Microstructure And Mechanical Properties Of AZ91D Magnesium Alloy By Homogenizing Annealing With Continual And Isothermal Cooling Methods

The microstructure of as-cast Mg-Al magnesium alloy was inhomogeneous and segregation existed in the solidification process, there were a lot of continual netβ-Mg17Al12 on the grain-boundary, therefore the mechanical properties were restricted. The key of the Mg-Al alloy strengthening is to improve the morphology and the distribution ofβ-Mg17Al12. In this paper, AZ91D as-cast magnesium alloy was treated by homogenizing annealing treatment with continual and isothermal cooling methods. The microstructure of heat-treatment specimens were studied by OM, SEM, XRD. As well as the mechanical properties, including Brinell hardness, micro-hardness, tensile, were used to study of AZ91D magnesium alloy after homogenizing annealing treatment. The processes of solidification and precipitation were analyzed, and also the effect of cooling methods on microstructure and mechanical properties of AZ91D alloy.After homogenizing annealing treatment, the net segregationβ-Mg17Al12 phase dissoluted into a-Mg and the lamellar-shapedβ-Mg17Al12 phase precipitated again during the homogenizing annealing cooling process. When the cooling rate was 1.206K/min,β-Mg17Al12 phase wouldn't precipitate thoroughly, the lamellar-shapedβphase was slender, lamellar spacing were different. As the cooling rate slow down, the precipitated quantity ofβ-Mg17Al12 phase increased and the interlamellar spacing was bigger. When the cooling rate was under 0.402K/min,β-Mg17Al12 phase precipitated thoroughly, the lamellar-shapedβphase was spacious, a few lamellar dissolved, the lamellar spacing turned to homogenizing.Theβphase precipitated mostly as lamellar-shaped after the homogenizing annealing isothermal cooling. On the certain temperature of isothermal cooling, theβ-Mg17Al12 phase precipitated increased when the time went. A few lamellar-shaped and massive-shapedβphase on the grain-boundary in 350℃isothermal cooling. In 300℃and 250℃, theβ-Mg17Al12 phase precipitated on the grain-boundary and the grain completely in 6h and 4h, the portion of lamellar dissolved and granulated. In 200℃and 150℃,β-Mg17Al12 phase unprecipitate in part grains in 6h. The interlamellar spacing increased when the isothermal temperature heightened. Compared to 420℃solution temperature, the precipitated quantity ofβ-Mg17Al12 phase decreased in 450℃, the grain size increased markedly too. Compared the two cooling methods, the microstructure were more homogenization and diversity by isothermal cooling method.The results of the AZ91D magnesium alloy mechanical properties showed that the brinell hardness increased from 57.7HB to 82.1HB by continual cooling mode, and to 83.9HB by isothermal cooling improved 45%. Theσ0.2 andσb increased respectively 36% and 37%, and elongation raised from 2% to 6.52%. Theσb and the 8 of increased and theσ0.2 decreased a little with the isothermal cooling than continual cooling. The fracture specimen changed from brittleness to a few plastic fracture after the homogenization annealing, and there were lamellar morphologies in the fractography.