In situ alloying of a modified inconel 625 via laser powder bed fusion: Microstructure and mechanical properties

This study investigates the in situ alloying of a Ni-based superalloy processed by means of laser powder bed fusion (LPBF). For this purpose, Inconel 625 powder is mixed with 1 wt.% of Ti6Al4V powder. The modified alloy is characterized by densification levels similar to the base alloy, with relative density superior to 99.8%. The material exhibits Ti-rich segregations along the melt pool contours. Moreover, Ti tends to be entrapped in the interdendritic areas during solidification in the as-built state. After heat treatments, the modified Inconel 625 version presents greater hardness and tensile strengths than the base alloy in the same heat-treated conditions. For the solution annealed state, this is mainly attributed to the elimination of the segregations into the interdendritic structures, thus triggering solute strengthening. Finally, for the aged state, the further increment of mechanical properties can be attributed to a more intense formation of phases than the base alloy, due to elevated precipitation strengthening ability under heat treatments. It is interesting to note how slight chemical composition modification can directly develop new alloys by the LPBF process

Processing of hybrid laminates integrating ZrB2/SiC and SiC layers

Tape casting technique was used to develop hybrid laminates constituting by SiC and ZrB2-SiC layers; the main aim is obtaining a structure which integrate the unique properties of these materials and potentially extent their application temperature range. Multilayer with ZrB2-SiC layers stacked in between SiC ones were successfully processed. Thin cracks propagated in the composite layers without affecting SiC ones; their formation was due to residual stresses developed in the two materials because of the differences in their shrinkage and coefficients of thermal expansion. However, these cracks did not significantly affect the material properties: relative density, elastic modulus and flexural strength of hybrid laminates was indeed only slightly lower than those of laminates made up of layers with the same composition

Microstructure and Residual Stress Evolution of Laser Powder Bed Fused Inconel 718 under Heat Treatments

The current work aimed to study the influence of various heat treatments on the microstructure, hardness, and residual stresses of Inconel 718 processed by laser powder bed fusion process. The reduction in residual stresses is crucial to avoid the deformation of the component during its removal from the building platform. Among the different heat treatments, 800 °C kept almost unaltered the original microstructure, reducing the residual stresses. Heat treatments at 900, 980, and 1065 °C gradually triggered the melt pool and dendritic structures dissolution, drastically reducing the residual stresses. Heat treatments at 900 and 980 °C involved the formation of δ phases, whereas 1065 °C generated carbides. These heat treatments were also performed on components with narrow internal channels revealing that heat treatments up to 900 °C did not trigger sintering mechanisms allowing to remove the powder from the inner channels

Investigating Complex Geometrical Features in LPBF-Produced Parts: A Material-Based Comparison Between Different Titanium Alloys

The Ti–6Al–4V (Ti64) alloy is a well-established material to be processed via laser powder bed fusion (LPBF). Recently, other α + β titanium alloys are receiving attention, such as Ti–6Al–2Sn–4Zr–6Mo (Ti6246). Their typical industrial fields of application (aerospace, automotive), often require critical design choices, such as low wall thicknesses and hollow channels. Thus, a comparative analysis between these two competitor alloys in terms of processability was conducted in this work. To do so, specific sample designs were developed. The specimens were analyzed in terms of geometrical compliance with the initial design, porosity, and microstructure. A correlation between the width of the specimens and their porosity, micro- structure and hardness was found. Overall, both the alloys proved to be well processable, even for very low wall thickness (300 μm) and channel diameter (1 mm) values. Nevertheless, the Ti6246 alloy seemed to behave better in specific scenarios. For instance, some Ti64 specimens provided delamination. The hollow channels proved to be challenging for both materi- als, mainly due to the high amount of residual powder particles adhered to the upper part of the holes. This works aims at giving a materials perspective on process-related issues, considering the LPBF-induced defectology and microstructural variations in these Ti alloys

Hot deformation behavior and flow stress modeling of Ti–6Al–4V alloy produced via electron beam melting additive manufacturing technology in single β-phase field

The hot working behaviour of additively manufactured Ti–6Al–4V pre-forms by Electron Beam Melting (EBM) has been studied at temperatures of 1000–1200 °C and strain rates of 0.001–1 s−1. As a reference, a wrought Ti–6Al–4V alloy was also analyzed as same as the EBM one. In order to investigate the hot working behaviour of these samples, all the data evaluations were carried out step by step, and the stepwise procedure was discussed. No localized strain as a consequence of shear band formation was found in the samples after the hot compression. The flow stress curves of all the samples showed peak stress at low strains, followed by a regime of flow softening with a near-steady-state flow at large strains. Interestingly, it is found that the initial microstructure and porosity content as well as the chemistry of material (e.g. oxygen content) as being possible contributors to the lower level of flow stress that could be beneficial from the industrial point of view. The flow softening mechanism(s) were discussed in detail using the microstructure of the specimens before and after the hot deformation. Dynamic Recrystalization (DRX) could also explain the gentle oscillation in the appearance of the flow softening curves of the EBM samples. Moreover, the hot working analysis indicated that the activation energy for hot deformation of as-built EBM Ti–6Al–4V alloy was calculated as ~193.25 kJ/mol, which was much lower than the wrought alloy (229.34 kJ/mol). These findings can shed lights on a new integration of metal Additive Manufacturing (AM) and thermomechanical processing. It is very interesting to highlight that through this new integration, it would be possible to reduce the forging steps and save more energy and materials with respect to the conventional routes

