Abstract
Laser beam welding is a commonly used technology for joining similar and dissimilar materials. In order to improve the mechanical properties of the weld, the introduction of ultrasonic vibration into the weld zone has been proposed [5]. The ultrasonic system consists of an electronic control, a power supply, a piezoelectric converter and a sonotrode, which introduces the vibration into the weld zone. Its proper design is of great importance for the process performance. Furthermore, the effects of ultrasound in a melt pool need to be understood to evaluate and optimize the process parameters. In addition, it is important to find out the limits of ultrasonic excitation with respect to a maximum vibration amplitude. Therefore, firstly different methods of ultrasonic excitation are investigated and compared with respect to their performance. A system which is based on using longitudinal vibrations turns out to be the best alternative. Secondly, the system design is described in detail to understand the boundary conditions of the excitation and finally, simulations about the influence of ultrasonic vibrations are done by using a simplified model. The system is used to perform experiments, which aim at detecting the maximum vibration amplitude doing bead on plate welds of EN AW-6082 aluminum alloy. The experiments reveal a significant change of the weld shape with increasing ultrasonic amplitude, which matches the simulative findings. If the amplitudes are small, there is a marginal effect on the weld shape. If the amplitudes are high, melt is ejected and the weld shape is disturbed. In the present case, amplitudes over 4 µm were found to disturb the weld shape.
Highlights
Laser beam welding is a joining technology that uses energy emitted by a laser to weld materials
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
The experiments aim to evaluate the influence of the ultrasonic vibration amplitude on the resulting weld seam shape, the limits of the excitation amplitude, and the underlying mechanisms
Summary
Laser beam welding is a joining technology that uses energy emitted by a laser to weld materials. 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. 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. 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
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