Magnesium Technology 2008 Edited by Mihriban O. Pekguleryuz, Neale R. Neelameggham, Randy S. Beals, and Eric A. Nyberg
TMS (The Minerals, Metals & Materials Society), 2008
Castability of Magnesium Alloys
Shehzad Saleem Khan1 , Norbert Hort1 , Ingo Steinbach2 , and Siegfried Schmauder3
1
GKSS Forschungszentrum GmbH, Max Planck Strasse 1, D-21502 Geesthacht
2
3
ACCESS e.V. RWTH Aachen. Intzestraße 5 D-52072 Aachen
Institut für Materialprüfung, Werkstoffkunde und Festigkeitslehre (IMWF), Universität Stuttgart, Pfaffenwaldring 32, D-70569
Stuttgart
Keywords: Fluidity Simulations, Microstructure Simulations, Magnesium Die-Casting
using MICRESS (Micro Structure Evolution Simulation
Software). The last section advises on the comparison of the
simulated and the experimental results and provides validation of
the results.
Abstract
Research and development of magnesium alloys depends largely
on the metallurgist’s understanding and ability to control the
microstructure of the as-cast part. This work comprises the
determination of experimental input parameters to run a
successful and vast informative simulation using state of the art
software. Various thermodynamic analyses have been used to
study the thermo physical properties of binary magnesiumaluminum alloys, and the resultant microstructures have been
simulated and then compared with the experimental output. The
calculated heat distributions have been used to simulate the
resulting microstructure using the phase-field method.
This paper presents an overview of a range of ideas that have been
undertaken to improve our understanding on the gravity diecasting behavior & solidification characteristics of Mg-Al alloys.
It follows the solidification process of binary alloys Mg-Al,
beginning with the nucleation and grain refinement. The
simulated and the experimental results are compared and used for
validation.
____
__
50m
Figure 1: The Mold made of Steel, maintained at constant
250 ºC for all casting experiments. (Left) shows the CAD
modeled geometry of the mold. (Right) Real world
geometry in a glance. Introduction
Currently few sources of magnesium solidification information
and as-cast microstructures exist. So far there has not been any
subsequent progress in the field of casting simulations for
magnesium fluidity. This research work comprises the
determination of input experimental parameters to run a
successful and vast informative simulation using state of the art
software. Therefore, the goal of this thesis is to increase the
general knowledge base of magnesium fluidity behavior with
different mold geometries and to simulate the resultant
microstructures.
Various thermodynamic equipments have been used to study the
thermo physical properties of different magnesium alloys with
including the (CALPHAD) Calculation of Phase Diagram
analysis. These analyses have been performed on 13 binary
magnesium-aluminum alloys. The resultant microstructures have
been simulated and then have been compared with the
experimental output. The temperature distribution and the heat
dissipation during casting has been simulated with the finite
difference method and compared with experiments for different
geometries.
This paper presents an overview of a range of ideas that have been
undertaken to improve our understanding on the gravity diecasting behavior & solidification characteristics of Mg-Al alloys.
It follows the solidification process of binary alloys Mg-Al,
beginning with the nucleation. The later section considers the
collection of the pre-requisite thermo physical properties for
simulation of the whole casting process using Finite difference
based Magmasoft® and the simulated microstructure obtained
Fluidity Measurement
Until recently, interest in metal fluidity has been concentrated
largely in the field of ferrous metals and Aluminum alloys.
Fluidity tests have been developed and are used commercially as
quality checks to
determine the flowing qualities of molten
metal [1]. Fluidity is, in casting terminology, the distance to
which a metal, when cast at a given temperature, will flow in a
given test mould before it is stopped by solidification [2], [3],[4].
Fluidity is therefore a length, usually in millimeters or meters. It is
an important property for casting alloys and determines the ability
of the liquid metal to fill the mold cavity.
The term Fluidity has come to a meaning quite different to the
foundry man than to the physicist. To the physicist, it is the
reciprocal of viscosity; to the foundry man, it is an empirical
measure of a processing characteristic [5]. Fluidity, in the casting
sense, refers to the property of a metal which allows it to flows
when it is poured into a standard fluidity test channel.
This channel may be straight or it may be in the form of a spiral,
the cross-section may be round, half round, trapezoidal, or
rectangular. In GKSS a familiar fluidity spiral has been used
(figure 1). The channel was wound into a spiral, thereby
simplifying handling and levelling problems. However, the
fluidity of magnesium alloys and the influence of alloying
elements have not yet been well studied. The purpose of this test
rig to develop a type test which would be simple and easy to use,
and would also:
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1.
