| Extrusion-shear process(ES)is a large deformation processing technology which combines traditional extrusion with equal diameter Angle extrusion(ECAE).Semisolid extrusion shear process is a newly developed deformation processing technology that combines semisolid forming with extrusion-shear process.This process integrates the advantages of ES and semisolid forming process,reduces the processing difficulty,improves the forming properties of materials,and further optimizes the microstructure and properties.Based on the mature research results of the research group,this paper firstly optimized the extrusion shear mold,and then explored the rules of the pouring and extrusion process through deform-3d extrusion simulation and AnyCasting casting simulation.On the basis of the simulation theory,three groups of experiments were conducted,namely 350 ? conventional extrusion,450 ?conventional extrusion,and semi-solid extrusion,after selecting extrusion shear molds with different shear internal angles(no corner,150°,135°).The as-cast AZ31 magnesium alloy was selected as the experimental material.Casting simulation of AZ31 magnesium alloy was conducted by AnyCasting,and the melt temperature-time curve was obtained,which provided theoretical support for subsequent semi-solid extrusion parameters.DEFORM-3D conducted extrusion simulation and studied the forming load,equivalent strain and velocity field rule of extrusion process.Afterwards,conventional extrusion and semi-solid extrusion experiments were carried out,and the extruded bar was observed for microstructure,tested for microhardness and tensile properties.The macroscopic texture measurement,tensile fracture analysis and microscopic texture analysis were conducted by X-ray diffraction and scanning electron microscope.Combined with the simulation results,the influence of shear Angle and extrusion temperature on conventional extrusion,the influence of shear Angle on semi-solid extrusion,the evolution law of semi-solid extrusion structure,and the comparison between semi-solid extrusion and conventional extrusion were investigated to explore the role of semi-solid extrusion in grain refinement,texture weakening and improvement of uneven microstructure performance.Finally,the following results were obtained:Casting simulation showed that when casting time t=18 ± 2s,the solidification rate was 80±5%,there was a small amount of liquid in the center of the casting,andthe core temperature was about 580±10?,which could be used as the initial slurry for semi-solid extrusion.Deform-3d extrusion simulation shows that the forming load increases with the decrease of the shear internal Angle,and the smaller the shear internal Angle is,the larger the final stable forming load will be,and the more difficult it will be for the material to extrude smoothly.The equivalent strain increases with the addition of the corner Angle,and the smaller the internal shear Angle is,the larger the accumulated equivalent strain of the bar is,the larger the difference of the lateral equivalent strain is,and the deformation unevenness increases.Velocity is affected by the Angle of law,through the conventional extrusion zone,presence of corner,almost the same on both sides of the velocity,and channels such as the corner area,there is no corner,central material flow velocity is the fastest,along with the shear Angle to join,the inner material flow rate is greater than the outer,along with the shear Angle decreases,and the velocity difference increases first and then decreases,and eventually converge on both sides of the velocity;The influence of temperature on the forming load is that the decrease of temperature increases the maximum forming load and it is more difficult to extrude the material smoothly.The influence of shear Angle on conventional extrusion at 350 ° c is as follows:with the addition of shear Angle,dynamic recrystallization becomes more complete and grain refinement occurs.The internal shear Angle decreases from 150° to 135°,and grain further refines,but the uneven structure increases.Hardness increased.When the internal shear Angle decreased from 150 ° to 135 °,the hardness increased slightly.Under the same shear Angle,the hardness of inner L to outer R decreased first and then increased,and the hardness of inner L was lower than outer R.The yield strength increased by 15~25MPa,the tensile strength increased by 10~15MPa,the elongation decreased by 1~3%,and the comprehensive mechanical properties were the best under the 150 ° internal shear Angle.The yield strength was 237.5MPa,the tensile strength was 299.2MPa,and the elongation was 11.3%.For the influence of extrusion temperature on conventional extrusion shear,compared to 350 ?,extrusion temperature to 450 ?,the microstructure of grow up grain and have not been eating small grain size,grain size increased,the average hardness is less than the average hardness under 350 ?,350 ? and the base surface texture intensity compared to 450 ? base texture intensity decreased significantly,yield strength,tensile strength decreased,elongation increased,and the same cross section structure and hardness distribution is uneven;At 450?,Angle added,tensilestrength decreased.The influence of shear Angle on semi-solid extrusion is as follows: after the introduction of shear Angle,the grain was significantly refined,and the shear Angle decreased from 150° to 135°.The hardness values in all areas increased,and the average hardness values of 135° and 150° were basically the same.On the whole,the hardness distribution was relatively uniform.The texture strength of the base plane increased by 3-4.The yield strength of the bar increased by 20~40MPa,the tensile strength increased by 10~20MPa,and the elongation decreased by 2~3%.The comprehensive mechanical properties of the bar at 150° shear internal Angle were the best,with yield strength 222.2MPa,tensile strength 308.5MPa and elongation9.6%.Contrast of semi-solid extrusion and 350 ?,450 ?,found that after the microstructures and properties of conventional extrusion semi-solid extrusion regions for even small organizations,uniformity is superior to the conventional extrusion,and compared with 450 ? conventional extrusion,semi-solid grain refinement effect significantly,compared with 350 ? conventional extrusion,after joining corner,has also a certain degree of grain refinement of semi-solid extrusion effect;The hardness values of semi-solid extrusion were close to that of conventional extrusion at 350?,which were significantly higher than that of conventional extrusion at 450?.Under semi-solid extrusion,there was no significant difference in the hardness values of the inner L,middle M and outer R of the bar cross section,and the hardness distribution was more uniform.The substrate texture strength of cross section(0002)under different processes is different.Semi-solid extrusion can effectively weaken the substrate texture of AZ31 magnesium alloy bar(0002).By comparing the tensile properties under different processes,it is found that the refinement of semi-solid extrusion on the microstructure and the weakening of substrate texture are beneficial to improve the material properties. |
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