Laser material processing

Laser beam welding is widely used in industrial processes and offers significant advantages over competing processes due to its high energy density and non-contact energy input. The work of the Production Technology Group focuses on process engineering, systems engineering, and materials science to develop innovative approaches to increase processing speed and weld quality, as well as novel strategies for processing hybrid material combinations.

The system engineering comprises hardware and software components for the execution of laser beam processes, taking into account the manipulation of the laser beam tool as well as the workpiece to be processed. Examples include the development of optics (in cooperation with the Optical Engineering Group  Technical Optics), remote laser beam processes and new approaches to fixture-integrated sensors and actuators. These enable, among other things, the development of adaptive clamping devices that allow in-process data acquisition and time-dependent adaptation of the process conditions to control the workpiece position.

Thermal joining of metals with thermoplastics is a novel process that can be performed by laser and resistance joining. Applications range from the automotive industry to mechanical engineering and white goods. Thermal joining enables the direct joining of a hybrid composite without additional joining elements or adhesives and allows the simultaneous use of both materials for the realization of optimized component structures. The focus of the work is on the link between process design and the resulting material properties, aging and fatigue behavior, as well as fundamental considerations of the interface and bonding mechanism. 

The analysis of the laser beam process with respect to the keyhole behavior, the interaction between keyhole and melt pool, and the resulting seam imperfections, e.g. spatter formation, is a major focus of our work. Numerous industrial applications have motivated the identification of relevant mechanisms as well as their targeted manipulation in order to improve the weld quality while increasing the processing speed. The use of suitable hardware, e.g. high-speed cameras, and the continuous development of specially developed software packages allow the description and quantification of the phenomena.

Mixed metallic compounds consist of materials that have limited or no suitability for welding, but whose different properties, when combined, offer numerous advantages in the fields of lightweight construction, electromobility and other research areas. Examples include aluminum-copper, aluminum-titanium, and steel-aluminum joints. Research activities focus on the modification of processes and system technology, on the one hand for adapted temperature control and control of diffusion between the materials involved, and on the other hand with regard to the implementation of control strategies in pulsed laser beam welding.

The laser beam-material interaction and the resulting temperature-time profile are largely determined by the intensity distribution of the laser beam on the workpiece, especially in transient laser material processing. Process optimization by adapting the intensity distribution to the material characteristics and the required process conditions has great potential for many laser processes. The adaptation to the process is done by empirical tests as well as by numerical simulation. In close cooperation with the Optical Engineering Group Technical Optics), the developed intensity distributions are implemented and tested in specially developed processing optics, e.g. by using diffractive optical elements.

The use of shielding gases enables the welding of materials that must be shielded from atmospheric oxygen in order to ensure the desired material properties. The focus here is on the entire flow path from shielding gas supply to atmosphere to welding process and clamping elements, which is represented in simulations and experiments using methods such as the schlieren technique and particle imaging velocimetry. In addition, the shielding gas supply can also make a significant contribution to process stabilization, e.g. to increase welding speeds for materials susceptible to spatter formation.

The interaction between the process and the metallurgy can be influenced by adjusting the cooling conditions and the melt convection. The temperature-time profile and the effective strains can be specifically influenced by pulse modulation in the laser welding process and by the use of adapted intensity distributions. These strategies allow, among other things, hot crack-free welding of aluminum alloys (EN AW 6xxx) without filler material or laser cladding of nickel-based superalloys without pre-heating of the base material.

Precise material processing at the micro level with highly brilliant laser beam sources, for example for laser beam welding of thin metal foils (foil thickness < 25 µm), surface processing as well as surface structuring forms a further research field with the aim of maximizing the processing quality, the reproducibility of the processes and the processing speed while at the same time locally limiting the thermal influence of the process.

The simulation of laser material processing processes depends on several influencing variables, including the intensity distribution of the laser beam and the material, which significantly affect the processing result.The use of modeling and simulation of material processing provides a deep understanding of the process and ranges from analytical considerations to numerical simulation of thermomechanical models to multiphase process simulation of laser beam deep penetration welding. 

On the fundamental research side, manipulation of the ambient conditions provides further opportunities to expand the understanding of the process with respect to the formation of weld defects and the characterization of influencing variables, e.g., surface tension. In addition to conducting studies under reduced ambient pressure or vacuum, it is also possible to selectively adjust atmospheric compositions while simultaneously measuring residual oxygen content and pressure.

On the one hand, the intensity distribution of the laser beam can be specifically manipulated by optical elements; on the other hand, deviations, e.g. a focus shift, often occur in practice due to contamination or the effect of the thermal lens. Precise knowledge of the beam profile, power distribution and power densities is essential for consistent processing results. In addition to devices for caustic and power measurement of continuously operated beam sources, there is also the possibility of time-resolved power measurement of pulsed laser beam sources.