There are many reasons for influencing the laser beam induced weld pool. The manipulation of the dynamics, the solidification, the resulting grain size, and in the end, as a result of the aforementioned influencing, the mechanical characteristics of the weld is the aim of different attempts to gain an impact on the melt. Aluminum alloys tend to porosity formation because of different solubility of hydrogen in solid and liquid states. For reliable welds, the porosity has to be limited. An ultrasound excitation is one possibility to allow a fast degassing, especially for the considered round bars, for which no welding through is possible for geometric reasons. The presented research shows the influence of the ultrasonic amplitude on the microstructure of laser beam welded round bars of the aluminum alloy AA6082-T6. Furthermore, the position of the weld pool in the vibration distribution is varied and the influence evaluated. Metallographic cross sections show in analyses the resulting weld characteristic and the microstructure of the weld metal. The grain size and the grain orientation are evaluated for the different ultrasound parameters. Additionally, the summed porosity area is compared to acquire knowledge about the correlation between ultrasound excitation (with regard to vibration amplitude and position in the vibration distribution) and pore formation.

Laser beam brazing is an established manufacturing process due to its low heat input and esthetically appealing seams. However, brazing of materials with high oxygen affinity, such as aluminum alloys, requires the removal of surface oxides prior to the brazing process, commonly through the application of chemical fluxes that may be harmful to the environment and to health. The approach presented here dispenses with the use of fluxes and involves oxide layer removal by means of ns-pulsed laser radiation within an atmosphere that is adequate to an extreme high vacuum (XHV) in regard to the oxygen content. By doping the process gas with monosilane (SiH4), an oxygen content equivalent to an extreme high vacuum with an oxygen partial pressure below 10−20 mbar is realized. Hence, a subsequent reoxidation is actively prevented so that wetting of the base material by the filler material and consequent diffusion processes are enabled. The wetting angle between filler material and material is used to evaluate the effectiveness of laser-based deoxidation under an XHV-adequate atmosphere.

The risk of weld defects increases when laser beam welding of round bars is performed in a rotational process. The reason is heat accumulation, which changes process conditions. The analysis of weld pool shape and weld defects in the course of a weld seam is essential for being able to evaluate the overall weld quality and to set up control measures. This study focuses on laser beam welding of round bars with partial welds and various welding speeds. The experiments are carried out with 1.7035 round bars of 30 mm diameter. For partial welds, a laser beam power of 6 kW and welding speed of 1 m/min are used for welding paths of 1/4, 1/2, and 3/4 of the circumference. Welding with various speeds is conducted with 0.5, 1.0, and 1.5 m/min and a constant energy per unit length of 240 kJ/m. The specimens are evaluated by metallographic microsections and scanning acoustic microscopy. The investigations reveal three major weld defects resulting from a gradient in linear welding speed between the specimen surface and the center and from heat accumulation due to specimen geometry. Porosity and hot cracks form under the surface and the weld root bulges, which also result in hot cracks. The weld depth increases to its final weld depth after approximately 1/8 of the circumference.

No 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.