It has been shown that the mechanical properties of the welds can be effected by superimposed vibrations. Campbell [
2] and Jose et al. [
3] have listed proven effects of vibrations on solidification processes. These include an improvement of the breaking strength, the ultimate tensile strength, the yield strength, and the hardness. All these improvements are caused by finer grains and a more uniform grain distribution. For laser beam welding, a grain refinement can be achieved possibly by acoustic cavitation inside the melt pool, which is stimulated by the ultrasonic vibration, if its amplitude is strong enough. Cavitation goes hand in hand with strong local pressure fluctuations in the melt. The dynamic pressure change effects small, hollow, high-energy bubbles. At sufficiently high pressure amplitudes, the bubbles implode and release their energy, which effects very high local temperatures and strong local streams as a result of shock waves. Jiang et al. [
4] investigated the effects of the excitation frequency on the microstructure, the mechanical properties, and the fracture behavior of A356 aluminum alloy obtained by expendable pattern shell casting. They found that the grain size decreased with increasing frequency. Additionally, the tensile strength, yield strength, elongation, and hardness increased due to the vibration. Tewari [
5] investigated the influence of transverse vibrations at frequencies between 0 and 400 Hz and vibration amplitudes between 0 and 40 µm on an arc welding process. The study concluded that grain refinement is caused by dendrite fragmentation. The influence of the ultrasonic amplitude on the weld seam properties has been investigated in [
6]. The extent to which local excitation influences the weld seam was also investigated. The local excitation depends on the excited vibration shape of the system and, in particular, on the relative position of the weld with respect to the vibration nodes and antinodes of the system. In [
6], the situation where the melt bath is in a vibration anti-node (which means that the entire melt bath is shaken) was compared with the situation where the melt bath is in a vibration node (which means that the borders of the melt bath are expanded and compressed). The ultrasonic frequency used was 20 kHz. It was found that higher vibration amplitudes and an excitation near the vibration anti-node leads to the best grain refinement. Since high frequencies and high amplitudes lead to high pressures, it seems reasonable to excite the melt with higher frequencies, e.g. ultrasonic vibrations [
2]. Zhoua et al. [
7] excited nickel-based alloy and austenite stainless steel ultrasonically during a laser beam welding process. The excitation direction was parallel to the laser. They concluded that the excitation leads to better melt mixing in the melt pool. Krajewski et al. [
8] investigated the influence of ultrasonic vibrations on an arc-welding process of aluminum alloys. The arc-welding process was used to create a weld seam on ultrasonically excited workpieces that were attached directly to the transducer of the ultrasonic system. Grain refinement and a reduction of porosity were achieved. Furthermore, no mechanical decohesion of the weld-face occurred when welding in a vibration node. The vibration had a negative influence on the quality, when the weld was at a vibration anti-node. A limitation, however, existed in the experimental setups: the samples were usually screwed to the transducer and the position in the vibration distribution was changing during the processes [
10]. To overcome this problem, the developed system is capable of butt welding round samples at approximately 20 kHz with an adaptable welding position in the vibration distribution at different ultrasonic vibration amplitudes. Different excitation methods are discussed in this paper. In a previous work [
9], it was shown that a similar setup, a longitudinal ultrasonic vibration system with a small water filled fluid container in a vibration anti-node, leads to high fluid dynamics. Another limitation in the previous studies concerns the restrictions of the excitation level. The amplitudes used in the experiments were rather small. However, since higher amplitudes have a higher influence on the microstructure, a high excitation amplitude is of major interest. This, however, leads to the question if there is a critical excitation amplitude leading to lower qualities. Additionally, the excitation can lead to a change in the weld seam shape. In [
10], the change of the bead geometry in laser beam welds was investigated. A change was observed; but the experiments had been limited to low power ultrasound and a small welding depth.
The present contribution focuses on both finding the limits of the excitation amplitude and understanding the dynamic processes in the molten pool while doing bead on plate welds of an aluminum alloy. The resulting seam geometry for vibration amplitudes of up to 8 µm is investigated experimentally and the observed effects are discussed with the support of a simplified fluid dynamics model of the melt pool.