Study On The Microstructures And Mechanical Properties Of Reciprocating Extrusion Mg₂Si/Mg-Al Composites

Mg₂Si/Mg-Al composites, which contain 10 % aluminum and 3 % silicon, were prepared by in situ reaction. The composites were reciprocally extruded (RE) by 2, 4 , 8 and 12 passes at 370℃ and then heat treated under different conditions. After repetitious experiments, the optimal process for preparation of Mg₂Si/Mg-Al composites was determined. The effects of RE and heat-treatment on microstructures of the composites were studied. OM and XRD were used to analyze the microstructures of the composites. The ambient and high-temperature mechanical properties of the composites were measured. The tensile fractures of the composites were also investigated.The main conclusions are as follows:1. The optimal process to prepare Mg-Si interalloy is recommended, that is put the industrial pure magnesium and silicon particles of about 4mm diameter into crucible resistance furnace, and then heated the melt to 810℃ and held it at the temperature for 4 hours and stirred it every 1hour while the melt is protected by flux and CO₂+SF₆ mixed gases all the time.2. The Mg₂Si particles in Mg₂Si/Mg-Al composites are very coarse, because of the existing of Mg₂Si, the hardness of the composites increase and is bigger than that of as-cast AZ91D by 10.5%. however, the hardness of the composites is not even or heterogeneous.3. Reciprocating extrusion can refine the microstructures of Mg₂Si/Mg-Al composites effectively. As the extruded passes increase, the hard Mg₂Si particles are breaked gradually and distribute dispersedly. After 12 passes, coarse Mg₂Si phase reduce obviously, and are almost 5-15μm, expect a few particles are about 50μm. After extruded for 4 passes, the fluctuating rate of hardness decrease to 2.4% from 24.6% in the as-cast composites. The uniformity of microstructure increase with the extruding passes.4. The experimental results show that the microstructures thermal stability of the composites is well. In particular, the Mg₂Si particles are refined greatly and distribute more uniformly after extruding by 12 passes. The dispersed Mg₂Si particles can pin the grain boundary effectively and thus advance the stability of microstructures.