Effect Of Alloying And Physical Field On Microstructure And Mechanical Properties Of Mg-9Al-3Si Alloy

Mg₂Si phase is an ideal reinforcing phase because of its excellent thermodynamic parameters and moderate elastic parameters,which can improve the mechanical properties of the alloy.Compared with other Mg-Al alloys,due to the addition of Si elements,the Mg₂Si phase can improve the strength of Mg-Al-Si alloy at room temperature and high temperature significantly,and it can be used in more complex and more demanding conditions.The fine and evenly distributed Mg₂Si phase can improve the comprehensive mechanical properties of the alloy,while the partially distributed Mg₂Si phase is beneficial to the preparation of gradient material.In this paper,Mg-9Al-3Si alloy was studied,and the primary Mg₂Si phase was refined by alloying.The mechanical properties of the alloy were improved by ultrasonic-alloying composite treatment.The pulsed magnetic field-alloying composite treatment were used to control the solidification structure of the alloy and prepare gradient materials.By using the hot extrusion deformation process,the hot deformation behavior of the alloy was studied and the constitutive model was established.Meanwhile,optimize the hot working parameters and improve the microstructure of the alloy.Therefore,in this paper,the changes of solidification structure and mechanical properties of the alloy after alloying with Sr and Y elements were studied.On the basis of this study,the effects of ultrasonic-alloying and pulsed magnetic field-alloying composite treatment on the morphology,distribution and mechanical properties of the primary Mg₂Si phase were studied.At last,the isothermal constant strain rate compression test was carried out on the sample under the optimum alloying process parameters,the constitutive model was established and the thermal deformation behavior and microstructure change of the alloy were analyzed.The results show that:1)After adding Sr and Y elements,the solidification structure and mechanical properties of the alloy were improved obviously.And the effect of composite modification was better than that of single element treatment.Compared with untreated samples,the average grain size of primary Mg₂Si phase decreased by 40%,and its tensile strength and elongation at room temperature increased by 31.2%and 300%,respectively.Under the condition of composite metamorphism,when the content of Y element is in the range of 0～1.17%,with the increase of Y element content,the size and Sr’value of primary Mg₂Si phase in the alloy first decrease and then increase,and a turning point occurs at 0.78%.The mechanical properties of the alloy are closely related to the size and distribution of the primary Mg₂Si phase in the alloy.The smaller the size of the primary Mg₂Si phase is,the more uniform the distribution in the matrix,and the better the mechanical properties of the alloy are.2)Under the condition of ultrasonic-alloying composite treatment,the solidification structure of the alloy has been significantly improved.At the same time,the mechanical properties have been improved obviously.The results show that the effect of ultrasonic-alloying composite treatment is the best.Single ultrasonic treatment and single alloying treatment were not as effective as composite treatment.Under the condition of primary Mg₂Si phase size reduced 72.9%,the room temperature tensile strength and elongation increased by 125%and 310%respectively.Under the condition of composite treatment,when the ultrasonic power increases and the ultrasonic time prolongs,the size of the primary Mg2Si phase decreases.The change of mechanical properties of the alloy corresponds to the size and shape of the primary Mg₂Si phase.The smaller the primary Mg₂Si phase,the better the mechanical properties of the alloy.3)After Mg-9Al-3Si alloy was treated by pulsed magnetic field-alloying composite treatment,when the pulse voltage increased in the range of 0～300V,the primary Mg₂Si phase was firstly refined and then coarsened,and the turning point was 100V.In the range of 1～10Hz,the effect of pulse frequency on the diameter of Mg₂Si equivalent circle was not obvious.When the preheating temperature of the mold increases in the range of 200～600℃,the size of the primary Mg₂Si phase increases gradually.After the alloy was treated by pulsed magnetic field-alloying composite treatment,the initial Mg₂Si phase showed different degrees of gradient distribution at the macro level,and its volume fraction increases gradually from the center of the sample to the edge.The hardness of the alloy increases gradually in the radial direction from the center to the edge of the sample,presenting a continuous gradient distribution.The changing rule of hardness and wear resistance is very similar to that of the volume fraction of primary Mg₂Si phase of the alloy.4)In the process of thermal deformation of Mg-9Al-3Si alloy,with the decrease of strain rate and the increase of temperature,the peak stress decreases gradually.Meanwhile,with the decrease of deformation temperature,the sensitivity of peak stress to strain rate increases gradually.The constitutive equation of the thermal deformation of Mg-9Al-3Si alloy Arrhenius with introduced strain was established.The model can predict the flow stress of the alloy in the range of 250℃～400℃and 0.001～10s^-1.In the range of 300℃,350℃and 0.001～10s^-1,the average absolute error of the model were 1.57%and 1.76%,respectively.Under the conditions of high strain rate at low temperature（250℃,1～10s-1）and low strain rate at high temperature（400℃,0.001～0.01s-1）,the error of the model were large.The average deformation activation energy of Mg-9Al-3Si alloy was183.58k J/mol in the range of 250～400℃and 0.001～10s^-1,and the average strain rate sensitivity index was 0.1616.At low temperature（250～300℃）,dynamic recrystallization only occurs at the grain boundary,and primaryα-Mg grains do not recrystallize.At high temperature（350～400℃）, the primaryα-Mg grain had obvious dynamic recrystallization.When the temperature of hot deformation increased and the strain rate decreased,the recrystallization area increased and the recrystallization grains grew up.