Direct Chill Continuous Casting Research Articles

Influence of homogenization preceding to cold-rolling on the microstructure of the AA-3003

The aluminum alloy AA3003 produced by a direct chill continuous casting process has a microstructure that significantly affects its potential use in engineering applications. This work studies the effects of the homogenizing heat treatment on the microstructure of AA3003 with cold working. Six conditions were studied, combining the variables initial condition (with and without homogenizing) and amount of cold working. All conditions were evaluated by means of optical and scanning electron microscopy, in combination with backscattered electrons and energy dispersive X ray spectroscopy techniques. Results suggest that for both initial conditions, the secondary phases present are Al6(Mn,Fe) and α-Al(Mn,Fe)Si, which vary in number, size, and shape. The homogenization caused the dissolution and precipitation of dispersoids, in addition to the spheroidization of primary particles, and minor variation of the size of secondary particles during cold working. Secondary phases are composed of primary and secondary particles, which differ in their Fe and Mn content, resulting in a lower Mn/Fe ratio for the primary particles (0,57 for the as‑received condition and 0,80 for the homogenized condition), whereas the dispersoids have a higher Mn/Fe ratio (1,56 after the homogenization). Homogenization increased ductility and reduced the likelihood of cracking during cold working. This was evidenced by the results obtained for strength, hardness, and ductility.

Influence of homogenization preceding to cold-rolling on the microstructure of the AA-3003

The aluminum alloy AA3003 produced by a direct chill continuous casting process has a microstructure that significantly affects its potential use in engineering applications. This work studies the effects of the homogenizing heat treatment on the microstructure of AA3003 with cold working. Six conditions were studied, combining the variables initial condition (with and without homogenizing) and amount of cold working. All conditions were evaluated by means of optical and scanning electron microscopy, in combination with backscattered electrons and energy dispersive X ray spectroscopy techniques. Results suggest that for both initial conditions, the secondary phases present are Al6(Mn,Fe) and α-Al(Mn,Fe)Si, which vary in number, size, and shape. The homogenization caused the dissolution and precipitation of dispersoids, in addition to the spheroidization of primary particles, and minor variation of the size of secondary particles during cold working. Secondary phases are composed of primary and secondary particles, which differ in their Fe and Mn content, resulting in a lower Mn/Fe ratio for the primary particles (0,57 for the as‑received condition and 0,80 for the homogenized condition), whereas the dispersoids have a higher Mn/Fe ratio (1,56 after the homogenization). Homogenization increased ductility and reduced the likelihood of cracking during cold working. This was evidenced by the results obtained for strength, hardness, and ductility.

Microstructure of AZCa912 Continuous Casting Bar after Hot Compression

AZCa912, 8.8mass%Al-0.68mass%Zn-0.25mass%Mn-1.9mass%Ca-Bal.Mg, continuous casting bars with 50 mm in diameter were formed by a direct chill continuous casting method. AZCa912 alloy is a noncombustible magnesium alloy. The surfaces of the bars had excellent metallic luster. A dendrite structure with a DAS of around 11-23 μm and grain sizes of about 200 μm was observed. Samples with almost the same grain size and different DAS were measured at hot compression test at 250-350°C, 0.01-1/s. Crack on the side surfaces of the samples were not formed by compression at 350°C, 0.01-0.1/s. All samples had an unchanged casting structure after hot pressing, and the percent of the structure remaining unchanged decreased with low-temperature compression and wide DAS.

Interface Formation During Fusion™ Casting of AA3003/AA4045 Aluminum Alloy Ingots

