Investigation on the influence of various welding parameters on the ...

Journal of Mechanical Science and Technology 28 (8) (2014) 3255~3261 www.springerlink.com/content/1738-494x

DOI 10.1007/s12206-014-0736-8

Investigation on the influence of various welding parameters on the arc thermal efficiency of the GTAW process by calorimetric method† Mohammad Bagher Nasiri1, Masoud Behzadinejad1, Hamidreza Latifi2,* and Jukka Martikainen2 1

Department of Materials Engineering, University of Applied Science and Technology, Boushehr, Iran Department of Mechanical Engineering, Laboratory of Welding Technology, Lappeenranta University of Technology, Lappeenranta, Finland

2

(Manuscript Received July 17, 2013; Revised January 20, 2014; Accepted April 18, 2014) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract Arc efficiency of Gas Tungsten Arc Welding (GTAW) was determined by calorimetric method. A water-cooled anode calorimeter was designed and manufactured to measure the arc thermal efficiency, which was determined as a function of current, arc length, polarity and gas flow rate for GTAW of mild steel. With Direct Current Electrode Negative (DCEN) polarity and 5 mm arc length, a thermal efficiency of 67±4% was obtained, which was independent of the welding current. With Direct Current Electrode Positive (DCEP) polarity and 5 mm arc length, arc thermal efficiency was determined as 52±4%. The experimental data show that the arc efficiency decreases from 67% to 58% and 51% as the arc length increases from 5 mm to 11 and 17.5 mm, respectively. The experimental results also show that the arc efficiency is not significantly affected by the shielding gas flow rate. Keywords: Arc length; Arc welding; GTAW; Polarity; Thermal arc efficiency; Water-cooled anode calorimeter ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction Arc welding is probably the most common manufacturing process for joining metals used in structural applications [1, 2]. An initial important step in creating an accurate basis for the design and analysis of welds is precise computation of the transient temperature field [3]. Perhaps the most critical input data required for welding thermal analysis are the parameters necessary to describe the heat input to the weldment from the arc [4-6]. Weld quality problems of distortion, residual stresses, inappropriate grain structure, fast cooling, high temperatures, and reduced strength of a structure in and around a weld joint result directly from the thermal cycle caused by the localized intense heat input of fusion welding [7]. Heat input is the amount of energy that is transferred to the base metal by a source of energy per unit of weld length [8]. Arc power and arc efficiency are two of the most important parameters connected to the heat input of arc welding processes [9]. The arc power is calculated as the product of the arc voltage V and the welding current I, and it defines the rate at which energy is being generated by the arc [10]. The arc efficiency h refers to the fraction of the arc energy that is delivered or transferred to the workpiece [11]. The arc power and arc efficiency result directly from the physical properties of the arc. *

Corresponding author. Tel.: +358 465599441, Fax.: +358 54117201 E-mail address: [email protected] † Recommended by Associate Editor Young Whan Park © KSME & Springer 2014

For example, in GTAW with DCEN, electrons travel from the tungsten electrode to the workpiece. As the electrons leave the cathode they must overcome the work function of the tungsten electrode, which is approximately 4 volts. As a result, the surface of the tungsten cathode is cooled by the emission of electrons. When the electron passes through the anode voltage drop region and enters the anode plate, it acquires significant kinetic energy. This anode voltage drop energy plus the energy of condensation of the electrons to the Fermi level of the anode results in energy release in the anode [12]. The energy is released in the arc region and transported by the hot plasma to the anode surface. Four principal modes of heat transfer contribute to the anode heat flux in welding: convection from the plasma, electron flow due to the current, radiation from the plasma, and vaporization of the anode [13]. Ushio [14] found that the contribution of plasma radiation to the anode is less than 5% of the total heat input. In addition, heat transfer due to vaporization of the anode is neglected according to a realistic assumption [13]. The energy carried by the electrons and plasma convection constitutes most of the heat on the anode surface. The convective heat transported by the hot plasma is very sensitive as the arc parameters are varied [15] .The energy transferred and released by the electrons also depends on the anode surface properties and shielding gas properties [15]. Thus, arc efficiency must be considered as a function of the GTAW welding parameters and materials involved. The arc efficiency can be calculated using calorimetric

