Abstract
An effective strategy to strengthen a superalloy processed by selective laser melting (SLM) is proposed. The aim is to increase the yield strength of Inconel 718 fabricated by SLM to beyond 1400 MPa, which has never been achieved before. In this study, various NbC additions (0.0%, 0.5%, 1.0%, and 5.0% by weight) were added in the powder bed of Inconel 718, and two types of post-SLM heat treatments were investigated, i.e., solution heat treated plus aging (STA) and direct aging (DA). With NbC addition, smaller depth of melt pool and finer dendritic cells were obtained. Both STA and DA promoted the precipitations of γ′ and γ″. STA eliminated the cellular dendrites and induced grain growth while DA retained the as-built cellular dendrites, grain size, and nano-carbide from NbC addition, rendering a significant 326.2 MPa increase in yield strength. In this work, 0.5% NbC addition exhibited a record-high yield strength of 1461 MPa and ultimate tensile strength of 1575 MPa for Inconel 718 processed by laser manufacturing process according to literature data to-date.
Highlights
Inconel 718 superalloy is a high strength structural material widely used in oil-gas industry [1,2]
Engineering components used in oil-gas industries such as drills for deep-water operation are made of Inconel 718 and it is required to possess a yield strength beyond 1400 MPa at ambient temperature [2,4], but the standard heat treated Inconel 718 could possess only a yield strength of 900~1200 MPa [5,6]; to meet such ultra-high strength demand, additional strengthening contributions from grain refinement [7] and strain hardening [8,9] have been applied in addition to precipitation strengthening [8]
Among various fused-based additive manufacturing (AM) technologies, powder bed selective laser melting (SLM) possesses the ability to make intricate parts with high cost-efficiency [25,26,27]; it permits part design with more complex geometries and better quality of surface finish compared to those made by powder jetted laser engineered net shaping (LENS) [25,26]
Summary
Inconel 718 superalloy is a high strength structural material widely used in oil-gas industry [1,2]. It is primarily precipitation strengthened by coherent L12 structured γ0 Engineering components used in oil-gas industries such as drills for deep-water operation are made of Inconel 718 and it is required to possess a yield strength beyond 1400 MPa at ambient temperature [2,4], but the standard heat treated Inconel 718 could possess only a yield strength of 900~1200 MPa [5,6]; to meet such ultra-high strength demand, additional strengthening contributions from grain refinement [7] and strain hardening [8,9] have been applied in addition to precipitation strengthening [8]. For SLM Inconel 718, Kuo et al [18] reported that residual strain from SLM process could be retained by direct aging process, and dislocation and sub-grains strengthening could render a yield strength of 1380 MPa; a similar result was reported by Deng et al [11]: tensile yield strength of
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