Laser process manipulation by axial beam shaping
Joerg Volpp, Adrien da Silva, Alexander Laskin
The laser beam is a highly flexible tool, which is used for many material processing applications. However, new beam shaping technologies open even further possibilities and processing options in order to control the heat input into the material. Beam shaping for high-power applications are usually done by manipulating the spatial intensity distribution in one layer to create e.g. several laser spots, tophat or donut profiles. A new beam shaping device offers the possibility to create up to four focal spots in axial direction, which enables an extended depth of focus (DOF) and thereby tailoring the distribution of the energy along the beam axis. In this work, experiments are shown that present the impact of different axial beam shaping settings on process behaviors during laser material processing. At low processing velocities, the amounts of measured spatters at the bottom side of the processed sheets show a reduced number of spatters compared to higher speeds. It is assumed that a stable keyhole opening is achieved that prevents the spattering.
Keywords: Multi-focus optic, Laser welding, Laser cutting, Melt pool spatter
Joining dissimilar materials: a new approach based on laser beam welding and melt displacement by electromagnetic forces
Jennifer Heßmann, Marcel Bachmann, Kai Hilgenberg
For vehicles the combination of steel and aluminium alloys offers the most promising compromise between weight, strength and formability. Thermal joining of these dissimilar materials is still a challenge to overcome. A possible approach is a new joining technology, whereby a combination of laser beam welding and contactless induced electromagnetic forces are used to displace the generated melt of one joining partner into a notch of the other. This paper presents the working principle and shows numerical analyses to improve the understanding of this joining process. The simulations help to calculate the thermal development of the joining partners, which is important for the formation of intermetallic phases. Furthermore, the calculation of the time required for a complete displacement is possible. The numerical results are validated by experimental results.
Keywords: Electromagnetic forces; Lorentz forces; Laser beam welding; Joining technology; Dissimilar materials; Steel; Aluminium alloy
Process control of aluminum-copper mixed joints during laser beam welding in vacuum
Uwe Reisgen, Simon Olschok, Niklas Holtum, Christian Frey
The creation of aluminum-copper mixed joints is difficult to produce in all welding processes. This is due to the unavoidable formation of intermetallic phases (IMP), which reduce the mechanical technological properties of the weld, and to the different melting points of pure aluminum and copper. Laser beam welding in vacuum with a single-mode fiber laser ensures precise temporal and local energy input, allows a controlled degree of melting copper, and homogeneously mixes both materials. This paper reports the recorded radiation characteristic of the mixed joint in vacuum to determine the degree of copper and consequently the IMP. Therefore, we detected the characteristic copper wavelength 521.8 nm and correlated the intensity to the copper content and IMP in the weld. The copper content and the IMP in the weld seam do not correlate to the resistance that we measured with the four-wire method.
Keywords: laser beam welding; mixed joint; aluminum; copper; LaVa
Investigations on the weld seam geometry of ultrasonic assisted laser beam welded round bars in and beside antinode position
J. Grajczak, C. Nowroth, J. Twiefel, J. Wallaschek, S. Nothdurft, J. Hermsdorf, V. Wesling, S. Kaierle
Laser beam welds are usually symmetric. They can form asymmetric weld seams by introducing ultrasound, which normally can be used for grain refinement and reducing segregation. The investigations comprise ultrasonic assisted laser beam welding around antinode position of a stationary ultrasonic wave.
The experiments are carried out with 2.4856 round bars, a laser beam power of 6 kW, a welding speed of 0.95 m/min and ultrasonic amplitudes of 0 μm and 4 μm. The welding positions are placed 0/7.5/15.0 mm on the right and left side of antinode position. Afterwards, specimen macrographs and micrographs of metallographic cross sections are made. As a result, the direction dependent asymmetry of the weld seams due to the position in the vibration distribution was proved. In addition, two different approaches are described to explain the fluid dynamic processes.
Keywords: laser beam welding ; ultrasound ; excitation methods ; melt pool dynamics ; nickel base alloy 2.4856
Laser in vacuum spot welding of electrical steel sheets with 3.7% Si-content
Krichel, T., Olschok, S., Reisgen, U.
The energy efficiency of electric motors is largely determined by the magnetic and electrical properties of the soft magnetic core material. For high-frequency applications in the automotive sector, the cores usually consist of iron-silicon alloys with several lamellae electrically insulated from each other to minimize eddy current loss. To join these lamellae, a novel method with individual, statistically distributed weld spots instead of continuous linear welds is used. The influence of beam power and beam intensity on the weld geometry and grain structure of the material is investigated.
