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.

The effect of the chemical composition of two low-carbon steels, C18E and 22MnB5, on their behavior after forming by fine blanking was investigated. A specific tool, adaptable to a tensile testing machine, was designed to replicate an industrial half-cutting process. This tool allows for the production of samples with simple geometries and easy modification of the processing conditions. Residual elements in the raw material, concentrated in segregation bands, appear to play a key role in crack initiation within the shear zone during the blanking process. The role of non-metallic inclusions is discussed to explain the presence of large cracks in C18E, while 22MnB5 only shows damage nucleation. After fine blanking, a carbonitriding heat treatment process was performed to modify the initial microstructure and achieve the required mechanical properties in the final parts. Continuous cooling transformation diagrams were created for both steels to guide this process. The results of this study demonstrate the better formability of 22MnB5 by fine blanking, compared to that of C18E.

Additive Manufacturing of metallic alloys offers many possibilities in the design and use of structural components. It allows obtaining intricate parts while reducing the quantity of raw material and creating lightweight in service structures. However, the microstructure of such materials turns out to be very different from those of conventional parts. Consequently, their properties, such as radiation resistance, may also be different. This work focuses on the irradiation response of a Ni-20Cr alloy produced by Laser Powder Bed Fusion (L-PBF) following three different building strategies by varying the laser power and its scanning speed. Irradiation with 5 MeV Fe5+ and 2 MeV Ni+ ions up to a dose of 4.5 dpa at 450 °C results in the production of homogeneously distributed “black dots”, identified as Frank loops. Their growth kinetic was followed in situ by Transmission Electron Microscope observations during irradiation. In comparison to a reference material, the L-PBF material presents lower density of Frank loops while the loop sizes are identical in both materials. This suggests that the resistance to irradiation is improved by the L-PBF process. The AM building strategies used in this study does not influence the response of the material to irradiation.

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.

The microstructure evolution during dissimilar friction stir welding of copper with stainless steel 316L is studied. The welding is performed in transparence configuration using tools of two different geometries. The joints are characterized by optical microscopy, scanning and transmission electron microscopy, electron backscattered diffraction, and microhardness. The microstructure and crystallographic texture of base metals are significantly affected. The joint produced using a shoulder of 8 mm in diameter is characterized by a copper nugget with refined grains. The texture of copper is dominated by component of the shear texture. The steel beneath the tool shows a grain refinement with grains of sub‐micron size and a texture which is close to the ideal simple shear texture. For the joint produced by the 16 mm diameter tool shoulder, the nugget consists of large grains of copper, while the steel beneath the tool shows small recrystallized grains of micrometer size. The texture in the steel is dominated by the component of the shear texture. The Cu/316L interface in both types of joint is of very good quality. In a detailed study of the interface, it is revealed that the welding of metals is achieved by mutual intense mechanical interlocking without formation of any intermetallic compounds.

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.

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.

If the grain size reduction is known to enhance the yield stress of alloys, its impact on the corrosion resistance is still misunderstood. In this work, corrosion and passivation mechanisms of stainless steel 316L manufactured by ball milling and spark plasma sintering were studied in Na2SO4. The elaboration by powder metallurgy and fast sintering route produces dense samples with grain sizes in the ultrafine domain without crystallographic texture. Electrochemical properties were analysed by potentiodynamic polarization experiments. The samples display weak passivation current density, typically around a few μA/cm2, with a grain size reduction. This was related to an increase in polarization resistance of the free surfaces for smallest grain sizes. Moreover, a passivation of the milled powder in HNO3 before sintering has a beneficial impact on the stability of the passive layer, especially for the smallest grain sizes. The physical properties of the oxide layer were analysed by impedance spectroscopy and Mott-Schottky formalism. Capacitive properties and thickness of the oxide do not strongly depend of the grain size. Smallest grain size specimens manufactured with passivated powder exhibit the higher electrical resistance of the interface. Donor density also decreases with grain size, leading to most compact and less defective oxide layer.

Additively Manufactured (AM) metallic alloys differ from their conventionally produced counterparts by complex multi-scaled microstructures leading to deeply modified mechanical behavior. The characterization of these new links between microstructure and mechanical properties is of first importance. Nevertheless, many alloys produced by Laser Powder Bed Fusion (LPBF) process exhibit multi-phase microstructures which makes difficult the understanding of these links. In this article, we aimed at simplifying this complexity by investigating the basic strain hardening mechanisms of AM (LPBF) alloys of a theoretically monophasic Ni20Cr alloy manufactured by laser powder bed fusion. Based on the analysis of the microstructure and the tensile mechanical behavior including loading-unloading-relaxation tests, a comparison with conventionally manufactured Ni20Cr alloy is performed. First, an increase in yield stress for the LPBF samples is observed due to both effective stress and backstress modification. Second, the strain hardening mechanisms are modified for LPBF manufactured samples compared to cast ones. Kocks-Mecking model is then employed to reproduce the tensile curves and better analyze the strain hardening mechanisms. Results are discussed in terms of specific LPBF microstructure feature contributions to stress and strain hardening. We reveal that dislocation cells associated to dendrites are proved to be responsible for about 50% of the improved yield stress of LPBF material and seem to control the dislocation production, forest interactions being inoperative for those materials.

