Droplet Temperature Measurement in Metal Inert Gas Welding. Process by Using Two Color Temperature Measurement Method*

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1 [ 溶接学会論文集第 35 巻第 2 号 p. 160s-164s (2017)] Droplet Temperature Measurement in Metal Inert Gas Welding Process by Using Two Color Temperature Measurement Method* by Sarizam Bin Mamat**, ***, Titinan Methong**, Shinichi Tashiro****, Manabu Tanaka**** Droplet temperature for Metal Inert Gas welding process was measured by using optical pyrometry; two colors temperature measurement method. High speed color video camera was used to capture an image of metal droplet during transfer to base metal. As a result, the movement of an arc to cover the droplet from bottom half to the entire droplet by increase of welding current had allowed the change in droplet heating process from concentrated to uniform heating. The concentrated heating in globular transfer mode had caused a higher current density at the bottom of the droplet. The change of heating process from concentrated heating in globular mode to a rather uniform heating during transition resulted in a smaller current density and reduced the heat flux into droplet, therefore decreased the droplet temperature. The further increase in welding current led the increase of droplet temperature and produced the minimum temperature in the middle of transition mode. Key Words: GMAW, Droplet Temperature, Two Color Temperature Measurement Method, Arc Envelopment, Transition Mode 1. Introduction Droplet temperature is one of the essential information in order to understand the weld metal geometry such as welding penetration and its shape, fume formation rates and so on during Metal Inert Gas (MIG) welding. In terms of safety and health at workplace, welding fumes that mainly originates from metal droplet is known to be very harmful to welder s health and the fume formation rates is closely related to droplet temperature 1). Basically, the droplet with higher temperature will produce more fumes and the same effect goes with droplet size. In order to understand and control the fume formation rates, information regarding droplet temperature and parameters related to it is required. To date, many researches have been done to study the behavior of droplet in MIG and its different transfer types, but scarcely done to study the droplet temperature especially by in-situ measurement method due to the difficulties in measuring the temperature without the influence of arc. There are several methods applied to measure the droplet temperature such as thermocouples, optical pyrometry and calorimetry 2). For example, E.J.Soderstrom et al measured the droplet temperature by means of calorimetry and found that the droplet temperature for mild steel wire reached a minimum at about 2200 K during transition from globular to spray transfer 3). The high welding current in spray transfer mode increase the heat content which in turn increase the droplet temperature. By using the same * Received: ** Student Member, Graduate School of Engineering, Osaka University *** Faculty of Bioengineering and Technology, University Malaysia Kelantan **** Member, Joining and Welding Research Institute, Osaka University method, C.McIntosh et al have studied the effect of CO2 content mixed with argon shielding gas to droplet temperature and found that the temperature of droplet could be controlled by adjusting a small amounts of CO2 despite of maintaining a similar sinusoidal temperature curve shape to be agreed with the results by E.J.Soderstrom 4). However in both studies, the mechanism of temperature decrement phenomena at transition mode was left without detailed explanation. The calorimetry, that utilized the changes of water temperature to be considered as an average droplet temperature, is known as a suitable method to calculate a droplet energy during welding. But, it is difficult to clarify the mechanism of temperature decrement at transition mode as the value acquired is only about the average value during welding. Therefore, it is worth to visualize each captured droplet at the time of arc vanished and its temperature distribution, as the information allow the correlation between arc and droplet temperature distribution to be evaluated. Hence, in this study, the optical pyrometry: two color temperature measurement method, which is able to evaluate a temperature distribution on droplet surface, is utilized in order to clarify the temperature decrement phenomena at early stage of transition mode until it comes to the minimum temperature. The use of high speed video color camera enables the moment at the time of arc vanished to be captured and the relationship between arc behavior as well as temperature distribution on droplet surface can be evaluated. The study is focused on the droplet temperature measurement in MIG welding for a wide range of welding currents, which covers the behavior from globular to spray transfer mode.

