**Abstract**

Magnesium based nanocomposites are new lightweight and high-performance materials for potential applications in automotive, aerospace, space, electronics, sports and biomedical sectors primarily due to their lower density when compared to aluminum-based materials and steels. Synthesis of magnesiumbased materials is relatively challenging and accordingly this chapter explicitly provides an insight into various techniques hitherto devised/adopted by various researcher for synthesizing magnesium based nano-composites (MMNCs). Overall processing of MMNCs often includes combination of primary and secondary processing. Primary processing fundamentally leads to the initial formulation and creation of MMNC ingots by solid, semi-solid or liquid state processing routes. This is followed by secondary processing that includes plastic deformation or severe plastic deformation to alleviate inhomogeneity, clustering of particles and fabrication defects to enhance the properties of the MMNCs. This chapter provides an insight into different fabrication methodologies, their benefits and limitations for MMNCs.

**Keywords:** magnesium, reinforcement, nano-composite, MMNC, synthesis

## **1. Introduction**

Research on magnesium based-composites has seen sustainable growth in last four decades due to their light weight, higher strength to weight ratio, ductility, hardness, wear resistance and biodegradability [1, 2]. Magnesium based materials, in general, are currently targeted for applications in automotive, aerospace, electronics, sports and biomedical engineering. The driving force for intense research into magnesium-based nano-composites is to utilize them to mitigate global warming, energy consumption and land, air and water toxicity. The presence of reinforcement at nano-length scale leads to grain refinement leading to Hall-Petch strengthening and Orowan strengthening due to presence of nano-particles/fibers with diameter less than 100 nm [3].

The primary processing of MMNCs can be categorized in two groups i.e. ex situ and in situ routes [4]. In ex situ processing during fabrication of MMNCs the major issue is particle clustering. High surface energy navigates to poor wettability of particles/fibers with the matrix during liquid and semi-solid processing. These clustered particles lead to reinforcement distribution inhomogeneity in the matrix leading to inferior properties in as-cast state. Such non-uniform distribution can only be reduced through judicious secondary processing step.

The common liquid and semi-solid ex situ process are stir casting/melt stirring, ultrasound cavitation, disintegrated melt deposition (DMD) and Rheocasting. The solid-state ex situ syntheses are powder metallurgy (PM), severe plastic deformation (SPD) by accumulative roll bonding (ARB) and plastic deformation by friction stir processing. The in-situ processes eliminate the reinforcement clustering, since the reinforcements are distributed in the matrix by thermodynamic and chemical reaction during the process.

The MMNCs processed by ex situ route commonly exhibit microstructural defects such as dendrites, pores, and micro cracks. These require careful characterization followed by using a secondary processing technique that can target the property enhancement required by end application. The common secondary processes are thermal treatment, hot extrusion, hot rolling, equal channel angular pressing (ECAP) and cyclic extrusion and compression (CEC).

The prime objective of this chapter is to provide an overview of various processing methods used currently for synthesizing MMNCs, their benefits and limitations. Critical observations made by other researchers are also highlighted in this chapter.

## **2. Processing methods**

The processing methodology for fabricating magnesium based nano-composites (MMNCs) normally includes coupling of primary and secondary processing. In primary processing (solid, liquid or two phase), the matrix (master alloy) and the reinforcements are blended together by the application of thermal or mechanical energy to form a composite. During primary processing some undesirable effects are introduced in the composites such as porosity and non-uniform distribution of reinforcement voids. To minimize these defects, secondary processing is utilized to attain a relatively homogeneous microstructure and enhanced mechanical properties.

#### **2.1 Primary processing**

Primary processing techniques are grouped into liquid state, semi-solid and solid state processing types. The liquid state processes are stir casting, ultrasonic cavitation (UST), disintegrated melt deposition (DMD), in-situ processing. Rheocasting is a semi-solid composite primary processing. The soil-state fabrication includes powder metallurgy (PM), accumulated roll bonding (ARB) and friction stir processing (FSP). These techniques are briefly introduced in following subsections.