Fusion™ casting is a unique Direct Chill continuous casting process whereby two different alloys can be cast simultaneously, producing a laminated ingot for rolling into clad sheet metal such as AA3003/AA4045 brazing sheet. Better understanding of the wetting and interface formation process during Fusion™ casting is required to further improve process yields and also explore use of other alloy systems for new applications. In this research, AA3003-core/AA4045-clad ingots were cast using a well-instrumented lab-scale Fusion™ casting system. As-cast Fusion™ interfaces were examined metallurgically and by mechanical testing. Computational fluid dynamic analyses of the FusionTM casts were also performed. It was shown that the liquid AA4045-clad alloy was able to successfully wet and create an oxide-free, metallurgical, and mechanically sound interface with the lightly oxidized AA3003-core shell material. Based on the results of this study, it is proposed that the bond formation process at the alloys interface during casting is a result of discrete penetration of AA4045 liquid at defects in the preexisting AA3003 oxide, dissolution of underlying AA3003 by liquid AA4045, and subsequent bridging between penetration sites. Spot exudation on the AA3003 chill cast surface due to remelting and inverse segregation may also improve the wetting and bonding process.

Microstructure Change of AZCa912 Magnesium Alloy Continuous Casting Bar by Hot Compression

AZCa912, 8.8mass%Al-0.68mass%Zn-0.25mass%Mn-1.9mass%Ca-Bal.Mg, continuous casting bars were formed and their hot compression properties were measured. AZCa912 alloy is a noncombustible magnesium alloy. AZCa912 bars 50 mm in diameter were formed by a direct chill continuous casting method. The surfaces of the bars had excellent metallic luster. Grain sizes in the bars were 148, and the dendrite arm spacing was 16.9 μm. Cracks on the side surfaces of the samples were formed by compression at 250-300°C, 0.01-1/s and 350°C, 1/s. However, inner cracks were not observed and-5 μm fine-grains were formed. Some samples had an unchanged casting structure after hot pressing, and the percent of the structure remaining unchanged decreased with low-temperature compression.

Study on Direct Chill Continuous Casting of 3003/4004 Clad Slab

The 3003/4004 clad slabs are prepared by direct chill continuous casting. The microstructures on the interface are investigated by optical and scanning electron microscopes, chemical composition distributions across the interface are detected by EPMA, and tensile strengths are evaluated by MTS NEW-810 Materials Test System. The results show that 3003 alloy and 4004 alloy are metallurgical bonding on the interface on the interface, which is results from fusion and diffusion bonding. The thickness of diffusion layers is about 30 μm on average. The tensile strengths of the top, center and bottom regions of the bimetal slab tend to be uniform. The tensile properties of the interface for 3003/4004 clad slab are better than that of 3003 alloy.

NUMERICAL SIMULATION OF FLUID FLOW AND HEAT TRANSFER DURING THE INITIAL PHASE LEADING TO STEADY STATE SOLIDIFICATION IN D. C. CAST ALUMINUM ALLOYS

In this paper, two dimensional unsteady flow and energy equations are employed for simulating the fluid flow, heat transfer and solidification during direct chill continuous casting of Al-Mg alloy billet. In these processes, the formation of some macro defects such as thermal cracking, hot tearing, surface cracking, etc, has been found to initiate during the starting phase of the operation. In this paper, a numerical study of these developing fluid flow and thermal phenomena from the beginning of casting operation to a steady-state is presented. The computations are based on an iterative deforming finite volume method and a single-domain enthalpy formulation to solve the growth of metal being solidified and the accompanying dynamic fluid flow and thermal behavior. The effect of phase change on convection is accounted for using a Darcy's law-type porous media treatment. The results indicate that the fluid flow and temperature fields change drastically at the initial stage but evolve slowly afterwards.

Modeling of transport phenomena in continuous casting of non-dendritic billets

A macroscopic model for simulating the phase change process and transport of solid fraction is developed for the case of solidification during direct chill continuous casting of a non-dendritic Al-alloy billet, in presence of electromagnetic stirring. Maxwell’s equations are solved to obtain the electromagnetic force field, which is incorporated in the momentum conservation equations as body force source terms. Thereafter, the complete set of equivalent single-phase governing equations (mass, momentum, energy, species conservation and transport of solid fraction) are solved using a pressure-based finite volume method. A variable viscosity approach is employed to model fluid flow in presence of phase change. The model is first validated against some experimental and numerical results available in the literature, pertaining to the case of conventional continuous casting without any externally imposed stirring. The model predicts the temperature, velocity, species and most importantly, the solid fraction distribution in the mold. These predictions are then used for studying the influence of initial superheat, stirring intensity and cooling rate on the macroscopic behavior of the system.

