Study Of Strengthening Fundamentals Of VCAE Deformation And Heat Treatment Of Mg-8.5Li-6.5Zn-1.3Y Alloy

With the advantages of high specific strength,low density,excellent damping performance,magnesium-lithium alloy is favored by aerospace and new energy vehicles.However,the low strength limits the application and promotion in practical production.Alloying,equal channel angle extrusion(ECAE)and heat treatment are the most common means of strengthening magnesium-lithium alloy.Most of the existing ECAE techniques are complex and costly to produce,and require multiple passes of deformation to meet the alloy performance requirements.To address the limitations of ECAE deformation,a new method of variable channel angular extrusion(VCAE),which combines extrusion,upsetting and shearing,is proposed to effectively improve the alloy properties.The problem of insufficient strength of Mg-8.5Li-6.5Zn-1.3Y(LZW861)alloy and the high cost of ECAE deformation are discussed.In this paper,a process method of VCAE deformation combined with heat treatment is proposed,which can realize the manufacture of high-strength Mg-Li alloy at low cost.In this paper,Mg-8.5Li-6.5Zn-1.3Y(LZW861)alloy is processed by extrusion,VCAE and heat treatment processes,and the effects of extrusion,VCAE deformation and heat treatment on the alloy are systematically investigated by combining microstructure characterization and mechanical property analysis.The effects of extrusion,VCAE deformation and heat treatment on the microstructure and mechanical properties of the alloy were systematically investigated,the microstructure evolution of the alloy under VCAE deformation temperature was thoroughly discussed,the strengthening mechanism of VCAE deformation and high temperature solution-aging treatment was elucidated,and the optimal heat deformation +heat treatment process of the alloy was determined,which provided some theoretical basis for the design and development of ultra-light and high-strength magnesium-lithium alloy.The first and foremost,the microstructure and mechanical properties of cast and extruded LZW861 alloy were studied and compared,and it was found that the cast alloy mainly consisted of α-Mg phase,β-Li phase,I phase,W phase,MgY phase and MgLiZn phase.The extrusion process caused the eutectic mesh compound composed of I and W phases to break into particles,and the morphology of MgY phase remained unchanged.α-Mg phase was distributed in a striped pattern along the extrusion direction,and β-Li phase was dynamically recrystallized and transformed into equiaxed crystal.The ultimate tensile strength,yield strength and elongation of the extruded alloy were 245.92 MPa,173.9 MPa and 27 %,respectively.There is one more port,I should touch on,that the effects of VCAE extrusion temperature on the microstructure and mechanical properties of LZW861 alloy were investigated and compared,and it was found that the second phase and grains of the VCAE state alloy were significantly refined.I phase,W phase and MgY phase acted as heterogeneous nucleation sites and promoted the refinement of DRX grains.The volume fraction of the fine grain region increases and then decreases with increasing temperature.β-Li phase in which the nanoscale MgLiZn phase maintains a good coherent relationship with the matrix,the grain size increases with decreasing volume fraction of the fine grain region.β-Li phase grain refinement mechanism is dominated by discontinuous dynamic recrystallization.At 275 ℃,dislocations accumulate at the grain boundaries and the α-Mg phase transforms into nanoscale sub-grain;above 300 ℃,continuous dynamic recrystallization of the α-Mg phase occurs.the ultimate tensile strength of the alloy reaches a maximum of 288.93 MPa at the extrusion temperature of 275 ℃.The last but not the least,the effects of solid solution + aging treatment on the microstructure and mechanical properties of LZW861 alloy were investigated and compared,and it was found that the high temperature solid solution treatment at 450 ℃ + 3h promoted the precipitation of diffusely distributed needle-like α-Mg phase in the β-Li phase,and a large amount of I phase transformed into W phase and needle-like α-Mg phase.Static aging hardness increases and then decreases with increasing aging time.high concentration of Li elements in the biphasic matrix during static aging at 130 ℃ led to the transformation of needle-like α-Mg phase into Static aging at 200 ℃ accelerated the formation of the reticular α-Mg phase and the MgLiZn softening phase,and the properties were severely degraded.280 ℃ high temperature static aging treatment suppressed the precipitation of the MgLiZn phase and promoted the precipitation of the I and W phases in the matrix.The best heat treatment process is high temperature solution(450 ℃ + 3 h)+static aging(280 ℃ × 30 h)process.