AN EXPERIMENTAL INVESTIGATION ON FIBER LASER WELDING AT CONTROLLED INERT GASATMOSPHERE

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1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India AN EXPERIMENTAL INVESTIGATION ON FIBER LASER WELDING AT CONTROLLED INERT GASATMOSPHERE Yadaiah Nirsanametla 1*, Swarup Bag 1, C. P. Paul 2, L. M. Kukreja 2 1 Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam , India; yadaiah@iitg.ernet.in * ; swarupbag@iitg.ernet.in 2 Laser Material Processing Division, Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh , India; paulcp@rrcat.gov.in, Kukreja@rrcat.gov.in Abstract Fiber laser is a desirable heat source for deep-penetration and high-speed fusion welding process due to its noticeable advantages such as high beam quality and high efficiency over other lasers. During fusion welding process, thermo-chemical reactions may take place among surrounding atmosphere particles and molten weld pool at high temperature gradients. The atmosphere particles such as oxygen, hydrogen and nitrogen may become part of final weld joint that severely affects the weld joint quality and weld metal properties. Therefore, the welding atmosphere and protection of weld pool plays a noticeable role on the quality of the final weld joint. Henceforth, in the present work, fiber laser welding of austenitic stainless steelhave been investigated in two different ambient atmospheres. Firstly, the experiments are conducted in open atmosphere and in argon ambient atmosphere to study the characteristic difference on weld joint with respect to weld bead size and dimensions, and microstructure formation at different welding speeds. The experimental investigation specifies that the weld bead dimensions are higher in case of argon atmosphere as compared to open atmosphere. The microstructures of heat affected zone (HAZ) and fusion zone (FZ) at both atmospheric conditions are analyzed. It is also obvious from the experimental results that the top surface profile is better and weld surface is very clear in case of welds at argon atmosphere as compared to open atmospheric condition. Keywords:Fiber laser, fusion welding, Inert gas atmosphere, Open atmosphere, Metallographic analysis 1 Introduction In recent past, high power density welding processes, like laser welding have been progressively employing in industrial manufacturing with respect to traditional welding processes due to lower dimension and shape distortion of pieces, greater processing speed, high energy density, deep penetration, focalization, and high efficiency. Moreover, recently, fiber lasers have been receiving greater attention because of its advantages like high beam quality which can produce an ultra-high peak power density of MW/mm 2 levels corresponding to a focused electron beam, and is favorable to become one of the desirable heat sources for deep-penetration and high-speed welding. During high power density fusion welding processes there is the possibility of thermo-chemical reaction among welding atmosphere particles and molten weld pool. These inclusions become part of final weld joint and initiate the weld defects such as pinholes. Therefore, most of the fusion welding processes involve protection of weld molten pool from the atmosphere by several means such as shielding gas or flux to avoid the reaction with atmosphere. Various molten pool protection techniques offer different degree of protection from atmosphere. Several shielding methods such as slag, gas, gas and slag, self-protection and vacuum offer different degree of weld molten pool protection from the surrounding atmosphere during various welding processes [Kim et al. (1998) and Kou (2002)]. Sahoo et al. (1988) have studied the influence of the surface active elements such as sulphur and oxygen in blocking vaporization sites on weld pool surface. The presence of oxygen and sulphur in weld metal leads to increase in metal vaporization rate. Ramirez et al. (1994) have investigated the influence of welding process parameters on solidification substructure to cause weld metal porosity. In their investigations the amount of porosity increases with increase of nitrogen gas introduced in argon shielding gas atmosphere. Dong et al. (2003) and Dong et al. (2005) have studied the nitrogen absorption and desorption during CO 2 laser welding of stainless steel in controlled atmosphere of Ar-N 2 mixed gas. Ramazan and Koray (2005) have studied the 875-1

