Systematic Computational Optimisation and Local Laser Processing for Steel-Based Architectured Materials – SCOLASTIC

Many industrial applications require materials with enhanced specific properties (performance per unit of mass), especially the transportation and biomedical sectors. Architectured Materials are an emerging class of advanced materials that bring new possibilities in terms of functional properties. The term architectured materials describes any heterogeneous material that exhibits improved specific properties due to a thoughtful and predetermined morphology and/or topology design. This usually induces characteristic length-scales comparable to the size of the final component being produced. Localised processing methods appear as natural candidates for developing such materials.

The main purpose of SCOLASTIC consists in developing architectured metallic materials through computational optimisation and laser processing. A numerical framework is developed for generating and optimising architecturation patterns. The local laser heat treatment of dual-phase (DP) steel sheets is considered first for controlling the overall plastic anisotropy, then for improving the fatigue behaviour, in comparison to untreated materials, hence demonstrating feasibility of the approach. When finely controlled, laser processing can generate deterministically graded or homogeneous topographical, mechanical or metallurgical alterations, in surface or volume, depending on laser parameters and on the configuration for the treatment.

By capitalising on the concept of localised treatment of thin structures, our project aims at developing a systematic approach to determine alteration patterns for a given set of requirements. SCOLASTIC consists mainly in setting-up an integrated computational methodology for designing patterns used for architecturing materials. This pattern will come as an output from a computational topology-optimisation loop that will be developed around a shape-generation module, e.g. cellular automata-based, and a cost function evaluation module, e.g. finite element method. This cost function has to be minimised for given constraints. The cost function value is then used as a feedback, and an optimised topology is generated accordingly. The spatial resolution associated with the shape-optimisation is chosen here as the relevant size for laser treatment, i.e. 1 mm, corresponding to a representative scale of the underlying microstructure.

Many applications could be envisioned, but for the sake of clarity, this study is focused on the localised laser tempering of DP steel sheets. As a matter of fact, the high content of martensite results in enhanced strength but reduced ductility. Such properties limit the overall formability of the material. When considering thinner DP steels sheets, tearing and fracture are more likely to occur during stamping, hence limiting the commercial use of such materials. Localised laser treatment can result in customised martensitic tempering, i.e. locally tuning the plastic yield stress/ductility trade-off, hence allowing the control of local softening in the architectured material. Optimised patterns can enhance the overall fatigue and fracture behaviour by locally modifying the fracture toughness/yield stress trade-off, since the critical transition crack size between plastic yield and fracture depends on this ratio. Moreover, the crack propagation can be controlled by blunting surface cracks, and adding a plastic dissipation contribution to the effective fracture energy of the architectured material. One can also think about channelling cracks in designed intricate paths, in order to enhance the fatigue behaviour. Regarding material functionality, we can design privileged strain localisation paths using the present approach. We can enhance the formability of sheets for processes based on plastic deformation by softening metal sheets only where needed, i.e. along optimised strain localisation paths obtained by local laser heat treatment.