Keywords: laser in vacuum; electrical steel; spot welding
Vapor plume behavior during a standalone laser pulse on thedissimilar aluminum/titanium interface: high-speed imagingwith 810 nm band-pass filter
Manoj Raja Kumar, Iryna Tomashchuk, Jean-Marie Jouvard, Mélanie Duband
The understanding of plume behavior in dissimilar laser welding is little studied and can bring an insight to the concurrent
vaporization process and keyhole dynamics. The present study is dedicated to the high-speed imaging and post-mortem
characterization of a standalone Yb:YAG laser pulse on aluminum A5754/titanium interface with different offsets to the
joint line. Plume morphology, dimensions, orientation and propagation velocities were characterized basing on the videos
performed with an 810 nm band-pass filter. The main observed feature of the plume was its pronounced initial inclination
towards aluminum side, accentuated by the beam offset on A5754 and reduced by the offset on titanium, followed by a
progressive redressing resulting in a close to vertical jet. This behavior was attributed to the domination of the vapor jet
from titanium side over that of aluminum side during the evolution of keyhole curvature. SEM observations of the melt
were found in agreement with this statement.
Keywords: laser welding; vapor plume; dissimilar welding; high-speed imaging.
High-speed synchrotron x-ray imaging of the formation of wedge-shaped capillaries during laser-beam welding at high feed rates
Eveline N. Reinheimer, Marc Hummel, Alexander Olowinsky, Rudolf Weber, Thomas Graf
In case of high welding velocities, the geometry of the capillary during laser-beam welding elongates in feed direction to a wedge shape. Following to the change of the capillary, the weld pool changes and the melt velocity increases. Therefore, high-speed X-ray imaging to study the time evolution of capillary geometry and weld pool shape during high-speed laser welding was performed at DESY's synchrotron. The investigation addressed the transition of the capillary geometry from a high aspect ratio to a wedge-shape geometry and how the weld pool geometry and the weld flow changed. In order to investigate the melt flow in the weld pool during welding, the movement of tungsten carbide particles was analysed in the acquired image sequences. The tungsten particles were deposited on top of the sample before welding. Further, a simple method to determine the average melt velocity around the capillary without tracking the tungsten particles is shown. A laser spot diameter of 100 μm and feed rates of up to 2 m/s were used to weld in aluminium.
Keywords: Laser High Speed Welding; Capillary; Weld pool; Synchrotron
Impact of sulphur content on spattering and weld seam shape insteel specimens using modified side-gas application technique
Goran Jovic, Axel Bormann, Johannes Pröll, Thomas Niendorf, Stefan Böhm
Laser welding experiments have been carried out using stainless steel specimens (1.4003/1.4005) with different sulphur
contents (S1:10ppm, S2:210ppm, S3:2770ppm). A parameter for spatter control, the capillary widening ratio mainly
affected by the side-gas jet is considered. This value can be controlled by welding speed, laser power and side-gas pressure.
Decreasing the sulphur content resulted in (1) a shift of the humping threshold to higher side-gas pressure and (2) an
increase in weld seam bulge while decreasing spatter quantity at the same time. For S1 and S2, aspect ratios of R>3 were
achieved while spattering could not be detected by high speed imaging at 2000 fps. By increasing the welding speed from
1.5m/min to 3.0m/min and a simultaneous adaption of laser power, differences in spattering and weld seam shape
between S1 and S2 were minor, indicating that higher melt pool velocities acting in welding direction dominate the surface
tension driven effects.
Keywords: Side-gas application, spatter reduction, weld shape modification, surface tension;
Influence of beam parameters on the capillary formation and the depth progress in laser spot welding of copper
Frauke Faure, Rudolf Weber, Thomas Graf
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.
Keywords: laser spot welding; copper; depth progress; X-ray imaging; capillary formation
Remote laser welding of die casting aluminum parts for automotive applications with beam oscillation and adjustable ring mode laser
Mikhail Sokolov, Pasquale Franciosa, Dariusz Ceglarek
Aluminum die casting alloys are frequently used in the automotive industry for front and rear rails, corner nodes and interface blocks to weld together varying cross sections of aluminum extrusions in lightweight chassis structures. However, these materials have limited weldability due to entrapped gases which generate pores or cavities. Therefore, the thermal cycle during welding as well as the overall heat balance need to be carefully controlled in order to reduce the porosity level and hence achieve the desired joint integrity. This paper focuses on the selection of process parameters for the material combination of Al die casting C611 to Al extrusion AA6063. Results showed that the porosity level can be significantly reduced from 5-6% to below 2% of the weld area through the combination of beam oscillation and dual beam welding with Adjustable Ring Mode laser. Additionally, the selected parameters resulted in an average ultimate tensile strength of 120 MPa.
Keywords: Remote Laser Welding; Die Casting Aluminium Welding; Weld Porosity Control; Beam Oscillation; Adjustable Ring Mode Laser
Advanced quasi-simultaneous welding – a new approach to laser welding of polymers
Lea Sauerwein, René Geiger, Christian Ebenhöh
Evosys Laser GmbH is developing a new variant of laser plastic welding, the so-called Advanced Quasi-Simultaneous Welding (AQW). It combines two monochromatic laser beam sources of different wavelengths in a sequential time pattern. By using two different wavelengths in a quasi-simultaneous welding process, the specific deposition of radiation energy and heat into each joining partner can be better controlled. This results into a more reliable welding operation with an enlarged process window.