Thermal modelling of DC continuous casting including submould boiling heat transfer

The steady-state thermal problem associated with the direct-chill continuous casting of aluminium–magnesium (Al–Mg) alloy cylindrical ingots is solved by an efficient numerical simulation method which also includes secondary cooling with submould boiling heat transfer. A nonorthogonal control volume discretisation and coordinate transformation technique has been applied for solving the heat transport phenomena in a multi-domain framework which involves detrmination of the solid–liquid interface (solidification front) and evaluation of the thermal profile during continuous casting operation. Conservation equations are reformulated in differential–integral form in terms of the transformed coordinates. All the terms arising from the nonorthogonality of the control volume have been retained in the numerical solution methodology and a front tracking procedure has been applied based on an iterative solution scheme. Using theories of nucleate boiling with forced convection and film cooling, a methodology has been devised to evaluate the external boundary conditions in the submould region of the ingot. These concepts are incorporated into the numerical model to account for the secondary cooling conditions during ingot solidification and simulation results show very good agreement with the published experimental data.

A finite element formulation for phase-change problems with advective effects

A finite element formulation for solving transient multidimensional phase-change problems considering advective effects is presented. This temperature-based formulation includes the definition of a phase-change function able to deal with classical isothermal and non-isothermal phase-change cases. Moreover, a new upwind weight function is defined in order to avoid numerical oscillations in problems with dominant advective effects. Further, some important aspects related to its numerical implementation are also addressed. The ability of this methodology is illustrated, firstly, in the solution of a one-dimensional test example. Finally, the numerical simulation of a direct-chill continuous casting process is performed. © 1998 John Wiley & Sons, Ltd.

Dual reciprocity boundary element method for convective-diffusive solid-liquid phase change problems, Part 2. Numerical examples

A newly developed dual reciprocity boundary element method for solid-liquid phase change problems has been verified in this paper. Tests include numerical convergence studies for the transient Dirichlet jump problem with and without phase change, and the steady-state convective-diffusive problem with different material properties of the phases and phase change. Results are compared with the existing analytical solutions and with the results obtained by the finite volume and finite element methods. Excellent agreement has been obtained. The method turns out to be robust over the whole broad spectrum of conducted numerical tests. The axisymmeeryryc problem of direct chill continuous casting, where nonuniform meshes have been applied, serves as a ‘real’ problem example exhibiting nonlinear material properties, solidification over a temperature interval, and nonlinear boundary conditions.

Modeling of macrosegregation due to thermosolutal convection and contraction-driven flow in direct chill continuous casting of an Al-Cu round ingot

MACROSEGREGATION continues to be one of the concerns in direct chill (DC) continuous casting of aluminum ingots. The variation in composition over the ingot cross section results in the need to adjust alloy composition in order to meet mechanical property specifications in finished products. The extent of macrosegregation is greatly affected by the casting conditions, such as casting speed, superheat, cooling rate, mold size, alloy composition, and grain-refining practices. Thus, it is of interest to develop a mathematical model to simulate macrosegregation in DC casting, enabling the selection of conditions that minimize macrosegregation while maintaining a high productivity. Various mechanisms for macrosegregation in aluminum DC casting have been discussed by Chu and Jacoby [1] and Yu and Granger. [2] In general, macrosegregation is caused by the relative movement of liquid and solid in the mushy zone, because the alloy constituents have different solubilities in the two phases. One cause of interdendritic liquid flow through the rigid solid structure in the mushy zone is the contraction of the liquid and the density difference between the solid and liquid phases during solidification. The commonly observed inverse segregation pattern near the outer ingot surface is known to be the result of such contraction driven backflow of solute-rich liquid. Positive segregation at the ingot surface can also be caused by exudation of interdendritic liquid through channels in the partially solidified shell into the contraction gap between the shell and the mold. This flow is driven by the metallostatic head. [3‐6] Less clear are the transport phenomena leading to either positive or negative segregation in the cen