3256

M. B. Nasiri et al. / Journal of Mechanical Science and Technology 28 (8) (2014) 3255~3261

measurements. Some calorimetric studies have been carried out using a seebeck envelope calorimeter [16-20]. In other calorimetric studies, the heat flow to the workpiece was measured by attaching thermocouples directly to the workpiece and measuring the bulk temperature rise [9]. Arc efficiency has also been estimated by recording point temperatures of workpieces and using inverse techniques [21]. Early calorimetric studies involved experiments where a welding arc was maintained between a tungsten electrode and workpiece immersed in cooling water [12, 22, 23]. In these experiments, the heat flow to the workpiece was estimated by measuring both the flow rate and temperature rise of the cooling water. With this approach, the heat sink of the water caused the temperature of the anode surface to decrease, and the measured arc efficiency was thus much higher than the arc efficiency of normal welding [12]. The differential temperatures of the anode boundary layer that connects the arc plasma with the weld pool are a key factor in welding thermal efficiency. As the anode surface temperature is decreased by the cooling water heat sink, the heat transported by the flow of hot plasma and electrons increases [13]. Niles and Jakson [24] investigated the influence of current level, travel speed, and argon and helium shielding gases on energy distortion at the welding arc of GTAW. A number of other studies have also investigated the power and efficiency of gas tungsten arcs [9, 16-22]. However, the majority of these studies have focused only on one parameter of the welding variables and there is a shortage of data about the influence on arc efficiency of variables such as arc length, shielding gas flow rate and polarity. The main objectives of the present study are: 1) to develop a water-cooled calorimetric method for estimating the influence of water flow rate on the arc efficiency and 2) to measure the arc efficiency of GTAW as a function of welding process variables.

2. Experimental procedure 2.1 Set-up The water-cooled anode method was used to determine the arc thermal efficiency of GTAW. The underlying principle of this method is based on heat transfer from the workpiece to the water that passes under the workpiece. Heat transfer to the water, which has constant flow rate, increases the temperature of the water and the temperature difference between the input and output water indicates the amount of heat transferred to the water. In this method, the heat content of the calorimeter is given by the temperature rise of the water multiplied by the water flow rate and the density and specific heat of the water. Various types of calorimeter have been investigated in Ref. [25]. The heat transfer from the workpiece to the calorimeter and thus the arc efficiency can be calculated by Eqs. (1) and (2), respectively [25]. ¥

h=

Q IV

(2)

where W is the mass flow rate of water, C the specific heat of water, Tout the outlet water temperature, Tin the inlet water temperature, tweld the welding time, Q the rate of heat transfer from the heat source to the workpiece, V the welding voltage, and I the welding current. The flow rate of the cooling water was measured by rotameter and differential K-type thermocouples were used to sense the temperature of the water. The arc voltage, arc current and temperature rise of the water were monitored and recorded by a computer using an analog digital board Advantech PCI1710HG. A Matlab-Simulink data acquisition program was developed. The program was used for digital filtering, calculation of differential temperatures, differential temperatures integration, and computation of the heat content of the calorimeter and arc efficiency during welding. A digital filtering technique was utilized to delete the high frequency component of the recorded data caused by fluctuations in the power supply. It is very difficult and subjected to error to measure simultaneously by welding. In the experiments, the voltage and the current were recorded so that the arc energy could be calculated over the duration of the weld. The welding current was recorded using a Hall effect sensor, which was attached to the power cable between the power supply and the welding torch. The voltage was measured between the torch and workpiece with a programmable voltmeter connected to the computer. A schematic representation of the data acquisition system used in the study is shown in Fig. 1. A fully automated welding system designed specifically for research was used for all the experiments. Fig. 2 shows the welding set-up used in this study. 2.2 Arc efficiency measurement errors

¥

Qtweld = ò WC (Tout - Tin )dt » WC ò (Tout - Tin )dt 0

Fig. 1. Schematic diagram of data acquisition system and experimental set-up.