Trials employing the new AQW process show that a significant improvement in weld seam quality is possible compared to the standard process with only one laser source. Due to the wavelength of the secondary laser, more energy is deposited in the transmissive joining partner. The increased volume of plasticized material in this part is leading to an increased weld strength. Furthermore, it facilitates processing high-performance thermoplastics which often impose challenges to the laser welding process.
Keywords: Laser Plastic Welding; Laser Transmission Welding; Quasi-Simultaneous Welding
The effect of temperature and joining speed on the joining quality for conduction laser joining of metals to polymers
Mahdi Amne Elahi, Max Hennico, Peter Plapper
Laser joining of metals to polymers offers several advantages to produce lightweight hybrid assemblies. An important one is the exceptional control over the heat input which defines the temperature at the interface of the materials. Initially, the in-situ heating observation of PA inside ESEM is considered. Then, aluminum and polyamide are joined in an overlap configuration while the temperature was recorded simultaneously at different areas between the materials. The results show that due to excessive heat input, polyamide degrades and leaves bubbles in the melted area. Finally, the materials are laser joined with several joining speeds to investigate different cooling rates of the polyamide during the joining process. It is concluded that joining with high cooling rates generates an amorphous melted layer of the polyamide which is different from the semi-crystalline structure of the bulk. This difference acts as a stress concentration zone and reduces the shear strength of the assembly.
Keywords: metal to polymer laser joining; temperature measurement; polyamide degradation, shear strength.
Prediction of Cu-Al weld status using convolutional neural network
Karthik Mathivanan, Peter Plapper
Welding copper (Cu) and aluminum (Al) result in brittle intermetallic (IMC) phases, which reduces the joint performance. The key for a strong joint is to maintain an optimum amount of Al and Cu composition in the joint. To implement this without the destruction of the sample is a challenge. For this purpose, high-resolution images of the weld zone are utilized after welding. With the image processing technique, the presence of (Al/Cu) material melted is distinguished. Therefore, the different weld type/status like insufficient melt, optimum melt, and excessive melt is detected from the images.
This paper analyses the weld images and applies the convolutional neural network technique to predict the weld type. The microstructure and Energy Dispersive X-ray Spectroscopy (EDS) analysis of the fusion zone for each weld types are correlated to the weld images.
Keywords: Aluminum-copper joints; Laser welding; weld analysis; Intermetallic phases; Energy Dispersive X-ray Spectroscopy (EDS) analysis; Image processing; Convolution neural network; weld type prediction
High-speed synchrotron X-ray investigation of full penetration welding of aluminum sheets
Jonas Wagner, Christian Hagenlocher, Marc Hummel, Alexander Olowinsky, Rudolf Weber, Thomas Graf
Full-penetration laser beam welding of aluminum alloys is widely applied in industrial welding processes of sheet metal components. It is characterized by a capillary, which fully penetrates two or more sheets in overlap configuration and is open at its top and bottom. Compared to partial penetration laser beam welding, full-penetration welding is associated with a stable capillary and therefore a more reliable process because the additional opening at the bottom side results in the avoidance of a collapsing capillary tip. The behavior of the capillary was analyzed by means of high-speed synchrotron X-ray imaging at the DESY for welding of aluminum sheets with thicknesses of 1 mm. The results prove the stabilization of the capillary if it opens at the bottom side of the sheet. Additionally, a stabilization of the melt-pool isotherm in front of the capillary is observed. Despite the reliable avoidance of capillary collapses, the formation of pores is still observed.
Keywords: laser beam welding; aluminum; full-penetration; X-ray imaging; synchrotron; pore formation
Investigations on the transmissivity and scattering behavior of additively manufactured components for laser transmission welding applications
Julian Kuklik, Verena Wippo, Peter Jaeschke, Stefan Kaierle, Ludger Overmeyer
Additive manufacturing (AM) of thermoplastic parts is a common technique for prototypes, small batches, and mass customization products. A widely used AM process is the fused deposition modeling (FDM), which generates parts with an inhomogeneous volume structure, because it is build up line by line and layer by layer.
An industrial established joining technology is the laser transmission welding (LTW), e.g. for joining injection molded parts in the automotive sector. For this technique, the transmissivity of one joining partner has a high influence on the resulting weld seam quality and the welding process itself. In order to use LTW for joining AM parts, the influence of transmissivity and scattering behavior of AM parts were investigated. The optical properties were analyzed with spectroscopy and shear tensile tests were performed with welded samples to enhance the knowledge about the relationship between the FDM process, the optical behavior, and the weld seam strength.
