Research On Hear Treatment Of Semisolid Fromed AM60B Magnesium Alloys

Microstructures and heat treatment characteristics (solid solution and ageing treatment) of thixoformed (TF) AM60B magnesium alloy have been studied and compared with those of permanent mould casting (PMC) AM60B. The results reveal that the microstructure of TF alloy is different from that of PMC AM60B, which is characterized by globularly primaryαparticles and secondarily solidified structures. The primaryαparticles appear in three shapes: spheroidal particles with halos, gear-like particles and cellular particles. The secondarily solidified process produces finer and more dispersive eutecticβphase in the TF alloy than that in the PMC alloy. Two stages, dissolution of eutecticβphase and coarsening of the particles, and normal growth of the particles, are suggested for the AM60B alloys prepared by these two technologies during solid solution treatment. The solution process will be accelerated as the solution temperature rises. When solid solution treated at 430℃, the first stage of the TF alloy completed in 0.5 h compared with 1 h of the PMC alloy. The growth forms of the particles or grains and corresponding models are different during the different stages. In the first stage, the secondaryαparticles of the TF alloy and the polygonalαparticles of the PMC alloy coarsen by consumingβphase basically and their growth obeys a square root relationship. However, the primaryαgrains of the TF alloy coarsen through combining their surrounding secondaryαgrains, and the growth mould fulfill a power function relationship. During the secondary stage, all the particles or grains turn to a combining form with a same growth mould of a cube relationship. Ageing process is characterized mainly by discontinuous decomposition (DC). Unlike the PMC alloy, the DC occurrs in the regions of the secondaryαgrains for the TF alloy. The DC amount increases as the ageing time increases or the solution temperature rises. Moreover, it is influenced by the relative liquid fraction, sizes of secondaryαgrains and supersaturation. Most of the precipitates, nucleating at grain boundaries, are in lamellar shapes growing in < 112|-0> orientation and (01|-1)β/ /(01|-10)α' direction with matrix. The hardness of the AM60B alloy relies on the precipitates content prior to peak value and the morphologies after the peak value. Due to lager interface areas in the secondαgrains region of the TF AM60B than in the polygonalαparticle regiones of the PMC AM60B, the nucleation and growth of theβprecipitates are easier to happen. The hardness of theTF alloy is higher than that of the PMC alloy.