Study on Microstructures and Interfacial Characteristics of Al<sub>2</sub>O<sub>3</sub> and Rare Earth Electrode Caps in Resistance Spot Welding

Resistance spot welding (RSW) is a key body-in-white welding technique. The copper alloy (CA) electrode cap (EC) is the most important factor that affects the quality and cost of welding. It is difficult for the existing CrZr EC to meet the high quality and low cost of welding new materials, such as thermoformed steel (TFS) plates. The microstructural and physicochemical characteristics and electrode cap service life of Al 2 O 3 and rare earth (RE) copper alloy ECs under welding heat-pressure cycle during their service life were studied to find new EC materials to solve the problems mentioned above. The results show that the Al 2 O 3 EC has high hardness, and Al 2 O 3 is distributed in the form of spherical or elliptical particles. Under the action of heat-pressure cycle, they are constantly broken and refined, which further improves the creep resistance at high temperature and the life of the EC. Under the same conditions, the microstructures of the RE EC gradually changed from regular twins to fine grid-shaped twins, which played an active role in preventing dislocation movement and improving creep resistance. Al 2 O 3 and RE can change the microstructures and interfacial behaviors via different mechanisms to improve the high-temperature creep resistance. Al 2 O 3 EC is suitable for welding AlSi-coated plates, such as TFS plate, and prevents further erosion of the surface owing to its ceramic properties. RE EC is suitable for welding galvanized plates because of its special twin microstructures, which can change the properties and structure of the erosion layer and reduce the interfacial tension. The study of the microstructures and interface layer characteristics of the two ECs provided a theoretical basis and research direction for the development and life improvement of ECs.