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									                                                             18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 




 CONNECTION BETWEEN TEMPERATURE OF MELTING AND COEFFICIENT OF THERMAL
                      CONTRACTION OF Al-Si ALLOYS

                                              Jiří MORÁVEK a, Iva NOVÁ b
               a
                   Technická univerzita v Liberci, Studentská 2, 461 17 Liberec CZ, jiri.moravek@tul.cz
                   b
                       Technická univerzita v Liberci, Studentská 2, 461 17 Liberec, CZ, iva.nova@tul.cz

Abstract

The article deals with measurements dependence of coefficient of thermal contraction on chemical
compound of Al-Si alloys, so-called silumin, during their solidification and cooling. For this purpose the
measuring apparatus, which applies a dilatometer, measuring frame, casting mould with cavity ∅ 60 x 70
mm and PC, was made up. Dilatation curves of pure Al and Al alloys were acquired during the casting
process and noted by computer. Melts were metallurgical treated with the refining salt before casting into the
moulds. We observed the influence of chemical compound of melt on coefficient of thermal contraction,
which was computed from obtained dilatation curves.

1.    INTRODUCTION

It is necessary at the casting production to ensure not only their quality (accuracy, geometry, surface
roughness, structure homogeneity, minimal internal stress, etc.), but also to supress of their dimensional
linear and volume changes. In the case of casting alloys it is necessary to observe the linear and volume
changes especially during solidification of Al-Si alloys, where the eutectic silicon crystallization is important.
The course of volume changes of castings defines the casting material ability to equalize the loss of volume
during solidification in an effective way. The volume changes at the process of Al-Si alloys solidification have
been observed in our Department of Engineering Technology at the Technical University of Liberec. In this
contribution we would like to get acquainted professional technical public with our research results.

2.    PHYSICAL PRINCIPLE OF DIMENSIONAL CHANGES DURING SOLIDIFICATION

Volumes solid and liquid state of substances depended on their temperature and pressure. Change of
temperature metal or alloy is connected with change of their volume. Major part alloys during solidification
reduces their volume, without rarities: bismuth, antimony and their alloys, theirs volume extended, table 1.
Metal density is depended on gram molecule and distance of atoms. Given temperature has the given
distance of atoms. Common relationship of metal density and atoms distance is:

      M .n
ρ=                                                                                                            (1)
     N A .Ve
where is: M – molar weight of metal;
           n – number of atoms in one element;
           NA – Avogadro´s constant;
           Ve – cube one crystallform element.

In conjunction with higher temperature is enlarging parameter of crystal lattice with falling of their density.
These effects are related to changes of volume. Crystalline structure of metal stays the same but his
distance of atoms is changed. During heating growing, during cooling falling, table 1. Volume of metal or
                                                        18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 



alloy stays constant at certain temperature and pressure. Change of alloy temperature means change of
their volume. For volume changes at atmospheric pressure we can write this formula:

Table 1. Properties of pure metals
                                                           Density                            Melting
                                          Melting                           Melting           volume
       Metal            Atom mass         temperature      20 [°C]          temperature       alternation
                                                                            [°C]              [%]
       Aluminium        27                660             2700              2380              5,1
       Silicon          28                1430            2300              2500              -5,0
       Magnesium        24                650             1700              1600              4,2
       Cupper           64                1083            8920              7900              5,3
       Lead             207               327             11300             10700             3,2
       Zinc             65                420             7100              6600              4,1
       Bismuth          209               271             9800              10 000            -3,3
       Gallium          70                29              5900              6100              -3,2

       1 ΔV
γ=      ⋅                                                                                           (2)
      V0 ΔT
where is: γ – coefficient of the volumetric shrinkage for temperature interval [K-1];
          V0 – initial temperature volume [m3];
          ΔV – volume alternation [m3];
          ΔT – temperature alternation [K].

Generalities of alloys shrinkage during their solidification, it appears from this that density of alloy in solid
state will be higher than in liquid state. These changes have connection with thermal expansivity or
contractility. Crystalline structure of metal stays the same but his atomic distance is changed. Given
temperature have given balance parameters. These parameters are growing during heating, against cooling
where they are falling. When these parameters cannot change due to external interference stress grows in
crystal lattice. These stresses can be tensile, pressure or shear. Pure metals are solidifying and shrinking at
constant temperature against alloys which are solidifying in temperature interval. Shrinkage is a physical
property of alloys. If we are observing the shrinkage of alloys during their casting, shrinkage is influenced by
technology of casting (casting temperature and time, construction of cast). Changes of cast volume are
featured technological decrease of volume.

2.1    Volume changes in liquid state

Volume changes in liquid state proceed from casting temperature to temperature of liquidus. Fall-off in liquid
level of melt is result of volume changes. Total volume change of cast in liquid state is:

ΔVL = ΔVL′ + ΔVL′′ = γ L ⋅ (Tlití − TL ) ⋅ V0                                                        (3)
where is:V0 – meeting volume initial;
          γL– temperature shrinkage volume coefficient;
         Tlití – casting temperature [K-1];
         TL – temperature of liquidus [K-1].

2.2    Volume changes during solidification

Volume changes during solidification give rise to centered shrink hole and porosity in cast. Shrinkage period
in cast during solidification will start in moment when the first solid state element is except from melt. During
                                                          18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 



solidification of pure metals and eutectic alloys are rising centered shrink hole. Shrink hole is created from
moment when is created a break free film of solid state which closed melt inside. After that follows:

a)     reducing volume of melt ΔV’’L

b)     expansion volume of solid state and reducing of volume ΔVL-S

ΔV L − S = γ S ⋅ (TL − TS ) ⋅ V0                                                                                    (4)
where is: V0 – volume initial [m3];
       γS – temperature shrinkage coefficient
       TL - TS – temperature interval liquidus – solidus [K-1].

c)     reducing of surface fall-off in liquid level of melt

d)     shrinkage solid states of cast to value ΔVS during cooling. It follows change of linear dimensions.

