Pulsed Current Gas Metal Arc Welding under Different Shielding and Pulse Parameters; Part 2: Behaviour of Metal Transfer

Transcription

1 , pp Pulsed Current Gas Metal Arc Welding under Different Shielding and Pulse Parameters; Part 2: Behaviour of Metal Transfer P. K. GHOSH, 1) Lutz. DON, 2) K. DEVAKUMAAN 1) and F. HOFMANN 2) 1) Department of Metallurgical & Materials Engineering, Indian Institute of Technology oorkee, oorkee , India. 2) Fügetechnik und Bechichtungtechnik, Sekr. PTZ 6, Pascal strasse 8-9, TU Berlin, Berlin, Germany. (eceived on June 24, 2008; accepted on November 13, 2008) The behaviour of metal transfer and arc stiffness understood in terms of arc pressure (P a ) of pulsed current gas metal arc welding (P-GMAW) using mild steel filler wire have been studied with respect to change in pulse parameters under different gas shieldings of Ar 2%CO 2 and Ar 18%CO 2. The arc environment revealing the droplet transfer from electrode to weld pool in bead on plate weld deposition has been studied by high speed video graphy. Effect of pulse parameters has been considered by their hypothetically proposed summarized influence defined by a dimensionless factor f, mean current (I m ) and arc voltage. The droplet diameter and velocity of droplet at the time of detachment are found to vary significantly with the variation of f. At a given f the experimentally measured behaviour of metal transfer is found well in agreement to their corresponding theoretical estimates. The average droplet diameter transferred per pulse predominantly reduces but, the velocity of metal transfer at the time of detachment enhances with the increase of f under both the shielding gases. In general a higher values of f, I m and arc voltage enhances the P a depending upon type of gas shielding. The use of Ar 2%CO 2 shielding gas reduces the droplet diameter and enhances the arc pressure than that observed under Ar 18%CO 2 gas shielding. KEY WODS: P-GMAW; mild steel; pulse parameters; shielding gas; high speed video-graphy; metal transfer. 1. Introduction The superiority of pulsed current gas metal arc welding (P-GMAW) over the conventional gas metal arc welding (GMAW) has been largely highlighted by its ability to more precise control of metal transfer by appropriate selection of pulse parameters. 1 4) The variation in behaviour of metal transfer along with a change in arc characteristics in P- GMAW can manipulate thermal behaviour of weld 3,5) and consequently affect the characteristics of weld joint such as its geometry, 6,7) microstructure 6 10) as well as mechanical properties. 6,9) Thus, it is often proposed that to improve the weld quality it is imperative to employ proper pulse parameters resulting desired behaviour of metal transfer, a deviation from which may harm the weld characteristics. 8 13) The variation in arc characteristics and behaviour of metal transfer with a change in pulse parameters also influences the arc stiffness significantly as it is observed in case of stainless steel weld deposition. 2) This may be one of the primary cause of developing porosity in weld as reported 6,14,15) in several cases of P-GMA weld. In fact a variation in degree of fluctuation in arc pressure under varying arc stiffness and arc length at different pulse parameters causes porosity in weld through air aspiration in arc environment primarily due to puncture in shielding gas jacket. 1,2) But the paucity of knowledge about the correlations amongst the simultaneously interactive pulse parameters and theier functions governing the behaviour of metal transfer and arc stiffness, in case P-GMAW of different materials under varying gas shielding, poseses major difficulties in right selection of pulse parameters to control all these phenomena. However, the criticality of selection of pulse parameters to control the P-GMAW process for desired characteristics can be successfully addressed by considering the factor f [(I b /I p )ft b ] as stated earlier in Part 1 of this presentation. The dimensionless factor f is defined as a hypothetically derived summarised influence of pulse parameters based on energy balance concept of the system under P-GMAW process. The role of f in realising the effect of mean current (I m ), base current (I b ), pulse current (I p ), pulse time (t p ) and pulse frequency (f ) on various weld characteristics has been amply justified in earlier works. 3,4,16) The factor f as a function of the basic parameters governing the P-GMAW process, as described earlier in presentation of Part 1 of this work, it is imperative to basically understand the influence of f on the behavior of metal transfer and arc stiffness in order to choose proper pulse parameters of desired f to control the weld quality. In view of this effect of f at various pulse parameters and the arc voltage on the behaviour of metal transfer and arc stiffness in P-GMAW process has been studied during weld bead on plate deposition of mild steel filler wire under Ar 2%CO 2 and Ar 18%CO 2 gas shielding. The studies have been carried out by analyzing the variation in physical ISIJ

