**1. Introduction**

Nickel alloys with an addition of about 20% chromium show high corrosion resistance and high strength at elevated temperatures, so they are commonly used for high-temperature applications and in corrosive environments, as example for fan blades in turbines or for parts in chemical plants [1]. There are several trade names for this material group like Inconel, Chronin or Nicrofer.

Welding of highly alloyed nickel leads to similar problems as in welding stainless steel. Segregation is likely to occur due to a high amount of alloying elements. Those can form lately solidifying phases between the primary grains, and e ffect hot cracking [1]. In addition, segregated alloying elements are missing in the residual material, which changes its properties like corrosion resistance in the case of chromium [2].

Ultrasonic excitation of solidifying melt leads to mixing of the melt [1]. Hence, it can prevent segregation including its consequences and foster a finer grain structure, because dendrites break through mixing, which in turn creates more nucleation sites [3]. A finer grain structure improves strength as well as ductility and hardness [4]. In addition, porosity and hardness in the heat a ffected zone can be decreased and penetration depth can be increased for aluminium welds [5,6]. Another e ffect of ultrasonic excitation at high amplitudes is acoustic cavitation, which occurs in fluids and can be

divided into gas bubble cavitation and vapour bubble cavitation. In vapour bubble cavitation the fluid turns to gas when its pressure becomes low enough. This gas bubble implodes immediately, when the pressure rises su fficiently high for condensation. In result, liquid is drawn into the generated vacuum and a pressure shock of around 1010 Pa with temperatures of about 10,000 K occurs [7]. Close to solid interfaces a liquid micro jet, coursed by an imploded bubble, generates a stress peak on the interface and may lead to damage. In ultrasonically assisted welding, cavitation may happen in the melt pool and for a stationary wave cavitation is unlikely to occur in a melt pool at nodal position. Gas bubble cavitation occurs when gas is present in a liquid. The gas itself can form bigger bubbles when the pressure changes periodically by ultrasonic excitation or the bubbles can grow at the surface of particles. Such big bubbles slowly rise up by buoyancy, even if they shrink slightly when the ultrasonic pressure increases again. In ultrasonic laser beam welding the keyhole interaction with cavitation e ffect has to be examined [7].

The melt dynamics are also influenced by ultrasonic excitation due to the e ffect of acoustic streaming. Besides an induced oscillating motion, a time-dependent unidirectional flow can be induced [8], which suggests the possibility of e ffecting asymmetrical welds. Wu [8] gave a mathematical overview of how a fluid flow can be generated by ultrasound. He assumed that the flow force in a standing wave originates at the wave antinode. If the molten pool lies in this position, it is loaded equally, resulting in a symmetrical seam. The same applies if the molten pool is located in the wave node. Due to the same distance to the left and right wave antinode, cf. Figure 1, a symmetrical influence is also created. If, however, welding is carried out in an intermediate position, the distance to one wave antinode is smaller than to the other wave antinode. Wang et al. [9] investigated the influence of acoustic streaming on the solidification process of an Al2Cu alloy. They describe that the force *F*n emanating from acoustic streaming depends, among other things, on the distance to the antinode. It leads to the assumption that the asymmetrical weld seam is a result of the force di fference due to the unequal distances which causes a material flow to one side.

**Figure 1.** Influence of the wave-position on the weld seam.

Porosity can be influenced by the melt solidifaction morphology: Exogenous solidification originates from the melt border with growing columnar crystals and endogenous solidification happens in the melt volume with growing equiaxed crystals. At high supercooling of exogenous solidification also equiaxed crystals form in the melt volume. Equiaxed crystals can as well be promoted by small particles for heterogeneous nucleation. Gas bubbles can move through a mushy melt with equiaxially solidifying crystals, but in a columnar dendrite network they are trapped [10].

Ultrasonic excitation was applied in many cases only for welding steel and aluminium alloys. Zhou et al. [1] conducted pulsed laser welding with ultrasonic excitation on dissimilar welds of nickel-based Hastelloy C-276 with austenite stainless steel 304 in overlap configuration. No grain refinement occurred, but segregation was strongly reduced.

In previous simulations [11] and experiments on stainless steel and aluminium alloy suitable ultrasonic frequency range and vibration amplitudes were found, which will be tested on a nickel alloy. The e ffect of antinodal ultrasonic excitation on a liquid pool surrounded by solid material at its sides and air at its top was investigated. The liquid is pushed up at the solid wall sides and can be ejected at very high excitation levels. In result, a V-shaped weld seam collapse forms [11]. Experiments with similar setups showed that with laser beam welding sagging appears in the weld and the keyhole's bottom part can be closed by ultrasonic excitation [4,12]. The experiments in our contribution investigate the ultrasonic excitation e ffects on partial penetration laser welded nickel alloy round bars, see Table 1, to improve the weld properties and minimize pore and crack formation. The welds will be evaluated by visual inspection and metallographic cross sections.


