Double Aging Treatment Research Articles

Influence of Ageing Treatment on Microstructure and Mechanical Properties of GH4169 Alloy Prepared Using Wire Arc Additive Manufacturing

The effects of solid-solution and ageing treatments on the microstructure and mechanical properties of a GH4169 alloy made by wire arc additive manufacturing, micro-casting, and forging were researched. The microstructure, along with the size and type of precipitated phase, were analysed using an optical microscope, scanning electron microscope, and transmission electron microscope. The strength and toughness were tested using a tensile testing machine. The results show that a polygonal austenite microstructure was obtained for the GH4169 alloy prepared through wire arc additive manufacturing, micro-casting, and forging, followed by solid-solution and double-ageing treatments at different times. There were a few twins in the austenite matrix. A large number of nano-sized γ″- and γ′-precipitated phases and a small number of Laves phases and MX phases were found in the matrix. The tensile strength and yield strength of the GH4169 alloy increased first and then decreased with the ageing time. After ageing for 16 h, the maximum yield strength was 1287 ± 22 MPa, the maximum tensile strength was 1447 ± 19 MPa, and the elongation was about 19.5%. The main strength mechanism is precipitated phase strength and solid-solution strength. The fracture exhibited obvious ductile fracture characteristics.

Effects of microstructures and double aging on high temperature yield strength in Ni–Co-base superalloy produced by a powder metallurgy route

Microstructural features and their effects on yield strength at high temperatures in the P/M HGN200 Ni–Co-base polycrystalline superalloy, subjected to a standard double aging treatment, have been clarified using both Thermo-Calc® and a unified approach. The matrix γ possesses a very fine grain size due to the used processes and pinning by primary γ'. The calculated antiphase boundary energy is very high, resulting in high yield strength. Dispersion of fine tertiary γ′ phases is remarkable to increase the yield strength compared with those of primary and secondary γ′; the contribution of the primary γ′ to the yield strength is negligible. The stoichiometric composition of primary and secondary γ′ is (Ni,Co)3(Ti,Al) and that of tertiary γ′ is (Ni,Co,Cr)3(Ti,Al) and the excellent high temperature strengthening is related to homogeneous dispersion of both the secondary and tertiary γ′.

Role of the aging procedure on the ω + β microstructure in the metastable β Ti-30Nb-3Fe alloy

This report investigated the effect of low-temperature aging on the ω + β microstructure in the Ti-30Nb-3Fe alloy (wt%). Samples were subjected to several aging procedures. The ω + β microstructure remained quite stable under conventional transmission electron microscopy across most temperature/time regimes. Only the double aging treatment led to the growth of ω particles and an increased in Vickers hardness, indicating that the pre-aging treatment was necessary to promote rapid ω growth in the subsequent treatment. Furthermore, the early stages of solute partitioning that accompanies ω growth did not drastically increase hardness, suggesting that no embrittlement occurred.

Influence of Single- and Double-Aging Treatments on the Mechanical and Corrosion Resistance of Alloy 625

Nickel–chromium–molybdenum Alloy 625 exhibits an excellent combination of mechanical properties and corrosion resistance. However, the high-temperature plastic deformation process and the heat treatment represent critical aspects for the loss in mechanical strength by grain coarsening. This detrimental behavior is worsened by the absence of phase transformation temperatures. However, the chemical composition permits slow precipitation-hardening response upon single aging. Therefore, when the soft- or solution-annealed condition is associated with insufficient mechanical properties, this potentiality can be exploited to improve the mechanical strength. Since the 𝛾″ precipitation can be accelerated by double-aging treatment, different time–temperature combinations of double aging at 732 °C and 621 °C are investigated. The simultaneous precipitation of intergranular carbides can dramatically affect the corrosion resistance. Such an undesired phenomenon occurs very quickly at 732 °C, but it is obtained only after very long exposure times at 621 °C. For this reason, a performance chart is developed to compare all the tested conditions. In particular, single aging at 621 °C for 72 h and 130 h are associated with an acceptable combination of mechanical and corrosion properties. Double aging permits a conspicuous acceleration of the aging response. For instance, with double aging at 732 °C 3 h and 621 °C 72 h, it is possible to obtain the same mechanical properties of single aging at 621 °C for 260 h. Such acceleration is accompanied by a more critical corrosion behavior, especially because of the primary step. However, even after its optimization, none of the tested conditions were acceptable.

