## 4 Discussion
### Application of Specific Point Energy to Welding of T-Joints  
Traditionally, the depth of penetration has been characterized through the concept of heat input, also called line energy, which describes the energy available for producing the weld through the relationship of available laser power and welding speed. \(E_{\text{SP}}\) also considers the diameter of the beam on the surface of the specimen and therefore provides higher accuracy in definition of the penetration depth and weld geometry [23]. [PERSON] and [PERSON] [22] have shown that, in the case of bead-on-plate welds, the penetration depth is defined by power density and specific point energy, and the width of the weld by interaction time, regardless of the optical setup used. T-joint fillet welds follow the same analytical model regarding the penetration depth as bead-on-plate welds; in this work, however, there was a deviation at larger beam diameters produced with the 600 \(\upmu\)m process fiber. The relationship between \(E_{\text{SP}}\) and penetration depth for all three setups is summarized in Figure 7.  
Figure 7 shows curves for penetration depth produced with each of the process fibers and macrographs of three welds obtained at similar \(E_{\text{SP}}\) using different process fibers. Beam diameters on the surface of the welds shown in Figure 7 were 0.82 mm, 1.00 mm and 1.61 mm (FPP \(-2\) mm). The following laser power and welding speed combinations were used: (a) \(P_{L}\) = 6 kW, \(v_{w}\) = 0.75 m/min; (b) \(P_{L}\) = 8 kW, \(v_{w}\) = 1.25 m/min; and (c) \(P_{L}\) = 4 kW, \(v_{w}\) = 1 m/min. Welds (a) and (b) both had penetration depths close to 10 mm, when measured from the top of the bead, thus exceeding the thickness of the material, while weld (c) had only partial penetration. At similar \(E_{\text{SP}}\), the 200 \(\upmu\)m process fiber produced the weld with the deepest penetration and largest melt area (a). Full penetration was not  
Figure 7: Penetration depth plotted as a function of specific point energy. \(E_{\text{SP}}\) values of the three welds shown: (\(\blacktriangle\)a) 396 J; (\(\blacklozenge\)b) 384 J; and (\(\blacksquare\)c) 386 J.  
obtained with the 300 \(\upmu\)m process fiber despite sufficient penetration depth, and the weld produced with the 600 \(\upmu\)m fiber was noticeably shallower.  
It is known that the diameter of the beam governs the dimensions of the keyhole, which in turn define the depth and width of the weld. As expected, beams with smaller diameters produced deeper penetrations at any given \(E_{\text{SP}}\). Within each setup, it can be seen that an increase in \(E_{\text{SP}}\) also increases the penetration depth, but comparison of all three setups shows that the power density of the beam has a more pronounced effect on the morphology of the weld than \(E_{\text{SP}}\). A possible explanation for this might be that the heat conduction in T-joints is different than in butt joints or bead on plate joints. Distribution of energy over a larger area leads to a slight decrease in molten area and reduced penetration depth. The power density of the beam determines the depth of the weld, while the width of the weld is determined by the diameter of the focused beam on the surface of the specimen.