To reduce the manufacturing costs as well as the environment footprint of laser powder bed fusion additive manufacturing, powder is generally recycled from a production to the next one which may affect the manufacturing and further properties of the final part. This study investigates the influence of powder recycling on Inconel 718 fabricated by Laser Powder Bed Fusion (LPBF), comparing two industrial protocols: with and without addition of virgin powder, over up to 54 recycling cycles. Powder characteristics (composition, size, circularity, flowability), sample microstructures (including porosity), and mechanical properties (tensile and fatigue) were evaluated with or without precipitation heat treatment. A slight increase in oxygen content and a reduction in particle circularity were observed with recycling, more pronounced for the accelerated protocol without virgin powder addition. However, all powders remained within industrial specifications. Powder flowability improved with recycling, and porosity evolution was found to depend on the protocol: fewer critical-sized pores were found with the refreshment approach with virgin powder addition. Mechanical properties in tension remained unaffected, while fatigue life showed no significant degradation across protocols and conditions. These results demonstrate that, when properly managed, powder recycling regardless of powder addition has limited impact on IN718 part quality in LPBF, supporting its implementation for cost and sustainability reasons.

Grain size strengthening in polycrystalline metals is traditionally described by the Hall–Petch relation, yet recent experiments indicate that the Hall–Petch slope in thin polycrystals depends on stress path, revealing a coupling between grain boundaries, free surfaces, and loading condition. Despite these observations, a quantitative mechanics framework explaining stress-path-dependent grain size effects remains lacking.In this work, a three-dimensional strain gradient crystal plasticity framework of the Evers type is extended with a tunable interfacial boundary condition governing geometrically necessary dislocation (GND) permeability. The formulation enables a continuous transition between micro-hard and micro-free limits and isolates the role of interfacial GND activity without modifying bulk hardening parameters. Representative volume element simulations of thin nickel polycrystals with varying grain sizes are performed under shear, uniaxial tensile, and large tensile loadings.The numerical results and the experimental campaign on thin Nickel sheets demonstrate that the Hall–Petch slope and surface softening effects strongly depend on stress triaxiality. According to the simulations, stress-path-dependent strengthening is shown to originate from the competition between slip-gradient-induced GND accumulation and interfacial GND annihilation or transmission. Increased triaxiality enhances slip gradients and GND storage, while interfacial permeability governs the effective accumulation of GNDs and thus the apparent grain size strengthening.The proposed framework provides a unified mechanics interpretation of stress-path-dependent grain size effects and establishes interfacial GND permeability as a key governing factor in size-dependent plasticity.

Notched bending tests were carried out to investigate the ductile fracture behavior at different scales of a Ni-20wt.%Cr alloy printed by Laser Power Bed Fusion. Three sizes of specimen were employed for evaluation of J-R curves covering a wide range of dimensions, from a few microns, through an intermediate size of several tens of microns, up to several millimeters. In microscopic specimens, there is evidence of severe ductile tearing and blunting, as a result of prominent plastic effects occurring at the notch tip. It is not possible to dissociate the influence of the local microstructure on fracture properties from the influence of blunting effects due to the change in measurement scales. Despite the documented approach, the validity of the results obtained for small specimens is not achieved. These results highlight that standard toughness testing methods are not suitable and directly applicable to micro-cantilever bending tests on ductile materials, mainly due to significant blunting. To overcome these limitations and extract local toughness values, it would be necessary to focus on developing different loading methods, alternative test geometries, or methods coupled with simulation. Finally, the wide range of dimensions tested allows to discuss the applicability of the test standard to micromechanical tests, as these standards have been initially developed for macroscopic testing. This work underscores the need for revised testing protocols tailored to microscale ductile fracture.

In recent years, materials have evolved across all fields. The goal is to develop materials that are increasingly resistant while also being lightweight, stainless, and tough. All these mechanical and physical properties require increasingly complex microstructures, which can directly affect the cutting process. Adding additive manufacturing to this equation—with its ability to produce even more complex structures—makes mastering the cutting process difficult and its prediction nearly impossible.The objective of this paper is to investigate, in the case of an additively manufactured stainless steel (316L) produced using different manufacturing routes, the relationships between microstructure and the cutting process. The analyses are based on the observation of the cutting process through post-mortem examinations of chips, chemical analyses of material evolution, as well as microstructural characterizations such as EBSD tests. In addition, a specific setup was developed to ensure that each test applies a mechanically oriented load with respect to the build plane. Finally, the cutting process partly relies on the creation of a crack initiated by the tool. This crack then becomes the beginning of the shear within the primary shear zone. One possible indicator of machinability is toughness. Part of this study therefore explores the impact of toughness modification on the cutting process.

