**1. Introduction**

54 Heat Treatment – Conventional and Novel Applications

Bearings, Management Art, 56 p., Žilina.

Jech, J. (1983). Thermal Processing of Steel. 384 p., SNTL Praha.

212 p. FVT TU Košice, ISBN 978–80–553–0636–0, Prešov.

Tech Publications, ISSN 1013-9826, Zurich, Switzerland.

chemical society Praha, HF TU Košice, ISSN 0009-2770, Praha.

Vasilko, K. (1988). Roller Bearings, 540 p., Alfa, Bratislava.

Blecharz, P.; Štverková, H. Product Quality and Customer Benefit. International Symposium on Applied Economics, Business and Development, Dalian, China. Communication in Computer and Information Science. BERLIN, SPRINGER-VERLAG,

Firm ZVL & ZKL Publ. No. TKS 1/96 S. (1996). Assembly, Disassembly and Failures of Roller

Firm ZVL AUTO spol. s r.o. Prešov (2008), Internal documentationfor tapered roller bearings

Panda, A. (2011). Automotiv production from development to serial production. Monograph,

Panda, A.; Duplák, J. & Jurko, J. (2011). Analytical expression of T-vc dependence in standard ISO 3685 for cutting ceramic. In: Key Engineering Materials, 317-322 p. Vol.480-481. Trans

Panda, A.; Duplák, J. & JURKO, J. (2011). Analytical expression of *T*-*v*c dependence in standard ISO 3685 for sintered carbide. *In: International Conference on Computer Science and Automation Engineering, CSAE 2011,*10-12.6.2011, Shanghai, art. no. 5952440, pp. 135-139, Publisher: Institute of Electrical and Electronics Engineers, ISBN 978-142448725-7,

Panda, A.; Duplák, J.; Jurko, J. & Behún, M. (2012). Comprehensive Identification of Sintered Carbide Durability in Machining Process of Bearings Steel 100CrMn6. In: Advanced Materials Research, Trans Tech Publications, vol. 340, p.30-33, ISSN 1022-6680, Zurich,

Panda, A.; Jurko, J.; Džupon, M. & Pandová, I. (2011). Optimalization of Heat Treatment Bearings Rings With Goal to Eliminate Deformation of Material. Chemické listy, Material in Engineering Practice 2011, vol. 105, Herľany, p.459-461, Special issue, Asociation of Czech

Panda, A.; Jurko, J. & Gajdoš, M. (2009). Accompanying phenomena in the cutting zone machinability during turning of stainless steels. International Journal Machining and

Panda, A.; Jurko, J. & Gajdoš, M. (2011). Study of changes under the machined surface and accompanying phenomena in the cutting zone during drilling of stainless steels with low carbon content, 113-117 p. Metallurgy No.2, vol. 50, ISSN 0543-5846 Zagreb, Croatia. Panda, A.; Vasilko, K. & Duplák, J. (2011). Evaluation of *T*-*v*c dependence in standard ISO 3685 for selected cutting materials. *In: Asia-Pacific Conference on Wearable Computing Systems,* pp. 129-132, *APWCS2011, 19-20.3.2011,* vol.2, Changsha, China, IEEE, ISBN 978-1-4244-9870-3,

Perez, M.; Sidoroff, Ch.; Vincent, A. & Esnouf, C. (2009). Microstructural evolution of martensitic 100Cr6 bearing steel during tempering: From thermoelectric power measurements to the prediction of dimensional changes. Acta Materialia 57, pp. 3170–3181.

Machinability of Materials, INDERSCENCE, ISSN 1748-5711, Switzerland.

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**7. References** 

production, Prešov

Shanghai, China.

Switzerland.

Changsha, China.

The trend of the automotive industry goes toward the construction of high-powered, comfortable, economical, ecological and safe vehicles. A few Al alloys containing Cu and a few containing Mg and Si are heat treatable in the cast condition due to the precipitation strengthening mechanisms. Two of the major families of heat treatable aluminum alloys containing magnesium and silicon are the 6xxx series in wrought aluminum alloys, and 3xx series in casting aluminum alloys. Al-Si-Cu/Mg alloys are well studied and there exists a lot of publication about the effect of alloying elements and solidification rate on the microstructure formation [1-3]. The influence of heat treatment on the mechanical properties including hardness and tensile strengths is also well studied, while the influence on plastic deformation behavior and elongation to fracture is less studied.

