不同制备条件下原位Mg_2Si/Al复合材料的组织演变和耐磨性

MICROSTRUCTURE EVOLUTION AND WEAR RESISTANCE OF IN SITU Mg_2Si/Al COMPOSITES UNDER DIFFERENT PREPARATION CONDITIONS

采用普通重力铸造、挤压铸造和等温热处理半固态挤压的方法制备了原位Mg2Si/Al复合材料,研究了其组织演变和耐磨性.结果表明,P孕育变质后,铸态组织中Mg2Si增强相由粗大的枝晶转变为细小的块状,经过等温热处理后的半固态组织,Mg2Si增强相分布均匀、尺寸细小,表现为规则的球形,a-Al的形貌也变得较为圆整,表现为规则的球状或椭球状.此外,等温热处理的半固态组织中的Mg2Si和a-Al尺寸还有较为明显的改变.与普通重力铸造Mg2Si/Al复合材料相比,挤压铸造复合材料的HB硬度提高了23.5%,半固态挤压复合材料的HB硬度提高了39%.在相同磨粒尺寸和载荷条件下,普通重力铸造复合材料的磨损率最大,挤压铸造复合材料的磨损率次之,半固态挤压复合材料的磨损率最小.

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 α-Al grains exhibit spherical or ellipsoidal structure. The size of α-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 α-Al is 1.78×10^-16m^3/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.