Influence of microstructure and applied force on the crack initiation and propagation of tungsten electrodes for spot welding

Abstract

In recent years, the electrical wiring works of automobiles and home electronics have become highly complicated, and a spot welding is being used as the interconnecting joining method. The high-conductivity materials such as copper and aluminum need a huge current for a short time in the welding. Therefore, tungsten and molybdenum materials having high melting point and high heat resistance are currently being used for the electrode. In this research, the welding operations were carried out repeatedly up to 3,000 cycles using two kinds of tungsten electrodes with two different microstructures (unrecrystallized and recrystallized), where the temperature of the boundary between the electrode and the work metal (tough pitch copper) at the initial cycle of welding was maintained constant with different forces applied onto the work metal. The influences of the electrode microstructure and the applied pressure on cracking behavior on the electrode surface were examined, and then the durability and lifespan of the electrode were discussed. As the results, the total crack length, the maximum crack width and the crack depth of the unrecrystallized tungsten electrode were all smaller than those of the recrystallized tungsten electrode. The total crack length increased with increasing welding cycles and applied force. The cracking was considered to be caused by a late shrinking of heat area during the cooling and an expansion of the outer part, and the crack progressed along the grain boundaries. From the point of view of the cracking of the electrode surface, it became clearly that the unrecrystallized tungsten material had high durability and would be able to reduce the exchange of the electrode. Therefore, the unrecrystallized tungsten electrode was considered to be effective in expansion of the lifespan of electrode in the spot welding of copper materials.

Keywords

• spot welding
• tungsten electrode
• unrecrystallized microstructure
• recrystallized microstructure
• cracking
• durability
• lifespan