Research On Rapidly Solidified AZ91 Magnesium Alloy And SiCp/AZ91 Composite

Fabricating magnesium alloy by rapid solidification/powder metallurgy (RS/PM), can refine the grain of the alloy ,reduce and eliminate the coarse precipitation phase, and then improve the plastic forming properties and mechanical properties of the alloy. It is vital significance for developing high-strength and high elongation property wrought magnesium and industrial appliations.In this study, alloy fluid flow was broken up, the AZ91magnesium alloy powder was prepared by twin-roll rapid solidification technique, and then the hot extrusion of AZ91 alloy solid materials 10mm and 8mm in diameter were prepared at 350℃. The SiC powder and AZ91 alloy powder were blended uniformly and then SiCp/AZ91 magnesium matrix composites have been fabricated using rapid solidification/powder metallurgy (RS/PM) process. By means of SEM, XRD and TEM, some analysis on grain structure, the change of precipitation phase, the distribution of SiC particles in matrix and mechanical properties of materials was carried out. Reinforcing mechanism and tensile fracture behavior were discussed, and the stability of microstructure and mechanical property of RS/PM AZ91 magnesium alloy and RS/PM SiCp/AZ91 composites at different technological conditions of heat treatment were studied too.The experiment results showed that RS/PM AZ91 magnesium alloy microstructures consist dominantly of supersaturatedα-Mg solid solution and little amount of the fine Mg17Al12 phase. The density of the hot-extruded RS AZ91 powder was approximately near the theoretical density of alloy. The optimal extrusion temperature and extrusion speed of RS/PM AZ91 magnesium alloy and its composites was 350℃×0.1m/min. The ultimate tensile strength of the as-extruded AZ91 alloy was 383.23MPa, the elongation was 9.4%. Increasing the extrusion ratio could refine the grains and improve the yield, tensile strength and elongation of the as-extruded AZ91 alloy. The as-obtained alloy shows ductile fracture feature.The silicon carbide particulates(SiCp)in AZ91 magnesium alloy metal matrix were near uniformly dispersed through the alloy matrix. The mechanical properties of the composite such as tensile strength, yield strength and elongation degraded with the increase of the content of SiCp. The Mg2Si appeared to originate between the Mg and SiO2 on the surface of SiC during the hot extrusion. For all content of the SiC particle reinforcement the mechanical properties of the composite was lower than the unreinforced counterpart.Heat treatment of RS/PM AZ91 magnesium alloy and its composites under different processing parameter was studied, then the microstructure and mechanical properties were analyzed. the results showed that the occurrence of static recrystallization in magnesium alloy bars during annealing can be observed, which began at annealing of 200℃. The influencing mechanism of annealing temperature and annealing time on the microstructure and hardness of the alloy bars were approached. The occurrence of static recrystallization in RS/PM SiCp/AZ91 composites bars during annealing could be observed too.