys: Additive manufacturing of aluminium alloys using selective laser melting. Progress in Materials Science. 2019;**106**:100578

[11] Parsons EM, Shaik SZ. Additive manufacturing of aluminum metal matrix composites: Mechanical alloying of composite powders and single track consolidation with laser powder bed fusion. Additive Manufacturing. 2022;**50**:102450

[12] Ozdemir F, Witharamage CS, Darwish AA, Okuyucu H, Gupta RK. Corrosion behavior of age hardening aluminum alloys produced by highenergy ball milling. Journal of Alloys and Compounds. 2022;**900**:163488

[13] Mahale RS, Vasanth S, Krishna H, Shashanka R, Sharath P, Sreekanth N. Electrochemical sensor applications of nanoparticle modified carbon paste electrodes to detect various neurotransmitters: A review. Applied Mechanics and Materials. 2022;**908**:69-88

[14] Pradeep N, Hegde MR, Rajendrachari S, Surendranathan A. Investigation of microstructure and mechanical properties of microwave consolidated TiMgSr alloy prepared by high energy ball milling. Powder Technology. 2022;**408**:117715

[15] Rajendrachari S. An overview of high-entropy alloys prepared by mechanical alloying followed by the characterization of their microstructure and various properties. Alloys. 2022;**1**:116-132

[16] Shashanka R, Uzun O, Chaira D. Synthesis of nano-structured duplex and ferritic stainless steel powders by dry milling and its comparison with wet milling. Archives of Metallurgy and Materials. 2020;**65**:5-14

[17] Shashanka R. Investigation of optical and thermal properties of CuO and ZnO nanoparticles prepared by *Crocus sativus* (Saffron) flower extract. Journal of the Iranian Chemical Society. 2021;**18**:415-427

[18] Shashanka R. Synthesis of nanostructured stainless steel powder by mechanical alloying-an overview. International Journal of Scientific & Engineering Research. 2017;**8**:588-594

[19] Nayak A, Shashanka R, Chaira D. Effect of nanosize yittria and tungsten addition to duplex stainless steel during high energy planetary milling. In: IOP Conference Series: Materials Science and Engineering. England: IOP Publishing; 2016. p. 012008

[20] Zhou S, Wu K, Yang G, Wu B, Qin L, Wu H, et al. Microstructure and mechanical properties of wire arc additively manufactured 205A high strength aluminum alloy: The comparison of as-deposited and T6 heat-treated samples. Materials Characterization. 2022;**189**:111990

[21] Rajendrachari S. Effect of sintering temperature on the pitting corrosion of ball milled duplex stainless steel by using linear sweep voltammetry. Analytical and Bioanalytical Electrochemistry. 2018;**10**:349-361

[22] Shashanka R, Chaira D. Optimization of milling parameters for the synthesis of nano-structured duplex and ferritic stainless steel powders by high energy planetary milling. Powder Technology. 2015;**278**:35-45

[23] Chaira D. Development of nanostructured duplex and ferritic stainless steels by pulverisette planetary milling followed by pressureless sintering. Materials Characterization. 2015;**99**:220-229

[24] Shashanka R. Fabrication of Nano-Structured Duplex and Ferritic Stainless Steel by Planetary Milling Followed by Consolidation. India: Educreation Publication; 2016. p. 146

[25] Rajendrachari S, Yilmaz VM, Karaoglanli AC, Uzun O. Investigation of activation energy and antibacterial activity of CuO nano-rods prepared by *Tilia tomentosa* (Ihlamur) leaves. Moroccan Journal of Chemistry. 2020;**8**:497-509

[26] Moghimian P, Poirié T, Habibnejad-Korayem M, Zavala JA, Kroeger J, Marion F, et al. Metal powders in additive manufacturing: A review on reusability and recyclability of common titanium, nickel and aluminum alloys. Additive Manufacturing. 2021;**43**:102017

[27] Farjana SH, Huda N, Mahmud MP. Impacts of aluminum production: A cradle to gate investigation using lifecycle assessment. Science of the Total Environment. 2019;**663**:958-970

[28] Wang H, Leung DY, Leung M, Ni M. A review on hydrogen production using aluminum and aluminum alloys. Renewable and Sustainable Energy Reviews. 2009;**13**:845-853

[29] Kaufman JG. Introduction to Aluminum Alloys and Tempers. USA: ASM international; 2000

*Introductory Chapter: Introduction to Aluminum Alloys DOI: http://dx.doi.org/10.5772/intechopen.113373*

[30] Toros S, Ozturk F, Kacar I. Review of warm forming of aluminum–magnesium alloys. Journal of Materials Processing Technology. 2008;**207**:1-12

[31] Knipling KE, Dunand DC, Seidman DN. Criteria for developing castable, creep-resistant aluminumbased alloys – A review. International Journal of Materials Research. 2022;**97**:246-265

[32] Min J, Xie F, Liu Y, Hou Z, Lu J, Lin J. Experimental study on cold forming process of 7075 aluminum alloy in W temper. CIRP Journal of Manufacturing Science and Technology. 2022;**37**:11-18

[33] Li G, Li X, Guo C, Zhou Y, Tan Q, Qu W, et al. Investigation into the effect of energy density on densification, surface roughness and loss of alloying elements of 7075 aluminium alloy processed by laser powder bed fusion. Optics & Laser Technology. 2022;**147**:107621

[34] Zhou B, Liu B, Zhang S. The advancement of 7xxx series aluminum alloys for aircraft structures: A review. Metals. 2021;**11**:718

[35] Bouali A, Serdechnova M, Blawert C, Tedim J, Ferreira M, Zheludkevich M. Layered double hydroxides (LDHs) as functional materials for the corrosion protection of aluminum alloys: A review. Applied Materials Today. 2020;**21**:100857

[36] Wang C, Yu Y, Niu J, Liu Y, Bridges D, Liu X, et al. Recent progress of metal–Air batteries—A mini review. Applied Sciences. 2019;**9**:2787

[37] Zhang J, Song B, Wei Q, Bourell D, Shi Y. A review of selective laser melting of aluminum alloys: Processing, microstructure, property and developing trends. Journal of Materials Science & Technology. 2019;**35**:270-284

[38] Georgantzia E, Gkantou M, Kamaris GS. Aluminium alloys as structural material: A review of research. Engineering Structures. 2021;**227**:111372

**Chapter 2**

## A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys

*Ehab Samuel, Hicham Tahiri, Agnes M. Samuel, Victor Songmene and Fawzy H. Samuel*

## **Abstract**

Grain refining is considered one of the most important liquid metal processing processes for aluminum alloys. Three different types of grain morphology are possible: columnar, twin columnar and equiaxed. The present work reviews most of the theories that were proposed during the past three decades. These theories were mainly based on thermal analysis and thermodynamics to explain the mechanisms of grain refining of Al-Si based alloys, including the role of the master alloy used i.e., Al-B, Al-Ti, and Al-Ti-B alloys. Other aspects were also examined, mainly the interactions between Si and/or Sr and the grain refining master alloy, superheating of the molten metal as well as holding time prior to casting. This phenomenon is normally termed "poisoning" since it reduces the effectiveness of the added grain refiners. The effects of grain refining on the alloy microstructural characteristics, mechanical properties, machinability, hot tearing etc. have not been addressed in the present article.

**Keywords:** grain refining, master alloys, poisoning, theories, thermodynamics

## **1. Introduction**

Grain refinement in Al-Si casting alloys is usually assessed by the presence of titanium Ti and boron B. Since the 1980s, thermal analysis has established itself as an important alternative for determining the degree of grain refinement and in predicting the degree of modification of the eutectic silicon. Grain refining is one of the most important liquid metal processing processes for aluminum alloys. Three different types of grain morphology are possible: columnar, twin columnar and equiaxed. It is well known that an equiaxed grain structure provides uniform mechanical properties, reduced hot tear, second phase distribution and microporosity on a fine scale [1–5].

Grain refining in aluminum alloys aims to increase the number of crystallization sites of the pro-eutectic phase (α-Al phase) and avoid columnar growth. In order to have a fine scale grain size, the most widely practiced way is to present effective nuclei in the liquid metal using the Al-Ti-B grain refiners which usually contain active seeds like TiAl3, TiB2, AlB2 or (Al,Ti)B2. Thermodynamic studies suggest that these latter particles convert to TiB2, so that the titanium would diffuse into the (Al,Ti)B2 particles while the aluminum diffuses out, resulting in the formation of TiB2 [6–8].

While the Al-Si alloy system is widely used in industry, constituting around 85– 90% of aluminum parts produced, the eutectic silicon in untreated alloys is often very coarse, leading to poor mechanical properties, mainly ductility. These properties are strongly influenced by the morphology of the eutectic silicon. The latter changes from its original raw structure of platelets to a less harmful and finer fibrous structure termed as eutectic Si modification which leads to a significant improvement in the mechanical properties of the products. Modification of the eutectic silicon is usually accomplished by adding certain modifying agents such as strontium Sr. However, over-modification can lead to the formation of porosities and returns the silicon to its original shape, again weakening the characteristics of the alloy. The addition of strontium in Al-Si alloys leads to a considerable increase in the amount of the α-Al dendritic phase and changes the shape of the dendrites. In the presence of a grain refiner like Al-Ti-B, the modifier reduction is considerable since the Sr-Ti interaction alternately changes the volume fraction of the dendritic phase of α-Al and the morphology of the silicon phase [9, 10].

