Microstructure And Performance Of HPDC Mg-Al Base Alloy

Improving the mechanical properties of magnesium alloys by adding rare earth（RE）elements has become an effective way to strengthen magnesium alloys.Among them,the high pressure die casting（HPDC）Mg-Al-RE system is relatively common,and its microstructure and mechanical properties are related to the type and content of RE.Three high-performance HPDC Mg-Al-RE alloys were successfully prepared by alloy design and process control,which effectively solved the problem of insufficient room temperature and high temperature strength of HPDC Mg-Al alloys.The innovative research in this paper provides a theoretical basis for the development of high performance HPDC Mg-Al-RE alloys.For the first time,based on the research of HPDC AE44 alloy,Mg-4Al-4Gd-0.3Mn（wt.%）alloy（AGd44 alloy）with a main intermetallic phase as a petal phase was prepared by adding a single rare earth element Gd,and then further increase the strength of the alloy by aging treatment.In order to further reduce the cost of the alloy,a high-strength and low-cost Mg-4Al-3La-1Ca-0.3Mn（wt.%）alloy（ALX431 alloy）was prepared by replacing part of Ca with a rare earth element La.The main intermetallic phases of the alloy are Al₃Laand（Mg,Al）₂Ca phases,and the strength of the alloy is further improved after aging treatment.Based on the above research,we further studied the effects of the composite addition of Gd and Laon the mechanical properties and microstructure of Mg-Al alloys,and prepared ultra-high strength Mg-1Al-1La-8Gd-0.3Mn（wt.%）alloy（AE19 alloy）.The yield strength,tensile strength and elongation of as-cast AGd44 alloy are 130 MPa,250 MPa and 25%,respectively.After peak aging treatment,the yield strength of the alloy is170 MPa,the tensile strength reaches 260 MPa,and the elongation is 19.4%.The excellent room temperature and high temperature properties of the peak-aging alloy are mainly due to the precipitation of lamellae and lath-shaped Mg₁₇Al₁₂phases in the Mg matrix.These precipitated phases precipitated at the matrix and grain boundaries will effectively hinder the dislocation and grain boundary sliding.Compared with the AGd44 alloy,the ALX431 alloy exhibits higher strength.The yield strength and tensile strength of the as-cast ALX431 alloy are 178 MPa and 267 MPa,respectively.The yield strength and tensile strength of the alloy at250°C are 113 MPa,134 MPa,respectively.The excellent room temperature and high temperature mechanical properties of ALX431 alloy are mainly attributed to fine grain strengthening and second phase strengthening.After peak aging treatment,the room temperature and high temperature properties of the alloy were significantly improved.A large number of nano-sized Al₂Ca particles precipitated near the grain boundary and in the Mg matrix.These precipitates strengthened the grain boundary and the Mg matrix.In addition,the Al₂Ca phase has a higher melting point,which further enhances the strength of the alloy at high temperatures.In order to further increase the strength of the alloy,we prepared the AE19alloy,which exhibits the current optimum yield strength and excellent high temperature mechanical properties.The ultra-high yield strength is mainly attributed to a large number of second phases distributed in the matrix and the grain boundary,as well as fine grain sizes.