# Self-Powered Fast Brazing of Ti-6 Al-4V Using Ni/Al Reactive Multilayer Films
## 3 Results and Discussion  
### Characterization of RMF and BAl-Coated Ti64  
The total thickness achieved is 32.9 \(\upmu\)m with a bilayer thickness of 110 nm as seen in Figure 2a. The average Ni layer thickness (white) is 44 nm and the average Al layer thickness is 66 nm, producing a 110 nm bilayer thickness (Figure 2b). The thickness ratio between Al and Ni was kept \(\sim\)1.5:1 to obtain an equiatomic ratio [15]. Equal Al and Ni layer thickness produces a Ni:Al atomic ratio of 1.5:1, which proved unable to join BAlSi-4 via electrical ignition. The XRD pattern in Figure 3a shows that Ni and Al were deposited with no significant intermixing between the as-deposited layers.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Length** & **Width** & **Thickness** & **Thermal** & **Density** & **CTE (10\({}^{-6}\)** & **Heat Capacity** \\  & **(mm)** & **(mm)** & **(mm)** & **(W/m-K)** & **(g/cm\({}^{3}\))** & **K\({}^{-1}\))** & **(J/g-K)** \\ \hline
**Ti-6 Al-4V** & 30 & 3 & 0.44 & 6.7 & 4.43 & 8.6 & 0.526 \\
**BAlSi-4** & 3 & 3 & 0.1 & 205 & 2.66 & 21 & 0.9 \\
**RMF** & 3 & 3 & 0.135 & 76 & 4.91 & 12.5 & 0.536 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Layer-wise Additive Manufacturing Predictions and Simulations (LAMPS\({}^{\copyright\copyright\copy\right\copy\right\copy\right\copy\right\copy\right\copy\right\copy\right\copy\right\copy\right\)  
_A. Characterization_  
Cross-sectional analysis was evaluated by embedding a lap joint sample in epoxy and then polishing with SiC paper, then diamond paste and, finally, colloidal silica particles. Scanning Electron Microscopy (SEM) images were collected on a Zeiss Auriga Scanning Electron Microscope (Zeiss, Oberkochen, Germany). X-ray diffraction measurements were conducted on a Panalytical Empyrean X-ray Diffractometer (Panalytical, Almelo, The Netherlands); the post-brazing XRD profile was obtained from the lap-joint cross-section embedded in epoxy the XRD beam line is perpendicular to the bond interface to detect all phases present in the joint. Lastly, Energy Dispersive X-ray Spectroscopy (EDX) measurements were performed using a Zeiss MA15 EVO SEM (Zeiss, Oberkochen, Germany) equipped with a Bruker xFlash 61 30 detector. A PowerView HS-650 highspeed camera (1500 frames/s) (DICKEY-john@Corporation, Auburn, IL, USA) was used to determine the reaction propagation speed of Ni/Al films. Differential scanning calorimetry (DSC) was performed using a Netzsch-Geratebau STA 449 C Jupiter Thermo-microbalance (AZO materials, Manchester, UK).  
The post-ignition XRD scan was performed on a free-standing film that was peeled off the polyimide substrate. The post-ignition XRD scan detects only AlNi, indicating complete reaction of the RMF. The total reaction heat is 1275 J/g as shown by DSC (Figure 4). The reaction propagation speed is 4.36 m/s. As seen by the high-speed camera images (Figure 5), the RMF reaction completes very quickly (4.6 ms), but the film cools down over 30 times longer (169.5 ms) as indicated by the diminishing light intensity. The Ni/Al reaction is known as a solid + liquid reaction [16]. In the context of the RMF reaction, first the Al layers melt, then, the solid Ni layers dissolve in the molten Al and this is concluded by crystallization of Ni-Al intermetallic compounds. Melting and dissolution occurs during reaction propagation and crystallization occurs after the end of the propagation phase [17; 18].  
Figure 3: XRD pattern of the Ni/Al RMF (**a**) as-fabricated and (**b**) post ignition.  
Figure 2: (**a**) SEM image showing the total as-fabricated RMF thickness and (**b**) the bilayer thickness (white is Ni and grey is Al).