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The inferior creep ductility of Inconel 718 (IN718) fabricated by selective laser melting (SLM), which is unable to meet aerospace material specification (≥4%), limits its engineering application. Despite persistent optimization efforts in the SLM process and post-heat treatment, this issue remains unresolved. This study involved SLM fabrication of the modified IN718 with trace amounts of LaB6 and/or P, followed by homogenization and double aging heat treatment. The addition of trace LaB6 notably influenced the recrystallized grain structure and carbide precipitation at grain boundaries, while trace P addition had minimal impact. All samples exhibited similar γ′ and γ″ strengthening precipitates. Creep tests conducted at 650°C/690 MPa revealed that the individual modification of LaB6 showed limited improvement in creep performance and P showed no improvement, but the combined addition of LaB6 and P led to a substantial enhancement in creep performance, especially reaching as high as 8.6% creep strain. This remarkable improvement in creep ductility is unlikely to arise from the altered grain structure and grain boundary precipitates induced by LaB6 but predominantly arises from the synergistic effect of P and B in enhancing resistance to crack propagation during the third stage of creep. [ABSTRACT FROM AUTHOR]

Superior elevated temperature properties are critical for the application of Inconel 718 in hot-end components. The utilization of high-deposition-rate laser directed energy deposition (HDR-LDED) increases the microstructural diversity of Inconel 718, which accordingly provides more opportunities for properties improvement. Herein, Inconel 718 alloy with a multi-scale microstructure were fabricated by HDR-LDED followed by customized heat treatment. The coarser columnar grains (width ∼300 μm) were obtained with γ" phase (diameter ∼33.74 nm and volume fraction ∼15.16 %) distributed homogeneously in the grain and the fine Laves phases (length ∼3.69 μm; width ∼1.94 μm; aspect ratio ∼1.99 and volume fraction ∼1.55 %) in the inter-dendritic region. This multi-scale microstructure possesses an excellent stress rupture life of 342.9 h with the elongation of 13 % at 650 °C, which far exceeds that of the forging (106 h, 4.2 %), It is attributed to the reduced number of transverse grain boundaries, the finer Laves phase accommodating many dislocations, and γ" phase promoting the formation of stacking faults, Lomer-Cottrell locks and nanotwins. The findings demonstrate the great potential of additively manufactured microstructure in properties enhancement, and may be enlightening for the development of novel customized high-temperature alloys. • A multi-scale microstructure of Inconel 718 was obtained via HDR-LDED followed by customized heat treatment. • The microstructure is composed of submillimeter columnar grain, micron Laves phase and nanometer γ" phase. • Inconel 718 with this microstructure presents an excellent stress rupture life of 342.9 h and elongation of 13 %. • Strengthening mechanism is associated with the edge dislocations, stacking faults, Lomer-Cottrell locks and nanotwins. [ABSTRACT FROM AUTHOR]

Improvements to the surface of high-power laser solid formed Inconel 718 by use of electrochemical methods were systematically investigated. Formation features (surface microstructure characteristics and surface morphology) and the corresponding electrochemical anodic behavior (electrochemical dissolution behavior and surface levelling mechanism) were analysed. The electrochemical results show that the surface energy difference between the horizontal section and vertical section causes no difference to their transpassive current densities in 10 wt.% NaNO 3 solution up to 2.5 V, owing to the existence of a passive film. Analysis of micro-morphologies indicates that the high current density leads to a smooth micro-morphology due to the higher detachment rates of the interdendritic phases and surface products. Numerical simulations suggest that surface peaks dissolve faster than depressions during the electrochemical levelling process owing to the higher local current density on the surface peaks. Importantly, a detailed microstructure selection map for as-deposited Inconel 718 and an anodic current-density-potential map were developed, and the electrochemical levelling mechanism of its wavy surfaces was proposed. This work has the potential to integrate understanding of laser solid forming-process conditions, formation features, and electrochemical anodic behavior. [ABSTRACT FROM AUTHOR]

Microstructural analysis and electrochemical behavior of Inconel 718 fabricated by laser solid forming on horizontal section (HS) and vertical section (VS) were investigated. The electrochemical results show that the corrosion resistance on HS and VS have no difference in 10 wt.% NaNO 3 solution. However, HS possesses lower surface products resistance at low current densities due to the tube-like distribution of secondary phases. Besides, their higher current efficiencies at low current densities are mainly attributed to the undissolved phases being flushed away by the flowing electrolyte. The fast detachment rates of secondary phases at high current densities improve the surface quality. [ABSTRACT FROM AUTHOR]

