Dennis Arntz

Advanced trim-cut technique to visualize melt flow dynamics inside laser cutting kerfs

Instabilities of the laser cutting front cause loss of quality due to the formation of striations on the cut flank. The mechanisms of striation generation during laser cutting are still not yet fully understood. To visualize the laser cutting process, the trim-cut technique was invented many years ago and is being continuously improved by different authors in the last years as well as in current studies. During a trim-cut, the laser beam is moved in-parallel along an existing sheet flank and cuts off a stripe of less than a standard kerf width. The cutting front and the transition to the resultant cut flank are directly visible. Without additional measures, the cutting gas jet expands during trimming in the half space freed by the lack of a second cutting edge. To maintain a guided supersonic gas jet path along the melt film and simultaneously enable the in-situ-observation of the cutting kerf with high-speed imaging, the missing cut flank is simulated by a transparent substrate, e.g., made of synthetic f...

Residual stress reduction of laser beam welds by use of low-transformation-temperature (LTT) filler materials in carbon manganese steels—In situ diagnostic: Image correlation

Low-Transformation-Temperature (LTT) materials are used for residual stress reduction in weld seams and, subsequently, for the prevention of distortion. Typically, LTT materials are highly alloyed Fe-based materials with levels of chromium and nickel that ensure that austenite transforms to martensite at reduced temperatures. The dilatation associated with the transformation is prevented by the surrounding material. Consequently, compressive stresses develop within the transformed region, and these counteract the accumulation of stresses due to thermal contraction. The precise chemistry of the weld metal determines the transformation temperature as well as the magnitude of the volume expansion. The chemical composition of the weld metal can, in turn, be influenced by the energy input during welding. In recent years, LTT-filler materials have been applied successfully in arc welding processes for residual stress reduction. Digital image correlation provides one avenue for the detection of surface displacements or deformations, as they occur during the cooling down process, in the vicinity of a weld. Observation of the movement of surfaces using a stereo camera setup allows the recording of displacements in three dimensions. Technically, this is achieved by identifying recognizable points on the surface under observation and by tracking their movement via comparisons between several time-shifted images. Visualization of surface contractions, as well as the effects of solid-state phase transformations during the joining process, especially during cooling, is key to understanding and influencing the residual stress state in welds as well to reducing distortion. In this study, the used installation setup, and the methodology for sample preparation, shows the production of information relating to surface displacements in the direct vicinity of the laser weld during the cooling down stage. Different welding parameters were investigated, resulting in different dilution levels for the LTT-Filler material, thereby influencing the distances to the weld pool and temperatures at which transformations took place. In order to provide a point of reference, comparable welds, made with conventional filler wires, were also investigated. The displacements after welding are always lower when using an LTT filler wire when compared to a conventional wire, proving that LTT wires can be used to mitigate distortion during laser beam welding.Low-Transformation-Temperature (LTT) materials are used for residual stress reduction in weld seams and, subsequently, for the prevention of distortion. Typically, LTT materials are highly alloyed Fe-based materials with levels of chromium and nickel that ensure that austenite transforms to martensite at reduced temperatures. The dilatation associated with the transformation is prevented by the surrounding material. Consequently, compressive stresses develop within the transformed region, and these counteract the accumulation of stresses due to thermal contraction. The precise chemistry of the weld metal determines the transformation temperature as well as the magnitude of the volume expansion. The chemical composition of the weld metal can, in turn, be influenced by the energy input during welding. In recent years, LTT-filler materials have been applied successfully in arc welding processes for residual stress reduction. Digital image correlation provides one avenue for the detection of surface displacem...

