Wetting Process during Laser Brazing of Aluminum Alloys with Aluminum-Based Braze Material

Authors: M.Sc. Till Leithäuser, Dr.-Ing. Peer Woizeschke, BIAS – Bremer Institut für angewandte Strahltechnik GmbH

1 Abstract

This application note is based on the authors’ article: “Influence of the Wire Feeding on the Wetting Process during Laser Brazing of Aluminum Alloys with Aluminum-Based Braze Material” published in the Journal of Manufacturing and Materials Processing 2019, 3(4), 83. The whole article can be found from: https://www.mdpi.com/2504-4494/3/4/83/htm. This document contains modifications and direct quotes from the article.

The wetting behavior in laser brazing can be designated as inconstant, caused largely by external process discontinuities such as the wire feeding. To reveal periodic melt pool propagation effects that occur during laser brazing of aluminum, and for a better understanding of those effects in laser brazing in general, the researchers analyzed high-speed recordings of the brazing process with aluminum alloy. It is demonstrated that two main effects of periodic melt pool behavior in different frequency scales occur during the process, both related directly to the wire feeding.

2 Setup

According to the article, the braze was done in a bead-on-plate configuration on a custom Power Automation CNC table with a moving specimen holder. An Nd:YAG laser was used as the laser source, emitting at a wavelength of 1064 nm. The filler wire was supplied to the workpiece laterally through a wire rope with a copper tip and argon was used to shield the braze from the surrounding atmosphere. The processing laser power of the Nd:YAG laser was 3 kW, the brazing speed was 2 m/min, and the wire angle was 30°. The wire feed rate was varied from 2.5 m/min to 3.5 m/min in 0.25 m/min increments.

The process was recorded with a Phantom VEO 410L high-speed camera, recording 768 × 312 px images at 18 kfps. CAVILUX HF illumination laser, emitting at 810 nm, was used as a light source for the recording together with an 810 ± 10 nm bandpass filter in front of the optics. This provided a controlled illumination of the process without disturbances from the surroundings.

The setup is visible in Figure 1. The created seam has a length of 100 mm.

Figure 1: Setup to visualize the laser brazing process.

The tracking of the wetting front, as well as the capturing of process parameters such as the wire feed rate and wire angle, was achieved with image processing in MATLAB. In Figure 2, a circular object is noticeable at the front of the wetting melt pool. This can be recognized as the reflection of the wire nozzle. The fact that the melt pool is forming a spherical geometry at the wetting front brings the reflection to the camera. This circular shape was used for tracking the wetting front with a circle detection algorithm that uses a circular Hough transform.

Figure 2: Image of the filler wire with reflection of the nozzle in the melt pool.

3 Results

During the imaging process all relevant process data were collected to study the wire feeding conditions during the brazing process.

Figure 3 demonstrates a representative sample of the wetting front deviation and the wire velocity signal. In the wetting front deviation, two periodicities are visible: one at 144 ± 3 Hz with an amplitude of 0.05 ± 0.01 mm, and superimposed a frequency of 13 ± 1 Hz with an amplitude of 0.12 ± 0.01 mm.

Figure 3. Section of the wetting front and the wire feed rate of a representative sample.

In Figure 4, periodic spikes of high velocity, of around twice the preset feed rate, are caused by a retained force that evolves when the wire is not sufficiently melted when it hits the base material. Due to a higher unmolten bottom edge of the wire in relation to the base material sheet, as shown in Figure 5, the load is smaller, resulting in a rather oscillating velocity signal. Finally, in Figure 6, the signal flattens when the bottom edge no longer touches the base material.

Figure 4. (Left) Wire velocity over time, when the wire hits the sheet and the retained force is released periodically. (Middle) Power spectral density (PSD) of the time signal. (Right) Snapshot of the high-speed recording during these events.

Figure 5. (Left) Wire velocity over time, when the wire feeding is hindered by the sheet. (Middle) PSD of the time signal. (Right) Snapshot of the high-speed recording during these events.