Electrochemical anisotropy has drawn substantial attention in the fields of electrochemical dissolution and corrosion, and it is related to the surface energy levels in a single-phase alloy. Here, laser solid forming (LSF) Inconel 718 alloy was found to be a multi-phase alloy with a strong fiber texture. Its horizontal section (HS), in which secondary phases have a tube-like distribution, mainly consists of the (001) crystallographic plane, while a vertical section (VS) with a strip-like secondary phase, consists of the (001) crystal zone. The HS was first measured to be more electrochemically stable but it has a higher anodic dissolution rate than VS in 10 wt% NaNO. Contrast analysis indicates that the electrochemical anisotropy can primarily be attributed to the columnar structure. Therefore, researchers should be very careful when using a surface energy model to predict the electrochemical anisotropy of a highly textured alloy. Graphical Abstract: For convenience, the red-colored secondary phases are continuous. Actually, they are continuous or discontinuous. (Color figure online) [ABSTRACT FROM AUTHOR]

The 50% laser additive manufactured (50%LAMed) tensile samples of Inconel 718 superalloy, i.e., the laser-deposited zone and the substrate zone, occupied 50% volume fraction respectively along the tensile direction, have been fabricated by using laser additive manufacturing (LAM) technology. The inter-dendritic Laves phases were reserved because solution heat treatment could not be used in case of the deteriorating mechanical properties of a forging substrate. Meanwhile, most of the γ″ phases were precipitated in the inter-dendritic area, leading to local stress concentration around the Laves phases in the process of a tensile test. With tensile stress increasing, the degree of deformation and fracture of the Laves phases was closely related to the morphologies of themselves. For the long striped Laves phases, in order to deform with the austenite matrix, they slipped and broke up into small parts. For most of the granular Laves phases, they did not break up and held original morphologies in the process of the tensile test. The broken Laves phases were separated from the γ matrix, and micropores formed at the surface between them. The fracture mechanism was the microvoid coalescence ductile fracture, and the Laves phases were the main nucleuses for the formation of micropores. This study indicates that the mechanical properties of 50%LAMed Inconel 718 can be improved by controlling the morphologies of the Laves phases. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure variation of as-deposited Inconel 718 superalloy sample in deposition direction fabricated by laser solid forming (LSF) was investigated. It was found that the as-deposited microstructure was mainly consisted of predominant γ columnar dendrites directionally growing along the deposition direction. Primary dendrite arm space measurement revealed spaces of 11.5, 17.5 and 38.0μm at the bottom, the middle and the top section of the LSF sample respectively. The result was further studied by comparing with the Kurz–Giovanola–Trivedi (KGT) dendrite growth model, which was in a reasonable agreement with the experimental result. The various microstructures and micro-segregation situations at different sections of the LSF sample, which were induced by the thermal accumulation during LSF process, were also carefully discussed. [Copyright &y& Elsevier]

Abstract: Two kinds of laser scanning paths, i.e. single direction raster scanning (SDRS) and cross direction raster scanning (CDRS), were used to prepare Inconel 718 alloy parts by laser solid form (LSF) technology. The microstructures and mechanical properties of LSF Inconel 718 samples were investigated. It is shown that the as-deposited microstructure of SDRS sample is composed of columnar dendrites which grow epitaxially along the deposition direction; but in the CDRS sample, the continuous directional growth of columnar grains is inhibited and the orientation deviation of dendrites in two adjacent layers increases. The as-deposited grains of the CDRS sample are finer than those of the SDRS sample. After heat treatment, recrystallization occurs and grains of both samples are refined. However, the recrystallized grains in SDRS sample are not as uniform as that of the CDRS sample. Tensile testing at room temperature indicates that the ultimate tensile strength of these two samples is similar; however, the ductility of the CDRS sample is much better than that of the SDRS sample. Fracture surface examination presents a transgranular mode of crack propagation for the SDRS sample and a mixture of transgranular and intergranular modes of crack propagation for the CDRS sample. The inhomogeneity of grain size is considered to be the prime reason for the worse ductility of the SDRS sample. [Copyright &y& Elsevier]

Abstract: The microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718 was investigated. The as-deposited microstructure of an LRFed Inconel 718 alloy is composed of columnar dendrites growing epitaxially along the deposition direction, and the columnar dendrites transformed to unevenly distributed equiaxed grains after annealing treatment at high temperature. Residual stress evaluation in microstructure scale by Vickers micro-indentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample, and it has a significant effect on the recrystallization during solution annealing treatment. The residual stress is introduced by rapid heating and cooling during laser rapid forming. There is an alternative distribution between high residual stress regions and low residual stress regions, within a single deposited layer, resulting in a similar distribution of recrystallized grain size. [ABSTRACT FROM AUTHOR]