Although the benefit of heat treatment is undisputed, there exist several challenges for heat treatment operators, including market expectations of higher performance and reliability, lower production costs and energy use, as well as concern over environmental impacts. The heat treatment of age hardenable aluminum alloys involves solutionizing the alloys, quenching, and then either aging at room temperature (natural aging) or at an elevated temperature (artificial aging). The enhancement in mechanical properties after thermal treatment has largely been attributed to the formation of non-equilibrium precipitates within primary dendrites during aging and the changes occurring in Si particles characteristics from the solution treatment. The age hardening response depends on the fraction size, distribution and coherency of precipitates formed. Al-Si-Cu-Mg alloys and Al-Si-Mg alloys generally have a high age hardening response, while Al-Si-Cu alloys have a slow and low age hardening response.

© 2012 Mohamed and Samuel, licensee InTech. This is an open access chapter 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. © 2012 Mohamed and Samuel, licensee InTech. This is a paper 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.

## **2. Solidification process**

During the solidification from a melt, chemical thermodynamics and kinetics are generally considered in terms of the enthalpy and Gibbs free energy changes, the solidification path, composition changes, and phase transformations etc.. Chemical thermodynamics describes the most stable phases at equilibrium conditions (i.e. temperature, pressure, compositions etc.) relating to only the initial and final states of a system. Accordingly, the solidification rate in a metallurgical system can be estimated by the enthalpy (H) and heat capacity (Cp), and how these thermodynamic properties reflect the system thermal state and heat energy requirements. Chemical equilibrium is controlled by the Gibbs free energy (G) of the system which is minimized for equilibrium conditions. In contrast, the dynamic system transformation between initial and final states controlled by chemical kinetics, indicates the path and phase changes of a chemical reaction in a system when the limited atomic movement (i.e. in solids, low temperatures, etc.) becomes dominant in a short process time. Hence the solidification rate under a real time condition will be greatly influenced by the nucleation efficiency and the atom diffusion between phases [4].

A Review on the Heat Treatment of Al-Si-Cu/Mg Casting Alloys 57

Dendritic Post-dendritic Pre-eutectic

Eutectic Co-eutectic

Heterogeneous nucleation should be the major approach to refine the grains, which nucleate on some of the foreign nuclei sites and grow slowly in the melt. Effective grain refiners, such as TiAl3 and TiB2, must match their lattice perfect coherently to the Al matrix with their lattice coherencies (Figure 1.b). In contrast, particles with a poor lattice matching have little influence on increasing the nucleation of grains (Figure 1.c), resulting in an unrefined grain structure [9]. Typical examples of the microstructure of unmodified,

Sr-modified, and Sb-modified alloys are shown in Figure 2.

600 Aluminum dendrites

550 Eutectic Al + Si

Temperature (°C) Phases precipitated Suffix

and (Al15(Mn, Fe)3Si2) and /or Al5FeSi

and Al5FeSi Mg2Si

**Table 1.** Sequence of phase precipitation in hypoeutectic Al-Si alloys [7]

matching to Al atoms (points); c) Poor lattice matching [9].

(a)

650 Primary Al15(Mn, Fe)3Si2 (sludge) Pre-dendrite

500 CuAl2 and more complex phases Post-eutectic

**Figure 1.** Schematics of a) Three essential elements (grains, Al dendrites, DAS, and eutectic Si in a basic hypoeutectic Al-Si microstructure; b) Perfect grain refiner particles (squares) with one to one lattice

(b)

(c)

The solidification rate determines the coarseness of the microstructure including the fraction, size and distribution of intermetallic phases and the segregation profiles of solute in the α-Al phase. Large and brittle intermetallic phases form during a slow solidification, which may initiate or link fracture, decreasing elongation to fracture. Additional, the defect size such as pore size, is also controlled to some extent by the solidification rate. The influence of defects on the elongation to fracture depends on their size, shape, distribution and fraction. Dendrite arms with smaller radius may remelt into the molten into the molten liquid along with the decreasing total interfacial energy. The Ostwaldripening effect on the formation of dendrite arm spacing (DAS) is determined by local solidification time, allowing smaller particles to grow and merge into the larger ones due to the reduced total surface energy in the system. DSA, which is proportional to (average cooling rate)-n where n =1/2 and 1/3 for the primary and secondary dendrites respectively, generally ranging from 10 to 150 mm and which are controlled mainly by the solidification rate [5]. To gain an optimum property of an alloy, the DAS therefore must be minimized and distributed homogeneously.

