Rudolf Weber

University of Stuttgart

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i-10 index

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Co-Authors

Thomas Graf

University of Stuttgart

199

Christian Hagenlocher

University of Stuttgart

Volkher Onuseit

University of Stuttgart

Christian Freitag

University of Stuttgart

Florian Fetzer

University of Tübingen

H. P. Weber

University of Bern

Daniel Holder

Stuttgart University of Applied Sciences

Andreas Heider

Robert Bosch (Germany)

Peter Stritt

Robert Bosch (Taiwan)

Peter Berger

University of Stuttgart

Cooperative Institutions

University of Stuttgart

120

TRUMPF (Germany)

University of Bern

S&F Systemtechnik (Germany)

TU Dresden

Robert Bosch (Germany)

RWTH Aachen University

Berufsgenossenschaft für Gesundheitsdienst und Wohlfahrtspflege

Karlsruhe Institute of Technology

Fraunhofer Institute for Physical Measurement Techniques

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Time-resolved x-ray analysis of the keyhole behavior during laser welding of steel and aluminum at reduced ambient pressure

Thomas Engelhardt Andreas Heider Rudolf Weber Thomas Graf

Previous investigations on laser welding at reduced ambient pressure have shown an influence on the penetration depth, a change of the seam cross sections and a reduction of the amount of spatters [1 – 3]. These effects assume a change of the process behavior due to a lower ambient pressure. The shape and the dynamics of the keyhole during deep penetration laser welding is a main factor for the resulting weld seam properties and the process behavior. The present investigations show the influence of the ambient pressure on the keyhole shape and dynamic behavior for welding steel and aluminum.Bead-on-plate welding experiments at reduced ambient pressure down to 5 mbar of 16MnCr5 steel and AlMg3 aluminum were carried out for the present paper. A 5 kW Yb:YAG thin-disk laser at a wavelength of 1.03 µm with focal spot sizes of 150 µm and 300 µm has been used. A vacuum chamber was set-up that enabled time-resolved observation of the keyhole and the melt pool using the IFSW-X-Ray system. The influence of the ambient pressure and different feed rates up to 6 m/min on keyhole behavior was investigated.It was seen for both materials that pressure reduction results in a significant change of the keyhole shape. An influence on the capillary front angle as well as bending at the capillary bottom could be observed for steel welds. The impact on the resulting seam shapes will be discussed. Furthermore, a correlation between keyhole behavior and an increased process stability that results in pore reduction of aluminum laser welds under vacuum conditions will be shown.

Keyhole

Ambient pressure

Penetration (warfare)

Weld pool

10.2351/1.5063174

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Citations (3)

Kilowatt average power short-pulse laser processing of CFRP - Quality challenges

Rudolf Weber Christian Freitag Margit Wiedenmann Volkher Onuseit Thomas Graf

Laser processing of carbon fiber reinforced plastics (CFRP) is more demanding than most other material concerning thermal damage due to its strongly inhomogeneous structure and the high heat conductivity of the fibers. Short-pulsed lasers conveniently provide the intensity, which is necessary to reduce the thermal damage of the matrix caused by the strong heat flow along the fibers during interaction of the laser with the material. However, average powers above 1 kW are necessary for high cutting velocities or ablation rates as needed for example in automotive applications. With increasing average power, the movement of the focal spot over the processed surface has to be increased in order to avoid local heat accumulation between subsequent laser pulses (HAP). In addition, depending on the contour length, the heat accumulation between consecutive scans (HAS) becomes an issue.The latest results of short-pulse laser processing of CFRP at average powers above 1 kW with a Nanosecond CO2-laser at the wavelength of 10 µm and a Picosecond Yb:YAG laser at the wavelength of 1 µm will be presented. The quality challenges arising from the HAP and HAS effects will be discussed and the limits regarding average power, feed rate, and number of scans will be estimated.

Nanosecond

Picosecond

Laser Ablation

10.2351/1.5063228

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Citations (3)

Optimization of the solidification conditions by means of beam oscillation during laser beam welding of aluminum

Materials & Design (2018)

Christian Hagenlocher Martin Sommer Florian Fetzer Rudolf Weber Thomas Graf

Aluminum (AA6016) sheets were welded in overlap configuration to investigate the influence of different beam oscillation patterns on the resulting temperature gradient, the local solidification rate and the resulting grain structure and to compare the results with those obtained with the conventional rectilinear welding. Two pyrometers were used to experimentally determine the temperature gradient. The weld pool boundaries on the surface plane of the work piece were determined by image processing of high speed videos, in order to evaluate the local solidification rates. Metallographic analysis of the weld seams proved that laser beam oscillation during welding can be used to reliably form an equiaxed dendritic grain structure, which reduces the susceptibility to the formation of hot cracks.