Enhanced strength and ductility of laser-directed energy deposition repaired IN718 superalloy via a novel tailored heat treatment

High-quality repair of damaged Inconel 718 (IN718) superalloy components can achieve great economic benefits. However, the directly double aging (DA) treatment by industrial standards, yields an inferior ductility on the repaired component than that of the wrought base metal. In this work, wrought IN718 components were repaired by laser-directed energy deposition (LDED), a novel tailored heat treatment (THT) schedule consisting of a short-term low-temperature homogenization, and subsequent DA was subsequently conducted to strengthen the repaired IN718 alloys. The microstructure evolution and mechanical properties of the DA and THT-treated repaired alloys were comparatively investigated. The results indicated that the THT effectively dissolved most of the hard brittle Laves precipitates in the deposition region with only slight coarsening of the grains in the substrate. As compared to the DA sample, the elongation of the THT sample increased remarkably by 88% with only a slight reduction of 19.2 MPa in yield stress. Moreover, the strain distribution of the THT sample was overall more even but then destabilized in a narrow abnormal coarsened grain region caused by the static recrystallization. In general, this study breaks through the limitation of the low ductility of the DA-treated repaired IN718 alloys and provides a promising way to further improve the mechanical properties.

Abstract B084: Triple checkpoint blockade, but not anti-PD1 alone, enhances the efficacy of engineered adoptive T cell therapy in advanced ovarian cancer

Abstract Background: Over 20,000 women are diagnosed with ovarian cancer annually, and more than half will die within 5 years. This rate has changed little in the last 30 years, highlighting the need for therapy innovation. Although immunotherapy has revolutionized cancer treatment, efforts to harness endogenous patient immune responses have yielded limited therapeutic activity in ovarian cancer patients. T cells engineered to express a T cell receptor (TCR) targeting proteins uniquely overexpressed in tumors have the potential to control tumor growth without toxicity. Mesothelin (Msln) is over-expressed in ovarian cancer, contributes to the malignant and invasive phenotype, and has limited expression in healthy cells, making it a candidate immunotherapy target in these tumors. Methods: The ID8VEGF mouse cell line was used to evaluate if T cells engineered to express a mouse Msln-specific high-affinity T cell receptor (TCRMsln) can kill ovarian cancer. Tumor-bearing mice were treated with TCRMsln T cells plus anti-PD-1, anti-Tim-3 or anti-Lag-3 checkpoint-blocking antibodies alone or in combination, ultimately targeting up to three inhibitory receptors simultaneously. Single-cell RNA-sequencing (scRNAseq) was used to profile the impact of combination checkpoint blockade on engineered T cells and the tumor microenvironment (TME). Results: In a disseminated ID8 tumor model, adoptively transferred TCRMsln T cells preferentially accumulated in established tumors, delayed ovarian tumor growth, and prolonged mouse survival. However, elements in the TME limited engineered T cell persistence and cytolytic function. Triple checkpoint blockade, but not single- or double-agent treatment, dramatically increased antitumor function by intratumoral TCRMsln T cells. scRNAseq of tumor-infiltrating cells revealed distinct transcriptome changes in engineered and endogenous T cells and myeloid-derived cells. Engineered T cells, when combined with triple checkpoint blockade, increased expression of genes associated with effector and memory gene signatures, including proliferation and metabolic function, and reduced expression of genes associated with exhaustion. Moreover, combining adoptive immunotherapy with triple checkpoint blockade significantly prolonged survival in the cohort of treated tumor-bearing mice, relative to mice that received TCRMsln T cells alone or with anti-PD1 or double-agent treatments. Conclusions: Inhibitory receptor/ligand interactions within the TME can dramatically reduce T cell function, suggesting tumor cells may upregulate the ligands for PD-1, Tim-3 and Lag-3 for protection from tumor-infiltrating lymphocytes. In an advanced ovarian cancer model, triple checkpoint blockade significantly improved engineered T cell function and outcomes in mice in a setting where single checkpoint blockade had no significant activity. These results suggest that disrupting multiple inhibitory pathways simultaneously, which can be more safely pursed in a cell intrinsic form through genetic engineering, may be necessary for improved efficacy in patients. Citation Format: Kristin G. Anderson, Yapeng Su, Madison G. Burnett, Breanna M. Bates, Magdalia L. Rodgers Suarez, Susan L. Ruskin, Valentin Voillet, Raphael Gottardo, Philip D. Greenberg. Triple checkpoint blockade, but not anti-PD1 alone, enhances the efficacy of engineered adoptive T cell therapy in advanced ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_2):Abstract nr B084.

