Gwenn Pallier, Jannik Lind and Jorge Luis Arias Otero provide an update on the results of the Custodian project
The partners of the Custodian project have developed a methodology1 for beam shape customisation, designed to eliminate defects in high-power laser processes such as cutting, welding and additive manufacturing.
The methodology has been outlined in figure 1. First, an in-depth analysis of a laser process/material combination is performed, which reveals the optimal thermal cycle under which the occurrence of defects is eliminated. In a second step, proposed beam shapes are introduced in the laser process model and tested in simulations, which conclude the beam shapes capable of providing the required thermal cycle2,3. These beam shapes can then be implemented by a beam shaper based on Cailab’s Multi Plane Light Conversion (MPLC) technology.
In parallel, a real time closed-loop control system is developed based on a high speed medium-wave infrared (MWIR) sensor from NIT Europe, and a fast field-programmable gate array (FPGA) hardware architecture. The information gathered during the initial process analysis is used at this stage, in order to determine which monitoring temperature-related features are correlated to those laser process parameters affecting the defect occurrence.
Figure 1: Methodology for laser beam shape customisation, aimed to guarantee quality in laser-based manufacturing
Eventually, both the MPLC beam shaper and the real time process control system can be integrated into a laser-based manufacturing facility in order to deliver the final solution.
Putting the methodology to work
In the case of laser cutting, the Custodian methodology was applied to identify, realise and test new beam shapes that optimise the cut quality and feed rate when processing thick materials.
Figure 2 shows the procedure for the determination of a new beam shape. First, a sample with a high cut quality was selected, previously cut using a standard cutting head from Precitec. In order to investigate how the beam shape influenced the cutting process, the geometry of the cutting front was observed by means of online high-speed X-ray diagnostics. A clear contrast between the solid sample material (dark, high absorption of X-rays) and the cutting kerf (light, low absorption of X-rays) is visible in the grayscale image. The grayscale value of the X-ray image provides spatial information about the geometry of the cutting front, from which the width of the cutting kerf and its shape at the front can be extracted and three-dimensionally reconstructed4.
Figure 2: Methodology to identify new beam shapes: a) surface of the cut edge of a stainless steel sample, b) X-ray image of the cutting process, c) 3D-reconstruction of the cutting front geometry, d) absorbed irradiance distribution on the cutting front geometry
The 3D reconstruction was used to calculate the distribution of the absorbed irradiance on the cutting front geometry. For this calculation ray-tracing software was used5. To optimise the process, different beam shapes were defined in the ray-tracing software and applied to the given cutting front geometry. The goal was to homogenise the distribution of absorbed irradiance on the cutting front and to reduce losses.
Using this optimisation approach, the defined beam shape could be realised with the MPLC and tested by cutting trials.
A complex beam shaping system
Different beam shapers were developed and tested within the Custodian project for different applications, including a unique beam shaper to improve laser cutting, based on MPLC.
MPLC is a unique technology capable of shaping beams based on mode propagation that enables any unitary spatial transform to be performed. It converts any set of orthogonal spatial modes into any other sets of orthogonal modes through a succession of transverse phase profiles separated by free-space propagation serving as a fractional Fourier transform operation. MPLC is implemented in a reflective way in order to get a more compact system: the light is classically going back and forward in between a textured mirror and standard mirror, as illustrated in figure 3.
Figure 3: Theoretical MPLC implementation with multiple fibred inputs (left). An actual telecommunication MPLC (right)
MPLC enables multiple inputs (combining) and multiple outputs (separated shapes), both being either fibred or free-space, with a high control over the amplitude profile and the phase profile.
The optimal implementation of MPLC for high-power (multi-kW) continuous-wave laser applications is within a laser-head with multiple mirrors, each of them being a phase plate6. With this implementation the cooling of each optic is optimal and the robustness of the shaping is improved with a reduced focus shift. In the case of the cutting beam shaper, the shaping is done through a succession of five phase plates, as illustrated in figure 4.
Figure 4: MPLC beam-shaper integrated in a laser-head
The transformation is theoretically lossless, as opposed to diffractive optical element (DOE) shaping, for example, for which the conversion efficiency implies a significant loss of power. Moreover, the fully reflective implementation with optimal optical coatings provides a transmission exceeding 99 per cent.