#### *2.1.1 Stir casting*

Stir casting is one of the most common liquid-phase technique utilized for processing MMCs for almost last four decades. The schematic of stir casting setup is shown in the **Figure 1**.

For processing MMNCs, the Mg ingots are melted in a crucible made of graphite or steel at a temperature between 680 and 750 °C in an electric resistance or induction furnace. The liquid melt is mechanically stirred using a coated impeller. The coating is provided onto the impeller to avoid erosion by abrasion and chemical reaction. Predetermined amount of nano-reinforcement is introduced in molten metal along the side of the vortex. The reinforcements were distributed in the melt due to the difference in pressure from the inner to outer surface of the liquid

**13**

**Figure 1.**

*Schematic of stir casting setup.*

*2.1.2 Ultrasonic cavitation method*

*Synthesis of Magnesium Based Nano-composites DOI: http://dx.doi.org/10.5772/intechopen.84189*

vortex. The vortex is shielded using inert atmosphere to avoid oxidation /ignition. Alternatively, to overcome agglomeration/clustering, the powders of magnesium and the nano reinforcement are ball milled together prior to addition. The liquid slurry is stirred for ~10 min to homogenize the mixture. After homogenization, the liquid slurry is poured into a permanent mold. MMNCs reinforced with Al2O3, SiC

The benefits of stir casting include: (a) ease of processing, (b) economical, and (c) scalability to ensure large volume production. Disadvantages of stir casting includes: (a) unavoidable agglomeration of reinforcement and (b) porosity.

One of the main drawback of the stir casting technique is its inability to avoid agglomeration of nano-reinforcement due to their larger surface energy. This causes inferior mechanical properties of the end composites. Ultrasonic (UST) cavitation method is a relatively more effective technique to disperse the reinforcement into the matrix material in MMCs [17]. By introducing the ultrasonic waves with power and frequency range of as low as 1.5–4.0 kW and 17.5–20 kHz the agglomerates can be fragmented, and a uniform distribution of reinforcement can be realized in the liquid melt. This method has been adopted so far to produce MMNCs reinforcing with CNTs, AlN, SiC, B4C and Al2O3 [17–35]. The schematic of UST setup is shown in the **Figure 2**.

and CNTs are commonly synthesized using stir casting technique [5–16].

*Synthesis of Magnesium Based Nano-composites DOI: http://dx.doi.org/10.5772/intechopen.84189*

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

(ECAP) and cyclic extrusion and compression (CEC).

reaction during the process.

**2. Processing methods**

**2.1 Primary processing**

chapter.

properties.

subsections.

*2.1.1 Stir casting*

shown in the **Figure 1**.

The common liquid and semi-solid ex situ process are stir casting/melt stirring, ultrasound cavitation, disintegrated melt deposition (DMD) and Rheocasting. The solid-state ex situ syntheses are powder metallurgy (PM), severe plastic deformation (SPD) by accumulative roll bonding (ARB) and plastic deformation by friction stir processing. The in-situ processes eliminate the reinforcement clustering, since the reinforcements are distributed in the matrix by thermodynamic and chemical

The MMNCs processed by ex situ route commonly exhibit microstructural defects such as dendrites, pores, and micro cracks. These require careful characterization followed by using a secondary processing technique that can target the property enhancement required by end application. The common secondary processes are thermal treatment, hot extrusion, hot rolling, equal channel angular pressing

The prime objective of this chapter is to provide an overview of various processing methods used currently for synthesizing MMNCs, their benefits and limitations. Critical observations made by other researchers are also highlighted in this

The processing methodology for fabricating magnesium based nano-composites

(MMNCs) normally includes coupling of primary and secondary processing. In primary processing (solid, liquid or two phase), the matrix (master alloy) and the reinforcements are blended together by the application of thermal or mechanical energy to form a composite. During primary processing some undesirable effects are introduced in the composites such as porosity and non-uniform distribution of reinforcement voids. To minimize these defects, secondary processing is utilized to attain a relatively homogeneous microstructure and enhanced mechanical

