Rapid sintering of FeAl based ultrafine composites by a mechanically activated field-assisted process was evaluated. The influence of applied load and isothermal holding time on the as-sintered microstructure and mechanical properties was investigated. Hardness of the nanocomposite was determined by micro- and nano-indentation techniques, while the grain size was ascertained from electron backscatter diffraction and image analysis of scanning electron micrographs. A higher applied load as well as the isothermal holding time led to better dispersion of the in situ grown Fe3AlCx carbide particles in FeAl matrix. Significant improvement in the hardness and marginal rise in elastic constant were also observed in the fast sintered ultrafine composites when compared to previous reports. The increase in hardness was attributed to the presence of a carbide phase and fine-grained microstructure. (C) 2014 Elsevier B.V. All rights reserved.

The dynamic response and impact resistance of wire-arc additive manufactured (AMed) and wrought ATI 718Plus in different heat treatment conditions are characterised by using a direct impact Hopkinson pressure bar system. In addition, microstructural analyses of the alloys, before and after impact, are characterised by using advanced microscopy techniques, including scanning electron and transmission electron microscopies. The experimental results show that the impact resistance of the AMed alloy in the as-processed condition is inferior to that of the wrought alloy. The lower impact resistance is attributed to the presence of eutectic solidification constituents in the interdendritic regions and to the inhomogeneous distribution of the strengthening precipitates in the deposit. After the application of the recommended heat treatment for ATI 718Plus, excessive formation of η-phase particles are observed in the microstructure in addition to Laves phase particles. Since the recommended heat treatment for ATI 718Plus is not sufficient to eliminate the deleterious phases and optimise the properties of the alloy, a novel heat treatment procedure is proposed. Dynamic impact study of the AMed alloy after the application of the proposed approach shows that the alloy exhibits a dynamic response and impact resistance comparable to those of the wrought alloy. Furthermore, under high impact momentum, both the wrought and the AMed alloys fail due to the adiabatic shear band. A transmission electron microscopy analysis of the deformed alloys suggests the dissolution of the γ’ precipitates in the shear band as well as in the adjacent regions to the shear band. Increase in the rate of dissolution of the precipitates due to strain-assisted diffusion coupled with an increase in the adiabatic temperature during deformation, are likely causes of the dissolution of the precipitates in the shear band regions. © 2018 Elsevier B.V.

Electron beam melting (EBM) and Selective Laser Melting (SLM) are powder bed based additive manufacturing (AM) processes. These, relatively new, processes offer advantages such as near net shaping, manufacturing complex geometries with a design space that was previously not accessible with conventional manufacturing processes, part consolidation to reduce number of assemblies, shorter time to market etc. The aerospace and gas turbine industries have shown interest in the EBM and the SLM processes to enable topology-optimized designs, parts with lattice structures and part consolidation. However, to realize such advantages, factors affecting the mechanical properties must be well understood – especially the fatigue properties. In the context of fatigue performance, apart from the effect of different phases in the material, the effect of defects in terms of both the amount and distribution and the effect of “rough” as-built surface must be studied in detail. Fatigue properties of Alloy 718, a Ni-Fe based superalloy widely used in the aerospace engines is investigated in this study. Four point bending fatigue tests have been performed at 20 Hz in room temperature at different stress ranges to compare the performance of the EBM and the SLM material to the wrought material. The experiment aims to assess the differences in fatigue properties between the two powder bed AM processes as well as assess the effect of two post-treatment methods namely – machining and hot isostatic pressing (HIP). Fractography and metallography have been performed to explain the observed properties. Both HIPing and machining improve the fatigue performance; however, a large scatter is observed for machined specimens. Fatigue properties of SLM material approach that of wrought material while in EBM material defects severely affect the fatigue life. © 2018 Elsevier B.V.

In this work, the effectiveness of residual stress relief annealing on a laser powder bed fusion (L-PBF) manufactured austenitic stainless steel, alloy 21-6-9 was investigated. Residual stress levels were gauged using geometrical distortion and relaxation testing results. In the investigated temperature interval (600–850 °C), shape stability was reached after subjecting the as-built material to an annealing temperature of 850 °C for 1 h. Microstructural characterization and tensile testing were also performed for each annealing temperature to evaluate the alloy’s thermal stability and the resulting tensile properties. In the as-built state, a yield strength (YS) of 640 MPa, ultimate tensile strength (UTS) of 810 MPa and 4D elongation of 47% were measured. Annealing at 850 °C for 1 h had little measurable effect on ductility (48% 4D elongation) while still having a softening effect (UTS = 775 MPa, YS = 540 MPa). From the microstructural characterization, cell-like features were observed sporadically in the annealed condition and appeared stable up until 800 °C after which gradual dissolution began, with the last remnants disappearing after subjecting the material to 900 °C for 1 h. © 2023 The Authors

Microstructural evolution during the early stages of ageing (less than one hour) in a Ni-Cr-Fe based superalloy Inconel 718 (IN718) has been investigated using Small-Angle X-ray Scattering (SAXS). The effects of precipitate kinetics on the precipitate size distribution are compared indirectly with SAXS measurements by using Vickers microhardness data. The microhardness increased after 4 min of ageing at a temperature of 760 degrees C, although the recorded SAXS data did not reveal the precipitate size distribution. This indicates that the precipitates had not evolved enough to be detected, but still a small number of precipitates increased the yield strength. After ageing the alloy for the shortest period for which data were available, 8 min, clear evidence of precipitates could be found from the SAXS data, showing that the gamma ‘’ - precipitates are about 6 nm in width and 3 nm in height. (C) 2014 Elsevier B.V. All rights reserved.

