Interfacial phenomena and joint strength in resistance microwelding of crossed Au-plated Ni wires

Resistance microwelding (RMW) of crossed Au-plated Ni wires has been performed at welding currents from 250 to 800 A and welding times from 1 to 80 ms under 4-kg welding force. The interfacial phenomena and the joint breaking force were investigated using an optical microscope (OM), a scanning electron microscope (SEM), an energy-dispersive X-ray (EDX) spectroscope, and a tensile-shear test. The results showed that mechanisms of joint formation involve brazing at low welding current, brazing and solidstate bonding at medium welding current, and solid-state bonding and fusion welding at high welding current. The joint breaking force first increased with increasing welding current, and after reaching a peak value, subsequently decreased due to recrystallization and softening of the Ni wire. With Au plating on the wire surfaces, the joint breaking force was remarkably improved. Comparisons among the RMW of Au-plated Ni wires, Au-plated Ni sheets, and bare Ni wires are also discussed.