1. TOPOGRAPHY IN ALUMINUM CASTING
COMBINED EXPERIMENTAL AND COMPUTATIONAL APPROACH FOR THE DESIGN OF MOLD SURFACE
TOPOGRAPHY IN ALUMINUM CASTING
DATE OF PRESENTATION : 11 NOVEMBER 2003
PRINCIPAL INVESTIGATOR : PROF. NICHOLAS ZABARAS
PERFORMING ORGANIZATION : MATERIALS PROCESS DESIGN
AND CONTROL LABORATORY,
CORNELL UNIVERSITY
PROJECT START DATE : 31 AUGUST 2002
PROJECT END DATE : 31 AUGUST 2005
Materials Process Design and Control Laboratory

2. Industrial Partners Alcoa Technical Center, Alcoa Center, PA
Ingot and Solidification Platform (Dr. Alvaro Giron, Coordinator)
Provide experimental data
Validate important results obtained through simulations
Perform pilot experiments to demonstrate and validate
key modifications and technologies developed
Materials Process Design and Control Laboratory

3. Project Background and Objectives
Aluminum industry relies on direct chill casting for aluminum ingots
Presence of defects in ingots
Surface defects removed by scalping – post casting process
Defects caused by non – uniform heat extraction, improper contact
at metal/mold interface, inverse segregation, meniscus freezing etc.
Post scalping operations remove significant amount of material to
eliminate defects
Substantial energy and cost requirements for defect removal
processes, re-melting, etc.
Materials Process Design and Control Laboratory

4. Project Background and Objectives
Surface defects in casting
(a)
(c)
(a) Sub-surface liquation and crack formation on top
surface of a cast
(b) Ripple formation
(c) Non-uniform front and undesirable growth with non-uniform thickness
(b)
Materials Process Design and Control Laboratory

5. Project Background and Objectives
Classification of direct cast surface defects in direct chill cast ingots
Surface tears
Cracks
Pre-solidification cracks
Blebs
Post-solidification cracks
Bleed bands
Subsurface
segregation
and
non-uniform
microstructure
Liquation
CAST
SURFACE
DEFECTS
Surface
irregularities
TCG
Duplex
micro.
Cold shuts
Ripples/Laps
Surface porosity
Folds
Cavities
Gas porosity
Sweats
Blisters
Oxide patches
Materials Process Design and Control Laboratory

6. Project Background and Objectives
Materials Process Design and Control Laboratory
Materials Process Design and Control Laboratory

7. Project Background and Objectives
Materials Process Design and Control Laboratory

8. Project Background and Objectives
Some mold surface
topographies used in
Alcoa
Profound effects on the
morphology of final cast
surfaces
Materials Process Design and Control Laboratory

9. Project Background and Objectives
Materials Process Design and Control Laboratory

10. Project Background and Objectives
Identify mechanisms that impact shell surface morphology and microstructure
Investigate the effect of these mechanisms individually in metals and alloys
Develop inverse techniques to design mold surface topographies for desired
cast surface morphologies
Propose design solutions that reduce post-casting operations
Materials Process Design and Control Laboratory

11. Milestones ID NO MILESTONE DESCRIPTION PLANNED COMPLETION
ACTUAL COMPLETION
1.1
FEM analysis modeling and parametric characterization of heat transfer through textured molds.
1/31/03
9/30/03
1.2
Modeling and parametric analysis of air-gap nucleation, shell growth morphologies during the early stages
1.3
With realistic material and process models, perform a preliminary sensitivity analysis to understand and demonstrate the importance of mold topography on the resulting shell surface morphologies
3/31/03
Current work
1.4
Compare computed and measured contact resistances for textured molds.
8/31/03
Materials Process Design and Control Laboratory

12. Milestones
2.1
FEM analysis modeling and parametric characterization of heat transfer through textured molds. (continued from Year I)
11/30/03
Current work
2.2
Modeling and parametric analysis of air-gap nucleation, shell growth morphologies during the early stages (continued from Year I)
12/31/03
2.3
With realistic material and process models, perform a preliminary sensitivity analysis to demonstrate the importance of mold topography on the resulting shell surface morphologies (for aluminum alloys)
1/31/04
2.4
Modeling and parametric analysis of inverse segregation, exudation.
3/31/04
2.5
Develop rigorous sensitivity analysis-based computational design algorithms to compute optimum texture molds for control of shell morphology and microstructure.
5/31/04
2.6
Compare computed results with experimental physicochemical and micro-mechanical mechanisms that have been shown to control shell morphology
8/31/04
Materials Process Design and Control Laboratory

