Effects of porosity on the thermal properties of a 380-aluminum alloy
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prepared by changing the temperature of the melt in the range 610–682 °C. The volume fraction of porosity in each sample was determined by optical microscopy image analysis. PA and differential calorimeter techniques were used to determine thermal diffusivity and heat capacity, respectively. From these results the thermal conductivity of each sample was obtained. The thermal conductivity of the porous 380-aluminum alloy was analyzed by using all the above mentioned effective models. It was found that, among all the models, the Maxwell model best fits the experimental results.
II. SAMPLES, EXPERIMENTAL SETUP, AND THEORETICAL BACKGROUND
A set of five samples of porous 380-aluminum alloys were prepared by varying the temperature of the liquid metal in a gas-fired furnace in the range 610–682 °C. In the vacuum solidification test, the atmospheric hydrogen precipitates during cooling. The amount of dissolved hydrogen in the 380-aluminum alloy, which in turn depends on the temperature of the liquid metal,30 was promoted to obtain different volume fractions of gas porosity.31 Hence the samples named B3 (0.004), B1 (0.005), B2 (0.012), A2 (0.043), and A1 (0.10) were obtained at temperatures of 610, 620, 633, 652, and 682 °C, respectively. The number in the parentheses indicates the volume fraction of porosity. The average chemical composition of the 380aluminum alloy was obtained with an emission spectrometer. The chemical composition is shown in Table I. For the PA and optical microscopy measurements, samples were cut in slices about 300–350 m thick and with an area of 1 cm2. The samples were polished with a 6-m and then a 1-m diamond powder impregnated cloth. The percent of porosity of each sample was determined on a computer-based image analyzer (OMNIMET II). For that, 30 different fields at ×100 were measured on each sample and a standard deviation from 0.10 to 2.91 was obtained. The results are show in Table II. The density of the samples was determined by the comparative hydrostatic displacement technique, which compares the weight of the sample measured in air with that measured in water. A precision balance with a sensitivity of about 0.1 mg was used to obtain the weight of the samples. The results are shown in Table II.
TABLE I. Average chemical composition of the 380-aluminum alloy samples obtained by vacuum solidification tests. Element (mass %) Si
Fe
Cu
Mn
Mg
Ni
Zn
Al
8.4
0.8
3.6
0.25
0.03
0.1
1.4
Balance
3902
To measure the thermal diffusivity of the samples, the open photoacoustic cell method was used.28,32 In this arrangement, the cavity of an electret microphone is the photoacoustic chamber, and the sample closes the window of the microphone. Each sample was fixed to the microphone with vacuum grease. The experimental method uses a white light source from a tungsten halogen lamp (with a maximum power of 300 W). The light is mechanically chopped and focused by an optical system on a small area (of about 0.03 cm2) at the external surface of the sample. The light is aligned to the aperture
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