Study on the temperature measurement of AZ31B magnesium alloy in gas tungsten arc welding

Study on the temperature measurement of AZ31B magnesium alloy in gas tungsten arc welding

Available online at www.sciencedirect.com Materials Letters 62 (2008) 2282 – 2284 www.elsevier.com/locate/matlet Study on the temperature measuremen...

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Available online at www.sciencedirect.com

Materials Letters 62 (2008) 2282 – 2284 www.elsevier.com/locate/matlet

Study on the temperature measurement of AZ31B magnesium alloy in gas tungsten arc welding Guoli Liang ⁎, Shaoqiang Yuan Department of Electromechanical Engineering, Tanshan College, Tangshan 063000, China Received 14 November 2006; accepted 26 November 2007 Available online 26 December 2007

Abstract This paper is based on the Finite Element Analysis (FEA) to study the AZ31B Magnesium Alloy welding temperature filed, using a convenient, non-contact and fast response measured temperature method—Infrared Radiation (IR), the welding temperature field of AZ31B magnesium alloy plate in Gas Tungsten Arc Welding (GTAW) is measured by IR, the isothermal map of magnesium alloy plate is measured using IR device. The cooling curves are measured by thermocouple. Experiments and simulations by FEA are carried out to investigate the welding temperature field. The simulated results showed good agreement with the experiment ones. © 2007 Elsevier B.V. All rights reserved. Keywords: Infrared radiation; Temperature measurement; Metals and alloys; Computer simulation

1. Introduction Accurate and reliable measurement of temperature history during arc welding process is quite challenging due to the severe environment. During arc welding, the arc produces a wide spectrum of wavelengths, electromagnetic field, high temperature, radiations and spatter [1]. This proves to be a very difficult environment for accurately measuring quantities such as temperature field history. Advanced technique of Infrared (IR) radiation is a convenient, non-contact and fast response method of making the base metal temperature measurements. Infrared (IR) temperature recording devices use measurements of the emissivity of a surface to determine the top-face temperature [2]. The problems of interference from both the arc and electrode were important sources of interference in the IR emissions from base metal [3,4]. The objective of this research is to implement and test a noncontact temperature measurement based on magnesium alloy plate [5,6]. Although temperature readings using IR devices are ⁎ Corresponding author. Tel.: +86 315 2010536; fax: +86 0315 2022378. E-mail addresses: [email protected] (G. Liang), [email protected] (S. Yuan). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.11.096

more difficult for materials with high reflectivity (materials such as steel that have low reflectivity can pose easy), the back-face isothermal map of magnesium alloy plate is measured using IR device called SAT-HY6000. The cooling curves are measured by K-type thermocouple. Experiments and simulations by finite element analysis (FEA) were carried out to investigate the welding temperature field. The simulated results based on the model showed good agreement with the experiment ones. 2. Experimental The temperature measurement methods were used during the gas tungsten arc welding (GTAW) of AZ31B magnesium alloy plates and the results were evaluated. These methods were: • Mechanically peened the thermocouple in the magnesium alloy plate. • Remote viewing of the welding process using IR camera [7]. The thermocouple from peening methods was connected to the INV-4 multifunctional anti-blend amplifier (100 times), the collected signals were transferred to an INV306DF data logger and PC in order to record the thermal cycling curves. The data

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Table 1 IR measurement temperature and thermocouple measurement temperature T (°C) 340 280 230 190 150 130 110 95 80 65 40 TR (°C) 308 258.2 216.5 183 150.5 133.8 117 104.6 91 78.7 58.7

piece during welding. Most metals, including steel, are defined as graybodies having emissivities less than one [10]. The welding conditions were: welding speed, 660 mm min− 1; shielding gas (argon), 5 L min− 1; welding current, 95 A; Arc voltage, 10 V. Because magnesium is very active and oxidize easily, it adopts the alternating current with 50 HZ to eliminate the oxidation film by cathode cleanup. When calculating the workpiece temperature by simulating the welding process using FEA the thermal physical properties depended on the temperature. 3. Results and discussion

Fig. 1. Temperature field measurement and IR thermograph. (a) Schematic diagram of temperature measurement. (b) IR thermograph.

logger was set to record a least 10 readings per second from thermocouple. All two methods were used to monitor the welding temperature field of GTAW on AZ31B magnesium alloy plates of 300 mm × 120 mm × 3 mm. When the thermocouple is attached to a plate, the following factors must also to be taken into consideration. The temperature will be recorded at the first place along the thermocouple that the two wires touch. If a junction of the thermocouple is made further up for some reason, the thermal cycling curve recorded will not be that from the point we want to measure. The response time of the thermocouple will depend on the diameter of the wire. A thinner wire will give a shorter response time; generally the thinner the wire diameter of thermocouple, the more satisfactorily it will perform [8]. Thermocouple used in present work was made from wire under 1 mm diameter, and the time lag to reach the peak temperature was typically under 0.6 s. Data obtained from the thermocouple were compared for the finite element analysis (FEA) predictions. Mechanical peening was chosen as the thermocouple surface attachment method for a robust joint to the section, shown as in Fig. 1(a). The thermocouple peened at the distance of 8 mm from the weld centerline often gave peak temperatures above 190 °C. An infrared camera was used to monitor a GTAW in magnesium alloy plate [9]. Before measuring temperature, the emissivity of IR must to be emended by nickel-chromium-nisiloy thermocouple which diameter is 0.2 mm. The temperature field by IR screening can be recorded by IR thermograph. In order to limit the amount of data handled, IR captured the images of the work-

