The effect of solid solution treatment on semisolid microstructure of a Zn alloy ZA27 with developed dendrites was investigated and a new approach to produce non-dendritic semisolid microstructure available for thixoforming was developed. The results indicated that a semisolid microstructure with small and spherical primary particles could be obtained when the alloy with developed dendrites was solution treated for 3 h at 370uC and then partially remelted at a semisolid temperature of 460uC. This method could not only overcome some inevitable shortcomings resulting from using some present popular approaches, but also simplify the procedure of thixoforming. Too short or too long solution treatment duration could increase the shape factor, size and inhomogeneity of the primary particles. All of these microstructural changes were attributed to the dendrite coalescence and phase transformation which occurred during solid solution treatment.
The effects of quench aging heat treatment on microstructure and tensile properties of thixoformed ZA27 alloy were investigated. The results indicated that the microstructure of the alloy became into polygonal b phase particles after solid solution treatment at 350uC for 48 h. The b particles then decomposed into a and g phases which would coarsen during the subsequent aging at 150uC. It was observed that the Zn concentration in the b phase near the polygonal boundaries was higher than that within the b particles. As a consequence, both the decomposition speed of the former b phase and the subsequent coarsening speed were faster than those of the latter b phase. Thus, a and g phases near the boundaries were always coarser than those within the particles during aging. Owing to the coarsening, the ultimate tensile strength continuously decreased with increasing aging time. The percentage elongation increased up to 10 h, but decreased with aging time owing to bad deformation accommodation and low bonding strength between particles. Cracks initiated from some defects (e.g. inclusions and porosities) during tensile test, and the path for the cracks to propagation changed with the aging time.
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