İkincil ısıl işlemlerin seçmeli lazer eritme yöntemi ile üretilen Inconel 718 alaşımının mekanik özellikleri üzerindeki etkisi

In this study, effect of the building direction and post-processing heat treatments on the microstructure and high temperature mechanical properties of as-fabricated Inconel 718 alloys have been investigated. Inconel 718 samples were fabricated in two different building directions such that their longitudinal axes were either 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 to satisfy 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 represents the maximum heat flow, and hence easy grain growth direction. Additionally, the columnar grains had a slight tendency to also align along the (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 element in the system. Among the applied heat treatment processes, PHT-8h condition, resulting in optimum 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 was performed at high temperatures mainly because of the dissolution of the strengthening γ precipitates and the formation of the δ phase in the matrix.