COMBINED EXPERIMENTAL AND COMPUTATIONAL APPROACH FOR THE DESIGN OF MOLD SURFACE TOPOGRAPHY IN ALUMINUM CASTING DATE OF PRESENTATION : 4 OCTOBER 2005 PRINCIPAL INVESTIGATOR : PROF. NICHOLAS ZABARAS PERFORMING ORGANIZATION : MATERIALS PROCESS DESIGN AND CONTROL LABORATORY, CORNELL UNIVERSITY PROJECT START DATE : 31 AUGUST 2002 PROJECT END DATE : 30 AUGUST 2005
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
Project Objectives • A design methodology will be developed with which casting mold • surfacetopologies can be tuned to produce required surface features • andmicrostructural features of Aluminum ingots. • Both static and continuous casting processes will be examined with instrumental molds. Mold surface topographies, which consist of unidirectional and bidirectional groove textures, will be generated using contact and non-contact techniques to elicit a radiator like effect at the mold casting interface.
Project 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)
Liquation TCG Blebs Sweats Folds Cold shuts Project Objectives Classification of direct cast surface defects in direct chill cast ingots Surface tears Cracks Pre-solidification cracks Post-solidification cracks Bleed bands Subsurface segregation and non-uniform microstructure CAST SURFACE DEFECTS Surface irregularities Duplex micro. Ripples/Laps Surface porosity Cavities Gas porosity Blisters Oxide patches
Project Innovations • This project is developing methodologies to reduce surface defects in castings by designing appropriate mold surface topologies. • These techniques can help reduce material, energy and monetary losses during post-scalping operations.
Project Innovations • Some mold surface • topographies used in • Alcoa • Profound effects on the • morphology of final cast • surfaces
Project Energy Savings • 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%, 18000 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.
Project Energy Savings • Amount of scalper chips would decrease from 900 million lb to 450 • million lb. • Total energy required for secondary ingot casting. • =115 kWh of electricity + 126 m3 of natural gas per 1000 kg of cast • ingot • = 2240 BTU/lb of cracked ingot that is re-melted and re-cast • The potential manufacturing energy savings from successful implementation of these technologies estimated around 1.01 TRILLION BTU/year.
Project Baseline and Critical metrics • Baseline Metrics: • Current Aluminum casting techniques result in the formation of • scalp depth and ingot scalping consumes approximately 5% of each • rectangular ingot scalped. Therefore, the volume of scalped chips is • 900 million lb/year with the totalamount of Aluminum cast taken as • 18000 lb per year approximately. • The total energy required for secondary ingot casting is 115 kWh of electricity plus 126 cubic meters of natural gas per 1000 kg of cast ingot, or 2240 BTU/lb of cracked ingot that is re-melted and re-cast. • The principal emissions from the secondary casting are 0.066 lb of CO2, 0.023 lb of CO and 0.080 lb of solid waste per lb of cast ingot.
Project Baseline and Critical metrics • Project Metrics: • Success in research efforts could achieve around 2.5% reduction in ingot scalping and the volume of scalp chips would come down to 450 million lb/yr. • The potential for manufacturing energy savings in the domestic Aluminum industry is estimated to be around 1.01 trillion BTU/yr. • A potential reduction of up to 29 million lb/yr of CO2, 10 million lb/yr of CO and 36 million lb/yr of solid waste can be accomplished.
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 fluid flow solidification and macrosegregation • Aluminum and Aluminum alloys. Uneven growth Plain mold plate Even growth Mold plate with grooves
Technical Decision Points Air gap related events and resulting defects Shell formation Cast Surface defects Shell distortion & mold movement • Bleed bands • Blobs • Liquation • Presolidification cracks Air gap formation Re-melting of shell Flow of interdendritic residual melt Reduction in heat transfer Crack initiation Ref: Anyalebechi, P. N., ALCOA (2000)
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
Project Accomplishments • Development of a robust, dimension independent stabilized finite element • simulator to model solidification of alloys under various process conditions. • Investigating the effects of varying mold surface topography, in the form of • sinusoids, on fluid flow and macrosegregation in solidifying Aluminum alloys. • Development of a simulator for modeling coupled deformation, air-gap • formation, thermal and inelastic stresses in solidifying Aluminum metal • and alloys. • Investigations of uneven cooling and non-uniform contact, due to air gap • formation, uneven surface topography and liquid pressure on thermal • stresses in solidifying Aluminum. • Parametric analysis of air-gap formation, equivalent stresses and segregation • in Aluminum alloys under varying process conditions.
