Laser material processing procedures yield numerous benefits, as non-contact manipulation of the workpiece, high precision, and extensive automation capabilities. For metal joining, laser beam welding is a widely process employed in industry, for example in automotive body construction and in the production of electronic components. With the same optical setups as usually used for laser beam welding, a melt ejection can be induced in sheet metal, resulting in the formation of a cut. This enables laser remote fusion cutting (RFC) based on a ejection without the need for tools near the process zone. A comprehensive understanding of the conditions mechanisms causing the melt ejection or preventing it is yet to be achieved. In this study, in-situ observations of the process zone in RFC were performed using high-speed X-ray imaging synchrotron radiation, achieving frame rates up to 18 kHz for steel samples and 28 kHz for AlMg3 samples. features of the process zone morphology are extracted by means of image processing from the recordings, such the angle of front wall inclination or the ejection direction for different process parameters. The front wall angle for RFC is in line with an established model for the front wall angle in laser beam welding. Propagation-based phase-contrast imaging reveals the melt film at the cutting front, showing a decrease of the melt film thickness with increasing feed velocities. Melt dynamics at transitions between process states of cutting and not cutting could be observed. The temporal resolution was insufficient to capture humps in the melt film at the front wall steel samples. These were resolved for AlMg3 samples, confirming that humps at the front wall play an important role in the melt dynamics.

Laser beam microwelding is a precise technique for joining miniature metal components with high feed rates, which is crucial for productivity. However, high feed rates provoke humping formation-periodic beadlike protuberances along the weld seam-that compromise weld integrity. While humping has been associated with the keyhole transition from a narrow to an elongated shape using standard laser intensity distributions (e.g., Gaussian, top-hat), the impact of complex beam profiles, like ring-shaped intensity distributions, remains less understood. In this work, the influence of core-only, ring-only, and superimposed core-ring intensity distributions on humping formation during laser beam microwelding is investigated by means of synchrotron x-ray imaging. Single-track experiments on stainless steel (1.4404) at 1000 mm/s reveal that the keyhole geometry shifts from deep and narrow with core-only power input to shallow and elongated with ring-only power input. Using a superimposed core-ring intensity distribution (P-c = 300 W, P-r = 600 W) results in a U-shaped capillary and the reduction of the humping amplitude by nearly 80% (from 45.61 mu m with core-only to 10.29 mu m). The additional laser power comes with the tripling of the melt pool width (from 81 mu m with core-only to 263 mu m) likely decreasing the melt flow velocity. The reduced variability of the capillary length present for the superimposed intensity distribution further indicates a stabilized evaporation behavior. This work provides valuable insights into mitigating humping formation during laser beam microwelding of stainless steel at elevated feed rates using core-ring intensity distributions.

In laser beam fusion cutting of metals, the interaction of the gas jet with the melt determines the dynamics of the melt extrusion and the quality of the resulting cutting kerf. The gas-dynamic phenomena occurring during laser beam cutting are not fully known, especially regarding temporal fluctuations in the gas jet. The observation of gas and melt dynamics is difficult because the gas flow is not directly visible in video recordings and access to the process zone for observation is limited. In this study, the problem of imaging the gas jet from the cutting nozzle is addressed in a novel way by utilizing the striation pattern formed at the cutting kerf as a background pattern for background-oriented Schlieren imaging (BOS). In this first feasibility study, jets of different gas nozzles were observed in front of a solidified cutting kerf, which served as a background pattern for imaging. The results show that imaging of the characteristic shock diamonds of cutting nozzles is possible. Furthermore, the resulting shock fronts from an interaction of the gas jet with a model of a cutting front can be observed. The possibility of high-speed BOS with the proposed method is shown, which could be suitable to extend the knowledge of gas-dynamic phenomena in laser beam fusion cutting.

The topographical information of a weld seam bears information about quality relevant characteristics such as humping or spatter. Optical coherence tomography (OCT) can be used for inline scanning the weld topography coaxially mounted at a laser scanning optic. Feature extraction from this topographical information is challenging due to finding mathematical representations for the identification of relevant features. Feature extraction based on scalable hypothesis tests (FRESH) allows for feature extraction by a combination of various time series characterization methods. FRESHs feature selection is supported with an automatically configured hypothesis test and hence allows for quick extraction of significant features from sensing data in laser welding processes. In this work, a proof-of-concept is demonstrated for weld result categorization from OCT data by feature extraction using the FRESH algorithm. Changes in weld topography are characterized in a vast variety of process parameters for weld categories such as spatter, deep penetration welding, humping and heat conduction welding. As a result, a quantified separation of weld categories is possible and shows the feasibility of the FRESH algorithm for future quality assessments with different sensing technologies in laser welding.

Laser remote fusion cutting (RFC) offers a method for laser fusion cutting of metal sheets without need for a cutting gas. It is closely related to laser keyhole welding, which can transition into RFC for suitable parameters, such that the pressures in the process zone lead to a melt ejection on the bottom of the metal sheet. The mechanisms leading up to the melt expulsion are not fully understood yet and need further investigation. In this work, the cutting front is observed through a borosilicate glass sheet using a high-speed imaging system. In the setup, the glass sheet and a sample metal sheet are clamped together and the process zone is set directly at the interface between metal and glass. This setup allows for the observation of the melt dynamics adjacent to the glass sheet. In the experiments, the keyhole formation and the cutting front are investigated using a 6 kW disk laser and different feed velocities for cutting.

During laser keyhole welding of metal sheets, a process regime has been observed where no stable welding process sets in. Instead, material is ejected at the keyhole bottom, leading to the formation of a cutting kerf. The forces driving the melt ejection originate from the laser material interaction itself opposite to conventional fusion cutting, where an assist gas has to be used to enforce the melt ejection. As there is no need for a local application of auxiliary materials or gases, this holds the potential for a remote fusion cutting process. So far, only initial research exists concerning its occurrence and the underlying physical mechanisms are barely understood. Therefore, we use high-speed imaging of the cutting front and the melt ejection to generate insights into the melt dynamics. An evaluation approach is presented which incorporates image processing to enable the automated processing of large datasets, allowing a quantification of the observed effects and the evaluation of large-scaled studies of the melt behaviour for different parameters. The presented methodology enables the investigation of the mechanisms leading up to stable remote fusion cutting.