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Multi-material WAAM: manufacturing of the holding ring

Grade2XL project aims to develop multi-material WAAM for industry through the production of 8 demonstrators.

One of them is a holding ring for hydroelectric power plants (Figure 1). This demonstrator is a part proposed by EDF and has been described in the article published in June 2023.

Figure 1: Holding ring demonstrator

The holding ring demonstrator is constituted of three different materials:

  • A low alloyed material for the structure of the holding ring (S460 in Figure 1)

  • A martensitic stainless steel in area where the holding ring is in contact with water (410 in Figure 1) to improve the corrosion behaviour.

  • A cooper alloy material on sealing contact areas (CuAl8 in Figure 1) for its good tribological properties.

Prior to the manufacturing of the final demonstrator scale 1:1, several developments were required for several aspects:

  • Firstly, to assess and ensure the WAAM materials integrity and performances.

  • Secondly, to derisk the geometrical singularities of the multi-material demonstrator and therefore implement adapted WAAM printing strategies.

This is why, mono-material blocks, multi-material blocks and then pre-production parts with increasing representativity and complexity were manufactured and characterized.

Then, once the materials selection, the printing strategies and parameters validated, the production of the final demonstrator started on the WAAM facility of the Naval Group ‘s technical direction.

The manufacturing of a multi-material WAAM part brings specificities. As for other additive manufacturing processes, the deposition is carried out layer by layer. Nevertheless, the multi-material deposition involves printing sequences to be adapted in order to build the part with the proper material at the proper location (Figure 2).

Figure 2: Printing sequences for the demonstrator

The final demonstrator produced needed then to be heat treated for metallurgical reasons but also to relieve residual stresses introduced by the WAAM process (Figure 3). Numerous tests were therefore carried out to define the heat treatment that would meet both the metallurgical and mechanical requirements for the 3 grades making up the piece, while ensuring a low residual stress rate.

After the heat treatment, the demonstrator was machined and finally dimensional controlled.

Figure 3: Demonstrator before machining process

While the demonstrator deposition, Naval Group experienced production limitations, mostly due to the welding material. All along the development stage, improvements were implemented in order to maximize the productivity (printing strategies promoting arcing times, printing parameters optimizing the deposition rates, etc.).

The manufacturing of the demonstrator globally run well, with no major issue jeopardizing the demonstrator feasibility in multi-material WAAM. To guarantee the respect of the quality all along the steps of the project, the deposition strategies and parameters remained the same for the blocks, the pre-production parts and the final demonstrator except for the copper alloy area where printing parameters and strategies were adapted to mitigate the risk of delamination (observed during the pre-production phase). The results of the characterizations demonstrate that the parameters and printing strategies implemented for the demonstrator are robust and adapted even in consideration of the scaling up.

Next steps are the non-destructive tests of the demonstrator at EDF. If results are conclusive, the demonstrator will undergo commissioning tests at EDF (off-load manoeuvring, underwater manoeuvring, pressure test, sealing test, lock control).

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