Project Accomplishments • Microstructure evolution during solidification of metals in two and three • dimensions using energy conserving level set methods. • Evolution of droplet surfaces and early stage solidification using surface • evolver and level set methods.
Solidification of Aluminum-Copper alloy on uneven surfaces vx = vy = 0 q = 0 vx = vy = 0 q = 0 q = h (T – Tamb) vx = vy = 0 vx = vy = 0 q = 0 vx = vy = 0 vx = vy = 0 Ls 0.13 m q = 0 q = 0 0.08 m • Surfaces from where heat is removed modeled as sinusoids • with fixed amplitude A and wavelength λ. • Varying surface topography by changing A and λ leads to • changes in heat transfer, fluid flow and macrosegregation. • The reference case consists of a perfectly rectangular • cavity. Ls y x q = h (T – Tamb)
Vertical solidification from uneven surfaces t = 121 sec t = 66 sec • Isotherms (b) liquid mass fraction (c) liquid solute concentrations • A = 1 mm, λ = 10 mm
Vertical solidification from uneven surfaces t = 121 sec t = 66 sec • Isotherms (b) liquid mass fraction (c) liquid solute concentrations • A = 0.5 mm, λ = 10 mm
Vertical solidification from uneven surfaces t = 121 sec t = 66 sec • Isotherms (b) liquid mass fraction (c) liquid solute concentrations • A = 0.5 mm, λ = 5 mm
Vertical solidification from uneven surfaces Midplane solute (Cu) concentrations for varying mold topography • Extent of inverse segregation found to increase with increase in amplitude or • decrease in wavelength. • For both cases, inverse segregation is lowest when surface is even.
Vertical solidification from uneven surfaces Midplane vertical velocities (vy) for varying mold topography • Vertical velocity magnitudes increase with increase in amplitude or decrease • in wavelength. • For both cases, the magnitude is lowest when the surface is even.
Horizontal solidification from uneven surfaces Evolution of solid/mushy zones and velocity for varying mold topography A = 0.5 mm, λ = 10 mm A = 1mm, λ = 10 mm A = 0.5 mm, λ = 5 mm A = 0 mm, λ = mm • Fluid flow primarily driven by thermal and solutal convection for all cases here. • Fluid flow much stronger here compared to the vertical solidification cases. • Magnitudes of velocity vary with amplitude and wavelength of the surface.
Horizontal solidification from uneven surfaces Overall macrosegregation evolution for varying mold topography A = 0.5 mm, λ = 10 mm A = 1mm, λ = 10 mm A = 0.5 mm, λ = 5 mm A = 0 mm, λ = mm • Macrosegregation driven by thermosolutal convection occurs throughout the cavity. • Solute depletion occurs at the top and solute enrichment at the bottom. • Degree and extent of macrosegregation vary with amplitude and wavelength.
Main observations and conclusions from this study • In vertical solidification, increase in unevenness either by increasing the amplitude or • decreasing the wavelength leads to increase in inverse segregation. • This is because, the increase in contact surface area accelerates the phase change rate and increases the intensity of shrinkage driven flow, which is the main factor behind inverse segregation. • Maximum velocity magnitudes and differences in maximum and minimum solute • concentrations also highlight this fact. • In horizontal solidification, increase in unevenness leads to increase in the overall • extent of segregation highlighted by the GES values, but differences in the maximum and minimum solute concentrations first increase and then decrease. • Maximum velocity magnitudes first increase and then decrease when either the amplitude is increased or wavelength decreased. • Thermosolutal convection, which is the dominating factor affecting macrosegregation in horizontal solidification, is suppressed when surface unevenness increases leading to higher rate of phase change.
Solidification of Al on Uneven Surfaces Hypoelastic model without plastic deformation (Hector et al. 2000) • Heat transfer in the mold, solid shell and melt. • Heat transfer causes deformation (thermal stress). • Gaps or contact pressure affect heat transfer. • Solidification after air-gap nucleation not modeled. Materials Process Design and Control Laboratory
Thermal resistance at mold/shell interface Contact resistance: • At the very early stages, the solid shell is in contact with the mold and the thermal • resistance between the shell and the mold affected 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
Mold – Metal Boundary Conditions Consequently, heat flux at the mold – metal interface is a function of air gap size or contact pressure: = Air-gap size at the interface = Contact pressure at the interface • The actual air – gap sizes or contact pressure are determined from the contact sub problem. • This modeling of heat transfer mechanism due to imperfect contact very crucial for studying the non-uniform growth at early stages of solidification.
