Phase-Field Study of Polycrystalline Growth and Texture Selection During Melt Pool Solidification

Grain growth competition during solidification determines microstructural features, such as dendritic arm spacings, segregation pattern, and grain texture, which have a key impact on the final mechanical properties. During metal additive manufacturing (AM), these features are highly sensitive to manufacturing conditions, such as laser power and scanning speed. The melt pool (MP) geometry is also expected to have a strong influence on microstructure selection. Here, taking advantage of a computationally efficient multi-GPU implementation of a quantitative phase-field model, we use two-dimensional cross-section simulations of a shrinking MP during metal AM, at the scale of the full MP, in order to explore the resulting mechanisms of grain growth competition and texture selection. We explore MPs of different aspect ratios, different initial (substrate) grain densities, and repeat each simulation several times with different random grain distributions and orientations along the fusion line in order to obtain a statistically relevant picture of grain texture selection mechanisms. Our results show a transition from a weak to a strong $\langle10\rangle$ texture when the aspect ratio of the melt pool deviates from unity. This is attributed to the shape and directions of thermal gradients during solidification, and seems more pronounced in the case of wide melt pools than in the case of a deeper one. The texture transition was not found to notably depend upon the initial grain density along the fusion line from which the melt pool solidifies epitaxially

Scaling laws for two-dimensional dendritic crystal growth in a narrow channel

We investigate analytically and computationally the dynamics of 2D needle crystal growth from the melt in a narrow channel. Our analytical theory predicts that, in the low supersaturation limit, the growth velocity $V$ decreases in time $t$ as a power law $V \sim t^{-2/3}$, which we validate by phase-field and dendritic-needle-network simulations. Simulations further reveal that, above a critical channel width $\Lambda \approx 5l_D$, where $l_D$ the diffusion length, needle crystals grow with a constant $V<V_s$, where $V_s$ is the free-growth needle crystal velocity, and approaches $V_s$ in the limit $\Lambda\gg l_D$.Comment: 6 pages, 4 figures, one supplementary material document with 1 figur

On the occurrence of buoyancy-induced oscillatory growth instability in directional solidification of alloys

Recent solidification experiments identified an oscillatory growth instability during directional solidification of Ni-based superalloy CMSX4 under a given range of cooling rates. From a modeling perspective, the quantitative simulation of dendritic growth under convective conditions remains challenging, due to the multiple length scales involved. Using the dendritic needle network (DNN) model, coupled with an efficient Navier-Stokes solver, we reproduced the buoyancy-induced growth oscillations observed in CMSX4 directional solidification. These previous results have shown that, for a given alloy and temperature gradient, oscillations occur in a narrow range of cooling rates (or pulling velocity, $V_p$) and that the selected primary dendrite arm spacing ($\Lambda$) plays a crucial role in the activation of the flow leading to oscillations. Here, we show that the oscillatory behavior may be generalized to other binary alloys within an appropriate range of $(V_p,\Lambda)$ by reproducing it for an Al-4at.%Cu alloy. We perform a mapping of oscillatory states as a function of $V_p$ and $\Lambda$, and identify the regions of occurrence of different behaviors (e.g., sustained or damped oscillations) and their effect on the oscillation characteristics. Our results suggest a minimum of $V_p$ for the occurrence of oscillations and confirm the correlation between the oscillation type (namely: damped, sustained, or noisy) with the ratio of average fluid velocity $\overline V$ over $V_p$. We describe the different observed growth regimes and highlight similarities and contrasts with our previous results for a CMSX4 alloy

Using multicomponent recycled electronic waste alloys to produce high entropy alloys

The amount of electronic waste (e-waste) recycled worldwide is less than 20% of the total amount produced. In a world where the need for critical and strategic metals is increasing almost exponentially, it is unacceptable that tons of these elements remain unrecycled. One of the causes of this low level of recycling is that recycling is based on an expensive and complex selective sorting of metals. Extracting all metals simultaneously is much simpler and if this were done, it would significantly increase the recycling rate. Meanwhile, it was demonstrated that high entropy alloys (HEAs), which are in great demand in applications where very high performance is required, can be made from mixtures of complex alloys, hence reducing their dependence on pure critical metals. Here, we show that it is possible to obtain competitive HEAs from complex alloy mixtures corresponding to typical electronic waste compositions, combining two needs of high interest in our society, namely: to increase the level of recycling of electronic waste and the possibility of developing high-performance HEAs without the need of using critical and/or strategic metals. To validate our hypothesis that e-waste can be used to produce competitive HEAs, we propose an alloy design strategy combining computational thermodynamics (CalPhaD) exploration of phase diagrams and phenomenological criteria for HEA design based on thermodynamic and structural parameters. A shortlist of selected compositions are then fabricated by arc melting ensuring compositional homogeneity of such complex alloys and, finally, characterised microstructurally, using electron microscopy and diffraction analysis, and mechanically, using hardness testing

Modélisation des cinétiques de transformations multiples dans les alliages métalliques : étude de la microségrégation lors de la solidification dendritique, péritectique et eutectique d'alliages aluminium-nickel