Afford precise control over metallostatic head, and
permit this pressure head to be reached before any metal
entered the fluidity spiral.
2.
Provide control over metal turbulence as the metal
entered the spiral.
3.
Filter any dross or other foreign materials from the
metal before the metal entered the fluidity spiral and do
so without altering the metal “head”.
4.
Be of flat cross-section to simulate problems
encountered in pouring sand castings of thin section in
the foundry.
microscopy, scanning electron microscopy. The material in billet
form was subjected initially to macro- and micro structural
observations. Samples for light microscopy, interference layer
microscopy and SEM were mounted for subsequent ease of
handling. The melt temperature was always kept 100˚C above
liquidous of the alloy.
Cooling rate is one of the essential criterion for the simulation of
micro-structure. It is very difficult to determine the cooling rate of
the cast alloy while it is casted in the spiral; this is why the same
mold material (as in figure 1) was taken. Figure 2 shows the cast
alloy after it has been cast in a straight channel mold.
It is well known that fluidity relates to the mold and metal
parameters, as well as to casting conditions imposed during mold
feeding. It is thus possible to quantatively determine the values of
mold filling ability at various pressure heads. In the present
investigation, the mold filing ability characteristics have been
evaluated for binary Mg-Al alloys. The evaluation has been
carried out in three phases:
¾
¾
¾
Figure 2: The sample scheme of the billet used for
microstructure analysis.
Evaluation (experimental & simulated) of the mold
filling ability values of all binary Mg-Al alloys
containing up to 12% of aluminium by weight at various
pressure heads and superheats.
Study of the influence of process parameter on mold
filling ability of these alloys.
Evaluation of alloy and optimization of the mold
geometry and mold parameters to maximize mold filling
ability values.
Thermal Analysis Techniques
There are many techniques available for investigating the
solidification of metals and alloys. There are standardized
techniques such as differential thermal analysis (DTA),
differential scanning calorimetry (DSC). These techniques,
although well documented and very accurate, prove to be
sufficient for investigating the thermodynamic properties of alloys
such as Solidus, liquidous, melting range & heat of fusion,
specific heat etc.
Experimental Procedures
Table 1: Illustrates the results from the DSC experiments.
Nr.
Alloys
T
T liquidous (K)
Freezing
Range
solidus
(K)
(K)
1.
Pure Mg
923.0 924.1
------*
2.
Mg-1Al
902.9 917.9
15.02
3.
Mg-2Al
893.1 912.7
19.68
4.
Mg-3Al
875.1 907.6
32.54
5.
Mg-4Al
848.0 902.5
54.51
6.
Mg-5Al
816.2 897.3
81.18
7.
Mg-6Al
794.5 892.2
97.72
8.
Mg-7Al
790.9 887.0
96.14
9.
Mg-8Al
767.0 881.8
114.82
10.
Mg-9Al
763.9 876.5
112.65
11.
Mg-10Al 748.5 871.2
122.72
12.
Mg-11Al 731.4 865.8
134.41
13.
Mg-12Al 738.6 860.3
121.73
Although magnesium can be fabricated by virtually all
manufacturing techniques [18], this research focuses on casting
and the alloys specifically designed for casting. Magnesium
casting processes may be divided into three groups, sand casting,
permanent mold casting, and high-pressure die-casting. Selection
of casting processes is determined by the size, required tolerance,
and production quantity, similar to other commonly cast material.
High-pressure die-casting is currently the most commonly used
method for magnesium alloys.
Mg-Al Binary System
The Mg-Al binary system is origin of some of the oldest and the
most commonly used casting alloys. Alloys such as AZ91, AM50
and AM60 still comprise a large portion of all magnesium alloy
casting [6], [7].The maximum solubility of Al in Mg ranges from
about 2.1wt% at 25 ºC to 12.6 wt% at the eutectic temperature of
437 ºC. The eutectic composition is 32.3wt% and the eutectic is
between α-Mg and the β-phase, which is Mg17Al12 [8]. As the
simplest case the Mg-Al binary alloys were considered in this
paper. Ternary alloys can make the process of simulations very
complex because of the formation of multiple phases.
On the other hand these are inadequate for investigating
solidification of metal alloys. The solidification of commercial
alloys is complex and under normal conditions, does not occur
under equilibrium conditions.