Gap nucleation time: effects of wavelength • At the very early stages of aluminum solidification, contact pressure between mold and solid shell will drop at the trough due to thermal stress development. When this contact pressure drops to 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 m2oCsec J-1
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 m2oCsec J-1 Wavelength=2 mm
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 m2oCsec J-1 Wavelength=10 mm, (20 mm, 30 mm in the next two slides)
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’ in the analytical studies of L. Hector. From this figure, we can say that the critical wavelength is slightly above 20 mm. In Hector’s analytical study, the critical wavelength is 16.60 mm, for iron mold and 14.03 mm for lead mold under the same conditions.
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 won’t decrease to 0 for an iron or lead mold, so a large wavelength is preferred. • But in practice, we can never get a such a smooth mold topography with amplitude 1 µm and wavelength 30 mm.
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 more its ability to prevent distortion or warping. • From calculations, high pressure is the preferred option to achieve larger shell thickness at gap nucleation time.
Modeling Deformation In Mushy Zone • Low solid fractions usually accompanied • by melt feeding and no deformation due to • weak or non – existent dendrites • leads to zero thermal strain. • With increase in solid fraction, there is an increase in strength and bonding ability of • dendrites to non – zero thermal strain. • The presence of a critical solid volume fraction is observed in experiment and varies for different alloys. The parameter wis defined as: • Liquid or low solid fraction mush • -any deformation induced by thermal expansion is permanent. (Without any strength) • Solid or high solid fraction mush • -plastic deformation is developed only gradually.
Modeling Deformation In Mushy Zone • For deformation, we assume the total strain can be decomposed into three parts: • elastic strain, thermal strain and plastic strain. • Elastic strain rate is related with stress rate through an hypo-elastic constitutive law • Plastic strain evolution satisfy this creep law with its parameters determined from experiments (Strangeland et al. (2004)). • The thermal strain evolution is determined from temperature decrease and shrinkage. Strain measure : Elastic strain Plastic strain Thermal strain
Parameters for simulation of deformation in mushy zone Critical solid fraction for different copper concentrations in aluminum-copper alloy Ref: Mo et al.(2004) Creep law for plastic deformation Ref. Strangeland et al. (2004) Strain-rate scaling factor Stress scaling factor Activation energy Creep law exponent Volumetric thermal expansion coefficient Mushy zone softening parameter Volumetric shrinkage coefficient
Schematic of the Problem Definition • An Aluminum-copper alloy is solidified on an sinusoidal uneven surface. • With growth of solid shell, air – gaps form between the solid shell and mold due to imperfect contact – which further leads to variation in boundary conditions. • The solid shell undergoes plastic deformation and development of thermal and plastic strain occurs in the mushy zone also. • Inverse segregation caused by shrinkage driven flow causes variation in air – gap sizes, front unevenness and stresses developing in the casting.
Solidification Coupled with Deformation and Air-gap Formation Important parameters 1) Mold material - Cu 2) CCu = 8 wt.% 3) ΔTmelt = 0 oC Air gap is magnified 200 times. • Preferential formation of solid occurs at the crests and air gap formation occurs at the trough, which in turn causes re-melting. • Because of plastic deformation, the gap formed initially will gradually decrease. • As shown in the movies, a 1mm wavelength mold would lead to more uniform growth and less fluid flow.