13. Milestones
3.1
Experimentally implement and test at Alcoa casting processes using textured molds obtained from the computational design analysis.
3.2
Develop correlation models of the large scale contact resistance to the contact resistance computed at the micro-scale.
3.3
With realistic material and process models, perform a preliminary sensitivity analysis to demonstrate the importance of mold topography on the resulting shell surface morphologies (continued from Year II)
3.4
Complete modeling and parametric analysis of inverse segregation, exudation (continued from Year II)
3.5
Use the developed computational design algorithms for various process conditions to produce texture molds for control of various shell morphologies and microstructures (continued from Year II)
3.6
Introduce pilot innovative casting processes for required surface features and micro-structure.
Materials Process Design and Control Laboratory

14. Technical Decision Points
Mold topography:
Use of distorted or grooved molds to arrest or suppress gap nucleation
Modify heat transfer/solidification rate, thermal contact resistance, wettability
by using grooved molds
Effect of varying mold topographies on solidification of pure Aluminum and
Aluminum alloys
Uneven
growth
Plain mold plate
Even
growth
Mold plate with grooves
Materials Process Design and Control Laboratory

15. Technical Decision Points
Contact resistance:
At the very early stages of casting, the solid shell is in contact with the mold and the
thermal resistance between the shell and the mold is defined by the contact conditions
Uneven contact pressure generates an uneven thermal stress development and
accelerates distortion or warping of the shell.
Before gap nucleation, the thermal resistance
is determined by pressure
After gap nucleation, the thermal resistance
is determined by the size of the gap
Example: Aluminum-Ceramic Contact
Heat transfer retarded due to gap formation
Materials Process Design and Control Laboratory

16. Technical Decision Points
Cast Surface defects
Bleed bands
Blobs
Liquation
Presolidification cracks
Shell formation
Shell distortion &
mold movement
Reduction in
heat transfer
Air gap formation
Re-melting
of shell
Crack
initiation
Flow of interdendritic
residual melt
Air gap related events
and resulting defects
Ref: Anyalebechi, P. N., ALCOA (2000)
Materials Process Design and Control Laboratory

17. Technical Decision Points
Meniscus freezing:
Occurs during mold filling
Repeated contact of melt with the mold during
solidification
Leads to ripple formation (periodic surface defects)
Meniscus formation without wave mechanism
(a)
(b)
Ripples on a cast surface:
(a) linear, (b) horizontal defects
Meniscus freezing using wave mechanism
(Ref: Stemple, D.K. and Flemings M. C., 1982)
Materials Process Design and Control Laboratory

18. Technical Decision Points
Meniscus freezing related events
and resulting defects
Meniscus freezing
Partial growth of the
shell over the meniscus
Cast Surface defects
Cold shuts/Laps
Ripples
Bleed bands
Inverse segregation
Overflow of
meniscus
Reheating
of shell
Mechanical
bending of the
solid tip
Ref: Anyalebechi, P. N., ALCOA (2000)
Liquid metal restrained from contact with mold wall by the
solidified meniscus
Materials Process Design and Control Laboratory

19. Technical Decision Points
Alloying elements:
Affect distortion characteristics and contact between the shell and the mold
Could lead to defects because of macro-segregation (e.g. inverse segregates,
A and V segregates)
(a) Macro-segregation
patterns in a steel ingots
(b) Close view of a freckle
in a Nickel based super
-alloy blade
(Ref: Beckermann C., 2000)
(a)
(b)
Materials Process Design and Control Laboratory
Materials Process Design and Control Laboratory

20. Technical Decision Points
Mold materials:
May improve or retard heat transfer between metal and mold
Affect gap nucleation time
(very important during the initial stages of solidification)
Fluid flow:
Improve heat transfer rate due to convection
Changes in solid-liquid front morphology because of convection
Affects macro-segregation and inverse segregation in alloys
Degree of superheat:
Increases thermal load
Improves wettability and metal-mold contact
Increases heat flux
Finer microstructure
Smooth solid-shell interface
Materials Process Design and Control Laboratory

