Effect Of Transition Metal Elements On High-temperature Properties Of Al-Si-Cu-Mg Alloys

Al-Si casting alloy are widely used in the manufacture of automobile engine cylinder heads due to their advantages of light weight,good casting performance and excellent comprehensive mechanical properties.However,with the contradictory development between high load stress and thin-walled structure of the high-performance engine,the requirements for the balance of the strength-toughness at room temperature as well as the high-temperature performance of aluminum alloy materials used in engine cylinder head and engine block are becoming stricter.The traditional Al-Si casting alloy’s application in high-tech fields has been seriously limited because of the rapid decline of its mechanical properties when the working temperature is above 200℃.In this thesis,by means of microalloying strengthening,transition metal elements with high melting point which are suitable for strengthening Al-Si alloy were added to the alloy,in order to observe the specific strengthening effect and explore the strengthening mechanism.The intermetallic phases that contribute more to improving the alloy’s high temperature performance have been studied in advance,then followed the experimental verification and comparison among them.The strengthening mechanism was investigated for the alloy with better comprehensive mechanical properties,then optimized heat treatment process of the alloy,and finally obtained the alloy with excellent comprehensive mechanical properties.The main results show:（1）By adding transition metal elements to strengthen the alloy,a more superior strengthening effect was displayed with the increase of the test temperature,in particular when the test temperature above 160°C.The heat resistant intermetallic phases rich in transition metal elements were usually formed in the alloy,and the strength properties of the alloy are improved by hindering the movement of dislocations and crack propagation;（2）The addition of Mn,Mo,and Cr can significantly improve the elongation of the alloy,resulted from the following two points.Firstly,Cr and Mn contribute to change the needle-like morphology of the Fe-rich phase.Secondly,with the combination of Mo and the intermetallic phase in the alloy which improved the ability to resist coarsening of the intermetallic phase;（3）The compound Al₃Zr formed by the addition of Zr to the alloy can not only strengthen the alloy through the Orowan mechanism,but also refine the grain sizes of the matrix;（4）In this study,the optimized alloy with the best mechanical properties is obtained by adding Zr and Mo elements to the base alloy after optimizing the heat treatment parameters,being respectively 370.4MPa,351MPa and 5.2%of the ultimate tensile strength,yield strength and elongation percentage at room temperature.When the test temperature is raised to 300℃,ultimate tensile strength,yield strength and elongation percentage are 173.6MPa,165MPa and 8.1%respectively.The ultimate tensile strength and yield strength are increased by 85.3%and 81.3%respectively compared with the base alloy.