Powder Bed Fusion – Laser Beam enables exceptional design freedom but suffers from limited microstructure control due to rapid solidification and steep thermal gradients that favor columnar grain growth and strong crystallographic textures. This work presents an in situ grain refinement strategy based on laser-induced photoacoustic excitation, achieved by temporally modulating the processing laser to generate thermoelastically pressure waves within the melt pool. In situ laser interferometry and post-mortem Electron BackScatter Diffraction (EBSD) analysis of 316L beads-on-plate demonstrate that acoustic excitation modifies melt pool dynamics and grain growth. The results establish a mechanistic link between laser-induced ultrasound and solidification behavior, offering a new pathway for online microstructure control in PBF-LB.

Over the past decade, additive manufacturing has made tremendous progress. In particular, Laser Powder Bed Fusion (L-PBF) applications have increased dramatically due to its ability to produce complex shapes as lattice structures. In parallel, optical fiber sensors have demonstrated their ability to measure physical parameters such as temperature or stress inline. Recent studies suggest the possibility of incorporating optical fiber sensors into L-PBF parts. This allows the sensor to be placed at the desired location within the complex structure during manufacturing. After a brief introduction of the study and the position of optical fiber sensors, the paper presents the proposed approach for inserting the sensors during L-PBF fabrication and the associated experiments. The results show the quality of the integration and the impact on the mechanical properties of the parts. An example of application, the monitoring of lattice structure manufactured by L-PBF for shock absorbers of radioactive waste storage, is finally given. (c) 2024 The Authors. Published by Elsevier B.V.

This study aims to assess the potential of Laser Additive Manufacturing (LAM) for the elaboration of Ferritic/Martensitic ODS steels. These materials are usually manufactured by mechanical alloying of powders followed by hot consolidation in a solid state. Two Fe-14Cr-1W ODS powders are considered for this study. The first powder was obtained by mechanical alloying, and the second was through soft mixing of an atomized Fe-14Cr steel powder with yttria nanoparticles. They are representative of the different types of powders that can be used for LAM. The results obtained with the Laser Powder Bed Fusion (LPBF) process are compared to a non-ODS powder and to a conventional ODS material obtained by Hot Isostatic Pressing (HIP). The microstructural and mechanical characterizations show that it is possible to obtain nano-oxides in the material, but their density remains low compared to HIP ODS steels, regardless of the initial powders considered. The ODS obtained by LAM have mechanical properties which remain modest compared to conventional ODS. The current study demonstrated that it is very difficult to obtain F/M ODS grades with the expected characteristics by using LAM processes. Indeed, even if significant progress has been made, the powder melting stage strongly limits, for the moment, the possibility of obtaining fine and dense precipitation of nano-oxides in these steels.

Versatility of powder metallurgy was used to design new duplex and compositionally graded steels. Rapid consolidation by Spark Plasma Sintering (SPS) and longer consolidation by Hot Isostatic Pressing (HIP) were applied on two austenitic 316L and martensitic Fe-9Cr powders, either homogeneously blended or uniaxially graded before sintering.The microstructural investigations by micro-hardness, SEM, EDX and EBSD showed a continuous crystal-lographic structure at the austenite-martensite interface and a similar grain size in SPS and HIP samples, whereas a larger interdiffusion length of Cr and Ni was observed in the HIP sample, as confirmed by diffusion calculations. The tensile behaviour of materials could not be described by a simple law of mixtures. To better understand this phenomenon, the obtained materials were described as a composite and iso-strain and iso-stress models were used and discussed. The microstructural characterizations show that during the consolidation, diffusion of the chemical species modifies the nature and the respective fractions of phases, which explains the discrepancy between the models and the experimental mechanical behaviour of the duplex alloys.

