Control of geometry and metallurgy in laser beam microwelding by influencing the melt pool dynamics via locally and temporally adjusted energy input

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Arnold Gillner

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1. General

According to the current state-of-the-art in research, there is no methodology as yet that remedies or compensates for the process instabilities in laser micro welding, which are due to the dynamics of the molten phases. Here, an approach using temporally and locally adapted energy deposition offers the greatest potential. The aims of the project include an analysis of the individual precision-determining aspects of laser beam micro welding, a model-based evaluation of different influence factors, and, as a result, the derivation of methods and process control strategies to significantly improve quality and precision. The project has a two-stage objective:

  • Determination of precision-determining time constants and process boundary conditions using high-resolution process visualization at a high temporal resolution and, at the same time, with increased process understanding (in cooperation with A03).

  • Increase of the weld seam precision with regard to geometric properties (welding depth consistency ≤ 10%, welding depth control ≤ 10%, surface roughness Ra ≤ 10 µm and porosity ≤ 5%).

These objectives are to be attained by modulating energy deposition, thus manipulating both the melt pool size – and with it the weld depth and weld seam width – and the dynamics of the molten pool with the convective energy transfer. Different local and temporal performance modulation strategies are to provide insights into the controllability of seam geometry and molten pool dynamics and contribute to stabilizing the vapor capillary.

In order to obtain a thorough understanding of the physical processes for an evaluation and weighting of quality-determining factors, a detailed analysis of energy input, the melting process, and the molten pool dynamics through innovative methodological diagnostics approaches in laser beam micro welding of technologically relevant metal alloys (in particular, Cu and Al materials).

2. Phase 2

2.1 Analysis using synchrotron radiation

The precise analysis of melt-based processes is the main focus of the subproject in phase 2 of SFB1120. In subproject A01, a full-scale laser beam processing facility was set up for the first time at the German Electron Synchrotron in Hamburg, with which in situ transmission experiments can be carried out. This allows stationary and highly dyanamic laser beam welding, cutting and drilling processes to be carried out using a high-speed axis. In the first iteration loop of the experiments, a spatial resolution of 5 µm and a temporal resolution of 1 ms have been achieved so far. This makes it possible to also record microscopic processes with a vapour capillary diameter of only 35 µm. Figure 1 shows the in situ X-ray phase contrast experiments performed so far.

 
  Figure 1: Illustration of the synchrotron experiments at the German Electron Synchrotron (DESY), a) Experimental setup developed and implemented by LLT; b) Laser beam welding experiment on Al99.5 with dF = 100 µm, PL = 2000 W, v = 600 mm/s, λ = 1030 nm; Copyright: © SFB 1120
 
 

Figure 1: Illustration of the synchrotron experiments at the German Electron Synchrotron (DESY), a) Experimental setup developed and implemented by LLT; b) Laser beam welding experiment on Al99.5 with dF = 100 µm, PL = 2000 W, v = 600 mm/s, λ = 1030 nm; b) Laser beam welding test on Al99.5 with dF = 35 µm, PL = 500 W, v = 200 mm/s, λ = 1070 nm; d) 3D reconstruction from 2D image data using Lambert-Beer's law and numerical ray tracing on the vapour capillary surface - false colour representation of the laser exposure intensity in W/cm2.

 
 

The experimental set-up allows flexible performance of the in situ X-ray phase contrast experiments by arranging the material sample, laser beam and synchrotron beam at right angles. The images acquired via the scintillator (Fig. 1, b) and c)) show the phase boundaries (solid-liquid and liquid-gaseous) formed in the process as well as the material surface. From the 2D data obtained, the 3D geometry of the vapour capillary can then be obtained subsequently using Lambert-Beer's law. Finally, this 3D surface structure serves as a template for performing ray tracing simulations in collaboration with SP A03 and A09, with which the propagation of the laser radiation in the vapour channel can be calculated in order to determine the influence of the exposure intensity on the process.

  Exemplary representation and comparison of heat conduction welding and deep penetration welding using synchrotron radiation for visualization.
 
 

2.2 Spatial and temporal power modulation

Functional principle of local power modulation in laser beam microwelding. In contrast to conventional laser beam welding, the laser beam is moved in a spiral over the component surface to be joined. The shape of the movement is created by a circular superimposition of the linear feed movement. In addition to stabilising the welding process, the local power modulation leads to an increase in efficiency and the joining cross-section.