The high-cycle fatigue (HCF) properties of Inconel 718 (IN718) superalloy prepared using laser directed energy deposition (LDED) were investigated at room temperature (RT) and 650 °C. A specified heat treatment scheme, namely homogenization + solution + aging (HSA), was employed for the as-deposited LDED IN718 alloy. The results indicate that the grains of the HSA LDED IN718 alloy have a typical bimodal grain (BG) distribution consisting of coarse grains (CGs) and fine grains (FGs). The fatigue limit (σ w) of the HSA IN718 alloy at RT was higher than that at 650 °C (~385 and ~280 MPa, respectively), and their σ w values were only ~77% and ~63% those of wrought IN718, respectively. Fractographic analysis revealed that the fatigue failure model changed from surface slip failure to large crystallographic facet failure with increasing testing temperature. The HCF properties of the HSA IN718 alloy with BG microstructure are strongly affected by CGs in the material, which eclipses the deleterious effects of the initial pore defects in the deposit. Finally, the Basquin model was fitted to extract the stress-life curves of the HSA IN718 alloy. A certain degree of deviation was observed when predicting the σ w value of the HSA IN718 alloy at 650 °C using the Murakami model. [ABSTRACT FROM AUTHOR]

• Combined with thermodynamic calculation, the formation mechanisms of grain morphology and Laves phase are discussed in detail. • The contribution of different strengthening mechanisms to yield strength is quantitatively calculated. • The cause of anisotropy of fatigue crack growth rate in vertical and horizontal specimens is clarified. The microstructure and the anisotropy of mechanical properties for Inconel 625 superalloy prepared by directed energy deposition (DED) were investigated in this study. The microstructure of as-deposited sample was mainly composed of epitaxial columnar grains, which showed a z-shaped distribution along the deposition direction. Combined with thermodynamic calculation, the formation mechanism of Laves phase in as-deposited Inconel 625 superalloy was discussed in detail. Owing to the anisotropic microstructure of as-deposited sample, tensile properties and fatigue crack growth (FCG) behavior showed significant anisotropy. Compared to vertical sample, the grain boundary volume fraction in horizontal sample was larger, which caused that the ability of grain boundary strengthening and the resistance of fatigue crack growth were higher in horizontal sample during tensile and fatigue crack growth process. As a result, the yield strength and the ultimate tensile strength of horizontal sample were higher, while the fatigue crack growth rate was lower. However, the elongation of horizontal sample was smaller than that of vertical sample caused by the Laves phase distribution. [ABSTRACT FROM AUTHOR]

Usually, the laser additive manufactured nickel-based superalloys require subsequent heat treatment to modify their microstructure and mechanical properties. In present work, the solution treatment (ST), solution + aging treatment (SA) and hot isostatic pressing (HIP) treatment were used to investigate the effect of different heat treatments on microstructural evolution and tensile properties of Inconel 625 prepared by directed energy deposition (DED). The microstructure of as-deposited Inconel 625 was mainly composed of epitaxially grown columnar grains, the chain-like Laves phase was formed in the inter-dendritic regions. During high temperature heat treatment process, the initial columnar grains were replaced by equiaxed grains since the occurrence of static recrystallization, and the Laves phase dissolved into matrix completely. The nanoscale γ ″ phase was formed after aging treatment, the yield strength and ultimate strength was larger than other samples. However, the steady working hardening stage of SA treated samples was shorter than that of other samples, and its ductility was the lowest. Considering the strength and ductility comprehensively, the HIP treatment was a better heat treatment for Inconel 625 fabricated by DED. [ABSTRACT FROM AUTHOR]

• After high temperature heat treatment, a static recrystallization of the deposit occurred. • Element segregation altered the precipitation kinetics of γ′′/γ′ phases. • A judicious post-heat-treatment increased Δ K th and decreased FCGR of LDED IN718. • The Δ K th of HSA specimen increased by ~ 2.6 times vs. as-deposited specimen. In this study, Inconel 718 (IN718) specimens, manufactured by laser directed energy deposition (LDED), are subjected to three different heat-treatments to have different microstructural characteristics. Fatigue crack growth (FCG) testing is performed on both as-deposited and heat-treated specimens. The coarse Laves phases with uneven γ′′/γ′ precipitates severely decrease FCG resistance of LDED IN718. The soft precipitate-free zone around short-acicular δ-phase partly reduces FCG resistance of LDED IN718. The FCG threshold (Δ K th) of a judicious heat-treatment specimen increases by ~ 2.6 times vs. as-deposited specimen and reaches 10.8 MPa∙m1/2, and its Δ K th and FCG curve are comparable to those of wrought IN718. [ABSTRACT FROM AUTHOR]