One of the samples is a reference sample, commonly an inert
material over the range of temperature being investigated. The
sample material and reference materials are not required to have
any similarities, although it can be advantageous to select
Micro structural Analysis
The alloys were being casted in cylindrical chills, producing the
castings with 17mm diameter and 160mm length. After casting,
samples were prepared for light microscopy, interference layer
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reference materials with thermal similarities, such as thermal
conductivity and heat capacity [9],[10].
*Pure metals (Table 1) do not to have a freezing range.
feeding temperature ‘TG’ once not being provided the value. The
empirical formula it uses is
1.
TG = Tsol + (Tliq − Tsol ) × 0.25
Numerical Simulations
Input parameters
To simulate the entire forming process, first of all, different
material parameters have to be determined as input data or
underlying mathematical models. The necessary data, such as
boundary conditions, material properties and process control
parameters, have to be investigated. All material properties have
to be experimentally (figure 3) or theoretically determined, since
these values are commonly only available for lower temperatures,
and not only for the range of semi solid forming.
Here, the term ‘physical properties’ includes variables like
density, expansion coefficient, specific heat capacity, latent heat
and heat conductivity. Furthermore, process boundary conditions
such as radiation number and heat transfer number have to be
correctly modeled in order to get reliable simulation result [11].
Figure 4: The pre-solidification of the cast alloy(Mg9%Al) in the feeding system.
Nucleation and growth of –magnesium equiaxed dendrites
The solidification sequence of Mg-Al alloys starts with nucleation
of primary magnesium (α-Mg) in the temperature range 650-600
ºC , ranging from the melting point of pure magnesium to the
liquidous temperature of 9 % wt Al, covering the aluminium
contents used in most commercial alloys. Solidification reactions
involve the formation of eutectic phases, with the Mg-Mg17Al12
eutectic reaction occurring at around 437 ºC. According to the
Mg-Al phase diagram the eutectic phase (Mg17Al12) is expected to
appear when the aluminium content reaches around 13 wt%.
However, the eutectic phase appears in alloys containing as little
as 2wt% Al for non-equilibrium cooling conditions normally
encountered in castings [12].
Figure 3: The Specific heat of all the binary Mg/Al alloys,
please note, there is almost no change in the specific heat
while the specimen is in solid state.
The filling of foundry molds with liquid metal, its flow through
the channels and cavities of the molds, is a complex hydro
mechanical process. To be able simulate this process, the
knowledge of the properties of the metals and various alloys in the
liquid state, such as density, viscosity, thermal conductivity,
surface tension, feeding effectivety, wetting capacity and
properties of oxides are so forth.
A
B
Feeding effectivity
Through simulations the calculations of feeding is a must,
provision of a value at this point that defines the range of feeding
is vital. This value describes the solidified fraction of the melt up
to which macroscopic feeding can occur. The solidified fraction is
expressed in percent and is strongly dependent on the
solidification morphology.
Figure 5: Micrograph of fully developed dendrites in a
Mg-9wt% Al alloy permanent mould casting
The microstructure of the alloy Mg-12%Al (figure 5) is similar to
9 wt% Al alloy, but the dendritic structure is more clearly visible
than normally observed in a 9 wt% Al alloy. The magnesium
dendrites have a characteristic sixfold symmetric shape (A). The
white phase between the dendrites is secondary eutectic phase
Mg17Al12 (B).
The feeding effectivity is taken from the database provided for the
according material. Knowledge of feeding effectivity is essential
as a wrong parameter can cause solidification rite in the feeding
basin (Fig.4). MAGMAsoft automatically calculates the effective
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Mg-1Al
Mg-1Al
accurate number is still in question. Human error refers to the
errors involved in experimental practices in the foundry shop.
Mg-3Al
The plot (figure 8) shows the effect of metallostatic pressure head
and degree of superheat on the simulated fluidity. Three different
sets of simulations were done on the alloys:
1. Simulations with 25mbar pressure head and 100°C
superheat (100°C above liquidus).
2. Simulations varying the pressure head only and keeping
the superheat const. at 100°C.
3. Simulations with varying superheat (150°C) and
keeping the pressure constant at 25mbar.
Figure 6: The micrographs
Grain Size
The simulations drew the following trends in the fluidity of the
alloys:¾ Rise in superheat temperature by 50°C (from 100150°C), increased the fluidity up by almost 15%.