Atomised powders of aluminium-nickel alloys may be processed to obtain Raney nickel, a catalyst used in numerous industrial processes. Catalytic activity strongly depends on the progress of the multiple solidification reactions during atomisation. A microsegregation model for metallic alloys solidification is thus developed. Considering finite diffusion fluxes, dendritic, peritectic and eutectic reactions kinetics and nucleation undercoolings, an advanced alternative to Gulliver-Scheil model or the lever rule is proposed. Coupling with thermodynamic equilibrium calculations is achieved to evaluate interfacial compositions and enthalpy terms in the energy balance. The model is applied to a binary alloy, with constant densities of phases, to simulate the atomisation process of Al-Ni alloy droplets. A dedicated model is chosen for the heat transfer boundary conditions. Results are compared to measurements from neutron diffraction experiments. Interpretations are thus given on the non trivial behaviour of rapidly solidified Al-Ni alloys. The proposed model allows estimating concurrent effects of different kinetics (chemical diffusion, heat balance, microstructures growth kinetics, etc.) during solidification out of equilibrium. Main prospective developments from this work include: extension to multicomponent alloys, introduction of variable densities, coupling with macroscopic calculations.Les poudres d'alliages aluminium-nickel produites par atomisation peuvent être traitées pour préparer du nickel de Raney, un catalyseur utilisé dans de nombreux procédés industriels. L'activité du catalyseur dépend fortement du déroulement des multiples réactions de solidification pendant l'atomisation. Un modèle de microségrégation pour la solidification d'alliages métalliques est alors développé. En considérant des flux de diffusion finis, des cinétiques de réactions dendritique, péritectique et eutectique et des surfusions de germination, une alternative plus évoluée est proposée aux modèles de Gulliver-Scheil ou de la loi des leviers. Le couplage avec des calculs d'équilibre thermodynamique est effectué pour évaluer les compositions des interfaces et les termes d'enthalpie dans le bilan d'énergie. Le modèle est appliqué à un alliage binaire, avec des densités de phases constantes, pour simuler le procédé d'atomisation de gouttes d'alliage Al-Ni. Un modèle dédié est choisi pour les conditions aux limites d'échange de chaleur. Les résultats sont comparés à des mesures expérimentales obtenues par diffraction de neutron. Des interprétations sont alors établies sur les comportements non triviaux des alliages Al-Ni solidifiés rapidement. Le modèle proposé permet ainsi d'appréhender les effets concurrents des différentes cinétiques (diffusion chimique, bilan d'énergie, vitesse croissance des microstructures, etc.) lors de la solidification hors équilibre. Les principaux développements envisageables autour de ce travail incluent : l'extension aux alliages multicomposés, l'inclusion de densités variables, le couplage avec des calculs macroscopiques

Modélisation des cinétiques de réactions multiples dans les alliages métalliques (étude de la microségrégation lors de la solidification dendritique, péritectique et eutectique d'alliages aluminium-nickel)

PARIS-MINES ParisTech (751062310) / SudocSOPHIA ANTIPOLIS-Mines ParisTech (061522302) / SudocSudocFranceF

Microsegregation modeling of multiple phase transformations - Atomization of Al-Ni alloys

International audienceA microsegregation model for the solidification of binary alloys is presented. It accounts for diffusion in all phases, and for me nucleation undercoolings and the growth kinetics of the solidifying microstructures forming from the liquid state. The model successively considers the occurrence of several phase transformations taking place in the presence of liquid, including one primary dendritic reaction, one or several peritectic reactions and one eutectic reaction. Volume averaged conservation equations for the mass of solute specie in each phase and at each interface are coupled with a heat balance of the domain assuming a uniform temperature. The diffusion fluxes at the interfaces between phases are calculated through the definition of characteristic microstructural diffusion lengths for which analytical expressions are derived. The model is applied to simulate the solidification of aluminium-nickel droplets produced by atomization. The model predicts the occurrence of a recalescence during the growth of each microstructure, and the progress of peritectic transformations consuming previously formed solid phases, as well as the average composition of each phase

A generalized segregation model for concurrent dendritic, peritectic and eutectic solidification

International audienceA microsegregation model for the solidification of binary alloys is presented. It accounts for diffusion in all phases, as well as nucleation undercooling and growth kinetics of the solidifying microstructures. It successively considers the occurrence of several phase transformations, including one or several peritectic reactions and one eutectic reaction. Volume-averaged mass conservation equations are coupled with a heat balance assuming a uniform temperature. The model is applied to simulate the solidification of Al–Ni atomized droplets. Coupled with an atomization model, it predicts recalescences, volume fractions of phases as well as average compositions during solidification. It eventually predicts the occurrence of a recalescence during the growth of each microstructure, and the progress of peritectic transformations consuming previously formed solid phases. The influence of several process parameters is evaluated. A comparison with experimental measurements is given, showing variation of phase fractions as a function of particle size for Al–Ni atomized droplets