Optimizing the microstructure to enhance the high temperature stress rupture properties of Inconel 718 (IN718) alloy fabricated via selective laser melting (SLM) is an urgent issue. In this study, SLM-built IN718 specimens were heat-treated with hot isostatic pressing, solution and aging treatment, and their microstructure and high temperature stress rupture properties were investigated, focusing on the influence of grain structure and precipitates. The results show that the coarsening of γ″ precipitate with the aging time at 720 °C follows the Lifshitz-Slyozov-Wagner theory. As the γ″ precipitate size increases, the rupture life gradually increases, area reduction gradually decreases, and the uniform elongation first increases and then decreases. Furthermore, a wide distribution of grain size results in the fine grains regions with much higher geometrically necessary dislocations compared with coarse grain regions during deformation. Hence, the micro-cracks preferentially initiate and propagate along fine-grain boundary. The main deformation mechanisms of specimens aged at 720 °C for 4 h (HSA4) and 8 h (HSA8) with the small γ″ precipitate are dislocation slipping and micro-twinning mode during stress rupture test, while those of specimen aged at 720 °C for 32 h (HSA32) with relatively large γ″ precipitate are dislocation slipping and isolated faulting mode. The HSA8 specimen with the best comprehensive stress rupture properties has the average life of 130 h and uniform elongation of 7.5%, meeting the standard specification for wrought IN718. Finally, some potential strategies for further improving the stress rupture properties of SLM-built IN718 were proposed by comparing to forged-IN718. [ABSTRACT FROM AUTHOR]

In order to improve the seawater corrosion resistance of Inconel 718 superalloy, a La2Zr2O7/NiCoCrAlY thermal barrier coating corrosion resistant to 3.5 wt.% NaCl aqueous solution was prepared by laser cladding on Inconel 718 superalloy. X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and electrochemical techniques were used to study the microstructure and the corrosion performance of the coating in 3.5 wt.% NaCl solution. The results show that the thermal barrier coating is mainly composed of primary La2Zr2O7 phase and γ + laves/δ phase eutectic structure. The corrosion potential and corrosion current of the coating in 3.5 wt.% NaCl solution are higher and lower than that of the Inconel 718 substrate, respectively, indicating that the corrosion performance of the coating is better than that of the Inconel 718 substrate. The presence of La2Zr2O7 phase in the thermal barrier coating is the main reason for its corrosion resistance to 3.5 wt.% NaCl solution. [ABSTRACT FROM AUTHOR]

• Evolution law of the crystallographic texture was studied for HDR-LSF Alloy 718. • Effect of passive film on anodic activity isotropy was analyzed for the first time. • Amorphous structure of the bi-layer film was confirmed. We report the crystallographic texture and passive behavior of nickel-based superalloys prepared by high-deposition-rate laser solid forming (HDR-LSF) additive manufacturing in deaerated 10 wt.% NaNO 3. The HDR-LSF Alloy 718, mainly composed of columnar grains, is detected to be weakly fiber textured due to the direction changed structure. However, passive current densities on horizontal and vertical sections have no difference and both increase linearly with the electrode potential. Furthermore, the identical steady-state passive current density on both sections is attributed to the same film/solution interface energy. Amorphous structure of the bi-layer film induces the electrochemical activity isotropy regardless of matrix texture. [ABSTRACT FROM AUTHOR]

The influence of post-heat-treatment on the resulting microstructure and room-temperature fracture toughness (K I C ) of Inconel 718 fabricated by laser solid forming (LSF), a kind of laser based directed-energy-deposition additive manufacturing technologies, is investigated. Detailed microstructure characterization was performed on as-fabricated and heat-treated samples using direct aging (DA), solution treatment plus aging (STA), homogenization plus STA (HSTA), respectively. The results indicate that as-fabricated sample mainly consists of γ columnar dendrites and a small quantity of (γ+Laves) eutectics in the interdendritic areas. After DA post-heat-treatment, the nonuniform γ′′/γ′ precipitates exist around Laves phases. After STA post-heat-treatment, the short-acicular δ-phases precipitate around/in the Laves phases, the micro-segregation reduces and the distribution of γ′′/γ′ precipitates in the dendrite arm is almost homogeneous. After HSTA post-heat-treatment, Laves phase mostly disappears, micro-segregation completely removes and the γ′′/γ′ precipitates (~30 nm) are distributed in bimodal recrystallized grains. The results of K I C testing indicate that as-fabricated sample possesses the lowest K I C mainly due to its lowest elastic modulus and yield strength. However, the K I C of DA sample (~89.8 MPa m) is close to that of as-fabricated sample, mainly due to its low ductility and strain-hardening exponent. But, the K I C of STA and HSTA samples increases by ~55.8% and 90.8% compared with as-fabricated sample respectively, mainly because of the better performance in strength–plasticity of HSTA sample (~1114 MPa, ~26.1%). Particularly, the K I C of HSTA sample with bimodal grains is ~60 MPa m higher than the lower limit of wrought IN718 (AMS 5662) and reaches ~164.1 MPa m. There exists significant difference in the K I C fractographs of as-fabricated and heat-treated samples. Overall, this research illustrates that an appropriate post-heat-treatment possesses obvious toughening effect on the LSF Inconel 718 alloy. [ABSTRACT FROM AUTHOR]