Dimensionless diagrams for the temperature distribution in direct-chill continuous casting

This paper deals with the temperature distribution in direct-chill (DC) continuous casting and describes how results from a few computer simulations can form the basis for the construction of more generally applicable process diagrams based on dimensionless groups. Such diagrams were constructed for the temperature distribution in the steady-state period of the casting process for different types of aluminium alloys. The accuracy of these diagrams was checked extensively against literature data and revealed good correlations. Since the constructed diagrams are valid for a range of different casting conditions, they can be applied for sensitivity analysis, i.e. to obtain a general overview of the process and the relative importance of the different parameters.

A numerical simulation of the D.C. continuous casting process including nucleate boiling heat transfer

The steady-state thermal problem associated with the direct-chill continuous casting of A6063 aluminum cylindrical ingots is solved using the numerical finite element technique. Excellent correlation is demonstrated between the numerical model and experimental data from ingots cast at two different speeds. By application of the model, effective heat transfer coefficients are calculated as a function of vertical position on the outside surface of the ingot. It is shown that direct application of these coefficients to the modeling of different casting situations will produce substantial errors in the region in which heat transfer is by nucleate boiling. Using theories of nucleate boiling with forced convection and film cooling, a method is developed to calculate the external boundary conditions in the submold region of the ingot, thus making it possible for the first time to define explicitly all of the thermal boundary conditions associated with this casting configuration. These theories are incorporated into the numerical model, and a subsequent simulation shows excellent agreement with experimental data from a third ingot.

Mathematical Models of the D.C. Continuous Casting Process

Mathematical models of the direct-chill continuous casting process have been reviewed with particular emphasis on the simplifying assumptions that were necessary during their development. The methods and assumptions used by various researchers have been brought together in tabular form for easy reference. In general, analytical models may be used to generate closed form solutions which are valuable in showing semi-quantitative relationships; however, the simplifying assumptions required to solve the governing equation usually preclude precise quantitative modelling. Alternatively, numerical models do not readily show the qualitative interactions between the various casting parameters, but are capable of simulating accurately specific casting configurations. Numerical models of the process have, however, reached the level of sophistication where they are no longer constrained by the mathmatics of the problem, but by the lack of experimental data that is available to describe the external boundary conditions. Résumé Des modèles mathématiques de coulée continue sont résumés et les assumptions nécessaires pour leur simplification sont discutées. Les méthodes et assumptions employées par les chercheurs sont tabulées afin de permettre une référence facile.En généralles solutions obtenues par les modèles analytique sont utiles en montrant des relations semi-quantitatives. Cependant les simplifications nécessaires pour obtenir la solution aux équations gouvernantes ordinairement empêchent l'arrivee d'un modèle précis quantitatif. Sinon, les modèles numériques ne montrent pas promptement les interactions qualitatives entre les differents paramètres de coulée mais sont capables de bien simuler une configuration spécifique de coulée continue. Récemment les modèles numériques de coulée continue ont atteint un niveau de sophistication où ils ne sont plus retenus par les mathématiques du problème surtout le manque de données expérimentales disponibles pour décrire les conditions de limite externe.

A numerical simulation of the D.C. continuous casting process including nucleate boiling heat transfer

The steady-state thermal problem associated with the direct-chill continuous casting of A6063 aluminum cylindrical ingots is solved using the numerical finite element technique. Excellent correlation is demonstrated between the numerical model and experimental data from ingots cast at two different speeds. By application of the model, effective heat transfer coefficients are calculated as a function of vertical position on the outside surface of the ingot. It is shown that direct application of these coefficients to the modeling of different casting situations will produce substantial errors in the region in which heat transfer is by nucleate boiling. Using theories of nucleate boiling with forced convection and film cooling, a method is developed to calculate the external boundary conditions in the submold region of the ingot, thus making it possible for the first time to define explicitly all of the thermal boundary conditions associated with this casting configuration. These theories are incorporated into the numerical model, and a subsequent simulation shows excellent agreement with experimental data from a third ingot.