Periodic Reporting for period 1 - GlobalAM (Enabling Laser Powder Bed Fusion for Large Scale Production of Multi-Material Components)
Période du rapport: 2024-01-01 au 2025-06-30
While laser powder bed fusion (LPBF) inherently allows the production of complex geometries, it hasn't yet been introduced to mass markets due to prohibitive cycle times and uncompetitive product precision or quality. A hybrid production approach, where complex components utilising LPBF’s geometrical flexibility are built on top of conventionally manufactured substrates at near-net-shape geometry, can dramatically speed up the production process, especially if applied to small component volumes. GlobalAM aims to advance and combine existing state-of-the-art approaches to produce superior components on a large scale.
The project’s objectives are demonstrated for a cooling device for power electronics, as it combines typical challenges: complex geometries made from challenging materials, such as copper, are built directly on a ceramic-based substrate with high precision requirements. The project objectives derived thereof include (1) a defect-free production process for intricate structures improving the demonstrator’s functionality, (2) an advanced machine concept which allows a high degree of automation and process control, (3) optimised materials for improved process and functional properties.
If the technological barriers towards the demonstrator can be solved, GlobalAM will introduce – but not limit – LPBF to the multi-billion euro mass market of power electronics. Due to its flexibility, the fact that material resources are used close to 100%, and its expected precision, it will improve functional performance, reduce costs and environmental impact and finally create unique selling points.
The project’s objectives are demonstrated for a cooling device for power electronics, as it combines typical challenges: complex geometries made from challenging materials, such as copper, are built directly on a ceramic-based substrate with high precision requirements. The project objectives derived thereof include (1) a defect-free production process for intricate structures improving the demonstrator’s functionality, (2) an advanced machine concept which allows a high degree of automation and process control, (3) optimised materials for improved process and functional properties.
If the technological barriers towards the demonstrator can be solved, GlobalAM will introduce – but not limit – LPBF to the multi-billion euro mass market of power electronics. Due to its flexibility, the fact that material resources are used close to 100%, and its expected precision, it will improve functional performance, reduce costs and environmental impact and finally create unique selling points.
To reach the project goals, the most pertinent approaches in terms of functional performance and technology readiness level were chosen from an initial state-of-the-art analysis (WP1). Subsequently, the selected solution approaches for each technology were evaluated and further developed in WP2 and WP3.
The defect-free production of the project demonstrator requires a deep understanding of the relevant physical mechanisms. Hence, modelling plays a key role. To address the multiphase-multiphysics nature of the problem, a meltpool model considering radiation, fluid dynamics and thermal phenomena was developed in OpenFoam and a thermo-mechanical FEM model in Abaqus. Both were validated against measured data.
Residual stress information was obtained from dedicated XRD measurements on simple design elements which consider the coarse-grained and textured nature of the microstructure of LPBF-built components by adapting a weighted sin²ψ method.
Extended experimental activities explored the processing window for different powder materials, intensity profiles and laser sources as a function of the geometry of simple design elements.
For an advanced machine concept, a fixation system was developed which allows the simultaneous fixation of the delicate substrates in a semi-automated system.
Besides, a methodology was developed to accurately determine the true substrate positions based on a rake-mounted high-resolution line scan camera. Also, process monitoring was employed to detect excessive heat accumulation and cracks within the insulating ceramic substrate layer. To achieve this goal, a thermal near-infrared camera and a structure-borne acoustic emission sensor were installed.
Additionally, an optical coherence tomography system was evaluated as a means to measure geometric deviations.
Further activities centre around the development of optimised LPBF powder material. To allow an almost complete material usage, a regeneration process for the non-molten, oxidised Cu powder material was developed. Also, Cu powder is difficult to process with standard infrared laser sources due to its high reflectivity. Hence, surface modification methods were tested with regard to improved absorption properties. Finally, efforts to create metal matrix composites from Cu and SiC aimed to improve the mechanical properties of the components.
The defect-free production of the project demonstrator requires a deep understanding of the relevant physical mechanisms. Hence, modelling plays a key role. To address the multiphase-multiphysics nature of the problem, a meltpool model considering radiation, fluid dynamics and thermal phenomena was developed in OpenFoam and a thermo-mechanical FEM model in Abaqus. Both were validated against measured data.
Residual stress information was obtained from dedicated XRD measurements on simple design elements which consider the coarse-grained and textured nature of the microstructure of LPBF-built components by adapting a weighted sin²ψ method.
Extended experimental activities explored the processing window for different powder materials, intensity profiles and laser sources as a function of the geometry of simple design elements.
For an advanced machine concept, a fixation system was developed which allows the simultaneous fixation of the delicate substrates in a semi-automated system.
Besides, a methodology was developed to accurately determine the true substrate positions based on a rake-mounted high-resolution line scan camera. Also, process monitoring was employed to detect excessive heat accumulation and cracks within the insulating ceramic substrate layer. To achieve this goal, a thermal near-infrared camera and a structure-borne acoustic emission sensor were installed.
Additionally, an optical coherence tomography system was evaluated as a means to measure geometric deviations.