2 溶接学会論文集 第 35 巻 (2017) 第 2 号 161s 2. Experimental procedure 2.1 Machine Setting Condition The main component of experimental set-up is shown in Fig.1. There were two main equipment used in this experiment; welding machine set and high speed color video camera. The welding machine was operated at DCEP polarity and pure argon gas has been used as a shielding gas. Low carbon steel plate with 9 mm in thickness was welded by using solid steel wire with diameter of 1.2 mm. The details of the welding condition are shown in Table Two Color Temperature Measurement Method High speed color video camera was used to measure a droplet temperature. As mentioned in previous section, the analysis on droplet was done at the timing of an arc effect vanished completely. The color camera image consists of three images, which are in red (R), green (G) and blue (B). These three images are taken by different color sensors corresponds to a particular wavelength sensitivities. The sensitivities of these color sensors are nm, nm and nm, subject to red, green and blue, respectively. Fig.2 indicates the spectral sensitivities of QR(λ), QG(λ) and QB(λ) (here λ refers to wavelength) for red, green and blue sensors in the high speed video color camera. In this experiment, low pass and high pass filters were used to limit the spectral sensitivities range. Use of high speed video color camera enables all the radiation intensities (IR meas, IG meas, IB meas ) of each color to be measured simultaneously during experiment. By assuming the grey body radiation from the droplet surface, the theoretical intensity ratios IR theo /IB theo, IG theo /IB theo, IR theo /IG theo can be calculated as follows: IR theo (T) = ) B(λ,T) (1) IG theo (T) = ) B(λ,T) (2) IB theo (T) = ) B(λ,T) (3) Where : B(,T) = (4) Fig. 1 Main equipment of experimental set-up Here, is the emissivity of the droplet, B(λ,T) [Wsr -1 m -3 ] is the black body radiation intensity, h is Planck constant, c is speed of light, kb is Boltzmann constant and T[K] is droplet surface temperature. Table 1 Welding Condition Contact Tip to Welding Welding Gas Flow Work Distance, Speed Current Rate CTWD [cm/min] [A] [lpm] [mm] Meanwhile, high speed color video camera was set so as to perpendicular to the arc axis for droplet temperature measurement. The frame rate of the camera was set at 2000 frame per second (fps) with exposure time of 20 μs. The image resolution was set at 512 x 512 pixels for the captured image size of 20 x 20 mm. The in-situ droplet temperature measurement against the pendant droplet as well as free flight droplet, which were captured after the arc vanished, were done using two color temperature measurement method. RGB Sensitivity Q 0.08 QG QB Wavelength [nm] Fig.2 The spectral sensitivities of QR(λ), QG(λ) and QB(λ) From equation (1), (2) and (3), the ratios of IR theo /IB theo, IG theo /IB theo, IR theo /IG theo only depend on T if is constant. Here, the temperature, T on droplet surface can be determined by comparing the value of the radiation intensity ratios (IR meas /IB meas, IG meas /IB meas, IR meas /IG meas ) that measured in the experiments with the theoretical curve as presented in Fig.3. However, in this study, we use the

3 162s 研究論文 MAMAT et al.: Droplet Temperature Measurement in Metal Inert Gas Welding Process by IR meas /IG meas curve for temperature evaluation since the intensity of the blue color is observed to be very low, besides only this curve shows the monotonic decrease. Also, in this experiment, is assumed to be the same because the difference in centre wavelength of green and red is small (approximately 50 nm). In order to assure an accuracy of the measurement, the calibration against IR/IG in comparison with the temperature from a standard illuminant was done. In calibration, the high stability tungsten ribbon lamp was used as a standard illuminant. shows the average temperature of different targeted region. ntensity Ratio I (a) Temperature analysis for globular transfer mode (160 A) Temperature [K] Fig.3 Intensity ratio in corresponding to the temperature 3. Result and discussion 3.1 Analysis of droplet temperature The advantages of using this two color temperature measurement method is that the droplet temperature is measureable without any influence of an arc and the information regarding droplet temperature distribution is also accessible. Fig.4(a), (b) and (c) show the analysis on droplet temperature and the temperature distribution on droplet surface. In this study, 160 A, 175 A and 260 A were selected as a representatives welding current for globular, transition and spray mode, respectively. For temperature analysis, pendant droplet images were captured for the globular mode and transition mode, and free flight droplet image was captured for the case of spray transfer mode. Here, the bottom region refers to the droplet base area inside arc root which is affected most by concentrated heating, while upper region refers to the area outside the arc root. Each of which is numbered as or. However, for the case of spray transfer mode, the arc root does not exist in the droplet surface, therefore bottom and upper region refer to the droplet bottom-half region and upper-half region, respectively. In addition, (i) and (ii) refer to the targeted droplet at the timing of 0.5 ms before and at 0 ms after the arc vanished as being shown in Fig.5, (iii) shows the temperature distribution obtained by using two color temperature measurement method, and (iv) illustrates the exact area for and. Table in each case (b) Temperature analysis at transition phase (175 A) (c) Temperature analysis for spray transfer mode (260 A) Fig.4 Analysis on droplet temperature and the temperature distribution in each transfer mode