Milling of hardened carbon steels like SKD11 generates complex physical interactions that can affect tool wear, surface quality, and process stability. This study presents a multiphysics experimental approach aimed at better understanding these physical interactions. The first objective is to monitor key features in real-time, including cutting force, vibration, cutting temperature, and sound signal, during the milling process. An integrated setup involving infrared thermography and thermocouples, a piezoelectric dynamometer, sound and vibration emission sensors was used, and data were collected under two different cutting conditions. The second measurement objective is to inspect the end-of-process features such as surface quality, tool wear, and chip formation. Tool wear was measured by different methods at the end of the cut in order to show the strong correlation between cutting forces, temperature, sound signal, and vibration patterns of the machining process. The results of this study are focused mainly on the design approach rather than on detailed numerical details. The tooth-passing frequency analyzed from the cutting-force signal was correlated with the sound signal. Cutting strategies affect the surface quality under the same cutting conditions. This work provides guidance on eliminating redundant measurements for multiphysics investigations of the milling process, and accounts for all features, in contrast to previous experimental studies. Besides those findings, there are some issues that need to be improved, such as repositioning the thermocouples, elongating the cutting path to inspect the tool state and surface quality; more cutting conditions should also be applied.

This study investigates the effectiveness of various strategies used as in-situ lack-of-fusion (LOF) pore removal method in AISI 316L stainless steel parts fabricated using Laser Powder Bed Fusion (PBF-LB). A high level of porosity was intentionally introduced through suboptimal initial laser scanning and subsequently corrected using rescanning at varying laser powers (60 %-150 % of the optimal value) and scan paths (superimposed or perpendicular). Rescanning with laser power at or above 100 % restored the relative density to levels close to standard specimens while producing a finer microstructure with smaller grain sizes and dendritic arm spacing. Mechanical testing also confirmed a recovery in tensile properties, with yield and ultimate tensile strengths approaching those of standard parts. In the edge regions of the specimens, reduced porosity, coarser grains, and larger primary dendrite arm spacing were observed, with the outermost 400 mu m beneath the surface exhibiting a 70 % reduction in porosity compared to the global surface average. While such edge effects are known to occur in PBF-LB, they remain sparsely characterized in detail. This study contributes to refining the physical understanding of these variations, particularly under thermal conditions modified by rescanning. To investigate the impact of laser rescanning on the thermal history of the part, a new multi-scale model was developed. This model includes scanning with misorientation between each layer as well as rescanning for both superimposed and perpendicular paths. This experimental and numerical framework facilitates the understanding of the mechanisms driving enhanced pore reduction and microstructural coarsening at the edges, primarily due to particular thermal conditions on the edges and enhanced by rescanning. Although this work does not demonstrate an improvement beyond standard manufacturing quality, it emphasizes the potential of rescanning for pore correction, especially if coupled with real-time defect detection, offering significant implications for industries requiring highly reliable PBF-LB-manufactured components, such as those in aerospace and medical sectors.

Both experimental and numerical evidence supports that blending grains of different sizes within a polycrystalline materials allows to increase the alloy strength while maintaining its ductility. Microstructure-based modeling approaches have been developed to uncover the mechanisms governing the strength-ductility synergy, thereby assisting in the strategic design of alloys with multimodal grain size distributions. Due to significant differences in grain size and the need for statistical representativity, many approaches resort to simplifying hypotheses regarding the transition from ultrafine to macroscopic scales. Although the limitations of these simplifications in unimodal polycrystals are well documented, their biases associated with the micromechanical analysis of multimodal systems have not been addressed. To tackle this general question, this paper considers the model problem of a bimodal polycrystal with a single coarse grain embedded in a matrix of ultrafine grains. To ensure unbiased representation and enable systematic multi-scale comparisons, the analyses are based on a unimodal ultrafine grain polycrystal and its paired bimodal polycrystal, both of which have an identical microstructure of ultrafine grains. In order to distinguish structural effects of a classical matrix inclusion problem from crystal related interactions, two types of constitutive behavior have been investigated, both in 2D and 3D: isotropic macro-homogeneous for each grain population or full-field crystal plasticity. The four related configurations of a bimodal polycrystal all share the same macro-scale constitutive behavior. The distortions introduced by each of the above simplifying hypothesis and their combinations have thus been comprehensively evaluated, paying a particular attention to the specific patterns of localization of stress, strain and plastic activity. The 2D approach has been confirmed to be efficient in describing characteristic interaction mechanisms, yet with a propensity to accentuate localization phenomena. However the volume fraction of the coarse grain to achieve a given macro-scale stress-strain behavior has been found to be different from that in 3D.

