Microstructure Tailoring And Strengthening Mechanism Of AZ91 Magnesium Alloy With La And Nd Co-addition

AZ91 alloy has the characteristics of good castability and easy molding,which is widely used in the automotive industry,information electronics,military defense and aerospace.However,the large grain size and coarse eutectic phase of cast AZ91 alloy result in inferior mechanical properties,limiting the development and application of this alloy.Therefore,an improvement of its microstructure and mechanical properties is necessary.Alloying is one of the effective methods to improve the microstructure and mechanical properties of AZ91 alloy,with light rare earth elements being particularly effective.Typically,the elements are added individually or mixed.However,the different proportion of mixing directly impact the structure and properties of the alloy,and the research on this area is still relatively lacking.In addition,the different proportion of mixing elements on the microstructure evolution and strengthening mechanism of AZ91 alloy remains understudied.Hence,the microstructure and properties study from the composition design perspective of altering the proportion of rare earth element mixture is of great significance to optimize the properties and applications of AZ91 alloy.In this study,AZ91-RE alloy was prepared by alloying light rare earth elements La and Nd into AZ91 alloy with different content ratios(W _La/W_Nd).The effects of W_La/W_Nd on the microstructure and secondary phase morphology of AZ91-RE alloy were systematically analyzed.The strengthening mechanism of AZ91-RE alloy was revealed.The hot tearing behavior of AZ91-RE alloys and its fluidity variation pattern were investigated and the precipitation behavior of AZ91-RE alloys was studied.The main research contests of this paper are as follows:The results show that the addition of La and Nd to AZ91 alloy leads to the formation of thermally stable Al-RE phases(Al₁₁RE₃ and Al₂RE phase),which suppresses the precipitation of Mg₁₇Al₁₂ phase.As W_La/W_Nd increases,the number density of Al₁₁RE₃ phase increases and the number density of Al₂RE phase as well as the content of Mg₁₇Al₁₂ phase decrease.At W_La/W_Nd of 3/2,the average grain size of the alloy and the average nearest neighbor distance of the secondary phase are both approximately minimal,while the total number density of the secondary phase reaches the maximum.The results of the first-principles calculations show that the formation enthalpy of Al₁₁La₃ phase is lower than that of Al₁₁Nd₃ phase,and the formation enthalpy of Al₂Nd phase is lower than that of Al₂La phase.La tends to form needle-like Al₁₁RE₃ phase,while Nd tends to form granular Al₂RE phase.The deformation resistance,shear deformation resistance,and stiffness of Al₁₁La₃ and Al₂La are better than those of Al₁₁Nd₃,Al₂Nd and Mg₁₇Al₁₂ phases.Al₁₁La₃ and Al₂La maintain a plasticity comparable to that of Mg₁₇Al₁₂.The Vickers hardness and room temperature tensile strength of AZ91-RE alloy increase with the increase of W_La/W_Nd,and the optimum hardness and tensile strength are 81.4 HV and 184 MPa,respectively,when W_La/W_Nd is 3/2.The variation pattern of hot tearing sensitivity and fluidity of AZ91-RE alloy was analyzed.The results show that the hot tearing sensitivity coefficient of AZ91-RE alloy decreases with the increase of W_La/W_Nd,and the spacing of cracks decreases with the increase of W_La/W_Nd.Meanwhile,the hot tearing susceptibility of AZ91-RE alloy decreases as the pouring temperature decreases.When the pouring temperature is lower than 670℃,the alloy has no obvious cracks at the hot node position.When the pouring temperature is lower than580℃,the fluids is filled incompletely in the mold.According to this result,the casting performance of AZ91-RE alloy is superior when poured at 580°C to670°C.The viscosity of AZ91-RE alloy increases and the fluidity decreases with increasing W_La/W_Nd.In addition,when W_La/W_Nd increases to 3/1,the liquid phase line temperature of the alloy decreases to 599.19°C.The Mg₁₇Al₁₂ phase fraction decreases to 8.5%and the Al₁₁RE₃ phase fraction increases to 1.3%.The effects of squeeze cast pressure,injection speed and mold temperature on AZ91-RE alloy were investigated by performing squeeze casting tests.The results show that the liquid phase line of AZ91-RE alloy increases significantly with the increase of the squeeze casting pressure,which lead to the increase of the supercooling of the alloy,the decrease of the nucleation energy,and the increase of the nucleation rate ofα-Mg.In addition,the increase in pressure also leads to a lower average length of the needle-like Al₁₁RE₃ phase and a more uniform phase distribution within the alloy.Appropriate increase of the pressure injection speed leads to refinement of the grain size of th e alloy.High injection speed generates defects such as porosity and slagging in the alloy.Increasing the mold temperature improves the surface defects such as cracks,cold shuts and misruns,and the best formability of the castings is achieved when the d ie temperature is 225°C.The aging precipitation behavior of squeeze casting AZ91-RE alloy was investigated.The results show that the appropriate addition of La and Nd to AZ91 alloy contributes to inhibit the diffusion of Al atoms in the alloy and improve the activation energy of the precipitated phase.The Al-RE phase and the continuous precipitation phase in the alloy hinder the matrix slip,and the mechanical properties of the alloy are further improved.The room-temperature tensile strength and elongation of the aged AZ91-RE alloy are increased by about 28%and 35%to 241 MPa and 6.1%,respectively,compared with those of AZ91 alloy.