2019 Volume 60 Issue 11 Pages 2277-2281
Cracking behavior of tungsten electrode for fusing joining (a kind of resistance spot welding where the work material is insulating-resin-coated conducting metal such as copper and aluminum) was investigated. The electrode specimens with a fine fibrous microstructure were produced by the heavy swaging process without intermediate annealing after sintering, while reference electrode specimens with normal fibrous microstructure conventionally swaged with intermediate annealing were also prepared. Both types of electrode specimens were subjected to repeated joining tests with tough pitch copper sheets as a workpiece material up to 3000 cycles. The effect of compressive applied force at the joining on the cracking behavior was also examined. The total crack length and the maximum crack width tended to be smaller in the heavily swaged electrode than the traditionally swaged electrode. Therefore, the resistance against cracking during many cycles of joining was revealed to increase by refining the fibrous microstructure. Microcrack initiation and growth was presumed to be caused by the mechanism proposed in the previous paper: the surrounding portion is cooled faster at the earlier stage of cooling followed by small amount of plastic deformation in the central portion, and shrinkage of the central portion occurs in the later cooling stage resulting in tensile stresses in radial and circumferential directions. The effect of applied force was complicated since, in this study, different currents were applied depending on the compressive force to keep the temperature constant at the first cycle.
