# Self-Powered Fast Brazing of Ti-6 Al-4V Using Ni/Al Reactive Multilayer Films
## 3 Results and Discussion
### Elemental Profiles and Temperature Evolution  
The EDS line scan of the cross-section reveals the general distribution of elements in the joint. The dips and spikes in the Al and Si in the region on both sides of the RMF is due to detection of Si-rich grains in an Al-rich matrix (Figure 6a,b). A high-resolution line scan of the Ti64-BAl interface reveals a reaction zone produced during the plasma-arc coating (Figure 6c,d). Additionally, there is an increased concentration of Si detected at the Ti64-BAl interface. Si has limited solubility in Ti in a solid state so, when Si attempts to diffuse into Ti from the molten brazing material, it stagnates at the Ti64  
Figure 4: DSC measurement of Ni/Al RMFs. Figure 5: High speed camera measurement of the RMF reaction. surface. Notice that there is a reaction zone at the Ti64-BAl interface. EDS point analysis of this region reveals that this region is very Al-rich. Therefore, the formation of the reaction zone arises from Ti from the base material dissolving into the molten Al-Si and solidifying prior to the rest of the brazing material. Si is a melting point depressant so it is likely that the Al remained in its liquid state longer than normal, especially near the interface where the Si concentration was elevated [19]. If the Al is liquid for a longer time, it would allow more Ti to diffuse into the brazing material and expand the width of the reaction zone. The RMF-BAl interface also yielded interesting results. As seen in Figure 6e,f there are no obvious signs of a reaction zone, but there is a 1.6 um thick Ni-rich zone 6.4 um from the RMF-BAl interface. The diffusion length for Al is approximately 2.0 um and Ni is approximately 1.2 um. No noticeable reaction zone forms for three primary reasons: (1) A lack of excess Ni (i.e., above the equiatomic amount), (2) short reaction time, and (3) the lower intrinsic diffusion coefficient of Ni compared to Al [20; 21]. During the reaction, virtually all the Ni is consumed in the RMF reaction and is largely unavailable for any significant reaction with the brazing material (i.e., not enough to produce a secondary phase in the reaction zone or on the interface). This is in direct contrast to a Ni-rich RMF (Ni:Al atomic ratio = 1.5:1), where the excess Ni reacted with the molten Al in the BAl and formed a dendrite structure. However, this non-equimolar RMF produces significantly weaker bonding (0.59 MPa) than the equimolar RMF in this current study [22]. EDS indicates near-equiatomic amounts of Ni and Al at points 1, 3, and 4 (indicated in Figure 6e), with slight Al enrichment at point 4 and slight Ni enrichment at point 3. According to the Ni-Al phase diagram, these concentrations correspond to the AlNi phase. Point 2 detects almost two times the amount of Ni in this region compared to Al, which is characteristic of the Ni\({}_{5}\)Al\({}_{3}\) phase. XRD of the brazed joint (Figure 7) revealed several phases belonging to the parent structures, but the two particularly interesting peaks belong to  
Figure 6: EDS line scan of (**a,b**) the entire brazed joint, (**c,d**) the Ti64-BAl interface, and (**e,f**) BAl-RMF interface with select point analysis. Al3 Ti and Ni5 Al3, which, in conjunction with the EDS data, belong to the reaction zone at the Ti64-BAI interface and the Ni-rich region in the RMF, respectively. The Al3 Ti phase was formed during the plasma coating process based on SEM observations prior to RMF brazing. The temperature evolution of the joint as a function of time helps in understanding the most probable pathway to the nucleation of the Ni-rich phase in the brazing joint. The temperature distribution of the brazing joint cross section, according to LAMPS, is shown in Figure 8. The BAI and RMF reach a maximum temperature of approximately 813 \({}^{\circ}\)C, which is far above the liquidus line of BAI. The LAMPS simulation shows that the temperature within the brazing region (i.e., the RMF and BAlSi-4) increases by 991 \({}^{\circ}\)C/ms.