**3. Deposition strategy**

Since in additive welding, the parts geometry influences the cooling rate and thus the microstructure and mechanical properties, in a first step, two different part geometries (thin wall and multi-pass block) were initially investigated (**Figure 2**).

Depending on the geometry, different substrate sheets were used. These sheets were welded to a 40 mm thick carbon steel plate to counteract distortion and produce high thermomechanical reactions. For welding of both geometries, a stick-out of 15 mm and 30% He, 2% H2, 0.05% CO2, bal. Ar as shielding gas with a flow rate of 18 l/min were used. This gas is recommended for welding Ni alloys, as it ensures good flow and wetting behavior of the highly viscous melt. During the welding, the direction was changed after each layer. Between the beads or layers, there was no brushing. For all welds, the maximum interpass temperature (IT) was 150°C. To influence the deposition rate and heat input at welding the walls, the wire feed speed (WFS) was varied between 6.0 and 9.0 m/min and the travel speed (TS) between 0.6 and 1.0 m/ min. To achieve a high deposition rate when manufacturing multi-pass block with stringer bead technique, a high wire feed speed (WFS: 9.0 resp. 9.5 m/min) and a low welding speed (TS: 0.6 m/min) were selected.

#### **Figure 1.**

*Experimental setup for additively GMAW (a) three-axis gantry machine with CMT advanced 4000 and (b) two-axis welding portal with CMT TPS 4000.*

#### **Figure 2.**

*Deposition strategy of (a) wall (b) block (above: side view, below: top view).*

In second step, the influence of different shielding gases on the seam quality was investigated. Ni-alloys are normally welded with inert shielding gas. Additions of active gases (CO2, O2) in the ppm range and of He or H2 improve the flow and wetting behavior of the melt. However, the presence of CO2 or O2 can also lead to a slight seam oxidation. This effect can increase the possibility of lack of fusion on multi-pass deposition. On the other hand, hydrogen has a reducing effect and can ensure an oxide-free seam appearance. The following shielding gases were used:


These investigations were also performed on walls and blocks with constant setting values (WFS: 9.0 resp. 9.5 m/min and TS: 0.6 m/min). The nominal dimensions were for walls 200 mm long and 60 mm high and for blocks 200 mm long and circa 25 mm– 30 mm high and 30 mm–35 mm wide. In contrast to the previous blocks, the welding direction was changed after each stringer bead. The remaining welding conditions (stick-out, IT, etc.) corresponded to the previously tests.

In multi-pass deposition, maintaining the correct center distance d between stringer beads is of elementary importance. If this distance is too big, there is no connection to the neighboring bead and lack of fusion can occur. If it is too small, the beads overlap too much and no even seam surface is created. Therefore, it is important to ensure that the overlap areas and valleys are of equal size. In [17] it is proposed to calculate the ideal center distance depending on the ratio of WFS/TS. If WFS/ TS > 12.5, the equations shown in **Figure 3** should be used.
