Peter Berger

Plasma effects during ablation and drilling using pulsed solid-state lasers

Plasma and vapor plumes generated by ultrashort laser pulses have been studied by various optical methods for both single pulse ablation as well as high-repetition rate drilling. Time-resolved shadow and resonance absorption photographs enable to determine the plume and vapor expansion behavior and, by means of an analytical shock wave model, allow to estimate an energy balance that can be refined by plasma transmission measurements. The results furthermore suggest that several types of laser-induced plasmas can be distinguished according to their origin: the material vapor plasma originating at the ablated surface even at moderate intensities, a breakdown plasma at increased power densities occurring in cold vapor or dust particles left from previous ablations during repetitively-pulsed processing and, finally, the optical breakdown in the pure atmosphere at high intensities. The latter also gives rise to nonlinear scattering phenomena resulting in a strong redistribution of the energy density in the beam profile.

Moving humps at the capillary front in laser welding

Welding is a widespread application in laser materials processing and the current development of laser systems with high beam quality helps to enhance the number of applications furthermore. However, the weld-pool dynamics is still a limiting factor leading to weld defects and it seems to be more important for lasers with high focusability and for radiation around 1 µm compared to the 10 µm radiation.In the time past a lot of investigations were performed to analyse the reasons for those instabilities: high speed imaging, X-ray analysis and also modelling. In addition to experiments with real workpieces (iron and aluminium alloys) a set of experiments with transparent materials such as water and ice were performed at IFSW in the last years.As a result, flow components of the melt parallel to the laser beam were identified to be a major reason of the unwanted behaviour of the melt, driven by humps at the capillary front. Although such humps were discovered in X-ray analyses already in the 1980s and described in some models, they were discussed relatively seldom.For this contribution, welding with tracer material in the specimen was used to identify typical flow patterns, whereas the “welding” in transparent material shows directly typical flow structures. Finally, calculations of the beam propagation within the capillary clarify interesting differences for different wavelengths and beam quality. In this context, also the role of the polarisation will be discussed.Welding is a widespread application in laser materials processing and the current development of laser systems with high beam quality helps to enhance the number of applications furthermore. However, the weld-pool dynamics is still a limiting factor leading to weld defects and it seems to be more important for lasers with high focusability and for radiation around 1 µm compared to the 10 µm radiation.In the time past a lot of investigations were performed to analyse the reasons for those instabilities: high speed imaging, X-ray analysis and also modelling. In addition to experiments with real workpieces (iron and aluminium alloys) a set of experiments with transparent materials such as water and ice were performed at IFSW in the last years.As a result, flow components of the melt parallel to the laser beam were identified to be a major reason of the unwanted behaviour of the melt, driven by humps at the capillary front. Although such humps were discovered in X-ray analyses already in the 1980s and describ...

Understanding the influence of the focal position in laser welding on spatter reduction

Particularly the automation and industrialization of thermal material processing with lasers of radiation of 1u2005µm makes high demands on the resulting welding quality. Indeed, current laser systems with strong focusability exhibit a high innovation potential in many application ranges, for example the possibility of adjusting the welding depth to even small material thicknesses. However, at such high focusability there are also disadvantages observed especially in laser welding of steel expressed in a strong spattering generated at the rear keyhole wall. This undesired instability mainly occurs by using lasers with radiation of 1u2005µm in contrast to CO2 lasers which is why it is essential to understand its fundamentals to push the industrial use of laser systems of 1u2005µm.In order to improve the spatter behavior, a sound knowledge of the interaction between the front keyhole wall inclination and the melt pool is required. In this study emphasis is laid on how to manipulate the inclination of the front keyhole wall by changing the focal position. In doing so, the keyhole formation is observed by a novel process monitoring system which allows determining the spatial inclination of the capillary. According to this, a displacement of the focal position below the surface yields a reduced number of spatters due to the steepening of the front keyhole wall. By using additional high-speed cameras it is possible to distinguish the main driving forces for spatter generation and to accent the importance of the inclination of the front keyhole wall.Particularly the automation and industrialization of thermal material processing with lasers of radiation of 1u2005µm makes high demands on the resulting welding quality. Indeed, current laser systems with strong focusability exhibit a high innovation potential in many application ranges, for example the possibility of adjusting the welding depth to even small material thicknesses. However, at such high focusability there are also disadvantages observed especially in laser welding of steel expressed in a strong spattering generated at the rear keyhole wall. This undesired instability mainly occurs by using lasers with radiation of 1u2005µm in contrast to CO2 lasers which is why it is essential to understand its fundamentals to push the industrial use of laser systems of 1u2005µm.In order to improve the spatter behavior, a sound knowledge of the interaction between the front keyhole wall inclination and the melt pool is required. In this study emphasis is laid on how to manipulate the inclination of the front keyhole ...