21. Technical Accomplishments
Development of a volume-averaged based stabilized finite element simulator
for modeling melt flow in alloy solidification processes
Development of a simulator for modeling deformation and thermal stresses
in solidifying bodies including air-gap formation and growth
Parametric studies of solidification of aluminum on sinusoidal surfaces
characterized by amplitude A and wavelength λ. Parametric studies of the
effects of mold topography, melt flow, superheating, mold material, etc.
Investigation of melt flow `periodic structure formation’ as a possible
mechanism affecting the solidifying shell growth.
Materials Process Design and Control Laboratory

22. Deformation Problem Definition
Heat transfer in the mold, solid shell and melt
Heat transfer causes deformation (thermal stress development)
Mold/shell deformation & contact/frictional conditions affect heat transfer
Materials Process Design and Control Laboratory

23. Gap nucleation time: effects of wavelength
At the very early stages of aluminum solidification, contact pressure between the mold and the solid shell will drop at the trough due to thermal stress development. When this contact pressure reaches zero, gap nucleation is assumed to take place.
For rigid mold (with an topography
amplitude =1 µm, wavelength=1-5 mm), under liquid pressure 8000 Pa, the gap nucleation time
is in the order of seconds.
Physical conditions:
Liquid pressure P=8000 Pa
Thermal resistance at mold-shell interface R=10-5 m2 oC sec J-1
Materials Process Design and Control Laboratory

24. Gap nucleation time: effects of mold conductivity
Mold conductivity affects gap nucleation time
The higher the conductivity, the quicker the gaps nucleate from the mold surface
In this calculations, the deformation of the mold is neglected to illustrate the effects of mold conductivity.
Physical conditions:
Liquid pressure P=10000 Pa
Mold thickness h=0.5 mm
Thermal resistance at mold-shell interface R=10-5 m2 oC sec J-1
Wavelength=2 mm
Materials Process Design and Control Laboratory

25. Gap nucleation time: effects of mold material (deformable mold)
When the wavelength is relatively small, the evolution of the contact pressure at the trough is mainly affected by the conductivity of the mold, i.e. the deformation of the mold does not play a crucial role.
Physical Conditions:
Liquid pressure P=10000 Pa
Mold thickness h=0.5 mm
Thermal resistance at mold-shell interface R=10-5 m2 oC sec J-1
Wavelength=10 mm,
(20 mm, 30 mm in the next two slides)
Materials Process Design and Control Laboratory

26. Gap nucleation time: effects of mold material (deformable mold)
When the wavelength increases, the Ptr-t line is about to show a turn-around pattern when pressure reaches zero. This is defined as the `critical wavelength’ as in the analytical studies of L. Hector (2001)
From this figure, we can say that the critical wavelength is slightly above 20 mm.
In Hector’s analytical study, the critical wavelength is mm, for iron mold and mm for lead mold under the same conditions.
Materials Process Design and Control Laboratory

27. Gap nucleation time: effects of mold material (deformable mold)
When the wavelength is greater than the critical value, the Ptr-t curve shows a turn- around pattern before the contact pressure reaches zero.
The pressure will not
decrease to 0 for an iron or
lead mold, so a large
wavelength is preferred
In practice, it is difficult to
obtain such a smooth mold
topography with amplitude 1
µm and wavelength 30 mm.
Materials Process Design and Control Laboratory

28. Shell thickness at gap nucleation time (rigid mold)
The shell thickness at gap nucleation time plays an important role in deformation.
The thicker the shell, the higher its ability to prevent distortion or warping.
From our calculations, using high
melt pressure is the preferred option
to achieve larger shell thickness at
gap nucleation time.
Materials Process Design and Control Laboratory

29. Coupling fluid flow, heat transfer and deformation
Oscillation of contact will cause oscillation of temperature and stress
Materials Process Design and Control Laboratory

30. Stress development
The sinusoidal mold topography would decrease the stress at the trough, but increase the stress at the crest due to small gaps formed at the trough (not visible in this figure).
Materials Process Design and Control Laboratory

