Effect of bismuth additions on the thermophysical and thermodynamical properties of E-AlMgSi (Aldrey) aluminum semiconductor alloy

The economic feasibility of using aluminum as a conductive material is explained by the favorable ratio of its cost to the cost of copper. In addition, one should take into account that the cost of aluminum has remained virtually unchanged for many years. When using conductive aluminum alloys for the manufacture of thin wire, winding wire, etc., certain difficulties may arise in connection with their insufficient strength and a small number of kinks before fracture. Aluminum alloys have been developed in recent years which even in a soft state have strength characteristics that allow them to be used as a conductive material. The electrochemical industry is one of the promising application fields of aluminum. E-AlMgSi (Aldrey) conductor aluminum alloys represent this group of alloys. This work presents data on the temperature dependence of heat capacity, heat conductivity and thermodynamic functions of the E-AlMgSi (Aldrey) aluminum alloy doped with bismuth. The studies have been carried out in “cooling” mode. It has been shown that the heat capacity and thermodynamic functions of the E-AlMgSi (Aldrey) aluminum alloy doped with bismuth increase with temperature and the Gibbs energy decreases. Bismuth additions of up to 1 wt.% reduce the heat capacity, heat conductivity, enthalpy and entropy of the initial alloy and increase the Gibbs energy.


Introduction
Aluminum and its alloys are widely used in electrical engineering as conductor and structural materials. As a conductive material, aluminum is characterized by high electrical and thermal conductivity (after copper, the maximum level among all technically used metals) [1].
Aluminum also has a low density, high atmospheric corrosion resistance and resistance to chemicals. However, aluminum alloys in specific states and under severe operation conditions may undergo dangerous corrosion types. Of special interest is aluminum corrosion in close-to-neutral solutions (6 < pH < 8). This corrosion occurs in natural environments: seawater, lacustrine and river water, potable water and atmospheric precipitation. Under these conditions and normal temperatures, the mobility of H + ions or H 2 O molecules during hydrogen emission is negligibly low [2].
The economic feasibility of using aluminum as a conductive material is explained by the favorable ratio of its cost to the cost of copper. In addition, one should take into account that the cost of aluminum has remained virtually unchanged for many years [3,4].
When using conductive aluminum alloys for the manufacture of thin wire, winding wire, etc., certain difficulties may arise in connection with their insufficient strength and a small number of kinks before fracture.
Aluminum alloys have been developed in recent years which even in a soft state have strength characteristics that allow them to be used as a conductive material [4].
One of conductor aluminum alloys is E-AlMgSi (Aldrey) pertaining to thermally hardened alloys. It has high strength and ductility. This alloy acquires high electrical conductivity upon appropriate heat treatment. Products made from it are used almost exclusively for overhead power lines [4][5][6].
Since overhead power lines made from aluminum and its alloys are used in open air, increasing the corrosion resistance of these alloys is an urgent task.
The aim of this work is to study the effect of bismuth additions on the thermophysical and thermodynamical properties of E-AlMgSi (Aldrey) aluminum conductor alloy, chemical composition (wt.%): 0.5 Mg, 0.5 Si, balance Al.

Experimental
The alloys were synthesized in a SShOL type resistance laboratory shaft furnace at 750-800 °C. A6 grade aluminum which was additionally doped with the calculated amount of silicon and magnesium was used as a charge in the preparation of the E-AlMgSi alloy. When doping aluminum with silicon, the metallic (0.1 wt.%) silicon present in primary aluminum was taken into account. Magnesium wrapped in aluminum foil was introduced into the molten aluminum using a bell. Bismuth was introduced into the melt in a form wrapped in aluminum foil. The alloys were chemically analyzed for silicon and magnesium contents at the Central Industrial Laboratory of the State Unitary Enterprise Tajikistan Aluminum Company. The alloy compositions were controlled by weighing the charge and the alloys. Synthesis was repeated if the alloy weight deviated from the target one by more than 1-2% rel.u. Then the alloys were cleaned from slag and cast into graphite molds in order to obtain samples for thermophysical study. The cylindrical samples had a diameter of 16 mm and a length of 30 mm.
The heat capacity of the alloys was measured using the setup shown in Fig. 1. The electric furnace 3 is mounted on the support 6 and can move up and down (arrow shows movement direction). The specimen 4 and the reference 5 (also movable) are in the form of cylinders with a diameter of 16 mm and a length of 30 mm with drilled cavities at one side for thermocouple insertion. The thermocouple contacts are connected to a DI9208L Digital Multimeter (7, 8 and 9). The electric furnace is powered via the laboratory transformer 1 by setting the required temperature with the thermocontroller 2. The initial temperature is read on the DI9208L Digital Multimeters. The specimen and the reference are placed into the electric furnace and heated to the required temperature which is controlled from DI9208L Digital Multimeter readings on the compu- Figure 1. Experimental setup for determining the heat capacity of solids in "cooling" mode ter 10. The specimen and the reference are simultaneously removed from the furnace, and the temperature is measured from that moment on. DI9208L Digital Multimeter readings are recorded by the computer every 10 sec while the specimen and the reference are cooled down to room temperature (30 °C).
The temperature was measured with a multichannel digital thermometer allowing direct recording of the data on the computer in tabular form. The temperature measurement accuracy was 0.1 °C. The relative temperature measurement error in the 40 to 400 °C range was ± 1%. The heat capacity measurement error for this method is within 4-6% depending on temperature.
The measurement data were processed in MS Excel and plotted in Sigma Plot. The correlation coefficient was R corr. > 0.989 which confirms the correct choice of the approximating function.
The experimental temperature vs time functions of the specimens (Fig. 2a) are described by the following equation: where a, b, p, k are constants and t is cooling time. Differentiating Eq. (1) by t we obtained the following equation for specimen cooling rate determination: Based on Eq.
(2) we calculated the cooling rates for the specimens of E-AlMgSi alloy (Aldrey) doped with bismuth (Fig. 2b). The coefficients a, b, p, k, ab, pk in Eq. (2) for the test alloys are summarized in Table 1.
Then based on the calculated cooling rates for the alloys and the A5N grade Al reference specimens we calculated the specific heat capacity of E-AlMgSi alloy (Aldrey) doped with bismuth: where m 1 = ρ 1 V 1 is the weight of the reference; m 2 = ρ 2 V 2 is the weight of the specimen and are reference and alloy specimen cooling rates for a specific temperature.
Using polynomial regression we obtained an equation for the temperature dependence of the heat capacity of E-AlMgSi alloy (Aldrey) doped with bismuth: The coefficients а, b, c, d in Eq. (4) are summarized in Table 2.
where Т, Т 0 are the specimen and environment temperatures, respectively, and S, m are the specimen surface area and weight, respectively. The temperature dependence of the heat conductivity coefficient of E-AlMgSi alloy (Aldrey) doped with bismuth is shown in Fig. 3.
To calculate the enthalpy, entropy and Gibbs energy as functions of temperature using Eqs. (6)-(8), we integrated the specific heat capacity calculated using Eq. (4):

