Florian Fetzer

Influence of the focal position on the melt flow during laser welding of steel

For keyhole laser welding of tempered steel, we show that the characteristic formation of eddies in the melt flow significantly depends on the focal position. The local melt flow velocities and accelerations were analyzed in-situ by means of x-ray imaging. It was observed that the keyhole geometry as well as the direction of rotation of the eddy close to the weld pool surface changes when the focal-position is shifted by one Rayleigh length.

Benefits of very high feed rates for laser beam welding of AlMgSi aluminum alloys

The present paper shows that laser beam welding of AA6016 aluminum sheets without filler wire but at high feed rates in the order of several tens of m/min is beneficial with respect to depth fluctuation, pores, and centerline cracks. Welds in the overlap configuration with a laser power of 16 kW (cw) at feed rates of 30 m/min and above were investigated. A comparably large laser beam diameter of 630 μm enabled welding at more than 50 m/min without humping. Full penetration and partial penetration welds were accomplished with depths of 2.4 and 2.0 mm, respectively, without pore formation. The partially penetrated welds show negligible local depth fluctuations in the range of only a few μm. Full penetration welds were performed at edge distances of 4, 5, and 6 mm without occurrence of centerline cracks. This constitutes a significant reduction of the critical zone for the formation of centerline cracks at the close edge position compared to conventional laser beam welding parameters.

TOWARDS MULTIPHYSICS SIMULATION OF DEEP PENETRATION LASER WELDING USING SMOOTHED PARTICLE HYDRODYNAMICS

Multiphysics simulations of deep penetration laser welding are performed with the meshless Lagrangian Smoothed Particle Hydrodynamics (SPH) method. Compared to meshbased 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 cosimulation 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. Haoyue Hu, Peter Eberhard, Florian Fetzer and Peter Berger

Observation of Laser Materials Processing by Means of High-Speed Imaging

High-speed imaging is a valuable tool for investigations on laser processes. The high temporal resolution of high-speed imaging allows a detailed observation of different laser processes which helps to understand process mechanisms like e.g. the formation of spatters during laser welding or the formation of a heat affected zone during laser cutting of carbon fiber reinforced plastics (CFRP). In the following the potential of high-speed imaging as a tool for laser process development is shown using different applications as an example. Initially it is described how high-quality images can be achieved during laser processing although the laser beam itself and process emissions make this a challenging task. Subsequently different applications like laser welding, laser drilling of metals with ultra-short pulsed lasers and laser processing of CFRP are introduced. During laser welding, the formation of spatters and hot cracks can be observed. Furthermore the influence of spatial beam modulation on the welding process can be investigated by means of high-speed imaging. The capillary dynamics during laser welding of metals can be studied using high-speed X-rays imaging while in transparent materials the capillary dynamics can be directly observed. During laser drilling the drilling process can be immediately seen using a suitable experimental setup. With the help of high-speed imaging it was revealed that the formation of a heat affected zone during laser processing of CFRP with ultra-short pulses is caused by heat accumulation. Furthermore the dynamics of process emissions like particles or hot vapor generated during laser processing of CFRP was investigated.

Comprehensive process monitoring for laser welding process optimization

Fundamental process monitoring is very helpful to detect defects formed during the complex interactions of capillary laser welding process. Beside the monitoring and diagnostics of laser welding process enlarges the process knowledge which is essential to prevent weld defects. Various studies on monitoring of laser welding processes of aluminum, copper and steel were performed. Coaxial analyses in real-time with inline coherent imaging and photodiode based measurements have been applied as well as off-axis thermography, spectroscopy, online X-Ray observation and highspeed imaging with 808 nm illumination wavelength. The presented diagnostics and monitoring methods were appropriate to study typical weld defects like pores, spatters and cracks. Using these diagnostics allows understanding the formation of such defects and developing strategies to prevent them.

Fundamental investigations on the spiking mechanism by means of laser beam welding of ice

In order to gain further understanding on the mechanism of rapid seam depth variations, the so-called spiking, in deep penetration laser beam welding, this effect is analyzed and explained using the example of laser welding of ice. Laser welding of ice provides the advantage that the temporal behavior of the vapor capillary can be analyzed by means of common imaging methods in the visible spectral range. The occurrence of spiking and its frequency is found to depend on the feed rate but is not influenced by instabilities of the rear wall of the capillary. Also, the effect of downward moving structures at the capillary front can be shown to be independent from root spiking. Spiking is found to be governed by a periodically alternating irradiation of the capillary front wall by the incident laser beam in the lower part of the capillary. This is verified by ray-tracing calculations on capillary geometries extracted from high-speed videos. These findings are further substantiated by a transient smoothed parti...

Pores in laser beam welding: generation mechanism and impact on the melt flow

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.

Comprehensive analysis of the capillary depth in deep penetration laser welding

Laser welding is the state of the art joining technology regarding productivity and thermal loads and stress on the workpiece. In deep penetration laser welding the quality of the resultant welds strongly depends on the stability of the capillary. The highly dynamic depth fluctuations are of major influence on the controllability of the laser welding process and on the prevention of weld defects. In the present paper the capillary dynamics is investigated by means of time- and spatially resolved in-process X-ray imaging and optical coherence tomography. The X-ray diagnostics allows measuring the geometry of the capillary with frame rates of 1 kHz, while the optical coherence tomography enables the determination of the capillary depth with an acquisition rate of up to 70 kHz. These measurements are correlated to time varying input laser power to provide profound insight in the dynamics of the laser welding process. The measurements are performed for copper, aluminum and mild steel. The capillary depth resulting from arbitrary laser power modulation was investigated. Thereby, the response of the capillary depth to laser power changes was determined. Based on these measurements the changes of the capillary depth in deep penetration laser welding were described by methods known from control theory. These analyses can be utilized to optimize control strategies, to calibrate transient simulations of deep penetration laser welding and to identify the influence of material properties.

Fast numerical method to predict the depth of laser welding

A simplified numerical method is presented that allows a fast estimation of the penetration depth during laser beam welding. The method is based on a physical heat conduction model embedded in an iteration scheme, which adapts the keyhole depth depending on an experimentally calibrated threshold condition that characterizes the temperature distribution on the surface of the vapor capillary. With simulation times lasting only in the order of minutes, the predicted penetration depths are in good agreement with experimental results.