The actual object is not a black body, from which emissivity is less than one. The emissivity is generally dependent on emissive body property, surface state, temperature and wavelength. Emissivity has a vital effect on measuring temperature results. In this test, the demarcation of AZ31 magnesium alloy emissivity is adopted by combining the thermocouple with IR measuring temperature. The demarcated zone temperature is 40 °C–350 °C. It is found when the emissivity of magnesium alloy AZ31 equals to 1.8 (ε = 0.18). There exists a good rule between thermocouple and IR measured value, as shown in Table 1. The formula is T = 1.198TR–29.320, T shows the thermocouple measurement temperature, TR shows the IR measurement temperature. It can calculate the actual temperature value by above formula. In present work, when using the IR camera to capture the isothermal profile on the surface, it was found that there are three radiation sources contributed to the measurement results: the arc, the electrode tip and the sensing spot in the base metal. The interference from both the arc and the hot tungsten electrode with the IR radiation during welding led to indistinct images being captured. There exists vital inference nearby melt pool zone due to strong arc and hot tungsten action, as shown in Fig. 1(b). The experiment data of nearby melt pool zone, therefore, are undependable, so the arc interference zone is obviated when to analyze the temperature field. The analyzed zone is chosen beyond the zone of arc interference (the backface of work chosen as the analyzed zone). It is found that the interference zone is about 10 mm× 6 mm around melt pool by experiments.

Fig. 2. Thermal recycling curves obtained from thermocouple and FEA simulation.

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Fig. 3. The back-face temperature field results. (a) IR measurement results. (b) Simulation results.

The thermal recycling curves obtained from mechanical peening of a thermocouple and from FEA calculating, plotted from GTAW weld in 3 mm thick magnesium alloy plate, were shown in Fig. 2. As shown in Fig. 2, it was found that the thermocouple curve recorded a peak temperature at around 15.6 s, while the FEA simulation curve recorded a peak temperature at around 15.0 s. The time lag to reach the peak temperature was about 0.6 s, which is because the response time of the thermocouple depended on the diameter of the wire. From the thermal recycling profile, it was found that the peak temperature by FEA simulation is about 181.4 °C, while the peak temperature by thermocouple recorded is about 196 °C. The error between the actual test and simulated value is about 7%. The simulated results based on the FEA shows good agreement with experimental test. The temperature distribution of the plates being welded can be displayed in different ways. The two methods that were used to display the data for analysis the isothermal contours (regions of equivalent temperature). Fig. 3 showed the isothermal map mode of representation of the temperature distribution of the back-face of AZ31B magnesium alloy plate. The isothermal contour map consists of several bands with the temperature increasing from the outer edges to the center of weld. The light part in the middle of the map is the high temperature zone. From Fig. 3, it can be concluded that the simulated results based on the FEA shows good agreement with the IR measurement results.

4. Conclusions Based on above experiments, the following conclusions were obtained by this study: 1) Through experiments, it is found that when using the IR to measure magnesium alloy welding field, the emissivity of

magnesium alloy AZ31 equals to 1.8. There exists a good rule between actual temperature value and the IR measured value. 2) The macroscopic temperature and local temperature changes were obtained by infrared measurement technique and mechanical peening thermocouple. From the experimental and simulation results, it was found that the simulation results were in a fairly good agreement with the experimental ones. References [1] L.P. Jang, Transactions of welding institute, Vol. 22, 2001, pp. 1–3. [2] B.A. Chin, N.H. Madsen, Welding Journal vol.62 (1983) 228–234. [3] Rui Sheng Huang, Liming Liu, Materials Science & Engineering A vol.447 (1–2) (2007) 239–243. [4] L.M. Liu, M.S. Chi, R.S. Huang, et al., Science in China, Ser.E vol.48 (2005) 705–715. [5] G.Y. Lin, D.S. Peng, H. Zhang, J. Nonfer. Metals vol.11 (2001) 79–82 (in Chinese). [6] X.F. Huang, H. Zhou, Z.M. He, J. Nonfer. Metals vol.12 (2002) 882–885 (in Chinese). [7] P. Henrikson, M. Ericsson, 6th International Conference on Trends in Welding Research, Pine Mountain, USA, 2002. [8] F.C. Eller, Welding Journal vol.62 (1983) 38–41. [9] AGEMA Infrared Systems, Thermovision 900 Series Users Manual-part 2, 1992. [10] L. William, Wolfe, 1978, Ibid, pp. 4–15.