*2.1.6 Powder metallurgy*

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.

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

#### **Figure 6.**

*(a) Diagram shows the steel vial with steel balls and (b) planetary ball mill [2].*

**Figure 7.** *Hybrid microwave sintering setup [52].*

Several magnesium-based alloys, conventional and nanocomposites (e.g. Mg/3wt%Al/0.1wt%GNP) have be synthesized using PM technique. Typical processing steps include blending or mechanical alloying the predetermined amounts of metal and ceramic powder with or without steel balls using a planetary ball mill at a speed of 200 rpm for 1 h. A typical planetary type ball mill setup is shown in **Figure 6**. The composite powders obtained from blending step are subsequently compacted using a 100 T hydraulic press to attain a billet of ~35 mm diameter and height of 40 mm. Compacted billets can be sintered using a conventional furnace or microwave based sintering. Heating time during microwave sintering is kept with 16 min. Conventional microwave 0.9 kW power and 2.45 GHz operating frequency can be used. The schematic diagram of microwave sintering set-up is shown in **Figure 7**.

#### *2.1.7 Accumulative roll bonding*

Accumulative roll bonding (ARB) is a solid-state severe plastic deformation technique used to produce layered composite stacked with either similar or dissimilar alloys sheet and reinforcements [61–64]. The final output of the ARB process depends on the process variables namely weight percent of the alloys and the reinforcement, number of rolling cycles and the temperature. The benefits of the process are grain refinement from fine to ultra-fine, fragmentation of reinforcement clusters and their subsequent uniform distribution. To realize best possible

**19**

composite.

**Figure 8.**

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

schematic setup of the ARB is shown in **Figure 8**.

*Steps in synthesizing nano-composites using ARB [61].*

*2.1.8 Friction stir processing (FSP)*

results, the sheets need to be pretreated by grinding and polishing for descaling oxides and to make friction free surface. Degassing process is also often used. The

Monolithic aluminum (Al) and AZ31 magnesium [61] strips of 1 and 0.5 mm thickness were cut to 150 × 50 mm rectangular strips. The strips were annealed at 400 °C for 2 h and furnace cooled near ambient temperature to soften the strips prior to rolling. The sheets were ground, polished, degassed and cleaned prior to rolling. In a steel vial Al and nano-alumina (<50 nm) powder were purged along with 0.5 and 1 mm in diameter steel balls. Milling of Al-alumina powder was conducted at 300 rpm and ball-to-powder ratio of 20:1 for six cycles, each of 45 min duration with a dwell time of 15 min to eliminate undesirable rise in temperature. The milled Al-alumina particles were uniformly spread between the strips for better wettability. The stacking was done as Al/AZ31/Al with reinforcement powder in between. The stack was fastened by copper wires to avoid slipping. The assembly was preheated in the temperature range of 300–350 °C for 15 min in an air furnace. 50% reduction was

maintained at each rolling stage. Rolling was carried out for four times.

enhanced microstructural aspects and is investigated further for scaling up.

Accumulative roll bonding has yielded good combination of properties due to

Friction stir processing is a solid-state plastic-deformation-based synthesis method (**Figure 9**). It can be used to build nano-composite layer/surface composite as well as bulk composites of limited thickness/dimensions. It often leads to uniform distribution of reinforcement and refined grain size [65–70]. The FSP process uses a shouldered rotating tool that pass over the matrix containing nano-ceramic particles. During the translational movement of tool, matrix get plastically deformed and reinforcement gets simultaneously incorporated in the matrix. The stirring action enables uniform distribution and refine grained structure of the fabricated

The α-Al2O3 nano-particle reinforced AZ31 composite [65] was fabricated using friction stir processing technique. The Mg rectangular plates of 600 × 100 × 10 mm were cut for preparing the composite. The ceramic particles were spread into a groove of width and depth 1.2 × 5 mm cut in the plates as shown in the **Figure 8**. Two types of shouldered tools were used to form the composite, one pin less shoulder and the other shoulder with a pin of 5 mm height

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

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

*(a) Diagram shows the steel vial with steel balls and (b) planetary ball mill [2].*

