Compositional Analysis

Quantitative EDS profiles performed across the NiTi-SS interface at different depths from the irradiated surface are shown in Fig. 4(a-c). Each profile shows a constant composition of primarily Fe in the stainless steel and a slightly Ni-rich composition in the NiTi side of the joint.

Between the two base metal compositions is the joined region showing a mixture between Fe, Ni, and Ti in varying proportions. The extent of this mixed region indicates the width of the joint. The uppermost EDS profile (Fig. 5a) shows a joint that is roughly 25µm wide. A steep dropoff in the iron composition, from roughly 75 at% Fe to 4 at% Fe, is observed at the same location at which the Ni and Ti compositions increase significantly, however, an appreciable amount of Fe is observed to exist roughly 25µm into the NiTi side of the joint. The second EDS profile (Fig. 5b), performed 125µm below the first, shows similar characteristics but with the mixed composition region extending only 15µm from the interface. The third EDS profile (Fig.

5c), performed 250µm below the first shows a mixed region between the two base materials of a similar thickness. The variation in the composition profiles suggests that different joining mechanisms are dominant along different regions of the joint. Toward the laser irradiated surface the melted layer thickness is greater, which indicates a longer melt duration, allowing greater dilution of the stainless steel into the molten NiTi. Toward the center of the wires, where minimal mixing of the two materials is observed, the melt layer thickness should be significantly smaller. The composition profile in this region resembles more of a diffusion-controlled process while the upper layers resemble more of a fusion-based joining mechanism. Figure 5 shows an EDS map of region I in Figure 3. A clear delineation between the two materials is observed with a small layer of mixing between the two. The darker blue layer on the top of the NiTi wire is nearly all Ti and is likely a Titanium oxide. Ti-oxides are expected to form preferentially in these joints due to Titanium’s high oxygen affinity.

Figure 4: EDX line scan composition profiles for scans (a) i, (b) ii, and (c), iii from Error! Reference source not found.. Note change in width of mixed zone (joint width) at different locations along wire thickness.

Figure 5: EDS Map of region I in Fig. 3 where red, green, and blue represent Fe, Ni, and Ti. Note transport of Fe along NiTi wire surface as indicated by the purple region at the top-right corner of the NiTi wire.

Figure 6: Thermal model of autogenous laser brazing process showing thermal accumulation at joint interface as laser beam approaches. “▼” symbol indicates position of laser beam. (a) Equilibrium temperature distribution far from

interface. (b) Beginning of thermal accumulation at interface. (c) Melting of interface.

Thermal and compositional modelling of the autogenous laser brazing process is performed to understand the resultant composition profile at the interface. Figure 6 shows temperature contours in a joint pair with the laser directed at the NiTi wire at a number of different times.

Figure 6a shows the steady-state temperature distribution around the laser spot far from the wire-wire interface. Thermal accumulation, as evidenced by the increase in peak temperature, is observed as the laser beam approaches the interface in Fig. 6b and 6c. Some non-uniformity of the temperature at the joint is observed in the thermal model as the laser approaches the interface with the upper region showing a higher temperature consistent with the wider joint observed experimentally toward the top of the wire. Temperature-time profiles at various distances from the interface are shown in Fig. 7. The peak temperature of each point is seen to be a decreasing function of its distance from the joint interface with the faying surface of the joint experiencing the highest temperature rise. This is attributed to the thermal accumulation at the joint due to the imperfect conduction across the interface.

Figure 7: Simulated temperature-time profiles for points at different distances from the joint interface. Note higher peak temperatures for points located closer to the interface indicating thermal accumulation.

Figure 8 shows the resultant composition profile of a NiTi-SS joint as predicted by the combined heat transfer and fluid flow numerical simulation for the same laser parameters along the upper portion of the joint as shown in Fig. 5. The coloring is based on the RGB scale with Red, Green, and Blue representing Fe, Ni, and Ti, respectively. The laser flux was applied to the top surface of the modelled geometry. A mixed zone is observed which is largest in width toward the irradiated surface and extends along the NiTi wire surface away from the joint. This is in good agreement with the experimentally observed profile shown in Fig. 5. This confirms that the positive sign for the surface tension coefficient is correct for the parameters and environment surrounding the NiTi wire during irradiation in autogenous laser brazing. This flow of Stainless Steel material is caused by Marangoni convection which pulls the molten material along the surface of the NiTi wire toward the laser spot which is located to the left of the joint interface.

No significant melting is observed in the lower portion of the joint in the simulation. This morphology is also in good agreement with the shape observed experimentally. The factors causing the formation of such a geometry will be discussed in detail in later sections.

Figure 8: Color contour map of mass fraction of Fe in NiTi-Fe joint formed by Autogenous Laser Brazing as predicted by numerical simulation. Fluid flow is driven by Marangoni convection with a positive surface tension temperature

coefficient. Resultant composition profile matches that observed experimentally as shown in Fig. 5.