Comprehensive Microstructural Evaluation of Precipitation in Inconel 718

Performance Characterization of Laser Powder Bed Fusion Fabricated Inconel 718 Treated with Experimental Hot Isostatic Processing Cycles

“…It is apparent in Table 2, and with reference to Table 1, that heat treatment schedule strategies can allow a wide range of choices for mechanical properties for SLM-fabricated Inconel 718 components. And while more conventional post-process heat treatments involving solutionizing and aging treatments produce a range of precipitation hardening, as shown for Variant 3 (Table 1) (Figure 4a) [6][7][8][9][10][11][12]18], heat treatments dominated by HIP temperatures in excess of 1120 • C uniformly result in recrystallized, equiaxed grains of only tens of microns containing annealing twins and carbides. Consequently, HIP treatments dominate the heat treatment strategies for manipulating microstructures and associated mechanical properties for Inconel 718 alloy.…”

“…Table 1 involves a wide range of heat treatment schedules designed to examine the ability to select desirable residual mechanical properties, and the efficacy of HIP in heat treatment considering that HIP generally involves temperatures above that which induces rapid recrystallization (~1050 • C) [13,14]. While, as described briefly in the Introduction, Inconel 718 can be variously manipulated by heat treatment to produce a range of mechanical property (tensile) behavior (including yield stresses in excess of 1.2 GPa and corresponding elongations >10%) as a consequence of a wide range of induced precipitation [5][6][7][8][9][10][11][12], recrystallization can restrict these strategies. Consequently, and as shown in Table 1, HIP temperatures well above the rapid recrystallization temperature are expected to restrict more conventional heat treatment strategies, especially those involving solution annealing (SA) and aging (Age) treatments.…”

“…With the recent development of additive manufacturing (AM) technologies, especially involving laser and electron beam powder-bed fusion fabrication, Inconel 718 alloy has received renewed attention because of its continuing prominence in the aerospace and energy industries, as well as the ability of AM to achieve complex and cost-effective component geometries often impossible to fabricate by conventional manufacturing [6][7][8][9][10][11][12]. The many recent selective laser melting (SLM) and electron beam melting (EBM) studies [6][7][8][9][10][11][12] have also included a wide range of post-process heat treatment strategies to continue to provide selective and optimized mechanical properties specifically suited to applications of these novel products. In addition, since precursor, pre-alloyed powders can contain inert gas bubbles as a consequence of the atomization process, and scanning errors can create poor powder sintering and melting, the resulting porosities in fabricated products can be a concern.…”

Metal fused filament fabrication of the nickel-base superalloy IN 718

“…These are mainly c 0 /c 00 co-precipitates with disk-like shape. They form during the slow furnace cooling after sintering when the respective precipitation windows are crossed [34]. Solution annealing completely dissolves these precipitations and creates a supersaturated solid solution.…”

“…A high hardness of 350 ± 45 HV 0.2 has been measured after vacuum processing. The high hardness indicates precipitation of c 0 -and c 00 -phases during slow cooling after sintering [34]. Consequently, hardness is reduced to a value of 191 ± 9 HV 0.2 by solution annealing with subsequent air cooling that prohibits the formation of hardening precipitates.…”