Effect Of 160° ECAP And EPT On Microstructure And Mechanical Properties Of AZ61 Alloys

Magnesium alloy is currently the lightest metal structural material,and it is expected to be widely used in the engineering field under the current background of energy saving and emission reduction.The Hall-Petch constant of magnesium alloy is as high as 320MPam^0.5,and the grain refinement strengthening is also considered to be the most effective strengthening method for magnesium alloy.This paper continues to focus on obtaining the fine grain structure of magnesium alloys in order to improve the mechanical properties of magnesium alloys.According to the research results of magnesium alloy grain refinement and the previous experimental results obtained by our research group,this paper designed a 160°equal channel angular pressing die?ECAP,the ECAPed samples are named according to the different processing temperatures and extrusion passes.For example,100?-3 means samples?with a small single-pass strain to enable cumulative deformation of AZ61 magnesium alloy at low temperature.Two grain refinement processes were proposed.One is to obtain deformation structure by ECAP at 100?,150?,200?and 250?,and then to obtain fine grain structure by electropulsing treatment?named as ECAP+EPT,The samples after EPT were named according to EPT different pulse duration and processing time.For example,30?s-5min sample means AZ61 magnesium was processed using EPT whose pulse width and processing time are 30?s and 5min?;The other is to deform 8 passes through ECAP at 150?then reduce the temperature to 100?to deform 3 passes through ECAP,and then obtain fine-grain structure by electropulsing treatment?named as ECAP-va+EPT,va means variable temperatures?.In this paper,100°C-4 sample and 150°C-4 sample with high deformation stored energy were obtained through low temperature ECAP deformation,while deformation stored energy was released through ECAP at 200°C and 250°C due to complete dynamic recrystallization.The results showed that when the output pulse width was 30?s and the treatment time was 10 min,the 100°C-4 sample after EPT was refined to 7.73?m and the high comprehensive mechanical properties were obtained,the yield strength reached 275 MPa and the tensile strength reached 380 MPa.After ECAP-va+EPT process,the results showed that when the pulse width was 25?s and treatment time was 10 min,the average grain size of the sample was 1.8?m,the yield strength reached 335MPa,the tensile strength reached 440MPa,and the comprehensive mechanical properties were excellent.The grain refinement effect by the two methods designed in this experiment is quite different.The grain size obtained by ECAP-va+EPT is much lower than ECAP+EPT,indicating that the important role of complete dynamic recrystallization in the plastic deformation stage for obtaining fine-grain microstructure.Based on the kinetic model analysis of grain growth in the process of EPT,it is considered that the distance between the twin and the shear band of 100°C-4 sample is too long,the newly formed fine grains during EPT will rapidly devour the undeformed structure and grow up.Therefore,the grain size of the sample obtained by the ECAP+EPT process is much larger than that of samples processed by ECAP-va+EPT.In addition,it is found that the pulse width of electropulsing treatment plays an important role in the initiation of recrystallization.Only when the pulse width exceeds the critical value,recrystallization can occur.Simply extending the treatment time will not induce recrystallization,but only make recrystallized grains grow up.Increasing the pulse width can increase the free energy of the EPT system and increase the nucleation rate,therefore,it is possible to obtain fine grains by reducing the processing time and increasing the pulse width above the pulse width threshold.