## 1 Introduction  
Laser welding with multi-kilowatt fiber lasers is fast becoming a highly advantageous joining technology in manufacturing industries such as shipbuilding, where it saves production time and cost compared to conventional arc based welding processes [1]. The growing acceptance and adoption of laser technology can be seen, for example, in sales statistics, which show an annual growth rate of over 10% for the last few years [2; 3]. Modern high power fiber lasers are low maintenance, easy to integrate with production robots, and produce welds with deep penetration and low heat input at high throughput rates [4; 5]. High beam quality at high power levels and the decreasing price per kilowatt of laser power are enabling previous limitations to be overcome and opening up new possibilities, especially in keyhole welding. The main limiting factor hindering more extensive utilization of laser welding is its demand for high accuracy in joint fit-up tolerances. A common way to compensate gap fluctuations and ensure welds of acceptable quality is to add an arc process working in synergy with the laser. While such hybrid laser-arc welding allows control of weld bead formation through adjustment of the arc parameters and extends the gap bridging ability of the welding system, it increases process complexity and production costs. One of the most important characteristics of a laser welding system is the beam quality that the lasers deliver. The beam quality affects the power density of the beam, which has a direct effect on the penetration depth and geometry of the weld [6; 7]. High power density of the beam means deeper penetration for the same level of power and welding speed. Single-sided welding of T-joints with fiber lasers is not an entirely novel concept, and its applicability and key parameters have been studied, for example, for aluminum welding in the aircraft industry [4]. The high depth-to-width aspect ratio of typical fiber laser welds can be a drawback in medium and thick section welding of T- and fillet joints. Space and maneuverability restrictions on the welding head can cause the laser beam to cross the joint plane at a certain angle, and a narrow melt pool may easily partially miss the joint plane and produce incomplete fusion. In addition, T-joint welds can be several meters long and heat-induced distortions during welding can thus cause variations in joint fit-up regardless of the accuracy of the original setup. Classification societies such as DNV (Det Norske Veritas) and IIW (International Institute of Welding) suggest avoiding fillet welds in parts of a construction that are subjected to fatigue, because partial penetration creates a possible crack initiation point at the root of the weld [8; 9]. Nevertheless, more than 80% of welded joints are fillet welds, because one of the plates serves as backing during the welding process and less post-welding correction is required. Figure 1 illustrates the principal differences in the geometry and location of the stress concentration of arc, laser and hybrid welded joints. The key problem with T-joints is ensuring fusion throughout the whole joint plane, preferably with single-sided welding. In autogenous laser welding, avoidance of the possible occurrence of underfill or undercut due to an absence of filler material is important, because throat thickness and shape of the weld toe influence the fatigue performance of the joint. In thin materials and flat assembly joints, this problem can be addressed by increasing weld width through wider distribution of the beam energy. Approaches used include but are not limited to: manipulation of the focal point position [10], and usage of dual focal point setups [11; 12; 13] and beam oscillation techniques [14; 15; 16; 17]. Scanning is also beneficial for bringing more heat into the material, which decreases the cooling rate and keeps the hardness of the weld at acceptable levels.