Primary processing techniques are grouped into liquid state, semi-solid and solid state processing types. The liquid state processes are stir casting, ultrasonic cavitation (UST), disintegrated melt deposition (DMD), in-situ processing. Rheocasting is a semi-solid composite primary processing. The soil-state fabrication includes powder metallurgy (PM), accumulated roll bonding (ARB) and friction stir processing (FSP). These techniques are briefly introduced in following

Stir casting is one of the most common liquid-phase technique utilized for processing MMCs for almost last four decades. The schematic of stir casting setup is

For processing MMNCs, the Mg ingots are melted in a crucible made of graphite or steel at a temperature between 680 and 750 °C in an electric resistance or induction furnace. The liquid melt is mechanically stirred using a coated impeller. The coating is provided onto the impeller to avoid erosion by abrasion and chemical reaction. Predetermined amount of nano-reinforcement is introduced in molten metal along the side of the vortex. The reinforcements were distributed in the melt due to the difference in pressure from the inner to outer surface of the liquid

**12**

**Figure 1.** *Schematic of stir casting setup.*

vortex. The vortex is shielded using inert atmosphere to avoid oxidation /ignition. Alternatively, to overcome agglomeration/clustering, the powders of magnesium and the nano reinforcement are ball milled together prior to addition. The liquid slurry is stirred for ~10 min to homogenize the mixture. After homogenization, the liquid slurry is poured into a permanent mold. MMNCs reinforced with Al2O3, SiC and CNTs are commonly synthesized using stir casting technique [5–16].

The benefits of stir casting include: (a) ease of processing, (b) economical, and (c) scalability to ensure large volume production. Disadvantages of stir casting includes: (a) unavoidable agglomeration of reinforcement and (b) porosity.

## *2.1.2 Ultrasonic cavitation method*

One of the main drawback of the stir casting technique is its inability to avoid agglomeration of nano-reinforcement due to their larger surface energy. This causes inferior mechanical properties of the end composites. Ultrasonic (UST) cavitation method is a relatively more effective technique to disperse the reinforcement into the matrix material in MMCs [17]. By introducing the ultrasonic waves with power and frequency range of as low as 1.5–4.0 kW and 17.5–20 kHz the agglomerates can be fragmented, and a uniform distribution of reinforcement can be realized in the liquid melt. This method has been adopted so far to produce MMNCs reinforcing with CNTs, AlN, SiC, B4C and Al2O3 [17–35]. The schematic of UST setup is shown in the **Figure 2**.

**Figure 2.** *Schematic representation of UST [4].*

In the UST process, the Mg alloy is placed in a graphite crucible and heated to the desired temperature using a resistance/induction furnace. Predetermined amount of reinforcement is than added depending on the size of the particles. During processing, Mg melt is protected using argon gas (flow rate of 20 lpm) to avoid oxidation. High-intensity ultrasound shock waves [16] are supplied to disperse nano-particles thoroughly in the melt at semi-solid temperature. The ultrasonic processing temperature is chosen so as to ensure better flowability of the slurry in the mold.

The selection of material for sonotrode plays a vital role in MMNCs melting due to the erosion of sonotrode surface during melting of liquid metal. For better sonification of MMNCs, niobium (Nb) and titanium (Ti) are recommended by the researchers. Ti-based sonotrodes are widely used for UST treatment due to lesser costs when compared to Nb-based sonotrodes. To note that Nb-based sonotrode exhibit less variation in the Young's modulus as the function of temperature while Ti-based sonotrodes are very stable in MMNCs melt as Ti is insoluble in Mg. Earlier findings have indicated that the high-intensity UST vibration needs an intensity of 100 W/cm2 [4]. For large scale volume production, the requirement of sonification is likely to be higher. The large-scale production of MMNCs require enormous power and frequencies. The key issue is reducing rate of sonification by decreasing the volume of the melt during UST. This can be achieved in two steps firstly, preparing the melt with reinforcement and secondly, passing the liquid melt into a sonotrode assisted UST chamber for fragmenting particle clusters. Further work is required in this area especially regarding scalability.

