Wrought Material Research Articles

In Envelope Additive/Subtractive Manufacturing and Thermal Post-Processing of Inconel 718.

This study investigated the application of an in envelope additive/subtractive (LPBF) manufacturing method (Matsuura LUMEX-Avance-25) to fabricate IN718 benchmarking coupons. The coupons were then examined comprehensively for surface finish both with and without high-speed micro-machining. The microstructure of the manufactured IN718 coupons was investigated thoroughly in the as-fabricated condition and following three different standard and one non-standard post-processing heat treatments. As built coupons revealed columnar grain morphology mainly along the <100> direction with a cellular dendritic sub-grain structure and without any strengthening precipitates. Grain size, aspect ratio, and texture were maintained after each of the applied four heat treatments. Only one of the standard heat treatments resulted in the δ phase formation. The other three heat treatments effectively dissolved the Laves phase preventing the δ formation while promoting the formation of γ′/γ″ precipitates. Despite the observed differences in their microstructures, all of the heat treatments resulted in similar yield and ultimate tensile strength values that ranged between 1103−1205 MPa and 1347−1387 MPa, respectively. These values are above the minimum requirements of 1034 MPa and 1241 MPa for the wrought material. The non-standard heat treatment provided the highest elongation of 24.0 ± 0.1% amongst all the heat-treated specimens without a significant loss in strength, while the standard heat treatment for the wrought parts resulted in the lowest elongation of 18.3 ± 0.7% due to the presence of δ phase.

Laser powder bed fusion of high-strength and corrosion-resistant Inconel alloy 725

The development of additive manufacturing, or three-dimensional (3D) printing, technologies has produced breakthroughs in the design and manufacturing of products by enhancing design freedom and minimising manufacturing steps. In addition, the complex, unique microstructures imparted by the additive processes offer prospects of unprecedented advances to produce high-performance metal alloys for high-temperature and corrosive environments. Here, we present the first additive manufacturing of Inconel alloy 725, an advanced nickel-base superalloy that is the widely accepted gold standard material of choice for oil and gas, chemical, and marine applications. We explore the printability of Inconel alloy 725 and identify a wide processing space to build material with a crack- and near-pore-free microstructure. The conventionally heat-treated Inconel alloy 725 has an equiaxed, near-fully recrystallised microstructure containing copious twin boundaries and nano-precipitates. It also displays promising tensile properties and corrosion resistance compared to its wrought counterpart. Our work opens the door toward additive manufacturing of Inconel alloy 725 components with optimised microstructure and topology geometry for applications in harsh environments. • We additively manufacture Inconel alloy 725, for the first time, by laser powder bed fusion. • We reveal that Inconel alloy 725 is highly printable exhibiting a wide processing window for near defect-free microstructure. • We find that Inconel alloy 725 responses well to industrial standard heat treatment compared to other common additively manufactured alloys. • We demonstrate that Inconel alloy 725 shows promising tensile properties and corrosive resistance compared to conventional wrought material.

Hot workability and microstructure evolution of wire arc additive manufactured 300M steel

Wire arc additive manufacturing (WAAM) technology offers a material-saving and efficient method of manufacturing the 300M steel components, but the obtained microstructure and properties hardly reach the level of wrought materials. By combining WAAM technology with forging process, a novel forming process is proposed. The preferred shape pre-forgings are easily prepared, and the number of forging steps significantly decreases. The WAAMed as-cast microstructure also can be transformed into the wrought state by the proposed forming process. The aim of this study is to investigate the hot deformation behavior and microstructure evolution of WAAMed 300M steel under various deformation temperatures and strain rates. The results show that the WAAMed 300M steel exhibits a higher plastic deformation resistance than the wrought 300M steel. With the increase of deformation temperature and strain rate, the difference of flow stress between the WAAMed and wrought 300M steel decreases. The hot deformation activation energy of WAAMed 300M steel is calculated as 374.1 kJ/mol, which is much higher than that of wrought 300M steel (332.3 kJ/mol). The possible processing windows obtained from the hot deformation activation energy maps are 1040-1120 °C/0.01-10 s−1 for the WAAMed 300M steel and 1010-1130 °C/ 0.1-10 s−1 for the wrought 300M steel. The constitutive model considering strain compensation is established to describe the hot deformation behaviors of WAAMed and wrought 300M steel. The high correlation coefficient and low average absolute relative error confirm the suitability of the developed constitutive models for predicting the flow stresses of WAAMed and wrought 300M steel.

