Laurentiu Nastac

Numerical modeling of solidification morphologies and segregation patterns in cast dendritic alloys

A comprehensive stochastic model for simulating the evolution of dendritic crystals during the solidification of binary alloys was developed. The model includes time-dependent calculations for temperature distribution, solute redistribution in the liquid and solid phases, curvature, and growth anisotropy without further assumptions on the nucleation and growth of dendritic crystals. Stochastic procedures previously developed by Nastac and Stefanescu (Modell. Simul. Mater. Sci. Engng, 1997, 5(4), 391) for simulating dendritic grains were used to control the nucleation and growth of dendrites. A numerical algorithm based on an Eulerian–Lagrangian approach was developed to explicitly track the sharp solid/liquid (S/L) interface on a fixed Cartesian grid. Two-dimensional mesoscopic calculations (i.e. at the dendrite tip length scale) were performed to simulate the evolution of columnar and equiaxed dendritic morphologies including the formation of the columnar-to-equiaxed transition. The effects of solutal and curvature undercoolings on the evolution of both the dendrite morphology and segregation patterns during the solidification of binary alloys were analyzed in detail.

Macrotransport-solidification kinetics modeling of equiaxed dendritic growth: Part I. Model development and discussion

An analytical model that describes solidification of equiaxed dendrites has been developed for use in solidification kinetics-macrotransport modeling. It relaxes some of the assumptions made in previous models, such as the Dustin-Kurz, Rappaz-Thevoz, and Kanetkar-Stefanescu models. It is assumed that nuclei grow as unperturbed spheres until the radius of the sphere becomes larger than the minimum radius of instability. Then, growth of the dendrites is related to morphological instability and is calculated as a function of melt undercooling around the dendrite tips, which is controlled by the bulk temperature and the intrinsic volume average concentration of the liquid phase. When the general morphology of equiaxed dendrites is considered, the evolution of the fraction of solid is related to the interdendritic branching and dynamic coarsening (through the evolution of the specific interfacial areas) and to the topology and movement of the dendrite envelope (through the tip growth velocity and dendrite shape factor). The particular case of this model is the model for globulitic dendrite. The intrinsic volume average liquid concentration and bulk temperature are obtained from an overall solute and thermal balance around a growing equiaxed dendritic grain within a spherical closed system. Overall solute balance in the integral form is obtained by a complete analytical solution of the diffusion field in both liquid and solid phases. The bulk temperature is obtained from the solution of the macrotrasport-solidification kinetics problem.

Macrotransport-Solidification Kinetics Modeling of Equiaxed Dendritic Growth: Part II. Computation Problems and Validation on INCONEL 718 Superalloy Castings

In Part I of the article, a new analytical model that describes solidification of equiaxed dendrites was presented. In this part of the article, the model is used to simulate the solidification of INCONEL 718 superalloy castings. The model was incorporated into a commercial finite-element code, PROCAST. A special procedure called microlatent heat method (MLHM) was used for coupling between macroscopic heat flow and microscopic growth kinetics. A criterion for time-stepping selection in microscopic modeling has been derived in conjunction with MLHM. Reductions in computational (CPU) time up to 90 pct over the classic latent heat method were found by adopting this coupling. Validation of the model was performed against experimental data for an INCONEL 718 superalloy casting. In the present calculations, the model for globulitic dendrite was used. The evolution of fraction of solid calculated with the present model was compared with Scheil’s model and experiments. An important feature in solidification of INCONEL 718 is the detrimental Laves phase. Laves phase content is directly related to the intensity of microsegregation of niobium, which is very sensitive to the evolution of the fraction of solid. It was found that there is a critical cooling rate at which the amount of Laves phase is maximum. The critical cooling rate is not a function of material parameters (diffusivity, partition coefficient,etc.). It depends only on the grain size and solidification time. The predictions generated with the present model are shown to agree very well with experiments.