0

(1)

The maximum error of the measurement tools was utilized in estimation of the arc efficiency measurement error. Current

3257


Table 1. Experimental variables in all experiments. Arc length (mm)

Polarity

Gas flow rate (Lh-1)

1. Water flow rate 60, 70, 80, …, 210

5

DCEN

10

2. Current

60, 70, 80, 90, …, 160

5

DCEN

10

3. Gas flow rate

60, 100, 140, 180

4. Arc length

140, 160, 200, 220

5. Polarity

30, 40, 50, 60

Experimental series

Fig. 2. Experimental set-up.

measurements obtained from the Hall effect transducer were cross-checked with an independent battery-powered current clamp and found to agree to within approximately 1%. Uncertainty in arc length occurs due to displacement of the arc resulting from the surface roughness of the sample. In estimation of arc power at a given arc length, there was uncertainty of approximately 0.4 mm in the arc length. This uncertainty equates to a variation in arc voltage of approximately 0.2 V. Based on this value, errors in voltage estimation were calculated as being less than 2%. Consequently, an uncertainty of 2% was assumed in all estimates for arc power. Errors in measuring the arc power are relevant to the arc efficiency measurements. In addition, errors in estimating the temperature and flow rate measuring contribute to uncertainty in estimates of the arc efficiency. The major source of error in the temperature measurements is noise due to the magnetic effect of the arc current. Although the thermocouples were crinkled to reduce this noise, based on the NernstEttingshausen effect [26], nevertheless some noise was observed. Investigation of errors in temperature measurement was done by measuring the boiling temperature of water during welding process and without welding process. An error of approximately 2% was noted. Based on this value, uncertainty in water flow rate values is approximately 1%. The summation of all four uncertainties gave a theoretical error of 6%, and this error margin was included in the arc efficiency calculations made according to Eq. (2). However, the relative error of the calorimeter, according to the measured data, is estimated as approximately 4%. This is seen in the data given in Figs. 4, 5 and 7.

3. Experimental conditions GTAW was done using dc power with a 2-mm diameter 2%-thoriated tungsten electrode and argon shielding gas. An ac/dc inverter power supply capable of delivering 250 A was used in all experiments. This power supply utilized high fre-

Current (A)

180,

5

DCEN

10, 20, 25

5, 11, 17.5

DCEN

10

5

DCEP, DCEN

10

quency arc starting. Workpieces with dimension of 200×150×5 mm were selected based on the dimensions of the calorimeter. The dimensions and geometry of the calorimeter were designed based on the welding system shown in Fig. 2 and research presented in Ref. [25]. Workpieces were machined from mild steel and placed on a cooling water circuit manufactured from polyvinyl chloride (PVC). This work assessed the effect of welding current, arc length, shielding gas flow rate, DCEN and DCEP polarities, and cooling water. One difference between the water-cooled anode calorimeter used in this research and those of other studies is the possibility of fast replacement and unlimited number of the workpiece. The device also exhibits high measurement accuracy. These features enabled multiple experiments to be carried out, thus the variables and values were adjusted and selected. When welding with DCEN polarity, various ranges of current could be selected for the defined welding conditions, shielding gas flow rate and arc length,. Higher ranges of electrical current were selected due to the stable arc obtained. When welding with DCEP polarity, it was not possible to produce a stable arc at higher electrical current due to heat centralization on the electrode and melting of the tungsten electrode. Therefore, a lower range of electrical current was selected for the experiments with DECP polarity. Five series of experiments were done; the variables under investigation are summarized in Table 1.

4. Results and discussion Before study of the welding parameters, the optimal water flow rate must be determined. For this purpose, the arc power divided by the cooling water flow rate was estimated as a function of arc efficiency. The water-cooled anode calorimeter receives the heat transferred to the work piece at constant water flow rate. The calorimeter measures the temperature increase of the water and the temperature increase of the workpiece that is in contact with the water can thus be calculated. Since phase transition is not considered in the calculations, the water flow

