Enhancing joint strength: investigating different interlayer effects on Ultrasonic spot-welded NiTi/304 stainless steel

Ultrasonic spot welding (USW) is an emerging solid-state joining technology with great potential for solving joining challenges in Ni-Ti shape memory alloys and stainless steel (SS). In this study, USW was utilized to successfully lap weld Ni-Ti shape memory alloy and 304 SS, where copper and aluminum were introduced as separate interlayers to enhance the strength of the joint. The results show that USW can achieve effective bonding between Ni-Ti foil and 304 SS foil with or without an interlayer. In the absence of an interlayer, a mechanical interlocking structure is formed between Ni-Ti and 304 SS with a tensile strength of 404.71 N, which is lower than that of the joint with the addition of an interlayer. With the introduction of the interlayer, a double-interface structure is formed. It should be noted that the highest bond strengths were obtained with the aluminum interlayer, with a peak load of 890 N and edge fracture, although trace amounts of the brittle intermetallic compound Fe3Al14 were generated. A slightly lower joint strength of 690 N was obtained with the copper interlayer, but the formation of brittle intermetallic compounds was effectively avoided. These results have important implications for the successful bonding of NiTi with dissimilar materials.