## 2 Experimental Procedure  
Austenitic stainless steel 304L and Inconel 718 cylindrical rods, with a diameter of 15 mm and length of 100 mm, were employed as parent materials in this study. Table 1 presents the nominal compositions of the parent materials. The Inconel 718 alloy was subjected to two different pre-weld heat treatments: ST and STA. Pre-welded IN718 samples in ST and STA conditions were friction welded to SS304L, and these samples were designated SS304L/IN718 ST and SS304L/IN718 STA, respectively. During the solution treatment (ST), the samples were heated to 990 \({}^{\circ}\)C and held for 1 h and 15 min, followed by air cooling to room temperature. The STA treatment consisted of (i) solution treatment followed by (ii) two-step ageing. The ageing of solution-treated samples consisted of a first step aging at 720 \({}^{\circ}\)C for 8 h, followed by furnace cooling to 620 \({}^{\circ}\)C; and then a second step of aging at 620 \({}^{\circ}\)C for 7.5 h, followed by air cooling to room temperature. The welded dissimilar SS304L/IN718 ST joints were subjected to PWHT (direct ageing) and were designated as SS304L/IN718 PWHT. PWHT involved a first aging step at 720 \({}^{\circ}\)C for 8 h, followed by furnace cooling to 620 \({}^{\circ}\)C; and then a second aging step at 620 \({}^{\circ}\)C for 7.5 h, followed by air cooling to room temperature. The typical solution treatment and two-step aging cycle are shown in Figure 1.  
The friction welding machine used is capable of high precision and good weld parameter repeatability. The joints were made with hydraulically operated continuous drive friction welding equipment (ETA Technologies Pvt Ltd., Bangalore, India) with a capacity of 100 KN. During the welding process, the IN718 was fixed into the stationary chuck, the and SS304L was fixed into the rotating chuck. Table 2 shows the optimal parameters used during the welding process. \begin{table}
\begin{tabular}{l l l l l l l l l l l l l l} \hline \hline
**Material** & **Ni** & **Cr** & **Fe** & **Nb** & **Mo** & **Al** & **Ti** & **V** & **Mn** & **Si** & **C** & **B** & **P** \\ \hline IN718 & 51.6 & 18.2 & 19.7 & 5.1 & 3.2 & 0.5 & 1.1 & 0.3 & 0.09 & 0.35 & 0.004 & 0.003 & 0.015 \\ \hline SS304L & 8.2 & 18 & 71.5 & - & 0.23 & - & - & - & 2.00 & 0.75 & 0.024 & - & 0.045 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Base material compositions of Inconel718 and SS304L. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Values** \\ \hline Rotation speed & 1800 rpm \\ Soft force & 28 MPa \\ Soft force time & 0.5 s \\ Friction force & 195 MPa \\ Friction burn off & 4 mm \\ Upset force & 390 MPa \\ Upset time & 4 s \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Optimum welding parameters used to fabricate the joint. The dissimilar rotary friction welded joints were sectioned and embedded in bakelite before being mechanically polished with 600 to 2000-grit SiC sheets. Cloth polishing with 0.5-1 \(\upmu\)m diamond paste was used to finely polish the surface. A macroscopic examination of the welded samples was carried out with a stereomicroscope. The microstructural investigation was carried out with an optical microscope (Motic, Model: AE2000 MET, New York, NY, USA) and a scanning electron microscope (TESCAN, Model: VEGA 3 LMU, Brno, Czech Republic) with a connected Energy-dispersive X-ray spectroscope (EDS). The parameters used for this examination were 20 kV accelerating voltage, with a working distance of 10 mm. Grain size measurements were taken at three to four distinct areas in the FDZ using a line intercept method and higher magnification micrographs. The etchants used to reveal the microstructures of the weldment are shown in Table 3.  
A Vickers microhardness tester ([PERSON], HMV 2T E, Kyoto, Japan) was used to conduct a hardness survey across the weldment in the transverse direction. The measurements were made using a diamond pyramid indenter under a load of 200 g and a dwell time of 15 s, with 0.5 mm intervals between the indentations.