#### *2.1.3 Disintegrated melt deposition method (DMD)*

DMD technique is a hybrid technique that incorporates the concepts of casting, melt stirring and spray forming process [36–45]. In DMD, the processing steps involve:


**15**

*Synthesis of Magnesium Based Nano-composites DOI: http://dx.doi.org/10.5772/intechopen.84189*

c.Bottom pouring of the slurry and disintegration of slurry through 10-mm

e.Deposition of disintegrated slurry into a steel mold to get 40-mm ingots.

a.Bottom pouring of the liquid slurry to ensure almost 100% yield.

particle distribution and microstructural homogeneity.

DMD processing is carried out under argon inert atmosphere to minimize oxidation. Experimental setup of DMD process is shown in **Figure 3**. The advantages of

b.Disintegration of molten slurry using low pressure gas jets to ensure improved

c.Use of argon gas rather than SF6 (greenhouse gas) to prevent oxidation.

In-situ casting of MMNCs is a very versatile and an economical process to synthesize MMNCs. The reinforcements are formed and controlled by metallurgical

annular diameter graphite nozzle.

the method include:

*Schematic drawing of DMD [35].*

**Figure 3.**

*2.1.4 In-situ casting processing*

d.Disintegration of slurry using two argon gas jets.

*Synthesis of Magnesium Based Nano-composites DOI: http://dx.doi.org/10.5772/intechopen.84189*

#### **Figure 3.**

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

In the UST process, the Mg alloy is placed in a graphite crucible and heated to the desired temperature using a resistance/induction furnace. Predetermined amount of reinforcement is than added depending on the size of the particles. During processing, Mg melt is protected using argon gas (flow rate of 20 lpm) to avoid oxidation. High-intensity ultrasound shock waves [16] are supplied to disperse nano-particles thoroughly in the melt at semi-solid temperature. The ultrasonic processing temperature is chosen so as to ensure better flowability of the slurry in the mold. The selection of material for sonotrode plays a vital role in MMNCs melting due to the erosion of sonotrode surface during melting of liquid metal. For better sonification of MMNCs, niobium (Nb) and titanium (Ti) are recommended by the researchers. Ti-based sonotrodes are widely used for UST treatment due to lesser costs when compared to Nb-based sonotrodes. To note that Nb-based sonotrode exhibit less variation in the Young's modulus as the function of temperature while Ti-based sonotrodes are very stable in MMNCs melt as Ti is insoluble in Mg. Earlier findings have indicated that the high-intensity UST vibration needs an intensity

[4]. For large scale volume production, the requirement of sonifica-

tion is likely to be higher. The large-scale production of MMNCs require enormous power and frequencies. The key issue is reducing rate of sonification by decreasing the volume of the melt during UST. This can be achieved in two steps firstly, preparing the melt with reinforcement and secondly, passing the liquid melt into a sonotrode assisted UST chamber for fragmenting particle clusters. Further work is

DMD technique is a hybrid technique that incorporates the concepts of casting, melt stirring and spray forming process [36–45]. In DMD, the processing steps

a.Heating matrix material and reinforcement in a graphite crucible (ceramic bonded clay graphite) to a desired superheat temperature. Mg and its alloys (in the form of chips or turnings) and reinforcement are placed in alternative layers.

b.Stirring of reinforcement at ~ 450 rpm for 5–10 min using a Zirtex 25 coated

stainless-steel impeller realize uniform distribution.

required in this area especially regarding scalability.

*2.1.3 Disintegrated melt deposition method (DMD)*

**14**

involve:

of 100 W/cm2

**Figure 2.**

*Schematic representation of UST [4].*

*Schematic drawing of DMD [35].*

c.Bottom pouring of the slurry and disintegration of slurry through 10-mm annular diameter graphite nozzle.

d.Disintegration of slurry using two argon gas jets.

e.Deposition of disintegrated slurry into a steel mold to get 40-mm ingots.