This study aims to establish suitable grain refining mechanisms in Al-Si alloys to study the consequences of the refining-modification interaction in these alloys. Different time and temperature parameters on the thermal analysis curves are analyzed.

#### **1.1 Solidification phenomenon**

Grain morphology in as-cast alloys can be categorized as equiaxed or columnar. The grains of an equiaxed structure are nucleated in the liquid pool ahead of the solidification front on particles deliberately added as inoculants or present as impurities. In engineering alloys, the supercooling required to launch a grain onto an inoculant particle is produced by constitutional effects: the splitting of the solute between liquid and solid creates a solute profile ahead of the solidification front, lowering the local liquidus temperature. In the absence of equiaxed grains, growth is typically columnar; columnar grains grow approximately perpendicular to the direction of heat flow and have high elongation. The growth of columnar grains is blocked if there are enough equiaxed grains. Given the importance of grain morphology to the properties of a cast alloy, it is important to predict the conditions that cause the transition from columnar to equiaxed growth [11, 12].

During their growth, equiaxed grains can adopt various morphologies, depending on the rate of cooling and the content of solutes in the liquid metal and the degree to which a grain can grow before impact or collision with other grains. Equiaxed grains initially grow as spheroids, but a smooth solid-liquid interface becomes unstable as the radius of the grain increases. The grain becomes globular and then adopts a strongly branched dendritic structure. This evolution is accelerated by high cooling rates and dissolved solids content.

The morphology of the columnar grains is influenced by cooling carried out constitutionally ahead of the solidification front. In situations where supercooling is greater ahead of this front than at the front itself, the solid-liquid interface is unstable and results in dendritic structures. The greater the constitutional cooling, the more pointed and branched the dendrites.

In the absence of a constitutionally cooled region, the solid-liquid interface remains planar. The observed microstructure can be influenced by magnification of the dendrite arms after solidification is complete; the structure of a solidification front that has been quenched to preserve its structure is often markedly different from one formed without quenching under otherwise identical solidification conditions.

## **1.2 Importance of thermal analysis**

Thermal analysis could provide a reliable method for determining grain size, modifying the eutectic silicon and even quantifying the iron content of the alloy if the latter is equal to or greater than 0.6% by weight [13]. Thermal analysis consists in recording the evolution of the temperature of an alloy as a function of time during solidification. During cooling, heat is released and the temperature rises to a value close to the equilibrium value. This warming process is called recalescence. Supercooling, which represents a thermodynamic force, appears on the solidification curve as a drop in temperature below the equilibrium temperature of the reaction [14].

Grain refinement is a result of two separate processes: nucleation of new liquid aluminum crystals, followed by growth to a limited size. Both of these processes need a driving force which must be provided to the system through supercooling and supersaturation with respect to the equilibrium conditions of the real system. **Figure 1a** shows that during the entire first period of the solidification process, only those parts of the liquid metal which are in contact with the mold walls are cooled to such a degree that the nucleation of new aluminum grains can take place. **Figure 1b** shows that nucleation begins above the steady state growth temperature. This means that new crystals can be formed, not only at the first contact of the liquid bath with the cold walls of the mold, but also in the liquid.

#### **Figure 1.**

*(a) First part of a cooling curve and its derivative obtained from liquid metal close to the wall of the casting mold. (b) Cooling curve and its derivative of a sample to which titanium boride particles have been added. The nucleation temperature is below the liquid metal growth temperature. The recalescence phenomenon shows a very low value of (dT/dt)max, indicating a sample whose grains are refined.*

This phenomenon occurs due to particles with a high nucleation power (titanium borides). The latter becomes active at a supercooling of only 0.1–0.2°C when added to the liquid metal. Seed particles added to liquid metal must be effective substrates for heterogeneous nucleation in order to achieve grain refinement. However, nucleation can occur only if the liquid alloy is sufficiently cooled. In a solidification system, the remaining liquid pool can be cooled only if there is some solute in this liquid pool to limit solid growth or columnar competition with equiaxed solidification. Thus, efficient refining requires heterogeneous nucleation and growth restriction. Al-Ti-B refiners which are generally used for this purpose, consist of particles of TiB2 with a diameter ranging from 0.1 to 10 μm and Al3Ti particles with a diameter ranging from 20 to 50 μm, dispersed in an aluminum matrix [15]. Al3Ti can be a very effective nucleant for aluminum, but this phase dissolves quickly when the refiner is added to commercial purity aluminum, because all the titanium content in the liquid metal is well within the limit of solubility [16, 17]. It is well accepted that some excess titanium (beyond that combined with B in TiB2) is required for efficient nucleation [14].

Other temperature parameters seen in **Figure 1** correspond to:

TE = The liquidus equilibrium temperature.

TG = The steady state growth temperature of the molten metal.

TN = The onset of nucleation temperature. TN is called the nucleation power of the particles present in the liquid metal. This point is most easily recognized by a sudden change in the derivative, as shown in the figure.

TMIN = The temperature at which the newly nucleated crystals have grown to such an extent that the latent heat released swings out of equilibrium. After this time, the molten metal actually heats up to the steady state growth temperature. The period of time required for this heating is called the recalescence period (tRec).

#### **1.3 Nucleation phenomenon**

The grain refinement is carried out by the addition of master alloys of the Al-5Ti-1B and/or Al-Ti type in an embossed form. The addition rate of 1 kg/1000 kg gives Ti and B additions of 0.005% and 0.001%, respectively. Such an addition would typically produce grains of equiaxed structure with a grain size ranging from 100 to 150 μm in a small commercial pure aluminum ingot. The phenomenon of grain refining is directly linked to the process of nucleation and growth of aluminum grains. This is based on the nucleation ideas of Volmer and Weber [16].

The theory involves homogeneous and heterogeneous nucleation. In a solidified pure metal, the critical nucleus size for survival is given by:

$$r\_{\text{homogêne}}^{\*} = \frac{-2\gamma\_{sL}}{\Delta G\_v} \tag{1}$$

The free energy barrier is given by:

$$
\Delta G^{\*}\_{\text{homogêne}} = \frac{16\pi \chi^{3}\_{sL}}{3\Delta G^{2}\_{v}} \cong \frac{\Delta H\_{f}\Delta T}{T\_{m}} \tag{2}
$$

where *γsL* is the surface energy of the interface separating the solid seed from the liquid in J/m<sup>2</sup> .

By substituting <sup>Δ</sup>*Gv* <sup>¼</sup> *Lv*Δ*<sup>T</sup> Ts* in Eq. (2), we obtain the following relation: *A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

$$
\Delta G^\* = \left(\frac{16\pi\chi\_{L}^3 Ts}{3L\_v^2}\right)\left(\frac{1}{\Delta T^2}\right) \tag{3}
$$

where Lv is the latent heat of solidification per unit volume, ΔT is the supercooling (Ts � T) and Ts is the undercooling temperature. As for heterogeneous nucleation, the critical size of a nucleus is given by:

$$r^\*\_{\text{hetroĝené}} = \frac{-2\gamma\_{sL}}{\Delta G\_v} \tag{4}$$

Eqs. (1) and (4) are identical for both types of homogeneous and heterogeneous nucleation. The potential barrier that the germ must cross to reach its critical size is given by the following equation:

$$
\Delta G^\*\_{\text{heterogène}} = \left(\frac{16\pi\chi^3\_{sL}}{3\Delta G^2\_v}\right) f(\theta) \tag{5}
$$

where f(θ) is a function of the contact angle θ on the substrate on which nucleation takes place. **Figure 2a** shows a nucleated solid on a substrate in a liquid. **Figure 2b** shows the variation of f(θ) with θ and since f(θ) is always ≤1, the critical free energy for heterogeneous nucleation is always less than or equal to that for homogeneous nucleation. However, it is clear that effective heterogeneous substrates are those with θ close to zero [10]. Undercooling ΔT values are of the order of 0.1–0.2°C for observable nucleation rates in commercial aluminum alloys with grain refiners. Therefore, clearly heterogeneous nucleation takes place. The simplified expression for the heterogeneous nucleation rate per unit volume in m3 s �<sup>1</sup> is:

$$I\_{\text{heterogéne}}^v = \mathbf{10}^{18} N\_v^p \exp\left[\frac{-\mathbf{16}\pi \chi\_{sL}^3 f(\theta)}{\mathbf{3}K\_B \Delta S^2 \Delta T^2}\right]$$

#### **Figure 2.**

*Schematic representation showing (a) the formation of a spherical wetting of a solid S on a substrate, contact angle and surface tensions, (b) the variation of f (θ) with θ where f (θ) is equal to (2–3 cos θ + cos<sup>3</sup> θ)/4.*

where *KB* is the Boltzmann constant J/°C, *N<sup>p</sup> <sup>v</sup>* is the number of nucleant/m<sup>3</sup> and *I v <sup>h</sup>*́*et*́*erogene* ̀ is the heterogeneous nucleation rate of nucleant/m<sup>3</sup> s. Therefore, if the contact angle is near zero, the wetting of the substrate for nucleation is promoted and the rate of nucleation is increased.