The major phases in as-cast microstructure of Al-Si alloys are large size grains and primary α-Al, acicular eutectic Si, coarse primary Si, and also other harmful intermetallic phases such as needle like β-Al5FeSi, with uncontrolled and unevenly distributed porosities etc. [6]. Table 1 summarizes the sequence of phase precipitation in hypoeutectic Al-Si alloys [7]. Al in the eutectic has been reported to have mainly the same crystallographic features as the primary α-Al dendrites in unmodified alloys [8]. Figure 1.a indicates a basic structure of hypoeutectic Al-Si alloys consisting of grains (sizes at 1~10 mm in general), dendrites (typical DAS - 10~150 µm), and eutectic Si which can be in acicular shapes as long as 2 mm or round particle as small as 1 µm. The acicular Si might be chemically modified to a fibrous morphology by using effective modifiers. Heterogeneous nucleation should be the major approach to refine the grains, which nucleate on some of the foreign nuclei sites and grow slowly in the melt. Effective grain refiners, such as TiAl3 and TiB2, must match their lattice perfect coherently to the Al matrix with their lattice coherencies (Figure 1.b). In contrast, particles with a poor lattice matching have little influence on increasing the nucleation of grains (Figure 1.c), resulting in an unrefined grain structure [9]. Typical examples of the microstructure of unmodified, Sr-modified, and Sb-modified alloys are shown in Figure 2.


**Table 1.** Sequence of phase precipitation in hypoeutectic Al-Si alloys [7]

56 Heat Treatment – Conventional and Novel Applications

nucleation efficiency and the atom diffusion between phases [4].

be minimized and distributed homogeneously.

During the solidification from a melt, chemical thermodynamics and kinetics are generally considered in terms of the enthalpy and Gibbs free energy changes, the solidification path, composition changes, and phase transformations etc.. Chemical thermodynamics describes the most stable phases at equilibrium conditions (i.e. temperature, pressure, compositions etc.) relating to only the initial and final states of a system. Accordingly, the solidification rate in a metallurgical system can be estimated by the enthalpy (H) and heat capacity (Cp), and how these thermodynamic properties reflect the system thermal state and heat energy requirements. Chemical equilibrium is controlled by the Gibbs free energy (G) of the system which is minimized for equilibrium conditions. In contrast, the dynamic system transformation between initial and final states controlled by chemical kinetics, indicates the path and phase changes of a chemical reaction in a system when the limited atomic movement (i.e. in solids, low temperatures, etc.) becomes dominant in a short process time. Hence the solidification rate under a real time condition will be greatly influenced by the

The solidification rate determines the coarseness of the microstructure including the fraction, size and distribution of intermetallic phases and the segregation profiles of solute in the α-Al phase. Large and brittle intermetallic phases form during a slow solidification, which may initiate or link fracture, decreasing elongation to fracture. Additional, the defect size such as pore size, is also controlled to some extent by the solidification rate. The influence of defects on the elongation to fracture depends on their size, shape, distribution and fraction. Dendrite arms with smaller radius may remelt into the molten into the molten liquid along with the decreasing total interfacial energy. The Ostwaldripening effect on the formation of dendrite arm spacing (DAS) is determined by local solidification time, allowing smaller particles to grow and merge into the larger ones due to the reduced total surface energy in the system. DSA, which is proportional to (average cooling rate)-n where n =1/2 and 1/3 for the primary and secondary dendrites respectively, generally ranging from 10 to 150 mm and which are controlled mainly by the solidification rate [5]. To gain an optimum property of an alloy, the DAS therefore must

The major phases in as-cast microstructure of Al-Si alloys are large size grains and primary α-Al, acicular eutectic Si, coarse primary Si, and also other harmful intermetallic phases such as needle like β-Al5FeSi, with uncontrolled and unevenly distributed porosities etc. [6]. Table 1 summarizes the sequence of phase precipitation in hypoeutectic Al-Si alloys [7]. Al in the eutectic has been reported to have mainly the same crystallographic features as the primary α-Al dendrites in unmodified alloys [8]. Figure 1.a indicates a basic structure of hypoeutectic Al-Si alloys consisting of grains (sizes at 1~10 mm in general), dendrites (typical DAS - 10~150 µm), and eutectic Si which can be in acicular shapes as long as 2 mm or round particle as small as 1 µm. The acicular Si might be chemically modified to a fibrous morphology by using effective modifiers.

