## 2 Materials and Methods  
The current study used sheets of 3 mm thick, rolled AZ31 magnesium alloy for welding experiments. Table 1 shows the chemical composition of the AZ31 base metal. A sheet with dimensions of 120 \(\times\) 100 \(\times\) 3 mm was used for welding coupons. Prior to welding, these coupons were wire-brushed and cleaned with acetone. A GTA welding machine (ORIGO TIG 4300 Iw AC/DC, Esab India Ltd., Chennai, India) was employed for welding AZ31 coupons, using a 2.4 mm \(\phi\) tungsten electrode. Full-penetration autogenous welds were made on the coupons. The machine had an electromechanical linear movement set up for moving the welding torch. Welding was carried out so that the weld beads were oriented perpendicular to the rolling direction. Ar was used as the shielding gas, and 50% Ar + 50% He mixture was used for the auxiliary trailing and backing gas, improving the weld quality. The gas flow rate of 20 l/min was maintained for the trailing and backing gas shields, and the shielding gas flow rate was 15 L/min. After completion of welding, the shielding gas was allowed to flow for another minute to prevent the oxidation of welded area, while the welded coupon cooled to room temperature. A K-type thermocouple of 0.2 mm diameter wire was attached to the back of the weldment to measure the temperature history curves, which were then recorded by a high-speed computer-based data-acquisition system connected to a personal computer. Table 2 contains a list of the welding parameters. Heat input values were calculated using the formula: heat input = (V \(\times\) 1)/S kJ/mm, where V is the voltage used, in volts, I is the current used, in amperes, and S is the weld travel speed (mm/s), for all the weldments.  
The samples for optical microscopy were appropriately sectioned, mounted, mechanically polished, and etched for microstructural analysis. For the microstructure examination of samples, a solution containing 10 mL H\({}_{2}\)O, 10 mL acetic acid, 4.2 g picric acid, and 70 mL ethanol was used for etching. The etched samples were observed using a Nikon SMZ745T stereomicroscope to the macrograph of the welds. A Leitz optical microscope was used to examine the microstructure of the base metals and welds. A scanning electron microscope, with an energy dispersive X-ray spectrometer (EDS), was used to examine the weld microstructure and tensile fracture surfaces. A PANalytical X-ray diffractometer with Cu k\(\alpha\) radiation was employed for the identification of the various phases present in the weld region and base metal.  
A Vickers microhardness tester (Shimadzu corporation, HMV-G20S (E, 230V), Japan), with a diamond pyramid indenter, was used to measure the microhardness of the specimens. The microhardness investigation of the welded samples was made across the weld region at intervals of 0.5 mm for 15 s, under a load of 1000 g. Tensile specimens were made from base metal and as-welded coupons in accordance with the ASTM E 8M standard. The  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Al** & **Mn** & **Zn** & **Mg** \\ \hline
2.75 & 0.31 & 0.80 & Balance \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of AZ31 Mg alloy (wt.%).  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Alternating Current GTA Welds**} \\ \hline Alternating current amplitude & 115 A \\ Arc voltage & 11 V \\ Travel speed & 3, 4, 5 mm/s \\ Frequency & 50 Hz \\ Heat input & 421, 316, 253 J/mm \\ \hline \hline \end{tabular}
\end{table}
Table 2: GTA welding parameters.  
tensile tests were carried out using a universal testing machine at room temperature for the samples with a 1 mm/min crosshead speed. The gauge length, gauge width, and thickness of the transverse tensile weld specimens were 25 mm, 6 mm, and 3 mm, respectively.