Dual role of trace elements in magnesium dissolved age hardened A356 alloy on microstructure and peak micro hardness

The present work reflects the effects of trace addition of copper (up to 1 wt.%) and zinc (0.5 wt.%) on the microstructure and hardness property of heat treated A356 (Al-7Si) alloy. Small amount of zinc and copper was introduced into A356 in both atomic form (alloy) and powder form (composite) and its age hardening behaviour were investigated. To enhance the wettability during composite fabrication and solid solution strengthening in alloys, a small quantity of magnesium (1 wt.%) was added. The main objective of this study is to introduce lower melting point zinc reinforcement into A356 matrix alloy by copper coating of zinc particles and then reinforcing it into A356 matrix. All stir cast specimens were subjected to T6 treatment by solutionizing at 520 °C for 2 h followed by 60 °C water quench and then aging at 100 and 200 °C to determine peak hardness value. Microstructure analysis showed that a minimum 1 wt.% copper was required to form Al2Cu intermetallic phase during solidification resulting in finer grain structure with hardness improvement. Under as-cast conditions, a maximum hardness of 85 VHN was obtained in 1 wt.% copper reinforced composite. Under peak aged conditions, aging at 100 °C showed 116% hardness improvement in 1 wt.% Cu reinforced composite.