Optimization of the Mechanical and Corrosion Resistance of Alloy 625 through Aging Treatments

In the as-annealed condition, the nickel-based Alloy 625 has excellent mechanical and corrosion properties compared to those of common stainless steels. This peculiarity enables its exploitation in several industrial fields at cryogenic and high temperatures and in the presence of severely corrosive atmospheres. However, in this alloy, when high-temperature plastic deformation processes and heat treatments are not carefully optimized, the occurrence of excessive grain coarsening can irremediably deteriorate the mechanical strength, possibly leading to incompatibility with the standard requirements. Therefore, this research work investigated the possibility of adopting single- and double-aging treatments aimed at improving such strength loss. Their optimization involved identifying the best compromise between the hardening effect and the loss in corrosion resistance induced by the simultaneous formation of intergranular chromium-rich carbides during aging. The investigation of the aging treatments was performed using hardness, tensile and intergranular corrosion tests considering different time–temperature combinations in a range from 621 °C to 732 °C. Double aging resulted in a considerable acceleration in the hardening response compared to single aging. However, even after its optimization in terms of both temperature and time, the intergranular corrosion resistance remained a critical aspect. Among all the tested conditions, only single aging at 621 °C for 72 h was acceptable in terms of both mechanical and corrosion properties. The influence of longer exposures will be investigated in a future study.

Tailoring functional properties of a FeMnSi shape memory alloy through thermo-mechanical processing

The influence of thermo-mechanical processing on the microstructure and functional properties of a Fe–17Mn–5Si–10Cr–4Ni-1(V–C) (wt%) shape memory alloy was systematically investigated. The as-received material was subjected to 25 % cold rolling followed by a recrystallization at 925 °C and single or double aging treatments. Transmission electron microscopy revealed the formation of the ε-martensite and annealing twin boundaries and Shoji-Nishiyama orientation relationships of ε-martensite and γ-austenite in double aged specimen. Cyclic tensile testing demonstrated that the recrystallized and double aged alloy exhibited excellent pseudoelasticity. In the incremental strain test, the alloy achieved the highest peak stress and pseudoelasticity at each cycle. In the constant stresses test, the alloy accumulated a minimal residual strain of only 0.12 % over 50 cycles. This stability was attributed to a strong precipitation strengthening and the interactions between the martensite and the refined microstructural features. In addition, the recrystallized and double aged sample resulted in the greatest recovery stress of 450 MPa upon heating after pre-straining, because of its high yield strength suppressing new martensite formation during cooling process. The results of high-resolution transmission electron microscopy identified a non-Shoji-Nishiyama orientation relationship between the stress-induced ε-martensite after the stress recovery test and γ-austenite matrix, inducing additional irrecoverable strain and raising the recovery stress. Overall, the study can demonstrate that the tailored thermo-mechanical processing enables optimizing the functional performance of FeMnSi alloys.