Current specimen designs for biaxial tension-tension fatigue tests of composites are not optimal and often generate unexpected failure in fatigue tests, due to the lack of a design standard and the complexity of composite materials and multiaxial loads. And designing composite structures with uniaxial testing is not sufficient due to the multiaxiality of the stress tensor. This study aims to present an optimized specimen for multidirectional carbon fiber reinforced composites in biaxial tension-tension fatigue tests, in which thermodynamic phenomena occurring inside the specimen will be monitored by infrared thermography. A feasible specimen could be designed by numerical simulation using the finite element method. Firstly, a design criterion is proposed to ensure a failure in the gauge region, a practical manufacturing approach, and optimal conditions for temperature measurement. An initial simulation model is established to find out the proper shape, followed by a more comprehensive simulation used for determining the dimension of the specimen. Consequently, the optimized specimen is designated as a cruciform shape with a reduced gauge region. Last, two stacking sequences of specimens, referred to as cross-ply [(0/90)6]s and quasi-isotropic [(0/45/90/-45)3]s, are tested in a simulation model that takes into account biaxial static and fatigue loadings, and then a thermal and fatigue simulations will be performed to validate the geometry. The favorable simulation results indicate that the optimized specimen and design approach is well-suited for multidirectional composites in biaxial fatigue tests with temperature monitoring.

Additive manufacturing and, in particular, Laser Powder Bed Fusion Processes (LPBF), are known to generate non-equilibrium microstructures having a strong impact on the mechanical properties such as tensile of fatigue ones. To better control this impact, optimized lasing strategies can be employed. Among them, relasing or remelting ones have been proved to reduce the porosity and the surface roughness as well as to increase the ductility [1,2]. This particular manufacturing strategy seems, hence, to be a promising tool to modify the microstructure and mechanical properties of LPBF alloys and to optimize the mechanical properties. The objective of this study is to assess the influence of such relasing and its characteristics on the fracture toughness of Hastelloy X superalloys.

Despite the great success of the metal additive manufacturing process by Laser Fusion on Powder Bed (L-PBF), this technique still lacks maturity in several areas. Among its major challenges, the low fatigue strength of the L-PBF parts is caused, among other features, by two main types of defects. Firstly, the L-PBF process generates a high surface roughness, mainly linked to the phenomenon of partial powder melting on the part surface. Secondly, internal defects are created during the building process and are randomly distributed in the material. The present study investigates the effect of a second lasing on the improvements of the surface roughness, defects population as well as fatigue properties of L-PBF parts. Based on preliminary experiments, two re-lasing strategies were selected. A high cycle fatigue campaign was subsequently carried out with a stress ratio of R= -1 on samples with or without re-lasing, in the as-built state or after polishing. Specimens printed in Net-Shape showed even after re-lasing and polishing, a much lower fatigue strength than those machined in the bulk after additive manufacturing. In addition, results show that the tested re-lasing conditions has a beneficial effect on the quasi-static and fatigue strength of the 316L obtained by L-PBF.

Additive manufacturing is a new-age technology specialising in intricate fabrication in the manufacturing industry. However, extremely high cooling rates and far-from-equilibrium kinetics produce heterogeneous microstructure and induce high initial dislocation density, porosity, and residual stress. These unique microstructural features are known to severely impact cyclic and fatigue properties of alloys. In such a context, we investigated the cyclic and fatigue properties of a Ni-20 wt%. Cr alloy manufactured via laser powder bed fusion process (LPBF) and compared with its cast counterpart. The fatigue testing was carried out with three different strain amplitudes depicting Low, Medium and High Cycle Fatigue (LCF, MCF, HCF). LPBF samples exhibited higher fatigue resistance than that of cast samples, however experienced early failure in all the conditions. Despite strong planar glide, LPBF samples exhibited softening behaviour; the degree of softening is similar for LCF and MCF and less pronounced for HCF. The cast samples did not undergo any softening whatsoever indicating huge differences in cyclic strain mechanisms. Masing analysis was proposed to graphically analyse this softening behaviour. Further, the flow stress was categorised into backstress and effective stress for LPBF and cast samples. Majority of softening in LPBF samples for LCF and MCF occurred via backstress, whereas effective stress is associated to HCF. To investigate the origin of this softening for LPBF samples, microstructure characterisations were performed during the softening and at fracture. Post-fatigue microstructure indicates a clear modification of the dislocation structures from those inherited from LPBF to those linked to fatigue. This gradual change in microstructure is expected to induce the cyclic softening.

