| Magnesium and magnesium alloys are known as environmental friendly structural materials in the 21 st century and have wide applications in aerospace,automotive,3C and other fields.ZK60 magnesium alloy is one of the most widely used wrought magnesium alloys in Mg-Zn-Zr alloy system.However,strong basal plane texture will be formed during hot working,which results in asymmetric tensile-compressive properties of material and limits the practical application of magnesium alloy.In this study,the Forward-Parallel Channel Extrusion(FPE)process is used to investigate the influence of different process parameters,pre-deformation and deformation temperature on the structure evolution,texture evolution and mechanical properties of magnesium alloy.The following results are obtained:(1)The grain size of the extruded sample by FPE1 process at 300℃ is 5.8μm refined to2.9μm.With the decrease of deformation temperature,the refinement effect is better and the structure is more uniform.The {10-10} cylindrical diffraction peak intensity of extruded samples by FPE1 process is weakened by 90% and the grain orientation has changed dramatically from X-ray diffraction patterns on the cross surface of samples.The grain orientation distribution on the extruded surface was detected by macro XRD.After extrusion with FPE1 process,the original <10-10> fiber texture of the sample weakened,and then the{0002} basal plane and extrusion axis are about 30° new types of texture.With the change of texture,the average Schmid factor of grains increases from 0.142 to 0.261 before extrusion.The mechanical properties of magnesium alloy,such as microhardness,compression and tension,were tested.It was found that the average microhardness of extruded samples at 200℃by FPE1 process was 77.8HV and the compression yield strength was 250 MPa,which were increased by 20.6% and 83.0% respectively compared with the pre-treated samples.The tensile yield strength and elongation after breaking of the extruded sample at 250℃ are230 MPa and 21.8% respectively,which are increased by 21.1% and 24.6%.The SDE value of ZK60 magnesium alloy extruded by FPE1 process increases with the decrease of extrusion temperature and the asymmetry of tension and compression disappears.(2)The influence of extrusion ratio on the structure and properties of ZK60 magnesium alloy by compound extrusion was studied.It is found that FPE2 process with extrusion ratio of 4 has better grain refinement effect at low temperature than FPE1 process with extrusion ratio of 2.78.The grain size of sample extruded at 200℃ is about 1.7μm.FPE2 process enlarges the influence of forward extrusion in composite extrusion by increasing the extrusion ratio of the forward extrusion part,the texture strength of the extrusion basal plane increases and the degree of freedom in the transverse direction decreases,and a slightly lower extreme density region appears at an angle of 45 to the extrusion direction,forming a mixed texture with extrusion basal plane texture and shear texture.The combination of fine grain reinforcement and texture reinforcement results in excellent mechanical properties of FPE2 compound extrusion specimens,which can also improve the tension-compression asymmetry of magnesium alloy.The comprehensive mechanical properties of the extruded specimens at200℃ are the best,with average microhardness of 83.1HV,compressive yield strength of 298 MPa,tensile yield strength of 268 MPa and elongation after break of 23.9%.(3)The effect of channel angle on the structure and properties of ZK60 magnesium alloy extruded by compound extrusion was studied.It is found that the grain refinement ability of FPE3 process is not as good as that of FPE2 process and the grain size distribution is not uniform.The cylindrical {10-10} diffraction peak intensity of the extruded sample by FPE3 process is weakened most obviously in the three kinds of compound extrusion,and the texture of the basal plane is weakened most.It is observed in the macro-polarogram that the texture of the initial extruded base surface of the extruded sample by FPE3 process almost disappears.Four polar density areas appear in the basal plane polarogram at an angle to the extrusion direction,of which the maximum extreme density is 7.1.This area is about 49 degrees from the extrusion direction,multiple shear textures coexist.The texture of the basal plane is weakened greatly,which is reflected in the increase of the Schmid factor of the base plane,in which the average Schmid factor of the sample extruded at 180℃ reaches 0.351.Due to texture weakening higher than fine-grained reinforcement,the tensile strength of the material is very low,the tensile yield strength at 180℃ is 134 MPa,which is lower than that before extrusion,while the opposite plasticity is greatly improved,the elongation after fracture reaches 35%,which is 101.1% higher than that of the original extrusion state.(4)The effect of pre-deformation on the structure and properties of ZK60 magnesium alloy extruded by compound extrusion was studied.The texture of the original base surface of the sample extruded by PE process is weakened greatly,the structure is uniform and the grain is fine.After composite extrusion,the structure is more uniform and the grain orientation develops disorderly.The texture strength of extruded specimens by PE+FPE2 process is much lower than that by other compound extrusion processes.The maximum polar density of the base plane is 3.58,which distributes around the normal direction of the test surface.Due to the weakening of texture strength,texture contributes little to the improvement of mechanical properties,and the material strength is not as good as that of FPE2 process,while the plasticity is relatively improved.The plastic properties of the extruded specimens at 180℃ are the best,and the tensile yield strength is 153 MPa,which is slightly higher than that of the extruded specimens at 180℃ by FPE3 process.After breaking,the elongation reaches 36% and the elongation increases 105.7% compared with the pre-treated specimens. |
