Abstract: The 4043 aluminum wire with a diameter of 1.2 mm was employed as additive material deposition on the 2 mm thick Q235 low carbon steel plates to study BC-MIG arc additive manufacturing process. The T-shaped structure obtained by welding aluminum/steel welded joints with 6061 aluminum alloy plate was beautiful in appearance. The microstructure and hardness distribution of the joint were analyzed by optical microscope and microhardness tester respectively. The results showed that due to the difference of temperature gradient and cooling rate, the aluminum side of the interface layer is a dendritic structure that grew vertically upwards, the middle part showed a relatively cluttered crystal structure, and the top grain is finer and has no directional growth. Along the steel base material area to the intermediate layer of the interface, and then to the aluminum alloy area, the hardness of the joint first increases and then decreases to a gentle degree, and the hardness of the joint layer in the aluminum/steel interface reaches a maximum of 142 HV.