ABSTRACT
Aluminium alloy castings represent a significant proportion of worldwide metal usage. Mechanical properties of the casting alloys are principally controlled by the cast structure. The microstructure evolution of the casting alloys thereby influences their mode of fracture.
In the present work, investigation of mechanical properties and fracture behavior of cast Al-12.5Si alloy, sand mould was used to cast aluminum alloy with silicon as the alloying element added, and refined with 0.1wt % antimony (Sb). Tensile, impact and hardness tests were conducted on Sb refined heat treated Al-12.5Si alloy and the unrefined non-heat treated alloy samples to evaluate their mechanical properties. Optical and SEM microscopy were also conducted to study the resulting microstructures and fractography.
From the results, specimen aged at 220oC/3hrs showed remarkable improvement in some mechanical properties with UTS of 73.17MPa, YS of 24.53MPa, 28% elongation, and increase in Brinell hardness number of 16.78HB, but a decrease in impact strength of 20.57J. Specimen aged at 160oC/3hrs also indicated improvement in some mechanical properties, having UTS of 66.93MPa, YS of 21.63MPa, 20% elongation, but with decrease in impact strength of 18.43J and harness of 12.82HB. Specimen aged at 180oC/3hrs showed reduction in UTS at 41.83MPa, but other properties increased; YS of 20.24MPa, 12% in elongation, 29.92J of impact strength, and hardness of 15.59HB. From optical microscopy, it is shown that heat treatment influenced microstructural changes from coarse large elongated nodule like (flake-like) silicon precipitates to refined spheriodized fine structure. SEM fractograph of tensile specimen showed a brittle fracture in unrefined non-heat treated alloy and Sb-refined specimen aged at 180oC/3hrs and 200oC/3hrs, except specimen aged at 160oC/3hrs and 220oC/3hrs which indicated ductile-brittle mixed fracture. Morphology of the fracture surface showed crack propagation influenced by fracture of intermetallic phases, precipitate fracture, eutectic silicon fracture, α-Al matrix crack, and transgranular-intergranullar fracture mode of failure.
