Sheet Ingot Research Articles

Modelling macrosegregation modification in dc casting of aluminium alloys in sheet ingots accounting for inlet melt flow, equiaxed grain morphology and transport

Macrosegregation is a severe defect present in direct-chill (DC) cast aluminium ingots and billets. In the recent years, experimental studies were conducted to modify and to an extent optimize macrosegregation formation by modifying the inlet melt flow. Due to several limitations, the grain settling behavior and corresponding liquid flow pattern is difficult to analyze using experiments. Simulations on the other hand can provide this insight. However, conducting 2D sheet ingot simulations, as has been previously done, provides an incomplete description of flow pattern. To avoid this and as a first qualitative study, full scale 3D sheet ingot simulation results with two different inlets are presented in this paper. A simplified three-phase multiscale solidification model accounting for solidification shrinkage, natural convection and equiaxed grain growth and transport is used to conduct this study. We show that modification of inlet flow results in modification of grain settling and eventually leading to modification of macrosegregation. The impact of grain morphology is also additionally analyzed.

Компьютерное моделирование формирования 50-тонного листового слитка

Компьютерное моделирование формирования 50-тонного листового слитка

Predictive Capabilities of Multiphysics and Multiscale Models in Modeling Solidification of Steel Ingots and DC Casting of Aluminum

Prediction of solidification defects, such as macrosegregation and inhomogeneous microstructures constitutes a key issue for the industry. The development of models of casting processes needs to account for several imbricated length scales and different physical phenomena. For example, the kinetics of the growth of microstructures needs to be coupled with the multiphase flow at the process scale. We introduce such a state-of-the-art model and outline its principles. We present the most recent applications of the model to casting of a heavy steel ingot and to direct chill casting of a large Al alloy sheet ingot. Their ability to help in the understanding of complex phenomena, such as the competition between nucleation and growth of grains in the presence of convection of the liquid and of grain motion is shown, and its predictive capabilities are discussed. Key issues for future developments and research are addressed.

Modeling of Heat Transfer and Interdendritic Strain for Exuded Surface Segregation Layer in the Direct Chill Casting of Aluminum Alloys

This investigation on the formation of exuded surface segregation layer “ESSL” is intended to provide experimental and simulated comparison to verify the model developed previously by El-Bealy. Preliminary verification and calibration of the previous 2D mathematical model are demonstrated by quantitative errors between the previous measurements and predictions of temperature and macrosegregation. Also, the results from these comparisons reveal that the errors are in the reasonable and within allowable limits. These comparisons lead to the fact that the exuded surface segregation layer mostly forms on the middle slice of broad sheet ingot face and in the early stages of mold zone. The model predictions point out also that the different interdendritic strain hypotheses associated with fluctuations of mold cooling conditions. This affects the interdendritic liquid flow between the equiaxed crystals which influences the severity of ESSL formation and its macrosegregation level. The mechanism of ESSL with heat flow and interdendritic strain generation has been analyzed and discussed. The quantitative comparisons between the pervious experimental results and numerical simulation in this investigation reveal also several solutions to prevent this defect for future work.

Hydro Casthouse Reference Centre

Since October 2006 the Hydro Casthouse Reference Centre has been operating. The centre is a full scale state of the art pilot casting centre for extrusion ingot, sheet ingot and foundry alloys, consisting of a 17Mtons furnace with a metal loop, a launder system including modular in-line melt treatment units such as ceramic foam filters (CFF) and inline melt refining units (Hycast SIR) and a casting pit with the possibility to cast full size geometries and a casting length of 5.5m. A two strand horizontal casting machine further adds the possibility of continuous casting of extrusion ingot and foundry alloy ingot. The centre has a state of the art superior control system (SCS) and a lay-out, including control room facilities, well suited for training and demonstration purposes. In addition the centre has access to state of the art software codes for simulating the casting process (Alsim) and the as cast microstructure (Alstruc). The present paper gives some examples on how the centre is operating and the support that is offered to casthouses in Hydro. This includes (i) simulation of the casting processes (hot tearing and as cast structures) applying the Alsim and Alstruc codes, (ii) pilot scale testing of casting and melt treatment equipment, (iii) testing of new parameters and procedures for melt treatment and casting (iv) production of trial orders of new alloys and (v) practical training of casthouse operators (basic for molten metal handling, emergency situations and response, casting principles and trouble shooting, etc.).