2 AN EXPERIMENTAL INVESTIGATION ON FIBER LASER WELDING AT CONTROLLED INERT GAS ATMOSPHERE influence of the welding atmosphere during gas metal arc welding (GMAW) of low carbon steel. In their study, the authors realized that the toughness of the weld metal is higher and mechanical properties of the weldment are better due to less porosity in controlled atmosphere of argon when compared with the classical GMAW. Bayram et al. (2008) have studied the influence of the welding atmosphere and heat input during resistance spot welding of 316L austenitic stainless steel. In their study, the authors observed that the tensile shear load bearing capacity increases when heat input increases due to the enlargement of the nugget size. This occurrence is slightly higher in nitrogen atmosphere conditions as compared to welding in open atmosphere conditions. Dursun (2008) have investigated the influence of welding atmosphere and welding current during resistance spot welding of 304L austenitic stainless steel. In their study the author determined in a nitrogen atmosphere welding that an optimum weld quality was achieved by using 9 ka peak weld current. Kang et al.(2009) have studied the GMA welding process with various shielding gases and its combinations such as with helium, argon, mixture of argon and helium, and supplying pure argon and pure helium alternatively. In their study, the authors observed that the lowest degree of weld induced distortion is obtained if an alternative supply of shielding gas is used for molten weld pool protection when compared to individual supply and mixture of them with similar welding conditions. Ostsemin (2009) have studied the temperature measurement of the electrode-metal drop during carbon dioxide controlled atmosphere of arc welding. Acritical review of literature survey suggests that the way of protection of molten weld pool and welding atmosphere plays a significant role in the weld metal properties and weld joint quality. Furthermore, the degree of molten weld pool protection can be enriched by avoiding poor methods of shielding media. Argon gas ambient atmosphere or self-protective atmosphere of argon welding process avoids chemical reaction among open atmosphere particles and molten weld pool. This type of self-created inert gas atmosphere leads to less degree of weld defects. Moreover, there is a lack of proper investigations on fiber laser welding in a self-created inert gas of argon. Therefore, in the present work, authors have investigated an influence of welding atmosphere and laser scan speed in fiber laser welding of austenitic stainless steel, SS 304, 5 mm thickness plate.the fiber laser welding experiments are conducted in argon gas ambient atmosphere as well in open atmospheric conditions to study the characteristic difference between them. The weld bead dimensions, weld bead profile, weld zones macrograph, microstructures and top surface appearance has been compared between both atmosphere welds. The weld bead dimensions and microstructure observed in the inert gas argon ambient atmosphere welding is better when compared with an open atmosphere welds with similar welding process variables. 2 Experimental Investigation A 2.0 kw fiber laser based welding system was used to perform bead-on-plate welding on commercially available 304 stainless steel plate of5mm thickness. The welding experimental setup consists of four major subsystems. They are computerized numerically controlled (CNC) workstation in a Glove box, 2 kw ytterbium continuous wave fiber laser source,vacuum pump and CNC controller. The experiments were conducted with similar welding process variables both in controlled and open atmospheric conditions to determine the characteristic difference between them. Tables 1 depicts the welding process variables for both controlled atmosphere of argon and open atmospheric conditions. A mixed mode fiber laser beam power which is the mixture of two fundamental modes, 60% of TEM01, and 40 % of TEM00, which gives a near flat top beam profile [Kumar et al. (2008) and Kumar et al. (2012)], is considered. Table 1Fiber laser welding processvariables Data Laser Velocity Heat input per set power (W) (mm s -1 ) unit length (J/mm) Theworkstation is CNC controlled and positioned stationary during welding process where as the laser head is mounted in Z-direction vertically and movable. Prior to the welding operation the work pieces were cleaned to remove oil, grease, and water vapour using an organic solvent such as acetone. The work piece is kept normal to heat source. The laser welding workstation also equipped with laser power on/off mode and gas on/off mode to preserve the necessities. The fiber laser welding experimental setup characteristics during performance of the welding experiments are specified in Table 2. During experiments in controlled atmosphere of argon welding, the five axes CNC workstation was in a glove box. The glove box is essential for controlling atmospheric conditions during processing. To protect the weldment from the atmosphere, the welding experiments were performed inside a high purity argon gas filled glove box. Air and moisture particle are the main impurities in atmosphere which affects the properties of the weld joint. Henceforth, a controlled atmosphere of argon fiber laser welding 875-2