Keywords: Laser transmission welding, additive manufacturing; fused deposition modeling; transmissivity; polylactide
Glass to copper direct welding with a rough surface by femtosecond laser pulse bursts
Qingfeng Li, Gabor Matthäus, Stefan Nolte
We report on the welding of the borosilicate optical glass to a surface-unpolished copper substrate (Ra of 0.27 μm and Rz of 1.89 μm) using bursts of femtosecond laser pulses. By investigating the effects of the pulse energy and the focal position on the weldability, we confirmed that the glass melting and having the molten glasses fill the gap are the prerequisite conditions for the successful glass-to-metal welding. The factors that lead to those conditions are discussed. Under the optimal conditions, without the assistance of any clamping system, our welded samples kept a breaking resistance of up to 10.9 MPa.
Keywords: femtosecond laser; micro-welding; ultrafast processing; glass-metal-bonding;
Observation of the weld pool shape in partial penetration welding and its influence on solidification crack formation for high-power laser beam welding
Nasim Bakira,*, Ömer Üstündaga, Andrey Gumenyuka,b, Michael Rethmeier
Solidification cracking is still a particular problem in laser beam welding, especially in the welding of thick-walled plates. In this study, the influence of weld pool geometry on solidification cracking in partial penetration welding of thick plates is a subject of discussion. For this purpose, a special experimental setup of steel and quartz glass in butt configuration and lateral with high speed camera was used to capture the weld pool shape. Additionally, laser beam welding experiments were carried out to compare the crack positions and the cross section with the high-speed camera observations. The results showed a bulge in the weld pool root separated from the upper region by a nick area. This leads to the fact that three different longitudinal lengths with different solidification areas are taking place. This temporal sequence of solidification strongly promotes the solidification cracks in the weld root.
Keywords: Laser beam welding; weld pool shape; solidification craking; partial penetration; bulging
Correlation between the spatial weld seam morphology and the spatial-temporal temperature profile in laser transmission welding of polypropylene
Edgar Mayer, Thomas Stichel, Thomas Frick, Stephan Roth, Michael Schmidt
Laser welding of thermoplastic polymers is a well-known joining technology that is particularly efficient for joining thermoplastic polymers. Although the process is already in industrial use, the basic process-structure-property relationships are not fully understood and still subject of current research. The key to understand the correlations between process parameters and final weld properties are the mechanisms of origin of crystallinity, crystal phase, and spherulite size. Understanding is made difficult by the fact that the laser welding process is a highly dynamic thermomechanical process and therefore very sensitive to experimental circumstances and parameters. In addition, the spatial and temporal distribution of the temperature leads to different melting, consolidation and crystallization conditions within the weld seam, so that it is to be expected that this will also influence the spatial distribution of the microstructural features.
In this study, the spatial distribution of microstructural features inside the weld seam is investigated. For this purpose, the size of spherulites in the weld seam is examined by microscopy as well as differently occurring phases of polypropylene (α- and β-phase). Moreover, differential scanning calorimetry was performed in order to measure the crystallinity. The results are correlated with the spatial-temporal temperature profile inside the weld seam which is derived by a thermal simulation model applied with COMSOL.
Keywords: Laser transmission welding; weld seam morphology; temperature field simulation; polypropylene
Thermal cycles and charpy impact toughness of single-pass hybrid laser-arc welded thick-walled structures
Ömer Üstündag, Nasim Bakir, Andrey Gumenyuk, Michael Rethmeier
The study deals with the influence of the heat input on the thermal cycles and Charpy impact toughness for hybrid laser-arc welding of 25 mm thick structural steel S355J2 using a 20-kW high-power laser in combination with an electromagnetic weld pool support. The main focus is on the change of the mechanical properties over the entire seam thickness. The cooling times were measured using a pyrometer in combination with an optical fibre in three different locations near to fusion lines corresponding to different heights of the seam. Also, Charpy impact specimens were taken from different parts of the weld joint corresponding to the different heights. The influence of the heat input was investigated for 1.8 kJ mm-1 and 3.2 kJ mm-1. Despite the observed decreased values of both t8/5-cooling time and the Charpy impact toughness in the root part of the seam, the required values could be reached in dependance on applied heat input.
Keywords: hybrid laser-arc welding; thick-walled structures; Charpy impact toughness; thermal cycles
Joining ultra-high strength steels by edge welds
Christoph Wendt, Martin Dahmen, Viktoria Olfert, Alexander Sagel
An advantage of edge welding is the possible reduction in the required flange length compared to conventionally welded lap joints. As part of a pilot study, this method has been applied to welding of a press hardened martensitic chromium steel in similar and dissimilar joints to current high and ultra-high strength steels. A dedicated optical set-up was used to implement these welds. High-frequency beam oscillation was used to ensure mixing of the different materials and to prevent crack formation which appeared mainly in the combination with the ferritic-pearlitic grade. The welding results are evaluated using micrographs, hardness tests and tensile tests. For the tensile testing, an adapted LWF-KS2-sample geometry was designed to generate for the first time reliable comparative results compared to conventionally overlap-welded laser welds.
