Centrale Lyon ENISE - Post-doctorate - Modeling and optimization of RF plasma spheroidization of metal powders

The position is based at Centrale Lyon ENISE, on the Saint-Étienne campus, which hosts over 1250 students and 160 staff, in an exceptional environment on the outskirts of Pilat. Centrale Lyon ENISE trains engineers specializing in Mechanical Engineering, Civil Engineering, and Sensory Engineering under student status (FISE) or apprenticeship status (FISA).

DESCRIPTION

Scientific context:

As part of an ANR project dedicated to the treatment and functionalization of metallic powders by radiofrequency plasma, the laboratory wishes to recruit a post-doc to develop an approach combining physical modeling, experimentation, and optimization of process parameters.

The work will primarily focus on the spheroidization of AlSi10Mg powders by inductive RF plasma, carried out on a TekSphero40 (40 kW) torch. The process aims to transform initially irregular particles into spherical particles suitable for metallic additive manufacturing processes, among others.

The project addresses a strongly coupled scientific problem. Increasing the energy supplied to the particles favors their melting and spheroidization, but can simultaneously accentuate magnesium evaporation, nanoparticle formation, and in-flight particle coalescence. The objective is therefore to find an optimal compromise between several powder quality criteria.

Scientific bottlenecks:

Metallic additive manufacturing by laser powder bed fusion (L-PBF) requires highly spherical, flowable, and chemically homogeneous powders. The radiofrequency (RF) inductive coupling plasma spheroidization process is an interesting industrial route to meet these needs. In this process, the powder passes through an argon plasma at 10,000 K, melts in flight (~5-150 ms), and resolidifies into a spherical droplet under the effect of surface tension.

The final quality of the powders depends on four main operational parameters: process power, powder feed rate, gas flow rates (central, sheath, carrier), and the axial position of the powder injection probe in the plasma. On the TekSphero40 (20-40 kW) torch and for AlSi10Mg, three scientific bottlenecks have been identified and must be addressed simultaneously:

• Bottleneck 1:

Particle coalescence: molten droplets, due to their low viscosity, merge during in-flight collisions, causing uncontrolled growth of the particle size distribution. This was particularly observed in our trials on AlSi10Mg spheroidization. In the literature, this effect has been reported on tungsten powder (Zhu et al. 2017).

• Bottleneck 2:

Evaporation of light elements: magnesium (Tvap = 1091 °C) preferentially evaporates in the plasma, causing magnesium depletion of the alloy (loss of the hardening Mg₂Si phase) and the formation of metallic fumes.

• Bottleneck 3:

Nanoparticle formation: metallic vapors produced in the plasma lead to condensation into polluting nanoparticles and sometimes satellite deposits on larger particles.

These three bottlenecks are coupled: increasing power or residence time improves sphericity but exacerbates coalescence and evaporation. The problem is inherently multi-objective, and no "off-the-shelf" solution exists in the literature for this alloy.

MISSION AND ACTIVITIES

The post-doc will develop a model to predict particle behavior in the plasma, considering three main physical phenomena:

• Melting and spheroidization: establish the energy balance between the plasma and the particles to determine the conditions for their complete melting and transformation into spherical particles.

• Magnesium evaporation: model the preferential loss of Mg when particles reach high temperatures, linking temperature, residence time, and evaporation kinetics.

• Coalescence: study collisions and fusion between in-flight particles to predict their particle size evolution.

These three phenomena will then be coupled in a multi-objective optimization approach, in order to identify the conditions that maximize spheroidization while limiting Mg loss, nanoparticle formation, and particle size distribution broadening.

The work will involve strong interaction between modeling and experimentation. Predictions will be compared with tests performed on the TekSphero-40 and characterization of the powders by SEM, laser diffraction, bulk density measurement, and ICP-AES. A Design of Experiments (DoE) will be used in particular to calibrate and validate the models.

Main activities:

1. Acquire knowledge about the RF plasma spheroidization process

2. Develop physical models

3. Develop a numerical modeling approach

4. Implement an optimization strategy

5. Conduct and analyze experiments

Degree required for contract employees: Doctorate

Specialty/field (optional): process engineering, mechanical engineering, materials science and engineering, metallurgy, plasma physics, thermal and energy engineering, fluid mechanics, or a related scientific field. Direct experience in thermal plasma or powder spheroidization would be appreciated, but is not essential. The candidate must primarily possess a solid scientific background enabling them to quickly acquire knowledge specific to the process.

Knowledge:

The candidate should have a good foundation in several of the following areas: heat transfer, thermodynamics, fluid mechanics, transport phenomena, materials science and metallurgy, physical modeling, numerical methods, statistics, and Design of Experiments.

A good understanding of phase change phenomena, heat transfer, mass transport, and kinetics is particularly sought.

The project also requires the ability to transition from a physical description of the phenomenon to a numerically usable mathematical model.

Interpersonal skills:

We are looking for an autonomous, curious young researcher capable of approaching a complex physical problem, formulating hypotheses, developing a model, confronting it with experiments, and improving it based on the results obtained.

The position offers the opportunity to work at the interface of several fields: RF Plasma → Heat Transfer → Powder Metallurgy → Modeling → Experimentation → Optimization → Additive Manufacturing.

Weekly working hours of 37.30 H - 48.5 days of leave - possible flexible working arrangements over 4.5 days - remote work under conditions.

Find more English Speaking Jobs in France on Arbeitnow