**18**

**Figure 7.**

**Figure 6.**

*Hybrid microwave sintering setup [52].*

*2.1.7 Accumulative roll bonding*

Several magnesium-based alloys, conventional and nanocomposites (e.g. Mg/3wt%Al/0.1wt%GNP) have be synthesized using PM technique. Typical processing steps include blending or mechanical alloying the predetermined amounts of metal and ceramic powder with or without steel balls using a planetary ball mill at a speed of 200 rpm for 1 h. A typical planetary type ball mill setup is shown in **Figure 6**. The composite powders obtained from blending step are subsequently compacted using a 100 T hydraulic press to attain a billet of ~35 mm diameter and height of 40 mm. Compacted billets can be sintered using a conventional furnace or microwave based sintering. Heating time during microwave sintering is kept with 16 min. Conventional microwave 0.9 kW power and 2.45 GHz operating frequency can be used. The sche-

Accumulative roll bonding (ARB) is a solid-state severe plastic deformation technique used to produce layered composite stacked with either similar or dissimilar alloys sheet and reinforcements [61–64]. The final output of the ARB process depends on the process variables namely weight percent of the alloys and the reinforcement, number of rolling cycles and the temperature. The benefits of the process are grain refinement from fine to ultra-fine, fragmentation of reinforcement clusters and their subsequent uniform distribution. To realize best possible

matic diagram of microwave sintering set-up is shown in **Figure 7**.

**Figure 8.** *Steps in synthesizing nano-composites using ARB [61].*

results, the sheets need to be pretreated by grinding and polishing for descaling oxides and to make friction free surface. Degassing process is also often used. The schematic setup of the ARB is shown in **Figure 8**.

Monolithic aluminum (Al) and AZ31 magnesium [61] strips of 1 and 0.5 mm thickness were cut to 150 × 50 mm rectangular strips. The strips were annealed at 400 °C for 2 h and furnace cooled near ambient temperature to soften the strips prior to rolling. The sheets were ground, polished, degassed and cleaned prior to rolling. In a steel vial Al and nano-alumina (<50 nm) powder were purged along with 0.5 and 1 mm in diameter steel balls. Milling of Al-alumina powder was conducted at 300 rpm and ball-to-powder ratio of 20:1 for six cycles, each of 45 min duration with a dwell time of 15 min to eliminate undesirable rise in temperature. The milled Al-alumina particles were uniformly spread between the strips for better wettability. The stacking was done as Al/AZ31/Al with reinforcement powder in between. The stack was fastened by copper wires to avoid slipping. The assembly was preheated in the temperature range of 300–350 °C for 15 min in an air furnace. 50% reduction was maintained at each rolling stage. Rolling was carried out for four times.

Accumulative roll bonding has yielded good combination of properties due to enhanced microstructural aspects and is investigated further for scaling up.

#### *2.1.8 Friction stir processing (FSP)*

Friction stir processing is a solid-state plastic-deformation-based synthesis method (**Figure 9**). It can be used to build nano-composite layer/surface composite as well as bulk composites of limited thickness/dimensions. It often leads to uniform distribution of reinforcement and refined grain size [65–70]. The FSP process uses a shouldered rotating tool that pass over the matrix containing nano-ceramic particles. During the translational movement of tool, matrix get plastically deformed and reinforcement gets simultaneously incorporated in the matrix. The stirring action enables uniform distribution and refine grained structure of the fabricated composite.

The α-Al2O3 nano-particle reinforced AZ31 composite [65] was fabricated using friction stir processing technique. The Mg rectangular plates of 600 × 100 × 10 mm were cut for preparing the composite. The ceramic particles were spread into a groove of width and depth 1.2 × 5 mm cut in the plates as shown in the **Figure 8**. Two types of shouldered tools were used to form the composite, one pin less shoulder and the other shoulder with a pin of 5 mm height

#### **Figure 9.** *FSP fabrication stages from 1 to 3 for Mg nano-composite [67].*

and diameter of 6 mm. The pin less tool was fed first to prevent the nano-phase getting distorted from the groove. The second tool with pin was then passed to complete the process. The tool rotation (800–1400 rpm) and the traverse speed (45 mm/min) were varied to obtain the desired strength and structure of the composite. Higher hardness was observed due to grain refinement at higher tool rotational speed.