A unified constitutive approach for creep response of AISI 316 steel produced by conventional technologies or additive manufacturing techniques

ABSTRACT The creep response of AISI 316 and AISI 316 L was analysed to provide a coherent picture of the material behaviour, valid for both conventional wrought and additively manufactured steels. Literature evidences were considered. The analysis demonstrated that the presence of a fine distribution of particles, precipitated during creep exposure in both AISI 316 and 316 L(N), should be taken into account. A recent model, expressly developed for particle-strengthened alloys, was successfully used to describe the minimum creep rate dependence on applied stress for AISI 316 and 316 L(N). The same set of constitutive equations, in combination with the composite model, was then used for describing creep data obtained by testing the AISI 316 L produced by additive manufacturing. The main difference with wrought materials was that the microstructure of the additively manufactured alloy was composed by cells (soft zones) and thick walls heavily decorated with ultrafine oxide particles (hard zones).

Numerical Design of CoNi-Base Superalloys With Improved Casting Structure

Numerical methods can accelerate the design of alloys with improved material properties. One approach is the coupling of multi-criteria optimization with CALPHAD-based models of alloy properties. While this technique has already yielded promising new Nickel-base superalloys, the applicability to CoNi-base alloys has not yet been investigated. These alloys show promising properties for application as wrought high-temperature materials. We designed three CoNi-base superalloys, which were optimized for either high strength or high chemical homogeneity. The alloys were cast, and mechanical and thermophysical properties were characterized. The alloy optimized for strength showed creep performance inferior to a conventionally designed CoNi-alloy but had a much lower density. For developing highly homogeneous alloys, Scheil calculations were implemented in the optimization routine to quantify the severity of segregation. Non-equilibrium phases could be predicted successfully, resulting in a degree of homogeneity that rivaled that of a low-segregation ternary Co-base alloy. A comparison of elemental partitioning behavior and phase transition temperatures with CALPHAD calculations showed that trends are well represented for the most part. Finally, the applicability of the alloy design approach for Co-rich superalloys is evaluated, and possible applications for the optimized alloys are discussed.

Fatigue response of additively manufactured as‐built 15‐5 PH stainless steel and effects of machining and thermal and surface treatments

AbstractAdditively produced 15‐5 PH stainless steel has wide industrial applications, but the combined effects of heat treatment, machining, and shot‐peening and their order have not been deeply investigated. This topic is addressed here by a 2‐by‐3 experimental plan that has involved S–N curve and fatigue limit determination, using vertically built cylindrical samples, tested under rotating bending. The obtained responses have been analyzed by an ANOVA‐based statistical approach for comparison of fatigue trends. Results indicate that heat treatment without machining may be even detrimental for fatigue due to embrittlement. Conversely, machining with subsequent shot‐peening, even without heat treatment, has a remarkable impact and leads to a doubled fatigue strength with respect to as‐built material. This strength is also quite close to that achievable for wrought material. The study has been completed by micrography and fractography, to reveal the dependence of microstructure, crack initiation sites, and failure mode on the performed treatments.

