Fusion welding of nickel–titanium and 304 stainless steel tubes: Part I: laser welding

Abstract

Due to their large blocking stresses, high recovery strains, and solid-state operation, nickel-titanium actuators can offer substantial weight and space savings relative to traditional electric or hydraulic systems. A challenge surrounding NiTibased actuators is integration of the NiTi components into a given system; this alloy is difficult and expensive to machine and challenging to weld to itself and structural materials. In this research, we join NiTi and 304 stainless steel tubes of 9.53 mm (0.375 in) in diameter through laser welding to create joints with weld depths up to 1.65 mm (0.065 in). By joining NiTi to a common structural material that is easily machined and readily welded to other materials, system integration is greatly improved. The joints prepared in this study were characterized through optical microscopy, hardness mapping, energy dispersive X-ray spectroscopy, mechanical testing, and analysis of the resulting fracture surfaces. The average ultimate shear strength of these joints is 429 MPa (62.2 10 3 lbf/in 2 ) and the resulting fusion zone has a maximum width of 21.9 mm with a maximum hardness of 929 HV, while a possible heat-affected zone in NiTi is limited between 1 and 2 mm over most of the weld.

Thermomechanical fatigue of post-weld heat treated NiTi shape memory alloy wires

“…Mechanical joining techniques are generally used to integrate NiTi SMAs into these systems, but these techniques have limitations in joint geometry and maximum load. These limitations can be overcome with metallurgical bonding, which has a number of advantages that include high strength connections to control systems [6], and reduction of cost by joining NiTi with less expensive materials [7][8][9]. The counterpoint to these advantages are the difficulties that can be encountered during joining of NiTi, which include solidification cracking, formation of brittle intermetallics and oxides, and degradation of the material properties [10,11].…”

Welding and Joining of NiTi Shape Memory Alloys: A Review

“…The welding procedure consisted on rotating the base materials while a laser was pulsed, creating a continuous FZ. While in similar joints three different regions are usually observed (base material, heat affected and fusion zone), two extra regions were identified by Hahnlen et al [173]: partially mixed zone and unmixed zone. It must be noticed that within the FZ it was possible to identify several partially mixed zones, with different compositions.…”

“…Such a precise control of the conditions to promote a sound joining between NiTi and stainless steel was already achieved by some authors [17,19,173,174,176,177]. Vannod et al [17] presented a very in-depth study on the characterization of NiTi/ stainless steels joints using wires.…”

“…mechanical properties exhibited by the welded joints in Hahnlen et al study [173] is of significant importance as far as structural applications are concerned, as these joints were able to overcome the onset for martensite detwinning and the yield strength of AISI 304 stainless steel.…”

“…An interesting approach was made by Hahnlen et al [173] when welding thick tubes (5.08 and 9.53 mm of outer diameter) of NiTi to 304 stainless steel using Ni filler between both base materials. The welding procedure consisted on rotating the base materials while a laser was pulsed, creating a continuous FZ.…”