Electron beam melting (EBM) produced Alloy 718 was subjected to thermal post-treatment involving hot isostatic pressing (HIPing) and heat treatment (HT). Subjecting the material to HIPing at 1120 degrees C led to significant densification. Study of microstructure evolution during HT (comprising of solution treatment and aging) showed possibility of significantly shortening the HT duration, particularly the time for two-step aging from the standard (8 h + 8 h) long cycle to possibly a shortened (4 h + 1 h) cycle. Another approach for shortening the post-treatment cycle by integrating the HIPing with HT inside the HIP vessel was also successfully implemented. The above observations were further substantiated by tensile response of the material subjected to the varied post-treatment cycles; out of all the post-treatments steps, tensile behaviour was observed to be mainly affected by the aging treatment. Further prospects for shortening the post-treatment protocol are also described, such as shortening of HIPing duration for the typical 4 h to 1 h cycle as well as possible elimination of solution treatment step from the entire post-treatment protocol specifically when prior HIPing is performed. Heat treatment with prior HIPing was found to be crucial for improving fatigue life, because subjecting EBM Alloy 718 to only HT, irrespective of the short or standard long protocol, rendered inferior fatigue response.

High pressure torsion (HPT) at room temperature was used for post-treatment of additively manufactured Fe?Cr?Ni stainless steel with 12.9 wt % Ni as a very strong austenite stabilizer. The results showed that HPT caused a considerable increase in nanohardness of the additively manufactured samples. In contrast with thermodynamic equilibrium-state modeling, a phase transformation from FCC to HCP structure occurred, leading to the formation of ?-martensite during HPT on high angle boundaries, low angle boundaries, and dislocation cells with no detection of deformation twins. It was demonstrated that the combination of additive manufacturing thanks to the high density of dislocations after solidification and HPT process expands the opportunities of both methods to control deformation mechanisms in stainless steels leading to different phase and microstructural features. Thus, the outcome of this study provides a fundamental basis to design advanced structural materials.

Induction hardening is a relatively rapid heat treatment method to increase mechanical properties of steel components. However, results from FE-simulation of the induction hardening process show that a tensile stress peak will build up in the transition zone in order to balance the high compressive stresses close to the surface. This tensile stress peak is located in the transition zone between the hardened zone and the core material. The main objective with this investigation has been to non-destructively validate the residual stress state throughout an induction hardened component. Thereby, allowing to experimentally confirming the existence and magnitude of the tensile stress peak arising from rapid heat treatment. For this purpose a cylindrical steel bar of grade C45 was induction hardened and characterised regarding the microstructure, hardness, hardening depth and residual stresses. This investigation shows that a combined measurement with synchrotron/neutron diffraction is well suited to non-destructively measure the strains through the steel bar of a diameter of 20 mm and thereby making it possible to calculate the residual stress profile. The result verified the high compressive stresses at the surface which rapidly changes to tensile stresses in the transition zone resulting in a large tensile stress peak. Measured stresses by conventional lab-XRD showed however that at depths below 1.5 mm the stresses were lower compared to the synchrotron and neutron data. This is believed to be an effect of stress relaxation from the layer removal. The FE-simulation predicts the depth of the tensile stress peak well but exaggerates the magnitude compared to the measured results by synchrotron/neutron measurements. This is an important knowledge when designing the component and the heat treatment process since this tensile stress peak will have great impact on the mechanical properties of the final component.

This study presents a unique melting strategy in electron beam-powder bed fusion of Alloy 718 to tailor the grain morphology from the typical columnar to equiaxed morphology. For this transition, a specific combination of certain process parameters, including low scanning speeds (400-800 mm/s), wide line offsets (300-500 mu m) and a high number of line order (#10) was selected to control local solidification conditions in each melt pool during the process. In addition, secondary melting of each layer with a 90. rotation with respect to primary melting induced more vigorous motions within the melt pools and extensive changes in thermal gradient direction, facilitating grain morphology tailoring. Four different types of microstructures were classified according to the produced grain morphology depending on the overlap zone between two adjacent melt pools, i.e., fully-columnar (overlap above 40 %), fully-equiaxed (overlap below 15 %), mixed columnar-equiaxed grains, and hemispherical melt pools containing mixed columnar-equiaxed grains (overlap similar to 20-25 %). The typical texture was <001>; however, the texture was reduced significantly through the transition from the columnar to equiaxed grain morphology. Along with all four different microstructures, shrinkage defects and cracks were also identified which amount of them reduced by a reduction in areal energy input. The hardness was increased through the transition, which was linked to the growth of the.” precipitates and high grain boundary density in the fully-equiaxed grain morphology.