31. Conclusions from calculation before gap nucleation
Shell thickness at gap nucleation time is proportional to wavelength and liquid pressure. So a high pressure or a large wavelength is preferred.
Before gap nucleation, the solid shell is in perfect contact with the mold. Heat flux would be larger, microstructure will be finer, and the growth pattern stable. So if possible, we should avoid gap nucleation.
In practice, the liquid pressure is normally not very high, and the cast surface cannot be very smooth. So gap nucleation is unavoidable and happens very quickly, especially when the amplitude is large or the wavelength is small. In our calculation, amplitude is selected to be 1μm.
Since gap nucleation happens very soon, the growth pattern is mainly determined after gap nucleation.
Materials Process Design and Control Laboratory

32. Shell growth after gap nucleation
After gap nucleation, the heat flux between the mold and the shell is determined by
the contact condition
1mm mm
Solid-liquid interface
Alcoa chill cast results Vcast = 25mm/s
Conclusion:
A smaller wavelength is preferred, because the growth pattern will become stable much faster.
Materials Process Design and Control Laboratory

33. Solidification on uneven surfaces
Solidification of Aluminum on uneven surfaces characterized by
a sinusoid of amplitude A and wavelength λ
ux = uy = 0
q = 0
Presence of a lateral heat transfer component between the
crests and troughs
Assumption of a rigid mold in contact with metal (not modeled here)
ux = uy = 0
Convective heat transfer coefficient assumed constant
ux = uy = 0
Amplitude A and wavelength λ varied for parametric analysis
of heat transfer, fluid flow and phase change
Ti = Tm + ΔT
q = 0
q = 0
Degree of superheat, ΔT = 50 oC
Ambient temperature, T0 = 25 oC
Convection heat transfer
Coefficient, hconv = 0.05 kW m-2 oC-1
Initial temperature, Ti = 710 oC λ y 2A x
q = h (T – T0)
ux = uy = 0
Materials Process Design and Control Laboratory

34. Isotherms and liquid volume fraction contours at different stages of solidification
A = 0.5 mm λ = 10 mm
(a)
(b)
(c)
(d)
(a), (b) liquid volume fraction contours (c), (d) isotherms
Materials Process Design and Control Laboratory

35. Isotherms and liquid volume fraction contours at different stages of solidification
A = 0.5 mm λ = 20 mm
(a)
(b)
(c)
(d)
(a), (b) liquid volume fraction contours (c), (d) isotherms
Materials Process Design and Control Laboratory

36. Isotherms and liquid volume fraction contours at different stages of solidification
A = 1 mm λ = 10 mm
(a)
(b)
(c)
(d)
(a), (b) liquid volume fraction contours (c), (d) isotherms
Materials Process Design and Control Laboratory

37. Streamlines as a function of time
A = 0.5 mm λ = 10mm
A = 0.5 mm λ = 20mm
Materials Process Design and Control Laboratory

38. Streamlines at different stages of solidification
A = 1 mm λ = 10 mm
(e)
(f)
Materials Process Design and Control Laboratory

39. Solidification on uneven surfaces
Times for start of phase change for different A-λ combinations
S. No
Wavelength
(mm)
Amplitude
tp (sec) 1 10
0.5
685.0 2 20
700.0 3
633.0 4
683.0
Streamfunction values for different A-λ combinations
S. No
Wavelength
(mm)
Amplitude
|Ψl,max|
(at t = 795.0) 1 10
0.5
1.21x10-7 2 20
1.8x10-7 3
1.34x10-7
Materials Process Design and Control Laboratory

40. Solidification on uneven surfaces
Starting time for phase change greatly affected by change in thermal conditions
Fluid flow in the vicinity of sinusoid significantly affected, a well defined melt flow
periodicity
Gap formation substantially affects heat transfer from the mold to the shell
Effect of heat transfer on the solid-liquid front morphology substantial (more than the
corresponding effect of fluid flow)
At constant amplitude, shorter wavelength preferable for greater heat transfer
At constant wavelength, higher amplitude preferable for greater heat transfer
Front grows faster but is more distorted for higher amplitudes, when wavelength is kept
constant
Formation of smaller flow cells near the sinusoidal surface, which later dissolve into a
larger cell
Materials Process Design and Control Laboratory

