Conference contributions

Abstract

The depth progress and the final depth of V-shaped grooves are described by an analytical model for laser ablation of metals with ultrashort laser pulses. The model assumes that the fluence absorbed along the walls is distributed with a linear increase from the edge to the tip of the groove. The depth progress of the machined groove is recursively calculated based on the depth increments induced by successive scans of the laser beam along the groove. Experimental results agree well with the calculated predictions in the case of titanium alloy and tungsten carbide and for different pulse energies, repetition rates, scanning speeds, and number of scans. This confirms the validity of the model and its assumptions, and highlights the model as a useful tool for estimating groove dimensions, optimizing of process windows for machining with high depth progress, and predicting the maximum achievable groove depth.

Abstract

Due to the increasing product variety, flexible production facilities are required that can already work productively from batch-size 1 onwards. This requires flexible and online-capable quality assurance of the processes to be able to detect defective parts as soon as possible. One important quality feature in laser welding is the resulting hardness, which usually must be determined post-process. In this paper, a method for the spatially resolved online determination of the hardness is presented at the example of laser welding of mild steel. The dependence of the hardness on the cooling rate was utilized for this purpose. Therefore, the cooling curves were determined spatially resolved during the welding process using a scanning pyrometer and the heat loss coefficient was determined from these cooling curves as a measure of the cooling rate. The corresponding local hardness was determined post-process. The results show a correlation between the local hardness values and the local heat loss coefficients within a set of process parameters, making this a promising approach for the spatially resolved online determination of the hardness.

Abstract

Light-weight and function-oriented, complex metal parts can be generated by additive manufacturing processes. However, the characteristic, rough surface limits the applicability of these parts when a certain wettability is required. From literature it is known that the wettability mainly depends on the surface topography and surface chemistry. In our work, three consecutive steps were performed to modify the surface topography and surface chemistry of LPBF-generated parts made from Ti6Al4V: (1) Mechanical sanding and polishing smoothed the surface and removed powder particles from the surface. (2) Surface structuring with ultrashort laser pulses generated fine ripples or rougher spikes depending on the processing parameters. (3) Cleaning with ethanol or hexane and subsequent storage in air or butane modified the surface chemistry. As a result, durable wetting states with arbitrary contact angles between 0° (superhydrophilic) and 150° (superhydrophobic) were achieved for deionized water on LPBF-generated parts made from Ti6Al4V.

Abstract

3D measurements of the cutting fronts during laser fusion cutting were performed with and without beam oscillation. The used 3D measurement equipment offers up to 75.000 frames per second and a local resolution less 50 µm. This allows high-speed recording of dynamic changes at the cutting front geometry, followed by a three-dimensional reconstruction. Out of an extensive study was picked out a parameter set for 8 kW laser power and a feed rate of 4.5 m/min cutting 10 mm stainless steel (AISI 304) , compared to a cut with additional longitudinal beam oscillation by a frequency of 600 Hz and an amplitude of 0.15 mm. The large bright spots of common high speed videos can be interpreted as melt accumulations, pushed down by the gas pressure. They leave behind cold spots on the cutting front, which are very steep or even have an undercut. The influence of beam oscillation on the cutting front is also clearly visible. In the places where the laser beam is not currently located, the melt and the resulting structures appear to solidify. As a result, melt is not continuously ejected, but rather melt bursts are created with the oscillation frequency.

Abstract

In this paper electron beam welding (EBW) was investigated with a thermal emission-based goniometer setup, which was installed in an electron beam chamber. The local surface angles of the vapor capillary during EBW were determined for penetration welding of 5 mm stainless steel (AISI 304). The records were carried out with a frame rate of 75 kHz and a spatial resolution of about 30µm coaxially to the electron beam (EB). This allowed to visualize the influence of transverse EB oscillation on the capillary shape. At a frequency of 1000 Hz, the shape of the capillary front changes with the respective frequency and amplitude, which results in an asymmetrical capillary shape in the feed direction. At 10000 Hz a quasi static shape is formed.

