Study On Forming Capability And Solidified Microstructures Of Magnesium Alloy By Extrusion In The Semisolid

In this paper, with the prototype machine made by ourselves for magnesium alloys formation in semi-solid, according to the orthogonal experiment design, the capability of filling cavity of AZ91D magnesium alloys that were compressed in semisolid with SIMA process was studied. Effects of process parameters on forming capability and the flowing modes of slurry in semisolid state were analyzed. Mechanical properties of products were tested; the fracture morphology of tensile tested specimens was examined by scanning electron microscopy (JSM-6700F). The solidified microstructures were observed by means of SEM (PHILIPS XL20) and EDS (EDAX DX4), characteristic and modes of Solidification were analyzed. The results showed as follows:①The effect of reheating temperature in semisolid on capability of filling cavity was the greatest, the influence of holding time was greater. With the rise of temperature or holding time, the capability improved. However, the effect of deformation rate was complex, as deformation rate increased, forming capability changed from increasing to decreasing rapidly, and to increasing again.②The optimum parameters were that the reheating temperature, holding time and deformation rate was 580℃,20min,20%,respectively.③Under the condition of semisolid state, when there are many original crystal grains,α -Mg grew mainly by means of cling to crystal grains. But when the fraction of liquid phase was high, α -Mg grew in the form of dendrite crystal.④With eutectic reaction occurring in liquid phase, the eutectic grew not only in the form of divorced eutectic but intergrown eutectic, which depended on the growing form of α -Mg. If a -Mg grew in the form of cling, eutectic microstructures formed divorced eutectic. When α -Mg grew by means of dendrite, eutectic formed intergrown eutectic.⑤The fracture morphology of tensile tested specimens that were cast was cleavage or/and quasi-cleavage and cleavage steps were obvious. But the tensile tested specimens that were treated by SIMA fractured along grain boundaries mainly. At the same time, the fracture morphology at some crystal boundaries was featured as ductile fracture, evidenced by the existence of dimples, which indicated that the mechanical properties of alloys in semisolid that formed non-dendritic microstructures improved...