The electrochemical behavior of Inconel 718 prepared by laser directed energy deposition with single γ-phase and dual (γ + γ″)-phase was investigated using 10 wt% NaNO 3 , 5 wt% NaNO 3 + 5 wt% NaCl and 10 wt% NaCl, respectively. It is found that the Cl- ion weakens the inhibitive effect of the transpassive film, resulting in a uniform electrochemical dissolution for deposits with the single γ-phase, but an inferior surface quality for deposits with the dual (γ + γ″) phases due to the γ″ phase inducing uneven pits expansion. Two qualitative models are proposed to demonstrate their electrochemical dissolution mechanism. • The loose transpassive film induces pits on the surface of LDED Inconel 718. • Pit expansion determines the surface quality of LDED Inconel 718. • γ″ phases induce the uneven pit expansion in chloride-containing solutions. • The Cl- ions improve the uniform pit growth in the sample with single γ-phase. [ABSTRACT FROM AUTHOR]

This study investigated the synergistic effect of transpassive film and γ″ phase on the electrochemical dissolution behavior of Inconel 718 manufactured by laser directed energy deposition. The investigation was based on samples after homogenization and aging with (γ + γ″) phases and homogenization with γ phase. The compositions of the transpassive films for two kinds of heat-treated samples were similar, mainly comprising porous Ni hydroxides. The incompact transpassive film induced the local preferential dissolution of the γ matrix and generated multiple dispersive pits on the surface. Compared with the sample after homogenization, the spatial distribution of the γ″ phase in the sample after homogenization and aging restricted the expansion of pits in the γ matrix, leading to superior surface quality. The detachment of the γ″ phase during electrochemical dissolution provided extra mass loss and higher current efficiency. A qualitative model was proposed to demonstrate the synergistic effect of the transpassive film and γ″ phase on the dynamic electrochemical dissolution process. • The incompact transpassive films generate many dispersive pits on the surfaces. • γ″ phases in LDED-HA Inconel 718 restrict the expansion of pits in γ matrix. • The detachment of γ″ phase provides a higher current efficiency. [ABSTRACT FROM AUTHOR]

In this study, we explain why the amount of Laves phase in Inconel 718(IN718) manufactured by selective laser melting (SLM) unexpectedly increases under extremely rapid solidification and clarify its distinct dissolution behavior during heat treatment based on a comparative analysis of laser directed energy deposition (L-DED)-built and cast IN718. Compared with the cast and L-DED, SLM has much higher cooling rate, which greatly improves the dendrite growth rate and the solute trapping during the solidification process of molten pool. Thus, more Nb atoms are trapped in the matrix, and less Nb atoms are expelled to form inter-dendritic Laves phase in SLM-built IN718. However, the volume fraction of Laves phase is the highest in SLM-built IN718, which is attributed to the lower Nb concentration in the Laves phase and few Nb-rich region. During homogenization treatment, the dissolution of Laves phase is controlled by three-dimensional diffusion process at the early stage, and then is mainly controlled by the interface reaction process. After homogenization, there are still many tiny carbides, oxides, and nitrides undissolved both along the grain boundaries and inside the grains. [ABSTRACT FROM AUTHOR]