DMD processing is carried out under argon inert atmosphere to minimize oxidation. Experimental setup of DMD process is shown in **Figure 3**. The advantages of the method include:


#### *2.1.4 In-situ casting processing*

In-situ casting of MMNCs is a very versatile and an economical process to synthesize MMNCs. The reinforcements are formed and controlled by metallurgical reactions between principal alloy and the additives [46–50]. The type and number of additives are chosen based on final formulation of the matrix and volume fraction of the reinforcement. Reaction temperature is a key parameter in process design of in-situ MMNCs to ensure the desired matrix and the reinforcement phase. An example of creating in-situ Mg-Zn/MgO composites includes the use of Mg and ZnO as starting materials and heating them to a predetermined temperature to ensure the feasibility of the following two reactions (Eqs. (1) and (2)):

$$\text{Mg} + \text{ZnO} = \text{>MgO} + \text{Zn} \tag{1}$$

$$\text{Mg} + \text{Zn} = \text{>Mg} \xrightarrow{} \text{Zn} \tag{2}$$

Chelliah et al. [46] synthesized magnesium-polymeric derived ceramic (PDC) silicon carbonitride (SiCNO) nano composite by liquid pyrolysis using stir casting technique. The liquid poly (urea-methyl-vinyl) silazane (PUVMS) was used to formulate magnesium nano-composite. In this method, magnesium was melted in a steel crucible using a resistance furnace at a temperature of 700 °C and shielded with Ar-5%SF6 gas. The melt was stirred mechanically at 600 rpm to form vortex and the liquid PUVMS was injected to the melt. The melt was stirred for 15 min to ensure thorough pyrolysis. The liquid melt was bottom-poured into a rectangular metal mold preheated at 300 °C. Mg/nano- SiCNO composite was fabricated exhibiting uniform distribution of the reinforcement (**Figure 4**).

The benefits of in-situ MMNCs include: (a) uniform distribution of the reinforcement, (b) elimination of particle wettability issue, and (c) clean and strong matrixparticle interface. The disadvantages of in-situ techniques, in general, are scalability and the amount of reinforcements that can be created using the in-situ reactions.

**17**

**Figure 5.** *Step in PM.*

*Synthesis of Magnesium Based Nano-composites DOI: http://dx.doi.org/10.5772/intechopen.84189*

ried out to avoid oxidation and formation of inclusions.

Rheocasting is a semi-solid casting method where the matrix is processed in liquidus-solidus (L-S) zone. In this so called semi-solid zone, the reinforcement particles are added, and the resultant slurry is thoroughly stirred to ensure uniform distribution of the reinforcement. Following stirring, the semi-sold composite melt is tapped into a permanent mold. Often cleaning and degassing of the slurry is car-

A MMNC of Mg (AZ91E) with Al2O3n (50 nm) was synthesized using a semisolid Rheocasting process [51]. The Mg ingots were placed in boron nitride coated mild steel crucible. The melt was formed in the metal crucible at 750 °C using electric resistant furnace. The slurry was degassed using argon to avoid oxidation. The reinforcement (Al2O3n) was then added to the slurry at semi-solid (L-S) temperature (~590 °C). The melt slurry was stirred using a mechanical stirrer. The MMNCs slurry was subsequently poured into a permanent mold for further

The benefits of this technique include: (a) spheroidal/equiaxed grains and no dendrites, (b) less shrinkage and porosity and (c) lower operating temperature.

Powder metallurgy (PM) [52] is one of the most common solid-state synthesis method for magnesium based nano-composites [53–60]. The steps followed in PM are shown in **Figure 5**. In the first step metal alloy and ceramic particle in powder form are blended/mixed together to get homogenous mixture. The mixing parameters are decided based on the density difference between metal/alloy and reinforcement powder. The blended powders are subsequently compacted using a cold press or hot press or hot isostatic press. Finally, the green compacts are sintered by heating to a predetermined temperature to regain mechanical properties. Near-netshaped components with simple geometries can be fabricated by PM technique.

*2.1.5 Rheocasting technique*

characterization.

*2.1.6 Powder metallurgy*

**Figure 4.** *Experimental setup of liquid pyrolysis stir casting [45].*
