## 2 Materials and Methods  
Shipbuilding steel AH36 is commonly used for shipbuilding and offshore structures. Hot rolled steel plates of AH36 have excellent weldability with a CEV (carbon equivalent value) of 0.248, calculated based on the chemical composition presented in Table 1. The yield strength of AH36 is 355 MPa.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Material** & **C** & **Si** & **Mn** & **P** & **S** & **Cr** & **Mo** & **Ni** & **Cu** & **Al** & **V** \\ \hline AH36 & 0.111 & 0.149 & 0.711 & 0.035 & 0.150 & 0.051 & 0.01 & 0.041 & 0.031 & 0.030 & 0.008 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of AH36 steel (wt %).  
Test specimens (100 mm \(\times\) 350 mm \(\times\) 8 mm) were cut with a CO\({}_{2}\) laser using oxygen-assisted cutting. The edges were grid blasted with aluminum oxide and cleaned with acetone to remove possible contaminants. The plates were tack welded from the root side from the ends and the middle using gas metal arc welding. The workpiece was fixed in the flat (1F) position using stiff fixtures to avoid heat-induced air gap fluctuations during the welding. Single-sided welds with a length of 165 mm were performed. The experimental setup is shown in Figure 2.  
All welding experiments were made with a continuous wave fiber laser IPG YLS-10000 having wavelength of 1070 nm and a top-hat focused beam profile. A Kugler LK190 mirror optics laser welding head was used. An air knife protected the focusing system from contamination with fumes and spatter, and no additional shielding gas was used. When comparing a T-joint with a butt joint, a further parameter, called offset, describing the distance of the beam center from the joint at the flange front edge, must be added. This parameter has a crucial effect on weld quality and must be considered for thick section joints of the type studied in this work. The experimental parameters of the welding process are presented in Table 2.  
The beam was delivered by a system consisting of a beam transfer fiber (with core diameters of either 200, 300 or 600 \(\upmu\)m), 120 mm collimating length optics, and a 300 mm focal length mirror. The properties of the laser beam emitted from each transport fiber were measured using a laser beam analyzer from Primes GmbH and are shown in Table 3.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Parameter Range** \\ \hline Fiber diameter & [\(\upmu\)m] & 200; 300; 600 \\ Laser power, \(P_{L}\) & [kW] & 6.0; 8.0; 10.0 \\ Welding speed, \(v_{w}\) & [m/min] & 0.75; 1.0; 1.25; 1.5; 1.75 \\ Focal point position, F\({}_{\text{PP}}\) & [mm] & \(-2.0\); \(-4.0\); \(-6.0\) \\ Beam angle from flange \(\alpha\) & [\({}^{\circ}\)] & 6; 10; 15 \\ Beam offset from flange & [mm] & 0.5; 1.0; 1.2; 1.5; 2.0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Welding process parameters.  
Figure 2: Experimental setup for laser welding of T-joints. Beam is positioned at the joint plane (offset 0 mm).  
The bead surface and root of each weld were visually evaluated based on standard EN ISO 13919-1, which classifies welds into three quality levels based on the type and severity of the imperfections that are present. The categories from best to worst are: B, stringent; C, intermediate; and D, moderate [31]. Metallographic preparation of the samples was carried out according to SFS-EN ISO 17639 [32]. The welds were transversely sectioned at the middle of the joint length, and polished and etched using a 2% Nital solution. Macrographs of the weld cross sections were taken for inspection of penetration, defects, and dimensions and shape of the fusion zone and HAZ (heat affected zone).