In laser powder bed fusion (L-PBF) process, a deposited powder layer is melted by a laser and solidifies when the laser moves away. During solidification, the microstructure of the part is formed due to both epitaxial grain growth and nucleation. This structure has a strong influence on the final mechanical properties of parts and is influenced by the choice of process parameters such as the laser power and velocity or the scanning strategy. As a consequence, the prediction and characterization of this microstructure is of prime interest considering size, crystallographic orientations and shapes of grains. Among the approaches reported in the literature to model microstructure development, the Cellular Automaton (CA) method is a relevant choice to describe grain structure evolution. This model has been adapted to investigate microstructure formation during L-PBF process applied on an IN718 nickel-base superalloy. The steady state thermal behaviour at the scale of the melt pool is analysed and used to compute the development of the structure. This structure is computed at the scale of the part also considering realistic scanning strategies.

Grain structures generated by laser powder bed fusion (L-PBF) of the Ni-based superalloy Inconel 718 (IN718) are studied using Cellular Automaton modeling. A hybrid methodology is developed to benefit from a full thermohydraulic simulation that predicts the shape of the consolidated track and the temperature field. The advantage is to reach a large size of the simulation domain while still taking advantage of a thermohydraulic numerical solution of the L-PBF process at the scale of the melt pool. The computed grain structure becomes available in a Representative Elementary Volume (REV) of an Additively Manufactured (AMed) specimen and can be studied statistically considering the distribution of crystallographic orientations and the grain density. It is also possible to vary process and material parameters to approach industrial practice. The strategy consisting in opposite bidirectional scanning of the powder with an additional 67° rotation at each new layer is demonstrated. It is compared with the standard opposite bidirectional monoaxial scanning strategy and the opposite bidirectional orthogonal biaxial scanning strategies with two perpendicular axes, all reproducing trends reported in experimental literature. The generation of virtual REVs of AMed microstructures by L-PBF for IN718 opens the way to study new processing parameters and coupling with models for the prediction of metallurgical properties.

To evaluate the interactions between hydrogen and the mechanical behavior of additively manufactured (AM) 17-4PH martensitic stainless steel (MSS), stress-relaxation tests were performed for both non-charged and hydrogen-charged samples. Similar tests were performed for the conventional 17-4PH MSS counterpart (CM). The results clearly indicated significant differences in the relaxation properties between the CM and AM MSSs in the H900 state, which were attributed to the higher reversed austenite amount in the AM MSS than that in the CM MSS. In particular, the influence of hydrogen on the dislocation mobility in the austenite was clearly shown.

Materials produced by additive manufacturing (AM) often have different microstructures from those obtained using conventional metallurgy (CM), which can have significant impacts on the materials' durability, and in particular, resistance to corrosion. In this study, we were concerned with the susceptibility to pitting and environmentally assisted cracking (EAC) of 17-4PH martensitic stainless steel (MSS). We focused on the evolution from pitting to EAC, and the behaviour of MSS produced by AM was compared with that of its CM counterpart. Potentiodynamic polarisation tests were combined with chronoamperometry measurements performed without and with mechanical loading to study both stable and metastable pitting and the influence of stress on these processes. EAC tests were carried out and combined with observations of fracture surfaces. MSS produced by AM was more resistant to pit initiation due to fewer and finer NbC particles. However, the propagation kinetics of stable pits were higher for this MSS due to a higher amount of reversed austenite. The stress was found to stabilise the metastable pits and to accelerate the propagation of stable pits, which resulted in an increased susceptibility to EAC of the MSS produced by AM. These results clearly highlighted the fact that the reversed austenite amount has to be perfectly controlled in AM processes.

The aim of this work was to compare the microstructures of 17-4PH martensitic stainless steels (MSSs) obtained by conventional manufacturing (CM), and additive manufacturing (AM) using laser beam melting (LBM) process. Both materials were studied after the same H900 heat treatment. Significant differences in microstructure were observed between the two MSSs, with a higher austenite content for the AM-H900 sample, as compared to the CM-H900 sample. Transmission electron microscopy (TEM) analyses allowed to identify both retained and reversed austenite in the AM-H900 sample, but most part of the austenite detected was found to be reversed austenite. The high amount of reversed austenite in the AM-H900 sample was attributed to a heterogeneous distribution in austenite-stabilising elements in the solution heat treated AM sample, leading to a decrease in Ms value and subsequent increase in the driving force for the martensite to austenite transformation during the ageing at 480 degrees C. Moreover, TEM analyses highlighted thinner martensite laths for the AM-H900 steel as compared to the CM-H900 one. This was interpreted as an increase in both nucleation sites and growth rate for reversed austenite. Despite those differences in microstructure, the AM-H900 and CM-H900 samples showed similar tensile behaviour, with similar UTS and Rp0.2 values, but a decrease in the strain to fracture was observed for the AM-H900 sample, probably related to the pores and/or to intergranular carbides.

Thermal loadings representative of a welding cycle at a point in the Heat Affected Zone (HAZ) of an Electron Beam Welding (EBW) have been reproduced experimentally. It means that the temperature is continuously varying, first increasing (during heating) and then decreasing (during cooling) without dwell-time at the highest temperature. Tensile tests have been carried out either during heating or during cooling of the specimen. The mechanical properties of two idealized phases - T6 temper and O temper - have been estimated as a function of temperature. To validate the accuracy of this thermo-mechanical database, a comparison between measured and calculated stress-strain curve is then presented which shows that this approach may be useful to predict the tensile behaviour of this alloy at high temperature whatever the experienced temperature history representative of EBW. Finally, a comparison between numerical and experimental residual stresses is presented for a beam girth weld application.

