Damage tolerant design of additively manufactured metallic components subjected to cyclic loading: State of the art and challenges.

Undoubtedly, a better understanding and the further development of approaches for damage tolerant component design of AM parts are among the most significant challenges currently facing the use of these new technologies. This article presents a thorough overview of the workshop discussions. It aims to provide a review of the parameters affecting the damage tolerance of parts produced by additive manufacturing (shortly, AM parts) with special emphasis on the process parameters intrinsic to the AM technologies, the resulting defects and the residual stresses. Based on these aspects, basic concepts are reviewed and critically discussed specifically for AM materials: Criteria for damage tolerant component design;Criteria for the determination of fatigue and fracture properties;Strategies for the determination of the fatigue life in dependence of different manufacturing conditions;Methods for the quantitative characterization of microstructure and defects;Methods for the determination of residual stresses;Effect of the defects and the residual stresses on the fatigue life and behaviour. We see that many of the classic concepts need to be expanded in order to fit with the particular microstructure (grain size and shape, crystal texture) and defect distribution (spatial arrangement, size, shape, amount) present in AM (in particular laser powder bed fusion). For instance, 3D characterization of defects becomes essential, since the defect shapes in AM are diverse and impact the fatigue life in a different way than in the case of conventionally produced components. Such new concepts have immediate consequence on the way one should tackle the determination of the fatigue life of AM parts; for instance, since a classification of defects and a quantification of the tolerable shapes and sizes is still missing, a new strategy must be defined, whereby theoretical calculations (e.g. FEM) allow determining the maximum tolerable defect size, and non-destructive testing (NDT) techniques are required to detect whether such defects are indeed present in the component. Such examples show how component design, damage and failure criteria, and characterization (and/or NDT) become for AM parts fully interlinked. We conclude that the homogenization of these fields represents the current challenge for the engineer and the materials scientist.

Thermal and Structural Characterization of a Titanium Carbide/Carbon Composite for Nuclear Applications.

In the framework of ISOL (isotope separation on-line) facilities, porous carbides are among the most employed target materials for the production of radioactive ion beams for research. As foreseen by the ISOL technique, a production target is impinged by an energetic particle beam, inducing nuclear reactions from such an interaction. The resulting radionuclides are subsequently released, thanks to the high target working temperature (1600-2000 °C); ionized; and extracted into a beam. Since the target microstructure and porosity play a fundamental role in the radionuclide release efficiency, custom-made target materials are often specifically produced, resulting in unknown thermal and structural properties. Considering that such targets might undergo intense thermal stresses during operation, a thermal and structural characterization is necessary to avoid target failure under irradiation. In the presented work, a custom-made porous titanium carbide that was specifically designed for application as an ISOL target was produced and characterized. The thermal characterization was focused on the evaluation of the material emissivity and thermal conductivity in the 600-1400 °C temperature range. For the estimation of a reference material tensile stress limit, the virtual thermoelastic parameter approach was adopted. In particular, for the aforementioned temperature range, an emissivity between 0.7 and 0.8 was measured, whereas a thermal conductivity between 8 and 10 W/mK was estimated.

Residual Notch Stress Intensity Factors in Welded Joints Evaluated by 3D Numerical Simulations of Arc Welding Processes.

Approaches based on calculating Residual Notch Stress Intensity Factors (R-NSIFs) assume the weld toe to be a sharp V-notch that gives rise to a residual singular stress distribution close to the weld toe. Once R-NSIFs are determined, they might be included in local fatigue criteria for the structural strength assessment of welded joints based on NSIFs due to external cyclic loading. However, the numerical calculation of R-NSIFs through finite element (FE) simulations of the welding process requires extremely refined meshes to properly capture the residual stress singularity. In this context, the Peak Stress Method (PSM) has recently been adopted to estimate R-NSIFs due to residual stresses by means of coarse meshes of 2D 4-node plane or 3D 8-node brick elements. The aim of this work is to investigate the applicability of the PSM to estimate R-NSIFs in a butt-welded joint using coarse meshes of 3D 10-node tetra elements. The R-NSIF distribution at the weld toe line is estimated by applying the PSM to coarse meshes of 3D 10-node tetra elements, and the results are in agreement with those obtained using 3D 8-node brick elements. Accordingly, the PSM based on tetra elements further enhances the rapid estimation of R-NSIFs using coarse meshes and could be effective in analyzing complex 3D joint geometries.

Thermal and Mechanical Characterization of Carbides for High Temperature Nuclear Applications.

In the facilities for the production of Radioactive Ion Beams (RIBs) according to the Isotope Separation On-Line (ISOL) technique, a production target is typically impinged by a high-power primary beam, generating radioactive isotopes for basic research and technological applications. With the aim to guarantee an efficient extraction of the aforementioned isotopes, the production target must work in a high vacuum environment, at temperatures that are usually between 1600 °C and 2200 °C. Its main components are often characterized by intense temperature gradients and consequently by severe thermal stresses. Carbides are widely used for target manufacturing, and in this work a specific method for their thermal and mechanical characterization is presented and discussed. It is based on the comparison between experimental measurements and numerical simulations, with the introduction of the novel Virtual Thermoelastic Parameters approach for the structural verification procedure. High-performance silicon carbides (SiC) are taken as a reference to describe the method. Measured emissivity and thermal conductivity data are presented and discussed, together with the experimental estimation of material limitations for both temperature and stress fields. The aforementioned results can be promptly used for the design process of high-power ISOL targets.