Laser powder bed fusion (LPBF) process is an additive manufacturing technique that focuses on intricate metal fabrication using laser processing of metallic powder. Various processing parameters like laser power, scanning speed, and hatch spacing giving out unique applied volumetric energies are involved in such fabrication. Those varied energies bring about microstructural changes leading to modifications in mechanical response such as yield stress and strain hardening behaviour. In this work, we investigated the influence of volumetric energy density on the aforementioned mechanical properties of a Ni-20 wt%Cr alloy manufactured via LPBF. First, an analytical model was employed to study the contribution of each microstructural feature on yield stress of LPBF samples. Dendritic cellular structures (and their sizes) are found to be the most important feature to govern this parameter. The Kocks-Mecking model was further extended to associate the different strain hardening mechanisms with dislocation production and interaction mechanisms via different channels like dendritic cellular structures, grains and forest dislocations. The production of dislocation via dendritic cellular structures is also the most significant mechanism for unique hardening behaviour in LPBF alloys. A modified equation of dislocation production mechanisms is finally proposed to simplify the application of this model for modelling the mechanical behaviour in tension of LPBF Ni20Cr.

Macrostructure, microstructure, and distribution of phases through the interface were analyzed for friction-stir-welded joints 5083 aluminum alloy and 316L steel. Several analytical techniques, including light microscopy, transmission and scanning electron microscopy, elemental analysis using X-ray spectroscopy, and electron diffraction, were used to thoroughly analyze the weld interface. The interface is characterized by a significant reduction in grain size for both aluminum alloy and stainless steel. New compounds, not corresponding to thermodynamically stable phases in the binary Al–Fe phase diagram, were found in the stirred zone (SZ) as dispersed particles. On the steel side of the welding, thin slabs of new compounds were found, as well, being interlaced with the stainless steel. The observations support that the grain refinement of stainless steel is likely due to a continuous dynamic recrystallization. The intermetallic compounds present as a layer at the interface, exhibiting nanometric grain size, were identified by electron diffraction as Al 13 Fe 4 and Al 5 Fe 2 phases. Concerning the intermetallic compound formed in SZ, the elemental analysis showed a compound containing principally Al and Fe, with admixture of Si and Mn. It was concluded that it is a pseudo-ternary compound with body-centered cubic structure, Im -3 space group, which is for the first time reported in this kind of dissimilar assembly, and is known as α-Al(Fe,Mn)Si.

The effect of the negative clearances between punch and die of a flange with different fine blanking half-cutting depths was investigated for two low carbon steels with various elongation rates, C18E (0.18%C) and 20MnB5 steels (0.2%C, with upper mechanical properties than the carbon steel), industrially used for forming car seat parts. A specific tool adaptable on a tensile machine was built to determine the force needed to fine blank a simple geometry, in the objective to further transpose the obtained results to the complex geometry of industrial parts. Heat treatment was carried out to optimize the microstructure, hardness and dimensional modifications for the combination material/clearance/half-cutting height tested. Increasing the elongation rate of the raw material allows to reach a better formability and reduces the force needed to create a given geometry, but cracks were detected for the extreme deformation rate. Moreover, after fine blanking and heat treatment operations, different microstructures were observed according to the material grade, stable for low alloy 20MnB5 steels (martensitic microstructure) while non reproducible for the low carbon steel (mix of ferrite, bainite and martensite).

We studied the influence of the architecture of Copper-Clad Aluminum (CCA) wires produced by cold-drawing process, on the mechanical properties of materials, and the development of intermetallic compounds at the interface after annealing. Simple and architectured-CCA (A_CCA) rods, fabricated by stacking and re-drawing of original CCA wires, have been characterized before and after a thermal treatment. Nanoindentation tests, performed to map the hardness and the elastic modulus at the microscale, allow to well catch the recrystallization phenomenon, which takes place in both the Cu and the Al subjected to severe plastic deformation, as well as the formation of three intermetallic compounds (IMC) at the interface. Our measures underline that the mechanical properties of all the components involved in the wires, i.e., Cu, Al, and eventually the IMC developing after annealing, are only slightly influenced by the structuration of the wires. IMC developing at the Cu/Al interfaces present a high hardness, and might deteriorate the mechanical properties of the wires, even more for A_CCA wires, which exhibit more interfaces that the traditional CCA samples.