## 2 Materials and Methods
### Fatigue Testing  
The fatigue testing machine, manufactured by Jinan Precision Testing Equipment (Jinan, China), is called the \"Pure Bending Fatigue Testing Machine,\" Model No. PQ 6, and was used for the fatigue test. Standards GB4337-84 and ZBN71006-87 have been applied. The working principle of this machine is to clamp the specimen at both ends and weight is used to determine test load [40]. Stresses include tension and compression changes on upper and lower side of specimen due to rotation of the specimen by the motor. Therefore, for an instant the upper most layers of the specimen are under maximum compression while lower most are under maximum tension. During testing, the specimen is clamped and rotated along the main shaft and the pulling rod applies the load at both ends. Hence, pure bending is evenly distributed along the entire specimen. The direction of load does not change but the specimen rotates, while on the same speed, the specimen faces alternative bending stress at each point. This bending fatigue causes fracture after many alternative cycles. The location of loading or line of four point forces is shown in a schematic diagram [36]. Schematics and real fatigue testing machines are shown in Figure 2a,b. Samples with standard dimensions can be tested for fatigue failure at different desired stress levels. The testing is carried out at constant amplitude and fully reversed cycling. For each load, three different tests were performed and averaged. Number of specimens tested in this study are presented in Table 3.  
Seven different loads were applied to the samples, and three samples were tested for each load. The S-N curves of base samples and welded samples with different filler materials were plotted. All the samples, i.e., base or welded, were subjected to reverse cyclic stresses, having a stress ratio of R = \(-1\). Figure 3 is the S-N curve for the base metal AA7075. Table 4 reveals the seven different loads that are applied to the samples. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sr. No.** & **Weld Parameters** & **Values** \\ \hline
1 & Current supply & 90 A \\
2 & Rotational speed during welding & 3.7 RPM \\
3 & Specimen gap during welding & 1 mm \\
5 & Welding spots & 2 \\
6 & Passes & 3 \\
7 & Length contraction during welding & 0.7 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Illustration of welding parameters. Figure 2: Illustration of the experimental set up (**a**) schematic (**b**) actual. The fundamental beam theory was used to evaluate the degree of repeated stress in the un-notched sample after load application. \[\frac{\sigma}{C}=\frac{M}{I} \tag{1}\]  
where,  
\(\sigma=\) Applied (alternating) stress (MPa)  
\(Q=\) Load Applied (N)  
\(C=\) Distance from neutral axis at which stresses are being evaluated = \(D/2\) (mm)  
\(M=\) Moment of load (N-mm) = \(Q\times\frac{1}{2}\)  
\(I=\) Moment of Inertia. Substituting these values in Equation (2):  
\[\frac{\sigma}{\frac{D}{2}}=\frac{Q\times\frac{l}{2}}{\frac{\pi l}{64}\times D ^{4}} \tag{2}\]  
Solving Equation (2) for \(\sigma\) and putting \(l\) = 100 mm,  
\[\sigma=\frac{Q\ (\text{N})\times 50\ (\text{mm})\times 32}{\pi D^{3}\ (\text{mm}^{3})}\]  
\[\sigma=\frac{509.55\ Q}{D^{3}}\ \Big{(}\text{N}\ \text{mm}^{-2}\Big{)}\!=\! \frac{32M}{\pi D^{3}}\]  
From the notch specimen, the alternating stress is:  
\[\sigma_{notched}=K_{t}\times\sigma_{un-notched} \tag{3}\]  
After fatigue testing, the fractured surfaces are evaluated through the fractography technique. The specimens are put under the lens of SEM to investigate the surface morphology throughout the fracture zone and the role of different phases of materials in the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Description** & **Load Steps SN Curve** & \
\begin{tabular}{c} **Each Load** \\ **Iterations** \\ \end{tabular} &
\begin{tabular}{c} **Number of** \\ **Specimens Tested** \\ \end{tabular} \\ \hline SN-Curve welded & 14 & 3 & 44 \\ SN-Curve base & 14 & 3 & 44 \\ \hline \hline \end{tabular}
\end{table}