Laser welding of aluminum alloys: Problems, approaches for improvement and applications

Laser beam welding of aluminum alloys is rendered difficult by their specific material properties. On the other hand the productivity and quality of the laser process is very attractive for the manufacturing of light-weight constructions for e.g. all kinds of vehicles. In many years of research a fundamental understanding of the relevant technological and metallurgical mechanisms has been acquired. This allows to offer guidelines for a successful adaptation of the welding process to specific needs of applications. Factors relevant for deep penetration threshold, process efficiency and process stability will be discussed. The results achieved allow us to recommend the laser technique for series application, today.Laser beam welding of aluminum alloys is rendered difficult by their specific material properties. On the other hand the productivity and quality of the laser process is very attractive for the manufacturing of light-weight constructions for e.g. all kinds of vehicles. In many years of research a fundamental understanding of the relevant technological and metallurgical mechanisms has been acquired. This allows to offer guidelines for a successful adaptation of the welding process to specific needs of applications. Factors relevant for deep penetration threshold, process efficiency and process stability will be discussed. The results achieved allow us to recommend the laser technique for series application, today.

Process optimization of pulsed laser welding

Different diagnostic techniques were used to investigate melt pool instabilities and droplet generation during the welding process with pulsed lasers. To find the causes for these effects, an experimental setup was built that allows the simultaneous use of three different diagnostic techniques. The results of these experiments are discussed and serve as boundary conditions for a theoretical model that allows the time-dependant simulation of the keyhole formation. This model helps to understand the complex hydrodynamic mechanisms because of its capability of considering the influences of various parameters that are not directly accessible in the experiments.Different diagnostic techniques were used to investigate melt pool instabilities and droplet generation during the welding process with pulsed lasers. To find the causes for these effects, an experimental setup was built that allows the simultaneous use of three different diagnostic techniques. The results of these experiments are discussed and serve as boundary conditions for a theoretical model that allows the time-dependant simulation of the keyhole formation. This model helps to understand the complex hydrodynamic mechanisms because of its capability of considering the influences of various parameters that are not directly accessible in the experiments.

Detection and repairing of weld defects

There are different strategies to react on a detected weld defect. A very expensive one is to check the generated weld seam after welding and to put the part back into the clamping device for a further weld, if the joint does not fulfill the requirements. If a joint is vulnerable to defects, this procedure results in high costs. Therefore, there are different approaches to inspect the weld seam during the welding process in an in-process or post-process monitoring. The inspection systems reach from single sensors to camera-based systems. Coaxial systems are often preferred because of their high flexibility and small interfering contour. Most of these systems can decide if there is a defect or not, but can’t obtain the shape and type of the defect.In the framework of the WELDone project (funded by German Federal Ministry of Education and Research), the Institut fuer Strahlwerkzeuge developed a camera system that uses two images on one camera chip, resulting from process emissions with different polarization orientations. With the aid of a fast FPGA (field programmable gate array) the orientation of the hot workpiece surface is obtained on-line. With this information attributes of several different weld defects can be recognized. In principle, it is even possible to reconstruct the topology of the surface and not only detect a defect, but also characterize it.Since different defects require different treatments of the weld, the weld repair system requires a method to determine if there are defects and which type and size they have. The structures observed by the newly developed monitoring system and the measures taken after decision will be discussed exemplarily for two different defects in overlap welding. For the case of a hole, generated by melt ejection, the preferred method to close the hole and keep the sheets connected is to weld again with adjusted power. An undercut needs a different treatment. Here a defocusing yields best results.There are different strategies to react on a detected weld defect. A very expensive one is to check the generated weld seam after welding and to put the part back into the clamping device for a further weld, if the joint does not fulfill the requirements. If a joint is vulnerable to defects, this procedure results in high costs. Therefore, there are different approaches to inspect the weld seam during the welding process in an in-process or post-process monitoring. The inspection systems reach from single sensors to camera-based systems. Coaxial systems are often preferred because of their high flexibility and small interfering contour. Most of these systems can decide if there is a defect or not, but can’t obtain the shape and type of the defect.In the framework of the WELDone project (funded by German Federal Ministry of Education and Research), the Institut fuer Strahlwerkzeuge developed a camera system that uses two images on one camera chip, resulting from process emissions with different polarizatio...