**2. Solidification process** 

**Figure 1.** Schematics of a) Three essential elements (grains, Al dendrites, DAS, and eutectic Si in a basic hypoeutectic Al-Si microstructure; b) Perfect grain refiner particles (squares) with one to one lattice matching to Al atoms (points); c) Poor lattice matching [9].

A Review on the Heat Treatment of Al-Si-Cu/Mg Casting Alloys 59

also reported that the Cu + Mg content of the alloys determines the precipitation strengthening and the volume fraction of the Cu-rich and Mg-rich intermetallics obtained.

Yi [16] adopt the enhanced solid diffusion coefficient of Cu in his model. The diffusion coefficient of Cu in α-Al phase is increased by 4-fold. The presence of Si-phase also has great influence on the diffusion of Cu in the matrix. It is assumed that the diffusion coefficient of Cu increases by 20-fold due to the presence of Si-phase. The distribution of Mg and Si across the dendrite arm spacing also changes due to the increase of Cu diffusion in the matrix. This

Heat-treatment is of major importance since it is commonly used to alter the mechanical properties of cast aluminum alloys. Heat-treatment improves the strength of aluminum alloys through a process known as precipitation-hardening which occurs during the heating and cooling of an aluminum alloy and in which precipitates are formed in the aluminum matrix. The improvement in the mechanical properties of Al alloys as a result of heat treatment depends upon the change in solubility of the alloying constituents with temperature. Figure 5 shows the major steps of the heat treatment which are normally used to improve the mechanical properties of aluminum. The alloy should first be solution treated at a temperature just below the eutectic temperature for long enough to allow solutionizing of the second phase. Then it should be quenched to room temperature. Finally it should be heated to a lower temperature to allow precipitation. Table 2 details a few of the

The T6 heat treatment is illustrated in Figure 4 for an Al-Si-Cu alloy as an example. The evolution of the microstructure is shown; from (1) atoms in solid solution at the solution treatment temperature, through (2) a supersaturated solid solution at room temperature

after quench, to (3) precipitates formed at the artificial ageing temperature.

**Figure 4.** Diagram showing the three steps for precipitation hardening.

is attributed to the change of solidification path.

**3. Heat treatment of cast al alloys** 

more commonly applied heat treatments.

**Figure 2.** Comparison of the silicon morphology in: (a) unmodified; (b) Sr-modified (300 ppm Sr); and (c) Sb-modified (2400 ppm Sb), hypoeutectic aluminum-silicon alloys [10].

Copper forms an intermetallic phase with Al that precipitates during solidification either as blocky CuAl2 or as alternating lamellae of α-Al + CuAl2 [11]. During solidification, in the presence of iron, other copper containing phases form, such as Cu2FeAl7 or Q-Al5Cu2Mg8Si6 [12]. The CuAl2 phase can be blocky shape or finely dispersed α-Al and CuAl2 particles within the interdendritic regions, as shown in Figure 3. The presence of nucleation sites, such as FeSiAl5 platelets or high cooling rates during solidification can result in fine CuAl2 particles [11]. The blocky CuAl2 phase particles are difficult to dissolve during solid solution heat treatment, unlike the fine CuAl2 phase particles that can dissolve within 2 hrs solid solution heat treatment [13]. Magnesium is present as Mg2Si in Al-Si-Mg alloys if Mg is not in solution. Mg can also form a true quaternary compound Cu2Mg8Si6Al5 with other alloy elements in Al-319 alloy. In the absence of Cu, high Fe and Mg result in the appearance of π-FeMg3Si6Al8. The π phase is difficult to dissolve during solid solution heat treatment [8].