Abstract

To achieve sustainable development in today's production facilities, quality assurance of manufacturing processes, such as laser welding is very important. In particular, online monitoring for direct detection of defective products is especially important. To develop a method for detecting pores during the process, this paper investigates the influence of pores on the temperature field during laser beam welding of AlMg3. Numerical simulations were carried out to theoretically investigate the influence of the pores in the weld seam on the temperature field and experiments were carried out to validate the simulations. The simulations show a clear influence of the pores on the temperature field, making the measurement of the temperature field suitable as a detection method. The detection of the pores from the measured thermal radiation turned out to be difficult because of the huge influence of the seam surface geometry on the emissivity. Especially for an automatic data evaluation, further research on sophisticated evaluation algorithms is required.

Abstract

In flexible production systems with distributed control systems which communicate with each other via a real time network, changes in the requirements for real time communication during operation must not lead to a temporary failure of the deterministic communication. Rather, the changes of the network configuration must become active at exactly predictable times in order to guarantee the functionality of the deterministic communication all the time. This paper shows a realization of how the configuration of a real time network schedule under Linux can be adjusted at predetermined times without interrupting the communication. For this purpose, the real time scheduler TAPRIO is integrated into the library libnl, and the performance of this extension is evaluated in a test case. It is shown that the modification of the network configuration is reliably possible in several successive communication cycles.

Abstract

Welding of AlMgSi-alloys of the 6000 series in T4 condition suffer from a local reduction of the hardness, i.e. local softening of the heat-affected zone and the weld zone. This local softening results in an inhomogeneous hardness distribution, the so-called metallurgical notch effect, which leads to a reduction of the tensile strength of the component. The paper presents an in-process method that allows local hardness recovery in the metallurgical notch effect area. A post-weld laser heat treatment process was designed, which reduces the process time to less than one minute. To achieve precipitation hardening in the zone of the metallurgical notch effect, the process parameters for the post-weld laser heat treatment were estimated with an analytical model. The benefit of the post-weld laser heat treatment strategy was demonstrated experimentally.

Abstract

Laser spot welding of copper with a penetration depth in the range of several millimeters has gained increasing attention due to the growing field of electromobility. Deep spot welds require the formation of a capillary and this again defines the processing time for each weld which is of particular interest for achieving high productivity. The capillary formation and the depth progress are influenced by the laser beam parameters. Spot welding with a laser power of up to 16 kW, a wavelength of 1030 nm and, beam diameters of 200 μm and 600 μm were investigated. High-speed X-ray imaging with a temporal resolution of 0.5 ms during the welding process was used to analyze the capillary depth progress. With the maximum power of 16 kW, a capillary depth of 4 mm was achieved in copper within 5 ms.

Abstract

For many applications, edge quality and shape accuracy of microholes are crucial. One assumption is that the fluence at the tip of the microhole during drilling is a key parameter for the quality of the microhole’s exit. It was therefore investigated experimentally how the fluence affects the edge quality. The experiments were performed in 0.5 mm thick steel using a Ti: sapphire laser system operating at a wavelength of 800 nm, a pulse duration of 1 ps, and a repetition rate of 1 kHz. For the quantitative analysis of the edge quality, microscope images were evaluated using a machine learning approach. Two key figures, striations amplitude and perimeter ratio, were defined that proved to be significant in characterizing the edge quality of exits. In the current talk it will be shown that the quality of exits of percussion-drilled microholes could be significantly improved if the fluence dependence is considered.

Abstract

The laser is the only tool that can address all six main manufacturing groups of the German standard DIN 8580 simply by applying different processing parameters. Given the present state of technology, however, different dedicated machine concepts are still being used for the respective applications. To enable digitalmanufacturing in its full consistency, novel, fully reconfigurable machines need to be developed. We therefore outline the further developments that are required to combine all presently known laser-based manufacturing processes on one and the same machine. As both the laser devices and the knowledge about the fundamentals of laser materials processing are already very advanced, research must now be intensified on system engineering. The vision is an intelligent machine, which is fed with CAD data, semi-finished products, or sub-components and that is capable to autonomously produce the desired components with a 100% quality guarantee at a batch size of 1 – “first time right” – and at the costs of comparable mass-produced items. The machine independently selects the best production strategy, i.e. the best combination and sequence of different manufacturing processes. Such a fully flexible and autonomous laser machine will not only boost the implementation of digitalmanufacturing on a broad scale and provide an approach to resolve the “polylemma of production” but will also enable the relocalization of value creation and manufacturing back into high-wage countries and by this potentially disrupt today’s globalized value creating networks.

