Study On Microstructure And Properties Of Cast Aluminum Alloy For Piston Strengthened By Rare Earth

The near-eutectic multi-component Al-Si alloy has a series of characteristics such as high specific strength,good wear resistance,and excellent thermal stability,thus becoming a widely used piston material.However,with the development of the automotive industry,the improvement of engine parameters put forward higher requirements on the performance of piston materials.The addition of rare earth elements is a commonly used strengthening method in Al-Si alloys,but there is some controversy about the modification of primary silicon.There is relatively little research in multi-component Al-Si alloys,and multi-component alloys are more likely to form coarse-sized rare earth-rich phases.Thus,there are still some limitations to this strengthening method.Therefore,studying the influence of rare earth elements in the near-eutectic multi-component Al-Si alloy to maximize the alloying effect of the rare earth elements is of great significance for improving the properties of the alloy.This paper takes Al-13Si-5Cu-2Ni-1Mg alloy as the research object,and selects the typical light rare earth element La and heavy rare earth element Er.A series of studies carry out on the influence of the two elements added separately,the compound addition with P element and the two rare earth elements added together on the microstructure and performance of the alloy.The effect of adding separately La and Er two types of rare earth elements on the microstructure and properties of the alloy was studied.The result shows that both elements can modify primary silicon and eutectic silicon simultaneously,but there are problems that the primary silicon has a limited degree of modification and the eutectic silicon is not uniformly modified.When the addition amount is 0.2%,La has a relative strong modification effect on eutectic silicon,and Er improves the primary silicon and secondary dendrite spacing more significantly.In addition,both elements form coarse rare earth-rich phases with complex composition.When the contents of La and Er in the alloy are both 0.2%,the mechanical properties at room temperature are the best.Before modification,the tensile strength and elongation of the alloy are 181 MPa and 0.59%respectively,and the wear rate is 5.53×10-3 mm3·m-1;after adding 0.2%La,the tensile strength increases to 210 MPa and the elongation rate is 0.61%,and the wear rate is 4.92×10-3 mm3·m-1;after adding 0.2%Er,the tensile strength and elongation are 219 MPa and 0.69%respectively,and the wear rate is 4.73×10-3 mm3·m-1.By studying the interaction effect between rare earth and P elements,the performance limitations of primary silicon and eutectic silicon size on alloy are further solved.The study found that on the one hand,the formation of AlP can be used as a heterogeneous core,which significantly improves the size and morphology of primary silicon,eutectic silicon and intermetallic compounds generated in the eutectic reaction;and on the other hand,it can interact with rare earth elements,and weaken the metamorphic effect of rare earth elements on eutectic silicon.The addition of P element can effectively achieve the purpose of further improving the performance of the alloy.At room temperature,when the P content is 0.005%,the tensile strength of the alloy without rare earth elements is 248 MPa,and the elongation is 0.79%;the tensile strength of the alloy with 0.2%La or 0.2%Er is 258 MPa and 271 MPa,and the elongation is 0.80%and 0.91%,respectively.After adding 0.005%P to alloys containing 0%RE,0.2%La,and 0.2%Er,the wear rate is 3.94×10-3 mm3·m-1,3.54×10-3 mm3·m-1,3.33×10-3 mm3·m-1,respectively.After adding 0.2%La and 0.2%Er in combination to the alloy containing P element,the room temperature tensile strength and elongation of the alloy can reach 274 MPa and 0.96%,respectively.The interaction of the two elements can inhibit the growth of the rare earth-rich phase,and significantly reduce the size of the rare earth-rich phase.The generate small and uniformly distributed Er-rich and La-rich phases in the alloy can further improve the high-temperature properties of the alloy.At 350?,the tensile strength and elongation of the alloy without rare earth elements are 82 MPa and 8.6%respectively;when 0.2%La or 0.2%Er is added alone,the tensile strength of the alloy is 88 MPa and 89 MPa,and the elongation is 6.6%and 7.6%;when 0.2%La and 0.2%Er are added together,the tensile strength and elongation of the alloy are 91 MPa and 8.4%,respectively.Compared with alloys without added rare earth elements,the tensile strength at high temperature is increased by about 11%when the elongation is equivalent.