When the nucleation sites are homogeneously dispersed in the liquid pool, the result is a fine grainy structure. The important topics for understanding the nucleation phenomena are summarized as follows: (1) the contact angle between the molten metal and the nucleation particles, (2) the interface energy between the molten metal, and (3) the nucleants and the coherence of the lattices of the nucleants and metal liquid. The presence of possible phases at different T in the liquid pool can be evaluated by comparing the free energy ΔG of the reactions.

Based on thermodynamic data, the calculated results are shown in **Figure 3**. It can be observed that ΔG(TiB2) is much more negative than ΔG(Al3Ti) and ΔG(AlB2) in the range of T from 700 to 1200°K, while ΔG(Al3Ti) is less negative than ΔG(AlB2). In other words, the TiB2 phase is easier to form than the Al3Ti and AlB2 phases. With increasing temperature, the changes of free Gibbs enthalpy of TiB2 and AlB2 are almost constant while that of Al3Ti became small. In fact, the theoretical prediction indicates that Al3Ti particles become unstable when the reaction temperature is increased. From the crystallographic point of view, and to further explain the high stability of the TiB2 nucleation sites, the hybridization of the 3d orbital of titanium and the 2p orbital of boron is the main reason for the strong bond between these two elements. The bonding behavior between Ti and B layers is a combination of covalent and ionic nature.

#### **1.4 Effect of overheating**

As the Ti/B ratio is decisive for better grain refining, the casting temperature also plays a very important role when determining the grain size. Li et al. [18] studied the effect of overheating on pure aluminum before casting. **Figure 4** clearly shows the relationship between the superheat temperature and the average grain diameter. It is clear that as the temperature is increased from 725°C to 950°C, the average grain sizes

**Figure 3.** *Gibbs free energy of TiB2, AlB2, and Al3Ti as a function of temperature.*

*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

#### **Figure 4.**

*Effect of undercooling temperature on the grain size of pure aluminum.*

increase linearly. Casting temperature is a significant factor during the performance of grain refining. If the holding temperature is too high after inoculation, some fading or degradation occurs. This can be attributed to the growth and arrangement of TiB2 particles, leaving a liquid alloy depleted of nucleating particles for efficient grain refining.

It is known that the size of the TiB2 particles that form inside the aluminum depends on the temperature of the liquid metal; at high temperatures, the particles being formed are so large that they can settle to the bottom of the liquid bath by virtue of their greater density.

In general, overheating increases the grain size [11, 19], but in some cases it reduces it [20].

### **1.5 Master alloys**

By adding three master alloys of the type Al-Ti-B, Al-Ti and Al-B with an excess of TiB2 (Ti/B = 2.22), Lu et al. [21] examined the grain size in an Al-7%Si alloy (composition close to that of the hypoeutectic A356 alloy). The performance of these master alloys is shown in **Figure 5**. Indeed, the binary Al-Ti alloy is found to be less efficient, while the Al-B alloy is the strongest grain refiner in the Al-Ti alloys, since the grain size changes from 2000 μm to only 200 μm. From a certain level (0.1% by weight), the grain size remains constant even if the amount of master alloy is increased, hence the plateau obtained during supersaturation in master alloy.

In contrast, Al-B alloys show inefficient behavior in pure aluminum [22]. Similar observations were made by Sigworth and Guzowski [6] Cooper et al. [23] proved that the efficiency of residual nucleants of the TiB2 and Al3Ti type decreases with the number of recyclings of the Al-Si alloys, which explains the increase in grain size as a function of the number of repetitions of castings [24].

#### *1.5.1 Al-Ti-B*

The only controversial point in the ternary system of Al-Ti-B is related to the two borides TiB2 and AlB2. Both crystallize in the same crystal structure (hexagonal shape) with similar lattice parameters. The question is to determine if they form a single-

**Figure 5.** *Grain refining of 356 alloy using different master alloys.*

phase continuous compound (Al,Ti)B2 or if they coexist in a two-phase balance AlB2+TiB2. A single phase was previously assumed by Hayes et al. [25] considering all experimental data available at that time. Roger et al. [26] investigated a 1000°C isothermal section in the titanium-rich region using quantitative microprobe phase analysis of three molten ternary alloys. They found that TiB2 is in equilibrium with all Al-rich Al-Ti binary phases. The aluminum-rich corner of the ternary phase diagram was first calculated by Hayes et al. [27]. No ternary parameter was used in this work.

The question whether TiB2 and AlB2 exist in two separate phases or as a solid continuous solution has not been resolved. Zupanic et al. [28] investigated arc melted alloys in the triangular composition Al-AlB2-TiB2 and found four solid phases: (Al), AlB12, AlB2 and TiB2. The AlB12 phase, which is stable at very high temperatures in the Al-B binary system, decomposes during annealing below 900°C. Both borides AlB2 and TiB2 were found to coexist even after 1000 h at 800°C. Formation of mixed diboride (Al,Ti)B2 was not observed [29]. Fjellstedt et al. [30] have produced Al-rich alloys by several different sample fabrication methods. They concluded that only the maximum solubility of aluminum in TiB2 (up to 0.15 wt% Al) and titanium in AlB2 (up to 0.2 wt% Ti) exists at 800°C, hence a continuous phase (Al,Ti)B2 is not stable.

Based on the entity of this data, the AlB2 and TiB2 phases were modeled in the work of Gröbner et al. [31] as two separate phases without any solubility. No ternary phase or ternary solubility exists in this system, no ternary parameter is needed for the calculation. The complete phase diagram for the Al-Ti-B system can be calculated from the binary data and the extrapolation. An isothermal section at 500°C is given in **Figure 6**. It is important to note that the phase equilibria in the Al-rich corner are not changed significantly if a homogeneous ideal range of (Al,Ti)B2 solution has not been assumed. The reason for this is the much higher thermodynamic stability of TiB2 compared to that of AlB2.

When an Al-Ti-B type master alloy is added to an alloy such as A356 (�7%Si), several intermetallic phases are created. Among these, there is mention made of intermetallics of the (Al,Si)3Ti [32, 33] and Ti6Si2B [31] type. Ramos et al. [34] carried out detailed research on the ternary phase Ti6Si2B which belongs to the Ti-Si-B system. Thanks to X-rays, this phase is characterized by a hexagonal crystalline structure with lattice parameters a = 0.68015 nm and c = 0.33377 nm, and it forms a liquid through the following peritectic reaction: L + TiB + Ti5Si3 \$ Ti6Si2B.

*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

**Figure 6.** *Calculated isothermal section of the Al-Ti-B ternary system at 500°C.*

#### **Figure 7.**

*Projection of the liquidus of the Ti-Si-B system in the Ti-rich part. The symbol Δ marks the composition of the Ti6Si2B phase.*

As shown in **Figure 7**, which presents a liquidus projection of the Ti-rich part of the Ti-Si-B system, five regions of primary solidification coexist, namely, Tiss, Ti5Si3, Ti6Si2B, TiB and TiB2. At 1200°C, the Ti6Si2B phase is formed in two-phase fields: Tiss, Ti5Si3 and TiB. Solidification ends with an invariant ternary eutectic Tiss + Ti6Si2B + Ti5Si3, which should correspond to the lowest liquidus temperature in the region shown in the figure. Based on the invariant reaction temperatures in the Ti-rich side of the Ti-Si [35] and Ti-B [36] systems, this ternary eutectic temperature should be less than 1330°C.

### *1.5.2 Al-Ti*

One of the effects of the presence of titanium Ti in an aluminum alloy is the reduction in grain size. However, this reduction is no longer achievable in pure aluminum since the number of grains per unit length increases, and consequently, the size of the grains also increases as shown in **Figure 8**. This size becomes almost constant when the content in titanium is between 0.08% and 0.13% before it begins to increase again with increasing percentage of titanium [36].

Grain refinement in aluminum alloys by the addition of Al-Ti master alloys has been widely applied and studied in recent years. The grain refining mechanism by Al-Ti is not much doubted and can be explained by the action of Al3Ti particles as heterogeneous nucleating centers [37] as well as by the peritectic theory. The size, morphology and quantity of the nuclei of the different microstructures of the Al-Ti master alloy seem to be important factors in determining the degree of grain refinement. The efficiency of the grain refiner depends on its chemical composition and its processing parameters such as the maintenance at such a temperature, the contact time, the mechanical agitation and the rate of cooling. To show the effect of increasing the percentage of titanium, Simensen [38] investigated a series of Al-7%Si alloys with a cooling rate of the order of 1°C/s. Al-10%Ti rods were added to the liquid metal making alloys with Ti in the range of 0.01–0.18%Ti. The cooling curves showed that at first the grains began to grow at some supercooling. The gradual addition of titanium increased the growth temperature of the alloys according to the following equation: Tgrowth = 613.2°C + 30.2%Ti (by weight). As for the grain size of the alloys, it was reduced from about 2000 μm to 250 μm when the titanium content increased from 0.01% to 0.12%. The best results were obtained when the Al3(Ti,Si) phases were nucleated on the TiB2 particles during cooling, whereas the aluminum grains which form on the Al3(Ti,Si) intermetallics yield the fine-grained material.

