Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PAM^2 (Precision Additive Metal Manufacturing)

Teaser

Additive Manufacturing (AM) is an increasingly used production method with its almost unlimited design freedom, capability to produce personalised parts and efficient material use. Currently several challenges remain such as limited precision due to shrinkage, built-in...

Summary

Additive Manufacturing (AM) is an increasingly used production method with its almost unlimited design freedom, capability to produce personalised parts and efficient material use. Currently several challenges remain such as limited precision due to shrinkage, built-in stresses and dross formation at overhanging structures and a limited process stability and robustness. Post-processing is often needed as as-built parts have high surface roughness and possibly remaining porosity.
PAM² (Precision Additive Metal Manufacturing), is a European MSCA project with 10 beneficiaries and 2 partners collaborating on improving the precision of metal AM for each process step going from design until assessment. A laser and powder-bed based fusion (LPBF) AM process is used. The overall objective of PAM² is to ensure the availability of high precision AM processes and design procedures. Detailed objectives to reach this overall goal are:
1. to develop advanced design tools enabling competitive designs, better use of AM possibilities against minimal design costs and reduced time-to-market
2. to develop better modelling tools for first-time-right processing
3. to optimize SLM process strategies for improved part precision and feature accuracy
4. to understand the link between post-process metrology and in-process observations, creating the basis for in-process quality control and process stability
5. to develop innovative in-process and post-process techniques to reduce/remove roughness, porosity and internal stresses and to improve dimensional accuracy and mechanical properties.

Work performed

Design:
A computationally inexpensive simplified thermal model, called a ‘hotspot detector’, is developed to detect zones of local heat concentration. Finite element implementation of the hotspot detector can be integrated with the density based topology optimization to generate robust designs free of local overheating zones. An advantage of this method is that it incorporates the temperature response of a geometry instead of imposing explicit prohibition of overhangs. The latter is found to be ocassionally overrestrictive, leading to sub-optimal designs.

Modelling:
Full numerical modelling of an LPBF process is usually very time-consuming. Simulating even a single layer of a tiny part needs a lot of computation time. As a result, full numerical simulations cannot be extended to bigger parts. Therefore, a novel strategy is developed for doing a full thermo-mechanical simulation of an actual metal part made by LPBF. The method is a combination of a full thermo-fluid dynamic model and a lumped model which reduces computation time while keeping an acceptable accuracy. This way complete parts can be simulated and possible issues can be detected before processing. Finally, cellular automata is used to model the microstructural evolution during post-processing.

Processing:
To evaluate different AM processes and machines, a benchmark design was made (Figure 1) which allows to evaluate accuracy and precision of the machine, residual stresses, homogeneity, build speed, mechanical properties and surface finish. It includes challenging features which allow to track new developments on improving precision. For example a dynamic focusing unit was used to combine high productivity for the inner part by using a large spot size with high outer precision by using the smallest spot size. The surface quality and dimensional accuracy of down-facing surfaces produced by LPBF were also investigated.

Post-processing:
Post-process laser polishing is investigated for reduction of surface roughness where different pulse, scan speed, repetition rate and average laser power are used and the impact on material surface, melt pool geometry and microstructure are analyzed.

Metrology:
Both in-process and post-process metrology are used. For in-process metrology both the use of optical meltpool monitoring and 3D geometrical measurements through the use of a compact focus variation system are envisioned. For post-process metrology, micro-focus X-ray CT and fringe projection profilometry are investigated.

X-ray CT
In X-ray CT, a sufficiently small focal spot size is necessary to analyse micro-features. A limiting factor of CT for high resolution inspection is the finite size of the focal spot and much effort has been put into the reduction of spot size. However, as existing standards cannot fully cope with the achieved reduction in spot size, new methods and standards need to be developed to close this gap. A redesigned resolution test chart with feature sizes of 36-100 nm has been developed to analyse the focal spot size.

Optical techniques
An accurate optical technique is being developed for 3D form measurements. The AI-enhanced data-fused optical measurement framework that is being developed allows the measurement of multiple types of AM materials by a combination of fringe projection, photogrammetry and deflectometry. The setup uses a high-resolution camera and AI network which can quickly select the appropriate technique for each part of the areal measurement. This setup reduces the system cost and complexity by combining all 3 measurement systems into 1 framework while enabling an expanded material application range.

Final results

Current progress in summary :
- Topology optimization was performed to avoid hot spots and resulting part deformations.
- Modelling was done by combining full thermo-fluid dynamic model and lumped model enabling the simulation of complete AM parts.
- To evaluate the process, a novel benchmark part was made with advanced features that are beyond current machine capabilities.
- Combining laser additive with laser subtractive processsing (either in-process or post-process) should enable us to go beyond the state-of-the-art in terms of precision.
- To measure parts with micron-size features, we use micro-focus X-ray CT or optical techniques.

Expected final results:
- advanced design tools enabling first-time-right designs and better use of AM possibilities
- efficient modelling tools (for processing and post-processing) that are verified by processing results
- optimised LPBF process strategies for improved part precision and accuracy
- understanding of in-process observations, creating the basis for in-process quality control and process stability
- innovative in-process and post-process techniques to reduce/remove roughness, porosity and internal stresses and to improve dimensional accuracy and mechanical properties
- highly accurate metrology techniques for post-processing measurements

Impacts:
1.Scientific: High precision AM processes through improved layout rules with better use of AM possibilities, better modelling tools for first−time right processing, in−situ quality control and if required optimised post−processing routes
2. Socio-economic:
- Intersectoral and interdisciplinary trained professionals in an industrial field that is important for the future of Europe, enhancing the ESR future career perspectives and advancing European industry.
- Increased market acceptance and penetration of AM.
- Through the early involvement of European industry: a growing importance of the European industrial players in this fast−growing field. This will help Europe reach its target of 20% manufacturing share of GDP.
3. Societal: As AM allows making personalised parts with efficient material use, PAM² will contribute to personalised healthcare, lightweight parts and less waste.

Website & more info

More info: http://www.pam2.eu.