Heat Conduction Welding Research Articles

Laser powder bed fusion of pure copper using ring-shaped beam profiles

Additive manufacturing of copper using laser powder bed fusion (PBF-LB/M) enables the production of highly complex components. However, processing of copper by means of near-infrared laser radiation is challenging due to its absorptivity of only 5%–20%. Using a keyhole welding process with a Gaussian intensity distribution increases the absorptivity up to 53% due to multireflection. This enables the production of components with a density larger than 99.5% and electrical conductivity larger than 90% of the International Annealed Copper Standard (IACS), but this type of welding leads to keyhole porosity due to keyhole instabilities. One way of counteracting is the use of a heat conduction welding process. However, due to the Gaussian intensity distribution, it is not possible to supply sufficient energy to eliminate lack-of-fusion porosity and concurrently avoid the formation of a keyhole. Ring-shaped beam profiles have proven their advantages in stabilizing the PBF-LB/M process with a tendency toward higher laser power, but pure copper has not yet been processed in this way. Therefore, this study investigates the potential of three ring-shaped beam profiles to produce specimens with a density of more than 99.5% and their respective electrical conductivity using a laser power of up to 1300 W. In order to understand the underlying welding process, the weld geometry of single-tracks is analyzed. Specimens with a density of up to 99.77% and an electrical conductivity of up to 101.62% IACS are produced, whereby the material properties and welding regime depend on the selected ring-shaped beam profile.

Tailored laser beam shapes for welding of copper using green laser radiation

AbstractThe rapid development of laser beam sources and adapted welding technologies in recent years lead to an increased use of laser welding techniques in automated production nowadays. Especially its precision and local energy input are key features for joining applications in electric vehicle components, where joints have to meet both mechanical and electrical requirements as current-carrying connections. However, the copper materials used are difficult to weld due to their physical properties, making a stable process with fewest seam imperfections only feasible within a limited process window. Recently available beam sources emitting visible laser radiation have proven to overcome the low absorptivity at process start, but spattering is still a prone defect significantly affecting process efficiency and quality. Literature approaches for modifying the energy input point to laser beam shaping as a method for reducing process imperfections, which, however, has not been extensively researched in copper processing using green laser radiation.Thus, this study investigates the influence of a shaped intensity profile for visible laser radiation created with a reflective diffractive optical element in laser beam welding with laser powers up to 3 kW. A characterization of the process dynamics is performed by use of high-speed imaging, and metallographic analysis is used to elaborate benefits of the applied beam shapes. With beam shaping, an enlarged heat conduction welding regime and an advantageous seam shape are found. Furthermore, a decrease in spatter formation during deep penetration welding is detected for the elliptical beam profile, which correlates with an oscillation movement of the capillary.

Blue diode lasers – Understanding and influencing melt pool dynamics in copper

Within the last years an increasing demand of copper components was observed, especially in the fields of energy transition, e-mobility, or digitalization. For joining those components, laser welding has become more and more popular. Infrared lasers are commonly used for metal welding. They are characterized by a low absorptivity in solid copper at room temperature. In contrast, lasers in the blue wavelength spectrum are characterized by a significantly higher absorptivity. This facilitates the initial energy input into the material and makes the welding process more efficient. Further, in the liquid state, the absorptivity change in copper is much smaller for the blue wavelength. This results in a continuous heating of the material, a more stable melt pool and a lower amount of spatter. One supplier of lasers in the visible wavelength spectrum is Laserline. In 2023, they launched a 4 kW blue diode laser, the currently highest available power for this wavelength. It makes accessible a broad variety of applications. But the scientific investigations for the blue wavelength at a power of several kilowatt are still limited. It is the paper’s purpose to extend them by analysing welding of copper and its alloys with a blue diode laser. One the one hand, synchrotron trials were performed allowing a look into the material during welding. A special focus was on the transition between heat conduction and deep penetration welding as well as the keyhole behaviour. On the other hand, conventional welding trials were performed analysing the impact of beam shaping on the melt pool dynamics.