Significant research efforts have been undertaken over the past forty years to replace the StelliteTM cobalt-based alloys, which boast outstanding performances when used as hardfacing coatings, but proved problematic especially in radioactive environments. This work's purpose is to contribute to this effort by coming up with viable substitutes made of Complex Concentrated Alloys (CCAs). Previous work evidenced the (CrFeNi)90Mo5Ti5 alloy as a promising base that relies on the formation of intermetallic phases within a ductile matrix for an increase in hardness and an improved tribological behaviour. In this particular framework, the in situ alloying capabilities of the DED (Direct Energy Deposition) process were used for further explorations around this composition. Compositionally graded samples were successfully made despite the especially high brittleness of the alloys of interest. Coupled with an extensive use of the CALPHAD method, this combinatorial strategy dramatically speeds up material development compared to what the more conventional ways can achieve. The present paper emphasizes on the methodology and the high-throughput tools that were developed and used in this study, as such elements are growing in importance in the current context of intensive global research for new materials, especially in the CCAs field.

Over the past decade, additive manufacturing has made a tremendous progress; This technology gets a great interest to the development of mechanical parts with complex geometries and compositions. The freedom of conception allows using additive manufacturing process in order to integrate optical sensors during the printing process and opens the way to the production of instrumented components for SHM (Structural Health Monitoring). The latter required a particular process with adapted printing strategy and an interruption of the procedure in order to implement the sensor. The impact of this insertion needs to be investigated in order to assess its influence on the mechanical parts behaviors.

Dynamic temperature monitoring along a stainless steel specimen additively manufactured by selective laser melting was performed using point-by-point written femtosecond Fiber Bragg Gratings packaged in a metallic capillary.

The use of different steel powders allows to obtain either multi-material junctions, when they are stacked, or new duplex alloys when they are blended.This study focuses on new materials obtained by Spark Plasma Sintering (SPS) or Hot Isostatic Pressing (HIP) with an austenitic 316L steel and a martensitic Fe-9Cr steel powders. These materials are characterized at different scales: from metallography to electron microscopy with EDX and EBSD, hardness and tensile tests.The mechanical behaviour of materials cannot be described by a simple law of mixtures. To better understand this phenomenon by describing the obtained materials as a composite, Reuss and Voigt models are used and discussed.The microstructural characterizations show that during the consolidation, the diffusion of the chemical species modifies the nature and the amount of phases, which makes it possible to understand why the models do not completely account for the experimental behaviour.

In this paper, the high temperature transformation kinetics of delta ferrite to austenite (delta -> gamma) phase transformation is modeled by thermodynamic and diffusion calculations. It appears that, in martensitic steels, the delta -> gamma transformation is very fast (a few milliseconds) as soon as the first austenite nucleus appears. Classically the austenitic phase will thus systematically be observed in the material during conventional elaboration processes. However, in powder metallurgy and additive manufacturing, it is possible to obtain sufficiently high quenching rates (up to 10(6) degrees C/s) so that the gamma phase does not have time to appear. The calculations presented here allow to rationalize the understanding of the microstructures of powders and different additive manufacturing materials. They enable to understand why ferrite or martensite is sometimes obtained in the final microstructure. From the calculations made, an original CCT (Continuous Cooling Transformations) diagram starting from the delta phase is proposed. This understanding is one more step toward the control of microstructure and properties of additively manufactured martensitic steels.

This article presents the Laser Beam Direct Energy Deposition (DED-LB) process as a method to build a graded austenitic-to-martensitic steel junction. Builds were obtained by varying the ratio of the two powders during DED-LB processing. Samples with gradual transitions were successfully obtained using a high dilution rate from one layer to the next. Long austenitic grains are observed on the 316 L side while martensitic grains are observed on the Fe-9Cr-1Mo side. In the transition zone the microstructure is mainly martensitic. Characterisations performed after building and after a tempering heat treatment at 630 degrees C for 8 h were compared to dissimilar Electron Beam (EB) welds. Before heat treatment the DED-LB graded area has high hardness (values of around 430 HV) due to fresh martensite formed during building. Tempering heat treatment reduces this hardness to 300 HV. EDS measurements indicate that the chemical gradient between 316 L and Fe-9Cr-1Mo obtained by DED-LB is smoother than the chemical change obtained in EB welds. Microstructures in DED-LB are quite different from those obtained by EB welding. Hardness values in DED-LB samples and in welds are similar; the weld metal and the Fe-9Cr-1Mo heat-affected zone are relatively hard after welding because of fresh martensite, as found in the DED-LB transition zone; both are softened by tempering heat treatment. Tensile tests show that DED-LB samples and EB welds have similar behaviour with failure in 316 L base metal at 20 degrees C and 400 degrees C and failure in Fe-9Cr-1Mo base metal at 550 degrees C. DED-LB samples have comparable mechanical properties to EB welds.