Figure 6. (Left) Wire velocity over time, when the wire feeding is uninfluenced by an interaction between the solid (unmolten) wire and the sheet. (Middle) PSD of the time signal. (Right) Snapshot of the high-speed recording during these events.

Based on the PSD (Power spectral density) of the feed rate velocity, each condition has a distinct amount of periodicity. During a strong interaction between the wire and the base material (Figure 4), the foremost frequency is at 106 Hz, accompanied by harmonics at 212 Hz and 318 Hz. In the second phenomenon (Figure 5), where there is less force between the wire and the sheet, there is no single dominating frequency but rather a range between 150 Hz and 400 Hz. When there is sufficient melting of the wire, and thereby no interaction in the wetting area, no frequencies are being noted, as in the PSD shown in Figure 6. It is feasible to classify the samples with dominant frequencies in the wire velocity as “noisy”, and those with a comparatively constant feed over the observed sequence as “smooth”.

More results from the measurements of the relation between wire velocity and wetting, as well as the influence of the wire feed rate on process frequencies, can be found in the original article.

4 Conclusions

The following conclusions can be drawn based on these studies:

Interactions between the unmolten wire and the base material cause oscillations in the wire feeding speed in the range of 160 Hz to 400 Hz and the oscillations being transferred to the wetting front movement with half of the frequency.

The oscillations of the wire velocity in the frequency range of 11 Hz to 15 Hz, caused by the wire feeder, affect the wetting front propagation with the same frequency.

5 References

M.Sc. Till Leithäuser, Dr.-Ing. Peer Woizeschke, BIAS – Bremer Institut für angewandte Strahltechnik GmbH, “Influence of the Wire Feeding on the Wetting Process during Laser Brazing of Aluminum Alloys with Aluminum-Based Braze Material” published in the Journal of Manufacturing and Materials Processing 2019, 3(4), 83 that can be found from: https://www.mdpi.com/2504-4494/3/4/83/htm.

Imaging Technology

Camera: Phantom VEO 410L
Illumination: CAVILUX HF System by Cavitar Ltd.

Authors

M.Sc. Till Leithäuser,
Dr.-Ing. Peer Woizeschke

BIAS – Bremer Institut für angewandte Strahltechnik GmbH
Klagenfurter Straße 5
28359 Bremen
Germany

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3 kW Laser Welding with Filler Wire

Laser welding and laser soldering sometimes require additional filler material to enhance the weld. This video shows how a 3 kW laser melts the filler wire and a solid weld bead is forming. The received footage is clear and without the blinding brightness a welding process generates. This is thanks to visualization being done using CAVILUX laser illumination that “sees through heat”.

Find other examples of welding imaging: http://bit.ly/welding_imaging

Read more about CAVILUX laser illumination: http://bit.ly/CAVILUX_laser_illumination

Video courtesy of Nobby Tech Ltd.

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Flux Coated Electrode Hand Welding

High-speed imaging in slow motion of a flux coated electrode hand welding process visualized with our CAVILUX illumination laser reveals in clear detail what really is happening during the welding.

Video courtesy of Nobby Tech Ltd.

Read the application note from a similar process by our customer Leibniz University Hannover.

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Welding Industry

Graphic

Welding processes typically emit very bright light that “blinds” the eye as well as machine vision camera systems. Without proper means the visual monitoring of a welding process is impossible.

Cavitar’s laser illumination enables the clear visualization of welding processes without any disturbances as if the process were cold. Our technology can be applied to all major arc welding processes such as GMAW (MIG, MAG) and TIG, as well as to all major beam welding processes including CO2 laser, fiber laser, diode laser, Nd:YAG laser and electron beam sources. In many cases we can also utilize structured laser illumination and provide information about seam or gap topography.

