DS0303 -

Laser Design of Superelastic Materials – ALMARIS

Laser Design of Superelastic Materials

Compliant structures, i.e. capable of reversibly deforming with huge amplitude, can be designed by combining the freedom of geometry offered by additive manufacturing and the intrinsic properties of a superelastic material. The applications are extensive and, for example, concern reusable shock absorbers, actuators for the control of the shape of aircraft wings, or a monolith for a drone wing.

Control the fabrication process of superelastic architectured parts by powder bed laser fusion (LBM = Laser Beam Melting) to make a compliant demonstrator.

The main issue of this project is to design and manufacture, via LBM (Laser Beam Melting) process, periodic metallic architectures as models. They should be able to reversibly deform in order to offer damping and compliance properties to the constituted structural parts or that integrate them. While this layer-by-layer manufacturing process offers attractive design flexibility for metallic materials, some scientific and technological issues need to be tackled to create favorable architectural effects (auxetic type, metamaterials,...). The material/architecture/process optimization approach which is proposed here belongs to a “materials-by-design” approach based on the development of both numerical and experimental tools. Beyond the elasticity of metallic architectures, the project aims at exploiting the properties of the NiTi alloy to increase the amplitude of elastic deformation and to offer actuation capabilities.

A first task concerns the numerical aspects and more precisely, the optimization of the shape of cells exhibiting high elastic deformability. The key point of these NiTi architectures is to limit the stress concentration and thus the plastic strains of the material. Therefore, the first step is to numerically reproduce the deformation of superelastic cells and in particular the propagation of the front of martensitic transformation by using an analogy with modeling of Lüders-type instabilities. Several designs with unfolding effects can then be compared.

A second task deals with the control of the LBM process for the manufacture of superelastic NiTi architectures The effect of the process parameters (laser power, scan speed, layer thickness, laser beam diameter, etc.) on the metallurgical, dimensional and topographical quality of different types of geometries have been examined. In order to experimentally study their influence on mechanical properties for both the material and the architectures, various characterization techniques are used such as instrumented indentation (nanoindentation), X-ray diffraction, and macroscopic deformation measurements.

The last task concerns the characterization of the material fabricated by LBM in thin walls. In fact, mechanical properties are usually given from tests carried out on massive specimens. Microstructure analysis, hardness and residual stress measurements, and mechanical tests which were carried out on Inconel 625 architectures, aimed at characterizing the mechanical properties of the material in the form of thin walls. An issue concerns the need for having a high yield stress in order to ensure a homogeneous stress distribution within the architecture.

NiTi-architected parts were manufactured by the LBM process. This type of material has been shown to be very sensitive to the quality of the powder and to the nature of the support. After identifying the parametric domain of melt pool stability and coating continuity, laser tests which were carried out on the additive manufacturing machine of the CdM Phenix PM100 made it possible to manufacture an architectured part of several centimeters in size (thickness = several mm), as well as a series of cylinders for mechanical characterization (compression and nano-indentation). The LBM material exhibits an austenite finish temperature Af slightly higher than the raw powder. Changes in transformation temperatures are attributed to the shift of composition towards higher titanium concentrations.


The numerical work highlighted two modes of behavior according to the design of cells under the uniaxial tensile test: propagative and non-propagative. Square and triangular architectures show a propagative behavior of instabilities in the connectivity walls parallel to the tensile direction. In the case of the hexagonal cell, on the other hand, the localization bands do not propagate in the cell walls, because the connectivity nodes play the role of plastic hinging.


Finally, characterizations that were carried out on Inconel 625 architectures, show that impulse elasticity stress measurements enable us to extract elasticity parameters in bending or torsion by considering the equivalent homogeneous medium. This can be applied to the characterization of NiTi-architected specimens. Instrumented indentation should make it possible to assess the superelasticity of the constitutive material.