On the other hand, Li et al. [39] investigated the effect of various microstructures on grain refinement by Al-Ti master alloys synthesized at high temperature by mixing aluminum with titanium. The results of their work showed that the variation in experimental parameters, such as the stoichiometric ratio of the initial powders, the particle sizes of the powders, the use of fluxes, etc., led to the formation of various structures of the master alloys, in particular sizes, morphologies and quantities of

**Figure 8.** *Number of grains per unit length as a function of Ti content in pure aluminum.*

*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

Al3Ti particles. The intermetallic particles showed needle-like morphology in the lack of aluminum powder in the initial mixture which was related to the higher reaction temperature (**Figure 9a**). The same particles had needle-like and blocky forms at the same time (mixed morphology) which resulted from the lower reaction temperature (**Figure 9b**). Block-like Al3Ti crystals were formed at the lower reaction temperature when excess aluminum powder was added to the initial mixture (**Figure 9c**).

In order to clearly highlight the effect of the increase in titanium on the grain size and on the growth temperature Tc, Tøndel and Arnberg [40] studied the behavior of a binary alloy of the Al-10% Si type, with Ti additions through an Al-6%Ti master alloy. Two series of alloys were prepared: series A cast in a cold mold (0.02–1.5% Ti) and series B cast in a preheated mold (0.02–0.2% Ti). It was essential to compensate for the effect of differences in silicon content on the liquidus temperature before studying the effect of titanium additions on the recorded Tc growth temperature. Temperature data therefore compensated for the true deviation in Si content from a chosen low composition, 9.6%Si, in the claim that Ti additions do not change the slope of the liquidus line of the Al-Si system in small temperature intervals. The calculation was made with a polynomial that describes the liquidus temperature as a function of the Si content in hypoeutectic Al-Si binary alloys: Tliq (°C) = 660–5.59Si–0.14Si.

The phase identification clearly shows that when the Ti content is increased, intermetallic particles will appear in the solidified material. An example of a large Ti (Al,Si)3 type crystal (250 μm) is found in a B-series sample. Microprobe analyses of different samples show that Al3Ti arises when silicon varies from 12 to 13% in the solution. This may, in fact, be an indication that the Al3Ti particles of the master alloy have survived in the Al-Si liquid metal which thereby reaches equilibrium composition. The illustration in **Figure 10** suggests that the peritectic composition at 0.15% Ti is shifted to lower Ti concentrations when silicon is present because the particle is too large to be a result of exceeding the 0.15% Ti limit by only 0.023% Ti [41–43].

#### *1.5.3 Al-B*

The use of Al-B type master alloys (with 1–4%B) to achieve grain refining in Al-Si alloys is very common since this type of refiner is the most powerful in this type of alloys [43, 44]. The main particles that favor germination sites are AlB2 (more stable in Al-Si) and AlB12. The average AlB2 particle size varies with the B content in the master alloy as shown in **Figure 11a**. It is obvious that the small additions in B have a remarkable effect on the solidification process of the alloy. **Figure 11b** shows the

#### **Figure 9.**

*Microstructures Al-Ti master alloys. (a) Master alloy with acicular Al3Ti particles. (b) Master alloy with a mixture of acicular and blocky Al3Tiparticles. (c) Master alloy with blocky Al3Ti particles.*

**Figure 10.** *Growth temperature Tc and grain size as a function of Ti.*

#### **Figure 11.**

*(a) Average particle size of AlB2 as a function of the content of B in Al-B, (b) cooling curve of the Al-9.6%Si alloy as a function of the added B.*

effect of boron additions on the first part of solidification of an Al-9.6%Si alloy. The cooling curve increased by 2 to 3°C when 161 ppm B was added. The addition of B eliminates the phenomenon of supercooling and recalescence on the solidification curve.

Several concepts of the grain refinement mechanism of B on hypoeutectic Al-Si alloys have been adopted: the effect of B grain refinement on the α-Al phase and on the eutectic silicon with different additions of master alloys at 850°C was studied by Wang and Bian [45], where master alloys formed under different temperature conditions were studied to explore the morphologies of the AlB2 particles; the sample slowly cooled with the addition of grain refiner was made to explore the mechanism of refinement. The master alloy can refine not only the α-Al dendritic phase, but the eutectic silicon. Theoretical analysis indicates that although the AlB2 particles do not

### *A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

participate directly in the pure Al nucleation process in the presence of silicon, they provide a substrate for the precipitation of a small silicon content on which α-Al will grow without supercooling. As the temperature decreases to the eutectic line, AlB2 later nucleates the eutectic silicon; AlB2 particles appear in two different morphologies, namely hexagonal platelet and tetradihedron morphology which depend on temperature processing conditions.

As grain refining is an important process in industrial practice and has been the subject of much study, microstructural characterization of master alloys is useful in monitoring their production to ensure consistency of performance [43, 46]. AlB2 particles appear in master alloys in two different morphologies, namely, as hexagonal platelets or as a regular tetradihedron. This difference in morphology depends on the state of treatment. The hexagonal platelet morphology is favored by low temperature production, the reaction between Al and KBF4 is inactive, and the solubility of B in liquid aluminum is rather low. The formation of the AlB2 phase is affected by the long distance diffusion of B atoms, which makes the obvious growth trend of the crystal. The growth of the AlB2 particle proceeds by the diffusion of B atoms along <1120> at the edge of the crystal platform, the diffusion rate of B along <0001> is negligible. The schematic representation is shown in **Figure 12**.

The tetradihedron morphology is tilted to be formed at high temperature. The chemical reaction of Al and KBF4 proceeds rapidly at high temperature, therefore, the content of B in the aluminum liquid bath is higher at high temperature than that at low temperature. The diffusion of B atoms has relatively little influence on crystal growth, each plane of AlB2 grows with almost the same speed. The diffusion velocity of B along <0001> cannot be neglected. When the diffusion velocity of B along < 1120> is a little higher than that along <0001>, the tetradihedron morphology will be formed as shown in **Figure 12b**. No evidence that the morphology has an influence on the efficiency of the refiner was found, however, it seems that the hexagonal plate is always located in the center of two silicon flakes. The tetradihedron morphology is tilted to lie at the center of α-Al [43].

Very small additions of boron to Al-Si alloys lead to the precipitation of aluminum borides. To the Al-Si eutectic liquid, an addition of about 0.01% by weight of B is sufficient for this effect. It is the dominant feature of the calculated Al-B-Si phase diagram section at 0.1%B by weight, as shown in **Figure 13**. It looks like the binary Al-Si diagram with just AlB2 as the additional equilibrium phase. The other boride,

**Figure 12.** *Schematic representation of AlB2 morphologies; (a) hexagonal insert, (b) tetra-dihedron.*

#### **Figure 13.**

*Section of the Al-B-Si system calculated at 0.1%B. The composition of the ternary liquid eutectic is 12%Si and only 0.01%B; the temperature is 0.1°C below 577°C of the Al-Si binary eutectic system.*

AlB12, precipitates out of the liquid at high temperature and high in Si. In order to completely dissolve 0.1%B by weight, the liquid must be heated above 775°C. Using the Si-B master alloy, all boride particles should be formed in-situ during solidification, presumably in fine distribution and acting as strong nucleation sites. Only the binary peritectic reaction in the reverse direction, L + AlB12 ! AlB2 is observed upon superheating at T = 972 � 5°C [41–47]. If AlB2 formation is also suppressed by cooling Al-Si-B liquid alloys, the metastable phase diagram should be considered. Such a calculation proves that the relevant phase boundaries in **Figure 14** are virtually unchanged, just the saturation phase AlB2 is replaced by AlB12. This minor difference is also demonstrated by the calculated ternary eutectic:

L Stable ¼ ð Þþ Al ð Þþ Si AlB2 at 576*:*9°C and L with 12*:*5%Si, 0*:*010%B wt ð Þ *:*% , L Metastable ¼ ð Þþ Al ð Þþ Si AlB12 at 576*:*89°C and L with 12*:*5%Si, 0*:*011%B wt ð Þ *:*% *:*

Both equilibria are just slightly below the calculated binary eutectic L = (Al) + (Si) at 577°C and L with 12.5% Si by weight. Due to the slow formation of AlB2 in the liquid metal, it is very likely that the next stable phase, AlB12, will be formed instead. The particles found in the center of the grains are in fact B-rich, but it can be difficult to distinguish between AlB2 and AlB12 by backscattered electron and X-ray mapping techniques [48, 49]. The work carried out by Gröbner et al. [31] proves that, from the ternary phase diagram and thermodynamics, the two Al borides could be equally well formed. The fact that boron is a very efficient grain refiner in Al-Si alloys, but not in pure aluminum [4], is convincingly explained by the additional presence of dissolved silicon with a high growth restriction factor [50].

**Figure 14** gives an overview of the grain size when the A356 alloy is treated by three grain refiners. In the absence of any addition, the grain size amounts to about 1850 μm in the base alloy, see **Figure 14a**. After an addition of 0.08%Ti using the Al-10%Ti master alloy, the size drops to approximately 800 μm, see **Figure 14b**.