Passivation characteristics of ultra-thin 316L foil in NaCl solutions

• A commercial ultra-thin 316L foil (20 μm thick) has fine grains with preferential orientation of (110), and no detectable MnS inclusion. • The 316L foil exhibits significantly lower passive current density and higher breakdown potential in NaCl solution than wrought 316L. • The passive film on the 316L foil is highly enriched in Cr and Mo oxides, and Ni is enriched underneath the passive film. • The enhanced diffusion of Cr and Mo leads to a Cr- and Mo-enriched passive film giving the superior corrosion resistance. Electrochemical behaviour and passive film characteristics of an ultra-thin 316L foil with a thickness of 20 μm in 3.5 wt.% NaCl solution were investigated using multiple techniques, focusing on the effect of microstructure, the applied potential, and the pH of the solution. The microstructure contains mainly fine grains (∼4 μm) with high-angle boundaries and preferential orientation of (220), and no MnS inclusion was detected. The electrochemical measurements show a significantly higher breakdown potential and lower passive current density for the 316L foil than traditional wrought 316L. The surface analyses using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) reveal that, compared to the wrought material, both the inner and out parts of the passive film on the 316L foil are more enriched in Cr- and Mo-oxides. The microstructure favourable for elemental diffusion and the absence of MnS inclusion facilitate the formation of a continuous compact Cr- and Mo-rich passive film, which effectively retards corrosion in NaCl solution and remains stable in acidic solution (pH 2) or at high polarised potential up to 600 mV vs Ag/AgCl.

Hybrid post-processing effects of magnetic abrasive finishing and heat treatment on surface integrity and mechanical properties of additively manufactured Inconel 718 superalloys

• The magnetic abrasive finishing and post heat treatment are hybrid to apply for the additively manufactured In718. • The hybrid effects on the surface integrity and mechanical properties are studied comparatively. • Excellent mechanical properties of the additively manufactured In718 are obtained by the proper post-processing sequence. • The influencing mechanism of the hybrid post-processes provides a promising pathway to improve the mechanical properties. Desired microstructure and surface integrity are critical to achieving the high performance of additively manufactured components. In the present work, the hybrid post-processes of magnetic abrasive finishing (MAF) and post-heat treatment (HT) were applied to the additively manufactured Inconel 718 superalloys. Their hybrid effects and influencing mechanism on the surface quality and mechanical properties of the additively manufactured samples have been studied comparatively. The results show that the MAF process effectively reduces the surface roughness by more than an order of magnitude due to the flexibility and geometric consistency of the magnetic particles and abrasives with the finished surfaces. The proper sequence of MAF and HT obtains enhanced mechanical properties for the homogenized-MAF-aged sample with the yield strength of 1147 MPa, the ultimate tensile strength of 1334 MPa, and the elongation of 22.9%, which exceeds the standard wrought material. The surface integrity, compressive residual stress field, and grain refinement induced by the MAF and subsequent aging heat treatment increase the cracking resistance and delay the fracture failure, which significantly benefits the mechanical properties. The MAF process combined with proper post-heat treatment provides an effective pathway to improve the mechanical properties of additively manufactured materials.

Strengthening Mechanisms and Strain Hardening Behavior of 316L Stainless Steel Manufactured by Laser‐Based Powder Bed Fusion

The microstructure–properties relations and strengthening mechanisms of additively manufactured 316L stainless steel are comprehensively investigated in this work. The orientation dependency and the strain hardening are studied by tensile testing of as‐built specimens fabricated by laser‐based powder bed fusion (LPBF) in different directions. The results are compared with those obtained for wrought material. The microstructure of the wrought and the LPBF materials are also comprehensively investigated. Equiaxed grains with random orientation and relatively uniform size (≈30 μm) are observed in the wrought material, where the LPBF samples show columnar grains inside as well as fine equiaxed grains in the bottom of the molten pool. A bimodal grain size distribution, higher values of geometrically necessary dislocations density (≈25–32%), and lower fractions of high‐angle grain boundaries (≈24–28%) are observed in LPBF 316L. A significant yield strength and considerable ultimate strength improvement without remarkable elongation decrease are obtained for the LPBF tensile specimens, resulting in a high strength‐elongation balance (up to 26 122 MPa%). Two‐stage strain hardening is depicted in both wrought and LPBF samples. This is successfully predicted with two‐stage Hollomon analysis. However, the LPBF samples illustrate lower strain hardening exponents in comparison with the wrought ones.

