Simulation Study On The Deterioration Of Compressor Piston Er And The Simulation Of Gravity Casting Process

As one of important parts of the reciprocating piston refrigeration compressor, the piston has an important influence on the operation performance, vibration and noise of the system. ZL108 has many advantages such as the low density, high specific strength, low thermal expansion coefficient and high thermal conductivity. The coarse flaky eutectic silicon in the a-Al phase matrix of the unmodified alloy has serious adverse effect on the properties. The modification effect and mechanism of Er on the eutectic silicon were researched by using the tensile test, quantitative metallography technique, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy spectrum analysis (EDS). The permanent mold gravity casting process of the aluminum alloy piston was numerical simulated by ProCAST software. According to the numerical simulation result, the shrinkage porosity of the piston casting was forecasted and experimental verified.Er had the significant modification effect on the eutectic silicon by the impurity induced twinning mechanism. And the modification of Er had a long acting and remelting modification efficacy. Except for a tiny amount dissolving into the a-Al phase, Er prevented as the intermetallic compound precipitation of Al3Er. When the addition of Er was more than 0.6%, some Er existed as the point-like AlSiEr and acicular AlSiCuEr phase. The AlSiCuEr phase had a cutting action on the alloy matrix and adverse effects on the properties. When the addition of Er reached the optimum amount 0.6%, the eutectic silicon was fully modified and changed from the coarse flake to coralline. Compared with the unmodified eutectic silicon, the average length and aspect ratio reached 5.84um and 3.89 and decreased by 73.81% and 62.99% respectively. Meanwhile the secondary dendrite arm spacing (DAS) of the a-Al phase reached 22.0um and decreased by 32.31%. The tensile strength and elongation of the as-cast rare earth aluminum alloys were 220.2MPa and 2.53% and improved 22.20% and 39.01% respectively. The tensile fracture mode changed from the brittle fracture to the mixed fracture. During the heat treatment, 540?×6h solution+170 ?×10h aging, the pelletization effect of the coral-like eutectic silicon was significant due to the high surface energy state and the properties were further improved.The aluminum melt flowed smoothly and filled the cavity gradually from the bottom up in the permanent mold gravity casting process of the aluminum alloy piston by adopting vertical slitting gate system. When the pouring temperature was 740?and the mold temperature was 250?, the casting solidification time by numerical simulating was 114.1s and the solidification time of the piston part was 66.0s. When the casting minimum temperature reached 400? and the opening time was 227s, the maximum stress which occurred in the bottom corner of the sprue was 100.2MPa. The larger stress at the upper part of the piston was 85.9MPa. The shrinkage porosity defects which were forecasted by the numerical simulation were at the upper part of the piston pin boss and the both ends of the reinforcing rib at the back of the top surface. The forecast was consistent with the experimental result. Meanwhile the rating of the piston macro structure was level 3 and reached the requirements of the relevant standard. By comparing the unmodified microstructure and modified microstructures of the piston, the addition of Er not only had the modification effect on the eutectic silicon, but also made the main existing form of Fe from the coarse fishbone-like AlFeMnSi phase to small point-like complicated compounds. The effect could reduce the harm of Fe and improve the quality of the aluminium alloy piston.