The selective laser-melted (SLM) Inconel 718(IN718) specimens were heat-treated with different solution regimes after the hot isostatic pressing (HIP). The results show that δ phase forms at grain boundary and presents needle-like pattern, and the size and volume fraction of δ phase increase with the increase of holding time and the decrease of solution temperature. The equilibrium volume fraction of needle-like δ phase in SLM-built IN718 is higher than that of short rod-like δ phase in forged-IN718 under the same heat treatment. The SLM-built IN718 specimen without δ phase at grain boundary has the best stress rupture properties, while the specimen with the largest volume fraction of δ phase at grain boundary exhibits the worst stress rupture properties. This is attributed to that the needle-like δ phase at grain boundary promotes the nucleation of void-formed cracks and accelerates the deformation damage. The analysis of the responses to the coordinated deformation capacity of grain orientation and grain size indicates that there exist the heterogeneous deformation and stress/strain concentration at grain boundary. The existence of δ phase influences the coordinated deformation capacity and promotes the stress/strain concentration around grain boundary region. Especially, the needle-like δ phase at grain boundary leads to the more detrimental effect on the rupture life and ductility of SLM-built IN718 alloy on compared with the forged-IN718 alloy. [ABSTRACT FROM AUTHOR]

• The melt flow behaviour of selective laser melting Inconel 625 is characterized. • The elevation in the beginning of the melt track can be attributed to recoil pressure. • Low powder packing density gives rise to porosity. • Porosity formation process under unsuitable energy input is displayed. • The computational efficiency has been greatly improved by the present model. To study the melt pool dynamics and resultant porosity formation mechanism of the selective laser melting (SLM) Inconel 625 alloy, a height function-lattice Boltzmann method (HF-LBM) coupled model that considers both the computational efficiency and the important physics was proposed. In this model, a novel interface captured technique was used to provide insights to simulate the melt flow by considering interfacial forces via surface tension, Marangoni convection and recoil pressure. It was discovered that the fused powders could form a continuous cladding layer under the effect of surface tension rather than gravity, as the strength of surface tension was a million times larger than that of gravity. And the visible elevation at the beginning of the melt track was strongly associated with the recoil pressure. The simulation results matched well with the experimental observations. The porosity formation mechanisms under different process parameters and powder packing densities have also been investigated. The results revealed that at a fixed laser power (120 W), the melt track exhibited with porosity in the form of a lack of fusion when the scanning speed was above 1200 mm/s and the discontinuous melt track was thought to yield subsequent porosity due to the elevated layer thickness. In contrast, the porosity displayed in the form of trapped gas owing to the keyhole effect under high energy input. When a low powder packing density was applied, the "necking" phenomenon, attributed to the persistent flow rate difference between the bottom and the rear, was observed and thought to increase the porosity similar to the situation of high scanning speed. [ABSTRACT FROM AUTHOR]

• Plastic flow behavior of the LMD Inconel 718 were tested over a wide range of temperature and strain rate. • Anisotropy, temperature and strain rate dependence of mechanical behavior were investigated. • A constitutive model was developed in consideration of effects of anisotropy, temperature and strain rate. • The mechanism of the effect of strain rate on third-type strain aging effect was discussed. The lack of comprehensive insight and accurate description on the dynamic thermomechanical behaviors in high-temperature and high-strain-rate loading environment could be one of the major obstacles to extend the laser metal deposited (LMD) Inconel 718 to engineering applications in aeroengine. In this study, to obtain in-depth understanding of plastic flow behavior of the LMD Inconel 718, uniaxial compressive experiments were conducted over a wide range of temperature (298–1273 K) and strain rate (0.001–5300/s). Based on a great number of experimental results, not only the anisotropy of compressive property, but the dependence of the plastic flow stress on temperature and strain rate were investigated systematically. The flow stress of the as-deposited alloy in laser scanning direction is higher than that in depositing direction, which is attributed to the columnar grains epitaxially growing along the build direction and featuring the high length-diameter ratio within the as-deposited sample. Since the small-size precipitates in the under-aged (as-deposited) Inconel 718 alloy could be cut by the moving dislocations directly during plastic deformation, they have various effect on the interaction between moving dislocations and interstitial elements at different strain rate. The third-type of strain aging (3rd SA) effect, exhibiting an anomalous bell-shaped flow stress vs. temperature relation, was observed more apparently than that in the post-aged Inconel 718 reported by other literatures. As for the strain-rate effect, the flow stress of the alloy exhibits inconspicuous strain-rate sensitivity over the range of strain rate below ∼103/s, while increases sharply with the strain rate once it exceeds ∼103 /s. Finally, taking into account the mechanical anisotropy, as well as the anomalous temperature and strain rate sensitivities of the flow stress, we developed a constitutive model based on a physical frame. In this model, an anisotropy component was introduced to consider the effective size of columnar grains in the corresponding loading direction, the strain rate hardening was enhanced as the strain rate gets close to a certain level, and a normal distribution with temperature was used to describe the 3rd SA effect. The developed model was shown to be able to accurately reproduce the plastic flow behavior of the LMD Inconel 718 over a wide range of temperature and strain rate. [ABSTRACT FROM AUTHOR]

