Resistance microwelding of fine crossed stainless steel wires is of increasing industrial importance for medical devices. Therefore, a study has been performed to clarify the basic joining mechanisms. The effect of main process parameters such as welding current, force and weld time were investigated by detailed mechanical testing and metallurgical examinations. Especially, joint strength and microstructures were sensitive for welding current and force. It should be noted that no fusion nugget was formed at the weld interface. Since the joint breaking force was determined by bond area and interfacial strength in the case of interfacial fracture, both of them are needed to be optimized to obtain sound joint. Moreover, heat affected zone should be minimized. A bonding mechanism with several main process stages, wire collapse, surface melting, liquid phase squeeze out and solid-state bonding, was proposed.
The feasibility and microstructural development of high-nitrogen-containing nickel-free austenitic stainless steel by small-scale resistance spot welding were studied. Almost fully austenitic cellular microstructure was developed in weld nuggets because the cooling rate of approximately 10 5 Ks À1 was much faster than that in conventional fusion welding processes. Only a small amount of delta ferrite was formed at the gamma grain boundary in the weld, and chromium nitride precipitation was observed both in the gamma grains and at the grain boundary. No significant defects and sensitization were observed in the weld nugget and in the heat affected zone, respectively, and an adequate joint strength was obtained.
Micro-resistance spot weldability of nickel free stainless steel sheet was studied to fabricate
medical implants. Weld lobe was established and the effect of welding parameters on joint strength
was investigated. Weld nugget represented a rapidly quenched austenitic cellular structure whose cell
size is several μm with a little ferrite. The amount of δ-ferrite in weld nugget is smaller than that
predicted by some Schaefller diagrams due to rapid solidification during micro-resistance spot
welding.
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