Advances in investment casting of Ti–6Al–4V alloy: a review

Abstract This paper presents a comprehensive review of the state of the art and challenges involved in investment casting of Ti–6Al–4V (Ti–6–4) alloy to improve the quality, costs and manufacturing of Ti parts. More than one hundred selected reports and articles on melting, casting and secondary processes such as hot isostatic pressing (HIPing) and heat treatment of the Ti–6–4 alloy system were reviewed to prepare this paper. The microstructure, composition and mechanical properties of Ti–6–4 alloy are discussed in detail. One of the main objectives of this paper was to identify means of improving the quality of castings, e.g. improving the mechanical properties through microstructure development and control to eliminate the casting factor, in particular for castings for use in aerospace applications. This work supports the M777 lightweight howitzer (LWH) programme, where investment casting was selected over machining and welding titanium plate to reduce part count and associated manufacturing expense for several LWH components. Another goal of this work was better to understand the investment casting process of Ti–6–4 alloy to eliminate/minimise defects in LWH components and provide useful insights to meet aggressive schedule requirements by minimising experimental production trials.

Experimental and Numerical Analysis of the 6061-Based Nanocomposites Fabricated via Ultrasonic Processing

There is strong evidence showing that microstructure and mechanical properties of a cast component can be considerably improved if nanoparticles are used as a reinforcement to form a metal-matrix nanocomposite. In this paper, 6061 nanocomposite castings were fabricated using the ultrasonic stirring technology (UST). The 6061 alloy and Al2O3/SiC nanoparticles were used as the matrix alloy and the reinforcement, respectively. Nanoparticles were injected into the molten metal and dispersed by ultrasonic cavitation and acoustic streaming. The applied UST parameters in the current experiments were used to validate a recently developed multiphase computational fluid dynamics (CFD) model, which was used to model the nanoparticle dispersion during UST processing. The CFD model accounts for turbulent fluid flow, heat transfer, and the complex interaction between the molten alloy and nanoparticles using the ANSYS’s fluent dense discrete phase model (DDPM). The modeling study includes the effects of ultrasonic probe location and the initial location where the nanoparticles are injected into the molten alloy. The microstructure, mechanical behavior, and mechanical properties of the cast nanocomposites have been also investigated in detail. The current experimental results showed that the tensile strength of the as-cast-reinforced 6061 alloy with Al2O3 or SiC nanoparticles increased slightly while the elongation increased significantly. The addition of the Al2O3 or SiC nanoparticles in 6061 alloy matrix changed the fracture mechanism from brittle dominated to ductile dominated.

Analytical modeling of solute redistribution during the initial unsteady unidirectional solidification of binary dilute alloys

Abstract Existing analytical models for calculating solute redistribution during the initial transient (unsteady) unidirectional solidification with an axially moving boundary of binary dilute alloys were reviewed. The analytical solution obtained by Smith, Tiller, Rutter (STR) [Can. J. Phys. 33 (1955) 723] for semi-infinite domains was derived independently in this work. In obtaining the solution, STR used Laplace transform technique. In this work, it was rigorously proved by using Laplace transform, nondimensional analysis, and by eliminating the advection term in Eq. (1) , that the analytical solution found by STR is indeed “exact” and “unique” under the stated assumptions. A thorough comparison between the exact solution and some approximate solutions is provided for partition distribution coefficients smaller and larger than one. Transient and quasi-steady-state results obtained with the exact analytical solution for segregation profiles in the liquid and at the solid/liquid interface, liquid concentration gradient at the solid/liquid interface, and solutal boundary layer are discussed in details. The size of the initial transient region is calculated. The exact solution is then applied to investigate based on thermodynamic arguments the instability of the solid/liquid interface during the initial solidification regime of dilute alloys.