2 characteristics of metal transfer and arc viewed through high speed video-graphy. The study provides a basic understanding of the mechanism of metal transfer and variation in arc stiffness with respect to a change in pulse parameters and arc voltage in P-GMAW process. The knowledge may help in wider application of P-GMAW process in shop floor practices with more ease and confidence under manual and automatic operations. Table 1. Under different shielding gases the pulse parameters of different f measured during welding. 2. Experimentation and Analysis 2.1. Welding The bead on plate weld deposition was carried out by employing 1.2 mm diameter mild steel filler wire of specification AWS/SFA 5.18E-70S-6 under Ar 2%CO 2 and Ar 18%CO 2 gas shielding at a flow rate of 18 L/min, where the distance between the nozzle to work piece was maintained at 15 mm. The typical values of measured arc voltage and mean current (I m ) at wide variation of pulse parameters giving different f under different shielding environment considered in this work are given in Table 1. In order to compare the behaviour of metal transfer under different shielding environments the studies were carried out by keeping the welding parameters practically similar with respect to the wire feed speed, mean current, pulse parameter and arc voltage. The pulse characteristics, such as I p, I b, t p and f were measured with the help of a transient recorder (maximum resolution of 1 MHz) fitted with the electrical circuit of the welding set up. The arc voltage (V) and the I m were estimated as mean values of the voltage and current plots respectively of the pulse behaviour captured by the transient recorder as typically shown in Figs. 1(a) and 1(b) respectively Behaviour of Metal Transfer The behaviour of metal transfer was studied by droplet diameter (D) and detachment velocity (V i ) from the electrode tip to weld pool as schematically shown in Fig. 2. The variation in behaviour of metal transfer with the change in pulse parameters under different gas shielding have been studied by high speed video graphy with the help of a camera operated at a speed 10 4 frames per second. The camera was placed on a rigid fixture in front of the arc along the line of welding and viewed through a lens filter grade-5 with normal shield glass of welding mask. The measurement was carried out in reference to the known diameters of the filler wire appeared in the photographs. The droplets occasionally revealed in the background of the glair of arc during their transfer to the weld pool at different welding parameters. In most of the cases it becomes visible especially during transition of peak current to the base current showing diminishing brightness of the arc. However, just as a matter of chance in some cases the droplets were also visible on certain consecutive frames of video-graphs from the point of initiation of their travel towards weld pool after detachment from the electrode tip. Often the droplets could not be visible during their entire path of travel primarily due to arc glair and excessive glow of molten metal resulting from high heat intensity. Thus, only in some cases the droplet velocity (V i ) on the way from the electrode tip to weld pool could be estimated by measuring the distance of Fig. 1. Fig. 2. The variation in (a) current and (b) arc voltage recorded during P-GMAW. The schematic diagram showing measurement of droplet diameter (D), arc root diameter (D ) and radius of electrode tip (r) ISIJ 262