**Figure 3.** Cu-rich phases in as-cast 319 alloy: (a) Eutectic Al2Cu and (b) blocky Al2Cu [14].

A comparative study of the mechanical properties of Al-Si-Cu-Mg alloys was carried out by Cáceres *et al*. [15] to investigate the effects of Si, Cu, Mg, Fe, and Mn, as well as solidification rate. The authors observed that increasing the Cu and Mg content generally resulted in an increase in strength and a decrease in ductility, whereas an increased Fe content (at an Fe/Mn ratio of 0.5) dramatically lowered the ductility and strength of low-Si alloys. They also reported that the Cu + Mg content of the alloys determines the precipitation strengthening and the volume fraction of the Cu-rich and Mg-rich intermetallics obtained.

Yi [16] adopt the enhanced solid diffusion coefficient of Cu in his model. The diffusion coefficient of Cu in α-Al phase is increased by 4-fold. The presence of Si-phase also has great influence on the diffusion of Cu in the matrix. It is assumed that the diffusion coefficient of Cu increases by 20-fold due to the presence of Si-phase. The distribution of Mg and Si across the dendrite arm spacing also changes due to the increase of Cu diffusion in the matrix. This is attributed to the change of solidification path.

## **3. Heat treatment of cast al alloys**

58 Heat Treatment – Conventional and Novel Applications

**Figure 2.** Comparison of the silicon morphology in: (a) unmodified; (b) Sr-modified (300 ppm Sr); and

Copper forms an intermetallic phase with Al that precipitates during solidification either as blocky CuAl2 or as alternating lamellae of α-Al + CuAl2 [11]. During solidification, in the presence of iron, other copper containing phases form, such as Cu2FeAl7 or Q-Al5Cu2Mg8Si6 [12]. The CuAl2 phase can be blocky shape or finely dispersed α-Al and CuAl2 particles within the interdendritic regions, as shown in Figure 3. The presence of nucleation sites, such as FeSiAl5 platelets or high cooling rates during solidification can result in fine CuAl2 particles [11]. The blocky CuAl2 phase particles are difficult to dissolve during solid solution heat treatment, unlike the fine CuAl2 phase particles that can dissolve within 2 hrs solid solution heat treatment [13]. Magnesium is present as Mg2Si in Al-Si-Mg alloys if Mg is not in solution. Mg can also form a true quaternary compound Cu2Mg8Si6Al5 with other alloy elements in Al-319 alloy. In the absence of Cu, high Fe and Mg result in the appearance of π-FeMg3Si6Al8. The π phase is difficult to dissolve during solid solution heat treatment [8].

**Figure 3.** Cu-rich phases in as-cast 319 alloy: (a) Eutectic Al2Cu and (b) blocky Al2Cu [14].

A comparative study of the mechanical properties of Al-Si-Cu-Mg alloys was carried out by Cáceres *et al*. [15] to investigate the effects of Si, Cu, Mg, Fe, and Mn, as well as solidification rate. The authors observed that increasing the Cu and Mg content generally resulted in an increase in strength and a decrease in ductility, whereas an increased Fe content (at an Fe/Mn ratio of 0.5) dramatically lowered the ductility and strength of low-Si alloys. They

(c) Sb-modified (2400 ppm Sb), hypoeutectic aluminum-silicon alloys [10].

Heat-treatment is of major importance since it is commonly used to alter the mechanical properties of cast aluminum alloys. Heat-treatment improves the strength of aluminum alloys through a process known as precipitation-hardening which occurs during the heating and cooling of an aluminum alloy and in which precipitates are formed in the aluminum matrix. The improvement in the mechanical properties of Al alloys as a result of heat treatment depends upon the change in solubility of the alloying constituents with temperature. Figure 5 shows the major steps of the heat treatment which are normally used to improve the mechanical properties of aluminum. The alloy should first be solution treated at a temperature just below the eutectic temperature for long enough to allow solutionizing of the second phase. Then it should be quenched to room temperature. Finally it should be heated to a lower temperature to allow precipitation. Table 2 details a few of the more commonly applied heat treatments.

