Design of alloys for additive manufacturing
Whereas classical alloys are often associated to certain processes (casting, forging, powder metallurgy…), the beginnings of additive manufacturing have above all seen its application to existing grades, poorly adapted to these new processes. Our works deal with the design of alloys (stainless steels, alloys of nickel or titanium, high entropy alloys…) specifically adapted to additive manufacturing, may it be to reduce defects and improve quality (cracking, porosity, surface aspect…), to optimise mechanical properties, or to deposit layered multi-materials with original characteristics (notably a high resistance to crack propagation).
Keywords: ALM, SLM, DMD, multilayer materials
The first example is tackled within the PhD thesis of Mariam Assi (2018-2021), as a co-supervision with LGF in Mines Saint-Étienne, on the design of austenitic stainless steels for additive manufacturing. We are aiming at:
- Improving mechanical properties.
- Avoiding hot cracking, through a control of the solidification path.
- Reducing the formation of pores due to the keyhole effect, by limiting vaporisation under the laser beam.
- Ensuring the geometrical quality of deposited beads and a good surface aspect, by avoiding projections (“spatter”) and the “balling” phenomenon, as well as by reducing surface roughness.
- Controlling the composition stability of the deposited metal by limiting the differential vaporisation of alloying elements.
- This is made through the development and/or use of a set of models describing the characteristics of the material as a function of composition: surface tension and viscosity of the liquid, vaporisation flux of elements, phase formation, solid solution hardening…
The second example, tackeld in the PhD thesis of Madeleine Bignon (2020), as a co-supervision with Lancaster University, deals with the simultaneous design of two alloys that will be deposited as alternating layers by a wire additive manufacturing process or by DMD (direct metal deposition). For instance, an alternating deposition of hard layers and more ductile layers aims at obtaining materials having at the same time a high tensile strength (thanks to hard layers) and a good fracture toughness via the plastic blunting of cracks (by ductile layers) and their deviation (at interfaces). Our design methods would have allowed to target separately the properties of both alloys, but the additive process leads to an inter-dilution of successive layers, so to a modification of their composition and hence to different properties. Our approach consists in taking into account the sequential dilution between layers, so as to design simultaneously the compositions of both added metals, in such a way that the deposited compositions (after dilution) lead to expected properties. The example below shows the simultaneous design of pairs of titanium alloys, one having the hardest possible β stable structure, the other displaying a low yield stress and a TRIP effect to provide it with a good ductility and strain-hardenability.
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Multi-objective optimisation allows to propose several pairs of so-designed alloys, which maximise the difference in solid solution hardening between both types of layers.
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Collaborations:
. Laboratoire des Sciences du Numérique de Nantes (LS2N) – Université de Nantes
. Laboratoire Georges Friedel (LGF) – Mines Saint-Étienne
. Lancaster University