Joining of dissimilar metals: shape memory NiTi and stainless steels

Abstract

Many researchers have proposed the possibility of welding nickel-titanium (NiTi) to other metal for different applications. The joining of NiTi is relatively well understood with a number of studies presented regarding the joining of two Nitinol (NiTi) specimens. However, it is very difficult to obtain an acceptable weld joint connecting NiTi and stainless steel. The formation of brittle intermetallic compounds is one of the biggest challenges faced during the welding of dissimilar materials. The presence of TiFe₂ are thought to be the most common brittle intermetallic compounds formed during the welding of NiTi and stainless steel. As the formation of intermetallic compounds cannot be avoided during the direct welding of NiTi and stainless steel, the addition of a third body, positioned between the two, might be a possible solution to the above. The successful joining of NiTi to other metal has the possibility of extending the NiTi application range. Thus, a strong interest exists in the outcomes of this study. During this research, laser welding, one of the reliable techniques for the joining of NiTi, was used with an interlayer of either Co or Ni to join NiTi and stainless steel. By altering the laser power, welding speed, the position of the focal plane of the laser beam, with respect to the surface of the specimens and the amounts/shapes of different filler metals, the chemical composition of the weld bead can be varied to minimize deterioration of mechanical properties of the joint. The main effects and the interaction effects of the process parameters on tensile strength are discussed to determine the best combinations of input process parameters for the production of good weld quality of NiTi-Ni-SS joint. The microstructure of the welded joint was studied using an optical microscope and a scanning electron microscope (SEM). X-ray diffraction analysis (XRD) was used to identify intermetallic compounds (i.e. TiFe₂, TiCr₂, NiTi₂) in the area of the weld pool. The stress-strain behaviour was evaluated with a tensile test and the hardness studied by the micro-indentation method. The shape memory effect was evaluated by the guided bend method. The corrosion behaviour in Hanks' solution of 37.5°C was studied using the potentiodynamic polarization test. The results of this research show that the mechanical and functional properties as well as the corrosion resistance in Hank's solution of the laser welded joint can be effectively improved by using nickel as the interlayer between NiTi and stainless steel. The combination of input process parameters with appropriate amounts/shapes of Ni interlayer, could possibly improve the tensile strength of the welded joint from 54% to a maximum of 108%, compared to that of the tensile strength of a directly welded NiTi and a stainless steel joint in this study. A comparison of the corresponding values of corrosion potential and corrosion current density for the joint with rectangular shape and circular shape Ni interlayer indicated that, as expected, welding reduced the corrosion resistance of NiTi. However, an appropriate combination welding process parameters and interlayer quantity could make a better passivation and a lower corrosion current, close to the base material of NiTi. Transmission electron microscopy (TEM) with energy dispersive X-ray spectrometry (EDX) was used in the analysis of fusion zone of welded NiTi-Ni of NiTi-Ni-SS joints and chemical characterization of compounds. A significant difference between the compositions of NiTi-Ni fusion zone with different widths of Ni interlayers was observed. Although monoclinic B19', hexagonal R phase, hexagonal Ni₂Ti and Ni₃Ti, rhombohedral Ni₄Ti₃, cubic NiTi₂ and Ni were identified, their amounts were different in different welded samples. The amount of B19' increased to 43% in the fusion zone of the joint, which was welded by the power density of 6.37 X 10⁶ W/cm² with interaction time of 1.82ms. Due to this high amount of B19', the welded joint allowed a recoverable bending distance of 5.00 mm enabling the original position of the welded specimen to be completely recovered after 18s. Diffraction intensity distribution in Debye-Scherrer ring patterns, which were generated from different welding conditions, indicated lattice parameters varied. Microanalysis revealed that Ni₄Ti₃ were presented in different welding conditions and contributed to the shape memory characteristics and strengthened the matrix to increase the yield strength of the welded joint. However, the increase of Ni₄Ti₃ to over 20% of the matrix would be likely to increase the Ms, Mf and Af. At the same time, two-stage phase transformation occurred during heating arising when the amount of Ni4Ti3 increased to 26%. When the amount of Ni₄Ti₃ was below about 10%, only a one-stage phase transformation occurred as the heating arising. According to XRD and TEM analysis, the B19' with a strong preferred orientation of (111) planes and Ni₄Ti₃ with a preferred orientation of (3-1-2) and (2 1-1) planes were found in the fusion zone, which contributed to the improvement of the tensile strength and recoverable bending deflection. In summary, in the study reported in this thesis, the microstructural evolution in the welding of NiTi and stainless steel has been explored. This study has revealed a variety of phenomena which occur during laser welding, and the relationship between the possible amount of different phases, welding process parameters and amount of Ni interlayer were investigated on the contribution of joint strength, corrosion properties and recoverable bending deflection improvement. The addition of rectangular shape Ni interlayer between NiTi and stainless steel has been demonstrated as a feasible alternative for controlling the chemical composition of fusion zone. Also, the decrease of NiTi₂ and increase of B19' were identified as the crucial phases that affect the joint performance.