Business

  • Process observation from a safe distance and in an ergonomic manner
  • Welders can adjust the process in real-time based on the accurate live view
  • Images can be used for image analysis and automation
  • Images or videos can be stored for quality documentation

Research

  • Our laser illumination provides dramatically improved performance as compared e.g. to LED, flash lamp or halogen illumination
  • The illumination is highly uniform and bright which results in high-quality images with low speckle content and no motion blur
  • Our illumination is compatible with all high and low speed cameras that enable synchronization

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The COMBILASER project has come to a successful end

Combilaser logo

Combilaser logoThe COMBILASER project has ended. The project took place 2015-2017, with the goal of creating new production processes by integrating industrial laser processes (e.g.  welding and cladding) with a seamless Self-Learning System. The project has been a success and the created system creates an autonomous process optimization loop for new applications to be applied.

Cavitar’s main role in the project was to generate high-quality image data from various laser processes for real-time monitoring purposes. In addition, we developed methods for minimizing disturbing reflections from shiny and curved metal objects.

We, Cavitar, would like to express our sincere gratitude towards the European Commission as well as our warmest thanks to all partners of the project consortium.

You can read the final press release from the project here.
Watch the project video here.
And find more information on the project here.

The COMBILASER project was a European research project, co-financed by the European Union HORIZON 2020 framework research program.

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Flux coated electrode hand welding – CAVILUX laser illumination

Flux core welding

Visualization of flux coated electrode hand welding with Cavitar’s CAVILUX illumination laser – front illumination. Video taken at 7.000 frames per second by Leibniz Universität Hannover.

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Spatter behavior in laser beam welding process

Authors: M.Sc. Falk Nagel, Prof. Dr.-Ing. Jean Pierre Bergmann,  Ilmenau University of Technology, Fakultät für Maschinenbau, Fachgebiet Fertigungstechnik, Lasermaterialbearbeitung

1 Description of process

The group of production technology at Ilmenau University of Technology investigates the spatter behavior in laser beam welding process. Spatter is the formation of metal droplets that leave the melt pool as the result of the flow conditions in the capillary and in the melt pool. It is known that the spatter formation depends strongly on the welding speed, but the industry requires high welding speed to increase output. The escaping droplets cause lack of material in the weld seam this leading to reduction of their mechanical properties. Furthermore, the droplets deposit on the work piece reducing the surface quality. The spatter can also deposit on the protective window of the laser optic which then needs to be replaced causing downtime that has to be avoided.

Hence, the task of the group is to understand the physical mechanisms of spattering and how it can be reduced.

The research group observes the formation of the capillary as well as the melt pool behavior around the capillary using a high-speed camera. Due to the high demands in terms of high frames rates and short shutter times, an external lighting source is needed. Here the group uses Cavitar’s CAVILUX HF illumination laser for lighting the area of interest. The reason for choosing CAVILUX HF lies in its ability to produce high qualitative and homogeneous illumination to the melt pool. The robust design of the CAVILUX system enables also easy handling. Furthermore, an integrated green laser pointer in the illumination laser unit permits a simple alignment of the focusing optic in relation to the area of interest. The involved operators welcome the comfort of easy configuration of the laser parameters and the simple synchronization of the lighting with the used Photron SAX 2 high-speed camera.

For observing the spatter behavior, the best illumination results were achieved using the transmitting light setup. Therefore, the optic of the illumination system was placed on the opposite side of the high-speed camera using the same angle of incidence like the camera.

Video 1 shows the formation of the capillary and the melt pool. Moreover, the development of a column of material on the back side of the capillary can be observed. The column increases in vertical direction with further time steps and disintegrates into several droplets. The droplets leave the melt pool resulting in the lack of material and reduction of mechanical properties of the weld.


Video 1: Formation of capillary in melt pool of laser welding. Captured at 20.000 fps.

Video 2 shows the influence of the superimposed diode laser on melt pool behavior using the same welding speed. It is clearly visible that the size of the melt pool is increased, whereby the dynamics of the melt flow is reduced. Particularly the last mentioned effect leads to a distinctive decrease of spattering.


Video 2: Influence of superimposed diode laser on melt pool. Captured at 20.000 fps.

