Study On The Hot Compression Deformation Behavior And Microstructure Of Die Cast AZ91D Magnesium Alloy

The recognition of magnesium alloys is continuing to improve in the industryowing to its unique performance, and its applications is also expanding. Currentlydie-casting is the main molding process of magnesium alloy. In the actual production,magnesium alloy die castings are usually need to be partially or entirely plasticprocessing due to factors such as working conditions or processes restrictions. Thedefects of die castings such as shrinkage and porosity can be eliminate and mechanicalproperties can be improved after being plastic processed. But the plastic of magnesiumalloys is too poor to be plastic processed at room temperature. Because the low meltingpoint reticulate pahse which lead it to be difficult to be plastic processed, exist on thegrain boundaries. These alloys always need to be heated when to be plastic processed. Itlimits the promotion and application of magnesium to some extent.In recent years, the many reserchs of deformation behavior and mechanism areabout AZ31, AZ61, gravity cast AZ91and so on. But the reserchs on compressivedeformation especially the hot deformation of die cast AZ91D magnesium alloy arerarely reported. In response, Hot compression tests of die-cast AZ91D magnesium alloywere performed on Gleeblle-1500D at strain rates ranging in0.01¹0s^-1anddefromation termperature ranging in50℃⁴50℃. Thedeformation behavior andmicrostructure of die cast AZ91D magnesium alloy were studied and analyzed.The main research work and results of this paper are as follows.â‘ The flow curves of die-cast AZ91D magnesium alloy under experimentalconditions were got after the analysis about the data of hot compression deformation.The constitutive equation,=1.41×10¹²[sinh（0.014σ）]^5.295exp（-159449.509/RT）, whichis described by Zener-Hollomom parameters for hot compression deformation ofdie-cast AZ91D magnesium alloy was established. Compared to gravity casting AZ91Dmagnesium alloy, the deformation activation energy of die-cast AZ91D magnesiumalloy, Q=159.45kJ·mol^-1, determined by hyperbolic sine function was lower.â‘¡According to the dynamic material model and processing map thory, the powerdissipation maps, the instability maps and the processing maps of die cast AZ91Dmagnesium alloy were graphed at the strain of0.3,0.5and0.7respectively bymathematical statistics software using the data from hot compression experiments. Thepower dissipation coefficient（η） and instability coefficient （ξ（））, on the maps, were analyzed to identify a ideal processing area which is260³50℃,0.01¹s^-1.â‘¢A lot of reticulate second phase exit on the grain boundaries in themicrostructure of die-cast AZ91D magnesium alloy. These second phase gradually solidsolution in matrix with the rising temperature. The observation on the microstructure inthe central region of die cast AZ91D magnesium alloy after being compressed showedthat temperature, strain rate and strain are main influence factors. The observations ondifferent areas of compression specimen indicate that the amount of deformation incentral region were a little large and the streamline banded structure perpendicular tocompression direction is more obvious. Dynamic recrystallization could be seen at grainboundary. Near to the edge region the amount of deformation was tiny. The grain wasjust simply grow.â‘£The morphology of die casting defects in hot compression deformation wereobserved. It shows that holes is the major defect of die casting magnesium alloy and itcan be reduced through compression deformation.