## 3 Results and Discussion  
### Fatigue Testing  
Fatigue testing was performed and S-N curves were formed according to ASTM standard E 739. The SN curve is basically the plot drawn between cyclic stress and the number of cycles until the failure of the material. At a high stress level, a lower number of cycles can cause failure. At a low stress value, a large number of cycles is required for failure and after a certain high number of cycles, unexpected failures can be seen. Base samples and welded samples are both tested and compared. Table 5 presents the data after testing 16 base samples of 7075, with a notch to concentrate the stress for fractures at point loading. Table 6 presents the data after testing 16 base samples of 2024, with a notch to concentrate the stress for fractures at point loading. Figure 4 is the S-N curve for AA2024. Under the same loading conditions, AA 7075 showed better fatigue strength as compared to AA 2024. The difference in fatigue life between the two materials is more prominent under high-intensity stresses as compared to lower stresses. The same trend of AA 2024 and AA 7075 fatigue life was observed in previous studies [41; 42]. AA7075 had a better fatigue life than AA2024 because it was stronger and had a better microstructure. Figure 5 shows the comparison of S-N curves of both base samples, i.e., AA 7075 and AA 2024, while in Figure 6, welded samples with different filler material are compared. Table 7 and Figure 7 present the results of welded samples of two dissimilar metals using AA 4043 as a filler material. The fatigue test results of welded samples of dissimilar aluminum alloys with AA4043 as a filler material can be seen in Table 8 and Figure 8. After carefully analyzing the results, it was clear that the fatigue life of welded samples of two dissimilar metals, i.e., AA7075 and AA2024 using AA4047 as a filler material, is better than \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sr. No.**} & \multirow{2}{*}{**Load (N)**} & \multirow{2}{*}{\(\sigma\)=\(\frac{509.55\ Q}{D^{3}}\) (N mm\({}^{-2}\))} & \multirow{2}{*}{**Alternating Stress**} & \multicolumn{4}{c}{**Fatigue Life (No. of Cycles)**} \\ \cline{5-8}  & & & & **S=\(\sigma\)\(\times\)K\({}_{t}\)** & **Trial 1** & **Trial 2** & **Trial 3** & **Average Cycles** \\  & & & **= 1.28 \(\times\)\(\sigma\)** & & & & \\ \hline
1 & 470 & 239 & 306 & 59,897 & 67,433 & 70,268 & 65,866 \\
2 & 390 & 199 & 254 & 838,546 & 947,437 & 855,535 & 880,506 \\
3 & 335 & 171 & 218 & 2,572,867 & 2,192,966 & 2,347,428 & 2,371,087 \\
4 & 295 & 150 & 192 & 3,698,042 & 3,163,914 & 2,963,263 & 3,275,073 \\
5 & 245 & 125 & 160 & 8,743,953 & 8,246,498 & 7,632,763 & 8,207,738 \\
6 & 195 & 99 & 127 & 9,355,500 & x & x & 9,355,500 \\
7 & 170 & 87 & 111 & x & x & x & \\ \hline \end{tabular}
\end{table} 
Table 6: Fatigue test results of base sample AA 7075. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline
170 N & 195 N & 245 N & 295 N & 335 N & 395 N & 470 N \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Various loads applied to the samples. -welded samples using AA4043 as a filler material when loaded under similar cyclic stress. The difference was due to the low melting point and low freezing range of AA4047 as compared to AA4043. AA4047 provided welded joints with more crack resistance, which is why joints with this filler metal have better fatigue life. Table 9 is the illustration of the welded sample results using AA4047 as a filler material. Figure 4: Depiction of the S-N Curves of base sample AA 2024. Figure 5: Comparison of S-N Curve of base samples AA 7075 and AA 2024. Figure 6: Illustration of the S-N curves of welded sample with filler AA 4043.