*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

#### **Figure 14.** *Evolution of the grain size of the A356 alloy: (a) no addition, (b) Al-10%Ti, (c) Al-5%Ti-1%B, (d) Al-4%B.*

The aluminum grain size continues to decrease when 0.08% Ti is added using Al-5% Ti-1%B, see **Figure 14c**. A minimum value is obtained thanks to the boron-based refiner (Al-4%B) since the grain size is reduced to 200 μm, see **Figure 14d**. Adding excess titanium or boron has no effect on reducing the aluminum grain size. On the contrary, an overdose of titanium or boron can lead to deleterious effects on the microstructure of the alloy, and consequently on its mechanical properties.

## *1.5.4 Effect of master alloys*

The literature concerning the influence of the addition of Al-Ti and Al-B master alloys on grain refining is very voluminous. Li et al. [51] investigated the effect of the Ti:B ratio on the solidification structure of a molten aluminum arc which is similar to the welding arc process. Al blocks were prepared, a hole with a diameter of 3.5 mm and another with a diameter of 7.5 mm deep were drilled in the center of each aluminum block in order to hold the two types of powder used (Al-Ti and Al-B). Grain size measurements were made using the linear intercept method, and were

conducted at the edge, middle and center positions of each weld. The results are plotted in conventional form in **Figure 15**. This plot shows a minimum grain size at about 0.07% Ti.

Such a minimum has never been reported previously in welding or casting. However, one must be careful when interpreting such graphs because the Ti:B ratio also affects the performance of grain refiners.

The benchmarks shown in **Figure 15** are obtained for different Ti:B ratios. The minimum occurs when the Ti:B ratio approaches an atomic ratio of 2:1, which is the stoichiometric ratio for the formation of TiB2 and the highest mole number of TiB2 in the solder. Thus, it is concluded from these data that a stoichiometric TiB2 is the most effective compound for a good grain refinement of aluminum under these experimental conditions. This disagrees with a few previous reports [52] which point out that excessive Ti is necessary for good grain refining under casting conditions.

#### **1.6 Theories of grain refining**

Grain refining is an important technique for improving the properties of aluminum products. Various explanations have been presented in order to provide, a suitable mechanism for grain refining such as nucleating particle theories and phase diagram theories. Both categories of the theories are about the two types of particles present in Al-Ti-B master alloys. Particle theories or boride theory suggest that nucleation occurs on the borides in the master alloy (TiB2, AlB2 and (Ti,Al)B2), while phase diagram theories explain grain refinement by nucleation on the TiAl3 properitectic phase.

#### *1.6.1 Particle nucleation*

Cibula [53] proposed that nucleation is produced on borides or carbides when the latter are present. Borides are added by a master alloy, whereas carbides are formed by a reaction of residual carbon present in the liquid metal with additional titanium which results in a TiC-like form. The nucleation behavior of all borides can be

**Figure 15.** *Effect of of Ti and B additions on the average grain size.*

### *A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

discussed concurrently, since TiB2 and AlB2 are known to be isomorphic and hexagonal, with lattice parameters changing only slightly, having a = 0.30311 nm and c = 0.32291 nm, and a = 0.3009 nm and c = 0.3262 nm, respectively. The boron mixed phase is formed by replacing titanium atoms in the lattice with aluminum atoms. The stability of the (Al,Ti)B2 phase is not known; however, it is thought to convert to TiB2 after a long hold time. **Figure 16** shows the crystal structure of these two locations [54, 55].

When the Al-Ti-B master alloy is added, the titanium is present in hypoperitectic amounts (less than 0.15% Ti, **Figure 16**), where boron particles are often found at the grain centers, with titanium-enriched dendrites growing outside of them. This evidence suggests that borides nucleate in the α-Al phase. However, for other reasons, borides were thought to be less efficient nucleation sites than Al3Ti. In the master alloys, the borides are pushed or rejected towards the grain boundaries while the aluminides are at the centers of the grains [21]. Recently, Schumacher and Greer [55] confirmed that borides are pushed to grain boundaries and no grain enhancement is observed when there is no dissolved titanium. Additionally, borides are known to need some supercooling to nucleate aluminum, while aluminides need none.

Al3Ti particles are known to be strong nucleating bodies. If titanium is present at hypo-peritectic concentrations, a dramatic grain enhancement is observed. Al3Ti are found in the center of the grains at concentrations where they are stable, and multiple orientation ratios have been recognized between Al3Ti and the aluminum matrix [49]. Obviously, it can be concluded that Al3Ti is a better nucleant than TiB2. Phase diagram theories have been developed to explain how Al3Ti could be an active nucleant with hypo-peritectic compositions.

### *1.6.2 Theory of phase diagrams*

The theories in this category are grouped under this heading because each theory suggests that the grain refinement is caused by a peritectic reaction on the primary Al3Ti particles. In general, it has been suggested that a peritectic point shifts at low titanium concentrations (e.g. 0.05% Ti) caused by the addition of boron, and that this is the reason for the grain enhancement [54]. Therefore, it has generally been assumed that there is a peritectic Al-Ti-B ternary, and speculations and theories have been based on this assumption. The first attempt to explain the grain refining mechanism dates back to 1951. Crossley and Mondolfo [49] proposed a peritectic theory based on the peritectic reaction in the phase diagram of the Al-Ti system as:

$$\text{Liiquid} + \text{TiAl}\_3 = \alpha - \text{Al} \text{ (solid solution)}.$$

**Figure 16.** *Crystalline structure of AlB2 and TiB2.*

It is reasonably clear that the titanium aluminum crystals added by the master alloy are active nucleants and that the observed fading is due to the dissolution of these nucleants with time. Davies et al. [56], and Maxwell and Hellawell [57] observed TiAl3 particles at the center of the grains of the α-Al dendritic phase. The cooling curves published by Arnberg et al. [58] also support the order of nucleation, i.e., they show no supercooling but a nucleation temperature (Tn) above the melting point (Tf) of the liquid metal. This observation implies that nucleation occurs by a peritectic reaction around the peritectic temperature (665°C) which is higher than the melting point of pure aluminum.

Although the peritectic theory successfully explains the behavior of Al-Ti type master alloys, no consensus has emerged to explain the increased efficiency of commercial grain refiners containing titanium Ti and boron B. The authors of the peritectic theory suggest that the improved performance of boron is due to the shift of the peritectic composition from 0.15% Ti towards the aluminum end of the phase diagram, which ensures the thermodynamic stability of TiAl3 at low levels of addition of Ti (0.02%). Determined phase relationships show that the solubility of TiAl3 is practically unaffected by the presence of boron B. However, contrary to these thermodynamic predictions, Mondolfo et al. [48] obtained experimental data indicating the effect of boron by shifting the peritectic to the Al-rich end of grain seems to be far from being achieved. **Figure 17** presents the aluminum-rich part of the Al-Ti system diagram which clearly shows the domain of existence of the different phases with their formation temperatures.

### *1.6.3 Theory of peritectic transformation*

This theory was very popular in the late 1980s and early 1990s supported by Vader and Noordegraaf [59], and Bäckerud et al. [60, 61]. This theory assumes that TiAl3 is a stronger nucleant than TiB2. Therefore, it tries to explain how borides could slow down the dissolution rate of TiAl3 when an Al-Ti-B master alloy is added to the liquid aluminum bath, so that the more powerful nuclei remain active longer. It suggests that the borides form a shell around the aluminides and therefore slow down the

**Figure 17.** *Al-Ti binary diagram (Al rich corner).*

*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

dissolution of the aluminides as the diffusion must proceed through the boride shell. The finally dissolved aluminide leaves a liquid cell inside the boride shell, approximately peritectic in composition. Peritectic reaction takes place to form α-aluminum and growth occurs from there. A schematic representation of the peritectic reaction is shown in **Figure 18**.

Although this theory appears to fit experimental results, there is strong evidence against it, particularly that described by Johnsson et al. [62]. Borides are very stable in liquid aluminum alloys, compared to TiAl3 particles, to hypo-peritectic titanium compositions ( 0.15% Ti by weight). The peritectic theory suggests that borides are more soluble than TiAl3, because the borides must dissolve in the liquid bath so that they can reprecipitate onto the more slowly dissolved TiAl3 particles in the titanium-rich region produced by its dissolution - which does not seem possible. Even with boron in the master alloy, TiAl3 still dissolves after a few minutes at high temperatures.

Johnsson [63] melted and re-solidified a hypo-peritectic alloy and found that the efficiency of grain refining did not change with the number of cycles. If the peritectic mechanism were to occur, one would expect the efficiency of grain refining to decrease with the number of repetitions, as this would further allow the diffusion of titanium; therefore, the peritectic reaction would cease to occur. Boride shells have been found in the grains of the aluminum, although it is inconclusive that these were the site of nucleation. If they acted as nucleants, this was not the dominant mechanism, as more often boride particles were found in the grain center at hypoperitectic concentrations of titanium. Therefore, the evidence suggests that the peritectic mechanism did not work.