The T6 heat treatment is illustrated in Figure 4 for an Al-Si-Cu alloy as an example. The evolution of the microstructure is shown; from (1) atoms in solid solution at the solution treatment temperature, through (2) a supersaturated solid solution at room temperature after quench, to (3) precipitates formed at the artificial ageing temperature.

**Figure 4.** Diagram showing the three steps for precipitation hardening.


A Review on the Heat Treatment of Al-Si-Cu/Mg Casting Alloys 61

The changes in the size and morphology of the silicon phase have a significant influence on the mechanical properties of the alloy. It has been proposed that the granulation or spheroidization process of silicon particles through heat treatment takes place in two stages: (i) fragmentation or dissolution of the eutectic silicon branches and (ii) spheroidization of the separated branches [20]. During solution treatment, the particles undergo changes in size and in shape. In the initial stages, the unmodified silicon particles undergo necking and separate into segments, which retain their original morphology. As a result of the separation, the average particle size decreases and the fragmented segments are eventually spheroidized. The spheroidization and the coarsening of eutectic Si can occur concurrently

The solution treatment process needs to be optimized because too short a solution treatment time means that not all alloying elements added will be dissolved and made available for precipitation hardening, while too long a solution treatment means using more energy than is necessary. The solution heat treatment may be carried out in either a single step or in multiple steps. Single-step solution treatment is normally limited to about 495C, in view of the fact that higher temperatures lead to higher thermal stresses induced during quenching and the risk of the incipient melting of the Cu-rich phases [21-23]. This incipient melting tends to lower the mechanical properties of the casting. Solution treatment at temperatures of 495C or less, however, is not capable of maximizing the dissolution of the copper-rich phases, nor is it able to modify the silicon particle morphology sufficiently. In Al-Si-Cu-Mg alloys having a low magnesium content (0.5 wt.%), Ouellet *et al*. [24] used a solution temperature of 500oC because, at 505oC, fusion of low melting point phases can occur; Wang *et al*. [25], on the other hand, reported that, for a similar alloy with a solution temperature of

520ºC**,** mechanical properties increase without any observable localized melting.

temperatures can have a deleterious effect on the mechanical properties [28].

Based on conventional solution treatment rules, the solution temperature of Al-Si-Cu-Mg alloys is restricted to 495oC, in order to avoid incipient melting of the copper-rich phase [26,27]. The time at the nominal solution treatment temperature must be long enough to homogenize the alloy and to ensure a satisfactory degree of precipitate solution. In alloys containing high levels of copper, complete dissolution of the Al2Cu phase is not usually possible. The solution time must then be chosen carefully to allow for the maximum dissolution of this intermetallic phase, bearing in mind nevertheless, that solutions treating the alloy for long times are expensive and may not be necessary to obtain the required alloy strength. Moreover, the coarsening of the microstructural constituents and the possible formation of secondary porosity which result after prolonged annealing at such

Studies by Gauthier *et al*. [19] on the solution heat treatment of 319 alloy over a temperature range of 480oC to 540oC, for solution times of up to 24 hours, showed that the best combination of tensile strength and ductility was obtained when the as-cast material was solution heat-treated at 515oC for 8 to 16 hours, followed by quenching in warm water at 60oC. A higher solution temperature was seen to result in the partial melting of the copper phase, the formation of a structureless form of the phase and related porosity upon

during the second stage.

**Table 2.** Common aluminum heat treatment designations

**Figure 5.** The T6 heat treatment process [17].

## **3.1. Solution heat treatment**

Solution heat treatment must be applied for a sufficient length of time to obtain a homogeneous supersaturated structure, followed by the application of quenching with the aim of maintaining the supersaturated structure at ambient temperature. In Al-Si-Cu-Mg alloys**,** The solution treatment fulfils three roles: [18,19]


The segregation of solute elements resulting from dendritic solidification may have an adverse effect on mechanical properties. The time required for homogenization is determined by the solution temperature and by the dendrite arm spacing. Hardening alloying elements such as Cu and Mg display significant solid solubility in heat-treatable aluminum alloys at the solidus temperature; this solubility decreases noticeably as the temperature decreases.

