Peter Stritt

Analytical expressions for the threshold of deep-penetration laser welding

Explicit analytical expressions are derived based on a simplified model as a convenient estimation of the requirements to reach the threshold of deep-penetration laser welding. For materials with high heat conductivity and low surface tension the simple formulas allow determining the material-dependent minimum power required for deep-penetration laser welding as a function of the diameter and the travel speed of the beam on the work piece surface. Within this area of application of the model the derived formulas agree well with experimental results.

High-power laser sources enable high-quality laser welding of copper

Copper and copper alloys are more and more in demand for industrial applications due to their high electrical and thermal conductivity. However, the optical and thermal material properties make laser welding of copper a very challenging task. Due to the low absorptivity of only a few percent (< 10%) at the wavelength of about 1 µm and the high heat conductivity comparatively low feed rates of less than 10 m/min are needed to achieve welds with penetration depths of several millimeters using commercially available lasers with a few kW (< 6 kW) of power. In previous work it was shown that weld defects such as spatters, melt ejections and pores are very likely to occur within this parameter range. As these weld defects degrade both, the mechanical and the electrical properties of the weld seam it is important to minimize the number of such weld defects.In this paper the approach using a 16 kW disk laser to weld copper is discussed. The number of weld defects was analyzed serving as weld quality criteria. Welds were made at different laser power levels, feed rates and focal diameters and the resulting weld quality was compared. For every weld the thermal efficiency was calculated and identified as a key indicator of the reduction of weld defects.It is shown that above a thermal efficiency of about 35% welds in pure copper with reduced or even completely without weld defects were generated with penetration depth of up to 9 mm.Copper and copper alloys are more and more in demand for industrial applications due to their high electrical and thermal conductivity. However, the optical and thermal material properties make laser welding of copper a very challenging task. Due to the low absorptivity of only a few percent (< 10%) at the wavelength of about 1 µm and the high heat conductivity comparatively low feed rates of less than 10 m/min are needed to achieve welds with penetration depths of several millimeters using commercially available lasers with a few kW (< 6 kW) of power. In previous work it was shown that weld defects such as spatters, melt ejections and pores are very likely to occur within this parameter range. As these weld defects degrade both, the mechanical and the electrical properties of the weld seam it is important to minimize the number of such weld defects.In this paper the approach using a 16 kW disk laser to weld copper is discussed. The number of weld defects was analyzed serving as weld quality criteria. Wel...

New hot cracking criterion for laser welding in close-edge position

Hot cracking is one of the major issues in laser welding of high-strength aluminium alloys. The considered aluminium, magnesium and silicon based alloys (6xxx series) are highly crack sensitive due to a large solidification interval and little residual liquid between the dendrites.When welding of 2.7u2005mm thick alloy sheets, hot cracking mainly occurs for seams placed at a distance of about 3 to 6u2005mm from the edge of the work piece. The existing theories based on strain considerations do not explain why no longitudinal hot cracking is observed at shorter edge distances.In order to obtain a better scientific understanding of this experimental evidence, we performed a theoretical analysis based on a finite element model. High speed videos from experimental welding processes were used to calibrate this simulation model as well as to identify the location of hot crack formation during the solidification phase. The simulations were used to calculate the transient temperature distributions, the resulting deformations, and the stresses during welding with varying distances from the edge of the work piece.From this we derived changing shapes of the melt pool in close edge condition, indicating different solidification paths. Such analysis together with the common structural condition of positive strain at the trailing edge of solidification led to the finding of a new hot criterion for the formation of hot cracks.The criterion implies that positive strain combined with multidirectional solidification conditions is responsible for hot crack formation.Hot cracking is one of the major issues in laser welding of high-strength aluminium alloys. The considered aluminium, magnesium and silicon based alloys (6xxx series) are highly crack sensitive due to a large solidification interval and little residual liquid between the dendrites.When welding of 2.7u2005mm thick alloy sheets, hot cracking mainly occurs for seams placed at a distance of about 3 to 6u2005mm from the edge of the work piece. The existing theories based on strain considerations do not explain why no longitudinal hot cracking is observed at shorter edge distances.In order to obtain a better scientific understanding of this experimental evidence, we performed a theoretical analysis based on a finite element model. High speed videos from experimental welding processes were used to calibrate this simulation model as well as to identify the location of hot crack formation during the solidification phase. The simulations were used to calculate the transient temperature distributions, the resulting deformat...