41. Macro-segregation in alloys
Macro-segregation major cause of defects in alloys
Current research focus on modeling inverse segregation,
driven by shrinkage and solutal convection in alloys
Surface segregation and exudation in alloys being modeled
simultaneously
Continuum single domain model based on volume averaged
transport equations used for this purpose
Analysis is being extended to model inverse segregation on
uneven surfaces
Effect of inverse segregation on microstructure to be modeled
Steady state macro-segregation
patterns for directional
solidification
Materials Process Design and Control Laboratory

42. Problems Encountered
Modeling transport phenomena on the scale of surface roughness
Simulation of gaps and their transient growth after gap nucleation
Incorporation of microstructure evolution into the current analysis
Effect of mold topography and surface parameters on microstructure
Deformation of alloys and the effect of macrosegregation on stress
development
Materials Process Design and Control Laboratory

43. Energy Metrics
Data collected from Aluminum Association’s Aluminum Statistical Review 2000
and Aluminum Association’s LCI report for North American Aluminum industry
Net shipments of sheet and plate (made from rectangular ingots) = 10800
million lb
With average semi-fabricating recovery of 60%, million lb of rectangular
ingot cast in 2000
Approximately 5% of each rectangular ingot lost in ingot scalping process
Success of research and subsequent implementations assumed to give a
reduction of 50% reduction in ingot scalping
Materials Process Design and Control Laboratory

44. Energy Metrics
Amount of scalper chips would decrease from 900 million lb to 450 million lb.
Reverbatory furnaces commonly used for melting aluminum
- 20 – 45% efficiency and energy consumption = 0.75 – 1.7 kWh/kg
- total energy requirement = 153 – 347 million kWh per year
The potential manufacturing energy savings from successful implementation
of these technologies are estimated around TRILLION BTU/year.
Materials Process Design and Control Laboratory

45. Ongoing research and future plans
Modeling macrosegregation and inverse segregation in alloys solidifying on
uneven surfaces and their effects on the microstructure
Modeling meniscus freezing and related defects, and coupling it with current
analysis on an uneven surface
Sensitivity analysis for aluminum solidifying on an uneven surface incorporating
all the mechanisms discussed herein
Inverse techniques for design of mold surface topography for desired characteristics
in the cast surface
Development of a mathematical model to study deformation of solidifying alloys
in the presence of a mushy zone
Characterizing the effects of surface parameters and surface roughness on
microstructure of a cast alloy
Materials Process Design and Control Laboratory

46. Ongoing Research and Future plans
Shell growth kinetics
uneven growth
distortion
Metal/mold
interaction
Meniscus
instability
Air gap formation
(non uniform contact
and shell remelting)
Varying stresses
in solid
Lap marks,
ripples,
cold shuts
Interfacial heat transfer
Inverse
segregation
Microstructure
evolution
Surface parameters
and mold topography
in transport processes
Macrosegregation
Materials Process Design and Control Laboratory

47. Commercialization Plan
The technology developed within this project is to be made available through
journals and presentations
Alcoa will incorporate this knowledge to facilitate commercialization
At the present fiscal year, commercialization is not on the agenda
Materials Process Design and Control Laboratory

48. Awards/Recognition
“A stabilized finite element method for flow in porous media and solidification systems”,
Proceedings of the Seventh U.S. National Congress on Computational Mechanics,
presented at the Symposium on ‘Stabilized and Multi-length scale methods’, Seventh
U.S. National Congress on Computational Mechanics, Albuquerque, New Mexico,
July 27-31, 2003
“A stabilized volume-averaging finite element method for flow in porous media and
binary alloy solidification processes”, International journal of Numerical Methods in
Engineering, in press.
“Solidification of Aluminum alloys on uneven surfaces”, submitted in the symposium on CFD Modeling and Simulation of Engineering Processes, 2004 TMS annual meeting and exhibition, Charlotte, North Carolina, March 14-18, 2004
“Effect of mold surface topography on the freezing front morphology during aluminum
casting”, submitted in the symposium on Solidification of Aluminum alloys, 2004 TMS
meeting and exhibition, Charlotte, North Carolina, March 14-18, 2004
Number of students supported = 2
Deep Samanta
Lijian Tan
Materials Process Design and Control Laboratory