Effect of heat treatment on microstructure and wear properties of laser additively manufactured 316 L stainless steel/Co-Cr-W alloy part using directed energy deposition method

The directed energy deposition additive manufacturing was applied to fabricate hard Co-Cr-W layers on the 316 L stainless steel substrate. The effect of different heat treatment cycles was investigated. The primary microstructure of deposited layers contained a cobalt matrix with interdendritic eutectic carbides mainly M7C3, M23C6, and M6C. Most of the primary carbides were dissolved in the matrix after the solution treatment at 1250 °C for 1 h. The aging treatment promoted the precipitation of (W/Co)6C carbides. The hard carbides caused the third-body wear and increased the wear volume loss of specimens. The solution & double aging treatment provided the optimum combination of hardness and wear resistance due to the best distribution of W/Co carbide. The wear volume loss after heat treatment was decreased from about 6.6 mm3 to 2.8 mm3 compared to the as-deposited specimen. The proposed solution & double aging heat treatment improved wear resistance by 50%.

Effect of solutionizing and double ageing treatments on the microstructural characteristics and tensile properties of Inconel 718 welds

Inconel 718 joints were fabricated using the pulsating direct current gas tungsten arc welding (PDCGTAW) and laser-arc hybrid welding (LAHW) techniques using Ni-Cr-Mo-rich filler. A cyclic post-weld heat treatment (PWHT) comprising 980 °C/20 min/air cooling; followed by two-stage ageing 720 °C/8 h/furnace cooling and 620 °C/8 h/air cooling was performed on both joints. The liquation cracking was noticed on the heat-affected zone (HAZ) of the LAHW joints. Microstructural characteristics were assessed on the joints in both as-welded (ASW) and PWHT conditions. Regardless of the joints, the coarse Mo-rich segregates were noticed in the fusion zone (FZ) in the ASW conditions. On the contrary, the partial dissolution of MC, precipitation of δ-Ni3Nb needles and the coarsening of strengthening precipitates were observed on LAHW and PDCGTAW joints after subjecting to cyclic PWHT respectively. Tensile studies corroborated that there is a considerable enhancement in the tensile and 0.2% offset yield strengths of LAHW joints after being exposed to PWHT, despite the formation of HAZ liquation cracking.

Synergetic regulation of hierarchical microstructure to improve the 704 ℃ tensile property in laser directed energy deposition IN718Plus superalloy

IN718Plus superalloy with hierarchical microstructure has been successfully manufactured by laser directed energy deposition (LDED). Along building direction, microstructure varies from composing of epitaxial columnar grains oriented along <001>γ direction to equiaxed grains in flower like configuration. Striped Laves phase and fine γ'/γ'' precipitates form in the columnar grains, and would transform into fine Laves phase particles and nanoscale γ' precipitates at equiaxed layer. To improve the compositional homogeneity and microstructural stability, high temperature solution treatment has been explored on the superalloy. The treatment would not affect the morphology and orientation of columnar grains, but would redissolve Laves phase precipitates. Double aging treatment has also been performed to obtain homogeneous distributed γ' precipitates, and the synergistic regulation of microstructure enhances the mechanical property significantly, as causing the room temperature yield strength and tensile strength increased from 0.88 GPa and 1.14 – 1.04 GPa and 1.28 GPa, respectively. Moreover, the excellent mechanical property would be maintained upon 704 ℃, with yield strength of 0.93 GPa, tensile strength of 1.04 GPa and high elongation of 27.3%.

