Study On Hot Rolling And Recrystallization Annealing Of Mg-8Al-3.5Sr(-0.8Y) Alloy

Magnesium alloy is one of the lightest materials in engineering application materials.It has excellent performance such as strong shock resistance,electromagnetic shielding,thermal conductivity and conduction,and the pollution recovery rate is up to 100%.Therefore,the application of magnesium alloy in the electronic communications,aerospace,biomedical and other fields become more and more widely.So the magnesium alloy has the reputation of the most development and application potential of the new material in 21 st century.However,the heat resistance of magnesium alloy is poor,the temperature higher than 120 ? the creep strength will be drastically reduced,thus limiting the use of magnesium alloy at higher temperatures.In addition,the dense hexagonal crystal structure makes the plasticity of magnesium and its alloys far less than steel,aluminum and other structural materials,and thus cause practicalapplication is greatly limited,so how to improve the heat resistance of magnesium alloy while improving its plastic deformation capacity become the focus of the current research.In this paper,Mg-8Al-3.5Sr and Mg-8Al-3.5Sr-0.6Y were used as the research object.The two kinds of alloys were subjected to solution treatment,hot rolling and recrystallization annealing respectively to explore a reasonable solution treatment process.The amount of deformation and the influence of the second phase on the microstructure during the deformation process and the complete recrystallization annealing temperature of the alloy samples with different hot rolling properties.The microstructures and mechanical properties of the alloy specimens were measured by metallographic microscope?OM?,scanning electron microscopy?SEM?,X-ray diffraction?XRD?,tensile testing machine and Hopkinson pressure test system.The results are as follows:?1?The microstructure of as-cast Mg-8Al-3.5Sr alloy consists of a-Mg,reticular Mg₁₇Al₁₂ phase and rod-shaped Al4 Sr phase;The microstructure of as-cast Mg-8Al-3.5Sr-0.6Y alloy is mainly composed of a-Mg,granular Al₂Y phase,point-like Al₂Sr phase and rod-shaped Al₄Sr phase.The optimum solution conditions of the two alloys were 415 ?,20 h.After the solution treatment,the grain boundaries became clear and the number of interstitial Mg₁₇Al₁₂ phases decreased significantly,and the second phase distribution of rod and dot shape was more uniform.?2?In the process of hot rolling deformation,Mg-8Al-3.5Sr andMg-8Al-3.5Sr-0.6Y alloy have a large number of twin crystals in the microstructure,and the dynamic recrystallization degree increased.The eutectic microstructure of Mg-8Al-3.5Sr-0.6Y alloy is much larger than that of Mg-8Al-3.5Sr alloy when the cumulative reduction is 80%.Due to the presence of work hardening,the hardness of the alloy increases with the amount of deformation,but when the cumulative deformation of 90%,the two alloys have different degrees of cracks.?3?After repeated annealing experiments with different temperatures,the optimum recrystallization temperature of Mg-8Al-3.5Sr and Mg-8Al-3.5Sr-0.6Y alloy with 30% reduction was 350 ?,50% and 80%reduction were 300? and 250?,respectively.The recrystallization temperature of Mg-8Al-3.5Sr and Mg-8Al-3.5Sr-0.6Y alloy decreases with the increase of the amount of deformation.?4?The hardness and mechanical properties of Mg-8Al-3.5Sr and Mg-8Al-3.5Sr-0.6Y alloys are closely related to the degree of recrystallization of microstructure.For the Mg-8Al-3.5Sr alloy,the mechanical properties obtained after the recrystallization process are 80%,the tensile strength is 310 MPa,the yield strength is 197 MPa.The tensile strength and yield strength of Mg-8Al-3.5Sr-0.6Y alloy with 80% reduction after annealed to complete recrystallization structure were 315 MPa and 190 MPa respectively.?5?Whether Mg-8Al-3.5Sr alloy or Mg-8Al-3.5Sr-0.6Y alloy,the greater the amount of deformation at the same recrystallization temperature,the greaterthe fraction of recrystallized tissue in the crystal.As the hot rolling makes the shape of the second phase of the high melting point change,and thus can be effectively pinned dislocation to obtain high-density dislocation plug,for the start of recrystallization to provide energy support.