In situ X-ray phase contrast imaging of the melt and vapor capillary behavior during the welding regime transition on aluminum with limited material thickness

The X-ray phase contrast imaging is a powerful method to understand the fundamental behavior of the melt and keyhole during the laser beam welding process. In this paper, the keyhole-induced vapor capillary formation in the melt pool is investigated by using an adjustable laser beam source. For this purpose, the aluminum A1050 specimen with a thickness of 0.5 mm is molten only with the heat conduction welding regime by using the ring-mode laser beam. Once the specimen is molten through, the core multi-mode laser beam is then applied to vaporize the melt and a transition to keyhole welding regime occurs. Therefore, the core multi-mode laser beam with an intensity value of 33.3 MW/cm2 is investigated. The correlation between the keyhole-induced vapor capillary and the melt behavior is further investigated in this paper which was recorded with a high sampling rate of 19 kHz. In addition, a theoretical calculation about the keyhole depth is discussed in this paper.

Keyhole-in-keyhole formation by adding a coaxially superimposed single-mode laser beam in disk laser deep penetration welding

Laser deep penetration welding is characterized by the formation of a vapor capillary (keyhole) and a high overall absorption (coupling rate) of the laser radiation in comparison to heat conduction welding. Due to multiple reflexions within the highly dynamic keyhole, the absorption mechanisms are very complex. The questions, how a laser beam interacts with the existing keyhole, and, if shadowing effects occur, are a matter of concern for several researchers. In this study, the hypothesis that the beam of a laser source can be transmitted to a significant extent through an existing keyhole and acts predominantly at the keyhole bottom was investigated. Experiments were carried out on mild steel and technical pure aluminum, using a specially designed optical system, which allows to coaxially combine two laser beams and to adjust their settings like the energy and the focal plane individually. A combination of a disk laser with a focal diameter of 390 µm and a single-mode fiber laser with a focal diameter of 30 µm was used. Weld depths and seam shapes were analyzed in cross and longitudinal sections. Based on the results, it is shown that the much smaller focal diameter of the single-mode laser beam acted at the bottom of the keyhole and caused a kind of “keyhole-in-keyhole formation” in the form of a local maximum in weld depth whose expression depended on the focal plane of the single-mode laser beam.

Seam Properties of Overlap Welding Strategies from Copper to Aluminum Using Green Laser Radiation for Battery Tab Connections in Electric Vehicles.

Laser beam welding of metals has progressed dramatically over the last years mainly arising from joining applications in the field of electromobility. Allowing the flexible, automated manufacturing of mechanically, electrically, and thermally stressed components, the process is more frequently applied for joining highly reflective materials, for example for battery tab and busbar connections. The local, non-contact energy input favors this welding technology; however, joining of copper and aluminum sheets still poses a challenge due to the physical properties of the joining partners and intermetallic phases from dissimilar metal interaction, which reduce seam performance. The use of green laser radiation compared to infrared laser radiation offers the advantage of a significantly increased absorptivity for copper materials. A changed incoupling behavior is observed, and a lower deep penetration threshold has been already proven for 515 nm wavelength. When copper and aluminum are welded with the former as top sheet, this welding mode is essential to overcome limited aspect ratios from heat conduction welding. However, the opportunities of applying these beam sources in combination with spatial power modulation to influence the interconnection area of copper-aluminum joints have not yet been studied. The aim of this work is therefore to investigate the seam properties and process stability of different overlap welding strategies using green laser radiation for dissimilar metal welding. A microstructural analysis of the different fusion zones and mechanical strength of the joints are presented. In addition, the experimental parameter sets were analyzed regarding their application in battery module busbars by examining the electrical resistance and temperature distribution after welding. A parameter window was identified for all investigated welding strategies, with the stitched seam achieving the most stable results.

