Controlling intermetallic compounds formation during laser welding of NiTi to 316L stainless steel

Dissimilar laser welding of NiTi to stainless steel is of great importance in designing medical devices but the formation of hard and brittle intermetallic compound results in low strength joints. Normally, different interlayers are applied as physical and chemical barriers to control the microstructure and to improve the mechanical properties. However, this procedure is a cost and time consuming process and may cause the formation of other types of intermetallics depending on the interlayer used. In the present work, laser offsetting welding (LOW) was introduced without inserting any interlayer by shifting the laser beam 100 mu m into the stainless steel from the NiTi/316L stainless steel interface. This led to a softer weld zone (similar to 570 H V), due to the formation of less brittle intermetallics compounds (Fe2Ti, Cr2Ti and Ni3Ti) compared to that (similar to 970 H V) when the laser beam was placed at the NiTi/316L stainless steel interface. For comparison purposes, an Ni interlayer was also used to control the chemical composition of the fusion zone. In terms of mechanical properties, both the laser offset welding and the use of an Ni interlayer, were seen to improve the tensile strength of the dissimilar joints (above 400 MPa) compared to the centerline welding condition (around 200 MPa). Hence, LOW was confirmed to be an effective method to laser weld the NiTi/Stainless Steels.