Phase Transformation for Primary Particles in the Surface Regions of an AA1200 Alloy

In the present investigation the particle structure in an AA1200 sheet ingot used for litho applications has been studied. Caustic etching of the as-cast material was seen to result in a zone close to the surface with a different etching response. This zone was identified as what is known as a fir-tree zone or an Altenpohl zone [1,2,3,4]. A variation in particle type over the cross section of the as-cast ingot was seen to follow the differences in etching response. After heat treatment of the material, the fir-tree zones were no longer visible, and the accompanying change in particle structure was studied. Samples from the subsurface regions and from a distance of ~20 cm from the surface has been investigated before and after heat treatment. In the as-cast material, the sample from the surface was dominated by featherlike particles with long strings of particles, identified as AlmFe. While closer to the centre Al3Fe and Al6Fe were seen to be the main phases, however, some AlmFe and probably some α-AlFeSi was also found in this sample. After heat treatment, the particle structure was seen to change, and the surface sample contained mainly Al3Fe in addition to a small amount of AlmFe. The change in particle structure during heat treatment is discussed with reference to the change in etching response.

Prediction of porosity contents and examination of porosity formation in Al–4.4%Mg DC slab

A new method for predicting porosity contents in Al–4.4% Mg DC sheet ingot (slab) isproposed. Hydrogen supersaturated mass in the inter–dendritic liquid and the pressure drop were taken into account for the porosity prediction. Using the method, the porosity contents were calculated and theywell agreed with the experimental results. Thus it becomes possible to predict the porosity contents in the slab quantitatively. Furthermore, the porosity generation mechanism was observed from the surface to the center in the slab. The quantities and area fractions of the porosity were measured quantitatively by using an image analysis apparatus (Luzex). An explanation of this kind of porosity generation mechanism and the distribution of the porosity was attempted by using a local equivalent pressure and hydrogen supersaturation in dendritic solidification. The effects of parameters such as therma gradient and cooling rate on the porosity contents were investigated analytically and the distribution of porosity and porosity contents in the slab of Al–4.4% Mg alloys were determined.

Equi-axed growth and related segregations in cast metallic alloys

Full-scale trials of DC ingots and laboratory scale directional solidification experiments have been performed to study the effect of grain structure on macro-segregation in industrial cast products. An Al alloy sheet ingot was cast with constant casting conditions (speed, superheat, cooling rate) except for the grain refiner: the first half of the ingot was non-inoculated, while the second half was inoculated. The results indicate that the extent and intensity of the centreline segregation is modified via the grain-refinement treatment: the finer the grains are, the more intense is the macro-segregation.Numerical simulations of directional solidification of binary Al-Cu alloys have been carried out with the help of a 2D finite volume software which takes account of the movement of the liquid with respect to the solid in the mushy zone. It is possible to account for the segregation pattern of the directionally solidified ingots that exhibit columnar or coarse equi-axed grain structures. Contrarily, the intense segregation of the fine-grained ingots is not yet understood.

Fine Recrystallized Grain Structure by Dispersion of Eutectics in Al-Fe-Mn Alloy

Al-1.5%Fe-0.5%Mn alloy sheet ingot was rolled up to 99.8% and annealed at temperatures between 573 and 723K. With increased cold reduction, the mean size and the mean spacing of second phase particles became smaller and the number density of the particles increased. With decreased mean spacing of particles, average grain size became smaller. In case of 99% cold rolled samples, the number density of particles larger than 1 μ m was 3∼3.5 × 10 4 /mm 2 and the mean spacing of these particles was 5∼6 μ m. The recrystallized grain size of 4∼5 μ m was obtained after annealing above sample at 573K for 300s. With the reduction of grain size from 25 μ m to 4∼5 μ m, 0.2% proof stress increased from 45MPa to 100∼120MPa by 2∼2.5 times.

