Abstract: Mg 2 Si reinforced Mg-Al based composite with high amount of silicon was prepared by permanent mould casting, and then extruded by reciprocating extrusion (RE) after the composite was processed by homogenization heat treatment. The effect of RE processing on the morphology and size of Mg 2 Si and the mechanical properties of the composite were investigated, to develop new ways to refine the Mg 2 Si phase and improve its shape. The result showed that RE was very useful in refining the Mg 2Si phase. The more the RE processing passes, the better the refining effect would be. Moreover, the uniform distribution of Mg 2 Si phases would be more in the composite. After the composite was processed by RE for 12 passes, most Mg 2 Si phases were equiaxed, with granular diameter below 20 um, and distributed uniformly in the matrix of the composite. The mechanical properties of the composite could be increased prominently by RE processing, and were much higher than that in the as-cast state. As the temperature rises, the tensile strength is reduced. For the composite RE processed for 12 passes, the tensile strength, yield strength, and elongation are 325.9 MPa, 211.4 MPa, and 3.3 % at room temperature, whereas, 288.2 MPa, 207.7~MPa, and 7.8 %, respectively, at 150 C. In comparison with the properties at room temperature, the tensile strength and yield strength are high and only decrease by 11.6 and 1.8 at 150 C. The Mg 2 Si reinforced Mg-Al based composite possesses good heat resistance at 150 C. The excellent resistance to effect of heat is attributed to the high melting temperature and good thermal stability of fine Mg 2 Si phases, which are distributed uniformly in the composite, and effectively hinder the grain boundary gliding and dislocation movement.

Key words: Reciprocating extrusion, Magnesium alloy, In situ composite, Elevated temperature property