## 4 Discussion  
### Geometry of the Welds  
The purpose of this study was to investigate geometrical differences in welds produced with three different beam delivery fibers and to determine the effect of process parameters on T-joint welds. Thirty welds were produced, evaluated, and their cross-sections analyzed. The acceptance criteria in visual inspection were smooth and plain face and root sides of the weld seam, lack of spatter, cracks or other defects listed in the Standard EN ISO 13919-1, and full visible penetration on the root side. Cracks or porosity were not present, qualifying the welds for class C of ISO 13919-1. Obvious undercut and lack of fusion produced by an inappropriately positioned beam or a lack of laser power were causes of rejection. The width of the weld fusion zone determines the largest acceptable inclination angle for producing full penetration at a given web thickness. It can be seen in Figure 3 that compared to  
Figure 5: Weld profiles at various focal point positions: AH36, \(t=8\) mm, \(v_{w}=1.25\) m/min, beam offset from flange 1.0 mm (up) and 1.5 mm (down), \(\alpha=15^{\circ}\). the other set-ups studied, the process fiber with a core diameter 600 \(\upmu\)m was least sensitive to increase of \(\alpha\). However, at given thickness of 8 mm, \(\alpha=6^{\circ}\) produced the largest weld throat in all set-ups, regardless of the diameter of the beam, since the keyhole formed strictly along the axis of beam propagation. Figure 6 illustrates the effect of beam inclination angle on melt distribution relative to the middle axis of the joint. Figure 6 shows the size of the melt area above and below the middle axis of the joint of the welds presented earlier in Figure 3. The bars illustrate the proportions of the melt area above (web) and below (flange) the joint plane, and the lines show the area of the melt. Process fibers with core diameters of 200 \(\upmu\)m and 300 \(\upmu\)m produced welds with similar properties: the melt area decreased with increase of inclination angle. The 600 \(\upmu\)m diameter process fiber produced welds in which the melt area increased while simultaneously keeping the same proportions of the fusion zone above and below the joint axis. The fiber producing the largest focal point diameter was least sensitive to change of inclination angle. From an engineering point of view, beam offset is an important parameter affecting the weld quality of fillet welds, where joint bridging ability and ensuring complete fusion are more important than the penetration depth itself. As can be observed in Figure 4, tolerance to beam offset is also determined by the width of the weld. An inclination angle of 15\({}^{\circ}\) was chosen over two smaller angles tested based on the width of the weld bead and possible accessibility restrictions of the welding head. Optimal offsets for acceptable top bead and full fusion at the root are: 1.0 mm with the 300 \(\upmu\)m transfer fiber, and, 1.0 mm and 1.5 mm with the 600 \(\upmu\)m transfer fiber. The diameters of the beams on the surface were 1.0 mm and 1.6 mm, respectively. When the 300 \(\upmu\)m fiber was used, deviation to either side from the 1.0 mm offset produced either lack of fusion at the root or undercut and lack of penetration at the face of the weld. Due to the small width of the weld/fusion zone, higher offset resulted in formation of severe undercut on the top of the weld, while part of the melt was pushed through the weld root. The transfer fiber with 600 \(\upmu\)m core diameter had a wider positioning tolerance window because the width of the weld throughout the fusion area is also wider. All of the welds produced with the 600 \(\upmu\)m process fiber had a smooth bead and root sides, complete penetration, and class B quality according to EN-ISO 13919-1. It seems  
Figure 6: Proportions of the weld and melt area distribution differences at three inclination angles tested. AH36, \(t=8\) mm, \(P_{L}=6\) kW, \(v_{w}=1.25\) m/min, F\({}_{\text{PP}}=-2\) mm.