The changes in the size and morphology of the silicon phase have a significant influence on the mechanical properties of the alloy. It has been proposed that the granulation or spheroidization process of silicon particles through heat treatment takes place in two stages: (i) fragmentation or dissolution of the eutectic silicon branches and (ii) spheroidization of the separated branches [20]. During solution treatment, the particles undergo changes in size and in shape. In the initial stages, the unmodified silicon particles undergo necking and separate into segments, which retain their original morphology. As a result of the separation, the average particle size decreases and the fragmented segments are eventually spheroidized. The spheroidization and the coarsening of eutectic Si can occur concurrently during the second stage.

60 Heat Treatment – Conventional and Novel Applications

Solution treatment

Rapid quench

**Figure 5.** The T6 heat treatment process [17].

i. Homogenization of as-cast structure.

alloys**,** The solution treatment fulfils three roles: [18,19]

iii. Change of the morphology of eutectic silicon.

ii. Dissolution of certain intermetallic phases such as Al2Cu and Mg2Si.

**3.1. Solution heat treatment** 

temperature decreases.

**Table 2.** Common aluminum heat treatment designations

**Treatment Solution Quench Aging**

T4 Yes Yes Room Temperature only T5 No No Elevated Temperatures

T6 Yes Yes Elevated (to yield increased strength) T7 Yes Yes Elevated (to yield dimensional stability)

Artificial aging

Solution heat treatment must be applied for a sufficient length of time to obtain a homogeneous supersaturated structure, followed by the application of quenching with the aim of maintaining the supersaturated structure at ambient temperature. In Al-Si-Cu-Mg

The segregation of solute elements resulting from dendritic solidification may have an adverse effect on mechanical properties. The time required for homogenization is determined by the solution temperature and by the dendrite arm spacing. Hardening alloying elements such as Cu and Mg display significant solid solubility in heat-treatable aluminum alloys at the solidus temperature; this solubility decreases noticeably as the The solution treatment process needs to be optimized because too short a solution treatment time means that not all alloying elements added will be dissolved and made available for precipitation hardening, while too long a solution treatment means using more energy than is necessary. The solution heat treatment may be carried out in either a single step or in multiple steps. Single-step solution treatment is normally limited to about 495C, in view of the fact that higher temperatures lead to higher thermal stresses induced during quenching and the risk of the incipient melting of the Cu-rich phases [21-23]. This incipient melting tends to lower the mechanical properties of the casting. Solution treatment at temperatures of 495C or less, however, is not capable of maximizing the dissolution of the copper-rich phases, nor is it able to modify the silicon particle morphology sufficiently. In Al-Si-Cu-Mg alloys having a low magnesium content (0.5 wt.%), Ouellet *et al*. [24] used a solution temperature of 500oC because, at 505oC, fusion of low melting point phases can occur; Wang *et al*. [25], on the other hand, reported that, for a similar alloy with a solution temperature of 520ºC**,** mechanical properties increase without any observable localized melting.

Based on conventional solution treatment rules, the solution temperature of Al-Si-Cu-Mg alloys is restricted to 495oC, in order to avoid incipient melting of the copper-rich phase [26,27]. The time at the nominal solution treatment temperature must be long enough to homogenize the alloy and to ensure a satisfactory degree of precipitate solution. In alloys containing high levels of copper, complete dissolution of the Al2Cu phase is not usually possible. The solution time must then be chosen carefully to allow for the maximum dissolution of this intermetallic phase, bearing in mind nevertheless, that solutions treating the alloy for long times are expensive and may not be necessary to obtain the required alloy strength. Moreover, the coarsening of the microstructural constituents and the possible formation of secondary porosity which result after prolonged annealing at such temperatures can have a deleterious effect on the mechanical properties [28].

Studies by Gauthier *et al*. [19] on the solution heat treatment of 319 alloy over a temperature range of 480oC to 540oC, for solution times of up to 24 hours, showed that the best combination of tensile strength and ductility was obtained when the as-cast material was solution heat-treated at 515oC for 8 to 16 hours, followed by quenching in warm water at 60oC. A higher solution temperature was seen to result in the partial melting of the copper phase, the formation of a structureless form of the phase and related porosity upon

#### 62 Heat Treatment – Conventional and Novel Applications

quenching, with a consequent deterioration of the tensile properties. A two-stage solution heat treatment suggested by Sokolowski *et al*. [29] is reported to reduce the amount of the copper-rich phase in the 319 alloys significantly, giving rise to better homogenization prior to aging and improving mechanical properties. Also, Crowell *et al*. [30] stated that the blocky Cu phase in Al-Si-Cu alloys dissolves with increasing solution time at the recommended solution temperature of 495oC; also the rate of dissolution increases with Sr concentration.