Effect of Heat Treatment on Creep Deformation and Fracture Properties for a Coarse-Grained Inconel 718 Manufactured by Directed Energy Deposition

The creep properties of a laser-directed energy deposition (L-DED) technique manufactured Inconel 718 (IN718) was investigated at 650 °C/700 MPa. Microstructure and creep properties of L-DED IN718 samples were tailored by various post heat treatments involving homogenization heat treatment with temperature ranging from 1080 to 1180 °C + double aging and hot isostatic pressing (HIP). Microstructural changes and their influence on the creep behavior and fracture mechanism were observed and discussed. The results show that L-DED sample heat treated by a simple double aging exhibits a 49% increase in creep lifetime tr and a comparable creep elongation ɛf when compared to the wrought material, due to the reserved coarse dislocation cell substructure from the L-DED process. The loss of dislocation cell structure and the coarsening of grains at higher temperature of heat treatments contributes to a shorter tr, εf, but faster ε̇min (minimum creep rate). The present work demonstrates that a simultaneous improvement of creep strength and creep elongation can be achieved in the case of a coarse-grained L-DED IN718 by a double aging treatment which can preserve both the strengthening precipitates and an appropriate size of dislocation cells.

Effects of aging treatment on the mechanical properties and corrosion resistance of an Al-Cu-Mg-Li alloy

In this paper, the effects of as-quenched, single aging, and double aging on the mechanical properties, microstructure, corrosion resistance, and electrochemical behavior of an Al-Cu-Mg-Li alloy were investigated. Most changes only improved the mechanical properties or corrosion resistance of aluminum alloy alone, and very few of them improved the mechanical properties or corrosion resistance at the same time. The results showed that double aging treatment improved the strength, hardness, and corrosion resistance of the alloy but decreased its elongation. During the aging treatment, the corrosion morphology of the alloy gradually transformed from pitting corrosion (as-quenched) to a combination of pitting corrosion and corrosion cracks (single aging) and finally to pitting corrosion (double aging). The fracture morphology gradually changed from an uniform distribution of a mixture of large and small dimples (as-quenched) to an intergranular fracture (double aging) as aging progressed. Transmission electron microscopy results showed that the T1 phase within grains and at grain boundaries (GBs) was different between the as-quenched and double aged states. In addition, a precipitate-free zone (PFZ) formed in the double aged state. The optimal process of Al-Li alloy was determined to be pre-aging at 100 ℃ for 4 h and final aging at 160 ℃ for 24 h.

Achievement of grain boundary engineering by transforming residual stress in selective laser-melted Inconel 718 superalloy

The complicated geometric parts produced by selective laser melting (SLM) are not amenable to grain boundary engineering (GBE), which is primarily implemented via thermomechanical processing. Herein, the high internal residual stress produced by SLM processing is utilized as stored activation energy for direct GBE in a SLM-fabricated Inconel 718 superalloy. The grain boundary distribution characteristics, grain size, texture, and corresponding mechanical properties of the as-built specimens subjected to different heat treatments were investigated. As the annealing temperature increased, the recrystallized fraction increased, accompanied by an initial increase and subsequent decrease in the number of special grain boundaries. The numerous annealing twin boundaries effectively divided the grains, despite grain coarsening induced by grain boundary migration at a higher temperature. Compared to the as-deposited sample, the yield strength and ultimate tensile strength increased from 707 to 1071 MPa and from 947 MPa to 1271 MPa, respectively, and the ductility increased from 17% to 21% in the sample annealed at 1200 °C for 1 h followed by double aging treatment because this sample had the highest fraction of twin boundaries (64.89%). The “Additive Manufacturing + GBE” strategy based on utilizing the residual stress offers a feasible pathway for broadening the application scope of additive manufactured alloys.

