https://hal.archives-ouvertes.fr/hal-02064113/document

Texture control of 316L parts by modulation of the melt pool morphology in selective laser melting

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 ( 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.

https://hal.archives-ouvertes.fr/hal-02391334/document

A competition between the contour and hatching zones on the high cycle fatigue behaviour of a 316L stainless steel: Analyzed using X-ray computed tomography

Grain refinement is an important opportunity to improve mechanical properties of fusion welds and the weldability (susceptibility to solidification cracking) of the base metal. In this study, grain refinement was achieved in aluminium welds through additions of grain refiner to the weld metal. Increasing grain refiner additions led to a decrease of the weld metal mean grain size (down to −86 %). The grain refinement efficiency was the highest in commercial pure Al (Alloy 1050A, Al 99.5), followed by Alloy 6082 (Al Si1MgMn) and Alloy 5083 (Al Mg4.5Mn0.7). To investigate this clear influence of alloy content on grain size, the undercooling parameters P and Q were calculated. Temperature measurements revealed that solidification parameters such as solidification growth rate or cooling rate vary significantly along the solidification front, dependent upon torch speed and alloy. On the basis of this comparison, an analytical approach was used to model the columnar to equiaxed transition (CET). Moreover, wavelength dispersive X-ray spectroscopy (WDS) and transmission electron microscopy (TEM) analyses revealed particles rich in Ti and B that are probably TiB2 particles coated by Al3Ti nucleating Al grains during solidification. Also, Ti/B contents needed in commercial filler wires to allow optimum weld metal grain refinement were calculated dependent upon base alloy and welding process.

Understanding grain refinement in aluminium welding

Le procede de fusion laser selective de lit de poudre, egalement appele SLM, permet de fabriquer des pieces metalliques en fusionnant des couches de poudre. Cette methode novatrice donne acces a un large eventail de pieces aux geometries complexes, permettant notamment d’alleger les structures. Toutefois, la bonne tenue mecanique de ce type de pieces, en particulier dans le domaine de la fatigue, reste un enjeu industriel majeur. Les pieces elaborees par SLM peuvent en effet contenir des pores (debouchants ou internes) pouvant deteriorer leurs proprietes mecaniques. Les travaux realises ont pour but de caracteriser l’influence de defauts poreux sur l’endurance en fatigue a grand nombre de cycles de pieces en acier 316L fabriquees par SLM, et s’articulent autour de trois parties. La premiere consiste a identifier les parametres de fabrication SLM controlant la densification et la microstructure des pieces. Une distinction sera faite entre les differents types de pores crees, dont la morphologie et les dimensions dependent des conditions energetiques de l’interaction laser-matiere. Les pores crees seront mis en perspective par la microstructure du materiau brut, dont l’orientation cristallographique et la taille de grain est principalement reliee au recouvrement et la morphologie des cordons. Le deuxieme aspect des travaux a consiste a utiliser les resultats de la recherche parametrique pour generer des eprouvettes de fatigue contenant differentes populations de defauts aleatoires (stochastiques) internes observees en tomographie a rayons X, tout en conservant des microstructures similaires. L’influence relative des populations de defauts internes creees sur l’endurance en fatigue est quantifiee et comparee a la tenue d’eprouvettes optimisees contenant un taux de porosite minimal. Enfin, des defauts modeles internes (deterministes) aux dimensions variables, et dont la position et la morphologie sont controlees, ont ete generes apres optimisation parametrique dans des pieces denses. Un seuil d’amorcage sur defaut interne par rapport aux defauts de surface a ainsi pu etre degage, et pourrait etre lie a l’environnement gazeux local lors de l’amorcage et la propagation de la fissure

Nocivité en fatigue et contrôle de défauts produits par fabrication additive

Grain refinement is an important opportunity to improve mechanical properties of fusion welds and the weldability (cracking sensitivity) of the base metal. In this thesis, grain refinement was achieved for aluminium welds by additions of a grain refiner. For this purpose, inserts consisting of aluminium base metal and small additions of commercial Al Ti5B1 grain refiner were cast, deposited in base metal plates, and fused in a gas tungsten arc (GTA) welding process. As a result, higher grain refiner additions increased the weld’s titanium and boron content resulting in a significant decrease in the weld metal mean grain size up to 86%. This grain size reduction led to a transition from predominantly columnar to equiaxed grain shape (columnar to equiaxed transition CET). The grain refinement was thereby found to be strongly dependent upon the base metal chemical composition. Accordingly, the grain refining efficiency was the highest in commercial pure Al (Alloy 1050A, Al 99.5), followed by Alloy 6082 (Al Si1MgMn) and Alloy 5083 (Al Mg4.5Mn0.7). In this regard, the parameters P and Q were applied to investigate the influence of alloying elements on the supply of constitutional undercooling during solidification and on final grain size. Also, WDS (wavelength dispersive x-ray spectroscopy) and TEM (transmission electron microscopy) analysis found an increasing number of particles rich in Ti and B. These substrates are probably TiB2 particles coated by Al3Ti likely nucleating Al grains during solidification. The variation in torch speed showed that increasing torch speeds support the CET effect leading to many small and equiaxed grains at high torch speed. To give explanations for this observation, the thermal conditions, that are controlled by welding parameters such as torch speed, were determined with temperature measurements via thermocouples. These measurements revealed that solidification parameters like solidification growth rate, cooling rate, (local) thermal gradient and solidification time vary significantly along the solidification front (from weld centreline to weld fusion line). In a further step, the solidification parameters were related to the corresponding grain size and shape. On the basis of this comparison, an analytical approach was used to model the CET. This allowed the prediction of critical values for both solidification growth rate and thermal gradient, at which the CET occurs in aluminium weld metal. The influence of grain refinement on the weld mechanical properties was investigated in tensile tests. Accordingly, the ductility of Alloy 5083 welds was increased through grain refinement whereas no improvement in weld metal strength was observed. Furthermore, tear tests with notched specimens revealed for Alloy 1050A that the resistance against initiation and propagation of cracks in the weld metal can be enhanced through grain refinement. In addition, when welding Alloy 6082, weld metal grain refinement prevented the formation of centreline solidification cracking that was present only in welds with unrefined grain structure. On the basis of the above experiments, the Ti/B contents needed in commercial filler wires or rods to allow optimum weld metal grain refinement were estimated. Accordingly, this work gives specific recommendations to filler material producers through a simple calculation that considers the influence of base alloy and welding process. The results show that the Ti/B contents defined by the corresponding standards for filler alloys are too low to allow weld metal grain refinement.

https://depositonce.tu-berlin.de/bitstream/11303/4108/1/schempp_philipp.pdf

Grain refinement in aluminium GTA welds

/pdf/effect-of-welding-condition-on-solidification-structure-in-4a0ou5q0wm.pdf

Akira Matsui

Papers

Effect of Welding Condition on Solidification Structure in Weld Metal of Aluminum Alloy Sheets