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.

In fast neutron reactors, some parts can be subjected to displacements between each other (as movable parts for example). On these parts, the contact areas usually need a hardfacing coating. The standard hardfacing alloy is a cobalt-base alloy (for example Stellite®6). Unfortunately, in the primary coolant circuit and on wear conditions, cobalt can be released. Under neutron flux, the stable 59Co can be transmuted into 60Co by radioactive capture of neutrons and, therefore, can contaminate the primary circuit. Therefore, it is desired to replace this cobalt based hardfacing alloy by a cobalt-free one. Previous presentations have shown the potential interest of some nickel base materials as Colmonoy® alloy. In parallel, 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 all the study, the base material is the stainless steel 316LN. In the first section of this article, the authors present previous results related to the selection of hardfacing materials and their evaluation in comparable tribology conditions. Then, Tribaloy® 700, another nickel based alloy that has been poorly investigated, is presented and evaluated. This nickel base has a completely different microstructure, and its tribological behavior related to the variation of the microstructure is not well known. First, the authors present the features of the selected materials. Then, the authors present various property characterization results obtained by changing several process parameters. The quality of the clad is considered, and the process window providing a good clad is determined (no crack, only a few porosities, etc.). The variation of the microstructure is analyzed, and solidification paths are proposed regarding the process parameters. Wear tests are performed on typical wear conditions. The movement is linear. Argon is used for the protection of the sample against oxidation. Tests are carried out at 200 °C. Wear tests are analyzed, and wear mechanisms are correlated with the microstructure of the material.

Using pulsed or modulated laser irradiation for laser powder bed fusion (L-PBF) manufacturing can be an attractive solution for better controlling heat input, mastering geometry, and tuning microstructures of built samples. A comparison was made between a continuous wave (CW) and a modulated pulsed wave (PW) L-PBF process, considering high pulse frequencies (50 kHz), various duty cycles, and an Inconel 625 powder. In the first step, considering the precise shape of laser pulses, the peak powers were adjusted to provide similar mean powers between PW and CW and analyze the specific effect of the pulse regime at constant mean power. In the second step, single L-PBF beads were considered. The PW regime was shown to reduce the process window on (P, V) maps, increase powder-denudated widths around scan tracks, and slightly reduce the dimensions (depth, width and area) of fusion beads. In the third step, 3D samples were manufactured with PW (with duty cycles of 0.9 and 0.7) and CW regimes. The PW regime was shown to slightly increase the porosity rate, reduce the mean grain size by 30–40%, and modify the crystallographic texture.

Online monitoring and control of laser keyhole welding is highly desirable, especially in safety-critical aeronautic applications, in order to detect and prevent possible defects in the weld. In this article we describe a closed-loop control system which ensures full penetration in keyhole welding by controlling the focus position and power of a 4 kW Nd:YAG laser. We present a laser power control system which determines the degree of penetration by analysis of an image of the welding keyhole. The focus position control system, meanwhile, is based on a previously reported system, exploiting the chromatic aberration of the focusing optics. We demonstrate performance in bead-on-plate ‘welding’ of Inconel 718 and Titanium. The focus control system maintained focal position on tilted and non-flat workpieces, and the penetration monitoring technique successfully controlled the laser power to maintain the full penetration regime in the presence of linear and step changes of thickness. Finally we discuss the performances and the limits of the systems when applied to a realistic complex aerospace component.

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.