Effect of precipitated phases on the mechanical properties and fracture mechanisms of Inconel 718 alloy

In this study, hot-rolled Inconel 718 alloys were subjected to three different heat treatment schedules to develop various precipitated phases (γ″ phase, γ′ phase, needle-shaped δ phase, and granular δ phase). Based on microstructural characterization and mechanical testing, the effects of these precipitated phases on the mechanical properties and fracture mechanisms were investigated. The results showed that a double-aging treatment process was the key to ensuring the complete precipitation and growth of the γ″ and γ′ phases. A small amount of δ phase (∼2.5%) precipitating in the solution treatment stage did not affect the precipitation of the γ″ and γ′ phases in the subsequent aging treatment stage. An Inconel 718 alloy without δ phase precipitation had excellent impact toughness, and the fracture mode was ductile fracture caused by dislocation slip separation. However, if it was present, the morphology of δ phase determined the effect on the impact toughness. The fracture mechanism changed from a mixed fracture mode of intergranular and transgranular fracture to quasi-cleavage fracture when the morphology of the precipitated δ phase was transformed from needle-shaped to granular-shaped. According to the theory of the dihedral angle (2θ) for precipitated phase morphologies, a quantitative relationship between the interfacial energies of the needle-shaped δ phase and granular δ phase was determined. The interfacial energy of the needle-shaped δ phase was only ∼76% of that of the granular δ phase, meaning that the needle-shaped δ phase was likely to debond readily from the matrix, resulting in the formation of intergranular cracks.

Enhanced tensile strength in an Al–Zn–Mg–Cu alloy via engineering the precipitates along the grain boundaries

An investigation was conducted to study the influence of peak-ageing and double ageing treatments in the precipitation along the different grain boundary orientations of an extruded Al–Zn–Mg–Cu alloy. The results display that the peak-aged grain boundary precipitates along the Low Angle Grain Boundaries (LAGBs) are mainly of θ'' with the existence of few η′ precipitates, while the η′ and θ'' precipitates are along the High Angle Grain Boundaries (HAGBs). The major grain boundary precipitates are mainly of θ'' precipitates along LAGBs and θ′, η′, and η along the HAGBs after the double ageing treatment. The variation in the precipitate types along different grain boundary orientations was attributed to the different diffusion rates of solutes along the grain boundaries. In addition, it is observed that the yield strength (YS) (∼ 690 MPa) of peak-aged Al–Zn–Mg–Cu alloy increased by ∼ 73% and ∼ 18% by comparison with the extruded and double-aged alloys, respectively. The double-aged alloy displays good elongation to fracture (∼16%) compared to the peak-aged alloy. Precipitation hardening contributes to the tensile strength of the alloy, and the improvement of the elongation owed to the growth of the grain boundaries precipitates and the corresponding absence of the precipitate-free zone area. The current study provides new insights in engineering the grain boundary precipitates to the required precipitate type and the corresponding mechanical properties.

Effects of homogenization temperature on creep performance of laser powder bed fusion-fabricated Inconel 718 at 650 °C

The effect of homogenization temperature (1000 °C, 1100 °C and 1200 °C) on creep performance of laser powder bed fusion-fabricated Inconel 718 at 650 °C was investigated utilizing the small punch creep test method and the small punch creep specimens loaded perpendicular to the building direction, which can well reflect the comprehensive performance of the specimen in all radial directions of the circular plane parallel to the building direction. The results show that the specimen homogenized at 1100 °C for 1 h followed by double-aging treatment exhibited better creep performance than the other two ones homogenized at 1000 °C and 1200 °C. Such enhancement of the creep performance is attributed to fewer crack initiation sites, the higher GB sliding rate near the precipitates at grain boundaries to release stress concentrations, the higher strengthening effect of γ″ phases in grains, and the more effective resistance to cracking by the special GBs than the other specimens. The basic mechanism for the small punch creep of laser powder bed fusion-fabricated Inconel 718 at 650 °C was elucidated by the careful characterization of the microstructures and theoretical calculations. A feasible homogenization strategy for further improving the SPC performance of laser powder bed fusion-fabricated Inconel 718 alloy is to make the specimen have fewer and smaller granular precipitates at GBs, and more tortuous GBs by introducing more special GBs with considering the grain size. Such a key point can be used to determine the appropriate subsequent heat treatment for laser powder bed fusion thin-walled Inconel 718 component.