Comparison of Cast, Wrought, and LPBF Processed IN718 Concerning Crack Growth Threshold and Fatigue Crack Growth Behavior

Abstract This study focuses on the influence of microstructure and ductility on the short crack behavior and the fatigue crack growth (FCG) threshold. For this, an additively manufactured batch of the nickel-based alloy IN718, made via Laser Powder Bed Fusion (LPBF) process, is compared to both conventionally wrought and cast IN718 material. An initial characterization revealed significant differences in grain structure, from regular fine grained wrought material to large grains of several millimeters in diameter in the cast variant and a chessboard-like grain structure typical for LPBF. The ductility parameters of IN718-LPBF and wrought IN718 are comparable at room temperature, but at 650 °C the LPBF-variant shows macroscopic brittle fracture. The fatigue crack behavior both in the short and long crack regime is investigated in air at 650 °C. To produce minimal crack growth increments of about 1 μm, a compressive precracking and subsequent threshold test procedure with stepwise load increase has been adapted for high temperature testing. To assess the short crack behavior, cyclic R-curves have been generated taking the influence of three different stress ratios (R = −1, 0, and 0.5) into account. As expected, increasing crack closure mechanisms at higher stress ratios lead to higher stress intensity amplitudes ΔKI necessary to initiate crack growth. The crack growth resistance of the LPBF-processed variant is higher compared to wrought IN718. In the long crack regime, the wrought alloy yields higher crack growth rates compared to LPBF and cast IN718. An expected R-ratio dependency is observed for all material states.

Microstructural Features, Defects, and Corrosion Behaviour of 316L Stainless Steel Clads Deposited on Wrought Material by Powder- and Laser-Based Direct Energy Deposition with Relevance to Repair Applications.

This work analyses the microstructural defects and the corrosion behaviour of 316L stainless steel clads deposited by laser metal deposition on wrought conventional material, which is a highly relevant system for repair applications. The different defects and microstructural features found in these systems were identified and analysed from a perspective relevant to the corrosion performance of these materials. The role of these features and defects on the corrosion process was evaluated by exposure of the samples to corrosive media and further examination of the corrosion morphology. The heat-affected zone, located on the wrought base material in close vicinity of the deposited clad, was identified to be the primary contributor to the corrosion activity of the system due to the large depletion of alloying elements in this region, which significantly decreased its pitting resistance. Alongside the heat-affected zones, relatively small (<30 µm in diameter) partially un-melted powder particles scattered across the surface of the clad were systematically identified as corrosion initiation spots, possibly due to their relatively high surface energy and therefore high reactivity compared to larger powder particles. This work highlights the need for more investigations on as-built surfaces of additively manufactured parts to better explore/understand the performance of the materials closer to their final applications. It demonstrates that the surface defects resulting from the additive manufacturing process, rather than the presence of the refined sub-granular cellular structure (as highlighted in previous works), play the predominant role in the corrosion behaviour of the system.

The optimisation of hot isostatic pressing treatments for enhanced mechanical and corrosion performance of stainless steel 316L produced by laser powder bed fusion

This work compares the mechanical and corrosion properties of 316L steel manufactured by Laser Powder Bed Fusion (LPBF) and post treated by Hot Isostatic Pressing (HIP) to wrought 316L. HIP is often used by default on LPBF components to reduce porosity and obtain the best mechanical properties, however, if the HIP temperatures are too high, there is a risk of reducing mechanical strength and corrosion resistance. The purpose of this work was to investigate the HIP parameters and understand the trade-off in properties. By choosing various HIP temperatures (700 °C, 1125 °C, 1200 °C), pressures (100 MPa, 137 MPa and 200 MPa) and hold times, optimal cycles were investigated based on the most favourable mechanical properties (density, hardness, tensile and low-cycle fatigue), and pitting corrosion resistance. Microstructural features associated with LPBF such as melt pools, melt pool boundaries and sub granular cells were observed. These features were found to disappear with longer and higher temperature treatments, accompanied by increased grain sizes. Low and mid temperature point HIP treatments resulted in higher ultimate tensile strength but lower fracture elongation. The decreasing hardness and tensile strength trends were consistent with decreased grain boundary strengthening and decreased dislocation strengthening (with disappearing sub grain boundary and granular cells). Only one HIP condition, consisting of a low temperature and medium pressure, produced samples that achieved runout under low cycle fatigue testing for both the lower and higher stresses. Despite this, most higher temperature HIP cycles reduced the fatigue resistance. This was again attributed to the coarsening of the microstructure at the higher temperature treatments. The spread of the pitting potentials of HIP treated samples was reduced by 52.46 % compared to the as-built material, although none were better overall compared to the wrought material. Of all the properties, porosity appears to play the most influential role on pitting corrosion, and to this extent, despite having a larger variation in results, some of the treated parts demonstrated improved pitting resistance and some demonstrated improved repassivation potentials compared to wrought 316L.

