MICROSTRUCTURE EVOLUTION AND WEAR RESISTANCE OF IN SITU Mg2Si/Al COMPOSITES UNDER DIFFERENT PREPARATION CONDITIONS

Abstract

Hypereutectic Al-Si alloys with high Mg content are in fact an in situ aluminium matrix composites containing a large amount of hard particles of Mg2Si, and the Mg2Si/Al composite has a potential as automobile brake disc material because the intermetallic compound Mg2Si exhibits high melting temperature, low density, high hardness, low thermal expansion coefficient(TEC) and reasonably high elastic modulus. However, the primary Mg2Si particles in normal Mg2Si/Al composites are usually very coarse and thus lead to room temperature brittleness and deficient wear resistance. Therefore, the composite with coarse primary Mg2Si particles need to be modified to obtain adequate mechanical strength and wear resistance. Numerous experiments have shown that development of a semi-solid microstructure in which dendritic characteristic is absent can lead to significant enhancement of the mechanical properties in the composite. The semi-solid forming has been recognized as a technique offering several potential advantages over casting or solid state forming, such as producing high quality components capable of full heat treatment to maximize properties, and reducing macrosegregation, solidification shrinkage and forming temperature. The key feature that permits the shaping of alloys in the semi-solid state is the absence of dendritic characteristics from the morphology of the solid phase. In the present work, in situ Mg2Si/Al composites were fabricated by using gravity casting, squeeze casting and semi-solid extrusion. The microstructure evolution and wear resistance of Mg2Si/Al composites were investigated. Mg2Si/Al semi-solid composites were fabricated by isothermal heat treatment technology, forming spherical reinforced phase and matrix structure. The effects of holding time on the microstructure and grain sizes of the composite were investigated. The results show that with P modification, Mg2Si particle in the as-cast microstructure of the composites is evolved from coarse dendrite into fine block structure with grain size of 35 μm. Furthermore, reinforcement Mg2Si with fine size and uniformly distribution exhibits regular spherical structure and a-Al grains exhibit spherical or ellipsoidal structure. The size of a-Al changes from 60 to 115 μm with increasing the holding time from 50 to 160 min. It is calculated that the cubic coarsening rate constants K of a-Al is 1.78×10-16m3/s according to the statistical data. In addition, the hardness of squeeze casting and semi-solid extrusion composites enhanced 23.5% and 39% in comparison with casting composite, respectively. The wear test results show that, the wear resistance of Mg2Si/Al composite fabricated by using semi-solid extrusion is higher than those of composites fabricated by using gravity casting and squeeze casting under same load and wear particle size.