3 shift in position of a droplet on consecutive frames of video-graph in reference to the given speed of video-graphy. Although the studies on behaviour of metal transfer by video-graphy is largely jeopardized by the glair of the arc but, in certain cases it revealed on the video-graphs taken at various stages of welding parameters. Accordingly the droplet diameter (D) and detachment velocity (V i ) was suitably measured. The reliability of the measured D and V i has been verified by comparing them with their theoretical values estimated at the same welding parameters with the help of the expressions reported earlier 1,2,5) as stated below. V i (2g/r d r) 1/2 [ q 1.226q ] 1/2...(1) D 4r/(1 3q/16)...(2) q m o I 2 p /(gp 2 r)...(3) The expressions have been evaluated with the help of the experimentally measured (Fig. 2) values of effective radius (r) of tapering of electrode as mm. The effective radius defined as the radius at which the liquid metal droplet is detaching has been measured from the high speed video-graph of metal transfer. The coefficient of surface tension (g), density of molten filler metal and the permeability of free space (m 0 ) is considered as 1.2 N m 1, 7.85 g cm 3 and 4p 10 7 NA 2 respectively ) Fig. 3. Table 2. Table 3. Schematic diagram of arc column showing its components considered for estimation of arc pressure. Under Ar 2%CO 2 gas shielding and arc voltage of 24 1V the estimated and measured diameter and detachment velocity of droplet at different f and I m. Comparison between estimated and measured diameter of droplet at different f and I m under Ar 18%CO 2 gas shielding Estimation of Arc Stiffness The electrode tapering and behaviour of metal transfer significantly affect the arc stiffness, 2,21) which as a function of welding parameters plays an important role to govern the arc deflection from its central axis. The arc deflection adversely affects the energy concentration in weld. Thus, it is imperative to avoid the arc deflection by appropriate control of arc stiffness, which is generally considered as a direct function of arc pressure dictated by plasma stream. The arc pressure (P a ) can be estimated 17,18) with the help of an equation derived from total pressure distribution at the perturbed boundary of solid liquid interface by assuming an arc of root radius ( a ) having a concentric hollow conducting fluid cylinder of radius equal to the radius of molten drop () as schematically shown in Fig. 3. The a is defined as the radius of root area of the arc over which current is transferred from electrode to arc. The P a is expressed as follows µ 0J a 2 Pa a 2 2 [ 2ε0cos( ωt) cos( kz)]...(4) 4 Where J a I p /p 2 a is the arc current density during pulse on period, m 0 is the permeability of free space (4p 10 7 NA 2 ), w is angular frequency, k is a wave number and e 0 is the amplitude of the perturbation parameter. The expression (4) can be resolved 17) as follows with the help of the expression of pressure (P 1 ) due to surface tension attributed to the cylindrical radius ( 1 ) at the perturbed boundary as proposed earlier. 0 P...(5) 1 γ γ 1 ε cos( ωt) cos( kz) 1 2 ε0 cos( ωt) cos( kz)...(6) 1 Considering the Eqs. (5) and (6) the expressions for estimation of arc pressure P a in P-GMAW is finally derived as follows. µ IP P...(7) a a 4 a 3 2 4π 1 Where, the and 1 are assumed as the size of droplet radius (D/2) and effective radius (r) of tapering of electrode respectively (Table 2 and Table 3)). As the arc pressure is directly related to the arc blow, the occurrence of such behaviour of arc may cause variation in heat distribution and related thermal characteristics of weld joint. The observed deviation of arc from the electrode axis (appeared to be arc blow) was also measured by a computerized scaling technique as schematically shown in Fig ISIJ

4 3. esults and Discussion The behavior of metal transfer and arc stiffness under different composition of gas shielding were studied with respect to the factor f, mean current and arc voltage. Under the Ar 2%CO 2 gas shielding the photographs were analysed by classifying the I m into four different ranges of about 160 2, 176 1, and 232 2A at the arc voltage of the order of 24 V. However, in case of Ar 18%CO 2 gas shielding they were analysed into two levels of I m of about and 229 2A at the arc voltages of the order of 24, 30 and 33 V Behaviour of Metal Transfer The composition of shielding gas due to its influence on thermal efficiency significantly affects the burn-off rate 22,23) and thus may consequently manipulate the behaviour of metal transfer of P-GMAW process. In consideration to this phenomenon the influence of pulse parameters on the diameter and velocity of droplets transferred from the