Good material properties are the best base for a good simulation. Properties could be found in several locations, such as litterature, material suppliers, universities, web, ...
www.matweb.com is material properties website which contains many useful data.
www.matdata.net is a search engine for material properties.
Version 2006.0 Page 136 No phase change
For thermal problems (with or without solidification), the minimum data which are required are the following (typically for mold materials) :
Thermal conductivity Specific heat
Density
These properties can be either constant or temperature dependant.
With phase change (solidification)
When solidification is present (i.e. for casting materials), one should define in addition the following properties :
Fraction of solid Latent heat
Liquidus and Solidus temperatures
The fraction of solid curve must be temperature-dependant. It should start at 0.0 at high temperature and increase to 1.0 towards the low temperatures. The fraction of solid should be a strictly descending curve and it should be strictly defined
between 0.0 and 1.0. If it is not the case, a warning will be issued. If there is a isothermal transformation (e.g. eutectic plateau), it should be "spread" over an interval of one degree.
The latent heat, liquidus and solidus temperatures are defined by constants. Please note that the liquidus and solidus temperatures should be consistent with the fraction of solid curve (no consistency checks are performed). The liquidus and solidus temperatures are used for the porosity models and for the calculation of the permeability of the mushy zone in the case of flow calculations.
ProCAST offers an alternative in the definition of the phase change. Instead of defining the specific heat, and the latent heat, one can define the corresponding enthalpy curve.
The enthalpy as a function of temperature, H(T), is defined as follows :
where cp(T) is the specific heat as a function of temperature, L is the latent heat and fs is the fraction of solid.
As there are two ways of defining the phase change, the software is automatically detecting if there is a conflict in order to have either :
specific heat Latent heat or
enthalpy
If this is the case, the following message is displayed :
and the user has to select which definition is preferred.
In previous versions (v4.x.x and v3.x.x), there was no check to prevent both definitions. Thus, if a model which was created in a previous versions is loaded into PreCAST v2006.0, the following warning will be displayed (during the load in PreCAST) :
The user will need to resolve the conflict, by selecting which data are to be kept (i.e. either enthalpy or specific heat/latent heat) for each material which has this duplicate definition.
Density
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For fluid flow problems, it is mandatory to define the viscosity. Then, optionnal definitions are available, such as "Surface Tension", "Permeability" and "Filter".
Viscosity
Several viscosity models are available in ProCAST : Newtonian
Carreau-Yasuda Power-cutoff
The Carreau-Yasuda model corresponds to Non-Newtonian flow, where the viscosity depends upon the shear rate (see the equation below) :
with :
strain rate
zero strain rate viscosity infinite strain rate viscosity phase shift
n Power law coefficient a Yasuda coefficient
The above parameters can be defined in the database (as constants or as function of temperature) as follows :
The Power-cutoff is used in the case of Thixocasting.
Surface tension
This option is not described as it has not been validated at this stage. One can say that in conventional casting processes, the surface tension effects are certainly negligible in comparison to the simplifications made in the free surface algorithms.
Permeability
For a casting material, the permeability is defined by a Karman-Cozeny model, modified by Beckermann at low fraction of solid. The user has also the ability to define its own permeability table, as a function of temperature. In this case, a high permeability corresponds to a "free flow", whereas a low value corresponds to "no flow". For "casting" materials, the permeability is applied only in between the solidus and the liquidus temperatures.
For mold materials (in the case of lost foam), a permeability should be defined. In this case, one can define a constant or a temperature dependant permeability.
For Filter materials, if the Permeability is defined, it will override the default permeability calculated from the Filter tab.
Filter
Filters are characterized by the following properties : Void fraction
Surface area Pressure Drop
The void fraction (Fv) corresponds to the amount of "porosity" or void inside the filter. This value is dimensionless [-]. The definition of this value is mandatory in all cases.
The Surface area (Sa) corresponds to the amount of "interface" between the filter material and the air (when the filter is empty) per unit volume (see example below). This value is used for the calculation of the thermal exchange between the filter and the liquid metal going through, as well as for the automatic
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The Permeability of the filter (i.e. its resistance to the flow) can be calculated in three different ways.
a) Automatic permeability calculation
From the Void Fraction (Fv) and the Surface Area (Sa) definitions, the permeability can be automatically computed (based upon Karman-Cozeny), according to the following relationship :
This mode is activated if the "Pressure Drop" and "Permeability" tabs are not defined.
b) Pressure drop calculation
If the "Pressure Drop" tab is defined, then, the permeability is calculated, using the following values (coming from simple experiments) :
and the following equation :
where v, ∆P and ∆x are measured values which can be made in a simple
experiment (or provided by the Filter supplier). Please note that the Flow rate, v, corresponds to the velocity used in the experiment and not the velocity of your casting model.