#### *1.6.4 Theory of hyper-nucleation*

Jones and Pearson [5] assumed the concept of hyper-nucleation theory at the TiB2/liquid metal interface. They proved that when titanium is in excess, the titanium atoms segregate at the TiB2/liquid metal interface, providing a stabilized layer of atoms on the surface of the TiB2 crystals. This layer, being a full solution of Ti and Al, was predicted to remain stable above the melting point of pure aluminum, i.e., it exists in the liquid metal before casting. During cooling, such a layer will allow the growth of the primary α-Al phase without any supercooling. Although this concept seems the most promising, no experimental evidence is there to support it. **Figure 19** presents a model of the hyper-nucleation theory [5, 64–66]:

**Figure 18.** *Schematic presentation of peritectic reaction.*

#### **Figure 19.**

*Schematic presentation of the theory of hypernucleation. (a) Excess of Ti (Ti/B > 2.21) in solution, (b) Ti segregation at the interfaces TiB2-liquid metal, (c) formation of layers of TiAl3 on TiB2, (d) nucleation of α-Al through peritectic reaction.*

#### *1.6.5 Duplex theory of nucleation*

Mohanty et al. [67–70] suggest that the formation of Al3Ti is caused by a concentration gradient of titanium towards the boride particles, constituted by an activity gradient towards the borides. Due to the local equilibrium near the borides, the Al3Ti would be stable and could subsequently nucleate in the α-Al phase, as for alloys whose titanium is found in hyper-peritectic concentrations. Jones [70] supported this titanium gradient theory of segregation, but there is no conclusive evidence that a titanium gradient exists. As early as 1977, Naess and Berg [71] tried to suggest that there would have been a high concentration of titanium around the borides in the liquid pool, but their evidence showed nothing more than the predicted solute profile on the solidification of an Al-Ti alloy.

The duplex theory of nucleation is not totally new. In 1971, Bäckerud [72] claimed to have observed Al3Ti on boride particles and to have proposed a series of reactions to explain this. It was also mentioned by Cornish [52], who proposed that the role of borides is to facilitate the formation of Al3Ti at hypo-peritectic concentrations due to a variation in the peritectic composition, which then induces nucleation via the peritectic reaction. They used some of the same arguments as Mohanty et al. [69] about the segregation of titanium to borides. If the TiB2 particles nucleate the Al3Ti particles, which, in turn, nucleate the α-Al, then the mechanism is still unexplained, especially since the difference in the expected nucleation temperatures and the nonapplication of the theory to the alloys of foundries are always a problem. If Al3Ti particles form on the surface of TiB2 particles that increase nucleation, then it is the borides that act directly or indirectly as nucleation sites.

#### **1.7 Effect of Si**

The alloy composition effect seems to be quite complex. It has been documented for the Al-Si [73–78] system that the grain size first decreases with increasing the alloying

### *A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

concentration and then, after reaching a minimum, the grain size increases with further additions. The minimum is obtained near the maximum solubility limit and some researchers have therefore reported the minimum in grain size at a maximum range of solidification, and therefore at the maximum time of solidification, suggesting that an alloy with a wide range of solidification grants a longer time for nucleation. However, Bäckerud and Johnsson [79] recently suggested that a cellular structure transition to dendrites with well developed orthogonal branches is responsible for the transition, i.e. a cellular-dendritic transition during the growth of equiaxed crystals. They proposed that the transition occurs at a growth restriction factor of 20. This factor is equal to ΣimiC0,i (ki-1) where m is the slope of the liquidus line, C0 is the composition of the liquid and k is the equilibrium distribution coefficient for all elements.

In order to identify the effect of increased silicon content on the morphology and grain size in hypoeutectic Al-Si alloys, Lee et al. [80] used six silicon concentrations, 1, 2, 3, 4, 5 and 8 (wt %) combined with five levels of master alloy (grain refiner) of Al-5%Ti-1%B. The different samples were solidified in a preheated cylindrical graphite crucible at a cooling rate of 0.7°C/s. The results indicate that grain size is controlled by a combination of nucleating power and constitutional conditions at the growing crystal interface. **Figure 20(a)** and **(b)** illustrate the evolution of the grain size according to the concentrations of silicon Si and titanium Ti.

The TiSi2 type phases are the main cause of such poisoning. The micrograph given in **Figure 21** proves the presence of these phases. Other researchers have proposed that they are Al3Ti particles instead of TiB2, which become coated by certain complex aluminides or silicides formed by the interaction of silicon with Al3Ti. These intermetallic compounds may be unable to nucleate aluminum during solidification, since they have always been observed at grain boundaries instead of grain centers [81]. However, since boron atoms are too light to be detected by energy dispersive X-ray spectroscopy, the possibility of the existence of borides cannot be ruled out by the absence of boron in the spectrum. Through X-ray energy dispersive spectroscopy, the TiSi2 phase was identified by the presence of the high intensity peaks relative to titanium and silicon elements. As for **Figure 22**, it illustrates the distribution of the elements of this phase in a 390 alloy treated with 0.4% Ti and cast after 120 minutes. The spectrum relating to the TiSi2 phase is given in **Figure 23**.

#### **1.8 Sr-grain refiner interaction**

The addition of strontium as a modifier in Al-Si alloys causes a transition in the morphology of the eutectic silicon from an acicular to a fibrous and fine form. However, strontium can also lead to the formation of a long and columnar α-Al dendritic phase. The Al-5Ti-1B master alloy is often used as a grain refiner to achieve a fine, equiaxed grain structure in aluminum and its alloys. The TiB2 or/and TiAl3 particles of the Al-5Ti-1B master alloy are thought to be able to act as nuclei for the primary α-Al phase. However, in Al-Si alloys with a high level of silicon (%Si more than 7% by weight), the grain refining power of the master alloy of Al-5Ti-1B is lower compared to that of Al-3B and Al-3Ti-3B master alloys [81, 82]. It was therefore thought that the silicon poisoned the nucleation nuclei. This is mainly related to the formation of silicon and titanium above the TiAl3 particles, and Kori et al. [83] specified that this poisoning effect could be neutralized by increasing the level of addition of the main alloy of Al-5Ti-1B. The sequence of the grain refiner and modifier addition has a significant influence on the grain grade of the α-Al dendritic phase when compared to the combined addition as shown in **Figure 24**.

**Figure 20.**

*(a) Average grain size as a function of Ti content for 6 levels of Si, (b) same as in (a) using Al-5%Ti-1%B master alloy.*

When the liquid bath is treated with both a grain refiner and a modifier, the evolution of the Sr concentration in the liquid metal is highly time dependent for higher levels of addition of the Al-Ti-B grain refiners, Ti-B **Figure 25** shows this phenomenon. The zero-time concentration is the level of Sr in the liquid aluminum before the addition of either Al-Ti-B grain refiner. After addition, a weakening of Sr is observed for the two liquid alloys; the liquid bath treated with Al-1.5Ti-1.5B master alloy loses its Sr much faster, especially at the initial stage after addition, compared to the liquid bath treated with Al-5Ti-1B. This explains the rapid loss of eutectic modification in the liquid alloy treated with Al-1.5Ti-1.5B, it means that there is insufficient free Sr in the liquid aluminum to modify all the eutectic silicon. The rapid loss of strontium can be explained by external oxidation and vaporization as shown in **Figure 25** [84].

As discussed before, boron B is the most efficient refiner for the A356 alloy. With the addition of strontium and the latter's reaction with boron B, the overall percentages of strontium and boron capable of acting as a refiner and modifier respectively decrease and therefore would not be as effective as those obtained when each is added *A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

#### **Figure 21.**

*Backscattered electron micrograph identifying the formation of TiSi2 phases in a 390 alloy (17%Si) treated with 200 ppm Sr, 0.4%Ti and cast at 750°C after 120 minutes of dwell time.*

#### **Figure 22.**

*Distribution of aluminum, titanium and silicon in a 390 alloy treated with Al-10%Ti, 200 ppm Sr, showing the TiSi2 phase and cast after 120 minutes of holding time.*

individually. In order to demonstrate the resulting reaction, strontium and boron were added to pure aluminum, and the casting was carried out under the same conditions as those existing during individual addition. Microstructural analysis proved that

*Dispersive X-ray (EDS) analysis of the TiSi2 phase proven by high intensity peaks.*

#### **Figure 25.**

*Sr concentrations as a function of time after AlTiB grain refiners were added to achieve 0.15% Ti in the liquid bath. The broken line refers to the case of Sr modification without addition of any grain refiner.*

## *A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

strontium and boron react with each other forming compounds of the SrB6 type according to the following reaction: Sr + 6B ! SrB6. This last product is confirmed by the results obtained by Li et al. [85] and Nafisi [86]. This type of compound, whose name is strontium hexaboride, is characterized by a very high melting temperature of 2500°C [87] with a weight ratio of Sr:B equivalent to 1.35:1.

This reaction proves that each atom of Sr could react with six atoms of B and consequently form a compound SrB6. The intermetallics SrB6 and TiB2 could act as nucleants but it should be considered that the consumption of boron in the compound is one of the main parameters. It means that AlB2 consumes less amount of boron in comparison with SrB6. Considering a constant amount of boron, the density of nucleating particles is much higher in the case of AlB2 since a lower number of boron atoms is associated with this compound. Therefore, the greater the number of effective nucleants, the greater the probability of having a smaller grain size. Using dispersive X-ray (EDS) analysis, the SrB6 phase was identified and confirmed by high intensity peaks. The strong affinity between strontium and boron is shown in **Figure 26** obtained using the electron microprobe. The size of SrB6 compounds varies between 5 and 10 μm, and their color is a mixture of dark gray and white [88].

