Reusable Molten Aluminum Distributor (ReMAD) for Sheet

Reusable Molten Aluminum Distributor (ReMAD) for Sheet Ingot Casting
Mathematical simulation results
S.P. Tremblay, Pyrotek High-Temperature Industrial Products Inc., Chicoutimi (Québec), Canada
A. Buchholz, VAW aluminium AG, R/D Casting Technology1, Bonn, Germany
M. Lapointe2, Aluminerie de Bécancour Inc. ( ABI ), Ville de Bécancour (Québec), Canada
Abstract
In the last decade, the molten aluminum distribution in the ingot heat of dc sheet casting ingots has been
achieved using mostly a combo bag made of fiberglass fabrics. Some of these fabrics are open-weave
materials while others are solid fiberglass fabrics sewn together. This bag can deform and this can effect
not only the distribution but also the molten metal temperature profile around the mold and at the end, the
final ingot quality.
This paper will review the use of a new distributor for dc sheet ingot casting. The paper is divided in two
parts. The first part will present the results of water modeling and the second part will present an extensive
mathematical modeling study.
Test results of the new reusable molten aluminum distributor used at ABI to cast AA-1045, AA-3003 and
AA-5052 alloys has been already presented last year1.
Introduction
Last year Pyrotek introduced the Reusable Molten Aluminum Distributor concept (Figure 1) to replace the
combo bag presently used to distribute aluminum in the ingot head. The standard sewn combo bag is shown
in Figure 2. Oppositely to the regular bag which can be used only once, the ReMAD can be used at least 20
times. The inner bag is changed every drop.
Tests made at ABI using the ReMAD have shown a more consistent cast start-up, a better temperature
profile, a less turbulent flow leading to a better ingot surface finish than the regular flexible sewn combo
bag. Finally, the ReMAD has been find user friendly by the casthouse operators.
The ReMAD has been designed using water modeling and based on Pyrotek’s previous water modeling
tests carried out in various conditions with different sewn combo bags2. A summary of the results will be
presented.
An extensive numerical DC casting modeling on the ReMAD and the flexible regular combo bag has been
sub-contracted to the VAW R/D group. The VAW R/D group has been chosen because of the excellent
numerical DC casting modeling studies presented in the last years3,4,5. The results of this study will be
presented here.
1.
2.
M. Lapointe is now with Alumiform Inc., Chicoutimi (Québec), Canada
Now: Hydro Aluminium Deutschland GmbH, R & D.
Figure 1. Reusable Molten Aluminum Distributor (ReMAD )
Figure 2. Typical combo bag used for DC casting
Water modeling tests
All experiments were carried out in a full-scale water model built of Plexiglas and stainless steel. The
maximum ingot size is 660 x 1860 mm. Red dye is used to characterize the changes in flow. The cast
start-up as well as the steady state regime can be evaluated. ABI uses a standard combo bag with the
following dimensions: 100 x 100 x 300 mm similar to the one illustrated in Figure 2. The combo bag
filtration fabric weave was 31L4. The tested inner bag weaves of the ReMAD were 34P4, 32L4 and 31L4.
The 34P4 weave is the most closed fabric having 48.4% opening compared to 50.3% for 32L4 and 55.5%
for the 31L4 weave. The effects of the filtration fabric opening on the combo bag exiting flow has been
already presented 6. In this case, the flow pattern from a standard bag is illustrated in Figure 3a. The exit
velocity from the standard bag is much higher than that in the ReMAD due mainly to the smaller
size/volume ratio. A part of the flow is also directed toward the surface because of the pronounced bottom
bag curvature. Figure 3b shows the typical flow pattern from the ReMAD. The flow is slower and less in
surface than the standard combo bag. The ReMAD delivers molten aluminum better in the corners than the
regular combo bag. The higher exit speed of the metal from the standard bag could increase the probability
of oxide patch detachment and entrainment of oxide films from the surface. The turbulence within the
ReMAD is also smaller than in the standard combo bag due to the presence of the dome. A calmer flow
within the ReMAD will result in less metal disturbance, less metal oxidation and thus better metal
cleanliness.
