Microstructure and properties of MIG and CMT+P welded joint of high strength Al-Zn-Mg alloy

Abstract

Two arc welding modes of melt inert gas welding (MIG) and cold metal transition + pulse welding (CMT+P) were used to weld 20 mm thick 7A52 aluminum alloy. The effects of different arc welding modes on the microstructure and mechanical properties of welded joints were studied from the aspects of welding defects, grain size and precipitation. The results show that the two welded joints are well formed and have no obvious defects. The weld microstructure is as cast equiaxed dendrite. Strip or block iron rich impurity phases AlFeMn and Mg2Si and Al3Mg2 phase with non coherent relationship with the α(Al) matrix are found in the weld. However, the porosity of MIG welding weld area is 2.73%, while that of CMT+P welding weld area is only 0.64%. At the same time, the weld grain size is small, the width of heat affected zone is narrow and the grain boundary segregation is small in CMT+P mode, so the joint strength in CMT+P mode is higher, with about 289 MPa. The pull-down fracture of the two modes is characterized by ductile fracture with dimples as the main feature.

Keywords

• cmt+p welding
• al-zn-mg alloy
• pore
• microstructure
• mechanical property