Laser power modulation at the threshold from heat-conduction to deep-penetration welding

While the cw mode laser welding is commonly used for larger structural welds, pulsed welding is mainly used for smaller components, as it allows precise matching of the pulse properties with the desired melting and solidification conditions.In this paper the investigations focus on an alternative approach with a sinusoidal modulation of the cw laser power at the deep-penetration threshold. Thereby the lower laser power levels lead to heat-conduction welding whereas the higher levels result in deep-penetration welding. An interesting welding process is generated where the variation of modulation parameters such as frequency or amplitude leads to significant changes in welding depth and welding efficiency.Experiments were performed with a 5 kW disk laser on AlMgSi1. The process is monitored using a high-speed camera and a coaxial measurement of back-reflected laser radiation.The experiments clearly show that the resulting changes are caused by opening and closing mechanisms of the capillary. Both, the time constant of the power modulation and the energy input determine the characteristics of the process.While the cw mode laser welding is commonly used for larger structural welds, pulsed welding is mainly used for smaller components, as it allows precise matching of the pulse properties with the desired melting and solidification conditions.In this paper the investigations focus on an alternative approach with a sinusoidal modulation of the cw laser power at the deep-penetration threshold. Thereby the lower laser power levels lead to heat-conduction welding whereas the higher levels result in deep-penetration welding. An interesting welding process is generated where the variation of modulation parameters such as frequency or amplitude leads to significant changes in welding depth and welding efficiency.Experiments were performed with a 5 kW disk laser on AlMgSi1. The process is monitored using a high-speed camera and a coaxial measurement of back-reflected laser radiation.The experiments clearly show that the resulting changes are caused by opening and closing mechanisms of the capillary. Both, the time ...

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.

Comparing the amount of laser welding spatters resulting from different analyzing methods

Spatter formation during welding using 1u2005µm wavelength lasers is still a not satisfyingly solved problem. In most of the published results one single method to quantify the amount of spatters was used. In this paper the results of using different measurement methods to analyze the amount of generated spatters for the same weld will be presented.The amount of generated spatters was analyzed by applying different welding speeds. By comparing the results it will be shown that the different methods show different results for the same weld. The results show that the loss of weight due to spattering could be reduced but at the expense of a significant increase of the number of spatters and vice versa. The use of multiple measurement methods enables a complete evaluation of a welding process regarding spatter formation.Spatter formation during welding using 1u2005µm wavelength lasers is still a not satisfyingly solved problem. In most of the published results one single method to quantify the amount of spatters was used. In this paper the results of using different measurement methods to analyze the amount of generated spatters for the same weld will be presented.The amount of generated spatters was analyzed by applying different welding speeds. By comparing the results it will be shown that the different methods show different results for the same weld. The results show that the loss of weight due to spattering could be reduced but at the expense of a significant increase of the number of spatters and vice versa. The use of multiple measurement methods enables a complete evaluation of a welding process regarding spatter formation.

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.

Temporally resolved measurement of temperature gradients during power modulated laser welding of copper to aluminum

Temporally resolved temperatures and their gradients were measured in the weld pool during power modulated laser welding of copper to aluminum. A spectrometer and two pyrometers were used to measure temperatures with a high sampling rate of 5 kHz. When welding with a modulation frequency of the laser power of 50 Hz the measured temperature signals show periodic oscillations that correspond to the frequency of the laser power modulation.Temporally resolved temperatures and their gradients were measured in the weld pool during power modulated laser welding of copper to aluminum. A spectrometer and two pyrometers were used to measure temperatures with a high sampling rate of 5 kHz. When welding with a modulation frequency of the laser power of 50 Hz the measured temperature signals show periodic oscillations that correspond to the frequency of the laser power modulation.

Effects of different joining geometries on cracking susceptibility and process efficiency using multi-alloy aluminum

In the recent years, laser technology has been steadily growing in the field of car body fabrication. Typical laser welding applications are the joining of doors, door steps, floor groups, roof joints and hood parts. Since the introduction of the remote laser welding technology, flexible weld shapes are possible. This enables new space- and material-saving lightweight design.With this remote laser welding technology three different flange-reducing weld types were investigated analyzing their effect on cracking susceptibility and process efficiency: An overlap weld, a fillet weld, and a frontal edge weld. The results of these on-the-edge welds were compared with the state of the art welds positioned 10 mm away from the sheet edge. For the experiments a standard AA6xxx series alloy and a special multi-alloy which is known to be less crack sensitive were used.It is shown that combining frontal edge welding with multi-alloy aluminum enables high efficient and low crack sensitive welding processes.

Energiebilanzierung in frühen Phasen der Bauteilentwicklung

Steigende Rohstoffpreise und strenge Emissionsvorgaben fuhren in der Automobilindustrie zunehmend zum Einsatz leichterer Materialien. Die Werkstoffsubstitution ist dabei eine haufig angewandte Strategie zur Gewichtsreduktion. In diesem Beitrag wird eine Methode vorgestellt, welche die Werkstoffsubstitution in fruhen Phasen der Bauteilentwicklung anhand der Energieaufwendungen fur Herstellung und Nutzung nachhaltig bewertet. Die praxisnahe Umsetzung dieser Methode wird beispielhaft in Form eines Berechnungstools vorgestellt.