Effect of process parameters of pulsed current tungsten inert gas welding on weld pool geometry of titanium welds

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1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.23 No. pp3-320 August 20 Effect of process parameters of pulsed current tungsten inert gas welding on weld pool geometry of titanium welds M. Balasubramanian Department of Mechanical Engineering, Velammal Engineering College, Surapet, Anna University, Guindy, Chennai 00 02, Tamilnadu, India V. Jayabalan Department of Manufacturing Engineering, Anna University, Guindy, Chennai 00 02, Tamilnadu, India V. Balasubramanian Centre for Materials Joining & Research, Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar-0 002, Tamilnadu, India Manuscript received July 200; in revised form April 20 The effects of pulsing current parameters on weld pool geometry namely front height, back height, front width and back width of pulse current gas tungsten arc welded (GTAW) titanium alloy was analysed. Four factors, five levels, central composite design were used to develop empirical relationships, incorporating pulsed current parameters and weld pool geometry. KEY WORDS Gas tungsten arc welding; Pulsed current; Titanium alloy; Design of experiments 1 Introduction Welding thin sheets is quite different from welding thick sections, because during welding of thin sheets many problems are experienced. These problems are usually linked with heat input. Fusion welding generally involves joining of metals by application of heat for melting of metals to be joined. Almost all the conventional arc welding processes offer high heat input, which in turn leads to various problems such as burn-through or melt-through, distortion, porosity, buckling, warping & twisting of welded sheets, grain coarsening, evaporation of useful elements present in coating of the sheets, joint gap variation during welding, fume generation from the coated sheets etc. Use of the proper welding process, procedure and technique is one tool to address this issue [1 ]. GTAW is a good process for joining thin sheets but it suffers with low welding speed and high equipment cost. Pulsed current gas tungsten arc (PCGTA) welding, developed in the s, is a variation of GTAW process that involves cycling the welding current at a selected regular frequency. The maximum current is selected to give adequate penetration and bead contour, while the minimum is set at a level sufficient to maintain a stable arc [,]. This Corresponding author. Professor; Tel: ; Fax: address: manianmb@rediffmail.com (M. Balasubramanian)

2 313 permits arc energy to be used efficiently to fuse a spot of controlled dimensions in a short time producing the weld as a series of overlapping nuggets. By contrast, in constant current welding, the heat required to melt the base material is supplied only during the peak current pulses allowing the heat to dissipate into the base material leading to a narrower heat affected zone (HAZ) []. Advantages include improved bead contours, greater tolerance to heat sink variations, lower heat input requirements, reduced residual stresses and distortion, refinement of fusion zone microstructure, and reduced width of HAZ. There are four independent parameters that influence the process are peak current, background current, pulse frequency, and pulse on time [,]. Experimental results have shown that the front height, front width, back height and back width of the weld pool in the GTA welding of stainless steel are greatly improved by using this approach [,]. Earlier AC was used to weld aluminium and magnesium alloy only. But recent study revealed that, pulsed AC TIG can be used for welding of titanium alloy to greater effect. Grain refinement in titanium alloy was stronger in AC PCTIG than DC welding. However, no thorough study has been reported so far on pulsed current GTA welding of titanium alloys and hence the present investigation was carried out. 2 Experimental Set-up 2.1 Materials and methods The titanium alloy (Ti-Al-V) sheet of 1. mm thick with yield strength (at 0.2% offset) of MPa, ultimate tensile strength of MPa, and elongation of % was single pass autogenously welded. High purity argon gas (.%) was used as a shielding gas and as trailing gas right after welding to prevent absorption of oxygen and nitrogen from the atmosphere. Since the sheet was very thin, no distortion was found and hence backing plate was not necessary. There was also no need of using purging gas because of very thin sheet. From the literature four important factors of pulsed current GTA welding as presented in Table 1 were chosen. The joints were established with 2 mm tungsten electrode under the welding conditions presented in Table 2. A large number of trial runs were carried out using 1. mm thick mill annealed sheets of titanium (Ti-Al-V) alloy to find out the feasible working limits of pulsed current GTAW process parameters. Due to wide range of factors, it was decided to use four factors, five levels, rotatable central composite design matrix to perform a number of experiments for investigation. 2.2 Measurement of weld bead geometry Three metallographic weld bead samples were cut from each joint, with the first sample being located at 1 mm behind the trailing edge of the crater at the end of the weld. The transverse face of the samples were surface-ground using 0 grit size belt with the help of a belt grinder, polished using grade 1/0 (2 mesh size), grade 2/0 (2 mesh size), and Table 1 Important factors and their levels Series. Factor Notation Unit Levels Peak current p A Base current b A Pulse frequency f Hz 0 3 Pulse-on-Time t s 3% 0% % 0% %