Abstract Long-striped Laves phases are considered to be crack sources in laser repaired Inconel 718 during stress rupture test. In order to obtain fine Laves phases, Nd:YAG pulse laser is used to fabricate Inconel 718 superalloy on a wrought Inconel 718 substrate in this study. The microstructures and stress rupture properties of pulse laser repaired Inconel 718 superalloy after DA treatment (720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) and 900STA treatment (900 °C × 20 min/air cooling+720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) are investigated by using SEM and TEM. The results shows that granular Laves phases can be obtained when pulse laser is used. The formation of granular Laves phase is related to high cooling rate (G·R) and low ratio of temperature gradient to growth rate (G/R). The critical values of G·R and G/R are 104 and 107, respectively. After 900STA treatment, δ phase precipitates around the granular Laves phases. But there is no δ phases existing in the DA samples. The surrounding δ phases can induce stress concentration, resulting in that the elongation of the 900STA samples is worse than that of the DA samples. However, the regions denuded in Nb element around the δ phases can delay the formation of the vacancies, which leads to that the stress rupture life of the 900STA samples is better than that of the DA samples. Graphical abstract Image 1 Highlights • The deformation behaviors of Laves phase under different loads have been analyzed. • The selection map of Laves phase morphology has been established. • The effects of granular Laves and the surrounding δ phase have been investigated. [ABSTRACT FROM AUTHOR]

The effect of crystal orientation and heat treatment on microstructure and wear properties of Inconel 718 superalloy fabricated by laser directed energy deposition (LDED) were analyzed and compared with forged samples. Fully heat treated samples presented the lowest wear rate and better resistance to friction compared with other condition samples. The direct aging treated samples also had better wear resistance attributed to its microhardness and microstructure and were mainly abrasive wear with a large amount of wear debris and deeper scratches on the worn surface. As for the anisotropy of microstructure in LDED samples, the wear properties in different crystal orientations were different, and better wear performance was obtained in the section which is vertical to the deposition direction. • Anistropic microstructure leads to different wear resistance. • The solution of Laves phase and precipition of γ″ phase enhance the wear property. • Heat treatment can decrease the difference in wear property of differnt sections. [ABSTRACT FROM AUTHOR]

To achieve high-performance nickel-based superalloys by laser metal deposition (LMD) technology for applications in aeroengines, we prepared Inconel 718 superalloys by LMD with three groups of process parameters and then heat treated them by two different protocols. We carried out compressive experiments for these Inconel 718 samples over a wide range of strain rate (0.001–5000/s) to evaluate the effects of process parameters and heat treatments on their microstructures and dynamic mechanical properties. We observed both the initial microstructures and the failure characteristics of the samples using the optical microscope and the scanning electron microscope. We found that a higher energy input density during laser additive manufacturing led to a wider range of primary dendrite spacing. The plastic flow stress of the alloy decreased near-linearly with increase in primary dendrite spacing. The anisotropy of the compressive properties of the sample resulted from the anisotropy of the as-deposited and the direct aged structures, while the microstructural and mechanical anisotropy almost vanished after full heat treatment. We further carried out compressive experiments over a wide range of strain rate (0.001–5300/s) and temperature (298–1193 K) to understand the mechanical properties of Inconel 718 by LMD in an extremely high-strain rate and high-temperature loading environment. We noticed an anomalous high-temperature peak in the flow stress, in the flow stress vs. temperature relation, under different strain rates, and we proved that it is attributed to the third type of strain aging effect. Finally, by observing the compressive failure characteristics, we found that the propagation path of a crack is dependent on the loading direction. The compressive fractography morphology could reflect the effect of heat treatment on the ductility of the samples. Furthermore, it was evident that the initial defects (gas and shrinkage porosities) in Inconel 718 samples caused by LMD can contribute to the generation, deflection, and branching of cracks. [ABSTRACT FROM AUTHOR]

In this paper, a comparative study of high-cycle fatigue tests ( T =650 °C, f =110 Hz, R =0.1, K t =1) were carried out with wrought Inconel 718 and LAMed Inconel 718. The results show that the influences of the Laves phases on high-cycle fatigue properties are based on the applied stress amplitudes. At a low stress amplitude, most of the Laves phases held their original morphologies. The fatigue cracks stopped in front or detoured around them, which means that the unbroken Laves phases play an important role in hindering crack propagation. In this way, the high-cycle fatigue life of LAMed Inconel 718 was superior to that of wrought Inconel 718. However, at a high stress amplitude almost all of the Laves phases in the crack propagation region splintered into smaller fragments, parts of which separated from the austenite matrix. Microscopic holes or cracks were formed at the interface, which provided passages for the fatigue cracks to propagate. In this case, the Laves phases were harmful, leading to the degradation of fatigue performance in LAMed Inconel 718 compared with wrought Inconel 718. [ABSTRACT FROM AUTHOR]