mild steel filler wire used under the commonly used shielding gases of Ar 2%CO 2 and Ar 18%CO 2 have been studied. Under different shielding gases of Ar 2%CO 2 and Ar 18%CO 2 the variation in estimated and measured diameter of droplet transferred at pulse on period with a change in P-GMA welding parameters has been shown in Tables 2 and 3 respectively Under Ar 2%CO 2 Gas Shielding Under Ar 2%CO 2 gas shielding the nature of transfer of droplet to the weld pool after its detachment from the electrode as captured in consecutive frames of video-graphy during welding at different I m and f respectively of 176 A and 0.11 and 205 A and 0.23 has been shown in Figs. 5(a) and 5(b) respectively. Such video-graphs have facilitated the measurement of droplet diameter (D) and its detachment velocity (V i ) from the electrode. The comparative data given in Table 2 depicts that in spite of inherited heterogeneity of welding process and considerable difficulties in proper revealing of droplet size due to intense glare, the measured D and V i are mostly found well in agreement to their theoretically estimated values with an average difference of 7.52% and 3.26% respectively. In pulsed current GMAW the D and V i primarily depends upon I p due to its significant influence on the mechanism of droplet detachment from the electrode. 1,2) Thus, at a given arc voltages of 24 1V the influence of I p on the measured D and V i under different I m and f lying in the range of A and respectively has been shown in Figs. 6(a) and 6(b) respectively. The figure shows that in spite of a relatively wide variation of I m and f the D and V i of metal transfer in P-GMAW process almost linearly enhances with the increase of I p. Irrespective of the variation of other pulse parameters the predomination I p on the D and V i of metal transfer is also in agreement to the earlier observations on aluminium and stainless steel. 1,2) However, in agreement to the earlier observations 1,2) here also it mostly appears (Table 2) that at a given I m and arc voltage the increase of f reduces the D but enhances V i which may have primarily attributed to the increase of I p as observed in Table 1. Unfortunately the D and V i could not be measured at every welding parameters as it did not reveal due to intense glare of arc environment, which should be studied further by using more appropriate lens filter. The empirical correlation of D and V i with the I p has been worked out as follows with a high coefficient of correlation of D I p...(8) V i I p...(9) Fig. 4. The schematic diagram of measuring arc deflection from electrode axis Under Ar 18%CO 2 Gas Shielding Under Ar 18%CO 2 gas shielding the influence of I m, f and arc voltage on nature of transfer of droplet after its detachment from the electrode during pulse on period (t p ) as observed in high speed video-graphy has been typically shown in Figs. 7(a) 7(e). The figure primarily reveals the comparative effect of f and arc voltage at different I m on diameter of droplet (D) while other parameters are kept constant. The estimated and measured droplet diameters at Fig. 5. Under Ar 2%CO 2 gas shielding and given arc voltage of 24 1V the typical behaviour of droplet transfer from the electrode tip to the weld pool at different I m and f respectively of (a) 176 A and 0.11 and (b) 205 A and ISIJ 264

5 Fig. 6. Under Ar 2%CO 2 gas shielding and a given arc voltage of 24 1V the effect of I p on (a) droplet diameter (D) and (b) velocity of the droplet at the time of detachment (V i ) when the I m and f widely vary in the range of A and respectively. lower and higher mean currents and 229 2A respectively and f of 0.07 and 0.35 respectively are given in Table 3. The table depicts that at a given arc voltage of 24 1V the measured values of D at different parameters are well in agreement to their theoretically estimated values with an average difference of 7.1%. At a given arc voltage and I m of 24 1V and 229 2A respectively the effect of f on the droplet diameter (D) has been shown in Fig. 8. The figure shows that as it is observed in case of the Ar 2%CO 2 gas shielding (Table 2) here also the D decreases significantly almost linearly with the increase of f following the empirical correlations having a coefficient of correlation of as given below. D f...