Homogeneous Age Research Articles

Impact of Heat Treatment and Building Direction on Tensile Properties and Fracture Mechanism of Inconel 718 Produced by SLM Process

In the selective laser melting (SLM) additive manufacturing process of Inconel 718, the emergence of Laves and δ precipitate phases in the γ matrix during or after heat treatment is a critical consideration. This study comprehensively assesses the microstructures and mechanical properties of Inconel 718 alloy produced through SLM under varied conditions: as-built (AB), homogenization + solution + aging (HSA), homogenization + aging (HA), and solution + aging (SA). Additionally, the impact of building orientation, whether horizontal (H) or vertical (V), is investigated. The AB specimens oriented horizontally display a columnar melt pool structure, with dimensions roughly between 300 and 400 μm. In contrast, the AB specimens aligned vertically show an elongated river-like structure of melt pools, with their sizes approximately at 250 μm. From the detailed microstructural analysis, the findings reveal that the as-built specimens lack γ′ and γ″ precipitates in their microstructure. Conversely, in heat-treated specimens, both the γ′ and γ″ phases are evident. Notably, Inconel 718 alloy specimens subjected to SLM fabrication and SA heat treatment demonstrate optimal mechanical performance. Notably, SA exhibits an average hardness of 476 HV for the horizontal specimen, which is 51.1% higher than that of AB specimens. The morphology and distribution of the δ phase in the γ matrix emerge as decisive factors influencing high-temperature performance. In SA specimens, the dissolution of brittle Laves phases occurs, and the presence of the δ phase at the grain boundary imparts superior properties during high-temperature tensile testing, including excellent yield and ultimate tensile strength. The presence of the granular-δ phase in the SA specimens resulted in a tensile strength of 1422 MPa and a yield strength of 1236 MPa, which are the highest values among all the specimens. SA has a tensile strength of 1120 MPa and a yield strength of 974 MPa at 650 °C.

Strengthening mechanisms in selective laser-melted Inconel718 superalloy

The microstructures and mechanical properties of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) are evaluated in as-built, direct aging (DA), and homogenization + aging (HA) conditions on considering the effect of applied loading direction. The results show that the microstructure of as-built and DA specimens consist of the intracrystalline fine degenerated dendritic structures (0.6–1 μm in spacing) with a high density of dislocations (10 13 –10 14 m −2 ). Fine dendritic structures disappear after HA treatment, and the average geometrically necessary dislocation (GND) density decreases to 10 12 m −2 . A quantitative analysis of strengthening mechanisms is established by taking into consideration the grain boundary, solid solution, precipitation and dislocation strengthening. For the as-built and DA specimens, the contribution to the yield strength by dislocation strengthening is ~85 MPa in vertical deposition and ~170 MPa in horizontal deposition, respectively, and the Laves phases provide approximately 100–110 MPa. The contribution of precipitation strengthening to the strength increment in the DA and HA specimens is 590–600 and 830–850 MPa, respectively. The precipitation strengthening is mainly contributed by shearing mechanisms among which coherency strengthening plays dominant effect rather than order strengthening. For as-built, DA, and HA regimes, the specimens always exhibit higher strength along the horizontal direction than that along the vertical direction. The anisotropy of yield strength in the as-built and DA specimens is mainly attributed to the difference in dislocation density and effective grain size, and the anisotropy of yield strength in the HA specimens is mainly caused by the difference in effective grain size and Taylor factor.

Comparison of microstructures and mechanical properties of Inconel 718 alloy processed by selective laser melting and casting

The paper comparatively investigates the microstructures and mechanical properties of Inconel 718 superalloy manufactured by selective laser melting (SLM) and casting. The finite element analysis (FEA) method is used to simulate the temperature fields during SLM and casting processes. Driven by ultra-high temperature gradient and ultra-fast cooling rate during SLM process, the fine grains (average grain size of 48 µm) and dispersed fine precipitation in SLM-ed sample even after HSA (homogenization + solution + aging) and HA (homogenization + aging) heat treatment significantly enhance its mechanical properties, which far exceeds that of casting with average grain size of 1300 µm, and is comparable to that of forging. The microstructure of casting with coarse irregular Laves phases, acicular δ precipitates and globular carbides in the interdendritic zones after HSA heat treatment and some defects existed possibly result in premature failure of tensile samples. The microstructure without δ phases but only some globular carbides in the grain boundary of SLM-ed sample after HA heat treatment possesses higher mechanical properties than that after HSA heat treatment, in which there is only some finer needle-like δ phase and few carbides are precipitated in the grain boundaries. The analysis shows the large amounts of δ phase precipitated in the matrix will deteriorate the plasticity of SLM-ed IN718 superalloy, the appropriate reduction of the δ phase will improve the strength and plasticity of material simultaneously.