Laser welding of different pure copper materials under consideration of shielding gas influence and impact on quality relevant surface topographical features

The high demand for electronic products increases the need for high-quality welds of copper. Laser welding can be applied but may result in undesired weld characteristics such as humping or spatter. Process control is needed to identify defective welds in the production line. Surface topographical features can be used to identify different weld characteristics by optical coherence tomography (OCT). The resulting surface topography of a weld can be influenced by process parameters like its material properties or the application of process gas. In this work, we investigate the influence of different pure copper materials and process gas on weld seam surface features for the classification of quality-relevant weld characteristics. First, the resulting changes in weld depth and metallographic cross sections are qualitatively and quantitively characterized for different pure copper materials under the consideration of weld categories such as melt ejection, deep penetration welding, humping, and heat conduction welding with and without the application of shielding gas. Afterward, a qualitative and quantitative analysis of weld surface features is performed for the beforementioned categories under consideration of the copper material and shielding gas. As a result, an influence on the achievable weld depth could be identified for pure copper with residual phosphor content. No significant changes in surface topographical features could be identified for different material properties of copper. The influence of shielding gas and pure copper material is found to be negligible on surface topographical characteristics for process control.

Algorithms for Weld Depth Measurement in Laser Welding of Copper with Scanning Optical Coherence Tomography.

In-process monitoring of weld penetration depth is possible with optical coherence tomography (OCT). The weld depth can be identified with OCT by statistical signal processing of the raw OCT signal and keyhole mapping. This approach is only applicable to stable welding processes and requires a time-consuming keyhole mapping to identify the optimal placement of a singular OCT measuring beam. In this work, we use an OCT measurement line for the identification of the weld depth. This approach shows the advantage that the calibration effort can be reduced as the measurement line requires only calibration in one dimension. As current literature focuses on weld depth measurement with a singular measurement point in the keyhole, no optimal algorithm exists for weld depth measurement with an OCT measurement line. We developed seven different weld depth processing pipelines and tested these algorithms under different weld conditions, such as stable deep penetration welding, unstable deep penetration welding, and heat conduction welding. We analyzed the accuracy of the weld depth processing algorithms by comparing the measured weld depth with metallographic weld depths. The intensity accumulation approach is identified as the most accurate algorithm for successful weld depth measurement with a scanning OCT measurement line.

In-situ x-ray phase contrast observation of the full penetration spot welding on limited aluminum material thickness

The laser-spot welding process of aluminum alloy 1050A with a limited thickness is observed with the x-ray phase contrast method to investigate the melt dynamic especially when the melt penetrates the material. The laser-spot welding is investigated with two different wavelengths of the laser beam source: 515 and 1030 nm to investigate the influence of the absorptivity. The melt progressively penetrates the material during the spot-welding process until reaching the bottom side of the material and when the melt penetrates the lower side of the material, the so-called “lens-like” melt appears at the lower side due to the surface tension. At a comparable beam intensity value, the oscillation of the “lens-like” melt at the lower side of the material is driven by the expansion of vapor capillary. This expansion occurs inside of the material and directly above the “lens-like” melt. The shape of the expanded vapor determines the volume as well as the geometry of the resulting melt volume. Furthermore, the transition from the heat conduction welding mode to the keyhole welding mode is investigated by defocusing the laser beam for the beam source with a 515 nm wavelength. At a given variation, a clear difference between either mode is observed with the x-ray phase contrast method.

Effect of surface treatments on the laser welding performance of dissimilar materials

The connection of metal and plastic dissimilar materials is a difficult point in the lightweight design of automobiles. In the paper, the surface pretreatment on metals is carried out to improve the strength of the joint between the metal and plastic. The metal surface is processed by a laser to produce micro-textures, and then the aluminum alloy and PA66 are connected by heat conduction welding using the laser. The results showed that aluminum alloy with the mesh micro-textures arrays can be obtained the maximum tensile shear strength compared with several different micro-textures arrays. The mesh array micro-textures have a good filling effect. At the same time, the arrangement of the mesh array micro-textures improves the stress distribution during the tensile shearing process of the welded parts. The XPS analysis of the welded cross section showed that a new chemical bond is formed between the metal and the plastic during the welding process. The chemical bond is beneficial to improve the welding strength.