Ivestigating the weld depth behaviour using different observation techniques: X-ray, inline coherent imageing and highspeed observation during welding ice

For a comprehensive understanding of the deep-penetration laser-welding process, it is of fundamental interest to understand the dynamic behavior of the capillary. The X-ray system of the Institut fuer Strahlwerkzeuge (IFSW) allows to record a two-dimensional projection of the capillary with frame rates up to 10 kHz.Hence the system is capable to gain information about the welding capillary, such as size and shape. However, the laser-welding process is a highly dynamic process with significant changes in time periods shorter than 0.1 ms.Queen’s University and Laser Depth Dynamics (LDD) have developed a sensor based on inline coherent imaging [1] which provides direct geometrical measurements of the keyhole depth and associated dynamics at rates >300 kHz with micron-scale precision. The technique is based on spectral domain low-coherence interferometry and is delivered through a camera port and combined co-axially with the process beam.The two measurement methods were set up in a combined experiment.The X-ray system recorded the shape and depth of the capillary with a spatial resolution of about 100 µm and a frame rate of 1 kHz, whereas the depth sensor from LDD provided 200 kHz at a single spot with axial resolution on the order of 10 µm.The present contribution compares the results of the two methods for steel and aluminum welds allowing new insights to the short-timescale behavior of the capillary.In order to understand the principal findings, the results were compared with high speed videos, taken during welding of the transparent material ice. Compared to the X-ray technique, a higher spatial resolution can be obtained at high repetition rates.At a first glance, it might be astonishing, that welding of ice can be compared with welding of metals. In a large number of experiments, however, we found that during welding of ice a lot of phenomena known from welding of metals (especially steel and aluminum) are also present during welding of ice, but can be observed much more clearly because of the low temperature and the transparency of the material.For a comprehensive understanding of the deep-penetration laser-welding process, it is of fundamental interest to understand the dynamic behavior of the capillary. The X-ray system of the Institut fuer Strahlwerkzeuge (IFSW) allows to record a two-dimensional projection of the capillary with frame rates up to 10 kHz.Hence the system is capable to gain information about the welding capillary, such as size and shape. However, the laser-welding process is a highly dynamic process with significant changes in time periods shorter than 0.1 ms.Queen’s University and Laser Depth Dynamics (LDD) have developed a sensor based on inline coherent imaging [1] which provides direct geometrical measurements of the keyhole depth and associated dynamics at rates >300 kHz with micron-scale precision. The technique is based on spectral domain low-coherence interferometry and is delivered through a camera port and combined co-axially with the process beam.The two measurement methods were set up in a combined experiment.The X-...

Heat accumulation in short-pulse laser processing of CFRP

Short-pulse laser processing is usually referred to as “cold” due to the very high intensities causing a large fraction of the processed material to evaporate. This minimizes the thermal interaction with the remaining material. However, very often recast layers and rims of molten material are observed in metal processing. In the case of CFRP processing much larger heat affected zones are observed than expected from pure heat conduction. Besides plasma energy redistribution and scattering the so-called heat accumulation is very often made responsible for such thermal effects.An analytical model was used to describe the process of heat accumulation. It allows estimating the amount of energy which does not contribute to the process itself and remains as heat in the material contributing to a time-averaged temperature increase. The model takes 3-D heat flow into account for different pulse repetition rates and beam profiles.Short-pulse laser processing is usually referred to as “cold” due to the very high intensities causing a large fraction of the processed material to evaporate. This minimizes the thermal interaction with the remaining material. However, very often recast layers and rims of molten material are observed in metal processing. In the case of CFRP processing much larger heat affected zones are observed than expected from pure heat conduction. Besides plasma energy redistribution and scattering the so-called heat accumulation is very often made responsible for such thermal effects.An analytical model was used to describe the process of heat accumulation. It allows estimating the amount of energy which does not contribute to the process itself and remains as heat in the material contributing to a time-averaged temperature increase. The model takes 3-D heat flow into account for different pulse repetition rates and beam profiles.