More complex cells (rotachiral, hexachiral, auxetic, 3D..) will be studied in Zset with the implementation of the mechanical behaviour of NiTi. Firstly, the work will be dedicated to the transposition of the conclusions Lüders-type instabilities.
The possibility of introducing non-linear constitutive behavior of NiTi into the numerical framework of topological optimization will be evaluated.

Within the experimental work, the superelastic properties of the manufactured NiTi samples will be measured by nanoindentation and compression test. These measurements will make it possible to highlight the impact of the laser fusion process on the superelastic properties of an equiatomic NiTi compared to an equiatomic NiTi obtained by arc fusion. These properties can be correlated with observed microstructures and measurements of the transformation temperatures.


Further work aims at proposing a superelastic alloy with modified composition dedicated to LBM manufacture. Candidate compositions have already been identified by mechanical properties characterization of samples obtained by mixtures of arc-melted powders and by the measurements of their transformation temperatures.

1. Antoine-Emmanuel Viard, Justin Dirrenberger, Samuel Forest, Propagating instabilities in architectured materials, 6th European Conference on Computational Mechanics, 11-15 June 2018, Glasgow, UK

The main objective of the project is to design and manufacture, using selective laser melting (SLM) process, periodic metallic structures able to deform collectively in a reversible manner, thus yielding enhanced mechanical damping properties and flexibility. Although this layer-by-layer processing route offers flexibility in terms of design for architecture metallic materials, several scientific and technological challenges remain to be tackled in order to generate favorable architecture effects within the materials, e.g. auxetic effect and mechanical metamaterial behaviour. The optimization proposed for the material/architecture/process triptyque is the result from a materials-by-design approach which relies on the development of both numerical and experimental tools. Beyond the elasticity of metallic architectures, the project aims at exploiting properties of NiTi shape-memory alloy in order to increase the amplitude of elastic strain, as well as the actuating behaviour.

The original contribution of the ALMARIS project is to consider the whole process of elaboration of SLM-made components, from powder atomization and shape optimization of the cells, to the metallurgy of the constitutive materials, towards the production and characterization of a demonstrator. Two materials are proposed within the project: on one hand, Ni-base superalloy (Inconel® 625) for which metallurgical behaviour is well known for SLM; on the other hand, a quasi-stoichiometric NiTi shape-memory alloy for which SLM process and post-treatment developments are needed in order to achieve superelasticity. Tackling the technological and scientific challenges inherent to the project will involve various skills and know-hows regarding metallurgy, microstructural and mechanical characterization, constitutive behaviour modelling, computational mechanics, topological optimization, and residual stresses analysis based on full-field measurements.

ALMARIS can be broken down into 6 work-Packages:
- WP 0 : Project management
- WP 1 : SLM process development
- WP 2 : Investigation of superelasticity
- WP 3 : Mechanical characterization
- WP 4 : Modeling and topology optimization
- WP 5 : Validation on a superelastic architectured demonstrator

The consortium consists of Onera the French aerospace Lab, the Center of Materials (CdM) of the National School les Mines, of the laboratory of Processes and Engineering in Mechanics and Materials (PIMM) of Arts et Métiers, laboratory of Mechanical Systems and Simultaneous Engineering ( LASMIS) of the University of Technology of Troyes, and the company Poly-Shape.

Project coordinator

Madame Cécile Davoine (French Aerospace Lab - ONERA PALAISEAU)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UTT/ICD-LASMIS LAboratoire des Systèmes Mécaniques et d'Ingénierie Simultanée (LASMIS) de l'Université de Technologie de Troyes
PIMM Laboratoire Procédés et Ingénierie en Mécanique et Matériaux
Poly Shape POLY SHAPE
ARMINES ARMINES Centre des Matériaux de Mines ParisTech
ONERA PALAISEAU French Aerospace Lab - ONERA PALAISEAU

Help of the ANR 685,588 euros
Beginning and duration of the scientific project: September 2016 - 48 Months

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