Chapter 2 Laser and Electron Beam Welding of 6xxx Series Aluminum Alloys – On Some Thermal, Mechanical and Metallurgical Aspects Daniel Nélias, Daniel NéliasSearch for more papers by this authorMuhammad Zain-ul-Abdein, Muhammad Zain-ul-AbdeinSearch for more papers by this authorDaniel Maisonnette, Daniel MaisonnetteSearch for more papers by this author Daniel Nélias, Daniel NéliasSearch for more papers by this authorMuhammad Zain-ul-Abdein, Muhammad Zain-ul-AbdeinSearch for more papers by this authorDaniel Maisonnette, Daniel MaisonnetteSearch for more papers by this author Book Editor(s):Jean-Michel Bergheau, Jean-Michel BergheauSearch for more papers by this author First published: 21 January 2014 https://doi.org/10.1002/9781118578759.ch2 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onEmailFacebookTwitterLinkedInRedditWechat Summary The work presented in this chapter provides the outline of two recent experimental and numerical analyses on the welding of 6xxx series aluminum alloys (AA). The first analysis, which was carried out by M. Zain-ul-abdein in 2009, mainly focused on the experimental investigation and numerical simulation of laser beam welding (LBW) induced residual stresses and distortions in thin plates of an aluminum alloy AA 6056-T4. The second analysis, which was carried out by D. Maisonnette in 2010, primarily discussed the effect of high temperature on the mechanical and metallurgical properties of an aluminum alloy AA 6061-T6 subjected to electron beam welding (EBW). The role of material properties, phase transformations, coupling of thermal and mechanical analyses, heat source model and boundary conditions has also been investigated to an appreciable extent. A brief literature review of these is presented in the chapter. 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In age-hardening alloys, high-temperature processes, such as welding, can strongly modify the precipitation state, and thus degrade the associated mechanical properties. The aim of this paper is to present a coupled approach able to describe precipitation and associated yield stresses for non-isothermal treatments of a 6061 aluminium alloy. The precipitation state (in terms of volume fraction and precipitate size distribution) is modelled thanks to a recent implementation of the classical nucleation and growth theories for needle-shaped precipitates. The precipitation model is validated through small-angle neutron scattering and transmission electron microscopy experiments. The precipitation size distribution is then used as an entry parameter of a micromechanical model for the yield strength of the alloy. Predicted yield stresses are compared to tensile tests performed with various heating conditions, representative of the heat-affected zone of a welded joint.

This paper describes the mechanical behavior of the 6061-T6 aluminium alloy at room temperature for various previous thermal histories representative of an electron beam welding. A fast-heating device has been designed to control and apply thermal loadings on tensile specimens. Tensile tests show that the yield stress at ambient temperature decreases if the maximum temperature reached increases or if the heating rate decreases. This variation of the mechanical properties is the result of microstructural changes which have been observed by Transmission Electron Microscopy (TEM). (C) 2010 Elsevier B.V. All rights reserved.

Le travail de these presente dans ce memoire s'inscrit dans le cadre des actions de qualification anticipee du caisson du RJH (Reacteur Jules Horowitz). Le caisson est une enceinte sous pression fabriquee en alliage d'aluminium 6061-T6. Il s'agit d'un alliage a durcissement structural dont les proprietes mecaniques sont etroitement liees a l'etat de precipitation dans le materiau. Des essais de traction ont permis de mesurer l'evolution des proprietes mecaniques du materiau ayant subi des chargements thermiques representatifs d'une operation de soudage par faisceaux d'electrons (soudage FE). Des essais ont ete menes a temperature ambiante et a chaud. Des observations metallurgiques par Diffusion de Neutrons au Petits Angles (DNPA) et par Microscopie Electronique en Transmission (MET) donnent par la suite la taille et la fraction volumique des precipites. Les resultats de ces campagnes d'essais permettent de comprendre l'evolution des proprietes mecaniques. Ces experimentations ont ensuite ete utilisees pour mettre en place un modele metallurgique permettant de calculer les caracteristiques de l'etat de precipitation. Ce modele etant couple a un modele de durcissement structural, il calcule la limite d'elasticite du materiau pour divers chargements thermiques. Un modele mecanique phenomenologique a egalement ete developpe afin de modeliser le comportement d'une structure soumise a un chargement thermique.

6061 aluminum alloy is an age hardening alloy. Its mechanical characteristics are dependant of the precipitates contained in the material. The precipitates grow or dissolve with temperature, depending on the thermal history applied on the material. The aim of the project is to study the influence of the manufacturing processes used to create a part in AA6061. The study cannot be accomplished directly on the part and specimens representing the part have to be used. Thermal histories corresponding to the processes would be applied on the specimen. To do so, an experimental device able to simulate the processes on a specimen will be designed. This device uses Joule effect in order to reach very high heating rate. The heating device is installed on a tensile-torsion :machine. So, it can be used to measure the mechanical properties of the material submitted to various thermal loading.