Abstract

Laser Powder Bed Fusion (LPBF) is limited in the achievable accuracy, surface quality and structure size due to its inherent melting process. The achievable structure sizes are mainly dependent on the focal diameter and the grain size of the powder. Smaller structures, especially deep and narrow slits with a width below 100 µm, are still a major challenge. Combining continuous wave and ultrashort pulsed lasers in the same optical system enables consecutive additive and subtractive processes. This results in a quasi-simultaneous manufacturing process, where the emerging part can be precisely machined with ultrafast laser ablation after each additively added layer. In the talk the system technology used for the superposition of the lasers as well as the results of the combined additive and subtractive processes for the fabrication of deep and narrow slits in stainless steel parts will be shown.

Abstract

Additive manufacturing by means of laser-based powder bed fusion (LPBF) offers high flexibility with respect to the generation of individualized and light-weight metal parts. However, the produced parts are typically attached to support structures and deviate a few tens of micrometers from the targeted final component in geometrical net shape and surface roughness due to the melt-based fusion process. Therefore, different post-processing techniques were examined in the past to resolve the mentioned quality drawbacks. In our work, we investigated the potential of post-processing of LPBF-generated Ti6Al4V parts with ultrashort pulse laser ablation. As a result, the support structures were effectively removed, the surface roughness was reduced by 81% and complex geometries with high shape accuracy were fabricated. Furthermore, the LBPF-generated parts were laser surface structured to investigate the potential of post-processing with ultrashort laser pulses for advanced functionality, such as water-repellent surfaces. The generation of surface structures on the LPBF-generated Ti6Al4V part changed the wetting behaviour from hydrophilic to hydrophobic with an increased contact angle from 73° up to 130°.

Abstract

Recently, laser processing of silicon with ultrafast lasers has gained widespread attention for manufacturing of optics for THz radiation, an emerging topic with applications in medical imaging, security and communication. Such THz-optics require high-quality surfaces with low roughness in order to provide high transmission and low scattering. In the past, the low average power of ultrafast lasers limited the achievable throughput in silicon laser micromachining. In this work a processing strategy for high-quality high-throughput micromachining of silicon with a 1-kW sub-picosecond laser is presented, which takes benefit of pulse bursts, low fluences and high feed rates. As a result, laser micromachining could be demonstrated as a suitable technology for manufacturing of smooth structures on silicon while maintaining a high throughput. Surfaces with an appropriate roughness of Sa ≤ 0.6 μm were produced with a high material removal rate of 230 mm³/min and a machining depth of up to 313 μm.

Abstract

Recently, it was shown that oscillating the laser beam during laser beam cutting can increase the maximum cutting feed rate compared to cutting with a static beam. In order to investigate this phenomenon, the geometry of the laser cutting front was observed by means of online high-speed X-ray imaging. Fusion cutting of 10 mm thick samples of stainless steel was recorded with a framerate of 1000 Hz. When the beam was oscillated in longitudinal direction, the maximum cutting feed rate could be increased by 24% compared to cutting with a static laser beam. In addition, the global angle of incidence on the cutting front decreases with an increasing feed rate for both cases. When comparing the global angle of incidence of the two cases at the maximum feed rate for each case, it can be seen that the angle is smaller in case of cutting with an oscillated beam. As a consequence, the absorptivity increases by approximately 20%, which explains an increase of the maximum feed rate due to beam oscillation.

Abstract

Laser ablation with ultrashort pulses allows to create precise and flexible geometries on various materials. However, the generation of complex surface geometries with low surface roughness, high contour accuracy and defined depth still represents a challenge on porous and inhomogeneous materials such as additively manufactured parts or carbon fiberreinforced plastics (CFRP). In the present work, optical coherence tomography (OCT) was utilized for high-resolution optical distance measurement. The measurement beam of the OCT-based measurement system and the processing laser beam were superimposed with a dichroitic mirror. Combining both beams allowed online, time-resolved recording of the ablated depth during laser processing. The comparison of actual ablation depth with the target ablation depth was used to select areas that had to be processed in the subsequent pass. This closed-loop control of the ablation process was used to generate complex 3D geometries in stainless steel. Furthermore, the closed-loop controlled ablation was utilized for postprocessing of additively manufactured aluminum parts in order to remove support structures and to significantly reduce the surface roughness. Moreover, the OCT-based measurements allowed to determine the orientation of the fibers during controlled laser ablation of CFRP for layer-accurate laser ablation, which served as preparation for repairing damaged CFRP parts.