In fast neutron reactors, contact areas of moving parts usually require cobalt-free hardfacing coatings, as cobalt is highly activated under neutron flux. This is particularly critical for the insert holes of the diagrid for the positioning of the hexagonal fuel tubes that have to be internally coated. In this article, we propose to present the development of the cobalt-free hardfacing material up to the manufacturing of the inner clads with a specific deep laser cladding nozzle. In previous presentations, laser cladding has been identified as a deposition process that could increase the performances of the hardfacing materials compared to the standard process (Plasma Transferred Arc Welding). In parallel, the potential interest of some nickel base materials such as Colmonoy® 52 or Tribaloy® T700 has been demonstrated. Unfortunately, the deposition of these fragile alloys requires a preheating of the substrate over 450 °C. More recently, Nucalloy® 453, a new hardfacing nickel base alloy has been evaluated and demonstrated simpler deposition conditions that requires lower preheating temperature (<300 °C). The article presents the evaluation of Nucalloy® 453 with material analysis and wear tests. The microstructural characterization is compared to Colmonoy® 52, which is a similar NiFeCrSiBC alloy. Finally, the laser cladding of a scale one demonstrator is presented: two inner zones of a 1 m cylinder of 100 mm diameter are laser cladded, thanks to a deep cladding nozzle. The advantage of the laser cladding process is compared to Plasma Arc Transferred Welding.

We report that 316L austenitic stainless steel fabricated by direct laser deposition (DLD), an additive manufacturing (AM) process, have a higher yield strength than that of conventional 316L while keeping high ductility. More interestingly, no clear anisotropy in tensile properties was observed between the building and the scanning direction of the 3D printed steel. Metallographic examination of the as-built parts shows a heterogeneous solidification cellular microstructure. Transmission electron microscopy observations coupled with Energy Dispersive X-ray Spectrometry (EDS) reveal the presence of chemical micro-segregation correlated with high dislocation density at cell boundaries as well as the in-situ formation of well-dispersed oxides and transition-metal-rich precipitates. The hierarchical heterogeneous microstructure in the AM parts induces excellent strength of the 316L stainless steel while the low staking fault energy of the as-built 316L promotes the occurrence of abundant deformation twinning, in the origin of the high ductility of the AM steel. Without additional post-process treatments, the AM 316L proves that it can be used as a structural material or component for repair in mechanical construction.

If Stellite® 6 is a cobalt base alloy with a hard coating well known for its good wear resistance, its use is not desirable for nuclear installations due to the activation of cobalt under neutron flux which makes maintenances more delicate as, later, decommissioning. If several studies have led to propose certain hard coatings without cobalt, for the moment, none equals the wear properties of Stellite® 6.

This study investigates the microstructure and mechanical properties of 316L stainless steel (316LSS) samples fabricated by additive manufacturing (AM), in order to optimise the process for improving the properties of 316LSS parts built by laser powder bed fusion (LPBF). Accordingly, a substrate heating was performed on an SLM 280 HL printer fitted with an experimental heating device. During the process, samples are built on a substrate heated up to 600 ∘ C. The substrate temperature significantly influences the microstructural evolution and mechanical properties of manufactured parts. Concerning tensile properties, the ultimate tensile strength (UTS) and yield strength (YS) decrease as a function of the substrate temperature whereas the elongation (El) increases as a function of the substrate temperature. These results are similar to those obtained with a post-heat treatment performed on parts manufactured by forging or LPBF. The tensile and impact energies reach values higher than the minimum requirement for 316LSS manufactured by forging, according to the RCC-MRx code used for materials dedicated to French nuclear applications. When the substrate temperature is equal to 350 ∘ C, the UTS, YS, El and impact energy reached 596 ± 5 MPa, 489 ± 3 MPa, 48 ± 3% and 123 ± 20 J, respectively. Finally, this study demonstrates that heating the substrate during the process is a promising solution to optimise the fabrication route by suppressing post-process heat treatment of 316LSS made by LPBF.

