Effects of solidification rate and ageing on the microstructure and mechanical properties of AZ91 alloy

Sand-cast plates of a commercial AZ91C alloy have been used for the study. Varying the solidification rate by placing large cast-iron chills in the mould produced a range of secondary dendrite arm spacing (SDAS) within the cast plates. The plates were solution heat-treated, quenched and aged at 165 °C for up to 350 h. The SDAS (μm) varied with the solidification time, tf (s), as SDAS=5.3 tf0.43. The tensile ductility in the as-quenched (T4) condition did not depend on the solidification rate whilst in the T6 condition it tended to decrease for slowly solidified material (SDAS>50 μm). The yield strength and hardness increased and the ductility decreased with ageing. The fracture mode changed from predominantly transgranular in the T4 condition to predominantly intergranular in the T6 condition. The properties of the sand-castings are compared with those of high-pressure diecastings and the possible strengthening mechanisms are discussed. A number of areas that require more research are pointed out.