A Review on the Heat Treatment of Al-Si-Cu/Mg Casting Alloys 63

components, however, it causes detrimental effects such as precipitation during quenching, localized over-ageing, reduction in grain boundaries, increase tendencies for corrosion and

The best combination of strength and ductility is achieved from a rapid quenching. Cooling rates should be selected to obtain the desired microstructure and to reduce the duration time over certain critical temperature ranges during quenching in the regions where diffusion of smaller atoms can lead to the precipitation of potential defects [40]. The effectiveness of the quench is dependent upon the quench media (which controls the quench rate) and the quench interval. The media used for quenching aluminum alloys include water, brine solution and polymer solution [41-43]. Water used to be the dominant quenchant for aluminum alloys, but water quenching most often causes distortion, cracking, and residual stress problems [44,45]. It has been reported that the water temperature affects the properties of the cast aluminum alloy A356 subjected to T6 heat treatment once the water exceeds 60-70oC, with UTS and YS being significantly more sensitive than ductility. Detailed TEM investigations on A356 alloy, reported elsewhere [46], revealed that, at the peak-aged condition and with a water quench at 25°C, the α-Al matrix consists of a large number of needle-shaped and coherent β″-Mg2Si precipitates. The size of the precipitates is approximately 3 to 4 nm in diameter and 10 to 20 in length. With a water quench at 60°C, they observed how the density of the precipitates decreases and the size of the precipitates increases slightly; at the same time a significant number of fine Si precipitates resulting from

With a slow quenching in air, very different precipitation features are normally evidenced. By air quenching, the material remains at high temperatures for a longer period, which enhances the diffusion of silicon and magnesium. Besides a high density of fine β″-Mg2Si precipitates, the α-Al matrix also contained a large number of areas with coarse rods β′- Mg2Si grouped parallel to each other [46]. While the first precipitates have an average size approximately 2 to 3 nm in diameter and around 40 nm in length, the latter show an average

Age-hardening has been recognized as one of the most important methods for strengthening aluminum alloys, which involves strengthening the alloys by coherent precipitates which are capable of being sheared by dislocations [47]. By controlling the aging time and temperature, a wide variety of mechanical properties may be obtained; tensile strengths can be increased, residual stresses can be reduced, and the microstructure can be stabilized. The precipitation process can occur at room temperature or may be accelerated by artificial

After solution treatment and quench the matrix has a high supersaturation of solute atoms and vacancies. Clusters of atoms form rapidly from the supersaturated matrix and evolve into GP zones. Metastable coherent or semi-coherent precipitates form either from the GP zones or from the supersaturated matrix when the GP zones have dissolved. The

result in a reduced response to ageing treatment [38,39].

precipitation of excess Si could be observed in the α-Al matrix.

size ~15 nm in diameter and 300 nm in length.

aging at temperatures ranging from 90 to 260oC.

**3.3. Aging** 

A two-step solution treatment, namely, conventional solution treatment followed by a hightemperature solution treatment, as suggested by Sokolowski *et al*., [31, 32] is reported to reduce the amount of the copper-rich phase in 319 alloys significantly, thereby giving rise to better homogenization prior to aging and thus also to improvements in the mechanical properties. The holding time for the first stage and the solution temperature of the second stage are both significant parameters. Sokolowski *et al*. [32] studied the improvement in 319 aluminum alloy casting durability by means of high temperature solution treatment. Their results showed that a two-step solution treatment of 495ºC/2h followed by 515ºC/4h produced the optimum combination of strength and ductility compared to the traditional single-step solution treatment of 495ºC/8h.

Dissolve the micro-segregation of Mg and Si elements to form a supersaturated solid solution in the primary Al matrix in order to enable the formation of a large number of strengthening precipitates during subsequent natural and artificial ageing processes. Homogenize the casting, and attain a globular morphology of the eutectic Si phase to impart improved ductility and fracture toughness to the component. Reduce micro-segregation of other alloying elements in the primary Al matrix.