This paper reports experimental results concerning the corrosion of 316L austenitic stainless steels produced by ball milling and spark plasma sintering in NaCl electrolyte. Specimens with grain sizes ranging from 0.3 µm to 3 µm, without crystallographic texture, were obtained and compared with a cast that is 110 µm in grain size and an annealed reference. The potentiodynamic experiments showed that the reduction in grain size leads to a degradation of the electrochemical passivation behavior. This detrimental effect can be overcome by appropriate passivation in a HNO3 concentrated solution before consolidation. The Mott–Schottky measurements showed that the semiconducting properties of the passive layer do not vary significantly on the grain size, especially the donor density, which is responsible for the chemical passivation breakdown by chloride anions. The total electrical resistance of the layer, measured by impedance spectroscopy is always lower than the one of a cast and annealed 316L, but it slightly increases with a reduction in grain size in the ultrafine grain range. This is followed by a slight increase in the thickness of the oxide layer. The effect of chloride ions is very pronounced in terms of passivation breakdown if the powder is not passivated prior to sintering. This leads to the nucleation and growth of subsurface main pits and the formation of secondary satellite pits, especially for the smallest grain sizes. Passivation of the 316L powder before sintering has been found to be an effective way to prevent this phenomenon.

This work provides a comparative study on the influence of the manufacturing process on the tensile and fracture properties of a Ni-20 wt.%Cr binary alloy produced by Laser Powder Bed Fusion (L-PBF). Two main processing parameters were investigated: the orientation of the samples with respect to the building direction (horizontal and vertical) and the rotation angle between layers (67 degrees and 90 degrees). Tensile and three-point bending tests were conducted to study the mechanical behavior. Monotonic tensile results suggest that the choice of the printing parameters affected the yield and ultimate tensile strength values, but was not influential to elongation. An increase in yield and ultimate tensile strengths of up to 15 % was noted for horizontal specimens compared to the vertical ones, and an increase of 10 % for an angle of 67 degrees rather than 90 degrees. Fracture toughness values remained in a range of 200 to 450 kJ/m2 regardless of the set of parameters considered. For a rotation angle of 67 degrees, the fracture toughness was similar for both building orientations, but vertical specimens exhibited better resistance to crack propagation after initiation, with a tearing modulus increased by 50 % compared to horizontal ones. In contrast, building orientation was detrimental to specimens built with a rotation angle of 90 degrees, as both fracture toughness and tearing modulus decreased drastically (up to 70 %) for a horizontal orientation rather than a vertical one. Microstructural observations highlighted mixed fracture modes for all L-PBF configurations, though crack propagation seemed mainly intergranular for horizontal specimens, and transgranular for vertical specimens. Fracture mechanisms were mostly governed by the grain orientation, size and morphology, while the crystallographic texture and the nano-oxides located in the dendrite arm spacing had a second-order influence. The pores and meltpool boundaries did not appear to play a significant role in crack propagation.

This study explored the relationship between process parameters and fracture behavior in 316L stainless steel printed by laser powder bed fusion. Fracture testing was conducted according to ASTM E1820 for single edge notch bending, and elastic mechanical properties were determined using ultrasonic surface wave analysis. Five test sets were considered in a vertical building configuration using five different volumetric energies belonging to conduction mode. The critical fracture toughness was calculated and discussed along with the plastic deformations at the crack tip. The study found that local plastic deformation for single edge notch bending was influenced by powder bed fusion process parameters. A correlation was observed between energy density, fracture toughness, and the dimensions of the fracture process zone. The R-curves showed different fracture behaviors depending on the energy density. The energy required to grow a crack was associated with larger plastic zones, resulting in fracture toughness values ranging from 43 (43 J.mm-3) to 427 kJ/m2 (68 J.mm-3). Results are discussed in terms of porosity and strain hardening capacity depending on the manufacturing conditions.

Mechanical properties of Laser Power Bed Fusion (LPBF) parts, and particularly fatigue properties, are heavily affected by defects including surface roughness or porosity. To mitigate the occurrence of these defects, in-situ and on-line corrective measures can be implemented to the fabrication process, among them, rescanning, which consists in remelting an already solidified layer. Initially porous LPBF samples were created and then rescanned using different scanning parameters and strategies. Results show that it is possible to regain part's health, compared to a standardly processed one, in terms of density, hardness and even improved roughness. This remelting process is known to refine microstructure of fabricated materials as well as reduce surface roughness and porosity without requiring further post-processing steps. Therefore, employing rescanning as a corrective technique appears to be a promising approach for rectifying detected defects during the fabrication process. The objective of this study is to assess the corrective capabilities of different rescanning strategies to restore the microstructure of an initially porous 316L LPBF simulating a defected part. This study shows that various rescanning strategies allow for densification of initially porous material from 98.83 +/- 0.20 % to 99.75 +/- 0.09 %, as well as lateral surface roughness reduction from Ra 20.2 +/- 5.2 mu m to Ra 12.7 +/- 0.1 mu m and microhardness increase from 243 +/- 5 HV0.5 to 253 +/- 3 HV0.5.