 
  Comparison of conventional laser beam micro welding and laser beam micro welding with spatial power modulation on copper.
 
 

At the same time, however, due to the different path speed along the weld seam, caused by the oscillation movement, a tilting of the seam bottom occurs Fig. 2 a). One approach to compensate for these process fluctuations is the synchronised superposition of a local and temporal power modulation. The resulting welds are shown in Fig. 2 b-c).

  Figure 2: Precise adjustment of the weld seam geometry by superimposing local and temporal power modulation: a) Weld seam with local power modulation; b) Weld seam with local and temporal power modulation "W" geometry; c) Weld seam with local and tempora Copyright: © SFB 1120

Fi gure 2: Precise adjustment of the weld seam geometry by superimposing local and temporal power modulation: a) Weld seam with local power m odulation; b) Weld seam with local and temporal power modulation "W" geometry; c) Weld seam with local and temporal power modulation "V" geometry; d)+e) Compensation of the seam tilt in unequal material combinations caused by different material properties.

 
 

By selectively superimposing a local and temporal power modulation, it was now possible to create other weld geometries in cross-section (W- or V-shape) in addition to the increase in the connection cross-section. The initial geometry, shown in Fig. 2 a), shows a tilt to the right side in the cross-section. This tilting could already be compensated for at the end of phase 1. By extending the superposition of the local power modulation with a temporal power modulation, "W"- and "V"-shaped profiles can now be generated in the cross-section of the weld. This can be used, for example, to join sensitive components in an I-joint arrangement with even more targeted energy input. The same system can be used for dissimilar material systems (Fig. 2 d) and e)).

2.3 Infrared and green laser radiation for laser beam welding

In today's world, where climate change and the depletion of fossil fuels are a daily topic of conversation, the importance of switching to renewable electrical energy sources is becoming increasingly urgent. For this change, new electrical storage technologies are needed on the one hand, and on the other hand, manufacturing processes to connect the electrical contacts of battery packs, for example, without defects and with short process times.

With the widespread use of laser beam sources in the manufacturing industry, a further increase in process reliability and reproducibility of results in laser beam welding is becoming increasingly important. This is necessary to reduce costs and save resources. In addition, the aim is to optimise energy efficiency in order to save further costs. Laser beam sources with a wavelength in the near-infrared range between 1030 and 1070 nm are widely used today for processing metallic materials.

 
  Figure 3: Laser beam welding of electronic components and energy storage devices with laser radiation in the visible wavelength range Copyright: © Fraunhofer ILT Figure 3: Laser beam welding of electronic components and energy storage devices with laser radiation in the visible wavelength range
 
 

Thanks to their high brilliance, they offer the possibility of focusing the laser beam to a few 10 micrometres beam diameter. Laser beam sources with a wavelength in the visible range seem to be an ideal alternative due to the increased absorption from 5% (IR) to up to 60% (VIS) of the laser radiation on copper, which will be investigated in detail in this subproject in terms of their mode of action with the material

 
  Laser beam welding with visible wavelengths
 
 

2.4 Laser polishing of welds seams

Laser polishing is investigated as a possibility to subsequently improve the surface quality of laser welds. In this process, the weld surface is melted again by means of laser radiation with a temporal and spatial offset. The laser parameters can be selected in such a way that the molten bath is guided as homogeneously and undisturbed as possible and thus the surface solidifies smoothed by the surface tension of the melt. In contrast to conventional mechanical polishing, polishing is not achieved by ablation but by redistribution of material in the melt.

In this project, the aim is to minimise the temporal and spatial offset of the subsequent laser polishing process by combining the two processes. For this purpose, a two-beam optical system was developed that enables a second polishing laser beam to be focused simultaneously with the welding laser beam at an adjustable distance on the workpiece surface.

The investigations were carried out on copper (Cu-ETP) and the copper-based alloy CuSn6 without inert gas. For laser polishing, green (515 nm) and blue (450 nm) laser radiation was used, as it is not possible to create a stable molten pool with the infrared (1 µm) wavelength commonly used in industry due to the absorption properties of copper. First, the separate, i.e. non-simultaneous laser polishing of weld seams was investigated and suitable process parameters identified. On both materials, the roughness could be halved, as exemplified in Figure 4. By using the same laser spot for both processes, the polishing melt pool was smaller than the welding melt pool, which is why overlapping polishing was performed.

