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Precision Additive Metal Manufacturing

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

Okres sprawozdawczy: 2018-12-01 do 2020-11-30

Additive manufacturing (AM) has many advantages, such as a high level of design freedom, efficient material use and the capability to produce personalized parts. However, several challenges remain for the adoption of AM, such as limited precision due to shrinkage, built-in stresses and dross formation at overhangs and a limited process stability and robustness. Post-processing is often needed due to high surface texture and remaining defects.
Precision Additive Metal Manufacturing (PAM²), is an MSCA project with 10 beneficiaries and 2 partners collaborating on improving the precision of metal AM at each process step going from design to validation. A laser powder-bed fusion (LPBF) process is used. The overall objective of PAM² is to ensure the availability of high-precision AM processes and design procedures. Detailed objectives are:
1. to develop advanced design tools enabling competitive designs, better use of AM potential against minimal design costs and reduced time-to-market;
2. to develop better modelling tools for first-time-right processing;
3. to optimize LPBF 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 surface texture, porosity and internal stresses and to improve dimensional accuracy and mechanical properties.

The personnel challenges of AM are also addressed by providing training to young researchers through innovative and interconnected research projects, combined with intersectoral secondments and network-wide training events.
The work done:

1. Design
A steady-state process model was used to integrate AM constraints into the topology optimization method in order to generate robust designs free of local overheating zones. This model outperforms currently used overhang based rules in avoiding overheating. It was successfully implemented to find an optimal mold design (at The LEGO group) and for designing supports optimized for maximum heat evacuation (at 3DS and NP TEC).

2. Modelling
The thermo-fluid-metallurgical-mechanical conditions during the metal AM process were modelled at different length-scales. Higher scanning speeds were found to lead to higher cooling rates and finer grains sizes, improving the mechanical strength of the samples. The formation and evolution of lack-of-fusion and keyhole-induced porosities were investigated in other studies. It was shown how the change in the morphology of the depression zone can change the absorption of the laser power via multiple reflections. The impact of the Marangoni effect on the heat and fluid flow conditions was also modelled. Higher magnitudes of the Marangoni effect were found to create more uniform temperature distributions within the melt pool. Finally, a part-scale model was developed for predicting residual stresses and deformations.

A model for investigating the effect of a heat treatment above the β transus temperature in Ti-6Al-4V showed the transition from columnar to equiaxed to lath structure. Another model evaluated the effect of the order of the different post-processing operations on the residual stress and the deformation of a cantilever part. A novel creep-based heat treatment model was added. This model was validated at 3DS and resulted in an easy-to-use process map to determine the optimal heat treatment. Modelling the intrinsic heat treatment during the LPBF process also led to a sound prediction of the microstructural evolution.

3. Process strategies
Statistical process control and a quadratic regression model that can predict the maximum achievable channel diameter based on inclination angle and wall thickness, provided important corrective feedback to the design phase improving the precision of AM parts. Re-melting strategies, in combination with fine powder or powder removal by using laser-induced shock waves could improve the surface texture of top or inclined up-facing surfaces of AM parts, respectively. In-situ or post-process laser ablation enhanced the precision of edges and the precision of downfacing surfaces was improved by studying the effect of a wide range of process parameters, developing a new set of offsets to compensate for melt pool instabilities or by applying contactless supports (Figure 1).

4. Understand the link between post-process and in-process observations
For in-process metrology both optical melt pool monitoring and 3D geometrical measurements through the use of a compact focus variation system were investigated. Micro-focus X-ray CT (XCT), CMM, optical microscopy (OM), and fringe projection profilometry are used for post-process metrology. In-process melt pool observations were linked with post-process XCT for Ti-Al-V and with OM for maraging steel.

5. Innovative in-process and post-process techniques
Precision could be improved in-process by combining laser additive with laser subtractive processing (see point 3) or by using post-processing. Ti-Al-V AM samples from 3DS were laser polished with evident surface quality improvement. Wettability, microstructure, surface texture, and surface chemical composition of AM parts could be influenced by surface functionalization with ultrashort laser pulses.

Overall integration of the technology was ensured by defining dedicated cases by the end-users ASML, The LEGO group and NP TEC. These use cases went through all stages of design, modelling, manufacturing, measurement and assessment (Figure 2).
1. advanced design tools, enabling first-time-right designs by avoiding hot spots and resulting part deformations
2. experimentally validated modelling tools for modelling keyhole porosity, deformation, microstructure, etc.
3. optimized LPBF process strategies (use of fine powder and contactless supports, smart designs, re-melting) for improved part precision and accuracy
4. a novel benchmark part
5. in-process melt pool monitoring linked with post-process measurements (XCT and OM)
6. innovative in-process (laser erosion, re-melting) and post-process techniques (laser polishing) to reduce surface texture, porosity and to improve dimensional accuracy and mechanical properties
7. highly accurate measurement techniques (XCT, fringe projection, compact focus variation system) for measurements of AM parts
8. 3 manufactured use cases
9. 65 publications
10. PAM² YouTube channel documenting learnings from the workshops (10 movies), final presentations of the fellows and some conference contributions

1. the availability of more competitive, high precision AM processes and products with a reduced time-to-market through improved modelling and design tools for first-time-right processing, improved part precision and accuracy, accurate measurement techniques and optimized post-processing routes;
2. the availability of inter-sector- and interdisciplinary-trained professionals in AM, enhancing the career prospects of the fellows whilst advancing European industry at the same time;
3. an accelerated market acceptance and penetration of AM products through the early involvement of European industry in this fast-growing field, helping Europe to reach its target of 20 % manufacturing share of GDP;
4. PAM² AM processes can contribute to personalized healthcare, lightweight parts and less waste
Regular versus contactless supports
Manufactured use cases