If both the "Pressure Drop" and "Permeability" tabs are defined, the
"Permeability" values are ignored (and replaced by the ones obtained from the above equation).
c) Specified permeability
It is also possible to define a given permeability value in the "Permeability tab". In this case, the "Void fraction" is not used for the automatic Karman-Cozeny
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Material properties, such as the enthalpy curve and the solidification path (i.e. the fraction of solid curve versus temperature), density, viscosity and thermal
conductivity can be computed automatically from thermodynamic databases.
ProCAST has an automatic link with thermodynamic databases to calculate these properties. It is thus possible to compute the enthalpy curve, the fraction of solid curve, the density, the viscosity and the thermal conductivity, based upon the chemical composition, for the following systems and the following alloying elements (the elements shown in blue were added in version 2006.0) : CompuTherm LLC databases
The other alloying elements which are not present in this list are not available in the database and will have no effect on the computed material properties. More details about composition limitations are given in the next section.
To activate the thermodynamic database, one should go in the "Composition" tab of the material properties window.
Then the Base alloy (i.e. Al, Fe, Ni, Ti or Mg) should be entered, as well as each alloying element with its concentration (in weight percent). Once the chemical composition is entered, the "Apply->" button is pressed and the "Scheil" or
"Lever" option is selected to start the computation of the fraction of solid and of the enthalpy.
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"Scheil" and "Lever" correspond to two different microsegregation models. In the case of "Lever", the Lever Rule is applied, which corresponds to a complete mixing of the solute in the solid (i.e. very good diffusion in the solid). On the other hand, the Scheil model corresponds to no diffusion at all in the solid phase (both model consider complete mixing or infinite diffusion in the liquid).
The main difference between the two models is the shape of the fraction of solid curve at the end of solidification, as well as the solidus temperature (see figure below).
For most alloys, it is recommended to use the Scheil model, except for low alloy steels where the diffusion in the solid is very fast.
In the example above, when the Scheil button is selected, the following curves appear :
One could see the fraction of solid curve, as well as the fractions of the different phases as a function of temperature.
In the same time, automatically, the fraction of solid curve, the liquidus and solidus temperatures, the enthalpy curve, the density, the viscosity and the thermal conductivity are stored in the database, as shown hereafter (of course the value are finally stored only when the "Store" button is pressed, before exiting the
database).
Version 2006.0 Page 146 Density
Enthalpy
Fraction of solid
Version 2006.0 Page 148 Liquidus-Solidus
Viscosity
Please note that when a thermodynamic database is used, as the enthalpy is calculated, the specific heat and the latent heat should not be defined (as they are contained in the enthalpy).
During the Thermodynamic database calculation, a file named "prefix.phs" is created. It contains for each temperature the phase fractions, as well as the composition of each phase. This information is not needed for a ProCAST calculation, but it can be interesting for other purposes (e.g. growth kinetics calculations).
For some chemical composition, it may happen that the software which extracts the data from the Thermodynamic database is not able to find the right set of stable phases at low temperature.
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as such. If such situation occurs, one should check the calculated data in order to be sure that it covers at least the temperature range of interest.
In very few cases, it is possible that the density calculation does not give relevant results, as shown in the figure hereafter. In this case, these result should not be used (or the wrong values should be erased).
As a general rule, if the results are not realistic, it is advised to suppress (i.e.
ignore) the elements which are present in very low concentrations. This is especially true for traces of Sulfur (S) and Phosphorus (P) in steels, which are sometimes "corrupting" the results.
Calculation of Stress Properties
Beside the thermal properties, it is possible to calculate automatically some Stress properties.
At this stage, the Young's modulus, the Poisson's ratio and the Thermal expansion coefficient can be calculated based upon the phases obtained from the
thermodynamic databases.
When the Properties calculation is started in PreCAST with either the Scheil or the Lever model, the following window appears :
The user has the choice of either not calculate the Stress properties, to create a new entry in the Stress database or to substitute/Add the data to an existing entry.
If a new entry is created the user has to specify its name (without spaces).
If the user would like to substitute or add the calculated data to an existing entry of the stress database, one should select the desired entry with the Browse button.
Once these choices are made, the computation can be started (of both the thermal and stress properties) with the "Compute" button.
Please note that the other Stress properties (i.e. Yield stress, hardening, viscoplastic,...) can not be calculated at this stage. Thus these properties will remain empty.
The following figures are showing examples of computed Stress data from an A356 alloy.
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Young's modulus
Poisson's ratio
Thermal expansion coefficient Databases limitations
The Computherm databases can be used for the following elements and in the following ranges. More information can be obtained on the
www.computherm.com web site.
