**7. Conclusions**

The microstructural structures of magnesium matrix composite were studied in three different casting processes. The results indicated that SiCp/AZ61 composites fabricated in stirring melt casting process, compared to those in fully liquid stirring casting process and in semi-solid stirring casting process, possessed fairly uniform distribution of SiC particulates and few porosity rate. It was an ideal metal matrix composites fabricated process.

Under the experimental conditions, the optimum processing plan of SiCp/AZ61 composites fabricated by a stirring melt casting method were the volume fraction of SiC particles 6%, stirring temperature 595℃ and stirring time 5 min. In addition, the effects of volume fraction of SiC particles on the mechanical properties of SiCp/AZ61 composites was the most important among three factors (volume fraction of SiC particles, stirring temperature and stirring time), the second were stirring time and stirring temperature.

Semi-solid isothermal heat treatment technology was used for the partial remelting of SiCp/AZ61 composites. A fine semi-solid microstructure was obtained, whose equal-area diameter size was between 60µm and 85µm, and the effective liquid volume fraction was about 31%~38%. The optimal technological parameters of SiCp/AZ61 composites were the reheating temperature of 595℃~600℃ and an isothermal holding time of 30min~60min.

Compression tests on semi-solid SiCp/AZ61 magnesium matrix composites were carried out. Influences of strain-rate, strain, temperature and volume fraction of SiC particles on flow stress were analyzed. The results show that the flow stress of semi-solid SiCp/AZ61 composites is sensitive to temperature and strain rate. Meanwhile the flow stress increases with the increasing of the volume fraction of SiC particles.

The influence of deformation temperature, strain rate, strain, liquid volume fraction, volume fraction of reinforcement on flow stress in composites thixotropic plastic deformation process was considered. A new constitutive model of composites in thixotropic plastic deformation process was proposed. The constitutive equation of SiCp/AZ61 composites was obtained with the multiple nonlinear regression method based on data of thixotropic compression test. The calculated results were good agreement with the experimental ones. It is used to guide composites thixotropic plastic deformation process.

Numerical simulation can provide a help for the analysis of thixoforging process, and behavior of metal flow has been obtained. The effective stress distribution was more uniform and its value was more smaller with the increasing of forming temperature. The effective stress was increased with the increasing of volume fraction of SiC particle. The temperature distrubition was worse with the increasing of volume fraction of SiC particle. The differences between traditional forging and thixo-forging processes were analyzed. Results indicated that thixo-forging was better in filling cavity than forging. So the complicated workpiece can be done once in thixo-forging. Numerical simulation results are accorded with experimantal ones.