Weld Classification With Feature Extraction by FRESH Algorithm Based on Surface Topographical Optical Coherence Tomography Data for Laser Welding of Copper

The topographical information of a weld seam bears information about quality relevant characteristics such as humping or spatter. Optical coherence tomography (OCT) can be used for inline scanning the weld topography coaxially mounted at a laser scanning optic. Feature extraction from this topographical information is challenging due to finding mathematical representations for the identification of relevant features. Feature extraction based on scalable hypothesis tests (FRESH) allows for feature extraction by a combination of various time series characterization methods. FRESHs feature selection is supported with an automatically configured hypothesis test and hence allows for quick extraction of significant features from sensing data in laser welding processes. In this work, a proof-of-concept is demonstrated for weld result categorization from OCT data by feature extraction using the FRESH algorithm. Changes in weld topography are characterized in a vast variety of process parameters for weld categories such as spatter, deep penetration welding, humping and heat conduction welding. As a result, a quantified separation of weld categories is possible and shows the feasibility of the FRESH algorithm for future quality assessments with different sensing technologies in laser welding.

Effects of superposition of 532 nm and 1064 nm wavelengths in copper micro-welding by pulsed Nd:YAG laser

Unstable and low absorption of laser energy is experienced in copper welding at around 1000 nm wavelength. At 532 nm wavelength, there is stable and high laser absorption by copper. Past researches have shown that transitional processing condition between keyhole and heat conduction welding results in a stable micro-welding process characterized by good surface quality and deep penetration. In order to adapt laser welding to copper using pulsed Nd:YAG lasers, investigations of welding quality and efficiency were addressed. Processing was done under transitional processing condition between heat conduction and keyhole welding. Copper C1020 specimens were processed using superposed laser wavelengths of 1064 nm and 532 nm. Effects of irradiation delay and power density on the process were clarified by taking measurements of molten volumes, and by analyzing the weld beads. In addition, the dynamics of molten area and keyhole formation were investigated through three-dimensional FEM analysis. A stabilized laser absorption and increased molten volume was achieved by superposition using 532 nm laser of an appropriate high power density coupled with a short irradiation delay for the 1064 nm laser, which resulted in high-efficiency welding of copper.

Local Shielding Gas Supply in Remote Laser Beam Welding

The use of shielding gases in laser beam welding is of particular interest for materials interacting with ambient oxygen, e.g., copper, titanium or high-alloy steels. These materials are often processed by remote laser beam welding where short welds (e.g., up to 40 mm seam length) are commonly used. Such setups prevent gas nozzles from being carried along on the optics due to the scanner application and a small area needs to be served locally with inert gas. The article provides systematic investigations into the interaction of laser beam processes and parameters of inert gas supply based on a modular flat jet nozzle. Based on the characterization of the developed nozzle by means of high-speed Schlieren imaging and constant temperature anemometry, investigations with heat conduction welding and deep penetration welding were performed. Bead-on-plate welds were carried out on stainless steel AISI 304 for this purpose using a disc laser and a remote welding system. Argon was used as shielding gas. The interaction between Reynolds number, geometrical parameters and welding/flow direction was considered. The findings were proved by transferring the results to a complex weld seam geometry (C-shape).

Influence of plume attenuation under high power laser welding of copper using visible wavelengths

Joining of copper materials has become a key factor in laser material processing of electric components like electric engines, batteries, or power electronics. A paramount challenge is the design of the laser welding process, which has to fulfil the requirements of high energy efficiency and highest seam quality at the same time. By now, high-power laser beam sources emitting visible laser radiation are available to promote the well-suited noncontact joining method, which is furthermore characterized by local energy input and high automation potential. These laser sources can now face the challenges of welding highly conductive and reflective materials, such as copper, which originate mainly in the low absorption of conventionally used infrared wavelengths at room temperature and the rapid jump of absorptivity at transition from solid to liquid state. An increase in absorption of electromagnetic radiation to more than 40% for copper at room temperature for λ = 515 nm leads to increasing energy input in the material and promotes a more efficient process therefore. However, up to now, mostly the heat conduction welding regime has been examined and the effects of shorter wavelengths on deep penetration welding have not been understood in detail. A strong deviation in weld seam depth between infrared and green laser radiation is observed for identical process parameters depending on the use of additional gas supply [F. Kaufmann, A. Meier, J. Ermer, S. Roth, and M. Schmidt, “Influence of defocusing in deep penetration welding of copper by using visible wavelength,” in Proceedings of the Eleventh International WLT-Conference on Lasers in Manufacturing, Munich, 21–24 June 2021 (M. Reth, Munich, 2021)]. Consequently, radiation attenuation by the metal vapor inside and above the keyhole plays a substantial role in the case of 515 nm laser welding [M. Haubold, A. Ganser, T. Eder, and M. F. Zäh, “Laser welding of copper using a high power disc laser at green wavelength,” Procedia CIRP 74, 446–449 (2018)]. In this work, we investigate in situ measurements of plume attenuation during laser beam welding of copper with 515 and 1030 nm laser beam sources. Both laser sources are equipped with comparable optical setups to achieve identical focal diameters on the material surface. We investigate the plume characteristics in the deep penetration welding mode of copper. A range of industry-relevant process parameters are investigated to create a basis for comparison with theoretical models. It is found that a significant difference in the attenuation of both laser wavelengths occurs in the case of deep penetration welding copper specimen and blowing the vapor plume out of the beam path is recommended therefore for an efficient welding process.

