# Technological Processes for Steel Applications in Nuclear Fusion
## 4 Fusion Welding
### Sample Fabrication by Tungsten Inert Gas, Electron Beam, and Laser Welding  
Electron beam (60 kV, 19 mA, 10 mm/s) and laser (5 kW, 750 J/cm, 4 m/min) welding were applied to 5 mm EUROFER97 plates in the condition as received, that is, after rolling and a heat treatment of 980 \({}^{\circ}\)C/30 min + 740 \({}^{\circ}\)C/2 h. From tests with a massive plate, it was assumed that an electron weld current of 19 mA combined with a welding speed of 10 mm/s would be sufficient to progress the melt down through the beam stopper. However, as can be seen from the micrograph in Figure 9, the beam intensity was too weak since the beam stopped about 0.5 mm above the bottom. That is, the beam stopper is still recognizable after welding, which should not be the case for a perfect EB weld. Also, the plates were not aligned exact. There is a remaining gap of about 0.2 mm beside the beam stopper. Grain coarsening in the weld is not as pronounced as in the TIG welds. Nonetheless, a small heat affected zone (HAZ) can be seen at the border of the symmetrically wedge-shaped weld. With a power of 5 kW and with a welding speed of 4 m/min the penetration depth of the laser beam was adjusted perfectly, but the weld was not quite centered on the beam stopper. Therefore, a 0.5 mm long crack remained (barely visible in Figure 9 at the bottom right of the laser weld). The micrograph shows only little grain coarsening at the top of the weld. HAZs are hardly recognizable. Of all welds tested, this laser weld looks most homogeneous, and its borderlines are nearly parallel. In addition, TIG welds on the same material (5 mm, 10 mm, and 12 mm plates) were produced using EUROFER97 filler wire (heat D83350). The TIG weld geometry was \(\frac{1}{2}\) V joint with a root height of 1 mm and a distance of 2 mm. All welds were fabricated parallel to the direction of the last rolling step that was applied during plate production. Immediately before welding, the joining plate surfaces were dry milled by 0.1 mm and, in the case of TIG welding, the filler wire was grinded. The TIG welds were manually fabricated in a glove box flooded with inert-gas. The weld engineer certified a good weldability of EUROFER97, but the diameter of the filler wire (1 mm) was revealed to be too small for large structures, such as 5 mm, 10 mm and 12 mm plates. Since there was no other wire at hand, the TIG welds were fabricated with the present one. It took five weld layers to fill the 2 mm gap in the 5 mm weld. The wire feeding speed was about 35 mm/min for the root layer and 100 mm/min for the middle and cover layers. The microstructure reveals clearly severe grain coarsening in the weld and a finer grained heat affected zone (HAZ) on both sides. The grain size obviously has a maximum at the top of the weld. For the 10 mm TIG weld, eight layers were needed to fill the 2 mm gap. Severe grain coarsening with maximum grain size in the upper part of the weld, as well as fine-grained HAZs, can be clearly identified in Figure 9.  
Even though we used the same parameters as with the 5 mm and 10 mm TIG welds for fabricating a further 12 mm weld, there are significant differences. The grain coarsening in the weld is not so distinct and fine-grained HAZs are visible even in the transition region at the top and bottom of the middle layers (see Figure 10). All TIG welds are asymmetrically shaped due to their \(\frac{1}{2}\) V joint geometry. Since they were fabricated manually, it was difficult to reproduce the weld microstructure, even by using the same parameters. In case of the 12 mm TIG weld, the reason for the obvious differences was probably the fact that the 5 mm and 10 mm plates were welded in one pass, while the 12 mm plates were welded in several passes over several days. That is, between fabricating different weld layers, the plates had time to cool down to room temperature.