The use of the CAVILUX illumination system in combination with the high-speed camera enables the possibility to visualize the impact of the superimposed laser spots on the weld pool behavior and hence, the formation of spatter. The observations are necessary in order to extend the knowledge of spatter formation and their reduction.

The investigations are carried out within the project ”Spatter reduction due to adapted laser intensity for high-speed welding” (01.07.2016 – 30.06.2018). The research project (IGF-18582 BR/2) is supported by the Federal Ministry of Economic Affairs and Energy within the Allianz Industrielle Forschung (AIF), which is based on a resolution of the German Parliament.

2 Imaging technology

Camera: Photron SAX 2

Objective: Navitar 12 x zoom

Illumination: Cavitar Ltd’s CAVILUX HF

Authors

M.Sc. Falk Nagel, Prof. Dr.-Ing. Jean Pierre Bergmann
Ilmenau University of Technology
Fakultät für Maschinenbau
Fachgebiet Fertigungstechnik
Lasermaterialbearbeitung
Gustav-Kirchhoff-Platz 2
98693 Ilmenau
GERMANY

 

Fakultät für Maschinenbau

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High-Speed Visualization of Electromagnetic Pulse Welding

Impact welding

Author: Christian Pabst, Darmstadt University of Technology – Institute for Production Engineering and Forming Machines (PtU)

1 Introduction

Impact welding is a process which enables metallurgical bonds even between dissimilar metals. Electromagnetic pulse welding has been developed as one method for impact welding. It draws its energy from charged high-voltage capacitors. The accelerating force on the workpiece is generated by a coil, through which the current is driven by discharging the capacitors.

The main applications of electromagnetic pulse welding are the construction of hybrid space frames, the gas-tight sealing of high-pressure containers or low-ohmic joints between aluminum to copper for the electromobility. The joints produced with impact welding are very tough, because the joint area is not weakened by thermal influences but exhibits fine grains.

The basic mechanisms are understood and illustrated in Figure 1: One workpiece has to impact another at velocities in the range of roughly about 250 m/s and above. The impact has to occur under a certain impact angle. This leads to a collision point (or line) travelling across the surface. Figure 1: Schematic illustration of impact welding. The impact is accompanied by a bright flash which is characteristic for this process. The metallically pure surfaces are then pressed together by the immense pressure of the impact, which finally evokes the metallurgical joint.

Figure 1: Schematic illustration of impact welding.

2 Experimental Configuration

For the basic investigations, a special test rig has been developed at the PtU. It avoids the drawbacks of explosion welding and electromagnetic pulse welding by colliding and welding flat sheets mechanically. The buildup consists of two rotors which are driven by one motor each. Each rotor holds an aluminum tube with a welding specimen mounted at one side. Figure 2 shows the test rig without housing.

Figure 2: Left: Test rig without housing. Right: Rotors with specimens in collision position.


Both rotors have the same sense of rotation, so the specimens’ velocity adds when they meet in the center. When the specimens are welded successfully, they rip off their clamped rest with the help of a predetermined breaking point. The welded specimens and the clamped parts are shown in Figure 3.

Figure 3: Welded specimens (center) with the clamped parts.

3 Imaging Setup

As the actual impact and the joint formation take place in only few microseconds, the process is hard to capture with a conventional high-speed camera. Hence for these research works, an image intensifier camera is used. It allows exposure times and frame delays in the range of nanoseconds at still remarkable spatial resolutions of up to more than 1000 pixels. Filming the impact is accompanied by two more obstacles in addition to the high speeds: During the impact, a bright flash covers the actual joint area. Its formation will be discussed in the following chapter. Exact triggering is the second challenge, because the camera technique does not allow pre-triggering. Due to the fast turning rotors, the exact measurement of the momentary angle is hardly possible. The bright process glare can be suppressed by a trick which is also used when conventional welding processes are investigated. As the glare is usually white, it can be concluded that its intensity is spread almost constantly across all (visible) wavelengths. The light source that is mandatory for high-quality high-speed images only emits light in a small wavelength range. Thus, its intensity is much greater than the process glare, even if the latter appears to be brighter to the human eye. Figure 4 illustrates this issue.