¾ Elevated pressure head of 15mbar (from 25-40mbar),
caused the fluidity to increase by 10%.
¾ For lower concentration of Al (wt %) in Mg the
superheat and pressure have no significant change as
the freezing range is very short.
Determination of thermo dynamical input-parameters
While attempting to simulate the microstructure evolution there
are some properties, knowledge of whom is vital. These properties
are as follows:
•
Nucleation density.
•
Heat Extraction.
•
Latent heat of Fusion.
•
Specfic Heat Capacity.
As seen in the former figures that nucleants have to be place on
the simulation window. Provided they fullfill a certain thermal
requirement, then and only then they start growing. Now, the
quantity of these nucleants have either to be based on assumption
or compared with the experimental results. As in our case we
compare simulated results with the experimental ones so the
nucleant density was altered iteratively till we have convergence
with the grain sizes.
In this regard, keeping in view the extensive work involved in the
simulating each alloy we took only three combinations Mg-2%Al,
Mg-5%Al and Mg-10%Al.
Figure 7: The average grain size shown as a function of
Al wt% content.
In the above figure the grain size has been plotted along with the
change in concentration of Al (wt %) and as it can be seen that till
5% Al there is a reduction in size. Results have been compared
with literature [13](figure 7) and there is a considerable scatter.
The error bars are set to 5%.The observed difference is because of
different casting conditions and different sizes of the crucibles. In
this work a small chill was used and the average effect of Al was
determined. There have been some awkward peaks but in general
the results were satisfactory and expected. There are numerous
factors effecting fluidity. The salient being the following
¾ Metallostatic pressure head.
¾ Degree of superheat.
ζcp ∗ = ζ (cp +
L
)
ΔTF
2.
Where ζ=Density of the particular alloy.
Cp =Specific heat. [KJ/Kg.K]
ζ C p=Heat capacity.[KJ/K.m3]
L =Latent Heat of Fusion.[KJ/Kg]
∆TF =Freezing Range of the alloy. [K]
.
Q = ξcp ∗ ×
ΔT
Δt
Where Q =Heat Extraction .[KJ/sec.m3]
∆T/∆t=Cooling rate, determined experimental. [K/sec]
Figure 8: Fluidity comparison (simulated) of different
metallostatic pressure head and degree of superheat.
The height of the pouring level 25mbar was calculated to be the
pressure head but the human error might hinder to guarantee and
200
3.
Table 2: Input parameters for Mg-12%Al
Property for Mg-12%Al
Calculation
14.16 K/sec
Cooling Rate
1.27509 J/g. K
Specific heat
2.2186566 J/cm3.K
Specific heat capacity
3.5950055422
J/cm3.K
Effective heat cap.
50.90527677 J/s.cm3
Heat Extraction
These properties were updated in the MICRESS driving input file
and the results were recorded.
.
Q
h=
(T Alloy12 − Tmold )
4.
Where h= Physical heat Transfer Co-efficient.
.
Q AlloyX = h.(TMelt AlloyX − Tmold )
5.
Figure 10: The Grain Morphologies of Mg-5%Al , Mg10%Al and Mg-2%A l Results & Conclusions
Figure 11: The simulated grains for Mg-10%Al, the
dendritic, microstructure can be seen.
Figure 9: Development of the Fraction liquid with respect
to Temp.
As the grain size of Mg-2%Al is larger than that of Mg-5%Al and
Mg-10%Al (Fig.13), so it can be said that the result are
understood and were expected too. This is because at higher Al
concentration the grain is restricted to grow because of the solute
redistribution, as described by the grain restriction factor (g r f).
Figure 11 show the dendritic simulated micro-structure of Mg10%Al, phases like Mg17Al12 can’t be simulated with this
computational methodology.
The trend has been identified and it can be seen that after reaching
a certain value the grains reduce their size a little and then mature.
This phenomenon was observed in the simulations and is due to
the Ostwald ripening.
As a conclusion it could be added, the state of the art commercial
codes have now enabled us to perform highly complicated
foundry processes. In the field of Magnesium and its alloys soft
wares with the passage of time are becoming more and more
robust. With simulations the whole expensive experimental
equipment could be saved, the material could be saved and above
all precise data could be achieved.
