A B S T R A C TIn this study, 316L parts were fabricated with the selective laser melting additive layer manufacturing process using unidirectional laser scan to control their texture. The melt pool shape, microstructure and texture of three different cubic samples were analyzed and quantified using optical microscopy and electron back-scattered diffraction. The samples scanned along the shielding gas flow direction were shown to exhibit shallow conduction melt pools together with a strong {110} < 001 > Goss texture along the laser scanning direction. The sample prepared with a laser scan perpendicular to the gas flow direction had deeper melt pools, with a weaker {110} < 001 > Goss texture in addition to a < 100 > fiber texture parallel to the scanning direction. Correlations were proposed between the melt-pool geometry and overlap and the resulting texture. The decrease of the melt pool depth was assumed to be linked to local attenuation of the laser beam effective power density transmitted to the powder bed.

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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.

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.

Laser additive manufacturing enables economical production of complex lightweight structures. To realize the potential benefits of additive manufacturing technology in industrial applications, the fatigue performance of parts additively manufactured materials must be modelized. The aim of this paper is to present a new modeling approach combining plasticity and damage, and appropriate for as-built Laser-Powder Bed Fusion (LPBF) structures. The model presented is an extension of the Dang Van criterion, including damage, defined as porosity in the case of LPBF. Attention is focused on the integration of damage in a fatigue criterion using the concept of elastic shakedown. Finally, the case of 316L will illustrate the results of the model by fatigue tests with deterministic defects.