Harnessing solid-state thermal cycling to Guide microstructure evolution of Additively Manufactured Metallic Alloys

Project Information

GAMMA

Grant agreement ID: 946959

DOI

10.3030/946959

Project closed

EC signature date 17 September 2020

Start date 1 March 2021

End date 28 February 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 499 877,00

EU contribution

€ 1 499 877,00

Coordinated by

ECOLE POLYTECHNIQUE
France

Periodic Reporting for period 3 - GAMMA (Harnessing solid-state thermal cycling to Guide microstructure evolution of Additively Manufactured Metallic Alloys)

Reporting period: 2024-03-01 to 2025-08-31

Summary of the context and overall objectives of the project

Designing alloy microstructures with desired properties and parts with desired responses is a significant challenge, particularly during metal additive manufacturing, popularly known as 3D printing. The heat-matter interactions occurring during 3D printing result in a sequence of highly non-equilibrium processes: melt-pool dynamics and fast solidification followed by solid-state thermal cycling (SSTC) until the end of the process. As a consequence of these non-equilibrium processes, the finished parts exhibit microstructures that have a plethora of physical and chemical heterogeneities that are more varied than the ones occurring in conventionally processed alloys. These heterogeneities are sensitive to a large number of 3D printing parameters and minor changes to these parameters can result in very different microstructures exhibiting a large spread in the alloy’s mechanical response. This makes it very difficult to perform quality control checks and certification of parts.

The key to control the process-microstructure-properties-performance relationship in metal 3D printing lies in understanding microstructure formation during manufacturing. Prior to the start of project GAMMA, most experiment and modeling efforts in metal 3D printing were overwhelmingly focused on studying the role of melt-pool dynamics and solidification. While these processes are necessary to study microstructure genesis, they are highly insufficient to understand the formation of final microstructures, which also requires investigating the role of SSTC. The thermo-mechanical driving forces occurring during SSTC can trigger a plethora of solid-state mechanisms e.g. dislocation dynamics, phase transformation, grain growth, recrystallization, precipitation, etc. These mechanisms can alter the solidified microstructure and induce residual stresses, all of which determines the mechanical response of as-built parts. Only once an understanding of different phenomena has been gained, can the knowledge be transformed for industrial use to design parts with desired responses.

To that end, the overarching aim of project GAMMA (conducted in the group of Dr. Manas V. Upadhyay https://www.manas-upadhyay.com) is to highlight the important role of SSTC on microstructure evolution during metal 3D printing, and to show how SSTC can be harnessed to design microstructures with desired mechanical responses.

The objectives to achieve the overarching aim are:
1) To propose and perform experiments to quantify the microstructural changes brought about solely by SSTC, and to identify the underlying micro-mechanisms.
2) To develop and validate theoretical and fast numerical models to gain deep insight on dynamics of micromechanisms and predict polycrystalline evolution due to SSTC.
3) To use the novel models in synergy with experiments to tailor 3D printing parameters and suggest in-process/post-process thermomechanical treatments to engineer microstructures with desired material properties and part performance.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During the first half of project GAMMA, the group of Dr. Manas V. Upadhyay (https://www.manas-upadhyay.com) has executed a two-pronged strategy that involved developing complex microstructure evolution models that work in synergy with newly developed advanced in situ characterization experiments.

On the experimental front, the developments have included the building of a new miniature machine to perform in situ synchrotron X-ray experiments during laser metal deposition (mini-LMD), a 3D printing technique, and the creation of a unique machine to perform lasering using a continuous-wave laser inside a scanning electron microscope (CW Laser-SEM). Novel experimental procedures have also been proposed to perform in situ rapid heating and cooling of thin-film lamellae and micropillars inside a transmission electron microscope. With these devices and procedures, Dr. Upadhyay’s group has demonstrated the occurrence of two solid-state micromechanisms during 3D printing viz., dislocation dynamics and precipitate growth, and studied other solid-state micromechanisms including phase transformations and recrystallization. Furthermore, using CW Laser-SEM, novel post-process strategies have been proposed to improve the overall mechanical performance (strength and fatigue limit) of materials.

In parallel, the first stage of theoretical developments and numerical (computer) implementation of a dislocation thermomechanics model and a polycrystal thermo-elasto-plasticity model have been completed. These developments have been performed based on the following three-fold strategy: (i) developing the theoretical models within the same continuum framework such that they share the same main governing equations, (ii) numerically implementing them in a single open-source finite element (FE) code to facilitate the switch between these models, and (iii) generating tools to facilitate comparison with experiments.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

On the experimental front, conclusively demonstrating that some solid-state micromechanisms, particularly dislocation dynamics, do occur during 3D printing has been a significant progress beyond the state of the art. Another significant progress has been the development of the novel coupling between a continuous-wave laser and a scanning electron microscope. Studies with this equipment have led to the development of a novel approach to improve the mechanical properties of 3D printed alloys.

These experiments have also opened up the path to conduct computer simulations to gain a deep understanding of microstructure and internal stress evolution, as well as local plastic deformation, during 3D printing of metals. The first study conducted with the developed polycrystal model has been performed in synergy with experiments conducted using the novel CW Laser-SEM device to reveal the generation of unexpectedly large intergranular residual stresses and plastic strains during laser scanning. Meanwhile, the first study conducted with the dislocation thermomechanics model has shown for the first time what the temperature field of moving dislocations with a finite core size should look like in a fully coupled thermomechanical setting within a 3D continuum framework.

The next steps in the project include better understanding these phenomena and eventually using a modeling, simulation and experiment synergy to propose in-process/post-process routes to guide metal 3D printing in order to obtain microstructures with desired properties and mechanical response.

Project description