Additive manufacturing of fatigue resistant materials: Challenges and opportunities

Inconel 718 superalloy has been fabricated by selective laser melting technology (SLM). Its microstructure and mechanical properties were studied under solution+aging (SA) standard heat treatment, homogenization+solution+aging (HSA) standard heat treatment and as-fabricated conditions. Precipitated phases and microstructures were examined using OM, SEM, TEM and X-ray analysis methods. The fine dendrite structures with an average dendrite arm spacing of approximately 698 nm accompanying some interdendritic Laves phases and carbide particles can be observed in the as-fabricated materials. After standard heat treatments, dendrite microstructures are substituted by recrystallization grains, and Laves phases also dissolve into the matrix to precipitate strengthening phases and δ particles. The test values of all specimens meet Aerospace Material Specification for cast Inconel 718 alloy, and the transgranular ductile fracture mode exists for the three conditions. The strength and hardness of heat-treated SLM materials increase and are comparable with wrought Inconel 718 alloy, whereas their ductility decreases significantly compared with the as-fabricated material. This is because of the precipitation of fine γˊ and γ〞strengthening phases and needle-like δ phases. For the as-fabricated alloy, the formation of finer dislocated cellular structures that develop into a ductile dimple fracture shows excellent ductility. Due to dislocation pinning from γˊ and γ〞strengthening phases and the impediment of dislocation motion caused by the needle-like δ phases, the ductility of the SA materials decreases and causes a transgranular fracture, compared with the as-fabricated samples.

Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy

Laser net shape manufacturing (LNSM) is a laser cladding/deposition based technology, which can fabricate and repair near-net-shape high-performance components directly from metal powders. Characterizing mechanical properties of the laser net shape manufactured components is prerequisite to the applications of LNSM in aircraft engine industrial productions. Nickel-based superalloys such as INCONEL 718 are the most commonly used metal materials in aircraft engine high-performance components. In this study, the laser deposition process is optimized through a set of designed experiments to reduce the porosity to less than 0.03 pct. It is found that the use of plasma rotating electrode processed (PREP) powder and a high energy input level greater than 80 J/mm are necessary conditions to minimize the porosity. Material microstructure and tensile properties of laser-deposited INCONEL 718 are studied and compared under heat treatment conditions of as deposited, direct aged, solution treatment and aging (STA), and full homogenization followed by STA. Tensile test results showed that the direct age heat treatment produces the highest tensile strength equivalent to the wrought material, which is followed by the STA-treated and the homogenization-treated tensile strengths, while the ductility exhibits the reverse trend. Finally, failure modes of the tensile specimens were analyzed with fractography.

Studies of Standard Heat Treatment Effects on Microstructure and Mechanical Properties of Laser Net Shape Manufactured INCONEL 718

Astonishingly 3D printing has excited the world of aerospace. This paper takes stock of the popular 3D printing processes in aerospace. Reasons for their popularity over the traditional manufacturing processes are dwelled upon. Materials developed specially for aerospace applications along with their characteristics are discussed. Ongoing activities related to 3D printing at various companies and organisations around the world are looked into. Project works in the area of extra-terrestrial printing are also highlighted. Even though 3D printing processes are operationally simple, they do have limitations in terms of the type, quality, and quantity of the materials they can handle. This paper underlines these points while discussing drawbacks of the printed components. Challenges associated with 3D printing in microgravity are also touched upon. Finally, a glimpse is taken into the future appearance of aerospace industry with 3D printing.

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3D printing in aerospace and its long-term sustainability

Inconel 718 produced by selective laser melting (SLM) has been characterized with focus on the microstructure, the dependence of sample orientation on the mechanical properties and the effects of post heat treatments. The as-manufactured IN718 has a very fine cellular-dendritic structure with fine Laves phases precipitating in the interdendritic region, and electron backscatter diffraction (EBSD) analysis shows that both the vertically and horizontally built samples have relatively weak texture. The vertically built samples show lower tensile strength but higher ductility than the horizontally built samples, and the mechanism is shown to be partly due to the crystallographic feature but more importantly due to the different amount of residual stress and dislocations accumulated in these two kinds of samples. Applying heat treatments can significantly increase the strength while decrease the ductility correspondingly, and difference in yield strength between the vertically and horizontally built samples decreases with increasing the heat treatment temperatures, mainly due to the removal of residual stress and dislocations.

Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments

Ni–Fe base superalloy, Inconel 718, was processed through powder metallurgy (P/M) hot isostatic pressing (HIP) route. In order to balance the strength and ductility, the HIPed material was given the standard heat treatment, viz. solution treatment at 980 °C for 1 h/water quenched (WQ) to room temperature and a two-step ageing treatment consisting of 720 °C for 8 h/furnace cooling (FC) at 55 °C h−1 to 620 °C and holding at 620 °C for 8 h before air cooling (AC) to room temperature. Optical microscopy and scanning electron microscopy (SEM) studies on the heat treated alloy have shown a homogeneous microstructure with fine grain size (25 μm) along with the presence of prior particle boundary (PPB) networks. Transmission electron microscopy (TEM) on the heat treated material has revealed the presence of oxides, MC carbides and δ-precipitates at the grain boundaries and a uniform precipitation of fine γ″ and γ′ strengthening phases in the matrix. Tensile and stress rupture tests were performed on the heat treated material. While the yield strength (YS) and ultimate tensile strength (UTS) of the HIPed and heat treated alloy at room temperature and 650 °C were comparable to those of conventionally processed wrought IN 718, its ductility was lower. The stress rupture life of the HIPed alloy improved marginally due to heat treatment and met the minimum specification requirement of life hours but the rupture ductility was found to be inferior to that of the wrought material. The fractography of the failed samples has revealed the transgranular ductile mode of fracture in the as-solution treated alloy, while intergranular mode of failure with the decohesion of PPBs occurred more predominantly in the aged condition. This change of fracture mode with ageing treatment shows the ductility dependence on the relative strength of the matrix and PPBs. The TEM studies on the deformed alloy have revealed that the brittle oxides and carbides at the prior particle boundaries coupled with the fine γ″ and γ′-precipitates in the matrix are responsible for low ductility at 650 °C. The investigations of the present study have led to better understanding of the structure–property relationships in HIP+heat treated alloy 718 and suggest that the standard heat treatment recommended for wrought IN 718 is not suitable for HIPed alloy and has to be modified to realise optimum properties.

Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy inconel 718

Inert gas atomised Inconel 718 superalloy powder was characterised for various important properties and subsequently consolidated by hot isostatic pressing (hipping) at 1200° C and 120 MPa for 3 h. The density of the as compacted material was nearly the same as its theoretical density. Optical microscopy of as hipped material showed a fine grained structure with no porosity but having annealing twins and prior particle boundaries (PPBs). Electron probe microanalysis (EPMA) studies revealed that the PPBs were decorated with Al, Ti oxides, and MC type carbides enriched with Nb and Ti. In addition to these phases, the presence of very fine γ"-Ni3Nb and γ'-Ni3(Al,Ti) precipitates in the matrix were revealed by TEM analysis, which indicates that the compacted material was partially aged during the slow cooling stage of hipping. Tensile tests conducted on the as hipped material showed that the ultimate tensile strength (UTS) and ductility values were comparable to those obtained in the (solution treated...

Characterisation of hot isostatically pressed nickel base superalloy Inconel* 718

Argon atomised austenitic stainless steel (AISI 304) powder was characterized for its physical properties such as particle shape, microstructure, median particle size, particle size distribution, apparent density, tap density, and jlowrate. Subsequently, the tap density of the as received powder was improved to the desired level by adjusting the powder distribution followed by mixing and blending. This powder was subjected to hot isostatic pressing (hipping) at two different combinations of temperature and pressure to optimise the microstructure and mechanical properties. Mechanical properties of the stainless steel obtained by the powder metallurgy (PM) hipping route were found to be superior to those of conventionally processed wrought steel. The superior performance of PM hipped steel is attributed to its low oxygen content, fine grain size, and high degree of chemical homogeneity. Although the production of billets by the hipping route does not appear to be economical owing to the high capital...

High performance stainless steel via powder metallurgy hot isostatic pressing

Inert gas atomized superalloy Inconel 718 powder was hot isostatically processed (HIPed) at 1200 "C / 120 MPa / 3h. This material was then subjected to a standard heat treatment schedule adopted for wrought IN 718 (955 "C / 1 h. / water quenching and two - step ageing at 720 "C / 8 h. / furnace cooling to 620 "C / 8 h. 1 followed by air cooling to room temperature), which showed substantial improvement in strength but a drastic decrease in ductility. Since, the solution treatment temperature has a significant influence on microstructure development, the HIPed compacts were subjected to various solution treatment temperatures between 850 and 1270 "C, while the same ageing treatment was retained. The microstructure of the compacts in the as - HIPed, solution treated and aged conditions was studied in detail. The tensile properties of these samples were evaluated at room temperature and at 650 "C. The microstructure corresponding to the solution treatment temperature of 1270 "C, showed incipient melting at the particle boundaries leading to enhanced bonding across the particles and resulting in a considerable improvement of ductility.

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G. Appa Rao

Papers

Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy inconel 718

Characterisation of hot isostatically pressed nickel base superalloy Inconel* 718

High performance stainless steel via powder metallurgy hot isostatic pressing

Effect of solution treatment temperature on the microstructure and tensile properties of p/m (hip) processed superalloy inconel 718