 
  Figure 4: Separate laser polishing (left roughness Ra 5 µm) of a laser weld (right unpolished Ra 11 µm) on copper (Cu-ETP). Both processes were carried out with the same laser spot with 515 nm wavelength without inert gas. Top: Microscope image. Bottom: Copyright: © SFB 1120

Fi gure 4: Separate laser polishing (left roughness Ra 5 µm) of a laser weld (right unpolished Ra 11 µm) on copper (Cu-ETP). Both processes were carried out with the same laser spot with 515 nm wavelength without inert gas. Top: Microscope image. Bottom: False colour representation of the surface topography. Melt splashes and unevenness are clearly smoothed and the roughness is halved.

 
 

The next step was to investigate simultaneous laser polishing using dual-beam optics. With this setup, a small infrared laser spot (approx. 50 µm), suitable for welding, can be combined with a large blue laser spot (approx. 400 µm), suitable for laser polishing in one pass. This simultaneous combination process made it possible to reduce the roughness by a factor of 3 to 6, depending on the welding parameters, and to achieve roughnesses of Ra = 1 µm. For copper, the result of a simultaneous laser polishing is exemplarily shown in figure 5.

 
  Figure 5: Microscope image of a simultaneously laser polished weld track on copper (Cu-ETP). Right: Initial state of the weld seam produced with 1030 nm (IR). Left: Simultaneously laser polished area by dual beam optics with 450 nm wavelength (process si Copyright: © SFB 1120

Bild 5 : Separate Laserpolitur (links Rauheit Ra 5 µm ) einer Laserschweißnaht (rechts unpoliert Ra 11 µm ) auf Kupfer (Cu-ETP) . Beide Bearbeitungen wurden mit demselben Laserspot mit 515 nm Wellenlänge ohne Schutzgas durchgeführt. Oben: Mikroskopaufnahme. Unten: Falschfarbendarstellung der Oberflächentopographie . Schmelzspritzer und Unebenheiten werden deutlich geglättet und die Rauheit halbiert.

 
 

Different distances of the two laser spots and thus of the melting baths were also investigated. As a result, the achievable roughness is not significantly dependent on the distance as long as the melting baths do not merge. In high-speed video recordings of the combined process, it can be clearly seen how much quieter and more undisturbed the polishing melt pool is compared to the welding melt pool with its steam capillary, which in this case also periodically circles due to the wobbling of the laser beam.

 
  High-speed recording of a simultaneous laser beam welding and polishing process with 1070 nm (welding) and 450 nm (polishing) wavelengths
 
 

3. Phase 3

3.1 Research question

The investigations in SFB1120 have so far shown the possibilities of influencing the processes with regard to precision in the geometric dimensions and increasing the quality of the welds by means of local and temporal modulation of the laser power and the use of shorter laser beam wavelengths. All approaches developed individually so far have been investigated on an analytical basis to examine the direct influences. In the continuing application for the research project, the focus of the work is now on the combination of these previous approaches and their mutual interaction in order to determine how precise control of the process can lead to compensation for process fluctuations in the form of, for example, different energy deposition and the resulting errors in laser beam welding. In order to validate the influences of these combinations and to analyse them directly on the basis of the capillaries produced in the process, extended investigations are being carried out using in situ analysis methods such as the phase contrast imaging techniques at the German Electron Synchrotron in Hamburg. The following questions will be specifically investigated:

  • How can the combination of different laser beam wavelengths in the visible (VIS) and near-infrared (NIR) range be used to compensate for absorption fluctuations and energy deposition on copper alloys and dissimilar material systems?

  • How can a superposition of spatial and temporal power modulation be used in dissimilar material systems when using several laser beams in order to influence variations in the welding depth along and across the feed direction?

  • How can a reduction of pore formation and surface roughness be achieved by superimposing several partial beams and their modulation in the molten pool, and how can the formation of defects be avoided during high-speed welding > 1 m/s?

3.2 Objective

A two-stage objective is defined for the project, which is oriented towards the scientific-technical questions and the solution variants for achieving high precision in laser beam microwelding. The result-oriented objectives are based on manufacturing accuracy and quality targets: Weld depth consistency ≤ 10%, weld depth control ≤ 10%, surface roughness Ra ≤ 10 µm, porosity ≤ 5%.

