Influence Of Extrusion Speed And Cooling Method On Microstructure And Aging Precipitation Behavior Of AZ80 Magnesium Alloy

With the advancement of the social modernization process,the scarcity issue of energy and resource has become more and more serious,and energy conservation and emission reduction have become imminent.Research on lightweight materials has attracted widespread attention from all walks of life.Magnesium alloy is an excellent choice for lightweight materials due to its high specific strength,specific stiffness,excellent damping and shock absorption capabilities,and great electrical and thermal conductivity.Additionally,AZ80magnesium alloy is a widely used commercial alloy in current society,and?-Mg₁₇Al₁₂phase is the main strengthening phase of this alloy.In this paper,the Gleeble 3500 thermal simulator was used to conduct backward extrusion experiments with different extrusion speeds and cooling methods.The obtained samples were subjected to T5 heat treatment experiment,aiming to study the influence of extrusion speed and cooling method on microstructure and ageing precipitation behavior of the samples.Finally providing a theoretical reference for the further development of AZ80 magnesium alloy.In the backward extrusion thermal simulation experiment,different extrusion speeds and cooling methods had different effects on the microstructure and dislocation distribution of the samples.A slower extrusion speed was more conducive to the refinement of grains,the crushing and dissolution of the second phase,and the accumulation of dislocations during the deformation of the sample.While the faster cooling method(water cooling)caused the number of second phases in the sample increasing.For slow-extrusion samples,water cooling was more conducive to the accumulation of dislocation density,and for fast-extrusion samples,air cooling facilitated the increase of dislocation density.Additionally,different positions of the sample had different average hardness values in the backward extrusion thermal simulation experiment,showing a trend of increasing from the dead zone,the wall,the bottom,and the corner area.Among them,the average hardness value of the inside corner in BE_0.02-A sample was 71.8 HV,which was the maximum hardness in 16 areas of 4 different samples.During the T5 heat treatment process,the microstructure of each sample had obvious changes.After aging at 175?for 4 hours,the microstructure of each sample was mainly composed of?-Mg matrix,twins,discontinuous precipitated phase(DP),interdendritic Mg₁₇Al₁₂phase and a small amount of Al-Mn phase.The number of twins in the fast-extruded sample was more than that of the slow-extruded sample.After 12 hours of aging treatment,continuous precipitated phases(CP)appeared in each sample,including highly oriented lath phases,triangularly arranged rhomboid phases,and their mixed phases.For fast-extruded sample,the CP phases mainly precipitated in twins,while for slow-extruded sample,the CP phases mainly precipitated in the?-Mg matrix.What's more,during the T5 heat treatment at175?,the precipitation kinetics of?-Mg₁₇Al₁₂phase were different for the samples with different extrusion speeds and cooling methods.At the dead zone,the precipitation kinetics of?-Mg₁₇Al₁₂phase in the water-cooled sample was more adequate than that of the air-cooled sample.At the bottom and corner regions,due to the larger dislocation density and smaller grain size of the slow-extruded sample,the precipitation kinetics of the?-Mg₁₇Al₁₂phase was stronger than that of the fast-extruded sample.At the inner and outer wall regions,the extrusion force and friction force of the fast-extruded sample were larger.The precipitation was induced by stress concentration,so the precipitation kinetics of the?-Mg₁₇Al₁₂phase was stronger than that of the slow-extruded samples.The twins forming at the early aging stage had different effects on the continuous precipitation(CP)phase and the discontinuous precipitation(DP)phase,which was more conducive to the precipitation of the CP phase precipitating at?-Mg matrix and had a negative effect on the DP phase precipitating at grain boundary.Different extrusion speeds and cooling methods resulted in the different lamellar distance of?-Mg₁₇Al₁₂phase.At the 24-hour aging stage,the average lamellar distance of?-Mg₁₇Al₁₂phase in the slow-extruded samples was generally smaller than that of the fast-extruded samples.The average lamellar distance of?-Mg₁₇Al₁₂phase in the BE_0.02-W samples was the smallest among that of all samples,with the value about 354.8 nm.After the direct aging treatment at 175?,the hardness values of different samples have been significantly improved,reaching the peak value at about 24 aging hours.The average hardness value of slow-extruded specimens was generally higher than that of fast-extruded specimens in all stages of the aging.The BE_0.02-W sample had the best aging hardening response with a peak hardness value of 97.8 HV.In each stage of the aging treatment,the BE_0.02-W sample also had the highest average hardness value.