On the arc wire-directed energy deposition of low thermal expansion Fe–Ni alloys
A. Sood (TU Delft - Team Marcel Hermans)
M.J.M. Hermans – Promotor (TU Delft - Team Marcel Hermans)
V. Popovich – Promotor (TU Delft - Team Vera Popovich)
C. Goulas – Copromotor (University of Twente)
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Abstract
The increasing demand for lightweight, high-performance materials in aerospace engineering has driven the adoption of composite structures that require thermally stable mould tooling for precision manufacturing. Low thermal expansion (LTE) alloys within the Fe–Ni system, particularly alloy 36, are key functional materials for such tooling owing to their exceptional dimensional stability arising from magnetovolume effects below the Curie temperature. However, their single-phase austenitic structure makes them susceptible to grain coarsening and ductility-dip cracking (DDC) during additive manufacturing.
This thesis investigates the behaviour of alloy 36 and related Fe–Ni LTE alloys during arc wire-directed energy deposition (AW-DED) using the gas tungsten arc welding (GTAW) process. The influence of heat input (200–550 J mm⁻¹) on microstructural evolution, mechanical integrity, and thermal properties was systematically examined. Reducing the heat input preserved high-angle grain boundaries, eliminated DDC, and improved tensile performance while maintaining a low coefficient of thermal expansion (CTE) consistent with conventionally processed material. A twin-wire feeding approach was subsequently adopted to enable in situ alloying and compositionally graded deposition of Fe–Ni alloys with tunable thermal and magnetic properties. Furthermore, the combined application of accelerated CO₂-jet cooling and TiC particle reinforcement under the high heat-input (550 J mm⁻¹) condition effectively refined the grain morphology and suppressed DDC.
Overall, this study establishes the key processing–structure–property relationships governing AW-DED of Fe–Ni LTE alloys. It also demonstrates scalable fabrication strategies for producing dimensionally stable and defect-free additively processed LTE alloy structures. The findings provide a foundation for the reliable manufacture and repair of precision mould tooling used in aerospace composite production.
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