The influence of Cu–15P master alloy on the microstructure and tensile properties of Al–25 wt% Mg2Si composite before and after hot-extrusion

This study was undertaken to investigate the effect of CuP master alloy on the microstructure and tensile properties of hot extruded Al–25 wt% Mg2Si in-situ metal matrix composite. Cu–15 wt% P master alloy was added to produce different levels of P (0–3 wt%) in the composite. Then, these small billets were hot extruded by using a hydraulic press at a ram speed of 1 mm/s with the extrusion ratio of 6:1 at 500 °C. Microstructural examinations were carried out by using optical and scanning electron microscopy (SEM). The results showed that P addition changes the morphology of primary Mg2Si from large dendritic to polyhedral shape and its average particle size decreases from 220 μm to 22 μm in as-cast condition. Microstructural studies also showed that extrusion process not only refines the primary Mg2Si particles, but also fragments intermetallic phases to fine particles. Tensile testing results also revealed that the optimum P level for the improvement of both ultimate tensile strength and elongation values of hot-extruded specimens is 1 wt%. The examination of fracture surfaces via scanning electron microscopy (SEM) revealed a transition from brittle fracture in as-cast to ductile fracture in hot-extruded and P-modified specimens. This can be attributed to the changes in the size and morphology of intermetallics and reduction in pore size after hot-extrusion.