| TiNi shape memory alloy (TiNi SMA) is a new kind of function material, which possesses special shape memory effect and superelasticity as well as excellent erosion resistance and good biocompatibility. It has broad application prospects in many fields, such as aviation and space-flight, atomic energy, ocean development, mechano-electronic and medical device, etc. The development history of materials has proved that successful application of any advanced material depend on not only its inherent properties, but also the development of joining technology for itself or other dissimilar materials. At present, due to the scare work has been carried on joining TiNi SMA to other materials, the dissimilar material joint with satisfactory performance has been not obtained, which restricts the wider application of the shape memory alloy to a certain extent. Therefore, it is of great theoretical significance and practical values to study laser welding of TiNi SMA and stainless steel. This study will not only enrich the basic theory of welding, but also benefit for the development of welding technology on dissimilar welding of TiNi SMA to other materials.In this paper, the effects of laser parameters (pulse energy/pulse duration) on the penetration rate, melted metal width, microstructures and properties of TiNi alloy or stainless steel joint have been investigated systematiclly. Its purpose was to provide necessary theoretical basis to reveal the weldability of TiNi/steel dissimilar materials and increase the joint properties. The results showed that laser welding parameters had significant effects on the penetration ratio, melted metal width, microstructures and properties for laser-welded TiNi alloy joints and stainless steel joints. The penetration rate and melted metal width increased with the pulse energy and pulse duration rising. The adjustable pulse energy range was wider and the defects such as splash, burn-through etc. could be avoided effectively by using relatively large pulse duration (≥5ms). The grain size in the weld zone of TiNi alloy joint is non-uniform. The weld zone mainly consisted of B2 and B19'phases and it also contained Ti2Ni intermetallics, dislocations, defects such as pores and cracks The characteristic of HAZ was grain coarsening. The maximum tensile strength of TiNi alloy joint was 367 MPa, reaching only about 35% of the base metal strength (1030MPa). Joint fracture occurred in the weld zone and the fracture surfaces exhibited the typical brittle fracture features. The low joint strength may be related to the as-cast structure in the weld zone, the non-uniform distribution of microstructures, the reduction of effective bearing area, the emerge of gas-pores and micro-cracks in weld zone and the intrusion of some harmful gases such as H2, O2 and N2. The microstructure of stainless steel joint was non-uniform. From weld zone to base metal, the microstructure was cellular and cellular dendrite, equiaxed and banded structure. The maximum tensile strength of stainless steel joint was 680MPa, much lower than the BM strength (1870MPa), and the fracture surfaces exhibited the ductile fracture features.The composition and microstructure of TiNi/steel joint were non-uniform. The weld zone mainly consisted of B2, B19',γ-Fe,α-Fe, TiFe2, TiFe, TiCr2, TiNi3 and Ti2Ni phases. The average tensile strength of joints was 187MPa, much lower than that of both base metals. The embitterment occurred in the joints and there was not strain during drawing. All the joint fracture occurred in the fusion zone and the fracture surfaces exhibited the typical brittle fracture features. The gas-pores and micro-cracks could also be seen on the fracture surface of the joints. The bending angle of joints was almost zero. The main reasons for the deterioration of mechanical properties of joints were the formation of intermetallics in the weld zone, also related to the non-uniform distribution of composition and microstructures and the emerge of gas-pores and micro-cracks in weld zone.Changing laser beam postion (laser beam move to stainless steel side 0.2mm) could decrease the fusion ratio of TiNi alloy obviously and the formation of intermetallics containing Ti, thus could alleviate the brittleness of TiNi/steel joint to a certain extent. The tensile strength of joints was 179-213MPa, but the mechanical properties have not been improved obviously.The TiNi/steel joint obtained by applying an axial force had a narrow weld zone. The plastic deformation occurred during drawing, and the joint tensile strength reached 280-298MPa, increased by more than 50% compared with no axial force applied. However, the tensile strength was much lower than that of base metal, only reaching 29% of TiNi alloy base metal strength. The joint fracture occurred in the fusion zone, owing to the formation of a large number of intermetallcs.Laser welding was carried out using pure nickel interlayer between TiNi alloy and stainless steel. With the increase of the thickness of nickel interlayer, the tensile strength and elongation rate of joints increased. When the thickness of nickel interlayer was 50μm, the tensile strength and elongation rate of joints reached 372MPa and 4.4%, respectively, and the joint strength increased by 98.9% compared with the joint without nickel interlayer. It is mainly attributed to the increase of y-Fe phase and the decrease of TiFe2,TiCr2 etc. intermetallics in the weld zone. Further increasing the thickness of nickel interlayer (70μm -100μm), the joint strength and elongation lowered due to the increase of TiNi3, B19'phases in the weld zone. The results showed that the defects such as gas-pores, shrinkage and micro-cracks trend to increase with the thickness of nickel interlayer increasing. The joint bending angle was less than 90°, so it was unable to measure the shape recovery ratio of TiNi alloy HAZ.Laser welding was carried out using pure cobalt interlayer between TiNi alloy and stainless steel. With the increase of the thickness of cobalt interlayer, the tensile strength and elongation rate of joints increased. When the thickness of cobalt interlayer was 20μm, the tensile strength and elongation rate of joints reached 347MPa and 4.2%, respectively, and the joint strength increased by 85.6% compared with the joint without interlayer. It is mainly attributed to the decrease of Fe-Ti and Cr-Ti etc. intermetallics in the weld zone. Further increasing the thickness of cobalt interlayer (50μm), the joint strength and elongation lowered due to the increase of Co-Ti intermetallics in the zone. The joint bending angle was less than 90°, so it was unable to measure the shape recovery ratio of TiNi alloy HAZ.Laser welding was carried out using pure copper interlayer between TiNi alloy and stainless steel. With the increase of the thickness of copper interlayer, the tensile strength and elongation rate of joints increased. When the thickness of copper interlayer was 80μm, the tensile strength and elongation rate of joints reached 521MPa and 5.1%, respectively, and the joint strength increased by 178.6% compared with the joint without interlayer. It is mainly attributed to the increase ofα-Cu phase and the decrease of Fe-Ti and Cr-Ti etc. intermetallics in the weld zone. Further increasing the thickness of copper interlayer to 120μm, the joint strength and elongation lowered due to the increase of Cu-Ti intermetallics in the zone. When the thickness of copper interlayer was between 40μm and 120μm, all the joint bending angles could reach to180°without fracturing. TiNi alloy HAZ has high shape recovery ratio, reaching 98.09%～99.10%.Composite archwires (CoAW) of TiNi shape memory alloy and stainless steel were developed successfully using the copper interlayer and laser welding technology. The joint tensile strength (>520MPa), the joint bending angle (180°) and the shape recovery ratio of TiNi alloy HAZ (>98%) can meet requirements of performance.
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