Effects of Ni powder addition on microstructure and mechanical properties of NiTi to AISI 304 stainless steel archwire dissimilar laser welds

Abstract

In this study, dissimilar laser welding of NiTi to stainless steel (SS) orthodontic archwires was investigated. As a novel and more operational approach, Ni powder was used as a weld filler metal to improve the properties of joints in laser welding of these alloys. Therefore, welding was performed in two conditions, with and without Ni powder addition, and the results were compared. Optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and focused X-ray diffraction (micro-XRD) analysis were used to study the microstructure, chemical composition and phase characteristics of joints. The Fe-Ni-Ti ternary phase diagram was used as a tool to analyze the phase changes in weld zone with chemical composition variations in two conditions. Results showed that brittle intermetallics such as Fe2Ti and Cr2Ti were formed in zones close to NiTi weld interface in joints without Ni powder addition. However, Ni powder had favorable effects on microstructure, chemical composition and mechanical properties of laser welded joints. In fact, use of Ni powder changed primary solidifying phase region from Fe2Ti to ductile phases of Ni3Ti and γ, and consequently promotes the formation of these ductile phases in the weld zone. Moreover, as a result of chemical composition dilution by Ni, the microstructure of weld zone changed from dendrite to dendrite-cellular structure by suppressing the constitutional undercooling in Ni powder added condition. Furthermore, microhardness tests revealed that the addition of Ni powder caused the average hardness of weld zone to decrease from 580 HV down to 325 HV due to aforementioned intermetallic phase replacement. On the other hand, tensile properties of joints were improved and reached to a tensile strength of 300 MPa and a fracture strain of 2.9 % by adding Ni powder. These values for the welding without Ni powder addition were 150 MPa and, 1.8 %, respectively.