Effects Of Mn, RE Addition On Extrusion Deformation Resistance And Miscrostructure Homogeneity Of AZ61Magnesium Alloy

AZ61magnesuim alloy has brilliant application prospect because of its excellent mechanical performance and mature technology, but there are some problems during the application of it, such as low rate of extrusion processing, high deformation resistance and uneven deformation during the process of deformation, which can’t satisfy the extrusion requirement of the large-scale and thin-walled profile. Based on the previous studies and the still existing problems, this paper used Gleeble-1500D hot compression machine and DEFORM-3D simulation software and the OM, SEM and TEM is also used to study the effects of Mn and RE on the deformation behavior of AZ61alloy, and its mechanism is researched.The hot compression tests of AZ61alloy, AZ61+0.68wt%Mn+0.67wt%RE, AZ61+0.54wt%Mn+0.91wt%RE and AZ61+1.0wt%RE are performed on Gleeble machine. The deformation temperature is260,300,380and430℃with the strain rate is0.01,0.1and1s-1, respectively. The maximum deformation is to60%. The results showed that true stress-strain curves of four kinds of alloys have the same change. The flow stress increased quickly until to the peak stress with the increase of the strain since the deformation beginning. Then it decreases to a steady value with deformation continuing. And also, the peak stress of alloys decreases with increasing the deformation temperature. Compared with the true stress-strain curves of four kinds of alloys, it can be seen that the peak stress of AZ61is lower than that of alloys with addition of Mn and RE under the same deformation conditions, of which the alloy AZ61+0.68wt%Mn+0.67wt%RE has the lowest peak stress among these alloys.Through the calculation of constitutive equations of four kinds of alloys, we can know that the AZ61alloy has the highest hot deformation activation energy, which is180.2KJ/mol, while the composition4(AZ61+0.68wt%Mn+0.67wt%RE) has the lowest one, which is171.4KJ/mol. This shows that the hot deformation activation energy can be decreased with the addition of Mn and RE. It indicates that the addition of Mn and RE make the plastic deformation ability improved.The microstructure of four kinds of alloys is mainly about twin when the deformation temperature is260℃, and the twin size increases with addition of Mn and RE. But when the temperature increased to300℃or above, the dynamic recrystallization grain produced in the microstructure of alloys during the compression progress. The degree of dynamic recrystallization of alloys increased with the deformation temperature increasing. The experimental results also show that the degree of dynamic recrystallization of alloys with addition of Mn and RE is greater than that of AZ61alloy, which illustrated that the addition of Mn and RE could accelerate the occurrence of dynamic recrystallization AZ61alloy and composition4are undertook different deformation level (ε=0.08,0.12,0.16,0.2and0.3). The results reveals that recrystallization grain have not seen in AZ61alloy when the true strain was0.08, while there are some small recrystallization grains produced partly in composition4. The phenomenon revealed that the addition of Mn and RE accelerated the occurrence of dynamic recrystallization, which is in agreement with the results of true stress-true strain curve. It indicated that the addition of Mn and RE can lower the deformation resistance.We know that there were some dynamic recrystallization grains around Al10RE2Mn7phase in the alloy with addition of Mn and RE by means of transmission electron microscopy. The reason for the dynamic recrystallization phenomenon is that the phases of Al10RE2Mn7at elevated temperatures are stable and impede dislocation slip during the hot compression process. The impediment of the Al10RE2Mn7phases in dislocation slip, and the concentrated dislocations were beneficial for the nucleation of dynamic recrystallization. The occurrence of dynamic recrystallization make the work hardening got fully recovery. This also the reason that the peak stress in the true stress-strain curve became lower with addition of Mn and RE. The decrease of deformation resistance is significant to the extrusion of large thin-walled complicated profiles.The extrusion processes of four kinds of alloys are simulated by finite element method analysis software DEFORM-3D.The constitutive equations which had been obtained are input to DEFORM-3D.Computer simulation results showed that the equivalent stress value and equivalent strain value of alloys with addition of Mn and RE are lower than that of AZ61alloy, and the uniformity of the equivalent stress and equivalent strain is better than that of AZ61as well. Composition4had the best uniformity of the equivalent stress and strain. The above mentioned data showed that the addition of Mn and RE to AZ61alloy could improve the deformation uniformity.