Compositional Design of 316 Stainless Steel for Wire Arc Additive Manufacturing: Avoiding Sigma-Phase Formation through Creq/Nieq Balance and Post-Deposition Heat Treatment
Description
Context and Objective
In wire arc additive manufacturing (WAAM), repeated thermal cycling and high heat input generate microstructures not found in wrought stainless steels, including sensitization, segregation, and brittle σ- or martensitic phases that reduce ductility, corrosion resistance, and fatigue life. This PhD aims to design 316 stainless steel compositions stable under thermal cycling and to develop post-deposition heat treatments that prevent σ-phase formation while preserving mechanical and corrosion performance.
Innovative Aspects and Rupture
The project departs from empirical alloy design by systematically exploring synthetic stainless steel compositions (grade316) with precisely controlled Creq/Nieq ratios (i.e. Chromium Nickel content ratio). Using the zone-remelting facility at IMT Mines Saint-Étienne, alloys will be produced from elemental feedstock to isolate compositional effects on σ-phase stability. CALPHAD modelling will guide phase predictions and heat-treatment design, while Cranfield University will validate the results through advanced WAAM processes with controlled thermal histories.
Scientific Approach
1. CALPHAD prediction of phase equilibria and σ-phase stability.
2. Alloy synthesis and screening: zone-remelted 316 alloys; corrosion potential tests to exclude weak compositions.
3. Deposition: wire feedstock processed by – Gas Metal and Plasma WAAM (Cranfield), – TIG WAAM (IMT Mines Saint-Étienne).
4. Characterization: diffraction and microscopy of σ-phase, segregation, and austenite/ferrite balance.
5. Mechanical testing: tensile and low-cycle fatigue to link phase control with performance.
6. Heat-treatment optimization to dissolve harmful phases and restore ductility.
The student will spend one year at Cranfield University, focusing on WAAM deposition and in-situ microstructural analysis.
Expected Results and Outlook
The project will establish a composition–process map defining safe Creq/Nieq regions for WAAM of 316 stainless steel, ensuring stability and minimizing σ-phase formation. It will deliver guidelines for reliable AM-grade stainless steels and reinforce Franco-British leadership in alloy design. The work forms the basis for future CALPHAD-driven additive alloy research at ERC level.
Bibliography
Bibliography
- Lippold, J.C., Welding Metallurgy and Weldability of Stainless Steels, Wiley, 2005.
- Koseki, T. et al., Metall. Mater. Trans. A, 2022 – Solidification and σ-phase stability in 316L steels.
- Ding, J. et al., Materials & Design, 2016 – WAAM of stainless steels: microstructure and process control.
- Beal, V. et al., Additive Manufacturing, 2019 – Process–microstructure–property relationships in WAAM.
- Saunders, N. et al., CALPHAD, 1998 – Thermodynamic modelling of multicomponent stainless systems.