(10) At relatively lower and higher values of f and I m respectively as of 0.19, A and 0.35, 229 2A respectively the effect of arc voltage on droplet diameter has been shown in Fig. 9. The figure shows that D reduces significantly with the increase of arc voltage. However, at a given arc voltage the increase of I m and f relatively reduces the D. In spite of insufficient data point due to difficulties in experimentation as stated earlier the empirical correlation of D with the arc voltage at different I m and f has been tentatively estimated as follows. D 179 A V...(11) D 229 A V...(12) 3.2. Arc Stiffness The performance of P-GMAW process is primarily characterised by its pulse current where the back ground current Fig. 7. Under Ar 18%CO 2 gas shielding typical appearance of droplet after detachment from the electrode tip at different I m, f and arc voltage respectively of (a) 179 A, 0.19 and 25 V, (b) 180 A, 0.19 and 33 V, (c) 232 A, 0.10 and 25 V, (d) 227 A, 0.35 and 24 V and (e) 227 A, 0.35 and 33 V. Fig. 8. Under Ar 18%CO 2 gas shielding at a given arc voltage and I m of 24 1 and 229 2A respectively the effect of f on droplet diameter (D) during detachment from electrode in pulse on period. Fig. 9. Effect of arc voltage on droplet diameter under Ar 18%CO 2 gas shielding at a given combination of relatively lower and higher values of f and I m respectively as of 0.19, A and 0.35, 229 2A respectively ISIJ

6 Fig. 10. Under Ar 2%CO 2 gas shielding at different I m the effect of f on arc pressure (P a ) during pulse on period at a given arc voltage of 24 1V. maintains the continuity of the process. Thus, the arc stiffness of P-GMAW process largely understood by the arc pressure P a may be primarily considered as a function of I p and the geometry of effective part of the arc along its vertical axis corroborating the metal transfer as stated in Eq. (7). The P a of P-GMAW estimated by using the Eq. (7) has been found in close approximation to the arc pressure of gas metal arc welding as reported by earlier workers. 2,21) However, the arc pressure of P-GMAW has been found to vary as a function of pulse parameters under different gas shielding as stated below Under Ar 2%CO 2 Gas Shielding Under Ar 2%CO 2 gas shielding at a given arc voltage of the order of 24 1V the influence of f on arc pressure (P a ) has been shown in Fig. 10, where the I m is varied in a range of the order of A. The figure shows that at any mean current the increase of f up to about 0.15 enhances the arc pressure (P a ) but, a further increase of f to 0.23 marginally reduces the same. However, at a given f the increase of I m relatively enhances the P a. At different I m the empirical correlation of P a with the f has been worked out as follows. P a(160 A) f f 2...(13) P a(175 A) f f 2...(14) P a(200 A) f f 2...(15) Fig. 11. Under Ar 2%CO 2 gas shielding at a given arc voltage of 24 1V the effect of f on arc deflection during (a) pulse on period at different I m and (b) pulse off period at different I b. P a(230 A) f f 2...(16) In view of the arc pressure governing the arc blow which is measured in terms of arc deflection (A 0 D ) the effect of f on it during pulse on period (t p ) at different I m ( A) and pulse off period (t b ) at different I b ( A) under Ar 2%CO 2 gas shielding at a given arc voltage of 24 1V has been shown in Figs. 11(a) and 11(b) respectively. The Fig. 11(a) shows that at a given I m the increase of f and at a given f the increase of I m almost linearly reduces the arc deflection significantly and the effect of I m on deflection is comparatively more at lower f. This behaviour which is practically independent of I p varying in the range of A (Table 1) can be expressed by an empirical expression as stated below having coefficient of correlation as A 0 D 86.68f 0.099I m 0.31I mf (17) However, the arc deflection has been found comparatively stronger in pulse off period (Fig. 11(b)) where a significant dimension of arc exists which is in agreement to the observations on stainless steel as reported earlier. 2) Here also irrespective of the extent of I b ( A) the arc deflection significantly reduces almost linearly with the increase of f by following an empirical correlation as follows. A 0 D f...(18) Under Ar 18%CO 2 Gas Shielding Under Ar 18%CO 2 gas shielding during pulse on period at a given arc voltage and I m of 24 1V and 229 2A respectively the effect of f on arc pressure (P a ) has been shown in Fig. 12. The figure shows that initially the arc pressure (P a ) reduces relatively at a sharper rate with the increase of f up to certain extent followed by at a lower rate with a further increase of f to about In order to find out the point of criticality of f influencing the rate of reduction of P a with the increase f, two best fit linear analyses of variation of P a as a function of f have been made at its lower and upper ranges in reference to about 0.1 (Fig. 12) as given below ISIJ 266

