*4.1. Weld Appearance*

The outer appearance of weld seams in the nickel alloy 2.4856 (see Figure 4), depends strongly on position according to both the vibration distribution and vibration amplitude. With ultrasonic excitation and weld in node position the weld surface becomes wavy starting with an amplitude of 4 μm.

**Figure 4.** Visual inspection of 2.4856 weld seams, depicted side opposite to welding start/end point.

For the weld in the centred position the ultrasound leads to an unsymmetrical material distribution. This effect increases with increasing amplitudes. In addition, sagging appears starting with 4 μm amplitude. For the weld in antinode position sagging appears with ultrasound. At an amplitude of 4 μm the sagging becomes narrower and the edges touching each other partially. At an amplitude of 6 μm strong spatter occur additionally where the edges touch each other. This shows the melt pool dynamics similar to our simulation in [11].

Evaluating the specimens' cross-sections, see Figure 5, reveals more effects due to the ultrasonic excitation. Pores appear mostly in the weld root area. The weld seam without ultrasonic excitation has one small pore in the root area with a diameter of about 0.5 mm. For the welds in the nodal position, the pores increase up to a diameter of 1.3 mm and the weld surface becomes wavier. For the weld in centred position at an amplitude of 2 μm a big pore with a diameter of about 1.0 mm is located in the root area and the upper weld area is not symmetrical anymore. At amplitudes of 4 μm and 6 μm big pores with a diameter of about 1.4 mm appear and the weld symmetry decreases further until one weld side is straight from the specimen surface to its root. On this side, a reinforcement appears on the specimen surface. In the antinode position, sagging appears in the middle of the weld surface. The welds with amplitudes of 2 μm and 4 μm show show no or little pores with a diameter of about 0.2 mm in the root area, whereas an amplitude of 6 μm shows a big pore with a diameter of about 1.0 mm in the root and a little pore in the upper area. The weld shapes in centred and antinode position are similar for amplitudes of 4 μm and higher, although symmetrical conditions are expected for antinode position.

**Figure 5.** Exemplary micrographs of metallographic cross sections of 2.4856 weld seams.

It is important to consider the laser spot position on the weld seam. On those specimens where ultrasound was applied and asymmetric welds and sagging has been found, the laser spot aims onto one edge of the welding seam. Also considering the cross sections, it becomes clear that the laser beam still creates a keyhole, which penetrates the specimen rectangular to its surface, but the upper weld area shifts towards one side. The reason is likely a directed melt flow, which removes solid metal from one side. The directed melt flow may result from changed weld pool dynamics by acoustic streaming [8,9]. Another reason for asymmetric welds could be the dynamics of the weld during the oscillation, what is simulated in [11]. E ffects such as non-parallel interfaces, caused by the V-shape of the melt, between the melt pool and the environment can promote the ejection of the melt towards one side. In order to validate the influence of acoustic streaming on the weld seam, further test series are planned for the future. Special attention will be paid to the ultrasonic amplitude distribution directly at the weld pool in order to investigate the additional influence on the weld shape due to the di fferent coupling between solid and viscous liquid material.

## *4.2. Pore Formation*

The pore areas in Figure 5 show large di fferences. Without ultrasonic excitation the pore area is low, with diameters of about 0.5 mm. Welds at the centred and nodal positions have similar sized pore areas with diameters of about 1.3 μm at all ultrasonic amplitudes. In contrast, welds at antinode position have very small pore areas with diameters of about 0.2 mm at an amplitude of 2 μm and no pores at an amplitude of 4 μm. However, at an amplitude of 6 μm the pore area increases strongly to a diameter of about 1.0 mm. Commonly pores appear in the middle or bottom weld area, see Figure 5. In general, the pores have round shapes, because they contain gas. This gas can be residual keyhole-gas. Pore formation is modified by an ultrasonically modified melt flow. According to [11], welding at antinode position can course an ejection of melt and a V-shaped opening of the melt due to the strong dynamics. The opening supports the keyhole and promotes gas escapement from the melt. In contrast, during welding at the centred position the melt is moved to one single direction, as described in Section 4.1, crossing and disturbing the keyhole, which e ffects turbulences. Hence, the melt absorbs keyhole-gas resulting in many times bigger pores. Welding at nodal position neither foster a V-shaped weld seam collapse nor turbulences, but increases the pore area due to compressing the melt, which either closes the keyhole's middle part before the bottom keyhole gas can escape or promotes combining of gas bubbles and holding the gas in the metal resulting in pores. Acoustic cavitation could as well influence this process by inducing growth and shrinkage, which promotes bubble merging. Welds at nodal and centred position show similar pore areas, which can mean that the keyhole is disturbed only little and that acoustic cavitation has a big influence on increasing pores by merging little pores. Another possibility is that keyhole-disturbance and node-e ffects are of similar influence. For welds at antinode position there should be an optimal vibration amplitude depending on the welding speed to support the keyhole with no keyhole-disturbance by flow direction. At an amplitude of 6 μm big pores form due to a previously described strong melt flow directing mechanism for welds at antinode position.

