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Development of Activated Flux for Deep Penetration in GTAW
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 4703–4710
www.materialstoday.com/proceedings
ICMPC-2019
Development of Activated Flux for Deep Penetration in GTAW Sanjib Jaypuriaa, Swagatika Khandaib, Trupti Ranjan Mahapatrab*, Amresh Singhc a
b
Mechanical Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur – 721302, India Department of Production Engineering, Veer Surendra Sai University of Technology (VSSUT), Burla – 768018, India c Indian Ordnance Factory Board, Kolkata -700001, India
Abstract Gas metal tungsten (TIG) welding is one of the best suited fabrication methods in industries due to lower capital cost, contamination free and sound quality of weld joints. However, there are few drawbacks like the limited penetration capacity and limited productivity in TIG welding. That is why to improve the productivity attempts are made every now and then through improving the penetration of the TIG welding. In this regard, the Activated Tungsten Inert Gas (ATIG) welding is a unique method, where developed flux combination is pasted onto the surface of the base metal. The flux formulations have been done essentially based on empirical data as well as the practical experience. The in house developed activating flux is applied as a thin coating to the metal surface before welding. It was observed that there was a significant increment in bead penetration and quality through ATIG. Here efforts were made to increase the weld penetration by applying the active flux and to optimize the process parameters. An optimized mixture of MnO2, SiO2 and TiO2 was found to be effective for enhancement of penetration. Content of MnO2 and beam current have significantly contributed for enhancement of penetration and width of bead, respectively. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: TIG; GTAW; Flux; Weld Penetration; ATIG; Marangoni; Surface Tension.
1. Introduction Two or more work pieces are welded through an arc welding method using a non-consumable tungsten electrode in Tungsten Inert Gas Welding (TIG), which is also regarded as the Gas Tungsten Arc Welding (GTAW). To a great extent it is analogous to Metal Inert Gas (MIG) Welding. TIG welding is used for welding a variety of metals and broadly used in all types of manufacturing industry due to good weld surface finish. A variety of materials including Stainless Steel, Aluminum, Copper, Alloy steel, Titanium, Nickel and Magnesium alloys can be welded using TIG
* Corresponding author. Tel.: +91-9861425597; fax: +91-6632430204 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Welding. However, at the same time Conventional Tungsten Inert Gas (TIG) arc welding suffers from the following disadvantages: (a) low productivity; (b) High skill requirement (c) Expensive setup as compared to other arc welding process (d) relative shallow penetration. To obtain main objective of company regards quality and productivity, there are continuous attempts to improve the productivity and penetration of the TIG welding. One of the emerging methods is Active tungsten inert gas (A-TIG). The A-GTAW process uses activating fluxes to increase weld depth [1, 2]. Researchers developed this welding process in the 60s at the Paton Electric Welding Institute (PWI) in Ukraine [3]. Activated flux is a form of flux in powder form which is mixed with acetone to make paint like constituent and then applied to joint with brush to certain thickness to achieve enhanced penetration with reduced heat input in TIG Welding. A-TIG process involves application of thin coating of an activating flux to the surface of the material before welding that can bring out a significant increase in weld bead penetration. The methodologies adopted to improve the penetration of conventional TIG welds by using active fluxes are reviewed and studied from open literature. Tithering of the electric arc originated due to the presence of some components (mostly oxygen and fluorine) of the flux in the arc is the most prominent mechanism for improved penetration. As a result of this effect the penetration of the weld bead increases because of increasing anode current density as well as the arc force acting on the welding pool [4]. It is also reported that the surface tension of the weld pool decrease by the activating fluxes and a deeper invading in the pool is caused by the arc pressure. Subsequently the arc pressure reaches a deeper penetration with the help of this invading [4]. Therefore, Marangoni effect is playing a significant role in enhancing penetration. Some researchers reported that use of MnO2, TiO2, MoO3, and SiO2 fluxes in TIG welding leads to variation of the temperature coefficient of surface tension on the molten pool from a negative to a positive value. This indicates that the surface tension at the pool edge is lower than at the pool center and therefore, centripetal Marangoni convection is introduced by the surface tension gradient in the molten pool. This change in surface tension gradient with temperature from negative to positive provides the desired flow of molten pool to enhance heat input in central region of molten pool [5]. The chemical composition of the weld metals does not undergo any significant change in comparison to that of the base metals due to the use of activated flux. Scholars also observed that during the welding of structural components made of 304LN and 316LN stainless steels, the activated