Particle crack damage and quality index of Al-Si-Mg casting alloys

摘要

The casting industry often uses an empirical parameter termed quality index (Q) to assess the effect of changes in the heat treatment or the chemical composition on the mechanical performance of Al-Si-Mg casting alloys. The quality index decreases when the dendrite arm spacing (DAS) is large, the eutectic Si particles are unmodified or the amount of inclusions and Fe-rich intermetallic particles is high. However, there is no functional relationship between these microstructural parameters and the Q-value. The effect of some of these microstructural parameters-as well as the effect of varying the temper of the alloy matrix-on the quality index can be incorporated into an analytical model assuming a simple functional relationship between the matrix flow properties and the stresses in the second-phase particles. Assuming that macroscopic fracture occurs whenever a set fraction of second-phase particles cracks, the model can be used to predict the ductility and strength of the material under different tempers. An increasing loss in Q is predicted as the material strength increases during the aging process. The effect is more apparent when the material has limited ductility, i.e., material with partially modified eutectic Si particles, or when the content of Fe-rich intermetallics is high and the DAS is large. Experimental data supporting these claims are presented.