3 31 grade 3/0 (1 mesh size) sandpaper. The specimens were further polished by using aluminum oxide initially and then by utilizing diamond paste and velvet cloth in a polishing machine. The polished specimens after cleaning with alcohol were macro-etched by using kroll s solution to reveal the geometry of the weld bead (Fig.2). Several critical parameters, such as bead height and bead width (Fig.1) were measured. The bead geometry was measured with toolmakers microscope. The front height varied between 0.03 and 0.2 mm, back height varied between 0.01 and mm, front width varied between.0 and.2 mm and back width varied between 3. and. mm. The experimental design matrix and the mathematical modeling of the corresponding results were already discussed elsewhere []. Table 2 Welding conditions Power source Lincoln, USA Polarity AC Arc voltage 22 V Electrode W+2% Thoriated (alloy) Electrode diameter 2 mm Shielding gas Argon Gas flow rate L/min Torch position Vertical Operation Automatic Welding speed 300 mm/min Fig.1 Typical weld pool geometry. Fig.2 Macrographs of weld pool. 3 Empirical Relationships The response function representing any of the weld pool dimensions is expressed as Y =f(p, b, f, t) [,]

4 31 The second order polynomial (regression) equation used to represent the response Y is given by k k k Y = b 0 + b i x i + b ii x 2 i + b ij x i x j (1) i=1 i=1 i j i,j=1 and for four factors, the selected polynomial could be expressed as Y = b 0 + b 1 I P + b 2 I B + b 3 F + b t + b I 2 P + b 22 I 2 B + b 33 F 2 + b t 2 +b I P I B + b 13 I P F + b 1 I P t + b 23 I B F + b 2 I B t + b 3 F t (2) where b 0 is the average of responses and b 1, b 2,, b 3 are the co-efficients that depend on respective main and interaction effects of the parameters. The value of the co-efficients has been calculated using the expressions []. All the co-efficients were tested for their significance at 0% confidence level applying student s t-test using the SPSS statistical software package. After determining the significant co-efficients, the final relationships were developed []. Front height (F h ) F h = { p + 3b 0.01f + 1t + p b 2 +f 2 + t 2 + 1ft 2pb pt + 1bf + 3bt}mm (3) Back height (B h ) B h = {3 p + b 32f + t Front width (F w ) +0p 2 + b f 2 + 0t 2 + 1bf}mm () F w = { p + 0.1b 0.01f t p b 2 Back width (B w ) +0.13f t 2 0.1ft + 0.1pb ft}mm () B w = {.3 0.3p + 0.1b t p f 2 Results and Discussion +0.3t pb + 0.2pt + 0.bf}mm () The empirical relationships developed above can be employed to predict the geometry of weld bead and shape relationships for the range of parameters used in the investigation by substituting their respective values in coded form. Based on these models, the main and the interaction effects of the process parameters on the bead geometry were computed and plotted as depicted in Figs.3a 3f. The results show the general trends between cause and effect.

5 (a) Front height Back height Base current / A..2 (b). Front width.0 Back width Base current / A (c) Front height Back height (d) Front width Back width (e) Front height Back height Peak current / A Back hieght / mm (f) Front width Back width Peak current / A Fig.3 Effect of pulsing current parameters on weld pool geometry: (a) effect of base current on height; (b) effect of base current on width; (c) effect of pulse frequency on height; (d) effect of pulse frequency on width; (e) effect of peak current on height; (f) effect of peak current on width..1 Direct effects of process variables on the bead geometry In contrast to constant current welding, in pulsed current gas tungsten arc welding, heat energy is supplied only during peak current pulses, allowing it to dissipate into the

