Microstructure And Mechanical Properties Of A Modified A390Alloy

Hypereutectic Al-Si alloys are commonly used in automotive applications, particularly in piston industry, due to their low density, low coefficient of thermal expansion, good wear resistance and high dimensional stability. But the microstructure of hypereutectic Al-Si alloys consists of a great number of coarse primary Si particles. The brittle primary Si leads to poor mechanical properties with the increase of silicon content in Al-Si alloys. As the engine power continuously improves, higher strength piston alloy is required. The performance of hypereutectic aluminum-silicon alloy is expected to be greatly improved via alloying, modification, heat treatment, et al.The investigation is based on A390alloy. By modifying composition, fining primary silicon, hot deformation and T6heat treatment, the study contrives a new type hypereutectic Al-Si alloy with slight primary silicon particles and outstanding mechanical properties at room and high temperature. According to a series of experiments and analysis, such achievements are educed:(1) The microstructure of Al-17.5Si-4.5Cu-0.7Mg-1Zn-0.1Ti-0.5Ni-0.3Zr-0.2Sc-0.2V DC cast alloy consists of a-Al, primary Si, eutectic Si and second phases such as block Al2Cu(θ), Al7Cu4Ni(γ), Al9FeNi(T), Al8FeMg3Si6(π) and network-like Al9(Fe,Ni,Cu,Sc,Zr)4Si2phase and so on.(2) The addition of phosphorus can refine the primary Si crystals and make it uniformly distributed in cast alloy. Compare with unmodified alloy, the unltimate tensile strength of P modified alloy is improved from267MPa to287MPa at room temperature. However, the ultimate tensile strength at300℃of two alloys maintains140MPa.(3) Hot deformation eliminates the defects formed in cast alloys. It breaks coarse silicon crystals in unmodified and P modified alloy. The fracture of primary Si particles resulted in the degradation of room-temperature and high-temperature mechanical properties.(4) After T6heat treatment, eutectic silicones in both unmodified and P modified cast alloys granulate significantly. Al9FeNi and AlgFeMg3Si6phases disappear while a new dispersively distributed phase Al5Cu2Mg8Si6precipitating from matrix. Owing to the T6heat treatment, the room-temperature ultimate tensile strength of both alloys increases over80MPa. Unfortunately, their tensile strength at300℃decreases approximately10MPa.(5) Fractographic studies indicate that the fracture of alloys is in the hybrid mode, i.e., the cleaved fracture of the primary Si particles and the ductile fracture of the Al matrix. The main fracture mode transforms from transgranular crack of Si crystals at room temperature to ductile rupture of α-Al at300℃. The strengthening mechanisms of alloys are the combining effect of microstructure refining strengthening, dispersion strengthening, solid-solution and precipitation strengthening and so on.