Wire + Arc Additively Manufactured Inconel 718: Effect of post-deposition heat treatments on microstructure and tensile properties

Cui E Seow,Harry E Coules,Guiyi Wu,Raja H.U Khan,Xiangfang Xu,Stewart Williams

doi:10.1016/j.matdes.2019.108157

Abstract

Wire + Arc Additive Manufacturing (WAAM) can be used to create large free-form components out of specialist materials such as nickel-base superalloys. Inconel (IN) 718 is well suited for the WAAM process due to its excellent weldability. However, during deposition, WAAM IN718 is susceptible to micro-segregation, leading to undesirable Laves phase formation in the interdendritic regions. Further, the WAAM process encourages columnar grain growth and the development of a strong fibre texture, leading to anisotropy in grain structure. This unfavourable microstructure can be addressed through specialised post-deposition homogenisation heat treatments. A new modified heat treatment was found to be effective in dissolving Laves phase, whereas a standard treatment precipitated δ phase. Tensile test results revealed that Laves and δ phases lead to low ductility when present in a precipitation-hardened matrix. The modified heat treatment also reduced the anisotropy in grain structure, leading to almost isotropic elevated temperature tensile properties, which meet minimum specifications for conventional cast but not for wrought material. Specialised post-deposition heat treatments, which address the unique microstructure of WAAM IN718, are crucial to achieving optimal mechanical properties.

Highlights

The interlayer boundaries were observed to correspond to differences in dendrite patterns in the light micrographs (Fig. 7), but may be be caused by a combination of weak segregation banding and solute partitioning in the heat affected zones, which has been recently observed in Wire + Arc Additive Manufacturing (WAAM) titanium alloys [23]
The slightly outward-angled dendrite growth direction at the edges of the sample suggests that there was a small shift in the thermal gradient, which could be due to heat dissipation from the edges of the deposit
Poor elongation was observed in the Aging only tensile sample, which has been attributed to the embrittling effect of Laves phase within a precipitationhardened matrix

Summary

Introduction

Unlike powder-based manufacturing methods, WAAM can be used to build large parts quickly, due to its higher deposition rates. WAAM builds are near-net shape, which reduces machining operations and material wastage in comparison to traditional manufacturing methods of machining from a casting or forging. The costeffectiveness of WAAM is multiplied when used with expensive materials such as titanium alloys and nickel-base superalloys. One of the most weldable nickel-base superalloys is Inconel (IN) 718. Its low Ti and Al contents decrease its susceptibility to strain age cracking [3]. It is a precipitation hardenable superalloy known for its high-temperature strength and is widely used in the aerospace, nuclear, oil and gas industries. The alloy is hardened predominantly by a body-centred tetragonal phase, gamma double-prime, γ” (Ni3Nb), which is typically precipitated in post-process aging treatments [4,5]

Methods

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Discussion

Conclusion