Keywords: Edge welds; beam weaving; dissimilar joints; mechanical properties
Metallurgy of dissimilar laser beam welded lap joints of supra-ductile and ultra-high strength steels
Martin Dahmen, Berkan Deniz, Dirk Petring
Results of research on laser beam welding of a high manganese X30MnCrN16-14 (1.4678) to a press hardened X46Cr13 (1.4304), a dual phase steel (1.0944), and a press-hardened manganese boron steel (1.5528) in lap joint configuration will be reported. A pre-assessment of the local mechanical properties by micro hardness measurements revealed their uneven distribution over the weld zone. Based on metallographic inspection the underlying microstructures were revealed. By EDS analyses the local alloy constitutions were determined and assed by of COHMS diagrams. It was confirmed that in the combination with the high manganese steel the formation of ferritic phases, ferrite is largely suppressed to the favor of austenite and α’ and ε martensite. These findings were in part confirmed by nano indentation. The results allow an insight into the properties linked to melt transport but indicate some further research.
Keywords: Metallurgy; High manganese steels; dissimilar welding; microstructure; phases
Temporally and spatially highly resolved reconstruction of vapor capillary geometry during laser beam welding using synchrotron radiation
Marc Hummel, André Häusler, Sören Hollatz, Christian Hagenlocher, Jannik Lind, Ulrich Halm, Christoph Schöler, Alexander Olowinsky, Arnold Gillner
Welds of electrical components made of aluminium and copper need high electrical and mechanical quality. Process instabilities do not allow a continuous reproducibility. To generate a deeper understanding of the process and to evaluate process dynamics, it is possible to use modern in situ analysis methods. In this work, in situ phase-contrast high-speed videography using synchrotron radiation is presented. The phase contrast method reveals the phase boundaries between solid, liquid, and gaseous phases. A spatial resolution down to < 20 μm is possible due to the high coherence of the synchrotron beam, which allows to observe small vapor capillaries of fiber lasers with < 100 μm diameter. 3D capillary geometries are reconstructed from 2D image data of welds on aluminium using a fibre laser and disc laser. Ray tracing is performed using this reconstruction, which allows the time resolved tracking of the laser beam inside the vapor capillary for quantitative estimations.
Keywords: synchrotron; laser welding; vapour capilarry dynamic; reconstruction;
Analytical approach for the transition to an equiaxed dendritic solidification during laser beam welding of aluminium alloys
Constantin Böhm, Yassin Nasr, Jonas Wagner, Christian Hagenlocher, Stefan Weihe
An equiaxed solidification in a laser beam welded seam is beneficial. It leads to grain refinement, which increases mechanical strength and hot cracking resistance. The effects of filler wire addition and welding parameters on the resulting grain structure are experimentally well studied. Up until now, there has been no description of the process window for equiaxed solidification in terms of the process parameters. This work presents an analytical approach to access the description of the columnar-to-equiaxed transition based on fundamental solidification theory for a wide-range of aluminium alloys and laser welding parameters. To validate the approach, the theoretically calculated powers are compared to experimental results of full-penetrated weld seams. This study provides an overview of the key process parameters and the material characteristics, which determine the solidification mode – in this case columnar or equiaxed dendritic. Furthermore, a process window for grain refinement of aluminium alloys is derived.
Keywords: aluminium alloys; laser beam welding; grain structure; equiaxed solidification; grain refinement
Welding mode identification using a combination of OCT and photodiode signal
Tobias Beck, Christoph Bantel, Meiko Boley, Jean Pierre Bergmann
The high absorptivity (~ 40 %) of copper for green laser light, allows for new process zones with a wide and shallow capillary. This can benefit process monitoring while laser beam welding, due to the improved accessibility of the capillary. In this publication, the usage of an OCT system in conjunction with a photodiode to determine the welding mode during copper welding of with green lasers is presented. A good agreement of the OCT depth and weld depth is shown. However, for parameter sets where the welding regime is presumably heat conduction welding, a capillary depth greater than zero was measured. Three different welding modes could be identified as a result: Welding without measured depth and no material evaporation, welding with measurable depth but without material evaporation, and welding with measured depth and material evaporation. In order to identify the welding mode correctly, a photodiode was used to measure the temperature radiation of the welding process. A correlation between the signal characteristic and the presence of material evaporation could be established. By using both sensors in conjunction, it was possible to correctly identify the different welding modes during copper welding with green lasers.
