Effect Of Electromagnetic Stirring And Heat Treatment On Microstructure And Properties Of AZ91 Magnesium Alloy

Magnesium alloy has attracted much attention in modern industry because of its outstanding properties on: specific strength,material recycle rate and electromagnetic shielding ability.However,restricted by its poor plasticity at room temperature,the use of magnesium alloy is,currently,not as wide as that of other metal materials.Electromagnetic stirring technology can refine the grain,increase equiaxed grains and reduce the dendrites.Meanwhile,convection of the melt caused by the electromagnetic force can generate a uniform temperature field which in sequence decreases the segregation.In addition,the mechanical properties of magnesium alloy can also be improved through the solution treatment and aging treatment.In this paper,electromagnetic stirring technology and heat treatment are adopted to improve the microstructure and mechanical properties of AZ91 magnesium alloy.The thermodynamic,electronic and mechanical properties of β phase and Mg based solid solution are studied by using the first-principle calculation.The results show that the mechanical properties and microstructure of AZ91 magnesium alloy are improved with electromagnetic stirring frequency increasing.The best mechanical properties are achieved when the electromagnetic stirring parameters are 10 Hz and 150 A,and the tensile strength,elongation and hardness can be 163 MPa,5.3% and 58.6HB,respectively.By this set of parameters,the grain is obviously refined and the average size of grain is 93μm.The α-Mg primary phase’s morphology turns from gross dendrites to rose shape crystals,and the coarse β phase is refined and broken.The twins are also found in the matrix.To conclude,because of grain refinement,fragmentation of β phase,solution strengthening of Al,Zn and twins,the mechanical properties of the alloy is improved.After the solution treatment of electromagnetic stirred AZ91 alloy,it is found that its mechanical properties first increase and then decrease with the solution time,the peak is 36 h.At this point,the tensile strength,elongation and hardness are 229 MPa,8.8% and 67.8HB,respectively.Most of the β phase have been dissolved and the remaining β phases have a disperse distribution.By applying the aging treatment on the 36 h solution treated AZ91 alloy,it is found that the tensile strength and hardness properties first decrease then increase and finally decrease with time,the peak is reached at 12 h with exact values 247 MPa and 78.1HB,respectively.Different from tensile strength and hardness,the elongation of alloy never increases but keeps decreasing with the aging time.During the whole process of aging treatment,the size of grain grows with time.Generally,the β phases have a discontinuous precipitation behavior,by very little chance,they have continuous precipitation.The first-principle calculation result verifies that β phase can spontaneously form and keep stable.It has a high young’s modulus and presents a brittle property.In this way,when the β phase is with small and sparse distribution,it strengthens the matrix.In the solid solution of Mg24,Mg23Al1,Mg23Zn1 and Mg₂₂Al₁Zn₁,Mg₂₂Al₁Zn₁ has a relatively low combination heat and binding energy and good plasticity,which indicates that Al and Zn elements have positive effect towards both formation and stability of Mg based solid solution and can improve its mechanical property.