Simulation and experimental analysis of nanoscale precipitation during ageing treatment of laser powder-bed fusion fabricated IN718 Ni-based superalloy

Temporal evolution of the nanoscale disc-shaped γ′′ (Ni 3 Nb) and spherical γ′ (Ni 3 (Al,Ti)) strengthening precipitates during double-stage ageing treatment of the laser powder-bed fusion (LPBF) fabricated IN718 Ni-based superalloy is studied by thermodynamic-based modeling, electron microscopy, and small-angle neutron scattering (SANS). The precipitation kinetics of both γ′′ and γ′ particles in the multi-component/multi-phase IN718 alloy system was successfully predicted by the Langer–Schwartz–Kampmann–Wagner approach. The simulated particle size and volume fraction during ageing was validated against electron microscopy and SANS results, respectively. The size and volume fraction of the γ′′ phase was monotonically increased during the first ageing stage up to about 100 nm and 14%, respectively, but interestingly unchanged during the second stage. On the other hand, the volume fraction of γ′ phase was sharply increased to about 15% after about 100 s and kept constant until the end of the first ageing stage, where it then decreased to about 7%. The γ′ particle size monotonically increased up to about 20 nm. According to the determined temporal evolution of the nanoscale precipitates, a shorter ageing treatment of the LPBF IN718 superalloy compared to the conventionally manufactured alloy could be suggested. • The precipitation kinetics during ageing of LPBF IN718 superalloy is simulated. • The γ′′ growth rate is faster in the wrought material than in the LPBF one. • The γ′′ volume fraction stagnated during the second stage of ageing. • The simulated concentrations of precipitates did not remain constant during ageing. • A shorter ageing for precipitation hardening of LPBF IN718 alloy can be suggested.

Additive manufacturing of functionally graded inconel 718: Effect of heat treatment and building orientation on microstructure and fatigue behaviour

This paper addresses the effect of the post-process heat treatments on the microstructure and fatigue crack growth behaviour of the functionally graded (FG) laser powder bed fusion (L-PBF) Inconel 718 (IN718) superalloy. Sets of samples were additively manufactured (AM) altering the process parameters, namely the laser power, the laser scanning speed, layer thickness, hatch distance, and beam distribution function, resulting in distinctly different microstructures. Two categories of samples underwent heat treatment (HT) and hot isostatic pressing followed by HT (HIP+HT), while one category was kept in the as-processed (AP) condition to reveal the effects of the post-treatments. Additionally, to study the effect of microstructural anisotropy, samples were printed in horizontal (H) and vertical (V) building directions. To better understand the behaviour of the FG materials, non-graded (NG) L-PBF samples and wrought material were investigated as references. Significant variations in terms of porosity, grain size and elongation, crystallographic texture, and content of the strengthening precipitates or detrimental phases were found in different AM groups. The fatigue behaviour of the NG and FG materials was also studied by conducting three-point bending tests. Findings in terms of the role of different microstructures on the fatigue-crack initiation and fatigue crack growth rate are presented and discussed for all samples. The study demonstrated that heat treatments can enhance the damage tolerance of L-PBF IN718 to the level of wrought material. Interestingly, the effect of the roughness induced crack closure was found to be a function of build orientation, especially in the low stress ratio regime.