The feedback produced by operating Sodium-cooled Fast Reactors (SFRs) has shown the importance of material tribological properties. Where galling or adhesive wear cannot be allowed, hardfacing alloys, known to be galling-resistant coatings, are usually applied on rubbing surfaces. The most used coating is the cobalt-base alloy named Stellite 6® because of its outstanding friction and wear behaviour. Nevertheless, cobalt is an element which activates in the reactor leading to complex management of safety during reactor maintenance and mainly decommissioning. As a consequence, a collaborative work between CEA, EDF and FRAMATOME has been launched for selecting promising cobalt-free hardfacing alloys for the 600 MWe Sodium-cooled Fast breeder reactor project named ASTRID. Several nickel-base alloys have been selected from literature review then deposited by Plasma Transferred Arc or Laser Cladding on 17Cr austenitic stainless steel 316L(N) according to RCC-MRx Code (AFCEN Code). Among the numerous properties required for qualifying their use as hardfacing alloys in SFR, good corrosion behaviour and good friction and wear behaviour in sodium are essential. First results on these properties are shown in this article. Firstly, the corrosion behaviour of all coatings was evaluated through exposure tests in purified sodium for 5000 h at 400 °C. All coatings showed an acceptable corrosion behaviour in sodium. Finally, the friction and wear properties of one alloy candidate, NiCrBSi alloy, were studied in sodium in a dedicated designed facility. The influence of the oxygen concentration in sodium on the friction and wear properties was evaluated.

In the nuclear field, efforts are made to find substitutes to cobalt hardfacing alloys since these alloys have a principal drawback, the transmutation from stable 59Co to 60Co under neutron irradiation. In case of wear, fragments could be deposed on the surface of primary circuit and thus contaminate it, causing a real issue for deconstruction.

In this article, we present an ongoing study on a Ni-Fe-Cr-Mo-Si based alloy, as one of the potential cobalt free hardfacing materials. Here, we analyze the effect of the variation of iron content on microstructure and wear property that is induced by a controlled dilution the 316L substrate, or by direct addition. First, we present the state of the art on the proposed Ni-Cr-Mo-Si based alloy. Then, the laser cladding setup is presented and the process parameter map search of the Ni-Fe-Cr-Mo-Si based alloy too. Then, iron is added to the base Ni-Cr-Mo-Si alloy by direct fusion of the powders coming from two different powder feeders and directly blended into the coaxial nozzle used. Finally the samples are analyzed in iron content, microhardness and microstructure. The tribology tests of the samples exhibit a clear change in wear behavior with the iron content and an improvement at a certain level of iron.

Innovation in thin-film deposition processes, thermal spraying and cladding technologies mostly rely on evolutions of their previous iteration. Along with other examples, five case studies of emerging elaboration processes for metallurgical coatings are described coupled with their applications. In the frame of the lifetime extension of components exposed to aggressive media or their functionalization, this article depicts all the developments of the detailed processes. Physical vapor deposition (PVD) of coatings with exceptional properties is possible thanks to sources generating highly ionized metallic vapors. The control of the average energy per incident species and particularly metallic ions strongly influences the characteristics of the deposited layer obtained, for example, with HiPIMS (High Power Impulse Magnetron Sputtering). While PVD techniques are mainly directive regarding the growth of the coating, chemical vapor deposition (CVD) processes manage to homogeneously coat complex 3D shapes. The use of specific precursors in DLI-MOCVD (Direct Liquid Injection - MetalOrganic CVD), carefully selected from the whole metalorganic chemistry, allows one to efficiently treat heat-sensitive substrates and broadens their application range. The third detailed example of emerging technology is suspension plasma spraying (SPS). Projection of various solutions containing nanoparticles leads to the growth of unusual morphologies and microstructures and to the generation of porous coatings with multi-scaled porosity. On the other hand, cold-spray uses metallic powders with higher granulometry and does not modify them during the deposition process. As a result, high-purity and dense materials are deposited with properties similar to those of wrought materials. Whereas cold-spray is suitable only for ductile metals, laser cladding can be applied to ceramics, polymers and of course metals. Laser cladding is a key technology for advanced metallurgical engineering and alloy development due to its capability for functionally graded materials production and combinatorial synthesis. (C) 2018 Academie des sciences. Published by Elsevier Masson SAS.