Figure 4: Emitted wavelength range of the light source and the wavelength distribution of the process light.


To suppress all other wavelengths, an optical bandpass filter is fitted to the camera in addition to the special light source. In the experiments, CAVILUX Smart laser illumination system with a nominal wavelength of 640 nm ± 10 nm is used together with a filter for 640 nm ± 5 nm. The laser light is comparably easy to handle, because the light is visible and the emitted beam itself is neither coherent nor collimated which prevents speckling. The laser pulses are synchronized with the camera because their length is limited to a few microseconds only due to the limited duty cycle which prevents a constant illumination. Triggering is realized by using the two rotors as a kind of switch for the trigger circuit.

The following Figure 5 shows the process in two independent experiments immediately after the finished impact with and without bandpass filter. It can be clearly seen that it is almost impossible to investigate the impact in detail without the filter.

Figure 5: Propagation of the jet 20 µs after the impact with (left) and without (right) optical bandpass filter.

Figure 6: Image series taken from the side the process with 2 mm aluminum plates at 3 Mio fps.

Figure 7: Image series taken from the side of the process with 0.74 mm steel plates at 2 Mio fps. The process creates also shockwaves which can be seen in the images.

 

4 Jet Formation and Process Glare

A common explanation for the process glare is that it is caused by the jet. The jet does not only consist of oxides, but also of parent material. The theory states that this parent material burns whilst being emitted and thus causes the intense light. However, it is generally accepted at the same time that high temperatures or even melting do not occur during the impact and welding process. Thus, a sufficient energy source which is capable of initiating the oxidation should actually not exist. In order to examine jetting and glare in more detail, the electromagnetic pulse welding process is investigated under different atmospheres. The complete housing of the test rig has a volume of approximately 1 m³ and is not gas-tight. For the experiments, the coil is covered by an acrylic glass box, which is filled with an inert welding gas. When the enclosure is completely filled with the inert gas, the pulse generator is charged and the weld is established after only few seconds. The following images show the welding of two aluminum sheets with a thickness of 2 mm by a peak current of 300 kA at 20 kHz. Figure 8 shows the welding process with the inert gas atmosphere (left) and with the surrounding air (right). The inert gas does not only significantly decrease the emitted light, but also strongly weakens the emitted pressure wave during the impact.

Figure 8: Electromagnetic pulse welding of two aluminum sheets with (left) and without (right) inert gas atmosphere.


Welds between two copper sheets (Cu-ETP, thickness 1 mm) show a different behavior: Neither the light emission nor the pressure wave is significantly influenced by the surrounding atmosphere. In both cases, it is comparable to aluminum welds with inert gas. These experimental results suggest that an oxidation proceeds during the impact. Aluminum burns with a bright white flame and the oxidation is a highly exothermal reaction. As it can be seen in Figure 5, the emitted jet looks like a cloud of dust. If this dust does not only consist of superficial oxides but also of pure aluminum from the base material, a huge surface is created. Thus, if an appropriate energy source is available, a strong exothermal reaction can occur. This theory is supported by the experiments with the test rig: The extent of the glare and the extent of the jet are almost identical as highlighted in Figure 8. This indicates that jetting and glare are the same phenomenon or at least correlate closely.

Figure 9: Electromagnetic pulse welding of two aluminum sheets with (left) and without (right) inert gas atmosphere.

 

The auto ignition temperature of aluminum strongly depends on the particle size. The past research work in this field shows that the auto ignition of aluminum powder with a particle size of under 10 µm can already happen at about 600 °C. As burning aluminum is even capable of splitting water molecules, a hydrogen explosion due to the atmospheric humidity might occur as well. Copper on the other hand does not burn, which explains why the surrounding atmosphere does not change the light emission.

 

5 Conclusions

Through ultra-high-speed imaging employing a CAVILUX Smart laser illumination and image intensifier camera, it was possible to investigate the jet formation and the process glare in electromagnetic pulse welding. The laser illumination enabled the efficient removal of the thermal light emitted by the process. This provided a detailed view behind the bright process light. With back illumination and short pulse duration it is also possible to observe shockwaves.