(Digital Presentation) Effects of Surface Finish on the Corrosion Performance of Additively Manufactured Metals

Metal additive manufacturing (AM) is becoming a transformative technology in the manufacturing industry. The as-built surfaces produced by metal AM are very different compared to wrought material, typically resulting in very rough surfaces that have different corrosion properties. There are a large number of studies that have attempted to understand the corrosion response of metal AM. However, these studies are typically performed on materials that have been printed on a single machine. As a result, there are significant inconsistencies in the literature due to the high variability in the quality of parts manufactured on different machines. We will present work that compares the corrosion response of AM 316L stainless steel (SS) that has been printed on multiple machines. Our results have shown there is significant variability observed in the corrosion performance of metal AM parts built on different machines and within a single build plate. The observed corrosion performance is likely affected by the stability of the passive film, chemical segregation, and microstructure of the surface. We also propose that surface finishing of AM 316L SS reduces the part-to-part variations observed in the corrosion performance of the material printed on different machines and within a single build plate.This talk will discuss the corrosion properties of AM 316L SS, A20x (AM aluminum alloy), and Ti-6Al-4V parts produced by direct metal laser sintering (DMLS). The majority of our work focused on understanding the susceptibility to localized corrosion of metal additively manufactured materials with respect to build machine, surface roughness, and build angle. As a result, we also examined the corrosion performance of 316L SS built on a single instrument as we varied the build parameters. Conventional electrochemical corrosion experiments were used to characterize corrosion performance, including open circuit potential, potentiostatic electrochemical impedance spectroscopy, and potentiodynamic polarization scans. White light interferometry, optical profilometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were utilized to characterize the surface of the samples.

Ultrasonic Rayleigh Wave Interrogation of Directed Energy Deposition Ti–6Al–4V Having a Rough Surface

Abstract In-situ monitoring techniques for additive manufacturing are in high demand to help produce reliable parts. The structural integrity of these parts depends on both the presence of flaws and their microstructure. Ultrasonic Rayleigh waves have the potential to identify flaws and assess the local microstructure during directed energy deposition (DED) additive manufacturing processes, but the scattering associated with the surface roughness degrades the ultrasonic signal and must be understood to extract useful information. Herein, the microstructures and surface profiles of DED and wrought Ti–6Al–4V are compared to provide context for measured Rayleigh wave speeds and second harmonic generation. The Rayleigh wave speed and second harmonic generation for DED and wrought Ti–6Al–4V materials having comparable surface roughness are significantly different. The wave speed measured in DED material is 3% slower than in wrought material, and the relative nonlinearity parameter, commonly used to characterize second harmonic generation, is 3.5–6.0 times higher for polished surfaces. Wave speed and second harmonic generation measurements are also made along the hatch and across the hatch for both as-built and glazed DED surfaces. Based on our results, we conclude that in-situ Rayleigh wave linear and nonlinear measurements are possible; although we acknowledge that in-situ angle-beam transducer generation would be challenging, and thus we will investigate pulsed laser generation in future work.

Comparison of wrought and additively manufactured IN718 concerning crack growth threshold and fatigue crack growth behaviour

Rotary components in turbine machinery such as gas turbine blades and discs or turbocharger wheels need to be designed so that high cycle fatigue (HCF) loads are well below the fatigue endurance limit to ensure safe operation. This requires an accurate measurement of the fatigue endurance limit and the related intrinsic fatigue crack growth (FCG) threshold. Furthermore, knowing the FCG threshold is crucial in order to assess the criticality of initial defects (e.g. cavities, carbide nests, etc.). In this study, the influence of the manufacturing route on the FCG threshold and the FCG behaviour at 650 °C in air of the nickel based superalloy IN718 is investigated. For this, a per Laser Powder Bed Fusion (LPBF) process manufactured version is compared to a conventionally wrought IN718 material state, both in the short and long crack regime. To measure crack growth increments of about 1 μm, an alternative pre-cracking and threshold test procedure is proposed for high temperature testing. To assess the behaviour of both material states in the short crack growth regime, cyclic R-curves have been generated taking the influence of two different R ratios into account. A further comparison of the FCG behaviour of the LPBF and the wrought material state is made on the basis of da/dN-ΔKI plots, with a focus on the Paris regime created from both classical FCG tests as well as continued threshold tests. The results observed are discussed in relation to the initial characterization of the two material states, which includes tensile tests at room temperature and at 650 °C as well as microstructural investigations.