Abstract

In contrast to cw laser beams, the beam delivery of ultrafast laser beams with high average power or high pulse energies using optical fibers is still a challenge. In this paper, we present a free-space delivery and distribution system, which is able to deliver ultrafast laser beams with an average power of up to 5 kW and pulse energies of up to 5 mJ. This system is designed for automated switching between up to three laser sources and four processing stations with state of the art safety measures. Currently it is in use at the IFSW, with one kW class ultrafast laser source and three processing stations with a distance of up to 15 m from the laser source. Due to its active beam stabilization, high precision processes are possible, like Direct Laser Interference Patterning with structure periods below 1 µm, OCT based ablation control or micro drilling

Abstract

For some years now, three-dimensional numerical investigations have been used to gain insights into many phenomena of laser beam welding. However, two-dimensional investigations may be better suited to clarify basic interaction phenomena. The problem to be particularly emphasized here is welding at high speed and with relatively large capillaries. The influence of different parameters on the shape of the weld pool is shown, including the influence of different material properties at different Péclet numbers. Different flow conditions around and behind the capillary are also considered. As a result, it was found that at high Péclet numbers zones with high temperatures and low temperature gradients occur behind the capillary. These can destabilize the capillary back wall, which could explain the transition from a circular to an elongated capillary with increased welding speed.

Abstract

In this study, productivity optimization was investigated for laser based ablation processes with online depth control using a fast scan path generation algorithm. In ablation processes, a depth control system utilizing optical coherence tomography can be implemented to improve geometrical accuracy and reduce surface roughness. The actuating variable for the control loop is pulse energy, which in the simplest case involves switching laser pulses on and off along the initial scan path as required. However, every suppressed pulse contributes to a decrease in productivity, which is a critical criterion for industrial applications. For this reason, an approach was taken to deploy the scan path as an additional actuating variable. The developed algorithm is designed for improving the scan path during the process by omitting already finished areas. Theoretical verification with sample geometries shows a significant improvement in productivity compared to bare pulse toggling.

Abstract

Based on the assumption that laser-drilled microholes can be approximated by a cone the maximum depth as a function of the laser fluence can be predicted by a simple analytical model. In this contribution, it will be shown, that the calculated maximum depth agrees well to microholes in stainless steel with drilling depth > 1mm. A Ti: sapphire laser at a wavelength of 800 nm and a pulse duration of 1 ps was used to percussion drill microholes with pulse energies up to 5 mJ and the corresponding maximum drilling depth was determined. Due to the low repetition rate of 1 kHz heat accumulation effects could be excluded. Furthermore, the quality of the microhole exit was investigated as a function of the peak fluence in 1 mm stainless steel, including the formation of side channels and the development of the shape of the microhole exit.

Abstract

The generation of complex surface geometries with a defined depth, high contour accuracy, and low roughness in steel with ultra-short pulse laser ablation is still a challenge. In the present work, optical coherence tomography (OCT) providing high-resolution optical distance measurement of the ablation depth was combined with a Galvanometer-Scanner for fast beam deflection. The actual ablation depth was measured during processing. The online comparison with the target ablation depth was used to select areas that have to be processed in the subsequent pass. This selective material ablation allowed to smooth surface defects caused by inhomogeneous material removal. Furthermore, the selective material ablation was used to generate locally different ablation depths in order to create complex geometries. As a result, the surface roughness could be significantly reduced compared to the uncontrolled process, and precise ablation of complex geometries on steel was demonstrated.

Abstract

The influence of superimposed intensity distributions on the resulting melt flows, capillary shapes, and generated spatters during laser beam welding of mild steel was investigated using online high-speed X-ray imaging and visual high-speed imaging. The shape of the capillary changed for different superimposed intensity distributions and the size of the eddies in the melt pool changed for welds with different amounts of generated spatters. For a high number of generated spatters, a higher number of trajectories of the tracer particles leading to the upper part of the melt at the rear side of the capillary was observed, which was also the location of the generation of the spatters.