Laser repairing technology, based on laser additive manufacturing technology, has been used in repairing damaged components for saving time and economic cost. Due to the existence of laser deposited zone, the microstructure and mechanical properties of laser repaired (LRed) components are different from original wrought parts. In this paper, the tensile deformation behavior of LRed Inconel 718 was compared with that of wrought Inconel 718 by using digital image correlation (DIC) method. The microstructure and the failure mechanism of LRed Inconel 718 were also investigated. The results showed that the LRed Inconel 718 sample could be divided into two parts: laser deposited zone (LDZ) and substrate zone (SZ). The laser deposited zone consisted of columnar dendrite growing epitaxially along the deposition direction, where the γ″ phases gathered around the interdendritic Laves phases. However, the microstructure of the substrate zone was equiaxed grain with the uniform distribution of δ and γ″ phases. It means that the microstructure of the LRed Inconel 718 was non-uniform. The deformation compatibility of the LRed Inconel 718 was inferior to that of the wrought Inconel 718 because of the existence of the Laves phases and the heterogeneous distribution of the γ″ phases. As a result, strain concentrated in the laser deposited zone resulting in that most plastic deformation was clustered in this region. The hard and brittle Laves phases slipped and broke up into smaller parts due to local stress concentration. Microscopic holes formed at the interface of the Laves phases and the γ matrix and combined together to cause the fracture of the LRed Inconel 718. [ABSTRACT FROM AUTHOR]

Recently, an innovative process chain has been investigated and proposed, using laser-based directed energy deposition technology to produce pre-forms for forging. The advantage of this technology is to get high-performance near-formed parts at a low cost, compared traditional processing. Clarifying the hot workability and the microstructural evolution of as-deposited samples is the key to obtaining high performance parts. In present work, the plastic behaviors and dynamic recrystallization mechanisms of as-deposited nickel-based superalloy are studied. The compression curves are corrected by eliminating adiabatic heating and friction effect. And then the hot deformation activation energy and the constitutive equation of as-deposited sample are obtained. Compared to the wrought Inconel 625, as-deposited Inconel 625 has lower hot deformation activation energy (466.511 kJ/mol). The peak stress gradually decreases, while the recrystallized grain volume fraction gradually increases when deformation temperature increases and strain rate decreases. The characterization results of TEM and EBSD show that the discontinuous dynamic recrystallization mechanism nucleating by the bowing of grain boundaries occupies an absolute dominant position. In addition, combined with the microstructural evolution and hot processing map, the best hot working conditions of as-deposited Inconel 625 are obtained (deformation temperature: 1050–1150 ℃, strain rate: 0.001–0.01 s−1). • Hot compression stress-strain curves of Inconel 625 fabricated by L -DED were corrected by eliminating the influences of adiabatic heating. • The constitutive equation and hot processing map of as-deposited Inconel 625 were established. • The dynamic recrystallization mechanism of Inconel 625 fabricated by L -DED during hot deformation process was discussed. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) has unique advantages in manufacturing oxide dispersion strengthened alloys due to its high cooling rate and tiny molten pool. In this study, the microstructure and mechanical properties of Inconel 625 alloy and Y 2 O 3 strengthened Inconel 625 alloy prepared by SLM were investigated comparatively. The highest density for Y 2 O 3 /Inconel 625 specimen with a value of 99.6% was achieved when the laser volume energy density is around 145.8 J/mm3. Compared with Inconel 625 specimen, there presents a larger molten pool, finer grain structure, and weaker texture along the deposition direction in Y 2 O 3 /Inconel 625 specimen due to the active flux effect of Y 2 O 3. During the heat treatment, the grain growth and annealing twin formation are inhibited in Y 2 O 3 /Inconel 625 specimen due to the pinning effect of Y 2 O 3 particles. The yield strength of Inconel 625 specimen increases by 11.4%, 22.0%, and 12.5% at 20, 800, and 1100 °C, respectively, due to the dispersion strengthening and grain boundary strengthening induced by the addition of 0.48 wt % Y 2 O 3 particles. In addition, the elongation of Y 2 O 3 /Inconel 625 specimen at high temperature is improved because of the contribution of Y 2 O 3 to the strengthening and oxidation resistance of grain boundaries. [ABSTRACT FROM AUTHOR]