*2.2. Ultrasonic System*

The specimens, as part of the ultrasonic system, are excited to resonant longitudinal oscillations in the ultrasonic range during the welding process. For this purpose, the specimens are pre-stressed in the ultrasonic system between two adapters, see Figure 3a. Beside the specimens the system consist of adapters as well as boosters and bearing boosters both amplifying the vibration amplitude of the ultrasonic transducer up to the joining point. The vibration amplitude is adjusted between 0 μm and 10 μm in the antinode (closest one to the weld), depending on the current amplitude set. Since at resonance the current amplitude is proportional to the vibration amplitude. The ultrasonic transducer (built by IDS, Institute of Dynamics and Vibration Research, Garbsen, Germany) is driven in the systems resonance frequency (7. Longitudinal mode) at approximately 20 kHz. The control of the phase between current and voltage of the transducer guaranties the stable operation at resonance as well as the control of the current amplitude keeps the vibration amplitude, at the closest anti-node to the weld, steady. Both is carried out utilizing a control unit, the DPC 500/100 [14]. With di fferent adapters, the joint can be placed at any point of the amplitude distribution. Three significant positions are defined for the investigations. In the antinode position, the vibration amplitude has the highest value, see Figure 3b, (this value is always used to describe the amplitude) and the mechanical stress amplitude is minimal. If the joint is placed within the node position, the vibration amplitude becomes zero and the mechanical stress amplitude has its local maximum. In addition, the joint is placed exactly between the antinode and the node of the wave in the centered position. Both vibration amplitude and mechanical stress have significant amplitudes here. The system is preloaded hydraulically. This preloading force is selected to be high enough, that the components cannot clatter during the excited longitudinal vibration. It is selected to be 120 kN, based on a specimen diameter of 30 mm.

**Figure 3.** Ultrasonic system for laser beam welding of round bars with vibration distribution (**a**) and selected positions for welding tests (**b**).

## **3. Experimental Procedure**

The specimen are round bars with a diameter of 30 mm and length of 30 mm made of the nickel alloy 2.4856. Those are used for all bead on plate welds. For joining, the optics are angled by 20◦. The laser focus point is adjusted to be 4 mm in the sample and 6 mm backwards over the specimen's surface from the angular point. This orientation of the laser spot avoids the formation of melt drops on the specimen's surface resulting from the weight of the melt while rotating the round bars. A laser beam power of 6 kW and a welding speed of 1.00 m/min are used. The focal lense (welding optics: BEO D70, Trumpf, Ditzingen, Germany) is protected by a crossjet and two flat nozzles provide argon as shielding gas with a pressure of 6 bar and a flow rate of 60 L/min at an angle of 45◦. The flat nozzles

are positioned with 50 mm distance to each other and to the specimen. One nozzle is aiming at the specimen bottom and the other one is aiming above the specimen. According to the test plan (see Table 3), for an ultrasonic amplitude of 6 μm only one welding test per wave position is conducted for testing the e ffects of excessive excitation, which e ffects spatter. The applied parameters were determined by previous experiments [4,12].


**Table 3.** Test plan for ultrasound assisted laser beam welding of nickel-base alloy round bars.

Metallographic cross sections of each specimen are prepared. The etching is conducted with Adler's etchant until the microstructure appears. The weld width, weld depth, weld metal area and pore area are identified and evaluated. In addition, scanning electron microscopy (SEM)-investigations including energy dispersive X-ray spectroscopy (EDX)-analysis (Seifert ISOVOLT 320, 80 kV, 20 mA, 3.6 min, Rich. Seifert & Co., Ahrensburg, Germany) are used for analyzing cracks and chemical composition.