The recommended composition ranges mentioned hereafter are not strict limits.
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Developed for Al-rich alloys such as commercial casting and wrought alloys.
Tested with more than 40 commercial Al alloys.
20 Components :
Major alloy elements: Al, Cu, Fe, Mg, Mn, Si, Zn
Minor alloy elements: Ag, B, C, Cr, Ge, Hf, Ni, Sc, Sn, Sr, Ti, V, Zr
Major phases: Liquid, Fcc_A1(Al), Diamond_A4(Si), Al5Cu2Mg8Si6, Al8FeMg3Si6, Eps, Sigma-(Al,Cu,Zn)2Mg, T-(Al,Cu,Zn)49Mg32, Al20Cu2Mn3, Al23CuFe4, Al7Cu2Fe, S-Al2CuMg, TAO(t), a-AlFeSi, b-AlFeSi, AL15_FeMn3Si2(a-AlMnSi), AlMnSi-Beta, AlCu_Theta(q), Al13Fe4,
AlMg_Beta, Al11Mn4, Al12Mn, Al4Mn, AL6_FeMn, Al3Ni1, AlSr4, Mg2Si,Al3Zr, Al3Sc_x
Recommended composition range (in wt%):
Al 80 ~ 100
59 Phases: Liquid, BCC_A2 (ferrite), HCP_A3,
FCC_A1(austenite), TCP phases, Carbides, and so on.
Recommended Composition Limits (wt%):
It was observed that alloy elements which are present is very small quantities (such as P and S) may cause problems in the phase determination. As these elements do not affect significantly the material properties (although it may have important effects in other fields), it is recommended to remove these elements for the computation.
Mg database
Developed for commercial Mg-rich alloys 16 components:
Mg, Ag, Al, Ca, Ce, Cu, Gd, Li, Mn, Nd, Sc, Si, Sr, Y, Zn, Zr
Contains more than 200 phases.
Recommended composition range (in wt%):
Not yet available
Ni database
Developed for commercial Ni-rich alloys.
17 Components: Al, B, C, Co, Cr, Fe, Hf, Mo, N, Nb, Ni, Re, Si, Ta, Ti, W, and Zr.
63 Phases: Liquid, Fcc_A1(g), L12_Fcc(g¢), TCP phases, Carbides, and so on.
Developed for commercial Ti-rich alloys such as alpha, alpha+beta, and beta
Version 2006.0 Page 156 Ti > 75
Al, V < 11
Mo, Nb, Ta, Zr < 8 Cr, Sn < 5
Cu, Fe, Ni < 3
B, C, H, N, O, Si < 0.5
Influence of alloying elements
The goal of this section is to illustrate the influence of alloying elements on properties and to show why properties obtain with thermodynamic databases may differ from experimental data.
An AlSi9Cu3Fe will be considered to illustrate this example. The usual average chemical composition of such alloy is the following :
When the Scheil model is used, with the above composition, 10 phases are found (in addition to the liquid phase) :
Starting from AlSi9Cu3Mg0.3, the other alloying elements are added
progressively. The effect on the solid fraction curve is shown in the following figures (please note that the Temperature scale is changing from one graph to the next one):
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AlSi9Cu3Mg0.3
AlSi9Cu3Mg0.3Fe1.3
(the effect of Fe is mainly visible at the liquidus)
AlSi9Cu3Mg0.3Fe1.3Mn0.55Ni0.55Zn1.2
(Mn, Ni and Zn are mainly affecting the second half of the curve)
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AlSi9Cu3Mg0.3Fe1.3Mn0.55Ni0.55Zn1.2Cr0.15 (Cr is rising the liquidus temperature from 612 to 640°C)
AlSi9Cu3Mg0.3Fe1.3Mn0.55Ni0.55Zn1.2Cr0.15Ti0.15
(When Ti is added, the Al3Ti phase appears, with a very high liquidus temperature above 760°C)
The following figure are showing the solid fraction curves for all the alloys together.
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In order to see the effect near the liquidus, the same figure is shown, with a different vertical scale. Only the first 5% are shown.
One can see very well on this latter figure the effect of Ti. Ti is added in order to create this Al3Ti phase which is stable at very high temperature, which is acting as inocculant. The amount of this phase is very small (around 0.5%).
The above example is showing that one should be careful with the use of Thermodynamic databases. In this case, for instance, it would be advisable to ignore the Ti for the thermodynamic computation, in order to avoid this
"artificially" high liquidus temperature.
This explains also why there are differences observed between literature values (measurements) and computed values for liquidus and solidus temperature. This is due to the fact that such values are measured usually by Thermoanalysis and that
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