3258


rate should be selected based on the welding power such that water evaporation is avoided. Therefore, water flow rate relative to welding power should not be so low that evaporation occurs or that water output temperature reaches the boiling point. On the other hand, selection of a high flow rate relative to the welding power causes increased heat removal from the surface of workpiece, and as a result, the temperature of the welding surface temperature is reduced and a smaller molten weld pool is formed. The surface temperature of the workpiece at the molten weld pool is an important parameter of heat transfer to the workpiece. A lower temperature and smaller volume of weld pool due to water refrigeration increases the heat transfer to the workpiece and as a result the thermal efficiency increases artificially. The water flow rate should be selected for the welding power in such a way that the welding conditions stay close to reality. Unreal welding conditions due to high water refrigeration have a highly sensitive effect on the ratio of arc power to the water flow rate; therefore, the thermal efficiency of the welding was measured at various powers (i.e. various different welding currents). The experiments showed that the arc efficiency is independent of the cooling water flow rate, if the ratio of the arc power to the flow rate is above the minimum limit. As shown in Fig. 3, the arc efficiency varies in a range of 65-70% when the ratio of the arc power to water flow rate is higher than 80 JhL-1 . As mentioned previously, anode surface temperature is an important parameter in arc efficiency investigation; this temperature is affected by the arc power and cooling water heat sink. From the experimental results, it can be seen that the arc efficiency has considerable dependence on the ratio of arc power to water flow rate: as the ratio decreases, the arc efficiency increases. This trend is consistent with data presented by Tsai and Eagar [12]. In their studies, Tsai and Eagar used a water-cooled copper anode and measured the heat and current flux to the anode. They reported that the arc efficiency of GTAW with 5 mm arc length and without anode melting is approximately 80%. Based on the results shown in Fig. 3, the optimum conditions for measuring arc efficiency are when the ratio of the arc power to the water flow rate is higher than 80 JhL-1 . The ratio of 80 was used as the value of the water flow rate in subsequent experiments. It should be mentioned that water flow rate is a factor relevant to calorimeter adjustment in this series of experiments and it is not an effective parameter in the thermal efficiency of welding in real welding conditions. Fig. 4 shows the arc efficiency as a function of welding current. In this series of experiments, the arc current was varied from 60-160 A and arc length was specified as 5 mm. The data show that there is little variation in arc efficiency. These values are in good agreement with arc efficiencies reported in previous research. Using a Seebeck calorimeter, DuPont and Marder [20] calculated arc efficiency for GTAW with DCEN polarity and 5 mm arc length at a range of currents. They reported that the GTAW process has an average arc efficiency

Fig. 3. Arc efficiency versus ratio of arc power to flow rate, at arc length of 5 mm.

Fig. 4. Arc efficiency for arc length of 5 mm as a function of welding current.

Fig. 5. Arc efficiency versus welding current at shielding gas flow rate 10, 20 and 25 LPH and arc length of 5 mm.

of 67%. The effect of shielding gas flow rate on the arc efficiency has hitherto not been investigated. Fig. 5 shows arc efficiency as a function of welding current for three shielding gas flow rates (10, 20, and 25 LPH) and four welding currents (60, 100,


Fig. 6. Arc voltage versus arc length, at welding current of 100 A with DCEN polarity, and the extrapolated line.

140, and 180 A). As can be seen, the arc efficiency does not vary significantly over the investigated range of gas flow rate. Calorimetric experiments were not possible for the higher shielding gas flow rates due to instability of the arc. As Fig. 5 shows, thermal efficiency with DCEN polarity was 67±4% for the range of currents studied. The highest arc efficiency, approximately 71%, was obtained with a welding current of 140 A, and the lowest one, almost 65%, with a welding current of 180 A. The maximum axial velocity at the outlet of the nozzle caused by the shielding gas flow rate is 1 ms-1 [27], while axial velocity in the argon gas tungsten arc region caused by the cathode-jet reaches around 300 ms-1 [15]. Consequently, the gas flow rate does not play an important role in convective heat transport by hot plasma and, as shown in Fig. 5, the shielding gas flow rate does not have a significant effect on arc efficiency. The estimates for arc voltage are plotted as a function of the arc length in Fig. 6. The figure shows that the arc voltage increases as the arc length is increased. The result of extrapolation of the experimental data to zero is 8.7 V, which is approximately equal to the sum of the cathode and anode work functions. Consequently, increase in arc voltage will lead to an increased contribution of released energy in the arc region that is transported by hot plasma convection. The effect of arc length on arc efficiency is plotted in Fig. 7. It can be seen that the arc efficiency decreases as the arc length increases; although an increase in arc length results in an increase in arc power. It would appear that the heat transported by hot plasma convection decreases as the arc length increases. The curve in this figure is extrapolated based on the calorimetry results. As the figure shows, the arc thermal efficiency of GTAW at zero arc length extrapolates to 77.95, indicating that the arc efficiency of GTAW cannot be greater than 77.95%. The measured arc efficiency is plotted for the DCEP experiments in Fig. 8. The results of experiments with DCEP polarity in this figure can be compared to the results of DCEN experiments given in the previous figures. As using high current level in welding with DCEP polarity was difficult due to

3259

Fig. 7. Arc efficiency versus arc length, at welding current of 140, 160, 180, 200 and 220 A with DCEN polarity, and the extrapolated line.

