Thermophysical Properties and Microstructural ...

Materials Science and Technology (MS&T) 2008 October 5-9, 2008, Pittsburgh, Pennsylvania • Copyright © 2008 MS&T’08® Structure-Property Relationships in Multi-Functional Materials

Thermophysical Properties and Microstructural Analysis of AZ80 Magnesium Alloys Designed for Automotive Industry Gabriela Popescu, P. Moldovan, S. Bejan Materials Science and Engineering Department, “Politehnica” University of Bucharest, Bucharest, Romania M. Miculescu, F. Miculescu Physical Metallurgy Department, “Politehnica” University of Bucharest, Bucharest, Romania Keywords: AZ80, Diffusivity, Thermal conductivity

Abstract Magnesium and its alloys are considered to be the lightest metals available and are of major interest in automotive industry due to their advantage in component’s weight reducing. Thermophysical properties of AZ80 plastic magnesium alloy are determined. At different dimensions (φ 5 mm and φ 28 mm) and states (F and T5) are determined: the variation of diffusivity (α), the variation of thermal expansion coefficient and its mean value in the same temperature interval (60 – 400 oC). The microstructural analysis is determined by optical microscopy, SEM and EDS microanalysis for two different bars of AZ80 alloy and presents a series of continuous rows precipitates or polyhedral particles containing Mg, Al, Si and Ca. Knowing the density of AZ80 and using the obtained results it can be determined the thermal conductivity (λ) of the alloy. Introduction There is great potential for the use of extruded magnesium profiles and forgings in lightweight construction applications in automobile manufacturing. However, the economical and technical viability of these materials for volume production depends on all links in the process chain from semi-finished product manufacture through to components production being optimized in terms of cost and component characteristics. The paper will outline the thermophysical properties and microstructures of AZ80 magnesium alloy that might have future applications for lightweight construction in automotive industry. Summary of FLASH Test Method A small, thin disc specimen is subjected to a high intensity short duration radiant energy pulse. The energy of the pulse is absorbed on the front surface of the specimen and the resulting rear face temperature rise (thermogram) is recorded. The thermal diffusivity value is calculated from the specimen thickness and the time required for the rear face temperature rise to reach certain percentages of its maximum value (Fig. 1). When the thermal diffusivity of the sample is to be determined over a temperature range, the measurement must be repeated at each

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temperature of interest. This test method is described in detail in a number of publications [1, 2] and review articles [3, 4, 5].

Figure 1 Characteristic Thermogram for the Flash Method

Significance and Use Thermal diffusivity is an important property, required for such purposes as design applications under transient heat flow conditions, determination of safe operating temperature, process control, and quality assurance. The flash method is used to measure values of thermal diffusivity, α, of a wide range of solid materials. It is particularly advantageous because of simple specimen geometry, small specimen size requirements, rapidity of measurement and ease of handling, with a single apparatus, of materials having a wide range of thermal diffusivity values over a large temperature range. Thermal diffusivity results, together with specific heat capacity (Cp) and density (ρ) values, can be used in many cases to derive thermal conductivity (λ), according to the relationship: λ = α ⋅ C p ⋅ ρ (1)

Figure 2 Schematic of the Flash Method

Apparatus The essential components of the apparatus are shown in Fig. 2. These are the flash source, sample holder, environmental enclosure (optional), temperature response detector and recording device.

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Test Specimen The specimen is a thin circular disc with a front surface area less than that of the energy beam. Specimens are 12,5mm in diameter. The optimum thickness depends upon the magnitude of the estimated thermal diffusivity, and is chosen so that the time to reach the maximum temperature falls within the 40 to 200 ms range. Measurement of Specific Heat Capacity and Calculation of Thermal Conductivity The fundamental relationship between thermal diffusivity (α), thermal conductivity (λ), specific heat capacity (Cp), and density (ρ),

α=

λ

ρ ⋅C p

(2)

allows the calculation of thermal conductivity, a much sought after property, with the knowledge of the other properties. There are several conditions that must be satisfied in order for this process to be valid: Both reference and unknown sample must be homogeneous and isotropic, as Eq. 2 only applies for those materials. Heterogeneous and anisotropic materials will frequently produce erroneous data. Experimental Thermal diffusivity Thermal diffusivity was determined on 2 samples: P1 (AZ80 - T5) and P2 (AZ80 - F). Experimental results are presented in the next tables (Table 1, Table 2) and figures (Figure 3 up to Figure11) Table 1 Variation of temperature with thermal diffusivity for P1 Test 1 (thickness 0.3610 cm, Clark and Taylor) Segment Temperature Average Shots A 60 0.4227 0.3266 0.3671 0.3721 B 107 0.4151 0.4057 0.4013 0.4073 C 156 0.4098 0.4153 0.4126 0.4126 D 202 0.4189 0.4190 0.4192 0.4191 E 254 0.4203 0.4167 0.4343 0.4238 F 302 0.3914 0.4012 0.3941 0.3956 G 350 0.3803 0.3828 0.3818 0.3816 H 400 0.3853 0.3848 0.3843 0.3848 I 348 0.3793 0.3793 J 307 0.3669 0.3669 K 260 0.3623 0.3623 L 210 0.3587 0.3587 M 162 0.3327 0.3327 N 113 0.3089 0.3089

