Study On Semi-solid Microstructure Of Magnesium Alloy For Upsetting In The SIMA Process

The advantages of magnesium alloy being engineered material are as follows: low density, high rigidity, damping capacity, easy machinability and recyclability etc. SSF (semi-solid forming) is new technology to manufacture near net shaped parts with a globular microstructure. Among several processes to fabricate semi-solid billet, the SIMA( strain induced melt activated) process is very simple and advantageous with respect to equipment.In this paper, the Mg alloy specimens for upsetting were studied. The deformation microstructure was observed. Influences of parameters on microstructure of Mg alloy were investigated in semi-solid quenching and forming experiments and the spheroidization mechanism of Mg alloy for upsetting was analyzed in comparison with extruding. The results indicated as follows:(1)There were effects of deformation parameters on microstructure of Mg alloy for upsetting. With the increase of strain at 280*C, the initial dendrite arms of AZ91D alloy got broken or stretched-out; with the rise of deformation temperature under 20% strain, the deformation style of dendritic structure of AZ91D alloy changed gradually from fragmentation to directional bending.(2)In the semi-solid isothermal process, there was evident relationship between thespheroidization of dendrite of deformed AZ91D magnesium alloy and the defect withinthe dendrite caused by deformation, which brought about the precipitation of liquidphase entrapped within solid phase. The penetration of liquid phase along the sub-grainboundary resulted in the separation of the dendrite. The isolated dendrite became globular due to the reduction of the surface energy. After the spheroidization of grains, with the increase in soaking time, the large grains grew up gradually and the quantity of small grains decreased; with the rise of soaking temperature, the rate of grain's spheroidization got fast and the average dimension of grains diminished.(3)There were influences of parameters on semi-solid microstructure of Mg alloy for upsetting. After holding at 580 for 10min in the semi-solid state, the greater the strain at 280 , the finer and rounder the size of the globular grains of AZ91D alloy and in the mean time, the agglomeration of solid particles deformed at more than 250 got less than that of less than 250 .(4)the differences in microstructure of Mg alloy between for upsetting and for extruding were clear: The average size of grains in deformation microstructure of Mg alloy for extruding was much less than that of upsetting. The main deformation for upsetting was twin and that for extruding was dislocation. In semi-solid isothermal process, the spheroidization of Mg alloy for extruding was faster and the grains were finer.(5)With the rise of temperature in squeeze process, the Theological property of semi-solid magnesium alloy slurry was improved. During the semi-solid forming process, the deformation texture in microstructure of Mg alloy at high solid content was found and the higher the temperature, the less the texture.