Influence of annealing and spheroidizing treatment on microstructure and mechanical properties of AZ91 magnesium alloy

The low-strength and high-brittleness of AZ91 cast magnesium alloy mainly result from the coarse divorced eutectic phase.To solve these problems,the annealing treatment of AZ91 cast magnesium alloy was carried out at 415 ℃ and held for 24 h in this study and the alloy was then slowly cooled to room temperature in furnace.The microstructures of the alloy were observed using a metallographic microscope,a transmission electron microscopy and an emission scanning electron microscopy,respectively.The phase analysis was performed using the X-ray diffraction,and the tensile test of the specimen at ambient temperature was performed on a material test machine.The results indicate that the coarse divorced eutectic phase dissolves into the Mg matrix during the isothermal process,and the lamellar β-Mg17Al12 phase precipitates from the magnesium solid solution with a type of pearlite precipitation during furnace cooling.Consequently,the spheroidizing treatment was carried out at 320 ℃ for 20 h following the annealing process and the lamellar β-Mg17Al12 phase was spheroidized.Compared with the as-cast alloy,the strength and ductility of the AZ91 magnesium alloy are increased obviously after annealing treatment;the yield strength and tensile strength are increased to 137.8 MPa and 240.4 MPa from 102.9 MPa and 199.3 MPa,respectively;and the elongation is improved to 6.12% from 4.35%.After being spheroidized,the strength and hardness decrease a little,but the ductility is elevated to 7.23%.The nucleation,growth and spheroidizing mechanism of the lamellar β-Mg17Al12 phase were also discussed.

The low-strength and high-brittleness of AZ91 cast magnesium alloy mainly result from the coarse divorced eutectic phase. To solve these problems, the annealing treatment of AZ91 cast magnesium alloy was carried out at 415 ℃ and held for 24 h in this study and the alloy was then slowly cooled to room temperature in furnace. The microstructures of the alloy were observed using a metallographic microscope, a transmission electron microscopy and an emission scanning electron microscopy, respectively. The phase analysis was performed using the X-ray diffraction, and the tensile test of the specimen at ambient temperature was performed on a material test machine. The results indicate that the coarse divorced eutectic phase dissolves into the Mg matrix during the isothermal process, and the lamellarβ-Mg17AI12 phase precipitates from the magnesium solid solution with a type of pearlite precipitation during furnace cooling. Consequently, the spheroidizing treatment was carried out at 320℃ for 20 h following the annealing process and the lamellar β-Mg17A12 phase was spheroidized. Compared with the as-cast alloy, the strength and ductility of the AZ91 magnesium alloy are increased obviously after annealing treatment; the yield strength and tensile strength are increased to 137.8 MPa and 240.4 MPa from 102.9 MPa and 199.3 MPa, respectively; and the elongation is improved to 6.12% from 4.35%. After being spheroidized, the strength and hardness decrease a little, but the ductility is elevated to 7.23%. The nucleation, growth and spheroidizing mechanism of the lamellarβ-Mg12TAI12 phase were also discussed.