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India system was also integrated with oxygen and moisture analyzers. The desired purity levels are accomplished by purging high purity grade argon gas. The purity level of the glove box is retained by keeping the differential pressure just above the atmospheric pressure. Table 2 Experimental setup conditions Laser spot diameter 200µm Mode of beam Multi-mode and flat on top surface Laser power mode Continuous wave Wavelength 1080 nm Beam angle 90 degrees Focal point 1 mm below surface to the top Fiber core diameter 50 µm Shielding gas Argon After carrying out the welding experiments in bothatmospheres, the metallographic analysis was done to measure the weld pool shape and dimensions, and microstructure for welded samples of both atmospheres. In the course of metallographic analysis, the welded samples were sectioned perpendicular to welding direction, polished with different grades of polishing papers (220, 400, 600, 800, 1000 and diamond polish) and etched with Villella s reagent. The etched samples were analyzed on optical microscope to take macro and microstructures. The weld bead shape and dimensions are calculated for each welded sample. Top bead profile, appearance of the weld joint and microstructures for various welding speeds under both the atmospheric conditions are also studied. 3 Results and Discussion Bead-on-plate welds were generated on commercially available SS 304, 5 mm samples using a 2 kw ytterbium continuous wave fiber laser source, employing different welding atmospheres, welding speeds and laser power, 2000 W. The experiments are conducted in a range of J/mm to 150 J/mm heat input per unit length. In continuation, the metallographic analysis results such as formation of weld bead and its dimensions and microstructure of the welds for both the welding atmospheres and various process variables reported. Figure 1 Weld macrographs corresponding to data set 1 given in table 1, (a) open atmosphere and (b) controlled atmosphere welding. Figure 1 (a)-(b) describes the measured weld macrograph corresponding to laser power 2000 W and welding speed, mm/s in open and controlled atmosphere of argon respectively. From this figure 1, it can be observed that the weld bead dimensions and aspect ratio is more in case of controlled atmosphere corresponding to similar welding conditions. Moreover, the weld bead dimensions are increaseswhen decrease the welding speed, where laser power is constant. However, the weld bead shape and dimensions of open atmosphere welds and a controlled atmosphere of argon are different. The most noteworthy observation in this experimental investigation is thatt the noticeable variance in the appearance of weld bead surface quality. Figure 2 (a)-(b) represents the top view weld bead appearance comparison corresponding to data set 1 and data set 2 given in table 1, respectively. From this figure 2it is obvious that more neat and clean surface is obtained in controlled atmosphere of argon welding as compared to open atmosphere welds with similar welding process variables. In case of welding in open atmosphere conditions, air particles such as hydrogen, oxygen and nitrogen react with molten weld pool at high temperature gradients. These reactions with atmosphere particles may result in high degree of porosity, rough surfaces and gas pores. However, welding in argon controlled atmosphere, the appearance of weld surface is clean with less porosity and low degree 875-3

4 AN EXPERIMENTAL INVESTIGATION ON FIBER LASER WELDING AT CONTROLLED INERT GAS ATMOSPHERE Open atmosphere Controlled atmosphere (a) (b) Figure 2 Top view appearance comparisons for open and controlled atmosphere welding corresponding to (a) data set 1 and (b) data set 2 given in table 1. of surface roughness, when compared with the open atmosphere weld surface. In open atmosphere welding the air particles may dissolve in molten weld pool and they may try to escape from the weld pool during solidification. This may leads to optical micrographs of heat affected zone (HAZ) and fusion zone (FZ) in open atmosphere weld made using laser power is 2000 W and welding speed is mm/s.controlled Controlled atmosphere of