Research On Preparation、Microstructure And Properties Of SiCp/Al-Cu-Mg Composite

SiC particulates reinforced aluminum composites (SiCp/Al) have some features of properties, low density, low coefficient of thermal expansion, high specific strength, high specific stiffness and processability, which have caused wide public concern at home and abroad. The foreign researches of SiCp/Al composites have been industrialized application, and reduce the cost of the composites by using element powder instead of alloy powder. However, domestic researches start relatively late, and exist a lot of problems. With the target to optimize the fabrication, heat treatment and isothermal forging process parameters of the composites, this research fabricated the15%SiCp/Al-Cu-Mg composite through powder metallurgy and isothermal forging process using SiC particles as reinforcement and after-heat-treated Al-Cu-Mg alloy as matrix. A systematic research was carried out on the microstructure, interface, and mechanical properties of this kind of composite as well. Main conclusions include:From metallographs and scanning electron micrographs, we can conclude that the structures of the15%SiCp/Al-Cu-Mg composite and15%SiCp/2009Al composite which was fabricated for comparison were fully compact without inclusions and holes. SiC particulates well-distributed in the matrix without clustering phenomenon; TEM images show that there was smooth and well-combined interface between reinforcement and matrix. XRD patterns show that both two as-HIPed composites had the same precipitated phase, namely Al2Cu and Mg2Si. After heat treatment, Al2Cu phase dissolved into the matrix gradually, but Mg2Si phase was always stable. The tough strength of after-heat-treated specimens increased by53%to579MPa than as-HIPed for15%SiCp/Al-Cu-Mg composite and by46%to570MPa for15%SiCp/2009Al composite, which also demonstrates that the mechanical properity of the composites fabricated by element powder or alloy powder are comparative.The optimum solution treatment parameters of15%SiCp/Al-Cu-Mg composite has been established:solution heat treated for2h at510℃, and quenched into20℃water with the transfer time during4s followed by four days’ natural aging. Then, the mechanical property of15%SiCp/Al-Cu-Mg composite was attained:Rm=579MPa, Rp0.2=390MPa, A=7%. Increasing the solution treatment temperature up to the optimum value improved the mechanical properties. Increasing the solution treatment temperature beyond the optimum value resulted in a rapid reduction in mechanical properties, which was due to the formation of over-heated structure. When the solution time exceeded a range, soluble second phase dissolved into the matrix adequately, the strength of the composite reached its peak, but extending solution time sequentially could not improve the strength of the composite. The microstructure and mechanical properties of the composite are not sensitive to water temperature in quenching. Transfer time also had a permissible range, and the range was16s for the15%SiCp/Al-Cu-Mg composite. It was too late for the decomposition of the supersaturated solid solution between16s, which less affected the microstructure and mechanical properties of the composite.The results have found that the addition of SiC particles provided free silicon which formed Mg2Si with Mg atom, and increased the ratio of Cu and Mg and changed the composition of precipitation phases. Al2Cu phase would re-dissolution after solution treatment, yet the content of MgaSi phase did not change.The intensity and plasticity of15%SiCp/Al-Cu-Mg composite were highly improved after isothermal forged, and with the strain increasing, distribution of SiC particles was better, and the mechanical property was higher. When the strain was70%, tensile strength, yield strength and elongation were580MPa,375MPa and7%, respectively. When the strain exceeded70%, the strength and plasticity of the composite would reach a stable state.The results of fracture morphology of15%SiCp/Al-Cu-Mg composite indicate:there were three parts, crack source zone, crack propagation zone and crack terminal zone; and dimple fractures which had a uniform distribution were easy to see.15%SiCp/Al-Cu-Mg composite had two main types of fracture, namely ductile fracture of the matrix and fracture of particles.