Novel monitoring system for spatially resolved topographical measurement of laser-based processes

In the last years, parallel to the introduction of laser systems with high focusability the demand for quantitative monitoring systems for laser material processing has increased. Indeed, current laser systems with strong focusability and wavelengths of about 1u2005µm exhibit a high innovation potential in many application ranges, for example the possibility of adjusting the welding depth to even small material thicknesses. However, the usability of these advantages is limited because the suitable process windows are considerably constricted at increased welding speed. In some applications even more than for lasers emitting radiation of 10u2005µm. Therefore, a reliable real-time monitoring of thermal material processing is of vital importance. A new promising approach is the exploitation of the polarization-dependent emission characteristics of hot radiating surfaces to get detailed information about geometrical surface structures.In addition to the dimensions of the melt pool, the raised welding bead or its underfill are important quality characteristics for industrial applications. Generally melt pool structures or seam imperfections result from the geometry and the dynamics of the capillary. This paper introduces a novel monitoring system to determine the three-dimensional keyhole or cutting front geometry based on the polarized thermal emission of the hot surface. Besides the possibility to ascertain the spatial inclination of capillaries, this sensor can be used to monitor melt pool structures or upcoming seam imperfections during laser material processing or to prevent the latter by an in-process control as well.In the last years, parallel to the introduction of laser systems with high focusability the demand for quantitative monitoring systems for laser material processing has increased. Indeed, current laser systems with strong focusability and wavelengths of about 1u2005µm exhibit a high innovation potential in many application ranges, for example the possibility of adjusting the welding depth to even small material thicknesses. However, the usability of these advantages is limited because the suitable process windows are considerably constricted at increased welding speed. In some applications even more than for lasers emitting radiation of 10u2005µm. Therefore, a reliable real-time monitoring of thermal material processing is of vital importance. A new promising approach is the exploitation of the polarization-dependent emission characteristics of hot radiating surfaces to get detailed information about geometrical surface structures.In addition to the dimensions of the melt pool, the raised welding bead or its unde...

Investigations on cw CO2-laser induced welding plasmas using differential interferometry

The stability of the deep penetration laser welding process is strongly influenced by the laser-induced plasma above the workpiece surface. The plasma plume can interact with the incident laser radiation due to absorption and refraction. The effect of these interactions leads to a spatially and temporally varying intensity distribution on the workpiece and, therefore, can influence the keyhole dynamics. In order to obtain first basic informations on the importance of the different mechanisms, investigations were carried out using a differential interferometer at two different wavelengths simultaneously to obtain the refractive index distribution. The interferometer allows investigations with variable sensitivity and, hence, enables to detect the large phase shifts of the probe laser light (shifts of some wavelengths are typical). In this first step the weld was performed in a chamber filled with shielding gas to avoid any influence of the flow-field induced by a nozzle. As a result of the interferometrical experiments the refractive index distribution within the plasma plume can be calculated at the two probe-laser wavelengths as long as partial symmetry can be assumed for the plasma. The laser-induced plumes at cw-laser welding show strong temporal fluctuations concerning the geometry of the plume and, therefore, complicate the evaluation of the refractive index distributions.The stability of the deep penetration laser welding process is strongly influenced by the laser-induced plasma above the workpiece surface. The plasma plume can interact with the incident laser radiation due to absorption and refraction. The effect of these interactions leads to a spatially and temporally varying intensity distribution on the workpiece and, therefore, can influence the keyhole dynamics. In order to obtain first basic informations on the importance of the different mechanisms, investigations were carried out using a differential interferometer at two different wavelengths simultaneously to obtain the refractive index distribution. The interferometer allows investigations with variable sensitivity and, hence, enables to detect the large phase shifts of the probe laser light (shifts of some wavelengths are typical). In this first step the weld was performed in a chamber filled with shielding gas to avoid any influence of the flow-field induced by a nozzle. As a result of the interferometrica...