argon welds microstructure (figure 4) show that the Figure 3 Optical microstructure of weldment in open atmospheric condition corresponding to data set 1 given in table 1 at various zones, (a) parent material (PM), (b) heat affected zone (HAZ and (c) fusion zone (FZ). cracking of the weld joint and reduces the weld solidification structure is dendritic and contains joint quality. In case of controlled atmosphere, the austenite and a few percent of delta ferrite at the defects due to the reaction of molten pool with dendritic boundaries. oxygen, hydrogen, and nitrogen present in the Figure 4 (a)-(b) illustrates the HAZ and ambient atmosphere can be minimized or FZ micrographs of weld metal in a controlled nullified.moreover, Moreover, the weld bead dimensions dime are atmosphere of argon corresponding to da data set 1 more for a controlled atmosphere of argon as given in Table 1. It is clearly detected the variation compared to open atmospheric condition with of the microstructure in open and a controlled similar welding conditions when compared with atmosphere of argon from figures 3 (b)-(c) and 4 open atmosphere welds. (a)-(b) with the similar process variables.this Microstructure of the as-receiv received material difference in the microstructure is may be due to is shown in figure 3 (a). In general the transverse the solidification of the weld metal varies for both section of weld sample after fiber laser welding the atmospheres; in addition to this, the chemical shows that the extension of the heat affected zone reactions between the molten weld pool and is very small. However, this is the distinctive surrounding atmosphere can be avoid in a advantageous effect associated to a highly focused controlled atmosphere ere of argon. This elimination of welding heat source [Zambonet al. (2006)]. ( The chemical reaction with atmosphere particle such as microstructure analysis reveals that there are no nitrogen, oxygen and hydrogen lead to minimum minim cracks formed in any one of the specimens porosity or pin-holes holes formation in controlled examined. Even though some isolated pores were atmosphere welds. Figure 4 (c)-(dd) represents the found mostly in open atmosphere weldment rather HAZ and FZ optical micrographss of weld metal than a controlled atmosphere welds. There are no n made using the laser power is 2000 W and welding noticeable inclusions are observed in a controlled speed is mm/s in a controlled atmosphere of atmosphere of argon welds when compared to open argon.it can be found that the trend of the influence atmosphere welds. In both atmosphere welds, the of the laser welding speed in a controlled obvious feature is the highly directional nature of atmosphere is following same trend of open the microstructure around the axis of the fiber laser atmosphere welds. However, the microstructures in beam eam due to the solidification of weld metal at high the both welds are not same. From figure 4 (a) (a)-(d), cooling rate. Figure 3 (b) and (c) describes the it is obviously noticed thatat the higher welding 875-4

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India speed the dendritic structure is finer as compared to lower welding speed structure. Due to fine protection of weld pool in a controlled atmosphere; oxide, nitride, porosity and inclusion were formed minimum level in the weld metal. This may lead to the high efficiency of the formed weld joint in a controlled atmosphere.moreover, the finer dendritic structure observed when the welding speed is relatively higher; since the cooling rate is higher at high welding speeds. Figure 5 describes the quantitative comparison of the weld pool dimensions of open and controlled atmosphere welds corresponding to welding conditions given in Table 1 along with aspect ratio. The controlled atmosphere welding achieved maximum depth of penetration as compared to open atmosphere weld dimensions. This resembles the fact that argon helps to constrict the laser beam and results in more concentrated heat flux. The maximum weld bead dimensions are achieved for the data set 4 given in Table 2. The aspect ratio which is well-defined by the ratio of weld depth of penetration to bead width of a weld joint is a characteristic quantity to specify the effectiveness of the formed weld joint. The maximum aspect ratio, is achieved in controlled atmosphere of argon welding process corresponding to data set 4 given in Table 1. Though, the minimum is 2.02 for data set 1 shown in Table 1. In case of the welding in open atmospheric conditions, the aspect ratio is maximum of 2.8 and minimum is 1.8 for the data set 4 and data set 1 given in table 1, respectively. The aspect ratio achieved in controlled atmosphere of argon welding process is more when compared with the process of welding under open atmospheric conditions for the similar welding conditions given in table 1. Figure 4Microstructure of weldment in controlled atmosphere of argon corresponding to (a) data set 1, HAZ, (b) data set 1, FZ, (c) data set 4, HAZ and (d) data 4, FZ

