Abstract
Subsurface grains of Alloy 718 additively manufactured via electron beam-powder bed fusion technique were refined using shot peening to improve the surface texture and oxidation performance. Oxidation of the specimens was performed at 650 and 800 °C in ambient air. Due to plastic deformation upon shot peening, compressive residual stress and high microstrain were generated in the subsurface region within a depth of approximately 50 μm. The shot-peened specimen exhibited lower surface roughness, finer subsurface grains, and higher hardness compared to the as-built specimen. Shot peening, coupled with hot isostatic pressing and heat treatment (HIP-HT), yielded superior oxidation performance with substantially low oxidation kinetics at 800 °C. The smooth surface, as well as dense and refined subsurface microstructure resulting from shot peening, facilitated the formation of a continuous, protective, and adherent Cr-rich oxide scale.
Highlights
Additive manufacturing (AM) has been identified as a promising technique for the manufacture and repair of critical parts in the power generation industry [1]
The LoF defects are usually caused by incomplete melting between subsequent layers of material or adjacent melt pools, confirming their formation to be significantly influenced by processing parameters [31]
The presence of the inclusions, such as oxides (e.g., Al2O3) or nitrides (e.g., TiN), which are typically reported in the electron beampowder bed fusion (EB-PBF)-built Alloy 718 parts [33], can result in the lack of sufficient fusion between the layers and the formation of the LoF defects [34]
Summary
Additive manufacturing (AM) has been identified as a promising technique for the manufacture and repair of critical parts in the power generation industry [1]. Gravity affects the melt pools created on unsupported layers, which sag into the underlying un-melted powders, resulting in high surface roughness on the downward-facing surfaces [18] This effect can be combined with the uneven heat dissipation rates into the underlying powders, compared to the solid material, which creates thermal gradients, impairs the melt pools, and eventually disrupts the shape of the layer edge [19]. Despite the technique showing promise regarding the enhancement in the oxidation behavior of metallic materials, the oxidation performance of the EB-PBF-built Alloy 718 parts that have undergone shot peening has not yet been reported in the literature. The influence of shot peening on the surface and subsurface features, the microstructure, and the oxidation behavior of EB-PBF-built Alloy 718 is examined in ambient air at 650 and 800 °C for up to 168 h
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