Research On Backward Extrusion Process And Microstructure Of AZ80 Magnesium Alloy Cup Shell

With the rising demand for lightweight structures, the magnesium alloy is widely used because of its low density and high specific strength. The FE simulation was conducted to analyze the effects of extrusion parameters on cup shell extrusion process. And the microstructure and properties of the cast AZ80 alloy, solid solutioned alloy, extruded alloy and the alloy after T5 aging were investigated by backward extrusion experiments. The process of homogenization treatment was concerned, and the T5 aging process on the temperature of 175℃ was researched then. The effects of different parameters on microstructure and properties were verified as well.The result shows that the cast AZ80 alloy consists of α-Mg matrix and β-Mg17Al12. The yield strength, tensile strength and elongation of cast AZ80 alloy are 80.95 MPa,128.68 MPa and 3.6%,respectively. The best homogenization process is 410 ℃× 10 h, and the tensile strength and elongation of homogenized alloy are 195.31 MPa and 7.7%.The FE simulations were conducted to investigate the influence of extrusion velocity, the friction coefficient, the temperature and the extrusion ratio on backward extrusion process of AZ80 alloy. The reasonable experiment parameters were verified and the distribution of strain of side walls and bottom of the cup shaped parts was obtained then.The effects of extrusion temperature and extrusion ratio on cup shell backward extrusion process were investigated by hot extrusion experiments in detail. Results show that the quality of the cup shell is good at 410℃,but the obvious cracks occur at 370℃ and 450℃. And the overburning was observed at 450℃. With the increase of the extrusion temperature, the grain size becomes bigger and the tensile strength decreases with higher elongation. With the increase of extrusion ratio, the grain is refined adequately and the full recrystallization takes place during the extrusion process. Meanwhile, the tensile strength of the extruded alloy increases but the elongation decreases due to the fine grain strengthening. The alloy with the extrusion ratio of 2.78 has tensile strength of 247 MPa with the elongation of 10.2%. Quasi-cleavage brittle fracture is the main fracture of the extruded alloy and the appearance of fracture appears to be tearing ridges with shallow dimples.The deformable zone in the bottom of the cup shell was studied. The results show that the plastic strain increases radially outward with three-compressive stress there. And thus higher strength occurs because of the refined grains. The place where the wall and the bottom intersect shows the state of plane stress with relatively bigger grain size and lower stress.The extruded alloy was aged at 175℃,and the peak micro-hardness reached 82.71 HV after 24 h. The black second phase(Mg17Al12) separates out among grain boundries. And the amount of second phase adds with the increasment of aging time. The strength of the aged alloy was improved because of the continuous separation of the hard brittle phase and the tensile strength reached 280.99 MPa with the elongation of 9.6%.