7 Fig. 12. P a at f 0.1 P a at f 0.1 Under Ar 18%CO 2 gas shielding the effect of f on arc pressure (P a ) during pulse on period at a given arc voltage and I m of 24 1V and 229 2A respectively. P a f...(19) P a f...(20) Fig. 13. Under Ar 18%CO 2 gas shielding the effect of arc voltage on arc pressure during pulse on period at a given combination of relatively lower and higher f and I m of (0.19, A) and (0.35, A) respectively. Considering the linear correlations of the variation of P a with f of two different orders of sensitivity as depicted by the change in gradient of the expressions at different ranges of f, the criticality of it has been analyzed by taking into account the point of intersection of the best fit lines of the lower and upper ranges of f. It shows that the f of the order of plays a critical role to control the P a. Thus it may be inferred that at comparatively lower and higher values of f than about 0.1 the P a enhances at higher and lower rates respectively. This is in contrast to the earlier observation (Fig. 10) of Ar 2%CO 2 gas shielding which shows that at a similar order of mean current of 232 2A the increase of f up to about 0.1 enhances the arc pressure (P a ) to a peak to follow a declination in it with a further increase of f up to about But here it also confirms the occurrence of some criticality of f at about 0.1 to affect the arc characteristics in different manner which is also in agreement to the observations discussed in earlier part 24) of this work. However, the magnitude of P a under Ar 18%CO 2 gas shielding is considerably lower than that observed earlier (Fig. 10) as depicted at the similar range of I m of 232 2A. The arc pressure (P a ) is estimated on the basis of arc root radius, tapering of electrode and radius of transferring droplet as explained earlier. At a given arc voltage and a close range of mean current of 24 1V and about 230 A respectively the D enhances significantly with the increase of f 24) under both the gas shielding of Ar 2%CO 2 and Ar 18%CO 2. But, the magnitude of D under Ar 18%CO 2 gas shielding observed as about 3.7 to 4.4 mm is much higher than that of Ar 2%CO 2 gas shielding noted as about 0.8 to 1.4 mm 24) which, may have adversely affected the arc pressure of the former one. It indicates that a high value of arc root diameter enhances the radial diffusion of ion and anode drop which reduces the efficiency of ionization 18,20) and the arc pressure. At relatively low and high values of combination of f and I m of (0.19 and A) and (0.35 and A) respectively the effect of arc voltage on arc pressure (P a ) during pulse on period Fig. 14. Under Ar 18%CO 2 gas shielding the effect of f on arc deflection during pulse on period at a given arc voltage and I m of 24 1V and 229 2A. has been shown in Fig. 13. The figure shows that the increase of arc voltage enhances the arc pressure but it is more significant in case of higher values of f and I m. However, at lower arc voltage of 24 V the P a of lower f and I m has been found comparatively more than that of higher f and I m, whereas at higher arc voltage the situation becomes radically opposite. At different I m and f the empirical correlation of P a with the arc voltage has been worked out as follows. P a(229 A) V...(21) P a(179 A) V...(22) At a given arc voltage and I m of 24 1V and 229 2A respectively the effect of f on arc deflection (A 0 D ) has been shown in Fig. 14. Similar to that discussed about variation of P a with f in Fig. 12 here also the Fig. 14 shows the presence of some criticality of f in governing the A 0 D. It appears that the arc deflection reduces almost linearly but at different rates with the reduction of f at its upper and lower ranges in reference to the value of about 0.1 as depicted in the correlations given below. A 0 D at f 0.1 A 0 D f...(23) ISIJ