Fig. 8. Arc efficiency for arc length of 5 mm as a function of welding current with DCEP polarity.

melting of the electrode tip, calorimetric measurements were performed for DCEP polarity only up to a current level of 60 ampere. The results for the arc thermal efficiency with DCEP welding show that the thermal efficiency, as with DCEN welding, is independent of the welding current. The arc voltage in DCEP welding with 5 mm arc length was measured as approximately 23 V. Consequently, for equivalent welding parameters and according to the lower current level used in DCEP welding, the arc power was higher with DCEP polarity than with DCEN, although the DCEP arc efficiencies were lower than the efficiencies achieved with DCEN polarity. With DCEP polarity, electrons have to be supplied by the workpiece, which is much colder than the tungsten electrode, and thus the temperature is too low for thermionic emission. It has been suggested that electron emission occurs owing to the electric field near the cathode surface [28]. The general effect is to strip oxide from the metal surface as each emission site is initiated. According to Guile [29], the cathode voltage drop for a non-thermionic cathode is generally between 10 and 20 V. In comparison, the cathode voltage drop for a thermionic cathode has been estimated as being in the range between 5 and 7 V [30]. Therefore, it would appear that the higher arc powers associated with DCEP polarity are due

3260


to greater increase in the cathode voltage drop compared to DCEN polarity. The additional power is generated at the cathode, owing to increased cathode voltage falls associated with the field emission of electrons. Lancaster [28] suggests that this energy is absorbed as either heat in the workpiece, chemical and electrical energy in dispersing oxide films, or energy in the vapour jet emitted by individual cathode spots. It is to be expected that DCEP arc efficiencies will generally be lower than efficiencies achieved with DCEN polarity as the electrons must condense on the tungsten electrode and in doing so they will transfer a considerable amount of energy to the electrode.

5. Conclusion A water-cooled anode calorimeter was designed and manufactured to measure the arc thermal efficiency of the GTAW process. A Matlab-Simulink data acquisition program was developed for digital filtering, calculation of differential temperatures, differential temperatures integration, and computation of the heat content of the calorimeter and the arc efficiency during welding. The device designed for this study enabled experiments for a wide range of parameter values. In the investigation, arc efficiency was determined as a function of current, arc length, polarity and gas flow rate. Measurement error of approximately 4% was calculated for the arc efficiency measurement. A ratio of arc power to water flow rate higher than 80 JhL-1 was considered as the optimum condition for measuring the arc efficiency. The experimental results showed that the arc efficiency is independent of the cooling water flow rate at these conditions. Furthermore, the results showed that the arc efficiency is not significantly affected by the shielding gas flow rate over the range 10 and 25 LPH. In DCEN polarity welding with 5 mm arc length, a thermal efficiency of 67±4% was obtained, which was independent of the welding current. The experimental data showed that the arc efficiency decreased from 67% to 58% and 51% as the arc length increased from 5 mm to 11 mm and 17.5 mm, respectively. At zero arc length, an arc thermal efficiency of 77.95% was obtained for the GTAW process. On the other hand, with DCEP polarity and 5 mm arc length, the arc thermal efficiency was determined as 52±4%. However, the arc power was higher with DCEP polarity than with DCEN polarity.

References [1] R. Komanduri and Z. B. Hou, Thermal analysis of the arc welding process: Part I. general solution, Metallurgical and Materials Transactions B, 31 (6) (2000) 1353-1370. [2] H. Y. Huang, Research on the activating flux gas tungsten arc welding and plasma arc welding for stainless steel, Metals and Materials International, 16 (5) (2010) 819-825. [3] V. Kamala and J. A. Goldak, Error due to two dimensional approximation in heat transfer analysis of welds, Welding