G T G T H T H T T S T S T
where T 0 = 298.15.
The calculation data for the enthalpy, entropy and Gibbs energy as functions of temperature obtained using Eqs. (6)-(8) with 25 K intervals are summarized in Table 4.   Compulsory impurities which determine the properties of Aldrey are magnesium and silicon the content ratio of which should meet that for the Mg 2 Si compound which forms in the alloy and acts as a strengthening agent ensuring the high mechanical strength of the alloy. However one should take into account that in practice the melt always contains iron which is a still unavoidable but often detrimental impurity in any technical grade aluminum, forming the silicon containing compound Al 6 Fe 2 Si 3 . Therefore to completely ensure the formation of the Mg 2 Si compound one should compose the melt with a certain excess of silicon (0,4-0,5%) above the theoretical level [1][2][3].
The strengthening action of the Mg 2 Si compound stems from the fact that its solubility is solid aluminum decreases with a decrease in temperature. For example the maximum solubility of Mg 2 Si in aluminum is 1.85% at 595 °С and only 0.2% at 200 °С. Therefore rapid cooling (quenching) of an Aldrey type alloy heated to above 500 °С at which all Mg 2 Si is in the solid solution produces a supersaturated Mg 2 Si solid solution in aluminum [1][2][3].
Long-term tempering causes precipitation of excess Mg 2 Si from the solid solution in the form of a fine-grained structural component which increases the mechanical strength of the alloy (precipitation hardening). This tempering of the alloy is referred to as natural ageing. Ageing can be accelerated by slightly heating the alloy (to 150-200 °С), i.e., artificial ageing. In the course of ageing Mg 2 Si impurity precipitates from the solid solution causing an increase in the electrical conductivity of the alloy [1][2][3].
Bismuth doping of E-AlMgSi alloy modifies the primary precipitates of binary and ternary phases in the al-loy, providing a generally positive contribution to its performance.
Thus we determined the heat capacity of bismuth doped E-AlMgSi alloy (Aldrey) in "cooling" mode based on the known heat capacity of a reference A5N grade aluminum specimen. Using the experimentally obtained polynomial dependences we showed that with an increase in temperature the heat capacity, enthalpy and entropy of the alloys increase while the Gibbs energy decreases. Bismuth additions in the experimentally studied concentration range 0.05-1.0 wt.% reduce the heat capacity, heat conductivity coefficient, enthalpy and entropy of the initial E-AlMgSi alloy (Aldrey) and increase the Gibbs energy. The increase in the heat capacity, heat conductivity coefficient, enthalpy and entropy with an increase in the bismuth concentration in the alloy is caused by the modification of the α-Al solid solution structure, i.e., a higher heterogeneity of the structure of the multicomponent alloys [16][17][18].

Conclusion
Experimental data on the temperature dependence of the heat capacity, heat conductivity coefficient and thermodynamical functions of E-AlMgSi aluminum alloy (Aldrey) doped with bismuth were presented for "cooling" mode measurements.
We show that with an increase in temperature the heat capacity and the thermodynamical functions of E-AlMgSi alloy (Aldrey) with bismuth increase and the Gibbs energy decreases. Bismuth additions of up to 1 wt.% reduce the heat capacity, heat conductivity, enthalpy and entropy of the initial alloy but increase the Gibbs energy.