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Thermal Diffusivity Mg1

0,42

Diffusivity [cm 2/s]

0,41 0,4 0,39

Series1

0,38 0,37 0,36 0,35 50

100

150

200

250

300

350

400

Temperature [˚C]

Figure 3 Variation curve of thermal diffusivity with temperature for sample 1- test 1

Table 2 Variation of temperature with thermal diffusivity for P1 Test 2 (thickness 0.3830 cm, Clark and Taylor)

Segment A B C D E F G H I J K L M N O

Temperature 59 106 156 204 254 302 351 399 349 307 261 210 162 113 70

Average 0.3832 0.4004 0.4129 0.4176 0.4180 0.3917 0.3826 0.3877 0.3773 0.3696 0.3564 0.3583 0.3402 0.3111 0.3163

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0.3938 0.4112 0.4133 0.4145 0.4240 0.3928 0.3817 0.3888 0.3773 0.3696 0.3564 0.3583 0.3402 0.3111 0.3163

Shots 0.3703 0.3945 0.4158 0.4178 0.4112 0.3908 0.3822 0.3867

0.3856 0.3956 0.4097 0.4205 0.4188 0.3916 0.3840 0.3876

Thermal Diffusivity Mg2

0,42

Diffusivity [cm2/s]

0,41 0,4 0,39

Series 1

0,38 0,37 0,36 0,35 50

100

150

200

250

300

350

400

Temperature [˚C]

Figure 4 Variation curve of thermal diffusivity with temperature for sample 1- test 2

Coeficient of thermal expansion

Coeficient of thermal expansion 35

30

30

25

25 Average Alpha

x10-6/˚C

x10-6/˚C

20 20

Instantaneus alpha 15

Average Alpha 15

Instantaneus alpha

10

10

5

5 0

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380

430

30

80

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Temperature [˚C]

Figure 6 Sample 1- Test 2 Dependency curve for coefficient of thermal linear expansion to temperature



35

35

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30

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25

20

Average Alpha

x10-6/˚C

x10-6/˚C


Instantaneus alpha 15

20

Instantaneus alpha

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0

Average Alpha

15

0 30

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Tempera ture [˚C]

80

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Tempera ture [˚C]



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For a proper examination and testing of AZ80 alloy bars were cut samples from each of the bars: • Sample no.1 (P1), bar of φ = 28 mm in T5 state • Sample no.2 (P2), bar (wire) of φ = 5 mm in F state The samples were mounted and polished with diamond paste and subjected to nital attack. To avoid water contact for a long time it was used as lubricant an ethanol suspension and the samples were dried with compressed air. The choise of attack reactive was done according to specialty literature from www.magforge.com [6] site. Figure 9 presents the microstructures of transversal P1 at two different sizes and in two different zones.

Figure 9 Microstructures of transversal section P1 samples. Attack 4% nital / 20 sec

It is to be observed a cellular structure with non-uniform precipitates, recrystallized, after extrusion and heat treatment T5. It can be point that the extrusion leads to a grain crumble of about 10 – 20 µm. In several zones are observed cristalls dispozed as rows in the direction of formation. In Figure 10 are presented the microstructures in details regarding the morphology and the colour of the presented compounds in sample P1.

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Figure 10 The microstructure of sample P1 with details.

In figure 11 is presented the EDS analysis of the particles disposed in gray colour rows containing Mg, Al, Zn and O2.

Figure 11 EDS analysis of gray particles, disposed as rows

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Conclusions The microstructures were determined for two types of AZ80 extruded alloy bars with diameters of φ5mm and φ28mm sent by GKSS (Germany), from TIMMINCO Enterprise. The study of the microstructures was done by Optical Microscopy (OM) and Spectral electron Microscopy SEM and EDS microanalysis. For AZ80 alloy the study pointed out a series of fine precipitates in continuous rows shape or polyhedral particles, their chemical composition being identified by EDS analysis.

Acknowledgments This work is supported by the FP 6 Program under Contract No. 030208-2 (MagForge). The authors would like to thank to the Coordinator of this project, Dr. W. Sillekens, TNO Nederland, for the financial support under the MagForge Project. References [1] Parker, W. J., Jenkins, R. J., Butler, C. P., and Abbott, G. L., “Flash Method of Determining Thermal Diffusivity Heat Capacity and Thermal Conductivity,” J. Appl. Phys., 32, Vol 9, 1979 (1961). [2] Watt, D. A., “Theory of Thermal Diffusivity of Pulse Technique,” Br.J. Apply. Phys., 17, Vol 231, 1966. [3] Righini, F., and Cezairliyan, A., “Pulse Method of Thermal Diffusivity Measurements, A Review,” High Temperature—High Pressures, 5, 1973, pp. 481–501. [4] Taylor, R. E., “Heat Pulse Diffusivity Measurements,” High Temperatures, 11, Vol 43, 1979. [5] Taylor, R. E., “Critical Evaluation of Flash Method for Measuring Thermal Diffusivity,” Rev. Int. Htes. Temp. et Refract., 12, 1975, pp.141–145. [6] www.magforge.com

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