Further activities centre around the development of optimised LPBF powder material. To allow an almost complete material usage, a regeneration process for the non-molten, oxidised Cu powder material was developed. Also, Cu powder is difficult to process with standard infrared laser sources due to its high reflectivity. Hence, surface modification methods were tested with regard to improved absorption properties. Finally, efforts to create metal matrix composites from Cu and SiC aimed to improve the mechanical properties of the components.
The models developed were applied to the demonstrator system and used for systematically studying the influence of process parameters. Thus, process conditions which are attractive in terms of minimum stress, distortion and surface roughness can be identified. E.g. it was found that due to the very high conductivity of Cu, the meltpool is circular and highly sensitive to the peak intensity, which in turn may lead to a very rapid change from a shallow meltpool in conduction mode to a deep melt pool in keyhole mode.
As input to the model validation process, accurate stress data is required, which requires a methodology to consider coarse grains and texture as a typical characteristic of LPBF-generated microstructure. Applying the method, it was found that the residual stress inside Cu components is low due to the limited yield strength, while the residual stress in the ceramic substrates is significantly higher. Also, the influence of different beam shapes on the microstructure could be revealed. With a standard Gaussian beam, relatively small grains but a strong texture were observed, while a top hat beam profile produced large grains with negligible texture for the Cu specimen.
One key result in terms of process development is the fact that IR laser sources are able to generate fully dense Cu species with a density of ≥ 99.7% at optimum process conditions. However, the process window is small, leading either to insufficient densities or the destruction of the sensitive substrate if exceeded. Also, it was found that differences in functional properties are rather small for samples built with a Gaussian vs. samples built with a top hat beam profile.
For secure positioning and handling, a simple yet efficient fixture system was developed. It proved to handle the delicate substrates without introducing any defects. An electromagnet-based mechanism allows a fast exchange of base plates in the LPBF system.
Furthermore, a camera-based positioning system is developed, which determines the position of individual substrates with an accuracy of ≤ 30 µm in x- and y-direction and ≤ 0.1° concerning rotation. Also, a structure-borne sensor system was evaluated with regard to the detection of substrate cracking. It was found that transforming the time frequency domain signal into an envelope description and reducing the latter to a 2D representation by PCA allows detecting larger cracks with an F1-score of > 99.9% applying the SVM algorithm. Additionally, the ability of an OCT to resolve the surface height was evaluated and a resolution in the range of ≤ 30 µm in z-direction was found.
On the material side, a robust process to regenerate oxidised Cu powder was developed. It reduces the oxygen content of used powder by reduction in H2-atmosphere without impacting flowability or process parameters. Preliminary tests with recycled powder showed a comparable dependence of porosity and surface roughness on laser power and scan speed as virgin powder. Also, the effects of surface modification by etching were evaluated. However, absorption improved little, and the process window remained limited. Efforts to produce test samples of metal matrix composites from SiC and Cu failed, though, due to the large density differences and a lack of adequate homogenization.
As input to the model validation process, accurate stress data is required, which requires a methodology to consider coarse grains and texture as a typical characteristic of LPBF-generated microstructure. Applying the method, it was found that the residual stress inside Cu components is low due to the limited yield strength, while the residual stress in the ceramic substrates is significantly higher. Also, the influence of different beam shapes on the microstructure could be revealed. With a standard Gaussian beam, relatively small grains but a strong texture were observed, while a top hat beam profile produced large grains with negligible texture for the Cu specimen.
One key result in terms of process development is the fact that IR laser sources are able to generate fully dense Cu species with a density of ≥ 99.7% at optimum process conditions. However, the process window is small, leading either to insufficient densities or the destruction of the sensitive substrate if exceeded. Also, it was found that differences in functional properties are rather small for samples built with a Gaussian vs. samples built with a top hat beam profile.
For secure positioning and handling, a simple yet efficient fixture system was developed. It proved to handle the delicate substrates without introducing any defects. An electromagnet-based mechanism allows a fast exchange of base plates in the LPBF system.
Furthermore, a camera-based positioning system is developed, which determines the position of individual substrates with an accuracy of ≤ 30 µm in x- and y-direction and ≤ 0.1° concerning rotation. Also, a structure-borne sensor system was evaluated with regard to the detection of substrate cracking. It was found that transforming the time frequency domain signal into an envelope description and reducing the latter to a 2D representation by PCA allows detecting larger cracks with an F1-score of > 99.9% applying the SVM algorithm. Additionally, the ability of an OCT to resolve the surface height was evaluated and a resolution in the range of ≤ 30 µm in z-direction was found.
On the material side, a robust process to regenerate oxidised Cu powder was developed. It reduces the oxygen content of used powder by reduction in H2-atmosphere without impacting flowability or process parameters. Preliminary tests with recycled powder showed a comparable dependence of porosity and surface roughness on laser power and scan speed as virgin powder. Also, the effects of surface modification by etching were evaluated. However, absorption improved little, and the process window remained limited. Efforts to produce test samples of metal matrix composites from SiC and Cu failed, though, due to the large density differences and a lack of adequate homogenization.