flux developed has greater potential for use. This is because 316 LN A-TIG weld joints showed no degradation in the microstructure and mechanical properties and 75% enhancement of creep rupture life is noticed [6]. Oxide powders (Al2O3, Cr2O3, TiO2, SiO2 and CaO) were applied through a thin layer of the flux and a bead on plate welds was produced. The increase in the penetration is found to be more significant with the use of Cr2O3, TiO2, and SiO2 [6]. The researchers applied, oxide powders (Al2O3, Cr2O3, TiO2, SiO2and CaO) was applied on a type 304 stainless steel through a thin layer of the flux to produce a bead on plate welds. The experimental results indicated that the increase in the penetration is significant with the use of Cr2O3, TiO2, and SiO2 [6]. Some researchers observe that mixture of SiO2TiO2 is used as an activated flux and welding of type 316L stainless steel was investigated. The 80% SiO2+ 20%TiO2 mixture can produce the greatest improvement in TIG penetration [7]. It is observed from the literature that there are few established studies on dependency of flux and bead parameters. It is also observed from the literature that most of the studies are focused on single element of flux and their effect on penetration. Most of reported literature also focused on flux not on activated flux. Therefore, in this study an attempt has been made to study the effect of different activated flux separately in enhancement of bead penetration. This study also focuses on performance comparison of different activated flux and to design different flux combinations for steel. In addition to this, optimized process parameters have been suggested for enhancing bead penetration and minimizing bead width. 2. Experimentation The material chosen in the present analysis for the experimentation purpose were 304L Austenitic stainless steel of size (150mm x 50mm x 6mm) and chemical composition as shown in Table 1. Table 1. Chemical Composition of 304 Stainless Steel (wt %) Type
Carbon
Manganese
Phosphorus
Sulfur
Silicon
Chromium
Nickel
Nitrogen
304
0.07
2
0.045
0.03
0.75
17.5–19.5
8.0–10.5
0.1
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As the texts about activated fluxes available are few, hit and trial method is adopted for analyzing the effect on weld bead shape with single component flux or examining the effect of individual materials, which is already being used as flux component. Here some process parameters were kept constant and it is tabulated in Table 2. Based on design of experiments (DOE), parameters like SiO2, MnO2, TiO2 and MoO2 are varied. Here, process parameter is current which varies from 90 ampere to 150 ampere by keeping electrode diameter and arc gap constant of 3mm and 3.5mm, respectively. First of all, welding was done without flux and then single component flux to observe the effect of different flux components on penetration. After welding, the sample were prepared and molded with Bakelite powder. Then, after polishing, samples were etched with etchant prepared from HCL, glycerol and HNO3 and penetration was analyzed from image analyzer. The obtained result is tabulated in Table 3. The effect of different activated flux on bead parameters is given in Fig. 1. Table 2. Process parameters Process parameter
Description
Process parameter
Description
Current
90 Amps. - 150 Amps.
Welding speed
0.4 m/min
Electrode diameter
3 mm
Shielding gas
Argon
Work piece to electrode gap
3.5 mm
Gas flow rate
10 lit/min
Table 3. Preliminary weld bead parameters Sample Number
Sample Description
Penetration (mm)
Width(mm)
Height(mm)
1
Without flux
0.56
1.85
0.1
2
With TiO2
1.36
3.33
0.13
3
With SiO2
1.36
3.43
0.06
4
With MnO2
1.85
5.34
0.46
5
With MoO2
2.43
5.68
0.38
Fig.1. Effect of different flux constituents on weld bead geometry.
Although MoO2 had positive effect on the penetration, but later it was decided to eliminate MoO2 due to health hazard and formulations were prepared with rest of the materials. Three different levels of weight taken for each constituent and experiments were conducted as per the design matrix using L9 of Taguchi technique (Table 4). The experiments were conducted according to the design matrix at random order to avoid systematic errors infiltrating
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the system. Here, potassium silicate, water as solvent media and acetone is used as binder in the proportion of binder: water: acetone as 0.8-1.2gm: 1gm: 5-10 drops for each gram of activated flux. After making these 9 formulations, the activated flux with binder, water and acetone was made and applied at center of the plate for bead on plate welding of 304stainless steel. During welding the arc stability was the problem that lead to introduce iron powder to increase its conductivity. Iron powder was also kept constant in the ratio of Fe: activated flux as 1:15. After applying the flux to the plate, it was dried in electric oven to remove wetness/moisture at temperature of 120°C for 10-15 minutes. During Experimentation some process parameters were kept constant which was explained in Table 2 expect current, which was varied with three levels (110A, 130A, 150A). After welding all the nine beads were molded and etched in solution of hydrochloric acid, glycerol and nitric acid in the ratio of 3:2:1. The experimental combination and results of weld bead geometry is shown in Table.4. Main effect plot for bead penetration is shown in Fig. 2. Table 4. Flux formulations based on Taguchi L9 with corresponding responses Sl. No.
