Abstract Scope |
NiTi is an important candidate for using in medical devices owing to shape memory effect (SME), superelasticity (SE) and biocompatibility. A thermoelastic transformation between B2 austenite phase and martensite phase with B19' crystal structure is responsible for occurring SME and SE in NiTi. Most of the medical devices are multi-component system to make benefits of a wide range of materials’ properties. For instance, combining the properties of stainless steel with high elastic modulus and NiTi with considerable superelasticity is a great solution to design a biomedical device. The other example is using the Pt-based alloys as markers due to its excellent x-ray visibility, whereas NiTi could be used as a guidewire or stent for having excellent superelasticity and biocompatibility.
Thus, the study of different joining and welding processes is inevitable in a multi-component medical device, and therefore the dissimilar joining of NiTi to other biomaterials such as stainless steel and PtIr has a great importance. The aim of the present study is to evaluate the microstructural evolution and intermetallic formation in dissimilar joints of NiTi to Stainless Steel and PtIr and its relationship to mechanical and functional properties of the welds.
The materials used in this study were a superelastic NiTi (50.2 at% Ni), Pt-10%Ir and Stainless Steel 316L wire all with 380 µm diameter. All wires were cleaned using ethanol, acetone and deionized water prior any welding to remove any surface contamination. Laser welding of NiTi wires to PtIr and Stainless Steel were performed using an Nd-YAG Miyachi Unitek LW50A pulse laser with 1.064 µm in a butt joint configuration. The laser profile was a square shape with 0.15kW peak power and 20ms pulse duration. Argon gas protection was employed as a shielding gas with a flow rate of 30 Cfh.
Microstructural analysis was conducted on the joint cross section prepared through mounting through epoxy resin, grinding polishing and etching with Kroll’s reagent. Microstructure observation and phase map were conducted by Olympus BX51M Optical microscope (OM) and Zeiss Ultra Plus field emission scanning electron microscope (SEM) equipped with Energy dispersive spectroscopy (EDS). Phase identification has been done via synchrotron X-ray diffraction using a 2D Perkin Elmer detector placed at 1.35m from the welded sample with a wavelength of 0.1426Å (87keV) at beamline P07 high energy materials science (HEMS) of Helmholtz-Zentrum Geesthacht at PETRA III. A 200×200μm pixel size of a detector with a measured accuracy of 2θ = 0.0084° was used. The mechanical properties of the welded samples measured by adopting an Instron 5548 micro tester at a gauge length of 15 mm which includes the fusion zone, the heat affected zones and the BMs. All tensile tests were performed at room temperature and at a strain rate of 3×10-4s-1 and three samples for each selected welding condition were tested. To evaluate superelasticity and residual strain upon each full load/unload cycle, cyclic tests performed at room temperature up to a maximum strain of 6% before unloading.
The microstructure of NiTi-PtIr and NiTi-316LSS laser butt weld joints showed no visible cracks or detectable gas porosity inside the FZ and a full penetration condition were achieved in both cases. In addition, in both welds a swirling effect occurred inside the weld zone which is related to the Marangoni effect and promotes mixing of the two base materials within the fusion zone. In NiTi-SS welds, the microstructure of WZ consists of extensive network intermetallics phases in the form of dendritic structure. These intermetallics compound are the main outcome of mixing elements, especially Ti which formed different IMCs and caused extreme brittleness and failure even under low stress conditions. To identify which IMCs formed in the different welded joint synchrotron X-ray diffraction (SXRD) was used and the formation of Fe2Ti, Cr2Ti, Fe3Ti and Ni3Ti have been confirmed which are highly brittle and detrimental for joint performance. On the other hand, in NiTi-PtIr joint, the dilution of Pt inside the FZ promoted the formation of ternary B2 NiTiPt with the same crystal structure, but different lattice parameters, of NiTi B2 austenite. Furthermore, since the concentration of Pt inside the FZ was low, there were no detectable IMC compounds inside the FZ.
The mechanical properties of NiTi-316LSS shows the fracture at around 200MPa, whereas the NiTi-PtIr failed in the stress level of 500MPa after showing the stress-plateau region in the stress-strain curve of this weld. The occurrence of this stress-plateau region is owing to stress induced transformation in NiTi material and also in B2 NiTiPt weld zone. As a result of this phenomena, the NiTi-PtIr weld showed significant superelasticity with 0.5% residual strain. The preserving the superelasticity in NiTi-PtIr joint is related to have an acceptable strength to reveal stress induced martensite phenomena and formation of B2 phase in the fusion zone.
In this study, it is shown that in the laser welding of NiTi to stainless steel, extensive intermetallic compounds (such as Fe2Ti, Cr2Ti and Ti2Ni) formed inside the weld zone which caused brittleness of the welds. However, in the laser welding of NiTi to PtIr weld, the solute Pt inside the FZ (up to 15 at. %) caused the formation of ternary NiTiPt with the same crystal structure of B2 NiTi. Nonetheless, based on the synchrotron X-ray diffraction measurements, there were no detectable IMCs inside the FZ. The tensile test of NiTi-PtIr joint showed occurring of stress plateau region which is a sign of achieving acceptable strength in the welded sample. Therefore, the NiTi BM preserved its functional superelastic behavior which is a great achievement in the dissimilar welding of NiTi shape memory alloys. |