Figure 3a. Flow pattern from standard combo bag
Figure 4b. Flow pattern from ReMAD
Numerical DC casting modeling study
In this study the heat and mass transfer in DC casting of an 1.68 m x 0.66 m AA3004 ingot is analyzed for
the regular combo bag and the ReMAD by numerical simulation. The first bag design is a conventional
flexible distributor with a highly permeable inner bag and a shorter almost impervious outer shell, while the
second construction consists of a reusable rigid refractory body and a flexible insert. Although the outer
dimensions of the bags are very similar the effective openings for the melt flow are different suggesting the
formation of different flow patterns. The two scenarios were modeled with the commercial finite element
code FIDAP. VAW R/D applies this software successfully to DC casting problems since several years. The
present study was mainly focused on the investigation of the flow patterns and their effect on the thermal
field in the vicinity of the distributor. This required a new approach in the mesh generation and special
techniques to describe the effect of the porosity of the bag fabric and also justified some simplifications in
the description of the heat transfer.
While performing the two basic cases two shortcomings of the approach became obvious:
1.
The deformation of the flexible bag under the dynamic load of the melt flow is not described by
the model. A fixed shape is assumed instead. Actually the deformation of the flexible fabric could
lead to some significant modifications of the geometry, for instance the effective distance from the
spout to the bag bottom and the shape and inclination of the bag opening.
2.
The effective aspect ratio of both designs is differing very much. These geometric effects probably
obscure other differences due to the bag material choices. The different heat resistance of a
flexible porous fabric and a low conducting refractory could have a strong influence on the
formation of buoyant flow.
Numerical DC casting modeling study - Numerical Model
The problem is modeled in the FEM code FIDAP. This code solves the momentum, continuity and heat
transport equation. In the present study only steady state solutions are considered. In the FIDAP
formulation the effect of solidification on the fluid flow is incorporated by a very strong increase of the
laminar viscosity in the solidification interval. The turbulent mixing effects are described by a standard kepsilon model. This model is modified by a special decay approach to suppress turbulent transport in the
mushy zone. Buoyancy effects are accounted for by a Boussinesq approach. The mold and the different
cooling zones are approximated by appropriate heat transfer coefficients and reference temperatures. For
the melt material properties of an AA3004 alloy were assumed. To capture the effects of the different mold
materials and fabrics three different methods are applied. The body of the rigid bag is described as a
conducting solid with very low conductivity compared to the melt. Within the solid only conductive heat
transport is computed. Actually this material works as a thermal isolation. The permeable inner fabrics
produce a very small pressure drop. This effect has been mimicked by a thin layer of cells where a resistive
body force has been applied. In these layers both conductive and convective heat transport are calculated.
The third method applies to the outer fabric of the flexible bag which is almost impervious for melt flow
but not for heat conduction. This fabric is defined by an internal surface where the velocities are set to zero.
Only conductive heat transport can pass this obstacle. The thermal contact at the interface is considered to
be ideal.
Numerical DC casting modeling study - Mesh generation and boundary conditions
The construction of the mesh follows the same principle for both cases. Symmetry is assumed and only a
quarter section of the 1680mm x 660mm ingot with a length of 1200 mm is considered. The heat transfer
by radiation and convection at the top surface is neglected what can be justified by the low emissivity of
aluminum. At the inlet of the spout a parabolic profile is imposed which matches the casting speed defined
at the ingot bottom. The effect of the primary (mold) cooling and gap formation is smoothed out over a
region of 80 mm length at the top of the ingot where a low heat transfer coefficient is assumed. The other
cooling regions (water impingement, nucleate boiling and free stream flow) are also smoothed and
approximated by constant heat transfer coefficients in three subsequent regions. The cooling is assumed to
be constant around the circumference of the ingot.
The grids mainly differ in the center region where the two different bags had to be modeled by blocks of a
different topology.
The model geometry is simplified compared to the real bags. Only the immersed parts of the bags are
modeled. The basic shape of the bag is a rectangular brick. The outer dimensions of the brick (quarter
section) are 170 mm (l) x 55 mm (w) x 50 mm (h) for the combo bag (blue in Figure 4) and 200 mm x 70
mm x 75 mm for the ReMAD (orange in Figure 5). The thickness of this wall is 5 mm of porous (combo
bag) or impervious (ReMAD) material.
At the top, the combo bag is surrounded by an impervious shield with an immersion depth of 10 mm (not
visible in Figure 4). At the center the main body of the flexible bag is surrounded by a curved layer of
impervious fabric (outside face of light green region in Figure 4). Its length is 100 mm, the corner marking
the largest cross section is immersed 65 mm and 75 mm from the mid plane. This impervious shell, which
covers almost only half of the total bag length, mainly determines the directional characteristic of the
combo bag.
A cone of 90 mm diameter at the basis and 32 mm at the top with a height of 20 mm is in the center of the
ReMAD (orange in Figure 5). The opening at the bottom of the narrow face of the bag has a height of 28
mm. Inside this rigid case there is an internal highly permeable bag of 145 mm length with an immersion
depth of 50 mm. The wall thickness of this insert is also 5 mm.