Temperature field and residual stress distribution for laser metal deposition

Laser metal deposition (LMD) process has been widely used in many industrial applications such as automotive, defense, aerospace, and so on. Modeling has to address physical phenomena like laser-powder interaction, heat transfer, fusion, solidification, and track formation. LMD induces a complex thermal stress field that results in residual stress distributions. Depending on its magnitude and nature (i.e., whether tensile or compressive), the residual stresses can cause unpredicted in-service failures. Therefore, the prediction of its distribution in the deposited structure as a function of the process strategy is essential to improve the process and the part quality. LMD represents mathematically a free boundary value problem. This means that the track geometry is part of the solution. The authors developed a three-dimensional time-dependent finite element model for LMD with coaxial powder feeding supply. The model encompasses the powder stream, its interaction with the laser radiation, and the melt pool computation. The model was validated by a comparison of the experimental and computed shapes of the melt pool surfaces concerning cross section, longitudinal section and high-speed photographs [Pirch et al., in Proceedings of the ICALEO Conference, 16–20 October 2016]. In this paper, the model was applied to overlapping tracks for single and multilayer processing for different process strategies. The simulation allows us to analyze the time- and space-resolved evolution of temperature and stresses. The influence of the powder feed rate on the residual stresses is investigated.Laser metal deposition (LMD) process has been widely used in many industrial applications such as automotive, defense, aerospace, and so on. Modeling has to address physical phenomena like laser-powder interaction, heat transfer, fusion, solidification, and track formation. LMD induces a complex thermal stress field that results in residual stress distributions. Depending on its magnitude and nature (i.e., whether tensile or compressive), the residual stresses can cause unpredicted in-service failures. Therefore, the prediction of its distribution in the deposited structure as a function of the process strategy is essential to improve the process and the part quality. LMD represents mathematically a free boundary value problem. This means that the track geometry is part of the solution. The authors developed a three-dimensional time-dependent finite element model for LMD with coaxial powder feeding supply. The model encompasses the powder stream, its interaction with the laser radiation, and the melt pool...

In situ visualization of multiple reflections on the cut flank during laser cutting with 1 μm wavelength

Instabilities of the laser cutting front cause loss of quality due to the formation of striations on the cut flank. The mechanisms of striation generation during laser cutting are still not yet fully understood. In this paper, the frequently disputed and not completely clarified effect of multiple reflections in the cut kerf during laser cutting with 1u2009μm wavelength is investigated during cutting of 6u2009mm thick stainless steel using a laser power of 5u2009kW and nitrogen assist gas. To visualize the laser cutting process, the trim-cut technique was invented many years ago and has been and still is continuously improved by different authors. During a trim-cut, the laser beam is moved in parallel along an existing sheet flank and cuts off a stripe of less than a standard kerf width. The cutting front and the transition to the resultant cut flank are directly visible. In order to maintain a kerf-guided supersonic gas jet along the melt film and simultaneously enable the in situ observation of the cutting kerf with high-speed imaging, the missing cut flank is substituted by a transparent substrate made of quartz glass. The existence and the effect of multiple reflections and their manipulation are demonstrated during laser cutting by means of an automated in situ trim-cut test bench with versatile control features. Depending on the focus position, the behavior of the melt flow and of laterally extended cutting fronts due to multiple reflections is visualized. Moreover, measurable results such as the profile of the cut flanks of trim-cuts and the corresponding real reference cuts are presented and discussed. The theoretical intensity distribution on the flank of trim-cuts is calculated taking into account multiple reflections during the beam propagation in the interaction zone and compared with corresponding high-speed videos.Instabilities of the laser cutting front cause loss of quality due to the formation of striations on the cut flank. The mechanisms of striation generation during laser cutting are still not yet fully understood. In this paper, the frequently disputed and not completely clarified effect of multiple reflections in the cut kerf during laser cutting with 1u2009μm wavelength is investigated during cutting of 6u2009mm thick stainless steel using a laser power of 5u2009kW and nitrogen assist gas. To visualize the laser cutting process, the trim-cut technique was invented many years ago and has been and still is continuously improved by different authors. During a trim-cut, the laser beam is moved in parallel along an existing sheet flank and cuts off a stripe of less than a standard kerf width. The cutting front and the transition to the resultant cut flank are directly visible. In order to maintain a kerf-guided supersonic gas jet along the melt film and simultaneously enable the in situ observation of the cutting kerf wit...