Laser cladding and wear testing of nickel base hardfacing materials: Influence of process parameters

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 stable59Co can be transmuted into60Co 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

Accuracy Assessment of the ESA CCI 20M Land Cover Map: Kenya, Gabon, Ivory Coast and South Africa

This working paper presents the overall and spatial accuracy assessment of the European Space Agency (ESA) 20 m prototype land cover map for Africa for four countries: Kenya, Gabon, Ivory Coast and South Africa. This accuracy assessment was undertaken as part of the ESA-funded CrowdVal project. The results varied from 44% (for South Africa) to 91% (for Gabon). In the case of Kenya (56% overall accuracy) and South Africa, these values are largely caused by the confusion between grassland and shrubland. However, if a weighted confusion matrix is used, which diminishes the importance of the confusion between grassland and shrubs, the overall accuracy for Kenya increases to 79% and for South Africa, 75%. The overall accuracy for Ivory Coast (47%) is a result of a highly fragmented land cover, which makes it a difficult country to map with remote sensing. The exception was Gabon with a high overall accuracy of 91%, but this can be explained by the high amount of tree cover across the country, which is a relatively easy class to map

The prospects for additive manufacturing of bulk TiAl alloy

International audienceThis paper deals with the prospects for additive manufacturing (AM) of bulk TiAl alloys. A number of AM processes have already been explored in the literature for these intermetallic alloys. The main trend developed in published works concerns a strong crack sensitivity of this relatively brittle material due to rapid successive heating and cooling cycles. Optimized processing conditions have already been achieved for producing sound and crack-free TiAl materials by means of EBM and LMD processes. This experimental work was particularly focused on the third process, i.e. SLM, to produce fully dense TiAl parts. A series of beads, surface layers and cubes have been manufactured to investigate the microstructural evolution. Post heat treatments allowed a uniform microstructure to be restored for the intermetallic TiAl alloy

Surface Finish Issues after Direct Metal Deposition

International audienceDerived from laser cladding, the Direct Metal Deposition (DMD) laser process, is based upon a laser beam - projected powder interaction, and allows manufacturing complex 3D shapes much faster than conventional processes. However, the surface finish remains critical, and DMD parts usually necessitate post-machining steps. In this context, the focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish. Our experimental approach is based upon: (1) adequate modifications of the DMD conditions (gas shielding, laser conditions, coaxial or off-axis nozzles), (2) a characterization of laser-powder-melt-pool interactions using fast camera analysis, (3) a precise check of surface aspects using 3D profilometry, SEM, (4) preliminary thermo-convective simulations to understand melt-pool hydrodynamics. Most of the experimental tests were carried out on a Ti6Al4V titanium alloy, widely investigated already. Results confirm that surface degradation depends on two aspects: the sticking of non-melted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters

On laser welding of thin steel sheets

International audienceThis paper presents a process-structure-property relationship study of laser welds as a continuous consolidation method for joining thin monophased steel foils, thereby providing a more effective, less costly method to construct automotive catalytic converters. A body centred cubic (bcc) iron-chromium-aluminium alloy doped with Mischmetal was utilised in this study. Both pulsed and continuous wave modes were used to establish the limit welding diagrams for lap joint configuration. Actual laser welding parameters were selected using several testing conditions. The laser welds behaved substantially different from the base material under creep and high temperature oxidation. The difference was mainly attributed to the changes in grain morphology, precipitation of aluminium nitrides and carbides, and relocalisation of the reactive elements during liquid metal flow upon keyhole formation, solidification and cooling

Direct fabrication of a Ti-47Al-2Cr-2Nb ally by selective laser melting and direct metal deposition processes

International audienceA Ti-47Al-2Cr-2Nb (at.%) material was fabricated using two laser-based methods, “Selective Laser Melting” (SLM) and “Direct Metal Deposition” (DMD), for potential uses in aircraft jet engines. Experiments were conducted under controlled atmosphere by changing the processing parameters. Optimal parameters were searched for this relatively low ductility material to prevent cracking due to built-up residual stresses during fast cooling. It was observed that these non-equilibrium cooling conditions were fast enough to generate ultra fine and metastable structures exhibiting high microhardness values. Post heat-treatments were successfully used to restore homogeneous lamellar or duplex microstructures and to relieve the residual stresses. A comparison of these two methods is provided in terms of powder requirements and of process parameters to achieve noncracked structures and fully dense materials

Targeted Grazing: Applying the Research to the Land

The discipline of range science is in part based on the observation that vegetation on rangelands changes in response to livestock grazing. For much of the history of range science, livestock grazing was considered to affect range plants and ecological condition negatively. Thus range plants were classified as increasers, decreasers, or invaders as a function of their response to grazing. The concept that grazing can be used to restore degraded rangelands is relatively new. It requires a paradigm shift for most people from grazing animals reaping the benefits of the land to the land reaping the benefits of the grazing animals. Using livestock to accomplish vegetation management goals is referred to as targeted grazing. Targeted grazing is defined as the application of a particular kind of grazing animal at a specified season, duration, and intensity to accomplish specific vegetation management goals. It is the last half of this definition that differentiates targeted grazing from traditional grazing. The focus is on the vegetation and the subsequent outcomes and changes in composition or structure, rather than the performance of the grazing animal. Where the potential for targeted grazing to create positive change on the landscape has been clearly demonstrated through research and the experiences of practitioners, it still struggles to gain recognition as a viable vegetation management option. The recently published handbook Targeted Grazing: A Natural Approach to Vegetation Management and Landscape Enhancement was organized and written largely by range scientists to provide the scientific basis for targeted grazing. However, it did not provide much information on the practical and daily management decisions required by contract graziers and land managers. While the scientific basis for targeted grazing provides the foundation for understanding and improving this technology, as with all grazing management it is the daily operations and decisions that determine its success. The diversity of situations to which this tool can be applied necessitates the exchange of real-life experiences to promote learning among practitioners and to inform land managers of the successful programs and potential pitfalls to avoid....The Rangelands archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform March 202