Click here to access/download;Manuscript;Advances in Pantographic Structures.pdf Click here to view linked References Francesco dell'Isola et al.

Derived from laser cladding, the direct laser metal deposition (DLMD) process is based upon a laser beam–powder–melt pool interaction and enables the manufacturing of complex 3D shapes much faster than conventional processes. However, the surface finish remains critical, and DLMD parts usually necessitate postmachining steps. Within this context, the focus of our work is to improve the understanding of the phenomena responsible for deleterious surface finish by using numerical simulation. Mass, momentum, and energy conservation equations are solved using comsol multiphysics® in a 2D transient model including filler material with surface tension and thermocapillary effects at the free surface. The dynamic shape of the molten zone is explicitly described by a moving mesh based on an arbitrary Lagrangian–Eulerian method (ALE). This model is used to analyze the influence of the process parameters, such as laser power, scanning speed, and powder feed rate on the melt pool behavior. The simulations of a single layer and multilayer claddings are presented. The numerical results are compared with experimental data, in terms of layer height, melt pool length, and depth of penetration, obtained from high speed camera. The experiments are carried out on a widely used aeronautical alloy (Ti–6Al–4 V) using a Nd:YAG laser. The results show that the dilution ratio increases with increasing the laser power and the scanning velocity or with decreasing the powder feed rate. The final surface finish is then improved.

Metal powder bed fusion techniques can be used to build parts with complex internal and external geometries. Process parameters are optimized in order to obtain parts with low surface roughness and porosity, while maintaining a high productivity rate. The goal of this work is to quantify the sensitivity to internal and surface defects on the fatigue endurance of additively manufactured metallic parts. 316L Stainless Steel samples were fabricated through powder bed fusion using identical contour parameters, but three different hatching strategies were applied by varying the scanning speeds in the internal portions of the parts. Samples were subsequently mirror-polished to smooth the rough as-built surface. X-ray computed tomography analysis revealed several defect populations in samples from all three parametric sets due to lack of fusion in the bulk, with a nearly fully dense external "shell". High cycle fatigue tests at R = 0.1 were then performed on the specimens and combined with the X-ray computed tomography scans, helping to identify the largest and the critical defect size at which crack initiation occurred. Most fatigue failures initiated within the external contour zone for small (< 100 μm) defects, even when larger (> 200 μm) lack of fusion defects were widely present below the surface. It was determined that the high porosity (1% in volume or above 5% in area at some fabricated layers) observed in the bulk of parts manufactured with high scanning speeds had little impact on the fatigue limit of the material.

The systematic occurrence of porosities inside selective laser melted (SLM) parts is a well-known phenomenon. In order to improve the density of SLM parts, it is important not only to assess the physical origin of the different types of porosities, but also to be able to measure as precisely as possible the porosity rate so that one may select the optimum manufacturing parameters. Considering 316 L steel parts built with different input energies, the current paper aims to (1) present the different types of porosities generated by SLM and their origins, (2) compare different methods for measuring parts density and (3) propose optimal procedures. After a preliminary optimization step, three methods were used for quantifying porosity rate: the Archimedes method, the helium pycnometry and micrographic observations. The Archimedes method shows that results depend on the nature and temperature of the fluid, but also on the sample volume and its surface roughness. During the micrographic observations, it has been shown that the results depend on the magnification used and the number of micrographs considered. A comparison of the three methods showed that the optimized Archimedes method and the helium pycnometry technique gave similar results, whereas optimized micrographic observations systematically underestimated the porosity rate. In a second step, samples were analyzed to illustrate the physical phenomena involved in the generation of porosities. It was confirmed that: (1) low Volume Energy Density (VED) causes non-spherical porosities due to insufficient fusion, (2) in intermediary VED the small amount of remaining blowhole porosities come from gas occlusion in the melt-pool and (3) in excessive VED, cavities are formed due to the keyhole welding mode.

Recently the exotic properties of pantographic metamaterials have been investigated, and various mathematical models (both discrete and continuous) have been introduced. However, the experimental evidence available up to now concerns only polyamide specimens. In this paper, we use specimens printed using metallic powder. We prove experimentally that the main qualitative and quantitative features of pantographic sheets in planar deformation are independent of the constituting materials, at least when they can be regarded as homogeneous and isotropic at micro-level. Of course, the absolute value of Young's modulus of constituent Communicated by Francesco dell'Isola.

Optimized 316L steel samples were manufactured using laser powder bed fusion and tested in high cycle fatigue at R=0.1. They showed microstructural crack initiation and outstanding fatigue properties.Additional fatigue testings were then carried out on samples containing deterministic defects of various sizes and positions. All results summarized in a Kitagawa-Takahashi diagram show that the critical defect size is around 20 µm for surface defects and reach 380 µm for internal pores. Fracture surface analysis revealed that the large size gap between surface and internal fatigue crack initiation could be linked to the local gaseous environment in the pores.

The use of large beams in the Laser Powder Bed Fusion (L-PBF) process has been receiving increasing attention for the past few years and may widen the dissemination of this technology in the industry, as well as help increase the production volume. In this paper, a detailed comparison is presented between a usual 80 μm diameter Gaussian laser spot and a 500 μm diameter top-hat laser beam. The following benefits of a large and homogeneous beam could be demonstrated: (1) a moderate increase of productivity by reducing the number of scan lines, (2) a nearly total suppression of spatters and powder bed degradation (local loss of powder homogeneity caused by the redeposition of spatters) due to the low volume energy densities carried out and the limitation of deleterious vaporization effects, (3) the manufacturing of near fully dense Inconel 625 parts, especially in the hatching zones. Last, the occurrence of larger thermal effects induced by the large beam L-PBF was discussed by comparing two distinct definitions of the laser energy density: at a local (melt pool) scale, and at a global (the whole manufactured part) level.

This article discusses the results of the first attempts to print metal pantographic structures with perfect pivots, i.e. with rotational hinges connecting the two families of fibers composing the network without any stiffness. On the one hand, it is observed that perfect pivots do not behave as expected theoretically. On the other hand, the force measured during a bias extension test is a few orders of magnitude lower than that measured for pantographic structures with standard pivots (where a certain stiffness is associated to the interconnecting hinges). This leads to considering the pivots as quasi-perfect (non-zero stiffness, but neither the theoretical one which can be computed by means of the geometrical features of the pivot). Numerical simulations complete the analysis by showing how, by modulating the torsional stiffness of the pivots, it is possible to reproduce the force-displacement plot both in the case of standard pivots and with quasi-perfect ones.