Deformation Behavior And Microstructure Evolution Of AZ31 Mg Alloy During High-throughput Gradient Thermal Compression

At present,the thermal deformation behavior and deformation mechanism of magnesium alloy are investigated by using traditional cylindrical compression sample.As a highthroughput experimental method,the double-cone compression sample can replace the traditional “one-step” or “one-step” cylindrical compression experiment,which can obtain more information about deformation behavior and microstructure evolution in a relatively short time and fewer experiments.Based on this,the finite element simulation method was used to optimize and design the double-cone sample.The effects of double-cone sample size,deformation temperature and strain rate on equivalent strain distribution and compression load were investigated.The deformation behavior of AZ31 magnesium alloys was analyzed by gradient thermal compression experiments by double-cone sample.Optical microscopy(OM)and electron backscatter diffraction(EBSD)were used to study the gradient microstructure distribution after double-cone compression.The distribution of grain boundary misorientation and grain boundary characteristics was obtained.The dynamic recrystallization(DRX)mechanism can be investigated.The main conclusions are as follows:(1)The 3D-Deform simulation software was used to analyze the different side dip angle and side heights of double-cone samples.It is found that the change of side dip angle has no significant effect on the equivalent strain,but the maximum load required in the compression process decreases with the increase of side dip angle.Furthermore,a larger equivalent strain range can be obtained and the maximum compression load required increases with the decrease of side height.It is found that equivalent strain with large gradient distribution can be obtained under different temperatures and different compression rate.The equivalent strain rate is basically the same on the double-cone sample inner center line,but it only decreases obviously at the very edge.(2)The analysis of the deformation behavior of the as-cast AZ31 double-cone sample under thermal compression shows that the flow stress will reach the peak stress rapidly first,then gradually decrease,and finally reach the stable stage of horizontal change with the increase of strain.The flow stress-strain curve in steady state is not smooth,but shows fine serrated shape.At the same strain rate,the peak stress of the flow curve decreases with the increase of temperature.At the same deformation temperature,the peak stress of the flow curve increases with the increase of strain rate.(3)Through a single thermal compression experiment of as-cast AZ31 double-cone sample,the microstructure corresponding to gradient strain can be obtained.As the strain increases,the degree of DRX increases gradually and the coarse grains change into fine equiaxed DRX grains.At different strain rates,the degree of DRX and grain size increase in the same observation area with the increase of deformation temperature.At 250 ℃ and 300 ℃,continuous DRX(CDRX)is DRX mechanism during the deformation of as-cast AZ31 double-cone samples.In addition to CDRX,discontinuous DRX(DDRX)also take place at 350 ℃ and 400 ℃,indicating that DRX mechanism changes with the increase of deformation temperature.(4)The deformation behavior of rolled AZ31 double-cone sample under thermal compression was analyzed.The flow stress-strain curve can be divided into three stages: work hardening state,flow softening state and flow steady state.The peak stress obtained by compression along ND direction is the largest,while the peak stress obtained by compression along TD and RD direction is almost the same.The peak stress obtained by compression along TD,ND and RD directions gradually decreases and the flow stress-strain curve enters the flow steady state gradually earlier with the increase of temperature.(5)Through a single thermal compression experiment of rolled AZ31 double-cone sample,the microstructure corresponding to gradient strain can be obtained.At the same thermal compression temperature,the degree of DRX compressed along TD,ND and RD directions increases gradually with the increase of strain.Under the same strain condition,the degree of DRX increases with the increase of thermal compression temperature.Twinning is more likely to occur along TD and RD compressed than along ND compressed.The number of twins decreases gradually with the increase of thermal compression temperature or strain.At deformation temperatures of 250 ℃ and 300 ℃,CDRX can be observed and plays a dominant role in samples compressed in all compression directions.Twin DRX(TDRX)is also observed at 250 ℃,mainly in the samples compressed along the direction of TD and RD.Except for CDRX,DDRX was observed at 350 ℃ compressed along the directions of TD,ND and RD.The DRX mechanism changed more completely with the increase of thermal compression temperature,and only DDRX can be observed at 400 ℃.