Solidification of Al-cu Alloy on Uneven Surfaces • Combined thermal, solutal and • momentum transport in casting. • Assume the mold is rigid. • Imperfect contact and air gap • formation at metal – mold interface Solidification problem We carried out a parametric analysis by change these four parameters 1) Wavelength of surfaces (λ) 2) Solute concentration (CCu) 3) Melt superheat (ΔTmelt) 4) Mold material (Cu, Fe and Pb) Heat Transfer (Mold is rigid and non-deformable) Deformation problem Both the domain sizes are on the mm scale
Transient Evolution of Important Fields (λ = 5 mm) • Temperature • Solute concentration • Equivalent stress • (d) Liquid mass fraction Important parameters 1) Mold material - Cu 2) CCu = 5 wt.% 3) ΔTmelt = 0 oC (b) (a) • We take into account solute transport and the densities of solid and liquid phases are assumed to be different. • Inverse segregation, caused by shrinkage driven flow, occurs at the casting bottom.This is observed in (b). (d) (c)
Transient Evolution of Important Fields (λ = 3 mm) • Temperature • Solute concentration • Equivalent stress • (d) Liquid mass fraction (b) (a) • For smaller wavelengths, similar result is observed: (1) preferential formation of solid occurs at the crests (2) remelting at the trough due to the formation of air gap. • For wavelength 3mm, the solid shell unevenness decreases faster than the case of 5mm wavelength. (d) (c)
Variation of Air-gap Sizes and Max. Equivalent Stress λ = 5 mm, CCu = 5 wt.%, mold material = Cu • Air-gap sizes increase with time • Increasing melt superheat leads to • some suppression of air gaps • Initially, stresses higher for lower superheat • At later times, the difference is small Increasing melt superheat leads to some suppression of air gaps and a smaller stress at beginning stages. At later times, the difference of equivalent stresses is however small.
Effect of Wavelength on Air-gap Sizes and Max Equivalent Stress ΔTmelt = 0 oC, CCu = 5 wt.%, mold material = Cu • Max. equivalent stress σeqvariationwith λ • σeq first increases and then decreases • Initially, σeq is higher for greater λ • Later (t=100 ms), stressis lowest for • 5 mm wavelength. • Air-gap size variation with wavelength λ • Initially, air-gap sizes nearly same for • different λ • At later times, air-gap sizes increase • with increasing λ
Variation of Air-gap Sizes and Maximum Equivalent Stress ΔTmelt = 0 oC, λ = 5 mm, mold material = Cu Increase of solute concentration leads to increase in air-gap sizes, but its effect on stresses are small. • σeq first increases and then decreases • Variation of σeq with Cu concentration • is negligible after initial times • Air-gap sizes increase with time • Increasing Cu concentration leads to • increase in air-gap sizes
Variation of Air-gap Sizes and Max. Equivalent Stress ΔTmelt = 0 oC, λ = 5 mm, CCu = 5 wt.% Gap nucleation and stress development are prominent for a mold of higher thermal conductivity like Cu. For Fe or Pb molds, heat removal is inhibited due to their lower thermal conductivity. This in turn inhibits air-gap formation and development of stresses.. • Air gap sizes higher for Cu molds than • Fe or Pb molds • Equivalent stress far lower for Cu molds • than Fe or Pb molds
Effect of Inverse Segregation – Air Gap Sizes (a) With inverse segregation (b) Without inverse segregation By comparing the result with modeling inverse segregation and without modeling inverse segregation, we can find that inverse segregation actually plays an important role in air-gap evolution. • Differences in air-gap sizes for different solute concentrations aremore pronounced in the presence of inverse segregation.
Variation of Equivalent Stresses and Front Unevenness Time t = 100 ms • Value of front unevenness and maximum equivalent stress for various wavelengths • one cannot simultaneously reduce both stress and front unevenness • when the wavelength greater than 5mm, both unevenness and stress increase-> implies wavelength less than 5 mm is optimum • Equivalent stress at dendrite roots • The highest stress observed for 1.8% copper alloy suggest that aluminum copper alloy with 1.8% copper is most susceptible to hot tearing • Phenomenon is also observed experi-mentally Rappaz(99), Strangehold(04)
Effect of Mold Coatings on Solidification of Al-cu Alloy Mold coating • Combined thermal, solutal and • momentum transport in casting. • Assumption of a rigid mold. • Imperfect contact and air gap • formation at metal – mold interface Solidification problem - Thickness of coating assumed to be of the order of mm. - Thermal conditions assumed to be similar as before. - Coefficient of friction μvaried for different mold coatings. Heat Transfer (Mold is rigid and non-deformable) Deformation problem Both the domain sizes are on the mm scale
Variation Of Air-gap Sizes And Max. Equivalent Stress ΔTmelt = 0 oC, λ = 5 mm, mold material = Cu Effect of change in the coefficient of friction (μ) on air gap sizes and equivalent stresses is negligible. • Magnitude of equivalent stresses • unaffected. • Transient behavior same as before. • Magnitude of air-gap sizes unaffected. • Transient behavior same as before.