The term Morphology refers to the ratio of the surface area of the
particular grain to the surface area of a sphere of equal volume
(smallest area of the grain of equal volume). While keeping the
emphasis on grain size, grain morphology is necessary to address
also. The plot (Fig. 10) reveals the result obtained and it can be
seen that Mg-2%Al having less Al concentration and bigger grain
size has lower Morphology values also. With the increase in Al
content (wt %) the morphology values increase too with respect to
time. Figure 9 reveals the diminishing liquid fraction with the fall
in temperature and subsequently the nucleation starts.
In figure 10 the temperature shown in the horizontal axis of the
plot refers to the change in temperature, taking the nucleation
point as 0.
201
[2] Hayashi, S. a. (1921). Investigation of the Fluidity of Metals
and Alloys. In Memoires of Kyoto College of Engineering (p. 83).
Kyoto: Kyoto Imperial University Press.
[3] Flemings, M., Niiyama, E., & Taylor, H. (1961). AFS Tarns.
69 , 625-635.
[4] Flemings, M., Niiyama, E., Niiyama, E., & H.F.:, T. (1962).
AFS Trans. 70 , 1029-1039.
[5] C.J.Cooksey, V. a. (SEp 1959). The Casting Fluidity of Some
Foundry Alloys. In The British Foundryman, vol 52 (p. 31).
[6] F. Blum, W. W. (2001). Comparative Study of Creep of the
Die-cast Mg alloys AZ91, AS21, AS41, AM60 and AE42. Material
Sciences and Engineering A , 319-321,735-740.
[7] A.K.Dahle, Y. M. (2001). Development of the As-cast
Microstructure in Magnesium-Aluminum Alloys. Journal of Light
Metals , 61-72.
Figure 12: The experimental fluidity done in GKSS
compared with the simulated results from Magmasoft.
[8]M. Pekguleryuz. (2000). Creep Resistance in Mg-Al-Ca
Casting Alloys. The Minerals, Metals & Materials Society (TMS) ,
12-17.
The experimental fluidity values didn’t defer much from the
simulated ones and the deviation limit was set to 5 %( Fig.12).
This scatter is because of the approximation of the input
parameters. An important role of a heat transfer model, beside
description of the temperatures and heat flows inside the system,
is to perform the so-called desmearing of the DTA/DSC signal.
Generally, the recorded DTA/DSC signal is not directly
equivalent to the actual heat flow (the heat of the reaction) in the
sample due to the observed thermal lag between the thermal event
and the corresponding thermocouple response [14].
[9] F.Speyer, F.Roberts (1994). Thermal Analysis of Materials.
NewYork [u.a].
[10] B. Wunderlich. (1990). Thermal Analysis. New York: Marcel
Dekker.
[11] H.Shimahara, G. (2006). Proc. 9th ESAFORM Conf., (p.
811). Glasgow.
[12] I.J.Polmear (1989). Light Alloys: metallurgy of the light
metals 2nd edition. London: Chapman and Hall. ISBN: 0-34049175-2.
[13]Y.C.Lee, A.K.Dahle (2000). Metallurgical & materials
Transactions. Vol 31A, pg-2895. [14] K.R. Loeblich. (1994). On the characteristics of the signal
curves of heat-flux calorimeters in studies of reaction-kinetics.1. A
contribution to the desmearing techniques. Thermochim. Acta.
231 , 7.
[15] W. Himminger, H. K. Cammenga (1989). Methoden der
thermischen
Analyse,
Anleitung
für
die
chemische
Laboratoriumspraxis. Springer.
[16] U.Ulbrich, H. K. Cammenga (1993). Thermochim. Acta. 229
, Coupling of calorimetric with optical methods and its application
to the deconvolution of DSC curves. 53-67.
Figure 13: Comparison of the simulated grain size to the
experimental.
[17] K.R.Loeblich. (1994). Thermochim. Acta. 231 , 7.
Distortion and delay of the DTA/DSC signal related to the
original thermal effects occurring in the sample are known as
smearing [15],[16],[17].
Experimental grain size possessed scatter but the same was not
seen in simulations.
[18] M.M. Avedesian, H. B. (1999). In ASM Speciality Handbook
Magnesium and Magnesium Alloys (pp. 4-6). Materials Park,
Ohio: ASM International.
References
The authors are grateful for the funding by GKSS Research center
for the completion of this project. Special gratitude to Prof. Rainer
Schmid Fetzer (TU Clausthal, Institute for Metallurgy) for
assisting with thermo dynamical data.
Acknowledgement
[1] T. West. (1902). Metallurgy of Cast Iron,7th Edition.
Cleveland: Cleveland Printing Company.
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