Keywords: copper welding; micro welding; process control; optical coherence tomography; process emissions
New strategies for joining aluminum alloys to steel by means of laser
Daniel Wallerstein, Antti Salminen, Fernando Lusquiños, Rafael Comesaña, Jesús del Val, Antonio Riveiro, Aida Badaouic, Juan Pou
Joining steel and aluminum is still a challenging task, mainly due to significant differences in their melting temperatures, expansion coefficients and thermal conductivity, and to the practically zero solubility of Fe in Al. This low solubility leads to formation of brittle intermetallic compounds (IMCs) at the interface between aluminum and steel, which can seriously deteriorate the mechanical properties of the joint. Laser welding is a very competitive process that provides high energy densities, which results in moderate heat input, allied to high productivity and flexibility. On the other hand, although this joining process has been successfully applied to join aluminum to steel, high complexity and costs still hamper the acceptance of the technology by the industry. In this article, recent developments and limitations of laser joining of aluminum alloys to steel are briefly reviewed. Then, a new approach to join aluminum to steel in butt join configuration is introduced. Lastly, a detailed microstructural and mechanical characterization of the resulting joints is presented.
Keywords: Multi-material design; dissimilar joints; mechanical properties; intermetallic compounds
Spatter formation in high-speed laser processing of high-alloyed steel
Peter Hellwig, Klaus Schricker, Jean Pierre Bergmann
The combination of high processing speeds and improved beam qualities enlarges the possible fields of laser applications enormously. Gas-free sheet cutting and precise mass corrections represent examples where spatter formation is applied as the primarily removal mechanism under the use of cw-mode laser radiation. For a deeper insight into spatter formation at processing speeds of several meters per second, the processing zone needs to be characterized in detail. In this study, a glass plate was used for flanking the processing zone to realize high-speed videography in a half-section setup. This approach allows performing measurements directly in the processing zone regarding several melt pool dimensions especially on inclination angle of the absorption front. In addition, high-speed records in a transmitted light mode were carried out to investigate spatter formation in more detail. Eventually, a mathematical model is developed that calculates the spatter radius depending on minimum and maximum flow velocity.
Keywords: high-speed videography; high-speed laser processing; spatter formation; loss of mass; spatter radius
Occurrence of coating-related accumulations within the seam in laser beam deep penetration welding of aluminum-silicon coated press-hardened steels
Benjamin Karwoth, Thorsten Mattulat, Peer Woizeschke
In the laser beam deep penetration welding of press-hardened and Al-Si-coated Mn-B steels coated to protect the base metal from scaling during press hardening, a reduced joint strength is observed in relation to the base metal. One influencing factor is the insertion of coating constituents into the weld seam during the welding process. The objective of this study was to investigate the influence of partially decoated sheets on the amount of accumulation occurring in the seam of the overlap weld of two sheets. For this purpose, the samples were partially decoated in different ways before the joining process. The welds were analyzed using cross-sections. The results indicate that especially the coating on the contact surfaces of the sheets in the lap joint affects the visible accumulation of coating constituents. The partial decoating of one or both contact surfaces was able to significantly reduce the amount and size of the accumulations.
Keywords: welding; deep penetration laser welding; ultra-high-strength steel; 22MnB5
High-Power Joining of Duplex Steels using Laser Beam-Submerged Arc Hybrid Welding
Rabi Lahdo, Sarah Nothdurft, Jörg Hermsdorf, Patrick Urbanek, Markus Puschmann, Frank Riedel, Ludger Overmeyer, Stefan Kaierle
Duplex steels are used in many application fields due to their outstanding performance in respect to strength, toughness and corrosion resistance. These properties result from a microstructure of delta ferrite and at least 30 % of austenite. Hitherto, duplex steels are welded using multi-pass arc welding characterized by a low efficiency. Beam welding has not been successful due to the low ratio of austenite formed. Aim of this study is the development of a reliable and efficient laser beam-submerged arc hybrid welding process for duplex steel 1.4462 (t= 16 mm) with a high proportion of austenite using a disc laser beam source (PL= 16 kW). The influence of the process parameters on the stability of the process are investigated by cross-section, EDX analyses and microstructure analyses. As a result, a stable and efficient one-layer hybrid process was archived. Furthermore, a higher ratio of austenite compared to laser beam welded seams forms.
Keywords: High-Power Joining; Duplex Steels; Laser Beam-Submerged Arc Hybrid Welding
Joining technology and mechanical properties of laser-beam welded joints with martensitic chromium steels
Martin Dahmen, Jörg Baumgartner, Benjamin Möller, Viktoria Olfert, Rainer Wagener
Investigations were undertaken on welding of press-hardened martensitic stainless steels in similar joints as well as in combination with state-of-the-art and modern steel grades. Parameter development was conducted for square-butt and lap joint configuration aiming at the production of defect-free welds. In order to homogenise the mechanical properties of the weld area, a heat treatment has been developed and applied successfully. Tests on load capacity of lap joints have been carried out under quasi-static and dynamic loading using the LWF-KS2 concept. Emphasis is laid on the investigation of contour welds. Especially for the chromium steel a strong dependence on the load angle was detected. Heat treatment led to a significant improvement of strength and ductility. Independent of heat treatment and seam weld shape, uni-directional fatigue testing of lap joints shows similar endurable load amplitudes. The critical location of failure is the intersection of the fused zone with the joint plane.