6 AN EXPERIMENTAL INVESTIGATION ON FIBER LASER WELDING AT CONTROLLED INERT GAS ATMOSPHERE Figure 5 Comparison of weld bead dimensions of open and controlled atmosphere welding corresponding to welding conditions given in table 1. 4 Conclusion This study demonstrates the performance of fiber laser welding in two different atmospheres, namely, argon and open atmospheres. The characteristic difference between two different atmospheres fiber laser welding on SS 304 plate is reported. The results showed that in controlled atmosphere of argon, the weld pool dimensions, depth of penetration and bead width are more as compared to open atmosphere welds. Moreover, the aspect ratio is also more in case of controlled atmosphere of argon. The most significant outcome of this investigation is the top surface profile and top view appearance thatis clean and neatin controlled atmosphere of argon. The full depth of penetration is achieved in controlled atmosphere for welding speed, mm/s and laser power, 2000 W. The metallographic analysis revealed that microstructure of welds in controlled atmosphere has less porosity as compared to open atmosphere welds. This is due to a fine protection of molten weld pool from oxygen and hydrogen etc. atmosphere particles. This may lead to high efficiency of formed weld joint in controlled atmosphere. The microstructure, at higher welding speed, the dendritic structure is finer as compared to relatively lower welding speed structure in both welding atmospheres. From this experimental investigation, it is recommended that the welding in controlled atmosphere of argon is far better than open atmospheric condition. References Bayram, K., Ramazan, K.K., Suleyman, G. and Fatih, H. (2008), An effect of heat input, weld atmosphere and weld cooling conditions on the resistance spot weldability of 316L austenitic stainless steel, Journal of Materials Processing Technology, Vol.195, pp Dong, W., Kokawa, H., Tsukamoto, S. Yutaka, S.S. (2005), Nitrogen desorption by high-nitrogen steel weld metal during Co2 laser welding, Metallurgical and Materials Transactions B, Vol. 36, pp Dong, W., Kokawa, H., Yutaka, S.S. and Tsukamoto, S. (2003), Nitrogen absorption by iron and stainless steels during Co 2 laser welding, Metallurgical and Materials Transactions B, Vol. 34, pp Dursun, O. (2008), An effect of weld current and weld atmosphere on the resistance spot weldability of 304L austenitic stainless steel, Materials and Design, Vol. 29, pp Kang, B.Y., Yarlagadda, K.D.V, Kang, M.J, Kim, H.J. and Kim, I.S. (2009), The effect of alternate supply of shielding gases in austenite stainless steel GTA welding, Journal of Materials Processing Technology, Vol. 209, pp Kim, H.J., Frost, H.R. and Olson, D.L. (1998), Electrochemical oxygen transfer during direct current arc welding, Welding Journal, pp Kou, S. (2002), Welding Metallurgy, 3 rd ed., Willey Inter Science, New York. Kumar, A., Paul, C.P., Pathak, A.K., Bhargava, P. and Kukreja, L.M. (2012), A finer modeling approach for numerically predicting single track geometry in two dimensions during laser rapid manufacturing, Optics and Laser Technology, Vol. 44 (3), pp Kumar, S., Roy, S., Paul, C.P. and Nath, A.K. (2008), Three-dimensional conduction heat transfer model for laser cladding process, Numerical Heat Transfer, Part B: Fundamentals: An International Journal of Computation and Methodology, Vol. 53, pp Ostsemin, A.A. (2009), Estimating the temperature of an electrode-metal drop when welding in a carbon-dioxide atmosphere, Russian Engineering Research, Vol. 29 (7),pp Ramazan, K. and Koray, K. (2005), Effect of controlled atmosphere on the mig-mag arc weldment properties, Materials and Design, Vol. 26, pp Ramirez, J.E., Han, B. and Liu, S. (1994), Effect of welding variables and solidification substructure on weld metal porosity,metallurgical and Materials Transactions A, Vol.25, pp Sahoo, P., Collur, M.M. and DebRoy, T. (1988), Effects of oxygen and sulfur on alloying element vaporization rates during laser welding, Metallurgical Transactions B, Vol. 19, pp Zambon, A., Ferro, P. and Bonollo, F. (2006), Microstructural, compositional and residual stress evaluation of CO 2 laser welded superaustenitic AISI 904L stainless steel, Materials Science and Engineering A, Vol. 424, pp