Research On Microstructure And Properties For Aluminium Piston Alloy Of Laser Remelting

With the liters power of high power density diesel engine continuous improvement, the higher requirements for piston in combustion chamber were needed. There is still a gap between high temperature fatigue performance of existing aluminum piston alloy material and the actual demand. In order to improve the high temperature fatigue performance of aluminum alloy piston material, we need to use the laser remelting to make its surface refining.This subject adopts the type HL3006D YAG continuous solid laser on aluminum piston alloy (Al-Si-Cu-Mg) surface to study the effect of laser process parameters on the microstructure and properties. The test used NIKON EPIPHOT300digital metallographic microscope, QUANTA400F scanning electron microscopy and J2010TEM to observe microstructure and morphology after laser remelting, and used SHIMADZU-M micro-hardness meter to test its micro-hardness. This thesis will mainly discuss the influence of laser remelting process parameters on penetration, the weld width, grain size and secondary dendrite spacing. The research results are like the followings:Primary silicon grain size in aluminum piston alloy matrix decreased from25μm to2μm-3μm after laser remelting, which is about1/10of the original. Laser remelting area includes remelting zone, heat affected zone, critical zone and matrix zone.Phases in aluminum piston alloy matrix before laser remelting are:white matrix α-Al, large polygonal primary silicon and eutectic silicon, τ phase-(CuNi)3Al5, intermetallic phase-Al7(Ni, Cu)2-(Fe, Si), M phase-Mg2Si, ε phase-Al3Ni, rich Ni phase, etc. After laser remelting, the surface of aluminum piston alloy changes into solid solution and the ingredients in it are average. While after artificial aging, the second phase is separated, such as θ phase Cu2Al, S phase-Al2CuMg and Q phase-Al5Cu2Mg8Si6. There are three kinds of strengthen mechanism to improve the comprehensive mechanics performance of the aluminum piston alloy by laser remelting, which are solid solution strengthening, grain refining strengthening and second phase strengthening.Micro-hardness test shows that the average micro-hardness of remelting zone after laser remelting is240HV, which increased60%over the matrix134HV. Micro-hardness gradually increased from the matrix area to the remelting area, which is the result of refinement of the gram size.When laser power is constant, increasing scanning rate is beneficial to grain refinement, but when the scan rate is constant; the power has no obviously significant effect on grain refinement.Laser power has great influence on the weld width,and scan rate has great influence on the penetration. The weld width increases as the laser power increases;while the penetration decreases as the scan rate increases. When the laser power is constant and the scan rate increases, the secondary dendritic space will became small.