Alleviating segregation and enhancing tensile properties of Inconel 718 superalloy by twin-roll casting and two-stage cold rolling

A novel processing route involving twin-roll casting and two-stage cold rolling was proposed to manufacture Inconel 718 alloy. Due to the sub-rapid solidification rate (412.5 °C/s) of as-cast strip, the element segregation can be alleviated and the growth of Laves phase was effectively suppressed. It was also observed that the dissolution of Laves phases during subsequent homogenization was significantly accelerated because of their refined size. The corresponding homogenization time can be shortened from dozens of hours to 1 h. Additionally, the non-uniform microstructure of cold-rolled and annealed samples caused by coarse grains from homogenization treatment can be eliminated by two-stage cold rolling method. Furthermore, a large amount of granular δ phases precipitated during intermediate annealing. After double aging treatment, the final products exhibited an excellent combination of strength and elongation. The ultimate tensile strength when deformed at room temperature and at 650 °C were about 1475 and 1120 MPa, and the corresponding total elongation were ∼ 15% and ∼ 22%. These results indicated that the twin-roll casting was a promising route to produce Inconel 718 alloy.

Microstructural evolution and mechanical properties of Inconel 718 alloy manufactured by selective laser melting after solution and double aging treatments

In this study, Inconel 718 (IN718) alloy was manufactured by selective laser melting (SLM), and the effects of solution and double aging treatments (SDA) on the microstructure and mechanical properties of the IN718 alloy were investigated. Results revealed that the increase of the solution temperature promotes the increase of the number of recrystallized grains of the IN718 alloy. When the solution temperature is 940 ℃, a large amount of needle-like γ″ are formed. With the increase of the solution temperature (980 ℃–1140 ℃), the needle-like γ″ phase in the alloy begins to decrease and gradually disappear, accompanying the precipitation of the nanoscale granular γ'/γ" composite phase. When the solution temperature is 1140 ℃, an enhanced ultimate tensile strength (UTS) of 1451 MPa and a slight sacrifice of fracture elongation (23.7%) are achieved for the IN718 alloy. The formed nanoscale granular γ'/γ" composite phase and annealing twins were responsible for the enhanced mechanical properties of the IN718 alloys. • Revealing the grain variation mechanism at different solution temperatures. • Providing the microstructure evolution of precipitates at different solution temperatures. • Establishing the relationship between microstructure evolution and mechanical properties at different solution temperatures.

Heat treatment for selective laser melting of Inconel 718 alloy with simultaneously enhanced tensile strength and fatigue properties

Inconel 718 alloy fabricated by selective laser melting (SLM) was used to investigate the evolution of precipitated phase from as-SLM to heat-treated (HT) samples. A heat treatment, including homogenization, solution and aging heat treatments, was proposed to obtain SLMed Inconel 718 alloy components for desirable mechanical performance. Post heat treatment process can reduce the number of original print defects due to grain boundary migration and grain growth. The solid solution plus double aging treatment (HT-1) results in the precipitation of large amounts of δ phase within the grain and at grain boundaries, as well as a small amount of massive MC. Homogenization, solid solution plus double aging (HT-2) eliminated a large amount of internal δ phases in the grain as well as needle-like δ phases at grain boundaries. However, the single aging (HT-3) still retains some of the needle-like δ phase. Heat-treated specimens increased tensile strength by 20–24% at room temperature and maintained high strength at 650 °C compared to the as-SLM. Further fatigue results also confirmed that the HT-2 treatment increases the fatigue life by an order of magnitude compared to the as-SLM to 10 8 cycles at a stress amplitude of 300 MPa. This work is anticipated to provide a potential route to obtaining attractive mechanical properties for SLM superalloy components. • The evolution of precipitated phase in SLMed Inconel 718 alloy under different heat treatment systems was studied. • Post heat treatment significantly improves strength of SLMed Inconel 718 alloy and maintains the high strength at 650 °C. • HT-2 heat treatment significantly increases the ultra-high cycle symmetric bending fatigue strength and life of the specimens .