Abstract

The spatter formation was investigated in a model arrangement by means of high-speed imaging with a frame rate up to 10,000 fps. The process parameters laser power and feed rate were varied in order to adjust the line energy. The correlation of the spatter formation and the process parameters was investigated. Advantageous parameter ranges with significantly decreased spatter formation, quantified by the mean ejection angle and the number of spatters, will be presented in this proceeding.

Abstract

Direct Laser Interference Patterning (DLIP) is a laser processing method that provides a tool for creating a wide field of functional surfaces. In the present work, the generation of antibacterial surfaces on stainless steel is shown with a high power capable DLIP setup. The used laser was an ultrafast laser with a wavelength of 1030nm and a pulse duration of eight picoseconds. Two different topographies were produced, which were generated with two different polarization orientation of the laser. Both topographies were investigated in their antibacterial behavior. The employed method for assessing the bacterial retention is based on ISO standards for measurement of antibacterial performance. The resulting topographies shows a retention of up to 99,8% for E. Coli and up to 79.1% for S. aureus bacteria.

Abstract

We report on a flexible ultrafast thin-disk laser system delivering sub-1ps pulses at 1kW of average power. To enable high-throughput material processing a fast modulation scheme based on polarization-multiplexing and a burst-mode operation were implemented.

Abstract

The geometry of the cutting front was determined with a high temporal and spatial resolution during cutting of 10 mm thick stainless steel with a solid-state laser. The geometry was reconstructed using the information of the polarization state of the thermal radiation emitted from the process zone. This reconstruction of the geometry makes it possible to investigate the influence of process parameters on the geometry of the cutting front, including structures on the front and the global angle of inclination.

Abstract

We report on a novel laser concept employing a thin Ti:sapphire disk, which is symmetrically cooled by single-crystal diamond heat spreaders. Laser operation and a five times higher cooling capability than for a previously reported conventional thin-disk laser are presented.

Abstract

In laser beam welding, excessive evaporation leads to bulging of the capillary tip which results in the generation of metal vapor-filled bubbles in the melt pool. These bubbles either collapse and dissolve, or solidify and remain as pores in the weld seam, thereby degrading the quality of the weld seam. We investigated the mechanism of bubble formation and collapse in detail for laser beam welding of aluminum by means of online X-ray videography. Capillary shapes were reconstructed from these high-speed videos and correlated to two kinds of processes either sensitive to pore formation or unsusceptible to pore formation. We show that the fluid dynamics in the melt pool is strongly influenced by subsequent bulging and collapsing of bubbles. Its influence on the melt flow was quantified by analyzing the trajectories of tracer particles in the melt pool. We found that the generation and collapse of bubbles is a major driver of the dynamics in the melt pool. The melt is accelerated to velocities of up to several hundreds of millimeters per second by collapsing bubbles. Similar effects were found in laser beam irradiation of transparent media, such as ice and water, which allows to resolve the generation and collapse of capillary bulging with higher temporal and spatial resolution.

Abstract

The influence of superimposed intensity distributions on the number of generated spatters, the capillary shape, and the melt flows in the melt pool during laser welding of mild steel was investigated using online high-speed X-Ray imaging. A change of the shape of the capillary was observed for different superimposed intensity distributions. A change in the size of the eddies in the melt pool was observed for welds with different amounts of generated spatters and a concentration of the streamlines of the flow field at the upper rear part of the capillary was observed for a high number of generated spatters, coinciding with the location of the generation of the spatters.

Abstract

We analyze the requirements and demonstrate good beam quality transportation over several hundred meters in highly multimode step-index fibers. The beam quality factor is preserved over 100m (M2$\approx$1.3) and degrades linearly to M2$\approx$2.0 at 300m.

Abstract

The 3-dimensional shape of the processing surface can be reconstructed for typical laser applications like cutting, welding, drilling and hardening. In this paper laser beam cutting of stainless steel was investigated. The cutting front was reconstructed with high spatial and temporal resolution, taking benefit of the polarization dependence of the thermal emission. The reconstruction made it possible to resolve and measure structures, moving downwards at the cut front. The velocity was investigated with 75 000 frames per second and the resolution of the cutting front of 61 x 155 pixel. The velocity of the moving structures found to be approximately 10 m/s.