The porosity shape changes with the position on the wave, see Figure 6. In the weld at nodal position at an amplitude of 4 μm a slightly rectangular-, compressed and collapsed-looking pores has been found due to node position compression. In the centred positioned weld with an amplitude of 4 μm elongated asymmetric pores are found due to the directed melt flow in centred position. At antinode position, the pores are rounder and smaller.

For further investigation of pore formation, the amount of equiaxed microstructure is evaluated visually regarding very low and high amount, see Figure 7. Without ultrasonic excitation there is a very low amount of equiaxed microstructure. With ultrasonic excitation the equiaxed microstructural amount is high for the welds at node and centred position In the micrographs of the welds at antinode position, the equiaxed microstructural amount is very low until an amplitude of 4 μm and becomes high with an amplitude of 6 μm.

**Figure 6.** Exemplary micrographs of metallographic cross sections of welding seams (Material 2.4856) regarding porosity shape.

**Figure 7.** Exemplary micrographs of metallographic cross sections of welding seams (Material 2.4856), the columnar solidification is indicated by lines on one half of the weld with indicated columnar solidification areas, root area.

The comparison between pore area and equiaxed microstructural amount, see Figures 6 and 7, reveals a relation. Equiaxially solidifying melt promotes mobility, combining and growing of gas bubbles. Therefore, gas bubbles are located in the equiaxed crystal core zone in the middle of the weld, see Figure 8.

**Figure 8.** Exemplary microstructural surrounding of pores.

Following mechanisms contribute to an equiaxial microstructure. For welds in centred position the induced melt flow crushes dendrites, which promotes heterogeneous nucleation. For welds at nodal position dendrites are crushed by nodal pressure, which again promotes heterogeneous nucleation. For welds at antinode position with amplitudes below 6 μm equiaxial solidification is reduced to a similar amount as without excitation because no dendrites are crushed and hence no heterogeneous nucleation is fostered. At an amplitude of 6 μm the equiaxed amount rises due to strongly enhanced directed melt flow, which crushes dendrites and supports heterogeneous nucleation. Additionally, welding at antinode position eliminates porosity by promoting the absorption of gas into the keyhole by the previously described V-shaped weld seam collapse.

## *4.3. Crack Formation*

Cracks appear in some specimens with welds in centred and nodal position beginning with amplitudes of 4 μm, see Figure 9. The cracks appear around the grain boundaries, which indicates, that those are hot cracks. They appear in the porosity containing equiaxed crystal core zone in the middle of the weld, because they originate from the segregation of alloying elements, which are mainly pushed ahead by the columnar crystal solidification front. As the equiaxed crystal core zone solidifies endogenously most segregated elements remain at the columnar-equiaxed interface and initiate hot cracks.

Cracking does not happen in welds in antinode position, because the amount of equiaxed microstructure is not increased for amplitude lower or equal than 4 μm. The low number of specimens including cracks does only allow assumptions, but cracks in welds in nodal position appear finer than those in welds in centred position and in general cracks become finer with increasing amplitude, because the melt is pressed together periodically and the segregations are pressed between the solidifying phases. With increasing amplitude at the antinode there is an increasing pressure at the node and the segregations are distributed over larger areas. In result, extremely fine cracks or detachments form, which result in severely weakened welds.

**Figure 9.** Micrographs of metallographic cross sections of specimens with cracks.

Scanning electron microscope (SEM)-investigations, see Figure 10, validate the hot crack assumption, since the grains are intact without sharp edges and the crack surfaces are coated by segregated melt. Energy dispersive X-ray spectroscopy (EDX)-analysis show only small differences in the chemical compositions of weld metal and crack surface because of the small segregation layer thickness. The niobium- and tantalum-contents increase at the crack surface from about 4 wt.% to 7 wt.% for niobium and from about 2 wt.% to 4 wt.% for tantalum; the nickel-content decreases from about 60 wt.% to 55 wt.%.

**Figure 10.** SEM-picture of a crack surface, centred position, ultrasonic amplitude: 4 μm.
