Abstract
In this work, the microstructure, texture, phases, and microhardness of 45° printed (with respect to the build direction) homogenized, and hot isostatically pressed (HIP) cylindrical IN718 specimens are investigated. Phase morphology, grain size, microhardness, and crystallographic texture at the bottom of each specimen differ from those of the top due to changes in cooling rate. High cooling rates during the printing process generated a columnar grain structure parallel to the building direction in the as-printed condition with a texture transition from (001) orientation at the bottom of the specimen to (111) orientation towards the specimen top based on EBSD analysis. A mixed columnar and equiaxed grain structure associated with about a 15% reduction in texture is achieved after homogenization treatment. HIP treatment caused significant grain coarsening, and engendered equiaxed grains with an average diameter of 154.8 µm. These treatments promoted the growth of δ-phase (Ni3Nb) and MC-type brittle (Ti, Nb)C carbides at grain boundaries. Laves phase (Fe2Nb) was also observed in the as-printed and homogenized specimens. Ostwald ripening of (Ti, Nb)C carbides caused excessive grain growth at the bottom of the HIPed IN718 specimens, while smaller grains were observed at their top. Microhardness in the as-fabricated specimens was 236.9 HV and increased in the homogenized specimens by 19.3% to 282.6 HV due to more even distribution of secondary precipitates, and the nucleation of smaller grains. A 36.1% reduction in microhardness to 180.5 HV was found in the HIPed condition due to γ ″ phase dissolution and differences in grain morphology.
Highlights
Inconel 718 (IN718) is widely used primarily due to its excellent mechanical properties and corrosion resistance at high temperatures [1]
Difficulties in machining finished products made from Inconel 718 arise due to its high hardness and low thermal conductivity, which can result in excessive tool wear [1,6]
Proce1s7s.5μ20m18, 2, 30 coarsening in heat treated Selective laser melting (SLM) specimens. This is evident through the presence of larger grains in the bottom horizontal cross-section of the HIPed cylinder compared to the top, which is discussed in JS. eMcatniounf
Summary
Inconel 718 (IN718) is widely used primarily due to its excellent mechanical properties and corrosion resistance at high temperatures [1]. Selective laser melting (SLM) is able to produce complex geometries using significantly less materials and requiring no tooling compared to traditional subtractive methods of manufacturing. Zhang et al [19] observed Laves phase and carbide precipitates in the inter-dendritic regions of as-printed specimens. The study described the replacement of dendrites with recrystallized grains and the dissolution of Laves phase, allowing γ , γ , and δ to precipitate along grain boundaries [19] They reported an improvement in the ultimate tensile strength of the solution treated, and homogenized, solution treated specimens (both 1371 MPa) compared to as-fabricated specimens (1126 MPa) [19]. The data presented correspond to an average hardness observed at each location
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