Effect of Grain Refinement on Macrosegregation in Direct Chill Semi-Continuous Casting of Aluminium Sheet Ingot

An experimental investigation of the macrosegregation occurring in a commercial size direct chill cast of an 5182 alloy ingot is described. The extent of the chemical homogeneity in a sheet ingot depending of the grain refining practice is discussed. A complete analysis of 2D macrosegregation maps and of the associated grain structure in a non grain refined part and a grain refined part of the ingot is compared and discussed. The intensity of the centerline segregation is modified via the grain refining practice. The change of macrosegregation measured in the slices with and without grain refiner may be attributed to the effects of the grain motion but also the liquid movement.

A mathematical model of the heat and fluid flows in direct-chill casting of aluminum sheet ingots and billets

A finite-element method model for the time-dependent heat and fluid flows that develop during direct-chill (DC) semicontinuous casting of aluminium ingots is presented. Thermal convection and turbulence are included in the model formulation and, in the mushy zone, the momentum equations are modified with a Darcy-type source term dependent on the liquid fraction. The boundary conditions involve calculations of the air gap along the mold wall as well as the heat transfer to the falling water film with forced convection, nucleate boiling, and film boiling. The mold wall and the starting block are included in the computational domain. In the start-up period of the casting, the ingot domain expands over the starting-block level. The numerical method applies a fractional-step method for the dynamic Navier-Stokes equations and the “streamline upwind Petrov-Galerkin” (SUPG) method for mixed diffusion and convection in the momentum and energy equations. The modeling of the start-up period of the casting is demonstrated and compared to temperature measurements in an AA1050 200×600 mm sheet ingot.

Ａｌ‐４．４％Ｍｇ合金スラブ内部ポロシティの生成量の予測

A new method for predicting porosity contents in Al–4.4%Mg DC sheet ingot (slab) was proposed. Hydrogen supersaturated mass in the interdendritic liquid and the pressure drop were taken into account for the porosity prediction. Using the method, the porosity contents were calculated and they well agreed with the experimental results. So it became possible to predict the porosity contents in the slab quantitatively. The effects of parameters such as thermal gradient, cooling rate, local equivalent pressure and initial hydrogen on the porosity contents were analytically investigated, and the distribution of porosity and porosity contents in the slab of Al–4.4%Mg alloys were determined. The following conclusions were obtained, (1) The porosity contents decrease with increasing the thermal gradient and cooling rate. But when the thermal gradient and the cooling rate are higher than 5000 K/m and 5.0 K/s, respectively, the porosity contents become almost invariable. When the thermal gradient and the cooling rate are lower than 2500 K/m and 1.0 K/s, respectively, the porosity content rapidly increase. (2) The lower the initial hydrogen contents, the lower the porosity contents become. (3) The controls of the local equivalent pressure and initial hydrogen content are necessary to reduce the porosity formation in the DC slab. (4) The distribution of porosity contents in the slab corresponds to the distribution of DAS. As the DAS becomes coarse the porosity contents become high.

Cracking in the Weld Heat-Affected Zone of COntinuously Cast Sheet and Ingot of Ni3Al

ABSTRACTWeldability is a key issue for successful applications of ductile Ni3Al in structural components. Here we report the results of an investigation of gas tungsten arc welding behaviors of continuously cast thin sheet and cast ingot of a Ni3Al alloy, known as IC-50, containing boron and zirconium. Cracking in heat-affected zone (HAZ) was prevalent in both types of specimens. Both in cast sheet and ingot, liquation cracking was observed to occur along the grain boundaries in HAZ. Microstructural studies showed that continuously cast thin sheet consisted of columnar grains along the thickness. Easy cracking along the boundaries of columnar grains made cast sheets more susceptible to hot cracking than ingots. The possibility of segregation of boron and zirconium, causing liquation cracking, will be discussed.

