Abstract Scope |
Due to the excellent shape memory effect, superelasticity and biocompatibility, NiTi alloys have been widely employed in various application scopes of biomedical devices, aerospace manufacturing and micromechanical systems. Facing with growing demand of multimaterial components in medical field, reliable welding of NiTi to other biomaterials, especially stainless steel is of great interest. The intrinsic material difference between NiTi and stainless steel in welding processes (e.g., laser welding) results in the formation of intermetallic compounds (IMCs) or an entire IMCs network in fusion zone, as well as the associated weld pores, macrosegregation and residual stress in joint.
In this work, NiTi wire and 316L ss wire with 1-mm diameter were welded using pulsed fiber laser (IPG). The output laser beam parameters are square pulse with the pulse duration of 10 ms, frequency of 50 Hz and peak power of 255 W. The welding time and laser beam defocus distance were set as 150 ms and +1.5 mm (the distance from up surface of the wires), respectively. The laser beam offset from centerline of the NiTi/ss interface was set as in NiTi side (-0.4 mm, -0.2 mm), on centerline (0 mm), and in stainless steel side (+0.2 mm, +0.4 mm). Argon was used as a shielding gas with a flow rate of ~10 L/min. The weld microstructure, alloying element distribution and interfacial phases was analyzed by field emission scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The Vickers microhardness profile of the NiTi-ss joint was measured with a test load of 200 g and a dwell time of 10 s. The tensile test was conducted using a Instron 8801 universal mechanical testing machining with a room-temperature loading rate of 0.5 mm/min at a gauge length of 30 mm.
The fiber laser welded NiTi-ss joint is without any obvious defects in large offset conditions (-0.4/+0.4 mm). The fusion zone shows a similar alloying element distribution and lower hardness compared with base metal. Distinct reaction layers consisting of Ti2Fe, Ti2Cr, Ti2Ni, Fe2Ti, etc. exist at the interface with beam offset. However, without offset (0 mm), the fusion zone exhibits IMCs network structure (with a higher hardness of ~1000 Hv) and some pores or microcracks. With 0.4 mm offset the joint tensile strength reaches 329.6 MPa, due to the reduced Fe-Ti phases at interface. The fractographic analysis indicates that tearing feature on the quasi-cleavage surface is visible.
In summary, IMCs network at the interface of NiTi-ss laser welded joints can be controlled with beam defocus and offset. The NiTi-ss joints exhibited improved microstructure to reduce the brittleness without filler metal addition.
Keywords: NiTi, stainless steel, laser welding, reaction layer, intermetallic compounds |