4 溶接学会論文集 第 35 巻 (2017) 第 2 号 163s temperature tended to increase in line with the increasing of welding current and became almost constant above 230 A. The transition from globular to spray transfer was observed to be from 170 A and the transfer completely became spray mode from 220 A. These results were in good agreement with the studies by other researchers 5). Fig.5 Timing of the arc vanished during welding current setting at 160 A The result showed that the temperature difference between bottom region and upper region in globular mode was calculated to be almost 200 K. However the difference become smaller in transition phase mode for about 160 K and less than 50 K in spray transfer mode. From these observation results, the presence of nonuniform temperature distribution on droplet surface is suggested to be a consequence of the concentrated heating at droplet base during globular mode and transition mode, which causes higher thermal gradient in droplet. While, the arc that overwhelms the whole free flight droplet in spray transfer mode contributes for the uniform temperature distribution on its surface. The entire region in the table for each case refers to the average temperature at the whole droplet surface. Droplet temperature for globular mode was 2303 K, decreased to 2074 K in transition mode and increased to 2451 K in spray transfer mode. Basically, the temperature in transition phase showed the decrement more than 200 K regardless of any region. Here, the decrement of the bottom region temperature in transition phase mode suggests the lesser effects of concentrated heating at droplet base. In other words, the heating region spread up to a higher region of droplet. In order to elucidate the reason for this phenomenon, an observation scope was enlarged to a wider welding current. Fig.6 Dependency of the droplet temperature to welding current in globular, transition and spray transfer mode area Regarding to the shape of the line in overall, there was a clear tendency of forming a sinusoid curve, in which at globular transfer mode area, the droplet temperature increased by increasing of welding current, but decreased at early stage of transition from globular to spray transfer and tends to re-increase after a threshold, or the stable projected spray. The same tendency were reported by E.J.Soderstrom and C.McIntosh in their studies 3,4). Meanwhile, the difference between upper region temperatures in comparison with entire droplet temperature seemed to exhibit a particular tendency. The temperature tended to show a large difference during globular transfer mode and then the upper region temperature gradually approached the entire droplet temperature during transition mode. Subsequently, the difference became less than 50 K during spray transfer mode. 3.2 Behavior of droplet temperature curve 3.3 Observation of arc behavior in transition mode The tendency of the droplet temperature behavior as a function of welding current is shown in Fig.6. The lines in different colors indicated in this graph are showing the temperature taken at bottom region, upper region and entire droplet. From the graph, the temperature became higher by the increasing of welding current in globular mode and was observed to be the maximum at 160 A. However, it started to slightly decrease from 170 A, and continue to decrease until the minimum value at 190 A. Afterwards, the In order to explain the temperature decrement phenomena in transition mode, further investigation towards arc behavior was done. To get a better insight of the concentrated heating phenomenon, 135 A of welding current was selected for globular mode and 170 A for transition mode. Fig.7(a) and (b) show an arc envelopment behavior around droplet at 135 A and 170 A welding current, respectively. The observation result in globular mode

5 164s 研究論文 MAMAT et al.: Droplet Temperature Measurement in Metal Inert Gas Welding Process by showed that there was a repeated up and down movement in arc envelopment around droplet especially in 135 A. The arc envelopment seemed to cover the entire droplet at early stage of droplet formation, but moving downwards to the bottom half of droplet to cause a concentrated heating after some while until the droplet detachment. The arc started to cover the entire wire tip soon afterwards. This process was observed to be repeated during the droplet formation until detachment process and was completely unviewable during transition mode. covered a larger fraction of droplet area, thus leading to a rather uniform heating 6). In addition, it is thought that the change of heating process from concentrated in globular mode to a rather uniform heating during transition mode has reduced the current density at the bottom of the droplet which simultaneously decreased the heat flux into the droplet, thus decrease a droplet temperature. This mechanism suggests the decrease of the droplet temperature during transition mode. The further increase in welding current leads the increase of droplet temperature which in turn results in an existence of the minimum temperature in the middle of transition mode. 5. Conclusions (a) Globular transfer mode (135 A) The conclusions for this study are as follows: 1. Droplet temperature and temperature distribution are measureable by using two color temperature measurement method. 2. The movement of an arc to cover the droplet from bottom half to the entire droplet by increase of welding current allows the change in droplet heating process from concentrated to uniform heating. 3. The concentrated heating in globular transfer mode causes a higher current density at the bottom of the droplet. The changes of heating process from concentrated heating to a rather uniform heating during transition results in a smaller current density and reduces the heat flux into droplet, therefore decreases the droplet temperature. Reference (b) Transition mode (170 A) Fig.7 Arc envelopment behavior around droplet at 130 A and 170 A welding current Through observation, it could be considered that the movement of an arc envelopment during the droplet formation process was closely related to the formation of anode spot on droplet surface. The further increase of welding current initiated more formation of anode spots which tended to climb up and 1) P.F.Mendez, N.T Jenkins and T.W.Eager: Effect of electrode droplet size on evaporation and fume generation in GMAW, Proceedings of the Gas Metal Arc Welding for the 21 st Century, American Welding Society, Miami, Florida (2000). 2) Halmoy E.: Wire melting rate, droplet temperature and effective anode melting potential, Arc Physics and Weld Pool Behavior, Vol.1 (1980), ) E.J.Soderstrom, K.M. Scott and P.F.Mendez: Calorimetric measurement of droplet temperature in GMAW, Welding Journal, Vol. 90 ( 2011), 77s 84s. 4) C.McIntosh, J.Chapius and P.F.Mendez: Effect of Ar-CO 2 gas blends on droplet temperature in GMAW, Welding Journal, Vol.95 (2016), 273s 279s. 5) K.Kadota, Y.Suzuki, Y.Hirata, T.Kataoka, R.Ikeda and K.Yasuda: Influence of shielding gas and electrode wire on metal transfer phenomena in GMAW process, Journal of The Japan Welding Society,Vol.30, No.1 (2013), ) E.J.Soderstrom and P.F.Mendez: Metal transfer during GMAW with thin electrodes and Ar-CO 2 shielding gas mixture, Welding Journal, Vol.87 (2008),124s 133s.