The high image quality enabled the verification of the theoretical models and to validate research in that area. More information about the theoretical models and considerations for the source of the temperature that is created for the ignition that creates the process glare can be found from: https://eldorado.tu-ortmund.de/bitstream/2003/33500/1/C.Pabst_Electromagnetic%20pulse%20welding.pdf

 

 

About the author

 

Mr. Christian Pabst (M.Sc.)
Darmstadt University of Technology
Institute for Production Engineering and Forming Machines
Otto-Berndt-Straße 2
64287 Darmstadt
Germany 

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Research and Development of Modern Variants of Classical Arc Welding Technologies with High-Speed Videography with Laser Illumination

Author: Prof. Regis Henrique Goncalves e Silva, Dr. Eng.

Over the last years a profusion of new versions of arc welding processes has overwhelmed the international welding scenario in the industry and academia. Innovations have been made possible not only by means of electronics and software developments, but also through new concepts in mechanical design and mechanisms.

With respect to the TIG process, one example is Dynamic Feed (Wire Oscillation). Low productivity is often a disadvantage attributed to conventional TIG, when compared to other arc welding processes. In order to manage this drawback, as well to better deal with hard wetting materials (Ni-Cr alloys for example), a forward and backward wire oscillation movement has been implemented in TIG systems and it finds good acceptability in the industry as well as great interest within the scientific community. A further benefit of reducing porosity may also be expected from the technique. For the study and development of such techniques, high-speed filming has been a powerful tool for observation and stability evaluation of the metal fusion and transfer, arc behavior and weld pool behavior. Main objectives are scientific investigations on parameters influence over the resulting physical phenomena and development of parameterization for different welding conditions (position of wire feeding, torch geometry, wire dynamics).


Video 1: Dynamic Feed TIG Welding imaged at 1.000 fps

With respect to MIG/MAG welding, new technologies aim at developing adaptive control methods, innovative current waveforms and mechanization techniques in order to improve arc stability, metal transfer regularity, process reliability and expansion of the application range. Here, examples are the rotary arc and the pulsed arc mode, which are promising in achieving outstanding results for cladding and thick walled narrow gap joints. In these cases, high-speed filming is applied for metal transfer phenomena observation, arc movement patterns and respective influences over the weld pool, arc geometry and generation of the weld bead. Also, high-speed filming has been being applied to evaluation of consumables and peripherals (like wire-electrodes, contact tips and wire feeders).


Video 2: Rotary Arc Pulsed MIG/MAG Welding imaged at 5.000 fps


Video 3: MIG/MAG Welding with forward/backward movement of the wire electrode imaged at 4.166 fps

For the videos shown above a CMOS, 1.0 megapixel array size color camera was applied with 105mm and 180mm macro lenses. Acquisition rates were adjusted in synchronization with electric welding data monitoring via a Data Acquisition System.

In the scope of these investigations and developments, CAVILUX HF has been intensively applied. The laser  illumination system allows us to finely adjust the arc intensity of the high-speed images produced, thus enabling the selection and isolation of specific welding process features (wire, arc, droplet, pool, etc.) which are goal sensitively, specifically meant to be monitored, analyzed and investigated.

About the author

Prof. Regis Henrique Goncalves e Silva, Dr. Eng.
Director of R&D
LABSOLDA – Welding and Mechatronics Institute
Mechanical Engineering Department – EMC
Federal University of Santa Catarina – UFSC – BRAZIL
www.labsolda.ufsc.br
regis.silva@labsolda.ufsc.br

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Welding PIV measurement of protective gas flows

PIV GMAW welding

Studying the flow of the protection gas of a Gas Metal Arc Welding (GMAW) process by applying welding PIV imaging method. By the Institute of Surface and Manufacturing Technology, Dresden University of Technology, Germany. Illumination is provided by CAVILUX HF with light sheet optics.

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