## 2 Materials and Methods  
The base material used in the experiments was an austenitic stainless steel grade, AISI 316L, with plate thickness of 10 mm. The test specimens for the welding tests were rectangular pieces 150 mm in width and 300 mm in length. Preparation of the fusion faces of the joints was done by machining. Both I- and V-grooves were prepared for the experiments. The test specimens were tack-welded at both ends and in the centre to achieve the required butt joint configurations. In welds with I-grooves, both closed (root gap = 0 mm) and open square (root gap = 0.4 mm and 0.8 mm) configurations were used, Figure 1a,b. In tests with a V-groove, the configuration was 10-degree groove angle, closed gap and without root face, Figure 1c. The nominal cross-sectional groove area for the 0.4 mm and 0.8 mm open square configurations and the V-groove configuration is 4 mm\({}^{2}\), 8 mm\({}^{2}\) and 8.6 mm\({}^{2}\), respectively. The filler material used was a type 2205 duplex stainless steel filler wire (\(\mathcal{O}\) 1.0 mm) that has an over-matched chemical composition compared to the base material. The exact chemical composition of the base and filler material is given in Table 1.  
Figure 1: Butt-joint configurations used in the experiments: (**a**) Closed square (root gap = 0 mm); (**b**) open square (root gap = 0.4 mm and 0.8 mm); and (**c**) V-groove with 10-degree groove angle, closed gap and without root face. The welding equipment used in the laser-arc hybrid and laser cold-wire welding experiments was an IPG 10 kW YLS-10000 (IPG Inc., New York, NY, USA) fibre laser and a Kemppi Pro 5200 (Kemppi Oy, Lahti, Finland) gas metal arc welding (GMAW) machine. In the laser cold-wire tests, the filler wire was guided through the same GMAW torch as used in the laser-arc hybrid experiments. A grounding cable shoe was disconnected from the fastening table to enable the use of cold-wire welding. Beam delivery from the laser source to the welding station and welding head was with a process fibre of 200 \(\upmu\)m core diameter. The welding head was a laser welding head-unit from Kugler GmbH with copper metal optics and an adjustable GMAW torch unit. The optical system of the Kugler laser welding head consisted of 120 mm collimation and 300 mm focusing lenses. The optical set-up can provide a focal spot diameter of 500 \(\upmu\)m. The laser system enables a beam parameter product of 7.1 mm \(\times\) mrad and a Rayleigh length of 13.86 mm. A custom-built gantry-based X/Y/Z- linear movement station was used to execute the required welding movements. Figure 2 shows the welding head set-up attached to the linear movement station. Experiments consisted of sets of single-pass test welds made on 10 mm thick butt-joints in flat position using laser-gas metal arc (GMA) hybrid welding and laser cold-wire welding. The shielding gas used is commercially available mixture for welding of stainless steels. It contained a mixture of Ar + 2% CO\({}_{2}\) + 30% He and was supplied via an arc torch with the flow rate of 24 L/min. Pulsed current GMAW was deployed in the laser-arc hybrid experiments. Table 2 presents the welding parameters used. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Element Weight-\%** & **C** & **Si** & **Mn** & **P** & **S** & **Cr** & **Ni** & **Mo** & **N** & **Cu** & **Ti + Ta** \\ \hline AISI 316L & 0.024 & 0.37 & 1.70 & 0.020 & 0.0002 & 17.6 & 12.3 & 2.43 & 0.07 & 0.01 & \textless{}0.01 \\
2205 & 0.010 & 0.36 & 1.60 & 0.017 & 0.0001 & 22.9 & 8.7 & 3.20 & 0.147 & 0.08 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the base (AISI 316L) and filler (2205 duplex) material. Figure 2: Welding head set-up used in the laser-arc hybrid and laser cold-wire experiments. (**A**) process fibre with connector; (**B**) laser welding head-unit; (**C**) adjustable gas metal arc welding (GMAW) torch unit.