Based on the analysis of the characteristics of cold metal transition welding technology and the welding problems of the thin plate and dissimilar materials, this paper summarizes its applications in automobile industry. Cold metal transition technology can effectively solve the problems of burn-through and deformation of steel sheet and welding of dissimilar metals because of its characteristics of low heat input and low droplet transition temperature. The heat input controllability, coupled with the feature of the special unified expert database system of the cold metal transition technology makes it particularly suitable for automatic welding. It is commonly used in the automotive industry for the welding of aluminum alloys, magnesium alloys, high-strength steels and dissimilar metals. With the continuous upgrading of materials, the technology still needs to be continuously improved and upgraded to enrich the unified expert database system to play a better role in automobile manufacturing.

The increasing demand for the lightweight production of advanced high-strength steel puts forward higher requirements for the quality of welded joint forming. The common CMT welding process has certain limitations and is difficult to meet the needs of lightweight manufacturing. In this study, the CMT + P welding technology was used to weld the DP980 dual-phase steel plate with 1.2 mm thickness. The ER120S-G welding wire was used as the filling material to conduct a 70° angle lap welding experiment. The effects of wire feeding speed (3 m/min~5 m/min) and welding speed (350 mm/min~600 mm/min) on the forming, microstructures, and mechanical properties of DP980 dual-phase steel welded joints were analyzed. The results show that the CMT + P welding process can produce lap weldments with good formability and properties. The welded joints can be divided into the weld zone, the HAZ, and the base metal zone, where the HAZ can be divided into the coarse-grained zone and the softened zone. The role of the elements Ni and Si is to promote the production of martensite and to increase the ferrite strength in welded joints. As the wire feeding speed increases, the grain size of the coarse grain zone in the HAZ increases from 31.90 μm to 50.93 μm; while the welding speed increases, the grain size of the coarse grain zone decreases from 45.48 μm to 35.73 μm. The average microhardness of the weld zone is 420 HV. In contrast, the average microhardness of the softening zone in HAZ is reduced to 250 HV. When the wire feeding speed is 4 m/min and the welding speed is 550 mm/min, the tensile properties of the weldment are optimal, its tensile strength can reach 973 MPa, and the tensile fracture is ductile fracture.

In the background of energy saving and environmental protection, the lightweight of automobile is the goal of its development .The use of ultra-high strength steel as the structural parts and safety parts of car body is to ensure the lightweight of automobile under the premise of ensuring the safety of the car. In order to improve the vehicle safety and save the cost, the welding process of ultra-high strength steel should be optimized continuously. As ultra-high strength steel, DP steel is in common use for bodywork. When they are welded by spot welding, laser welding, MAG and CMT welding, the heat input and welding speed should be controlled so as to prevent the occurrence of burn through and incomplete welding. DP steel reduced the generation of tempered martensite while welding, they will be able to obtain the welding joints with good hardness and strength. CMT welding technology of DP steel is the development trend of automotive sheet welding.