#### **Figure 26.**

*Mapping produced by the electron microprobe showing the association of strontium and boron in pure aluminum forming SrB6 type phases [88].*

## **2. Conclusions**

Based on the presented survey of literature on the fundamental aspects of grain refining of Al-Si cast alloys, the main points could be summarized as follows:


*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

## **Author details**

Ehab Samuel<sup>1</sup> , Hicham Tahiri<sup>1</sup> , Agnes M. Samuel<sup>1</sup> , Victor Songmene<sup>2</sup> and Fawzy H. Samuel1 \*

1 Université du Québec à Chicoutimi, Chicoutimi, QC, Canada

2 Department of Mechanical Engineering, École de Technologie Supérieure (ÉTS), QC, Canada

\*Address all correspondence to: fhsamuel@uqac.ca

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Maxwell I, Hellawell A. The constitution of the system Al-Ti-B with reference to aluminum-base alloys. Metallurgical Transactions. 1972;**3**: 1487-1493

[2] Backerud L, Yidong S. Grain refining mechanisms in aluminium as a result of additions of titanium and boron, Part I. Aluminium. 1991;**67**:780-785

[3] Vatne HE. Efficient grain refinement of ingots of commercial wrought aluminium alloys part I: Method for grain refining. Aluminium. 1999;**75**:84-90

[4] Easton M, StJohn D. Grain refinement of aluminum alloys: Part 1. The nucleant and solute paradigms—A review of the literature. Metallurgical and Materials Transactions A. 1999;**30A**: 1613-1633

[5] Jones GP, Pearson J. Factors affecting the grain-refinement of aluminum using titanium and boron additives. Metallurgical Transactions B. 1976;**7B**:223-234

[6] Sigworth GK, Guzowski MM. Grain refining of hypoeutectic Al-Si alloys. AFS Transactions. 1985;**172**:907-912

[7] Kearns MA, Thistlethwaite SR, Cooper PS. Recent advances in understanding the mechanism of aluminium grain refinement by TiBAl master alloys. In: Hale W, edtiors. Light Metals 1996. Warrendale, PA: TMS; 1996. pp. 713-720

[8] Spittle JA, Keeble JM, Al Meshhedani M. A study of grain refinement efficiency in Al-Si alloy castings. In: Proceedings of the 4th Decennial International Conference on Solidification Processing. Sheffield; 1997. pp. 273-276

[9] Dahle AK, Tondel PA, Paradies CJ, Arnberg L. Effect of grain refinement on the fluidity of two commercial Al-Si foundry alloys. Metallurgical and Materials Transactions A. 1996;**27A**: 2305-2313

[10] Fuoco R, Correa ER, de Andrade Bastos M. Effects of grain refinement on feeding mechanisms in A356 aluminum alloy. AFS Transactions. 1998;**106**:401- 409

[11] Dahle AK, John DHS, Attavanich P, Taopetch P. Grain formation in AlSi7Mg0.35 foundry alloy at low superheat. Materials Science Forum. 2000;**331–337**:271-276

[12] Bäckerud L, Chai G, Tamminen J. Solidification Characteristics of Aluminium Alloys. In: Foundry Alloys. Vol. 2. Des Plaines, IL, USA: AFS/ Skanaluminium; 1990. pp. 71-84

[13] Arnberg L, Bäckerud L, Klang H. Intermetallic particles in Al-Ti-B-type master alloys for grain refinement of aluminium: Part II. Metals Technology. 1982;**9**:7-13

[14] McCartney DG. Grain refining of aluminium and its alloys using inoculants. International Materials Reviews. 1989;**34**(5):247-260

[15] Pearson J, Birch MEJ, Hadlet D. Recent advances in aluminium grain refinement. In: Proceedings of the Conference Solidification Technology in the Foundry and Cast House. London: Metals Society; 1980. pp. 1-5

[16] Volmer M, Weber A. Nucleation in super-saturated products. Zeitschrift für Physikalische Chemie Leipzig. 1925;**119**: 277-301

*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

[17] Yue NL, Lu L, Lai MO. Application of thermodynamic calculation in the in-situ process of Al/ TiB2. Composite Structures. 1999;**47**: 691-694

[18] Li H, Sritharan T, Lam YM, Leng NY. Effects of processing parameters on the performance of Al grain refinement master alloys Al-Ti and Al-B in small ingots. Journal of Materials Processing Technology. 1997; **66**:253-257

[19] Tuttle BL. Principles of thermal analysis for molten metal process control. In: AFS/CMI Proceedings of the Conference on Thermal Analysis of Molten Aluminum. Rosemont, IL; 1984. pp. 1-36

[20] Mondolfo LF, Barlock JG. Effect of superheating on structure of some aluminum alloys. Metallurgical Transactions B. 1975;**6 B**:565-572

[21] Taylor JA, Wang H, StJohn DH, Bainbridge IF. Anomalous grain coarsening behaviour in grain-refined aluminium alloys cast using low superheat. In: Angier J, editors. Light Metals 2001. Warrendale, PA: TMS; 2001. pp. 935-941

[22] Cook R, Cooper PS, Kearns MA. Benefits of master alloy melt treatments in the aluminum foundry industry. TMS Light Metals. 1996;**1996**:647-654

[23] Cooper P, Hardman A, Boot D, Burhop E. Characterisation of a new generation of grain refiners for the foundry industry. In: Crepeau PN, editor. Light Metals. San Diego, CA: TMS; 2003. pp. 923-928

[24] Lu HT, Wang LC, Kung SK. Grain refining in A356 alloys. Journal of Chinese Foundryman's Association. 1981;**29**:10-18

[25] Hayes FH, Lukas HL, Effenberg G, Petzow G. Thermodynamic calculation of the Al-Rich corner of the Al-Ti-B System. Zeitschrift für Metallkunde. 1989;**80**:361-365

[26] Roger P, Bauer J, Bohn M. Internal report, University of Vienna. 1996

[27] Zupanic F, Spaic S, Krizman A. Contribution to ternary system Al-Ti-B part 2 - study of alloys in Al-AlB2-TiB2 triangle. Materials Science and Technology. 1998;**14**:1203-1212

[28] Zupanic F, Spaic S, Krizman A. Contribution to ternary system Al-Ti-B Part 1: Study of diborides present in the aluminium corner. Materials Science and Technology. 1998;**14**:601-607

[29] Abdel-Hamid A, Hamar-Thibault S, Durand F. Nature and morphology of crystals rich in Ti and B in Al-Rich Al-Ti-B alloys. Journal of Crystal Growth. 1984;**66**:195-204

[30] Fjellstedt J, Jarfors AEW, Svendsen L. Experimental analysis of the intermediary phases AlB~2 AlB~1~2 and TiB~2 in the Al-B and Al-Ti-B systems. Journal of Alloys and Compounds. 1999; **283**:192-197

[31] Gröbner J, Mirkovic D, Schmid-Fetzer R. Thermodynamic aspects of grain refinement of Al-Si alloys using Ti and B. Materials Science and Engineering A. 1995;**395**:10-21

[32] Youdelis WV, Yang CS. Ti(Al,Si)3 compound formation in nonequilibrated Al-Ti-Si. Metal Science. 1980;**14**:500-501

[33] Sokolowski JH, Kierkus CA, Brosnan B, Evans WJ. Formation of insoluble Ti (Al,Si)3 crystals in 356 Alloy casting and their sedimentation in foundry

equipment: Causes, effect and solutions. AFS Transactions. 2000;**108**:491-496

[34] Ramos AS, Nunes CA, Rodrigues G, Suzuki PA, Coelho GC, Grytsiv A, et al. Ti6Si2B a new ternary phase in the Ti-Si-B System. Intermetallics. 2004;**12**: 487-491

[35] Seifert H-J, Lukas HL, Petzow G. Thermodynamic optimization of the Ti-Si system. Zeitschrift für Metallkunde. 1996;**87**:2-13

[36] Youdelis WV, Yang CS. Nonperitectic grain refinement of aluminum by titanium. Aluminium. 1980;**1980**: 411-413

[37] Sigworth GK. Theoretical and practical aspects of the modification of Al-Si alloys. AFS Transactions. 1983;**91**: 7-16

[38] Simensen CJ. Grain refining of Al-7wt%Si alloys. The Minerals, Metals & Materials Society. 1999;**1999**:679-684

[39] Li P, Kandalova EG, Nikitin VI. Grain refining performance of Al-Ti master alloys with different microstructures. Materials Letters. 2005; **59**:723-727

[40] Tøndel PA, Arnberg L. Grain refinement of an Al-10%Si alloy by Ti-additions. In: The 3rd International Conference on Aluminium Alloys. pp. 129-134

[41] Quested TE. Understanding mechanisms of grain refinement of aluminium alloys by inoculation. Materials Science and Technology. 2004; **20**(11):1357-1369. DOI: 10.1179/ 026708304225022359

[42] Wang X, Liu Z, Dai W, et al. On the understanding of aluminum grain refinement by Al-Ti-B type master

alloys. Metallurgical and Materials Transactions B. 2015;**46**:1620-1625. DOI: 10.1007/s11663-014-0252-3

[43] Mirkovic D, Gröbner J, Schmid-Fetzer R, Fabrichnaya O, Lukas HL. Experimental study and thermodynamic re-assessment of the Al-B System. Journal of Alloys and Compounds. 2004; **384**:168-174