Effect of Si on the recrystallization of Al-Mn alloy

These studies were made to investigate the effects of Si on the recrystallization of Al-Mn alloy.(1) Si had remarkable effects on the recrystallization temperature of Al-Mn alloy. When its sheet ingot was not preheated, Si raised the recrystallization temperature. Whereas, when it was preheated. Si lowered the temperature.(2) Heating rate also had effects on the recrystallization temperature of non-preheated Al-Mn alloy containing Si. The elevation of temperature was remarkable in slow heating, and it was maximum at the heating at 160°C.(3) Si decreased the effects of heating rate on the grain size of recrystallized Al-Mn alloy when the sheet ingot was not preheated. It gave a fine grain size even at slow heating rate. Whereas, Si increased the effects of heating rate on the grain size of the preheated alloy and a large grain size was produced by slow heating.These phenomena resulted in that Si prevented the effects of preheating on the refining of the recrystallized grains of Al-Mn alloy; and the grains obtained at the normal heating rate, which was slower than that in a salt bath, were coarser than those obtained after no preheating.

Study on the aluminium (alloy) sheet ingot (5th Report)

After classified by micro- and macro-structures, the parts of the sheet ingots obtained from experiments mentioned in the previous reports was examined on the correlation between the reduction and structures.The findings are as follows:(1) Through compression test, it is recognized that the behaviour of the sheet ingot at the time when the reduction is given is related to its micro-structure but not to its macro-structure.(2) Through examination and statistical study of the relation between grain size in cast structure and recrystallized grain size under the different reduction rates and annealing temperatures, it is recognized that the ingot which has fine micro-structure is rolled into the sheet which has fine grain size.

Study on the aluminium (alloy) sheet ingot (3rd Report)

Several pieces of pure aluminium sheet ingots were trially produced from identically charged molten metal by semi continuous casting process. Under the different casting conditions, particularly the height of molten metal in the mold, and casting speed, the comparative examination was made on the components and structure of thus produced sheet ingots.The findings were as follows:The height of molten metal in the mold is a very important factor for getting good ingot. By lowering the height of molten metal, the segregation of ingot surface can be reduced and the structure gets uniform. The effect of casting speed is much less than that of the height of metal in the mold.

Study on the aluminium (alloy) sheet ingot (4th Report)

Study on the aluminium (alloy) sheet ingot (4th Report)

Study on the aluminium (alloy) sheet ingot (2nd Report)

This is to report of a study on the annual-ring like structure which appears in slab of commercially pure aluminium produced by the semi-continuous casting process. This structure is seen in the central part of the slab but not in the outer part. It is recognized through interferrometer that the part of rings is dented by etching. This structure disappears when the slab is heated at 500°C for more than 48 hours or rolled down to 68.8% or more reduction. It is considered, therefore, that this does not give any influence to the rolling work in practice. On the other hand, the three dimentional variation of this part has been observed by means of repeating the polishing. From the above-mentioned study, it is considered that this annual-ring structure is formed by the nonuniformity of dcnsity of the solid solution on the condition of formation of which has been further studied.

STUDY ON THE CASTING STRUCTURE OF PURE ALUMINIUM SHEET INGOT

It is very hard in practice to get a sound sheet ingot without gases and cavities in the casting of pure aluminium. The gas content of aluminium has been hereto examined by many investigators and its contentt was recently brought to clear to be as small as less than 1c.c/100grs.The authers studied the comparative value of gas content and the porosity of 60kg's sheet ingot with 99.9%, 99.5% and 99.2% purities which were cast in book mould and direct-chill mould, and the following results have been obtained.(1) The higher the purity of aluminium, the lower the value of gas content.(2) The gas content increases towards the surface layer from its centre layer of sheet ingot.(3) The percentage of hydrogen in total gas is 64-71%.(4) The gas content of sheet ingot by C. C. P. is smaller by 50% than that of book mould process.(5) The percentage of porosity in the sheet ingot by C. C. P of 99.5% purity is lesst han 1%, on the other hand that of book mould is generally more than 3%.(6) There is no direct relation between the blister on sheet and the gas content of sheet ingot, but no blister occurs on the sheet rolled from C. C. P sheet ingot.