Comparison of the EHLA and LPBF Process in Context of New Alloy Design Methods for LPBF

Additive Manufacturing (AM) processes are becoming more and more important for production of parts with increasing geometrical complexity and functionality. However, to draw on the full potential of AM technologies, alloys that exploit process inherent particularities such as extremely high cooling rates (ca. 106 K/s) have to be developed. One of most important AM-processes is Laser Powder Bed Fusion (LPBF), a batch-wise process. This complicates experimental alloy development and increases the use of powder resources since only one chemical composition can be tested within one test job and the process chamber has to be cleaned carefully in between. The process Extreme High-Speed Laser Material Deposition (EHLA) has been found to have similar cooling rates as LPBF, however it uses an in situ supply of powders which allows an easy switching between materials and has potential for rapid alloy development methods. Since the mechanical properties of materials primarily depend on chemical composition and microstructure, which in turn depends heavily on the cooling rates in the production process, the EHLA-process could be used as a means for an accelerated alloy development for LPBF. However, to explore this possibility, a thorough comparison of the two processes has to be performed.In this work, EHLA and LPBF processes are compared and evaluated regarding the following characteristics: process parameters, laser intensities and volume energy densities, resulting microstructure (primary dendrite arm spacing, DAS), melt pool size and shape. The reference samples were manufactured using one set of LPBF process parameters and EHLA samples were manufactured using three different sets of process parameters.The volume energy densities Ev [J/mm³] of the processes were found to differ by a factor 2.4 with higher Ev observed in LPBF. However, considering that approximately 2 to 3 layers are remelted with each pass of the laser beam, the introduced Ev per pass approximates the Ev introduced in the EHLA process. The melt pool size as seen in a cross section in the EHLA-manufactured samples is approximately 25 times larger than in the LPBF-manufactured samples and its depth to width ratio (d/w ratio) can be attributed to a heat conduction welding process while the d/w ratio observed in the LPBF-manufactured sample suggests a transition process between heat conduction welding and deep welding. The observed DAS is in the same order of magnitude for both processes ranging from 0.55 to 1.15 µm in EHLA-manufactured samples and 0.73 µm in the LPBF-manufactured reference sample. Since the resulting microstructures of samples manufactured with both processes show this common feature and EHLA process parameters can be adjusted to control the cooling rates, the transferability between EHLA- and LPBF-processes is supported in this first investigation. Research for a more efficient alloy development for LPBF using EHLA will be continued by e.g. examining chemical compositions and performing mechanical testing.

