Microstructure and fracture toughness properties of CMT repairing welded 7075-T651 MIG welding joint

The cold metal transfer (CMT) technique was introduced to repair the heat-affected zone (HAZ) of the 7075-T651 aluminium (Al) alloy MIG welding joint using wire ER5356 as filler. The microstructural characteristics and fracture toughness of the welded joint before and after CMT repair welding was investigated. The results indicated that the sound welded joint was successfully produced through CMT; the repaired heat-affected zone (RHAZ) exhibited fine grain and small grain boundary precipitates (η phase) with a larger inter-particle spacing distance compared to that of the HAZ. Additionally, the J₀ integral value of the RHAZ (43.38 KJ m⁻²) and the repaired weld metal (42.68 KJ m⁻²) were higher than that of the HAZ of the MIG welded joint (32.88 KJ m⁻²) and the CMT welded joint (32.81 KJ m⁻²). The fracture toughness properties of the joint repaired with CMT were improved without sacrificing its overall fracture toughness properties. This study proposes a new method for repairing MIG welding joints made of 7075-T651 Al alloy, further reducing the cost of manufacturing high-speed train. (paper)