Journal, 72 (9) (1993) 440-446. [4] E. Friedman, Thermomechanical analysis of the welding process using the finite element method, Journal of Pressure Vessel Technology, 97 (3) (1975) 206-213. [5] E. A. Bonifaz, Finite element analysis of heat flow in singlepass arc welds, Welding Journal, 79 (5) (2000) 121-125. [6] E. A. Bonifaz, Multiscale simulations in welds, Advances in Science and Engineering, 4 (2) (2012) C1-C5. [7] J. Goldak, A. Chakravarti and M. Bibby, A new finite element model for welding heat sources, Metallurgical Transactions B, 15 (2) (1984) 299-305. [8] V. Malin and F. Sciammarella, Controlling heat input by measuring net power, Welding Journal, 85 (7) (2006) 44-50. [9] G. M. D. Cantin and J. A. Francis, Arc power and efficiency in gas tungsten arc welding of aluminium, Science and Technolology of Welding & Joining, 10 (2) (2005) 200-210. [10] N. Pepe, S. Egerland, P. A. Colegrove, D. Yapp, A. Leonhartsberger and A. Scotti, Measuring the process efficiency of controlled gas metal arc welding processes, Science and Technolology of Welding & Joining, 16 (5) (2011) 412-417. [11] M. Jou, Experimental study and modeling of GTA welding process, Journal of Manufacturing Science and Engineering, 125 (4) (2003) 801-808. [12] N. S. Tsai and T. W. Eagar, Distribution of the heat and current fluxes in gas tungsten arcs, Metallurgical Transactions B, 16 (1985) 841-846. [13] C. S. Wu and J. Q. Gao, Analysis of the heat flux distribution at the anode of a TIG welding arc, Computational Materials Science, 24 (3) (2002) 323-327. [14] M. Ushio, D. Fan and M. Tanaka, Contribution of arc plasma radiation energy to electrodes, Transactions of the Japan Welding Research Institute, 22 (2) (1993) 201-207. [15] M. Tanaka, H. Terasaki, M. Ushio and J. J. Lowke, A unified numerical modeling of stationary tungsten inert gas welding process, Metallurgical and Materials Transactions A, 33 (7) (2002) 2043-2052. [16] W. H. Giedt, L. N. Tallerico and P. W. Fuerschbach, GTA welding efficiency: calorimetric and temperature field measurements, Welding Journal, 68 (1) (1989) 28-32. [17] P. W. Fuerschbach and G. A. Knorovsky, A study of melting efficiency in plasma arc and gas tungsten arc welding, Welding Journal, 70 (11) (1991) 287-297. [18] P. W. Fuerschbach and G. R. Eisler, Effect of laser spot weld energy and duration on melting and absorption, Science and Technology of Welding and Joining, 7 (4) (2002) 241-246. [19] R. R. Unocic and J. N. DuPont, Process efficiency measurements in the laser engineered net shaping (LENS) process, Metallurgical and Materials Transactions B, 35 (1) (2004) 143-152. [20] J. N. Dupont and A. R. Marder, Thermal efficiency of arc welding processes, Welding Journal, 74 (12) (1995) 406-416. [21] C. V. Goncalves, L. O. Vilarinho, A. Scotti and G. Guimaraes, Estimation of heat source and thermal efficiency in GTAW process by using inverse techniques, Journal of


Materials Processing Technology, 172 (1) (2006) 42-51. [22] N. Christiansen, V. Davies and K. Gjermundsen, Distribution of temperatures in arc welding, British Welding Journal, 12 (2) (1965) 54-75. [23] S. Kou and Y. Le, Heat flow during the autogenous GTA welding of pipes, Metallurgical and Materials Transactions A, 15 (6) (1984) 1165-1171. [24] R. W. Niles and C. E. Jakson, Weld thermal efficiency of the GTAW process, Welding Journal, 54 (1) (1975) 25s-32s. [25] S. Kou, Welding Metallurgy, Second Ed., John Wiley & Sons Inc., Hoboken, New Jersey, USA (2003). [26] T. W. Kerlin and R. L. Shepard, Industrial temperature measurement, Instrument Society of America, Philadelphia, USA (1982). [27] J. Hu and H. L. Tsai, Heat and mass transfer in gas metal arc welding. Part I: the arc, International Journal of Heat and Mass Transfer, 50 (5-6) (2007) 833-846. [28] J. F. Lancaster, The Physics of Welding, Pergamon Press, Oxford, UK (1986).

3261

[29] A. E. Guile, Studies of short electric arcs in transverse magnetic fields with application to arc welding, Welding in the World, 8 (1) (1970) 36-53. [30] K. Hiraoka, N. Sakuma and J. Zijp, Energy balance in argon-helium mixed gas tungsten (TIG) arcs. Study of characteristics of gas tungsten arc shielded by mixed gases (3rd report), Welding International, 12 (5) (1998) 372-379.

Hamidreza Latifi received his M.Sc. degree in Mechanical Engineering from Lappeenranta University of Technology (LUT), Finland, in 2013. He is currently a Ph.D. candidate at the department of mechanical engineering and welding laboratory of Lappeenranta University of Technology. His research interests include materials science (composites and advanced ceramics) and welding technology (arc welding and adaptive welding).