Solution of these processes is centered shrink hole. Volume of centered shrink hole can be computed from
this formula:

V st = ΔV L′′ + ΔV L − S − ΔV S                                                                                    (5)

3.     EXPERIMENTAL OBSERVING COEFFICIENT OF TEMPERATURE CONTRACTION

Experiments were aimed for observing changes of dimensions during solidification and cooling of casts from
Al-Si alloys with different content of silicon for identification coefficient of thermal contraction α. 16
experiments aimed for observing changes of dimensions during solidification and cooling, was made. For
observing of these properties was made up measuring equipment with dilatometer, figure 1.




       Fig. 1a Measuring equipment                      Fig. 1b Component parts of measuring equipment
                                                              1 – base plate; 2 - measuring frame; 3 mould;
                                                              4 - isolation; 5 - magnet; 6 – silicone pipe; 7 - roller

Experiments were carried out in moulds from CT mixture with cylindrical profile ø 60 × 80 mm. Figure 2
shows using measuring equipment with measuring frame, inductive sensor and installed mould from CT
mixture, before and after casting. For measuring of dilatation changes was used the dilatometer of polish
provenience Crystaldiagraph PC-4T2L with A/D convertor and PC for data processing. Dilatation of
solidification cast is registered trough silicone pipes which are connected with inductive sensor and
displaying on PC. Along with dilatation is measured time dependence of temperature in heat axis of cast by
thermocouple NiCr-Ni. Time dependence of temperature is recording to PC.
                                                       18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 




     Fig 2a Measuring equipment is ready        Fig. 2b Measuring equipment during
               for measuring                          measuring process

From realized experiments has been obtained 16 graphical time dependences of temperature and dilatation.
Experiments were carried out with pursuit dependence coefficient of temperature contraction on chemical
compound of melt, content of Si in melt of Al-Si alloy near melting point. Using melts were AlSi1 to AlSi29
and pure Al. From these experiments were obtained dilatation curves dependent on temperature. Values
coefficient of temperature contraction were computed for all types of alloys in temperature interval 500 to
                                                              550°C. Alloys were melted-down in electrical
                                                              resistance furnace in graphite melting-pot on
                                                              725°C and before casting refining by salt T3.
                                                              Figure 3 shows example of graphical dependence
                                                              dilatation and temperature on time of cast ø 60 ×
                                                              80 mm from eutectic alloy AlSi11, which is solidify
                                                              in CT mould. Other dilatation curves were obtained
                                                              in the same way.



Fig. 3a Time dependence dilatation and temperature of AlSi11 alloy

Coefficient of temperature contraction for all types of alloys was obtained from dilatation-temperature curves
and calculated in temperature interval 500 to 550°C from this formula:


α=
              Δd
                      [ ]
       d o * (T2 − T1 )
                        K −1                                                                       (6)

where is: α – coefficient of temperature contraction [K-1],
          Δd – casting contraction [mm],
          d0 – original diameter of casting [mm],
          (T2-T1) – temperature interval (500 to 550) [°C].

Table 2 shows computed values coefficient of temperature contraction of casts for single types of alloys.
                                                        18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 



Table 2 Computed values of coefficient of thermal contraction for Al-Si alloys

                                                     Coefficient of thermal contraction α
                    % Si in alloy
                                                     [K-1]

                    0                                19,8.10-6
                    1                                19,0.10-6
                    2                                18,8.10-6
                    3                                18,4.10-6
                    4                                18,3.10-6
                    5                                18,1.10-6
                    6                                18,3.10-6
                    7                                18,6.10-6
                    8                                19,7.10-6
                    10                               21,3.10-6
                    11                               22,1.10-6
                    12,5                             22,2.10-6
                    15                               20,1.10-6
                    19                               17,7.10-6
                    25                               15,7.10-6
                    29                               14,2.10-6

Figure 4 shows summary of graphical dependence coefficient of thermal contraction on weight percentage of
silicone in Al-Si alloy in context of binary constitution diagram Al-Si.
                                                                 18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 




Fig. 4 Dependence of coefficient of thermal contraction on % Si in alloy

4       CONCLUSION

Observations about relationship between coefficient of thermal contraction and melting point of different
types of silumins confirmed theoretic physical premises: the higher melting point the lower value of
coefficient of temperature contraction. Values of coefficient of temperature contraction are about 10-6 K-1.
Alloy AlSi12,5 (eutectic) have the lowest melting point, cca. 577°C. For this alloy type was found (in interval
500 to 550°C) value of coefficient of temperature contraction 22,2×10-6 K-1.

ACKNOWLEDGEMENTS

                     The article was prepared within the score of project MSM 4674788501

LITERATURE

[1]     KRÝSLOVÁ, S. Monitoring of foundry properties of the Zinc alloys for casts production. [Thesis].FS, KSP, TU of Liberec 2008.

[2]     GRÍGEROVÁ, T., LUKÁČ, I., KOŘENÝ, R. Zlievárenstvo neželezných kovov. 1. vyd. Bratislava-Praha 1988 (Slovakia).

[3]     MICHNA, Š. et al. Aluminium materials and technologies from A to Z. Printed by Adin, Prešov 2007

								
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