| Piston alloys work in a severe condition of high temperature and high pressure for a long time,and sustain high cyclical fatigue load.The high-cycle fatigue crack is mainly caused by the coarse primary and eutectic silicon,which show irregular shapes.So,the refinement of Si phase is an effective way to improve the comprehensive performance of the piston alloy.In this paper,the effects of Si contents,RE element Nd,mixed RE La-Ce and external ultrasonic field on the microstructure evolution and mechanical properties of the Al-Si-Cu-Mg-Ni system piston alloys were studied.The influences of the above factors on the solidification precipitated phase morphology and distribution in the Al-Si piston alloys were observed by the optical microscope(OM),scanning electron microscope(SEM).The mechanical properties of the piston alloys were tested by electronic universal testing machine.The optimum amount of Rare Earth was determined.The optimal ultrasonic power and ultrasonic treatment temperature were also determined.The major results in the present study were as follows:(1)With the increasing of Si contents,the thermal expansion coefficient of the piston alloy decreases and the fluidity increases.The equal-area-circle diameter and the occupied area fractions of the primary Si phases in microstructure increase,the elevated temperature(350?,425?)tensile strength and the elongation decrease.(2)When element Nd content is below 0.3%,?-Al matrix and eutectic silicon are refined.The equal-area-circle diameter of primary silicon decreases by 73% compared with that in alloy without Nd addition.When the Nd content exceeding 0.8%,a new noodle-like phase is found,which results to the mechanical properties decrease seriously.The tensile strength of 350? increases by 29.8% compared with that of the Al-Si alloy without Nd addition when the content of Nd was 0.3%.The constitutional supercooling is caused by Nd element,which gathers around AlCuNi phase,inhibits the growth of AlCu Ni phase.The thick bones shape of AlCuNi phase turns into small bars.(3)The misch metal of La-Ce has a good metamorphism effects on the Si phase in Al-Si piston alloys.When the mixed RE La-Ce content is 0.5%,the equal-area-circle diameter of the primary Si and the average lengths of the eutectic Si are the smallest,which are 16.7?m and 22.12?m respectively.However,when the amount of mixed RE of La-Ce is more than 0.5%,a new long needle-like RE-rich phase is found.The appearance of this new RE-rich phase can seriously damage the mechanical properties of the piston alloys.When the contents of La-Ce was 0.35%,The tensile strength at room temperature and at 350? reached the maximum,which are 318 MPa and 124 MPa respectively.The tensile strength at room temperature and at 350 ? increase by 10.0% and 30.5% respectively compared with that of the alloy without misch metal La-Ce addition.(4)The isometric crystal is formed,and the sizes of primary Si and eutectic Si are refined after the ultrasonic processing.In 720? melting temperature and 0~1400W ultrasonic power conditions,the average equal-area-circle diameter of primary Si decreases from 40.73?m to 23.35?m;and the average length of eutectic Si reduces from 39.6?m to 27.1?m.There is no evident change for the morphological characterization of other second phases.The tensile strength at room temperature and tensile strength at 350? increase by 21.9% and 22.8% respectively,compared with that without ultrasonic processing.The ultrasonic melting treatment is contributed to refinement of silicon phase and the improvement of mechanical properties. |
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