**5. Conclusions**

The di fferent welding positions (regarding the vibration distribution) show remarkably di fferent effects compared to samples without ultrasonic excitation, see Table 4. No clear cavitation e ffects were observed, but cavitation may support the porosity-related e ffects. In terms of the few incidents of crack formation, segregation is subordinated to the change of microstructure caused by nodal and centred position.


**Table 4.** Effects of di fferent ultrasonic excitation positions.

For welds in nodal position the melt underlies pressure, which crushes the dendrites and e ffects a fine microstructure. Still a small area at the weld margin solidifies columnar and alloying elements segregate at the columnar-equiaxial interface, which can result in hot cracks. Pores form due to enhanced gas mobility in the equiaxially solidifying melt area.

In welds at centred position cracks and pores form due to the enhanced equiaxial solidification. It may result from an ultrasonically modified directed melt flow by acoustic streaming. This would crush dendrites and foster equiaxial solidification. In addition, a typical weld shape results, where the top area is shifted towards one side.

For welds at antinode-position a V-shaped weld seam collapse is promoted, cf. [11], which supports the keyhole and therefore gas escapement from the melt. The equiaxial microstructural amount is reduced, because directed melt flow is enhanced only by a small extent, which only reduces supercooling. Concluding, in antinode position porosity is eliminated and neither segregations nor hot cracks are formed.

Ultrasonic excitation in antinode position appears to be the most suitable excitation option for the welding of 2.4856 nickel-base alloy round bars due to the absence of cracks and due to the suppression of pore formation. It correlates with sagging on the welded round bar's surface. To avoid sagging, positions close to antinode position with suitable keyhole-support and without sagging as well as pores and cracks should be investigated.