Effect of Porosity on Tool Wear During Micromilling of Additively Manufactured Titanium Alloy

Abstract Porosity is a major quality issue in additively manufactured (AM) materials due to improper selection of raw material or process parameters. While porosity is kept to a minimum for structural applications, parts with intentional (engineered) porosity find applications in prosthetics, sound dampeners, mufflers, catalytic converters, electrodes, heat exchangers, filters, etc. During postprocessing of additive manufactured components using secondary machining to obtain required dimensional tolerance and/or surface quality, part porosity could lead to fluctuating cutting forces and reduced tool life. The machinability of the porous AM material is poor compared to the homogenous wrought material due to the intermittent cutting and anisotropy of AM materials. This paper investigates the tool wear progression and underlying mechanisms in relation to the porosity of AM material during their machining. Micromilling experiments are carried out on AM Ti6Al4V alloy with different porosity levels. Insights into tool-workpiece interaction during micromachining are obtained in cases where pore sizes could be comparable to the cutting tool diameter. Findings of this research could be helpful in developing efficient hybrid additive-subtractive manufacturing technologies with improved tool life and reduced costs.

Near-threshold fatigue crack growth in laser powder bed fusion produced alloy 718

The role of microstructure in influencing fatigue crack growth behavior for laser powder bed fusion (LPBF) produced nickel superalloy 718 was examined. Two common post-build heat treatments were applied to produce two distinct microstructures, one which retained much of the solidification structure from additive manufacturing, while the other was mostly recrystallized. For both groups, residual stresses were assessed along the crack path by neutron diffraction. At low load ratios (R = 0.1), the non-recrystallized LPBF material had the lowest and most varied fatigue crack growth thresholds at 6–7.2 MPa m1/2. This is attributed to a reduction in crack path roughness induced crack closure and high superimposed residual stresses. The recrystallizing heat treatment coarsened and homogenized the grain structure substantially, thus increasing the threshold to 7.3–7.6 MPa m1/2 due to a substantially rougher crack path and completely relieved residual stresses. Both LPBF heat treatments showed negligible effect of build orientation and gave significantly lower fatigue thresholds compared to wrought material tested in the L-T orientation (∼11 MPa m1/2). At high load ratios, the difference in fatigue behavior between microstructures was reduced but still present and was attributed to differences in geometric shielding from crack path roughness, grain size, and degree of microstructural homogeneity.

Investigation of short-term creep properties of a coarse-grained Inconel 718 fabricated by directed energy deposition compared to traditional Inconel 718

Short-term creep performance of a coarse-grained Inconel 718 alloy fabricated by Directed energy deposition (DED) technique has been investigated at 650 °C in comparison to a conventional wrought material. In order to study the effect of heat treatment and sample orientation on the creep behaviour, as-deposited specimens oriented parallel and perpendicular to the building direction have been treated by double aging (DA), solution annealing + double aging (SA) and homogenization + solution annealing + double aging (HSA) conditions. Microstructural changes along with their impacts on high temperature mechanical properties have been examined with respect to grain size, crystallographic orientation, precipitates, and dislocation substructure. Results show that, superior creep strength and creep ductility of DED-manufactured DA specimen can be achieved compared to wrought specimen. The process-induced dislocation substructure and the irregular columnar grains with the random network of grain boundaries primarily contribute to the superior creep behaviour. However, when the unique columnar grain features formed during DED process are replaced by recrystallized, equiaxed grain structure in the HSA case, the worst creep performance is observed due to the coarse grains with regular-shaped grain boundaries facilitating the crack propagation. In addition, anisotropy of creep performance with respect to sample orientation observed for the heat-treated DED specimens can be significantly alleviated by a higher temperature of solution treatment.