A high control of the intensity profile is possible thanks to the capability to have multiple phase plates: the shaped beam can reach the diffraction limit quality. In addition to the shaping quality, MPLC enables complex shaping such as non-symmetric shapes, which is not possible with double-core fibre lasers, for example. A high control of the phase profile is possible too, which is unique, and means that the depth of field is improved compared to other shaping technologies.
This unique capability has been implemented in order to provide a complex shape tailored to the cutting application: a dot and a C-shape separated in space, as described in figure 5. The energy ratio Ec/Edot target is 4.3.
Figure 5: (left) Target shape for the laser cutting beam-shaper. (right) Optical simulation results
Cutting with customised beam shapes
The beam-shaper has been developed to be compatible with a Trumpf 8001 laser with the following specifications: 1µm wavelength, 8kW power, 0.1 numerical aperture, 100µm fibre diameter and a BPP of 4mm.mrad. The shaper is integrated in between a collimation mirror with a 100mm focal length, and a focusing module from Precitec with a 150mm focal length. The shaper, integrated in a laser head, fits in an industrial environment, and may be integrated on a robot arm, as it was done at the Institut Maupertuis for the first performance test, as illustrated in figure 6.
Figure 6: MPLC beam shaper tested at the Institut Maupertuis integrated on a robot arm (left). Analysis of the output shape (right)
At Precitec facilities sheets of stainless steel with thicknesses ranging from 5mm to 30mm were successfully cut with a maximum laser power of 8kW, as illustrated in figure 7. The next steps are to compare the cut quality of the samples of the cutting tests with cut samples produced with a conventional cutting head (without beam shaping).
Figure 7: Cutting process using an MPLC-cutting head (left). Surface of the cut edge with thicknesses ranging from 5mm to 30mm (right)
Beam shaping for welding and additive manufacturing
In addition to the improvement of the laser cutting process, dynamic beam shapers tailored for laser welding and laser powder bed fusion (L-PBF) have also been developed within the Custodian project.
For welding, the objective is to guarantee geometry quality of the welding bead when there is a gap between the two pieces to be welded. The targeted shape is an intense spot with a lower rectangle background shape to reduce the thermal gradient. The shape can be adjusted dynamically: the width of the rectangle and the power ratio in between the shapes are tunable. The system, shown in figure 8, is currently being tested at the Aimen Technology Centre.
Figure 8: MPLC-based laser head designed for laser welding optimisation
In the case of L-PBF, the aim is to solve hot cracking during additive manufacturing of nickel superalloy components. The targeted shape is an intense spot with a lower energy disk background shape. The shape can be adjusted dynamically: the diameter of the disk and the power ratio in between the shapes are tunable. The system, shown in figure 9, is currently being tested at the Aidimme Metalworking Technology Institute.
Figure 9: MPLC-based laser head designed for laser powder bed fusion optimisation
Conclusion
The methodology developed within the Custodian project has provided three different target shapes to improve different high-power laser processes with beam-shaping: cutting, welding, and LPBF. Dynamic beam-shapers based on MPLC have been developed and are currently under testing for both the welding and L-PBF applications. Cutting with a C-dot shape has already been demonstrated, capable of processing stainless steel sheets up to 30mm in thickness.
This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement nº 825103. Custodian is an initiative of the Photonics Public Private Partnership.
Gwenn Pallier is project & product manager at Cailabs
Jannik Lind is a PhD candidate at the University of Stuttgart’s IFSW in research collaboration with Precitec
Jorge Luis Arias Otero is a senior researcher at Aimen and director of the Custodian project
References
[1] Matthieu Meunier, et al. Freeform Beam Shaping with Multi-Plane Light Conversion for 1.07µm Ultra-High Throughput Laser-Based Material Macroprocessing. ICALEO 2019; Paper ID # 0339_0501_000175
[2] Pierre Drobniak, et al. Simulation of keyhole laser welding of stainless steel plates with a gap. Procedia CIRP; 10:1016
[3] Michele Buttazzoni et al. A Numerical Investigation of the Laser Beam Welding of Stainless Steel Sheets with a Gap. Applied Science ; 10.1190
[4] Lind Jannik, et al. Transition from stable laser fusion cutting conditions to incomplete cutting analysed with high-speed X-ray imaging. J Manuf Process 2020; 60:470–80
[5] Lind Jannik, et al. Geometry and absorptance of the cutting fronts during laser beam cutting. J Laser Appl 2020;32(3):3201
[6] Matthieu Meunier, et al. High throughput laser beam welding with 16 kW 1.03 µm annular beam shaping. Photonics West 2021; Paper number 11679-8