Keywords: Ultra-high strength steels; welding; fatigue; load capacity; heat treatment
Contacting of cylindrical lithium-ion batteries using short pulse laser beam welding
Lukas Mayr, Lazar Tomcic, Michael K. Kick, Christoph Wunderling, Michael F. Zaeh
The increasing demand for electric vehicles requires innovative manufacturing processes to reduce the production costs of energy storages while ensuring or improving the electrical performance. Laser beam welding (LBW) is a highly flexible and fast process for automated connecting of battery cells. A possibility to overcome the challenges in terms of copper welding, such as the high reflectivity and thermal conductivity, is the use of nanosecond laser pulses with peak powers up to several kilowatts. In this study, a pulsed laser welding process was found to achieve narrow weld seams with high aspect ratios and a low heat input. Despite the small joint areas, good electrical resistances and high tensile strengths could be obtained. By using tin-coated copper material, the joint strength as well as the process efficiency in terms of the possible feed rate were enhanced. Pulse shapes with longer pulse durations proved to require a lower feed rate due to system restrictions in terms of pulse frequencies.
Keywords: laser beam micro welding; nanosecond pulsed fiber laser; copper welding; electric mobility
Solutions of laser material processing for electric mobility – evaluation of the Technology Readiness Level
Christoph Wunderling, Christian Bernauer, Christian Geiger, Korbinian Goetz, Sophie Grabmann, Lucas Hille, Andreas Hofer, Michael K. Kick, Johannes Kriegler, Lukas Mayr, Maximilian Schmoeller, Christian Stadter, Lazar Tomcic, Tony Weiss, Avelino Zapata, Michael F. Zaeh
Battery technology and lightweight design are central fields of research and development when it comes to making electric mobility technically and economically attractive for producers and customers. In this context, laser material processing will be a driver to enable innovations in future product generations. For this reason, the publication addresses the most relevant laser-based production technologies that are currently being researched or about to be transferred to applications in electric mobility. In order to give a structured and uniform overview, the advantages of individual processes are mentioned and the technology-specific state of the art is quantitatively presented based on a methodical procedure for the evaluation of the Technology Readiness Levels. Upon this, the challenges for the deployment in industrial production are specified, which is the basis to describe the need for adaption and further development in laser material processing.
Keywords: Technology Readiness Level; e-mobility; lightweight design; battery technology; laser material processing
Methodology for analyzing the influence of contact temperatures in laser beam brazing
Sven Müller, Peer Woizeschke
In laser beam brazing, the contact temperature between the brazing material and the substrate as well as the wetting behavior play decisive roles. Thus far, the process has been analyzed using model experiments via either droplet tests or tests with small amounts of brazing material pre-placed on the base material. While in the first case a comparatively high overheating of the molten brazing material is required to enable droplet formation, in the second case it is not possible to determine the emerging contact brazing temperature in the interface with sufficient accuracy. The current study presents a novel setup that can characterize the influence of the contact temperature on the laser beam brazing process. The setup enables the investigation of process temperatures slightly above the liquidus temperature of the brazing material and facilitates the surface temperature measurement of the brazing material shortly before contact generation.
Keywords: Laser brazing; Contact temperature; Joining; Wetting; Interface
Remote laser beam welding of copper to aluminum using a frequency-doubled disk laser
Lazar Tomcic, Christoph Neumeier, Michael F. Zaeh
When joining copper and aluminum, intermetallic compounds with complex properties can form and negatively affect the mechanical and electrical properties of the weld seam. Laser beam sources with emission in the visible wavelength range enable welding of copper with high process efficiency since its absorptivity is significantly higher there than for the near-infrared wavelength range. In this study, copper was welded to aluminum in an overlap configuration, using a continuous wave laser beam source emitting at 515 nm. Preliminary experiments were carried out to identify a suitable process window for further experiments. Subsequently, the electrical resistance and the tensile joint strength in dependence of the feed rate were determined. Through metallographic cross-sections, the weld seam and the formation of intermetallic compounds were investigated. The results show that dissimilar joints with good physical properties can be obtained using green laser radiation, enabling new possibilities for joining copper and aluminum.
Keywords: green laser radiation; copper; aluminum; dissimilar metals; welding
The higher the power and energy density in the focus of the laser beam, the more efficient is the process in terms of traverse speed, hardness increase, deformation and other effects. Even a minimal focus shift or an imprecisely adjusted tool center point (TCP) shows extremely negative impact on the quality of the weld. Traditional measurement methods are too complex to capture the focus shift or to quickly check the focus position. The non-contact beam profiling technology developed by MKS Instruments eliminates limits in terms of power, and the measurement takes only seconds. Especially for 24/7 industrial use, the company developed Ophir® BeamWatch® Integrated laser measurement system, a fully automated, robust device with an easy-to-use operating interface.