8 Fig. 15. Under Ar 18%CO 2 gas shielding the effect of arc voltage on arc deflection during pulse on period at a given combination of relatively lower and higher f and I m of (0.19, A) and (0.35, A) respectively. A 0 D at f 0.1 A 0 D f...(24) As it was analyzed earlier the point of intersection of the two linear best fit lines noted as the critical value of f to consider in controlling of A 0 D has been evaluated as This is in contrast to the earlier observation (Fig. 11(a)) of Ar 2%CO 2 gas shielding which shows that at a given mean current of 232 2A the reduction of f from 0.23 to 0.07 enhances the arc deflection. Although, it has not been duly considered there due to scattering of data point but, a tendency of similar criticality showing a relatively sharp change in trend of variation in droplet diameter with a decrease of f below about 0.1 can also be apparently marked in Fig. 8. However, the critical aspect of the influence of f at a value around 0.1 on various characteristics of arc and weld in P-GMA welding should be studied further in detail to establish it more precisely. At a given f and I m the increase of arc voltage also reduces the arc deflection (A 0 D ) of pulse on period significantly as shown in Fig. 15. However, the figure also shows that at a given arc voltage keeping a relatively higher combination of f and I m of 0.35 and 229 2A respectively reduces the arc deflection significantly than that observed in case of comparatively lower combination of f and I m as 0.19 and A respectively. This is largely in agreement to the observations of Fig. 13, which corroborates that the increase of arc pressure reduces the arc deflection. At different I m and f the empirical correlation of A 0 D with the arc voltage has been estimated as follows. A 0 D(229 A) V...(25) A 0 D(179 A) V...(26) 4. Conclusions The use of summarized influence of pulse parameter given by dimensionless factor f can be a potential technique for the control of behaviour of metal transfer and arcing in P-GMAW process. The study gives basic understanding of the effect of f at various pulse parameters including the arc voltage on these aspects under different gas shielding during bead on plate weld deposition of mild steel. The major understanding of the process can be concluded as follows. (1) Under Ar 2%CO 2 and Ar 18%CO 2 gas shielding the average droplet diameter transferred per pulse and its velocity of transfer at the time of detachment predominantly reduces and enhances respectively with the increase of I p dictated by any pulse parameters as I m and f. The measured values of diameter and velocity of droplet are well in agreement to their corresponding theoretically estimated values. (2) Under Ar 2%CO 2 gas shielding at any mean current the increase of f up to about 0.15 enhances the arc pressure (P a ) but, a further increase of f to 0.23 marginally reduces the same. (3) Under Ar 18%CO 2 gas shielding at relatively low and high values of combination of f and I m respectively of 0.19 and A and 0.35 and 229 2A respectively the increase of arc voltage enhances P a and consequently the arc stiffness. (4) In concurrence to its effect on arc characteristics the factor f appears to influence also the arc pressure at different rate at its relatively lower and higher range in reference to a critical value of about 0.1. (5) The use of Ar 2%CO 2 shielding gas reduces the droplet diameter but enhances the P a more than that observed under Ar 18%CO 2 gas shielding. Acknowledgements The authors thankfully acknowledge the financial support provided by the Alexander von Humboldt Foundation, Bonn to Prof. Dr. P. K. Ghosh to carry out this work in TU Berlin and also the Council of Scientific and Industrial esearch (CSI), India to support K. Devakumaran by research associateship during analysis of the work. Nomenclature I b : Base current, A I p : Peak current, A t b : Base current duration, s t p : Peak current duration, s I m : Mean current, A f : Summarized influence of pulse parameters L : Arc length, mm D : Droplet diameter, mm V i : Droplet velocity at the time of detachment from the electrode tip, m s 1 r : Effective radius of tapering of electrode g : The coefficient of surface tension, 1.2 N m 1 r d : Density of molten filler metal, 7.85 g cm 3 m 0 : Permeability of free space, 4p 10 7 NA 2 P a : Arc pressure, Pa : adius of the molten metal, mm a : Arc root radius, mm J a : Arc current density during pulse on period w : Angular frequency k : Wave number e 0 : Amplitude of the perturbation parameter P 1 : Pressure due to surface tension, Pa 1 : Cylindrical radius, mm A 0 D : Arc deflection, mm 2009 ISIJ 268

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