The grain structure of laser welded AlMgSi-alloys was varied by adapting the welding parameters and alloying additives. The hot cracking susceptibility was determined by the critical strain rate, measured by the application of digital image correlation in a self-restrained hot cracking test. A direct relation was found: The hot cracking susceptibility decreases with increasing number of grain boundaries across the width of the weld seam, i.e. along the effective load direction of transverse strain rates. It was shown, that the hot cracking susceptibility can be reduced by an optimisation of both the welding parameters and the alloying additives, which promote nucleation.
A combination of a cylindrical lens with a spherical lens was used to create an elliptical beam shape on the surface of the processed workpieces in order to increase the ablation efficiency producing linear grooves. Near-infrared 8 ps laser pulses with energies of up to 2.3 mJ were applied at a repetition rate of 300 kHz to ablate the grooves on the surface of the samples. The ablation rates obtained at different feed rates were examined. At a peak fluence of 4 J/cm2, an power specific ablation rate of almost 0.4 mm3/min/W was achieved for a beam with an ellipticity of 0.9955.
We present a model to predict the final depth of percussion-drilled holes that are produced with picosecond laser pulses in metals. It is based on the assumption that boreholes always have conical geometries when the drilling process terminates. We show that the model is valid for various process parameters when drilling in stainless steel. This was even confirmed by drilling with 3 mJ pulses, which resulted in a 10 mm deep borehole without thermal damage.
The investigation of capillary and melt pool dynamics in laser welding processes with conventional diagnostic equipment is very challenging. High-speed cameras in the visual and infrared spectrum are offering brilliant image quality and outstanding frame rates but they only access the behavior of the process surface. With these conventional techniques it is thus not possible to observe the key mechanisms inside the volume of the material which essentially determine the process.To gain insight into the process dynamics phenomena, such as the shape and movement of the capillary or the actual melt flow in the weld bead, X-ray videography is an ideal instrument. However, state of the art high-brightness laser processes require an X-ray system which is capable of imaging capillary diameters of about 100 µm at frame rates between 1,000 fps and 5,000 fps.This paper describes different experiments and their results, realized with the high-speed micro focus X-ray system of the IFSW that was presented and characterized in recent publications like [1] and [2]. The experiments include different materials like steel, aluminum and bronze, which were welded with focal diameters between 150 µm and 600 µm at feed rates of up to 30 m/min. For the visualization of the fluid dynamics, tracer materials were used in order to visualize melt flows with minimum falsification of the welding process.
One of the main factors influencing the dimensions of the welding process and its dynamics is the geometry of the keyhole. To optimize the welding results it is mandatory to understand the influence of the keyhole behavior on the weld quality and geometry. A better knowledge of these relations enables an improved parameter selection to achieve optimal welding results. However, it is nearly impossible to gain information on the keyhole geometry since it is located below the surface of the welding process and thus is not visible to conventional diagnostic methods.X-ray videography, in contrast, reveals this hidden information from the inside of the welding sample, namely the depth, the length and the orientation angles of the keyhole.The motivation for the experiments, described in the following, was to identify the actual cause for the differences in weld seams with different process parameters. This includes different joint configurations, welding speeds, and focal positions. To this end we observed the keyhole geometry by means of X-ray videography and compared it for butt-joint and bead-on-plate welds with different welding parameters.