Laser powder bed fusion (PBF-LB) is an additive manufacturing (AM) process that has several advantages to conventional manufacturing, such as near net-shaping capabilities and reduced material wastage. To be able to manufacture a novel material, however, one needs to first optimize the process parameters, to decrease porosity content as low as possible. Therefore, in this work the process parameters of PBF-LB built JBK-75 austenitic stainless steel, and its influence on porosity, microstructure, and hardness have been investigated. The least amount of porosity was found by using 132 W laser power, 750 mm/s scan speed, layer thickness 30 μm, and 0.12 mm hatch distance. These process parameters were then used to manufacture material for tensile testing, to investigate the tensile properties of PBF-LB built JBK-75 and potential anisotropic behavior. Hot isostatic pressing (HIP) was also performed in two sets of samples, to investigate the effect of pore closure on the tensile properties. The ultimate tensile strength (UTS) for the un-HIPed specimens was 1180 (horizontally built) and 1110 (vertically built) MPa. For the HIPed specimens, it was 1160 (horizontally built) and 1100 (vertically built) MPa. The anisotropic presence was explained by the presence of texture, with a multiple of random distribution (MRD) up to 4.34 for the {001} planes, and defects.

Cu-Ni-In powder was coated on Ti-6Al-4V fatigue test samples using plasma spray and detonation gun (D-gun) spray processes. Coatings were characterized in terms of microstructure, porosity, microhardness, residual stresses and surface roughness. Uniaxial plain fatigue and fretting tests were carried out at room temperature on uncoated and coated specimens. D-gun sprayed coating was dense with lower porosity compared with the plasma sprayed coating. D-gun sprayed coating was harder than the plasma sprayed coating and substrate because of its higher density and cohesive strength. Surfaces were very rough in both the coatings. While D-gun sprayed coating surface had higher compressive residual stresses, plasma sprayed coating surface exhibited lower values of compressive residual stresses and even tensile residual stresses. The ill effect of surface roughness was overcome by the beneficial influence of higher compressive residual stresses on the surface and higher surface hardness and so the D-gun sprayed samples exhibited superior plain fatigue lives compared with uncoated specimens. Though the plasma sprayed samples had relatively lower hardness, higher surface roughness and almost similar values of residual stresses on the surface compared with the uncoated specimens, they exhibited longer plain fatigue lives. This may be attributed to the layered structure of the coating. Though D-gun sprayed samples experienced higher friction forces, they exhibited superior fretting fatigue lives due to the presence of higher compressive residual stresses, higher surface hardness and higher surface roughness compared with uncoated specimens. The very rough surface of plasma sprayed samples enhanced their fretting fatigue lives compared with the uncoated samples. Higher surface hardness and higher compressive residual stress of the D-gun sprayed specimens were responsible for their superior fretting fatigue lives compared with the plasma sprayed specimens. (c) 2007 Elsevier B.V. All rights reserved.

A unique melting strategy was implemented in electron beam-powder bed fusion (EB-PBF) of Alloy 718, resulting in the formation of a bimodal grain morphology consisting of fine equiaxed and columnar grains. The microstructure was preserved following various thermal post-treatments. The post-treated specimens were exposed to low cycle fatigue (LCF), and fatigue crack growth (FCG) tests in ambient air at 600 °C under pure and dwell-time (120 s) fatigue cycles. Clustered inclusions spanned a region of 100-600 µm in length acted as the crack initiation site, reducing the specimens' total fatigue life. When compared to pure fatigue cycles, dwell-time fatigue cycles reduced LCF life by approximately 35%, regardless of the thermal post-treatments. Due to a high fraction of grain boundary area in the as-built EB-PBF specimens, oxygen diffusion across the grain boundaries was enhanced. The intergranular fracture mode was favored in the plastic zone ahead of the crack tip, leading to rapid crack growth. No unbroken ligaments behind the crack front were found by high-resolution X-ray computed tomography, which was consistent with a large crack opening displacement linked to severe deformation around the crack tip. 

homogenisation heat treatments. The hot ductility deteriorated significantly after long-dwell homogenisation heat treatments for 24 h at temperatures of 1120 and 1190 °C as compared with those treated at a short dwell time of 4 h at the same temperatures. The observed ductility deterioration was related to more extensive liquation along the grain boundaries caused by different mechanisms, e.g., liquation by solute segregation mechanism, Laves melting, constitutional liquation of MC carbides and supersolidus grain boundary melting, with the effect and extent depending on the solute changes after the homogenisation heat treatments. Furthermore, the role of Nb as the solute element and as the precipitate former, as well as the effect of minor alloying elements segregating along the grain boundaries, is discussed in connection to grain boundary liquation, which contributes to a better understanding of heat-affected zone liquation cracking susceptibility of cast ATI® 718Plus®. © 2020 The Authors