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Effect of post-processing heat treatment on the mechanical properties of inconel 718 fabricated by selective laser melting

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2020
Özer, Seren
In this study, effect of the building direction and post-processing heat treatments on the microstructure and high temperature mechanical properties of as-fabricatedInconel 718 alloys have been investigated. Inconel 718 samples were fabricated in two different building directions such that their longitudinalaxes wereeither parallel (horizontally built) or perpendicular (vertically built) to the building plate. During the selective laser melting (SLM)process, some undesired features can appear in the microstructure of the as-fabricated parts due to high temperature gradient and rapid solidification resulting from the nature of this additive manufacturing method. Therefore, possible post-processing heat treatments, including solutionizing and aging, were studied to eliminate these aspects in the as-fabricated parts to enhance the high temperature mechanical properties of the Inconel 718 alloy to be able tosatisfy the requirements of the demanding working conditions. For both as-fabricated and peak heat treated (PHT) samples, the texture of the horizontally built samples was commonly concentrated in (001) plane, which representsthe maximum heat flow,and hence easy grain growth direction. Additionally,the columnar grains hada slight tendency to also align along the vi(111) and (101) planes.Otherwise, the texture of the vertically built samples had an intense texture on (001) plane. After all heat treatment conditions, the average microhardness of the Inconel 718 alloy significantly increased,and the maximum microhardness of ~462 HV was achieved for PHT-8h condition (solutionizing at 1040 ºC for 2 h, subsequently aging at 700 ºC for 8 h followed by air cooling). A slight decrease in hardness was observed in aging heat treatments applied for more than 8 hours,since the formation of δ phase hindered the precipitation of the strengthening γphase by consuming Nb elementin the system. Among the applied heat treatment processes,PHT-8h condition, resulting inoptimum hardness, was selected for the samples to be tensile tested. The tensile test results performed at room temperature revealed that post-processing heat treatment can significantly increase the yield strength (YS) and ultimate tensile strength (UTS) while reducing the elongation at failure for both horizontally and vertically built samples. Aging treatment promoted the formation of the finely distributed strengthening γand/or γ′ precipitates in the matrix, which serve as a barrier to dislocation movement, and therefore improve the mechanical properties. However, both the YS and UTS of the age hardened Inconel 718 alloy decreased when the tensile test wasperformed at high temperatures mainly because of the dissolution of the strengthening γprecipitates andthe formation of the δ phase in the matrix.