The spout has an inner diameter of 38 mm and an outer diameter of 90 mm. The immersion depth in the
rigid case is 35 mm, in the flexible case 40 mm. The distance between spout and bottom plate of the bag is
10 mm in both cases.
To limit the amount of cells the grid was coarsened between the bag and the ingot surface by an
unstructured mesh. This leads to different cell numbers of about 115000 and 130000 cells for the ReMAD
and the combo bag case respectively.
Figure 4. Mesh at ingot center for combo bag.
Figure 5. Mesh at ingot center for ReMAD.
Numerical DC casting modeling study - Results
The Figures of temperature, flow and pressure fields of the simulations are arranged in a way that the
similar cases, i.e. flexible bag and rigid non-conducting bag can be compared directly.
The Figures 6 & 7 show the temperature distribution and the extent of the mushy zone for both computed
cases, Figures 8 & 9 present the temperature in the liquid with a higher resolution of temperatures, Figures
10 &11 show the global flow patterns and finally, Figures 12 & 13 show an enlarged section of flow near
the distributor.
Figure 6. Temperature distribution and mushy zone extent for the combo bag.
Figure 7. Temperature distribution and mushy zone extend for the ReMAD
Figure 8. Temperature distribution in effective liquid for the combo bag
Figure 9. Temperature distribution in effective liquid for the ReMAD
Figure 10. Flow pattern for the combo bag
Figure 11. Flow pattern for the ReMAD
Figure 12. Buoyancy driven flow next to the combo bag.
Figure 13. Buoyancy driven flow next to the ReMAD
Comparing the basic cases of the flexible bag and the rigid distributor, the rigid distributor seemed to
provide a better result since it directs more hot melt towards the narrow ingot face. Besides the deformation
of a flexible bag, which is beyond the scope of the modeling approach, the main difference lies in the
conductivity of the bag material. To approximate a flexible bag with similar dimensions the rigid bag case
was modified by setting the heat conductivity to the value of the surrounding aluminum. The flow pattern
observed in this computation is actually a mixture of features observed in the basic cases for the flexible
and the rigid bag. The flow leaving the bag exit forms a horizontal vortex (Figure 14) very similar to the
ReMAD (Figure 11). In the wake of the distributor the buoyancy driven convection generates a vertical
eddy, which is sufficient to modify the heat transfer between the ingot center to the rolling face (Figure 15)
The temperatures near the center of the rolling face are higher than in the basic case. The conductive heat
transport through the fabrics modifies the heat transport in the wake regions of a bag considerably. This can
even effect the heat transport to the rolling faces, which now reveal higher temperatures in the center. For
the time being it is difficult to say if they higher heat exchange by a conducting bag can be considered as an
advantage or disadvantage. This requires further investigation.
Numerical DC casting modeling study - Conclusions
The results of the computations are in good agreement with other simulations and confirming the
experience that short bags generate a diverging diffusive flow pattern, while long bags are more directional
and conduct more hot melt to the narrow ingot face. In these terms the combo bag design belongs to the
"short", and the ReMAD to the "long" bag category.
The deformation of a flexible bag can have a significant influence on the solution and should not be
neglected. Due to the deformation the distance between spout and bag can be significantly larger compared
to the "cold" setup. This modifies the flow field and can have a strong impact on the heat distribution.
The conductivity of the bag material can affect as well the flow field and the thermal field. A highly
effective heat transfer can generate strong thermal gradients in the liquid around the distributor that are the
driving force for thermal buoyancy. In this case the distributor works similar to an immersed heating.
According to the computations the performance of both designs is equivalent with respect to the heat
transfer. Therefore, no design can be recommended as superior to the other.
In how far some of the observations - for instance an increased heat exchange due to a higher conductivity
of the bag material - can become important under other casting conditions is not yet clear and has to be
evaluated by further investigations.
Figure 14. Flow pattern for the ReMAD with conductive walls
Figure 15. Temperature distribution in effective liquid for the ReMAD with conductive walls
Conclusions
The numerical DC casting modeling study results agree very well with the water modeling study results.
Better hot metal delivery in the corners as well as less metal flowing toward the surface with the ReMAD
have been shown.
The better performances of the ReMAD versus the regular combo bag in operations have confirmed the
results of both water modeling and numerical DC modeling studies.
Numerical CD modeling study is great tool to have a better comprehension of the molten aluminum
distribution in the ingot head.
References
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4.
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