6 31 base metal during the background current and thus lowering heat build up in the adjacent base material, thus leading to a narrower heat affected zone. From Fig.3a and 3b it is apparent that the back height decreases up to 3 A of base current and increases thereafter. The front height behaved constant up to 2 A and then increased thereafter. With respect to bead width, there is a slight dip in the front width up to 30 A and then increases as the base current increases. Back width gradually increases from the lower level (20 A) to higher level (0 A) of base current. From Fig.3c and 3d it is apparent that the back height decreases up to Hz of pulse frequency and increases thereafter. Same trend was observed in the case of front height too. With respect to bead width, there is a sharp dip in the front width up to Hz and then increases as the pulse frequency increases. Similar trend was observed in the back width also. This may be due to difference of heat input caused by variation in pulse frequency. From 0 to Hz of pulse frequency the interval between pulses is high and hence the heat input which enters the system at a moment increases and bead width and height is high, but when the frequency increases the heat input decreases and hence the width and height decreases (Fig.). Similar effect was observed in the case of pulse-on-time also. The bead height and width displayed two different trends. It followed a decreasing trend up to % of pulsing time and then thereafter it was found to be increasing as seen in Fig (a) Peak current 0 A Peak current 0 A (b) Peak current 0 A Peak current 0 A (c) Peak current 0 A Peak current 0 A (d) Peak current 0 A Peak current 0 A Fig. Interaction effect of pulsing current parameters on weld pool geometry: (a) pulse frequency and peak current vs back height; (b) pulse frequency and peak current vs front height; (c) pulse frequency and peak current vs back width; (d) pulse frequency and peak current vs front width.

7 (a) Front height 0. Back height (b) Back width Front width (c) Peak current 0 A Peak current 0 A (d) Peak current 0 A Peak current 0 A (e) Peak current 0 A Peak current 0 A (f) Peak current 0 A Peak current 0 A Fig. Effect of pulsing current parameters on weld pool geometry: (a) pulse-on-time vs front height; (b) pulse-on-time vs front width; (c) peak current and pulse-on-time vs back width; (d) peak current and pulse-on-time vs front width; (e) peak current and pulse-on-time vs back height; (f) peak current and pulse-on-time vs front height.

8 31.2 Interaction effects of process variables on the bead geometry From Fig.a to Fig.d it is apparent that the bead height and bead width decreases up to Hz of pulse frequency and increases thereafter. This may be due to difference of heat input caused by variation in pulse frequency. From 0 to Hz of pulse frequency the interval between pulses is high and hence the heat input which enters the system at a moment increases and bead width and height is high, but when the frequency increases the heat input decreases and hence the width and height decreases. After certain level of pulse frequency say Hz in this case, the pulse frequency is increasing and seems to be like a continuous current which causes more heat input and hence the width and height increases. The same trend is observed in the case of peak current also. Initially as the peak current is less the bead width and height is more and as the peak current increases, the height and width reduced up to say 0 A and then it increased for different values of pulse frequency. From Fig. it is evident that there exist an interaction effect between the peak current and pulse-on-time as the peak current increases from 0 A. A pulsing frequency of Hz is found to produce optimum results. At low frequencies, the effect of succeeding pulses on a solidifying bead is only minimal. On the other hand, at very high frequencies the amplitude of the vibrations induced in the weld pool and of the temperature oscillations are considerably reduced. Thus there exists an optimum frequency at which the greatest effect is produced. In the current investigation, the minimum bead width and height has been achieved at the middle level of the process parameters. Conclusions (1) A five level factorial technique was employed for developing empirical relationships for predicting important weld bead dimensions of pulsed current GTA welded titanium alloy. (2) The empirical relationships developed can be employed easily in automated or robotic welding in the form of a program, for obtaining the desired weld bead dimensions. (3) Out of the four process variables considered, pulse frequency had a significant positive effect on most of the important bead parameters. Acknowledgements The authors sincerely acknowledge the help and facilities extended to them by the Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, TamilNadu, India. The authors are grateful to Mr.K. Anbazhagan, Chennai for making necessary arrangements to procure the base metal for investigation. The authors are also grateful to the management of Velammal Educational Trust for rendering their full support. The authors wish to thank Mr.S. Babu, DRDO Project Associate, Annamalai University for rendering helping hand to carry out the statistical analysis. REFERENCES [1] T. Ueyama, H. Tong, S. Harada, R. Passmore and M. Ushhio, Weld J (2) (200) 0. [2] G. Mathers, Welding of Aluminium and Its Alloys (England: Wood Head Publishing Limited, 2002) p..

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