Electron backscattered diffraction to estimate residual stress levels of a superalloy produced by laser powder bed fusion and subsequent heat treatments

Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h–hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150◦ C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980◦ C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications

Effect of heat treatment on microstructure and oxidation properties of Inconel 625 processed by LPBF

This paper presents a study of the microstructure evolution due to oxidation exposure of Inconel 625 (IN625) alloy produced by Laser Powder Bed Fusion (LPBF). IN625 is a nickel-based superalloy characterized by good mechanical properties, excellent oxidation, and corrosion resistance from cryogenic temperatures up to 980 °C, allowing its wide use in various harsh environments. In order to enable the application of LPBF IN625 components at high temperatures, the oxidation properties and microstructure of as-built and post-heat treated LPBF IN625 alloy must be carefully investigated. For this reason, an extensive characterization of the oxidation behavior of the alloy in the as-built condition and after solution treatment was performed. For both these conditions, the oxidation treatments were performed at 900 °C up to 96 h. The characterization was performed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and scratch test analysis. The characteristics of the oxide layer and formed phases were investigated. The as-built IN625 state presented greater oxidation resistance compared to the solutionized IN625 one. The latter condition showed a defected oxide layer with the presence of Nb and Ni oxides inside the Cr oxide layer

Influence of focus offset on the microstructure of an intermetallic gamma-TiAl based alloy produced by electron beam powder bed fusion

It is well established in literature that, when processing intermetallic gamma-TiAl components by electron beam powder bed fusion, a banded microstructure is frequently formed because of an inhomogeneous Al distribution since more pronounced evaporation of Al occurs at the top of the melt pool. This feature is particularly promoted when highly energetic process parameters (high beam currents, slow beam speeds, narrow line offsets) are used. Therefore, an approach already suggested in the literature to reduce the Al loss is to minimize the energy level of the process parameter during production. However, there is a limit to such kind of approach: minimizing the beam current or increasing the beam speed, or increasing the line offset will, at a certain point, results in not being able to achieve a completely dense material and thus some process -induced porosity, the so-called lack-of-fusion defects, starts to occur in the produced parts.In this study, the effect of an additional parameter of the electron beam powder bed fusion process is taken under consideration: the focus offset (FO), i.e. the distance between the focusing plane of the electron beam with respect to the powder bed. The effect of the FO on the residual porosity, microstructure, phase composition, hardness as well as chemical composition is investigated, thus having the possibility to demonstrate that also the FO can affect the Al loss and play a fundamental role in the generation of a homogenous microstructure, contributing to mitigate the appearance of a banded microstructure

Net shape HIPping of a Ni-superalloy: A study of the influence of an as-leached surface on mechanical properties

Hot Isostatic Pressing (HIP) is a net-shape powder metallurgy technique where powders densification is achieved through the application of high temperature and pressure at the same time. Powders are allocated into a hollow steel mold called capsule or canister which gives the final shape to the particles. This technique is particularly useful for manufacturing complex components made of materials which are extremely difficult to process via forging or casting. Thus, HIP is particularly indicated to handle superalloy powders such as Astroloy, which is the object of the following study. One of the most attractive peculiarities of HIP is the low material waste obtained since the overstock is limited to the layers immediately beneath the steel capsule. At the end of the HIP cycle, the canister is typically removed using an acid leaching bath which is responsible for the alteration of the outermost layers of the final product. Only a little number of research papers deal with the optimization of the removal of these layers; Consequently, manufacturers often apply a very conservative approach by eliminating more material than is actually needed with a final machining procedure. This paper aims to optimize this procedure by systematically assessing the total thickness of the altered layer of material deriving from the HIPping and leaching process together. To achieve this goal, a set of samples were prepared by removing progressively thicker layers of material and then they were bend tested. Finally, the recorded mechanical properties were compared with those obtained with the samples machined from the core material. One of the main findings is that the removal of 500 μm of material is enough to recover mechanical properties which are comparable with those observed in samples coming from the core. More specifically, by eliminating the first 100 μm material, all the corroded layer is removed, which results in an overall increase of all the mechanical properties except for ductility. This property strongly depends on the number of prior particle boundaries arising from the HIPping process itself. Thus, the correct amount of overstock material must include both these layers

Role of the chemical homogenization on the microstructural and mechanical evolution of prolonged heat-treated laser powder bed fused Inconel 625

Ni-based superalloy components for high-temperature applications rely on the long term stability of the microstructure and mechanical properties at service temperatures. Nowadays, the production of such types of components is frequently performed via Additive Manufacturing (AM) technologies. Nevertheless, few studies are dedicated to understanding the behavior of AM Ni-based superalloys upon prolonged exposure to high temperatures. This work aims at studying the effect of prolonged thermal exposures on the microstructure and mechanical properties of Inconel 625 processed by laser powder bed fusion. Thermal exposures within the range of 600 °C and 900 °C for 200 h were performed on this material. The as-built and solution annealed Inconel 625 conditions were selected. The solution annealed state implies a complete chemical homogenization, typically recommended for working at high temperatures, whereas the initial as-built state is characterized by segregations and fine dendritic structures. Upon the studied prolonged thermal exposures, the peculiar as-built microstructure formed a higher quantity of phases with smaller dimensions with respect to the solution annealed condition under thermal exposures. The smaller phases of the as-built state resulted in similar mechanical properties evolution under different temperatures. Differently, the prolonged heat-treated solution annealed conditions exhibited more marked mechanical properties variations due to coarser phases