[44] Kori SA, Murty BS, Chakraborty M. Development of an efficient grain refiner for Al-7Si alloy and its modification with strontium. Materials Science and Engineering A. 2000;**283**:94-104

[45] Wang L, Bian X. Refining effect of boron on hypoeutectic Al-Si alloys. Journal of Materials Science and Technology. 2000;**16**(5):517-520

[46] Kori SA, Murty BS, Chakraborty M. Development of an efficient grain refiner for Al-7Si Alloy. Materials Science and Engineering A. 2000;**280**:58-61

[47] Easton MA, Qian M, Prasad A, StJohn DH. Recent advances in grain refinement of light metals and alloys. Current Opinion in Solid State and Materials Science. Feb 2016;**20**(1): 13-24

[48] Mondolfo LF, Farooq S, Tse C. Grain refinement of aluminium alloys by titanium and boron. In: Solidification Processing 1987. Sheffield: The Institute of Metals; 1988. pp. 133-136

[49] Crossley FA, Mondolfo LF. Mechanism of grain refinement in aluminium alloys. Journal of Metals – Transactions of the American Institute of Mining and Metallurgical Engineers. 1951;**191**:1143-1148

[50] Hu B, Li H. Grain refinement of DIN226S alloy at lower titanium and boron addition levels. Journal of

*A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

Materials Processing Technology. 1998; **74**(1):56-60

[51] Li H, Chandel RS, Sritharan T. Effect of Al-Ti and Al-B master alloy addition on the grain refinement of stationary arc-melted Al weld. Journal of Materials Science Letters. 1996;**15**:1886-1887

[52] Cornish AJ. The influence of boron on the mechanism of grain refinement in dilute aluminium-titanium alloys. Metal Science. 1975;**9**:477-484

[53] Cibula A. The mechanism of grain refinement of sand casting in aluminium alloys. The Journal of the Institute of Metals. 1949–1950;**76**:321-360

[54] Kiusalaas R, Bäckerud L. Influence of production parameters on performance of Al-Ti-B master alloys. Solidification Processing, The Institute of Metals. 1988;**1988**:137-140

[55] Schumacher P, Greer AL. Studies of the action of grain-refining particles in aluminium alloys. In: Grandfield JF, Eskin DG, editors. Essential Readings in Light Metals. Cham: Springer; 2016. DOI: 10.1007/978-3-319-48228-6\_44

[56] Davis IG, Dennis JM, Hellawell A. Metallurgical Transactions. 1970;**1**: 275-279

[57] Maxwell I, Hellawell A. A simple model for grain refinement during Solidification. Acta Metallurgica. 1975; **23**:229-237

[58] Arenberg L, Bäckerud L, Klang H. Metall Technologie. 1982;**2**:465

[59] Vader M, Noordegraaf J. Light Metals. In: Campell PG, editor. Warrendale, PA: TMS; 1989. pp. 937-941

[60] Bäckerud L, Gustafson P, Johnsson M. Grain refining mechanisms in

aluminum as a result of additions of titanium and boron, part II. Aluminium. 1991;**67**(9):910-915

[61] Bäckerud L, Krol E, Tamminen J. Solidification characteristics of aluminium alloys. In: Wrought Alloys. Vol. 1. Oslo, Norway: AFS/ Skanaluminium; 1986. pp. 34

[62] Johnsson M, Bäckerud L, Sigworth GK. Study of the mechanism of grain refinement of aluminium after additions of Ti- and B-containing master alloys. Metallurgical Transactions A. 1993;**24A**: 481-491

[63] Johnsson M. On the mechanism of grain refinement of aluminium after additions of Ti and B. In: Light Metals 1993. Warrendale, PA: The Minerals, Metals and Materials Society; 1993. pp. 769-777

[64] Samuel FH. Studies on addition of inclusions to molten aluminum using a novel technique. Metallurgical and Materials Transactions B. 1995;**26B**: 103-109

[65] Easton MA, John DHS. A model of grain refinement incorporating alloy constitution and potency of heterogeneous nucleant particles. Acta Materialia. 2001;**49**:1867-1878

[66] Guthrie RIL. Studies on the fading behavior of Al-Ti-B master alloys and grain refinement mechanism using LiMCA. Light Metals. 1995;**1995**:859-868

[67] Mohanty PS, Samuel FH, Gruzleski JE. Studies on addition of inclusions to molten aluminum using a novel technique. Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science. 1995;**26**(1):103-109

[68] Mohanty PS, Samuel FH, Gruzleski JE, Kosto TJ. Studies on the mechanism of grain-refinement in aluminium. In: Mannweiler U, editor. Light Metals 1994: Proceedings of the Technical Sessions Presented by the TMS Light Metals Committee at the 123rd TMS Annual Meeting, San Francisco, February 27– March 3, 1994. San Fransisco, California, USA; 1994. pp. 1039-1045

[69] Mohanty PS, Samuel FH, Gruzleski JE. Mechanism of heterogeneous nucleation of pores in metals and alloys. Metallurgical Transactions A: Physical Metallurgy and Materials Science. 1993; **24**(8):1845-1856

[70] Jones GP. Grain refinement of castings using inoculants for nucleation above liquidus. Solidification Processing, The Institute of Metals. 1987;**1987**:496-499

[71] Naess SE, Berg O. Zeitschrift für Metallkunde. 1974;**65**:599-602

[72] Bäckerud L. On the grain refining mechanism in Al-Ti-B alloys. Jernkontorets Ann. 1971;**155**:422-424

[73] Samuel AM, Mohamed SS, Doty HW, Valtierra S, Samuel FH. Some aspects of grain refining of Al-Si cast alloys. International Journal of Cast Metals Research. 2019;**32**(1):1-14

[74] Samuel AM, Mohamed SS, Doty HW, Valtierra S, Samuel FH. Grain refining of Al-Si alloys using Al-10% Ti master alloy: Role of Zr addition. International Journal of Cast Metals Research. 2019;**32**(1):46-58

[75] Samuel AM, Doty HW, Valtierra S, Samuel FH. A metallographic study of grain refining of Sr-modified 356 alloy. International Journal of Metalcasting. 2017;**11**(2):305-320

[76] Tahiri H, Mohamed SS, Doty HW, Valtierra S, Samuel FH. Effects of grain refining on columnar-to-equiaxed

transition in aluminum alloys. In: Sivasankaran S, editor. Aluminium Alloys - Recent Trends in Processing, Characterization, Mechanical Behavior and Applications. London, UK: Intech; 2017

[77] Samuel AM, Doty HW, Valtierra S, Samuel FH. Effect of grain refining and Sr-modification interactions on the impact toughness of Al–Si–Mg cast alloys. Materials & Design. 2014;**56**: 264-273

[78] Habibi N, Samuel AM, Samuel FH, Rochette P, Paquin D. Effect of grain refining and Sr modification on Prefil measurement sensitivity in 356 alloys using electron probe microanalysis technique. International Journal of Cast Metals Research. 2004;**17**(2):79-87

[79] Bäckerud L, Johnsson M. The relative importance of nucleation and growth mechanism to control grain size in various aluminum alloys. In: Hale W, editor. Light Metals 1996. Warrendale, PA; TMS; 1996. pp. 679-685

[80] Lee YC, Dahle AK, StJohn DH, Hutt JEC. The effect of grain refinement and silicon content on grain formation in hypoeutectic Al-Si alloys. Materials Science and Engineering A. 1999;**259**: 43-52

[81] Rao AA, Murty BS, Chakraborty M. Influence of chromium and impurities on the grain-refining behavior of aluminum. Metallurgical and Materials Transactions A. 1996;**27**:791-800

[82] Sritharan T, Li H. Influence of titanium to boron ratio on the ability to grain refine aluminium-silicon alloys. Journal of Materials Processing Technology. 1977;**63**:585-589

[83] Kori SA, Murty BS, Chakraborty M. Development of an efficient grain refiner *A Review on Fundamentals of Grain Refining of Al-Si Cast Alloys DOI: http://dx.doi.org/10.5772/intechopen.112987*

for Al–7Si alloy and its modification with strontium. Materials Science and Engineering. 2000;**83**(1–2):94-104

[84] Lu L, Dahle AK. Effects of combined additions of Sr and AlTiB grain refiners in hypoeutectic Al-Si foundry alloys. Materials Science and Engineering A. 2006;**435–436**:288-296

[85] Li JG, Zhang BQ, Wang L, Yang WY, Ma HT. Combined effect and its mechanism of Al-3wt.%Ti-4wt.%B and Al-10wt.%Sr master alloy on microstructures of Al-Si-Cu alloy. Materials Science and Engineering A. 2002;**A328**:169-176

[86] Nafisi S. Effects of grain refining and modification on the microstructural evolution of semi-solid 356 Alloy. [PhD thesis]. Chicoutimi, Canada: Université du Québec à Chicoutimi (UQAC); 2006. p. 358

[87] Massalski TB, Scott WW. Binary Alloy Phase Diagrams. 2nd ed. USA; 1990. p. 540

[88] Paradis M, Abdelaziz MH, Doty HW, Samuel FH. On the mechanical properties of lost foam Cast A356 automotive components: Effects of melt treatment and solidification conditions. International Journal of Metalcasting. 2017;**11**(3):494-505

## Section 2