Ultrasonic Cavitation-Assisted Molten Metal Processing of Cast A356-Nanocomposites

There is strong evidence that the mechanical properties of a cast component can be considerably improved if nanoparticles are used as a reinforcement to form a metal-matrix-nano-composite (MMNC).In this paper, Al2O3 and SiC nanoparticles reinforced A356 matrix composite castings were fabricated by using ultrasonic technology (UST). The A356 alloy and Al2O3/SiC nanoparticles were used as the matrix alloy and the reinforcement, respectively. Nanoparticles were inserted into the molten metal and dispersed by ultrasonic cavitation and acoustic streaming to avoid agglomeration. The microstructures and mechanical properties of the cast nano-composites were investigated in detail. The results showed that microstructures were greatly refined and with the addition of nanoparticles, tensile strength, yield strength and elongation increased significantly. Since the ultrasonic energy was concentrated in a small region under the ultrasonic probe, it is difficult to ensure proper cavitation and acoustic streaming for efficient dispersion of the nanoparticles without determining the suitable ultrasonic parameters via modeling and simulation. Accordingly, another objective of this paper was to develop well-controlled UST experiments that will be used in the development and validation of an UST dispersion modeling and simulation tool.

Multiscale modeling of the solidification microstructure evolution in the presence of ultrasonic stirring

Ultrasonic treatment (UST) was studied to improve the quality of cast ingots as well as to control the solidification microstructure evolution. Ultrasonically-induced cavitation consists of the formation of small cavities (bubbles) in the molten metal followed by their growth, pulsation and collapse. These cavities are created by the tensile stresses that are produced by acoustic waves in the rarefaction phase. The cavitation threshold pressure for nucleation of the bubbles may decrease with increasing the amount of dissolved gases and especially with the amount of inclusions in the melt. A UST model was developed to predict the ultrasonic cavitation and acoustic streaming. The developed UST modeling approach is based on the numerical solution of Lilley model (that is founded on Lighthillss acoustic analogy), fluid flow, and heat transfer equations, and mesoscopic modeling of the grain structure. The UST model was applied to study the solidification of Al-based alloys) under the presence of ultrasound. It is found that the predicted ultrasonic cavitation region is relatively small, the acoustic streaming is strong and thus the created/survived bubbles/nuclei are transported into the bulk liquid quickly. The predicted grain size under UST condition is at least one order of magnitude lower than that without UST, which is in excellent agreement with the experimental data.

Numerical modelling of fluid flow and desulphurisation kinetics in an argon-stirred ladle furnace

A full-scale, three-dimensional, transient CFD modelling approach capable of predicting the three-phase fluid-flow characteristics and desulphurisation behaviour in an argon-stirred ladle was developed. The model can accurately predict the molten steel flow and slag eye behaviour. The predicted sulphur content in ladle as a function of time agrees well with the experimental data. The effects of the initial sulphur content, the gas flow rate and the slag layer thickness on the desulphurisation efficiency were also investigated. The predicted results show that the desulphurisation efficiency improves with the increase of the initial sulphur content, the gas flow rate and the slag layer thickness. Higher gas flow rate can improve the slag–steel interaction, which, in turn, helps improving the desulphurisation rate. The thinner the slag layer, the larger the slag eyes and the smaller the interfacial area between the slag and steel phases. The consequence is the decrease in the desulphurisation rate.

Numerical modeling of the gas evolution in furan binder-silica sand mold castings

Modelling of gas evolution during sand-mould castings is one of the most important technical and environmental issues facing the metal casting industry. The current effort focused on developing the capability of numerically predicting the gas evolution for the furan binder-silica sand system. Specifically, the decomposition of furan was experimentally analyzed and then predicted based upon the work developed in the current project. This methodology can be easily implemented into existing commercial casting codes. A parametric study was also performed for steel 4340 and aluminium A356 cylinders (D100 × H200 m) and bars (H50 mm × W50 mm × L250 mm) cast into silica sand moulds (furan binder) of 50-mm mould wall thickness to investigate the effects of superheat and heating/cooling conditions of the mould on the gas evolution. Such information would enable more technically and environmentally friendly decisions to be made concerning the process design used to make a given casting.