In addition, the following process engineering objectives are defined for the 3rd phase of the project:

  • Realisation of a flexible irradiation unit with dynamic multi-wavelength adjustment of the energy deposition.

  • Realisation of a dynamic beam scanning unit with multi-beam approach and dynamic modulation of the individual partial beams.

Both process-technical goals serve to homogenise and maintain a stable keyhole in which the melt dynamics and the fluctuations of the energy deposition can be significantly reduced. These approaches are used to evaluate final joining configurations for laser beam microwelding that are not yet technically possible, especially on components with sensitive welding depth dependence.

The investigations at the German Electron Synchrotron DESY continue to serve as an important tool for evaluating the various technical approaches and determining the necessary beam and scan parameters for setting a stable energy coupling, with which both the physical principles of the processes and the superimposed compensation measures are investigated. From the above-mentioned measurements, further quantities can be derived with the help of numerical models, such as the viscosity and the surface tension of the melt, which are decisive for the design of the compensation methods. In addition to the planned work for TP A01, the setup at DESY potentially serves for other TPs of the SFB, such as A04, A10 and B07, and can also serve as a platform for further investigations at other research facilities.

3.3 Methodologies

To achieve the objectives, several process and analytical methods are combined. The methodological focus of the work is the use of different steel combinations, with which both the absorption and thus the energy deposition can be controlled via wavelength combinations and the keyhole and its geometry can be influenced via multi-beam approaches and beam shaping. In situ X-ray videography at the German Electron Synchrotron DESY continues to serve as an analytical tool for evaluating the effectiveness of these process engineering approaches. The methodological approach is again divided into three stages. The analysis and understanding of the processes will continue to form part of the scientific approach. In phase 3, however, the main focus is on combining and influencing the factors that affect quality. The three-stage approach is shown in Figure 6.

 
  Figure 6: Methodical procedure in the investigation and development of laser beam welding in SP A01 Copyright: © SFB 1120 Figure 6: Methodical procedure in the investigation and development of laser beam welding in SP A01
 
 

The main focus of the investigations will continue to be the energy coupling in the process. It has been shown that by precisely adjusting the local and temporal energy deposition by varying the wavelength, laser power and beam distribution, it is possible to precisely control the seam geometry and compensate for the formation of defects. Superposition with other laser beams in the visible wavelength range as well as shaping of the partial beams with geometries deviating from rotationally symmetrical ones and multi-beam approaches are being investigated. It is expected that the energy input into the material, the cooling rates and the flow dynamics in the melt can be influenced in a targeted manner. The setup at the German Electron Synchrotron will be used as a tool for investigating the following physical-dynamic processes and properties for copper, aluminium and steel and base alloys derived from them:

  • Spatial distribution of energy deposition on the material surface

  • Influence of the laser wavelength on the material-laser interaction

  • Geometry of vapour capillaries and melt films and speed of fluctuations

  • Melt acceleration and flow profiles (vector and velocity) in the melt

Besides the geometry of the keyhole, melt movements that continuously change the walls of the keyhole are decisive for the exact influence of the melt dynamics in the weld pool and the absorption in the keyhole. The possibilities of X-ray in situ analysis are used to analyse these melting movements as a function of the set irradiation conditions. In combination with corresponding simulations from SP A03, information on temporal and spatial processes of energy input, vapour formation and melt flow can be determined. Currently, there is no method for an in-situ analysis of these important process properties that provides information about important process interactions. The use of synchrotron radiation closes this gap and enables the fundamental in-situ investigation of laser beam processes with high temporal and spatial resolutionröntgenographischen In situ Analyse herangezogen. In Kombination mit entsprechenden Simulationen aus dem TP A03 lassen sich hierüber Angaben zu zeitlichen und räumlichen Prozessen des Energieeintrags, der Dampfbildung und des Schmelzflusses bestimmen. Derzeit gibt es keine Methode für eine in-situ-Analyse dieser wichtigen Prozesseigenschaften, die Aufschluss über wichtige Prozessinteraktionen gibt. Der Einsatz von Synchrotronstrahlung schließt diese Lücke und ermöglicht die grundlegende in-situ-Untersuchung von Laserstrahlprozessen mit hoher zeitlicher und räumlicher Auflösung.