New approaches on laser micro welding of copper by using a laser beam source with a wavelength of 450 nm

Laser micro welding is specified with welding geometries below 1 mm and is used with increasing demand for contacting electronic components such as battery and fuel cells. Fiber lasers with a wavelength in the near infrared range (IR, λ ≈ 1 µm) have established themselves for this purpose. The laser welding process allows processing of parts in the micron range but reduces the surface quality of the processed parts at the same time. Furthermore, weld defects can occur due to process instabilities caused by the low absorptivity (<5%) of copper for infrared radiation. Therefore, alternative laser beam sources and processes have to be established, to avoid these negative effects on the weld seam quality. Laser beam sources in the visible wavelength range (VIS) prove to be an alternative due to an increased absorption of the laser energy in copper-based alloys.This paper presents the observation of laser micro welding of Cu-ETP and CuSn6 specimen with a thickness between 150 µm and 1 mm. The diode laser is specified by a wavelength of 450 nm and a nominal output power of 150 W. The surface roughness of the weld seam and the overall weld seam geometry for heat conduction welding are investigated. The laser energy absorption is measured using two integrating spheres to compare the results quantitatively to measurements conducted with laser sources of 1070 nm and 515 nm. For detailed observation high speed imaging is used to observe the melt pool dynamics. Simulations are conducted, to optimize the dimensioning of optics and laser beam source. Finally, the possible use of the novel laser beam source for various technical joining applications is discussed and evaluated and the influence of the use of protective gas is observed.

Blue high-power laser sources for processing solutions in e-mobility and beyond

The laser based processing of copper components with blue lasers with 450nm wavelength gained significance in the last years due to the increasing demand in eMobility applications. The high absorption level allows new process approaches, which were formerly however restricted by the limited power level of blue laser systems. With the availability of multi-kilowatt blue laser systems with 450 nm wavelength, a further step has been achieved to extend the application range for copper components. Process stages from heat conduction welding to keyhole welding can be precisely adjusted. The properties of these process stages however vary from the conventional behavior known from IR laser sources. In this contribution, the properties and differences compared to IR sources are investigated. This is achieved by an evaluation of the process stages by high speed videography and an investigation of the process robustness e.g. gap bridgeability in the example of battery applications. Furthermore, we will review the latest application results regarding blue laser welding for the battery cell manufacturing and the battery module interconnection. This consists of welding results of copper to copper connections and also dissimilar material combinations.

Characterizations of welding mode transformation process during 1-μm and 10-μm laser welding

To reveal the enhancing energy coupling effect of plasma, a comparison of the welding mode transformation process during 1-µm and 10-µm laser welding of aluminum alloys is carried out through experimental observation and theoretical analysis. The heat conduction welding stage hardly exists in 10-µm laser welding and obviously exists in 1-µm laser welding. Alloy composition and surface roughness of the welded plate hardly influence the deep penetration welding threshold (DPWT) of the 10-µm laser, whereas they have a significant impact on that of the 1-µm laser. In addition, 1-µm laser welding and 10-µm laser welding have similar DPWTs. These phenomena are attributed to the formation of plasma near the workpiece surface during 10-µm laser welding owing to metal vapor breakdown by 10-µm laser irradiation. The plasma remarkably enhances the energy coupling between the 10-µm laser and workpiece by thermal conduction, and the DPWT of the 10-µm laser is thus reduced to approximately equal to that of the 1-µm laser.

Hybrid Solution Moves Boundaries of Copper Welding

AbstractFor copper joining, blue diode lasers have made it possible to control heat conduction welding for the very first time. The technology, however, has reached its current limits with copper deep welding, where infrared lasers have also not achieved satisfactory results. A hybrid concept by Laserline that connects blue and infrared diode lasers has now opened up new manufacturing options. The approach does not only prove itself at the joining of sheets, but also at the static welding of hairpins – and in so doing moves the boundaries of copper welding altogether.

A study on welding mode transition by electrical detection of laser-induced plasma at varying energy levels

The transition between heat conduction and deep penetration laser welding is investigated by the electrical detection of laser-induced plasma with an electrical passive probe. A further improved physical model based on plasma sheath effect is established to clarify the mechanism of plasma electrical signal. Different electrical signals are detected at varying levels of laser input energy and analyzed in the time and frequency domains. The amplitudes and the fluctuation frequency of the collected signals are demonstrated to be effective in reflecting the electron temperature and the plasma oscillation features respectively. These assessments are identical with the experimental results obtained by a high-speed camera. Combined with the signal characteristics and the weld profiles at different laser powers, the critical state and laser power density of mode transition are discussed and identified by an analytic calculation model.