Abstract

We present compression of spectrally broadened output pulses from an Yb:YAG thin-disk multipass amplifier, obtaining pulses with peak power of about 10GW at sub-200fs pulse duration at an average output power of 400W.

Abstract

Laser welding of pure aluminum (Al99.5) to pure copper (Cu-OF) in overlap configuration was investigated. Spatial modulation of the laser beam perpendicular to the welding direction was applied with frequencies of up to 1 kHz and amplitudes of up to 1 mm. The mixing behavior, seam geometry, and averaged copper content of the welds could be controlled by the application of the spatial modulation of the laser beam. This allowed to produce aluminum to copper dissimilar welds with low electrical resistances of down to 1 μ.

Abstract

Laser welding of pure copper (Cu-OF) to aluminum (Al99.5) in overlap configuration was investigated. It is shown, that temporal modulation of the laser power reduces the electrical resistance by suppressing inherent instabilities in the welding process when using copper as the top sheet. The resistance could be reduced by about 12% down to 1.7 μ in this configuration. Spatial modulation of the laser beam allows to adjust the averaged copper content of the weld seam when using aluminum as the top sheet by controlling the width and depth of the junction. With this, aluminum to copper welds with low electrical resistance of 1 μOhm could be achieved.

Abstract

Periodic micro structures on surfaces offer unique properties, such as hydrophobic behavior, holographic light reflection or friction and wear minimization. Multi-beam interference patterning is a technology to produce micro structures from several micrometers down to below 1 µm, dependent on the wavelength and angle between the interfering beams. A high power capable four-beam interference setup, designed for infrared ultrashort pulsed laser sources in the picosecond regime will be shown. The setup provides independent variation of the period, intensity distribution and pattern size of the interference pattern. The period is variable from 1 µm with a working distance of 100 mm up to 5 µm with a working distance of about 800 mm. The intensity distribution can be modified to different shapes, such as lines, holes, ripples etc. by controlling the polarization of each beam separately. The fluence of the pattern is controllable by changing the pattern size. To change the pattern size, the beam diameter can be varied by a telescope with variable focal length. The setup is built in a stable design of a size of about 300x300x300 mm³ and a weight of 4.6 kg. To avoid aberrations at high laser power, especially focus shift, the setup was designed with high reflective mirrors. An experimental verification shows the comparison between the experimental results and the calculated design and features.

Abstract

Titanium doped sapphire (Ti:Al2O3) is a gain medium used in rod-type lasers with a very broad emission bandwidth exceeding 200 nm (FWHM) allowing the generation and amplification of very short pulses (<100 fs). On the other hand the thin-disk type lasers have proven to offer both power and energy scaling to a wide extent 1-3. In this talk we present results on power scaling of a Ti:sapphire thin-disk oscillator in multimode operation. Due to the small gain of Ti:sapphire compared to Yb:YAG in cw-pumping and the temperature-dependent fractional heat, the task of building an efficient oscillator is very challenging. For the experiments presented here we used an a-cut crystal with a thickness of 260 µm and an absorption coefficient of 3.9±0.6 cm−1 for the pump wavelength of 532 nm. In order to increase the absorption of the pump light we modified the standard multi-pass pump optics from 24 pump light passes to 48 passes.

Abstract

We report on a SESAM-modelocked Yb:CaF2 thin-disk laser delivering an average output power of 17.8 W at 10 MHz repetition rate with 285 fs pulse duration i.e. pulse energy of 1.78 µJ and peak power of 5.5 MW. Furthermore, we show results on chirped pulse amplification using Single Crystal Fiber.

Abstract

We present a SESAM-modelocked Yb:CaF2 thin-disk laser designed for pulse durations below 300 fs and high peak powers of more than 5 MW. A cavity for fundamental mode operation (beam quality factor M2 below 1.2) was set up and modelocked using a SESAM. An average output power of up to 17.8 W was obtained at a repetition-rate of 10 MHz, corresponding to a pulse energy of approximately 1.8 µJ. The pulse duration was measured to be 285 fs, therefore a peak power of 5.5 MW was attained. Our research enables a comparison of the potential of Yb:CaF2 with other Yb-doped crystals with broad gain bandwidth in thin-disk laser technology. We conclude from our results that this gain material is very promising for high pulse energies and high peak powers.