Keywords: high power lasers; power density; focus shift; focus position; non-contact beam profiling;
Determination of thermophysical process limitations for the laser-based droplet brazing process using different droplet and substrate materials
Jakob Ermer, Florian Kohlmann, Markus Müller, Florian Kaufmann, Stephan Roth, Michael Schmidt
The demand for microelectronic components is rising constantly over the last few decades due to the progress in digitalization. Along with this, the requirements for joining technologies are growing since they are highly responsible for progress in microelectronics. Laser-based droplet brazing shows unique features like a quasi-force-free joining process and high-temperature stable connections and is therefore on the way to its industrial implementation. While the first developments have been limited to contacting silver metallizations on piezo actuators, today the transferability of the process with regard to different brazing and substrate materials is a major goal. In this work the relation between the solidification time of the droplet and diffusion processes depending on the substrate layout and material properties is investigated via highspeed imaging and EDX analysis. Based on the thermophysical behaviour of the process, the scalability for future applications can be derived for different material combinations.
Keywords: micro joining; droplet brazing; electronic production; highspeed imaging
Influence of defocusing in deep penetration welding of copper by using visible wavelength
Florian Kaufmann, Andreas Maier, Jakob Ermer, Stephan Roth and Michael Schmidt
High-quality joining of copper materials has become a key factor in many electric applications like electric engines, batteries or power electronics. By now high-power laser beam sources emitting visible laser radiation are available to promote the already well-suited joining method of laser beam welding. Consequently, this process can now face the challenges of welding highly reflective materials, such as copper, which originate mainly in the low absorptivity of conventionally used infrared wavelengths at room temperature and the rapid jump of the absorption at the transition from solid to liquid state. However, up to now mostly the heat conduction welding process has been examined and the effects of shorter wavelengths on deep penetration welding have been neglected. Thus, for this work the scope lies on the wavelength dependent intensity needed to overcome the deep penetration welding threshold and the alteration of energy incoupling into the vapour capillary using green wavelength and defocusing.
Keywords: visible wavelength, green laser beam, copper, deep penetration welding, energy incoupling
Investigations on the influence of the material selection of the clamping device during laser transmission welding of multi-layer polymer films with wavelength-adapted laser beam sources
Maximilian Brosda, Phong Nguyen, Alexander Olowinsky, Arnold Gillner
In laser transmission welding process of polymers with wavelength adapted laser beam sources, the joining partners are fixed in an overlap arrangement. A sufficient energy absorption is ensured by addressing the material-dependent intrinsic absorption bands. The fixing is realized by a clamping device to achieve a technical zero gap between the joining partners in order to ensure heat exchange and melt permeation. The materials are in direct contact with the polymers and influence the heat dissipation as well as the propagation of the laser beam. While the upper material must be transparent for the laser radiation, a variety of materials are available for the lower part. Hence, it is investigated how the individual material combinations affect the process. To investigate the influence of material combinations as well as the influence of roll-to-roll processing or possible process limits welding tests are performed. The weld seam is analyzed by cross sections and weld seam width.
Keywords: Laser transmission welding; Polymer; Multi layer films; Roll 2 Roll; Adapted Wavelength
Effects of separately laser-induced metal vapor amounts on the stability of a TIG arc
Insa Henze, Thorsten Mattulat, Peer Woizeschke
Arc stability during welding can be improved by using a laser process and the associated implementation of a hybrid welding process. Various effects are assumed to be the causes of the process stabilization by the additional laser beam. To investigate the metal vapor influence in a more decoupled manner, the metal vapor in this study is generated by a laser beam guided on an external substrate. The laser beam axis is oriented horizontally and thus perpendicular to the simultaneously ignited arc between a TIG welding torch and a counter electrode. The amount of metal vapor is adjusted by varying the laser power. The laser process causes the arc voltage to increase with the amount of metal vapor. This implies an increasing electrical resistance, which affects arc stability.
Keywords: laser welding; arc welding; metal vapor; arc stability
Melt-track merging and instabilities in multi-laser powder bed fusion
Craig B. Arnold, Wenxuan Zhang, Wenyuan Hou
Control over laser beam shape can enable precision control over the resulting materials properties in any laser processing application. One simply way to control the intensity profile of the material illumination is through the use of multiple laser sources or beamlets. However the use of multiple beams can introduce unexpected phenomena and instabilities that can create undesired effects in the material. In this work, we use synchronized laser beams to create two molten pools running parallel to each other in a powder bed fusion system where the beams are separated by a controlled spatial and temporal offset. Through varying the offset, results reveal that besides the completely merged and completely separated regimes, there exists a third regime in which periodic coalescence occurs between the two molten pools. We examine the instability that leads to this periodic structure as well as how to control its formation.
Keywords: Laser powder bed fusion