Microstructure effects on dynamic welding of Ag/W contacts

A computer-controlled device was used to simulate contact bounce on closing to evaluate the dynamic weld strength of some silver-tungsten contact materials. The effects of composition, microstructure modifications (such as different tungsten particle sizes), and the thickness of the silver-rich surface layer on the tendency to weld were evaluated. Probability analysis showed that silver has the highest tendency to weld. The addition of tungsten lowers the weld strength, with a minimum in the range of 35-62 vol.% of tungsten. The weld probability is lower than that predicted by a volume-mixing-rule calculation. The tungsten particle size was found to be a very important parameter in determining weld tendency. The optimum tungsten particle size for lowest weld tendency was dependent on the volume percent of tungsten. In general, an increase in tungsten particle size increased welding tendency. Silver enrichment of the contact surface by electrochemical etching also increased weld tendency. The welding tendencies were found to be related to the metallurgical properties of the contacts and the welding process. >