Effects Of Partial Rolling And Heat Treatment On The Microstructure And Mechanical Properties Of AZ Series Mg Alloy Joints

Magnesium alloys, which are recognized as the lightest commercially structural alloys, have great develop potential in automotive, electronics, aerospace and defense industry due to their advantages such as high specific strength, good damping property, good electromagnetic shielding property, good machinability, high recovery rate, etc. However, the application and development of Mg alloy are severely restrained owing to their poor welding ability. The chemical properties of Mg alloy are very active, and their melting point, boiling point and flash point are relatively low. Welding defects such as porosity and grain coarsening usually form during welding, resulting in poor mechanical properties of Mg alloy joints. In order to reduce the strength gap between Mg joint and Mg alloy base material(BM), the present study investigated the effects of post-weld rolling and heat treatment on the microstructure and mechanical properties of AZ series Mg alloy joints. The non-heat treatable AZ31 Mg joints and heat treatable AZ80 Mg joints were obtained by double-sided TIG welding. Post-weld treatment including partial rolling, solution treatment and aging treatment were conducted. The evolution of microstructure, porosity and mechanical properties together with the strengthening mechanisms were focused on. The major conclusions are as follows:(1) Simulation results indicate the strain distribution on the Mg joints is not homogeneous during the partial rolling process, but is closely related to the rolling reduction. The strain distribution on the joint can be divided to three zones including the biting zone, the steady deformation zone and the separation zone. Deformation occurs in the fusion zone(FZ) and its adjacent region, resulting in strain hardening in these areas during partial rolling process for Mg alloy joints.(2) The strength and elongation of AZ31 Mg joints can be significantly improved with increasing rolling reduction during partial rolling. The mechanical properties of AZ31 joints are almost as same as those of the base material when the rolling reduction reaches a certain value, and then the tensile fracture position can be transferred from the FZ to the BM. Strain hardening is the main strengthening mechanism of partial rolling process for AZ31 Mg joint. The mechanical properties of the AZ31 joints can be largely improved at room temperature, but the FZ is easy to crack once the rolling reduction is greater than 20%. When the rolling process is conducted at elevated temperature, the ductility of the joints can be improved and thus the crack tendency can be reduced. However, the strain hardening effect would be weakened and then more rolling reduction is needed to strengthen the joints to the level of the BM so that the fracture location can be transferred from the FZ to the BM.(3) During welding of AZ80 joint, a large amount of second phases formed in the FZ while the second phases in the heat affected zone(HAZ) were dissolved. The second phase in the FZ can enhance strain hardening effect and influence the dynamic recrystallization(DRX) behavior during subsequent rolling process. Abnormal grain growth would occur in the HAZ during the subsequent solution treatment due to the dissolution of second phase. The FZ can be effectively strengthened and the grain growth can be largely restrained by proper multiple post-weld treatment. Thus, the mechanical properties of AZ80 joints can be fully improved.(4) Owing to the high Al content in AZ80 alloy, more porosity would form in the FZ during welding process. Partial rolling process can effectively reduce the porosity. With increasing rolling reduction, pores are gradually compressed and shrank. When the rolling temperature is insufficient, cracks may initiate from the pores which is harmful to mechanical properties. The crack tendency can be reduced by elevating the rolling temperature.(5) The deformation resistance of AZ80 alloy is large. The FZ tends to crack and rolling process is difficult to carry out at temperature lower than 300 oC. The mechanical properties can be largely improved with increasing rolling reduction at proper rolling temperature. The main strengthening mechanism for AZ80 joint during the partial rolling process includes strain hardening, fine grain strengthening and the reduction of porosity. These strengthening mechanisms get a good combination at rolling temperature at around 350 oC.(6) During the aging treatment for AZ80 joint, the morphology, size and distribution of Mg17Al12 phase are closely related to the microstructure characteristics such as twins, DRX, substructures, etc. The microstructure of the aging treated joints can be controlled by the multiple post-weld treatment, and thus affect the mechanical properties. The AZ80 joint with welding pores can reach 200 MPa in yield strength,(85% of the BM), 314 MPa in ultimate strength(88% of the BM) and 4.5% in elongation(45% of the BM), by proper multiple post-weld treatment.(7) Post-weld hot rolling is practical for the Mg alloy joint without enough excess weld bead. The grains can be refined by DRX, and the microstructural differences between the FZ, HAZ and BM can be reduced. The strength of AZ31 joints can be enhanced with increasing rolling reduction while the ductility remains poor. Subsequent annealing treatment is good for improving the ductility of the joint.