Laser Powder Bed Fusion (LPBF) processing enables scrap free and sustainable fabrication of alloys. However, LPBF microstructural features and heterogeneities are governed by a different set of process variables than that of wrought materials due to rapid solidification and out-of-equilibrium conditions. Combined with powder and processing contamination, it can produce metastable precipitates or oxides affecting the mechanical properties. This work aims to investigate such particles formed due to LPBF processing in a Ni20Cr alloy. As-built specimens reveal the presence of nano-oxides in the inter-dendritic regions of Ni-Cr solid solution, having a core-shell structure with Ni-Cr as shell and Al-Si-Cr-O as core. ThermoCalc was used to predict the chronology of formation of such core-shell-precipitates. The impact of processing contamination on microstructure, especially the formation of inter-dendritic precipitates, is anticipated to be more detrimental than that of powder contamination.

Additive manufacturing is a powerful process to build complex geometry. Besides the numerous process parameters influencing the mechanical part performances, other parameters related to the initial powder feedstock or component machining are of most importance. In this study, the combined effect of a wide particle size distribution, surface machining and stress-relief heat treatment on the microstructure and mechanical properties (tension and fatigue) of a stainless steel AISI 316L, produced by laser powder bed fusion, is investigated. In order to correctly investigate those parameters separately, the netshape/machined character of the sample, alongside with the heat treatment, is studied for two kinds of powder having different particle size distributions, i.e. narrow and widely spread. Results show that a large spread of particle size is only slightly detrimental to the fatigue life, in particular in high-cycle conditions due to a larger porosity related to a weakly more uneven particle spatial distribution in the bed. Nevertheless, this effect is of a second order compared to machining or heat treatments which greatly affect the mechanical behaviour. Surface machining and moderate heat treatment are then the best post-operational steps to increase the fatigue life in high-cycle fatigue conditions independently of the particle size distribution. Results are discussed in terms of defects, microstructural modifications, surface roughness, martensitic transformation and mechanical loading.

The present study investigated the influence of Y2O3 addition by mechanical alloying (MA) on the microstructure evolution of a BCC High Entropy Alloy (HEA). The characterisation and mechanical properties of the alloy were explored using X-ray diffraction, SEM, EBSD, and nano-indentation. The sintered Al1.8CoCrCu0.5FeNi HEA shows a microstructure formed by an ordered BCC phase (Al-rich) and a second disordered BCC (Cr-rich), while a minor FCC (Cu-rich) appears. These BCC phases show a wide morphology evolution from cuboidal and wave-like structures to irregular shapes. The minor FCC phase also adopts several morphologies as the MA is performed. The introduction of oxide reinforcements and microstructure refinement through mechanical alloying yields a change in phase quantification and grain structure. In accordance with the hardness and elastic modulus values from ordered/disordered BCC phases, the disordered BCC shows higher values than the ordered one. The grain size reduction as well as the solid solution strengthening from the microstructure evolution consequence of the MA are shown to be the main contributors to the increase in hardness and elastic modulus in the consolidated samples.

Additive Manufacturing (AM) brings about an array of modifications in microstructure with respect to conventional routes transforming mechanical performances. These new microstructure features depend on process parameters and especially on volume energy-density delivered by the laser on powder layer. Among the different alloys manufactured by AM, Ni-alloys exhibit high-strength at elevated temperature opening the way of fabrication of gas turbines and jet-engine parts. Ni-superalloys experience precipitation hardening due to the formation of γ′ and γ′′ phases leading to complex microstructures. To better study the influence of the AM microstructure on Ni-alloys mechanical properties, in particular at elevated temperatures, a theoretically monophasic and binary Ni20Cr-alloy manufactured by laser powder-bed fusion was studied in this work. Remarkable Yield Strength (400 MPa) and Ultimate Tensile Strength (UTS) (600 MPa) were observed at 500°C with hardly any loss of properties from room temperature, owing to the thermal stability of cellular dendrites till 700°C. Ductility drop was reported at 700°C due to anomalous brittle behaviour of Ni-alloys. Hardening behaviour vanished at 900°C signifying the deletion of dendrites, disappearance of dislocations, diffusion of Cr from dendritic walls and growth of oxides.