Equiaxed crystals

Temperature Gradient

Weld pool

Oscillation (cell signaling)

Pyrometer

10.1016/j.matdes.2018.11.009

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Citations (123)

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

Peter Stritt Rudolf Weber Thomas Graf Steffen Müller Christian Ebert

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.

Penetration (warfare)

Modulation (music)

10.2351/1.5062028

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Citations (12)

Comprehensive analysis of the capillary depth in deep penetration laser welding

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE (2017)

Florian Fetzer Meiko Boley Rudolf Weber Thomas Graf

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.

10.1117/12.2250500

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Citations (11)

Thermal beam distortions in end-pumped Nd:YAG, Nd:GSGG, and Nd:YLF rods

IEEE Journal of Quantum Electronics (1994)

C. Pfistner Rudolf Weber H. P. Weber S. Merazzi R. Gruber

The thermally induced beam distortions in end-pumped Nd:YAG, Nd:GSGG, and Nd:YLF rods were analyzed and the influence of edge- and face-cooling was investigated. The distributions of temperature, stress, and strain in the crystals were calculated by finite element analysis. Based on these data, the space-resolved changes of the refractive index were determined considering thermal dispersion, surface deformation, and strain-induced birefringence. The resulting optical path difference for one round-trip in the end-pumped rods was integrated numerically. For each rod, the induced thermal lens was determined over the extent of the pump spot radius. The calculations of the optical path difference were experimentally confirmed by investigations using a modified Twyman-Green interferometer with a polarized HeNe probe beam at 633 nm under lasing and nonlasing conditions.< >

Rod

Optical path length

10.1109/3.299492

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Citations (268)

Influence of laser beam oscillation on the cutting front geometry investigated by high-speed 3D-measurements

Procedia CIRP (2022)

Michael Sawannia Madlen Borkmann Patrick Herwig Andreas Wetzig Rudolf Weber

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.

Oscillation (cell signaling)

Laser cutting

Undercut

10.1016/j.procir.2022.08.100

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Citations (8)

Remote Laser Welding of Multi-Alloy Aluminum at Close-Edge Position

Physics Procedia (2013)

Daniel Weller Cyrille Bezençon Peter Stritt Rudolf Weber Thomas Graf

6000 series (magnesium and silicon based) aluminum alloys are highly susceptible to hot cracking during laser beam welding. The occurrence of hot cracks depends on thermo-mechanical strain at the solidifying stage of the melt and in the micro-scale on the solidification structure. A new metallurgical approach to avoid these cracks by affecting the solidification process is using a multi-alloy aluminum with a silicon-rich layer. Within this paper infrared videos were used to investigate and compare the behavior of this new material with the 6xxx monolithic alloy at hot crack sensitive close-edge positions.

10.1016/j.phpro.2013.03.065

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Citations (16)

Cooling schemes for longitudinally diode laser-pumped Nd:YAG rods

IEEE Journal of Quantum Electronics (1998)

Rudolf Weber Beat Neuenschwander M. Mac Donald Matts Roos H. P. Weber

Longitudinal diode laser pumping causes pronounced inhomogeneous heating of the laser material. The effect of the thermal load on the mechanical and optical properties is significantly influenced by the method used for cooling the pumped surface. Four methods for longitudinally pumped Nd:YAG rods were compared experimentally and with finite-element (FE) analysis. The pumped surface exposed to air or to flowing water, a sapphire plate pressed onto the pumped surface and composite rods, having an undoped pumped end, were investigated. The comparison includes temperature and stress distributions with the emphasis on thermal lensing. Careful validation of the FE code with experiments allowed to numerically comparing the cooling methods under identical conditions.

Rod

Optical Pumping

Neodymium

10.1109/3.678602

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Citations (127)

Calibrated Heat Flow Model for Determining the Heat Conduction Losses in Laser Cutting of CFRP

Physics Procedia (2014)

Patrick Mucha Rudolf Weber Nicolai Speker Peter Berger Bernd Sommer

Laser machining has great potential regarding automation in fabrication of CFRP (carbon-fiber-reinforced plastics) parts, due to the nearly force and tool-wear free processing at high process speeds. The high vaporization temperatures and the large heat conductivity of the carbon fibers lead to a large heat transport into the sample. This causes the formation of a heat-affected zone and a decrease of the process speed. In the present paper,an analytical heat flow model was adapted in order to understand and investigate the heat conduction losses. Thermal sensors were embedded in samples at different distances from the kerf to fit the calculated to the measured temperatures. Heat conduction losses of up to 30% of the laser power were determined. Furthermore, the energy not absorbed by the sample, the energy for sublimating the composite material in the kerf, the energy for the formation of the HAZ, and the residual heat in the sample are compared in an energy balance.

Vaporization

10.1016/j.phpro.2014.08.036

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Citations (12)