It is of great significance in the field of engineering to repair the surface defects of ZM6 cast magnesium alloy by an arc welding method. Compared with the traditional tungsten inert gas (TIG) welding repair technology, cold metal transfer (CMT) welding repair has the advantages of low heat input, small repair deformation, and high efficiency. It is of great research value to repair the surface defects of ZM6 cast magnesium alloy by CMT welding. In this paper, the effect of CMT welding repair parameters on defect repair forming is systematically studied, and a repair process window free of unfused defects is obtained. The effects of preheating temperature of base material, wire-feeding speed, welding speed, stick-out length of welding wire and shielding gas flow on the spread of magnesium alloy melt and weld formation were investigated by a surface surfacing method. During the welding process, a camera was used to capture images of the arc and droplet features. A pit defect with a depth of 11.5 mm was machined on the surface of the casting, and the effect of five different repair paths on the formation of the repair area was studied. In order to make the repair area have better fusion, reasonable repair parameters are as follows: The preheating temperature range is 310–450 °C, the wire-feeding speed range is 5–7 m/min, the welding speed range is 8–10 mm/s, the stick-out length of the welding wire is 12 mm, the shielding gas flow rate is 20 L/min, and the repair path adopts a continuous linear reciprocating welding path. This study has important significance for guiding the development of CMT repair technology of cast magnesium alloy.

Ultrasonic energy is introduced into the Metal Inert Gas (MIG) welding arc and weld pool by superposition of an ultrasonic frequency current. In this study, the arc shape, arc energy, and ultrasonic energy that responded to ultrasonic excitation voltage and frequency is investigated. The comprehensive influence of arc and ultrasonic energy on weld formation, microstructure, and mechanical properties is further studied. The arc and ultrasonic energy are analyzed by using a high-speed camera and microphone, respectively. The results showed that the arc width increased, and the arc energy density decreased after the superposition of ultrasonic current. The arc height could be compressed under certain ultrasonic excitation parameters. The ultrasonic excitation voltage and frequency had a direct influence on the ultrasonic energy. The arc height, arc energy density, and ultrasonic energy together determined the weld width. Ultrasound could effectively refine the microstructure of the weld zone and fusion zone but had little effect on the heat-affected zone. Ultrasound improved the hardness of the joint by refining the grain and the second phase. The joint hardness was the highest when the ultrasonic excitation voltage was 100 V, and the frequency was 30 kHz.