Abstract

Multiphysics simulations of deep penetration laser welding are performed with the meshless Lagrangian Smoothed Particle Hydrodynamics (SPH) method. Compared to mesh-based methods, SPH has advantages in handling phase transitions, free-surface melt flow, and fluid-structure interaction. Based on previous work on simulating conduction mode laser welding using SPH, the numerical model is extended to include further physical effects such as evaporation and exertion of recoil pressure on the melt due to evaporation. Particular emphasis is placed on modeling the energy input through the laser beam. A co-simulation approach is developed by coupling an SPH code with a ray tracer that tracks the propagation of the laser beam in the keyhole in order to achieve spatial distributions of energy transferred to the melt layer. A surface detection and reconstruction algorithm is implemented to exchange current surface data. Simulation results of spot welding and seam welding are shown using this co-simulation approach. The developed model serves as a basis to investigate the influence and sensitivity of process parameters on the weld and to better understand transient effects around the keyhole leading to weld imperfections.

Abstract

We report, for the first time, on the generation, in a highly multimode gradient-index fiber, of supercontinuum (from visible to near infrared) with up to 54 mW of average power and emitting in LP0n modes.

Abstract

The crosstalk effect considerably limits the capability of holography-based modal wavefront sensing (HMWS) when measuring wavefronts with large aberrations. In this contribution, we introduce a curvature-based measurement technique into HMWS to extend the dynamic range and the sensitivity of HMWS via a compact holographic design. If the input aberrations are large, the dominating aberration modes are first detected via curvature sensing and compensated using a wavefront correcting device, e.g. a membrane mirror. The system then switches to HMWS to obtain better sensitivity and accuracy with reduced aberrations. Different approaches for the reconstruction of the wavefront have been tested and extensive simulations for different aberrations have been analyzed.

Abstract

To enable the direct-spinning process of super-micro fibres (< 0.5 dtex) suitable for novel medical, hygienical and technical products microhole arrays with diameters down to 25 μm in very high quality are required. Using ultrashort pulses together with a helical drilling optics microholes with high accuracy were manufactured in metals of a thickness in the range of 0.3 mm. However, the required process time for a single microhole ranges up to several ten seconds. Simple energy balance considerations show that higher averaged powers - either achieved with larger pulse energies or an increased repetition rate - considerable reduce the process time. In this case plasma formation and heat accumulation show an increased formation of melt and recast. Thus, the objective is to increase the productivity while maintaining consistent quality of the microholes. With this aim, the influence of pulse energy and repetition rate on the borehole geometry, processing quality and process efficiency was investigated for helical drilling. In the present research work a TruMicro 5250 laser source (tp = 8 ps, λ=515 nm, fR=800 kHz) was used. To determine the process time of the microhole the transmitted laser radiation was recorded. A systematic evaluation of the process quality and process time dependent on pulse energy and repetition rate will be presented in this contribution. First laser manufactured spinning nozzles with microhole diameters down to 25 μm processed in 0.24 mm thick AuPt alloy were used to fabricate unique super-micro fibres with yarn counts down to 0.2 dtex.

Abstract

We demonstrate an Yb:YAG single crystal fiber laser with 251 W output power and 44 \% efficiency. Oscillator performance shows potential power amplification from 40 W to 200 W in a double pass configuration.

Abstract

Particularly the automation and industrialization of thermal material processing with lasers of radiation of 1 µm 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 1 µm in contrast to CO 2 lasers which is why it is essential to understand its fundamentals to push the industrial use of laser systems of 1 µm. 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.

Abstract

Theoretical investigations on the generation and propagation of density perturbations have been performed using a two-dimensional unsteady finite-difference method. In comparing typical results for CO2 and excimer lasers it becomes evident that both the formation and the physical impacts of the perturbations differ substantially. At equal values of deposited energy density, the relative density variation is more than an order of magnitude higher in the CO2 laser, whereas, due to their different wave lengths, the resulting phase shift is about the same in both systems. In the excimer laser, the beam quality will be detracted by only the perturbations of the preceding pulses, whereas, in the CO2 laser, the density change due to the pulse under consideration itself additionally leads to phase distortions.