MnO2(g)
SiO2(g)
TiO2(g)
Current (amp)
Bead Penetration (mm)
Bead Width (mm)
1 2 3 4 5 6 7 8 9
2.5 2.5 2.5 5 5 5 7.5 7.5 7.5
2.5 5 7.5 2.5 5 7.5 2.5 5 7.5
2.5 5 7.5 5 7.5 2.5 7.5 2.5 5
110 130 150 150 110 130 130 150 110
0.73 1.12 0.42 1.77 0.46 0.69 1.13 0.87 1.21
2.47 5.28 3.16 4.73 3.48 3.28 3.23 1.99 4.66
MnO2(g)
1.4
SiO2(g)
1.2 1.0 0.8 0.6 2.5
5.0
7.5
2.5
TiO2(g)
1.4
5.0
7.5
Current (amp)
1.2 1.0 0.8 0.6 2.5
5.0
7.5
110
130
150
Fig. 2. Mean effect plot for bead penetration based on results in Table 4
From the observation and graph, it was established that penetration increases with MnO2 concentration and vice versa in case of SiO2. It was also observed the mixed response in case of TiO2. We also note that welding current has a linear relation with penetration. From the analysis it was decided to apply the same set of experimentation with modified flux formulation and designed flux formulation is shown in Table 5 below. Thus for the experimentation purpose SiO2 was varied between 10-20%, MnO2 as 40-50% and TiO2 between 20-50%. The finally decided levels for the fluxes for the best result are shown in Table 5.
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Table 5. Final flux formulations with their responses Sl. No.
MnO2(g)
SiO2(g)
TiO2(g)
Current (amp)
Bead Penetration (mm)
Bead Width (mm)
1 2 3 4 5 6 7 8 9
8 8 10 10 10 12 8 12 12
2 4 2 3 4 2 3 3 4
10 4 7 4 10 4 7 10 7
110 150 150 110 130 130 130 150 110
2.84 1.88 4.15 4.14 3.06 2.09 3.00 3.54 2.53
5.24 7.20 6.50 7.18 5.92 5.07 5.52 6.59 5.42
3. Results and Discussion The observations along with latest combination of flux mixture taken from experiments are shown in Table 5. The welded samples then moulded manually with araldite (an epoxy based polymer) and the final measured welded specimens from image analyzer are shown in Fig. 3.
Fig. 3. Final measured welded specimen from image analyzer
Penetration in TIG welding is significantly controlled by surface tension gradient over the pool and this surface tension is function of melt temperature. Surface tension is also affected by the surface-active elements, which helps to increase the surface tension gradient and induces thermo-capillary flow deep into the weld and deeper penetration can be achieved [8]. A-TIG is such a welding process where surface tension gradient of melt pool is altered by the active elements and helps to achieve deep penetration without filler wire and any edge preparation. Fig.4 shows the change in convection movement of the activated flux in the weld pool to centripetal from centrifugal during A-TIG welding. This is an indication of the occurrence of centripetal Marangoni convection in the fusion pool by the surface tension gradient. The temperature coefficient of surface tension on the molten pool altered from a negative to a positive value in A-TIG. Consequently, the surface tension at the pool center was higher
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as compared to at the pool edge. Corresponding to this condition the flow of the fluid in the molten pool surface transfers easily towards the center from the pool edge and then downwards [9]. Mn O 2(g)
S i O 2(g)
3 .5
3 .0
2 .5 8
10
12
2
Ti O 2(g)
3
4
C u rre n t
3 .5
3 .0
2 .5 4
Fig. 4. Schematic diagram of Marangoni Convection
7
10
110
130
150
Fig. 5. Mean effect plot for bead penetration based on Table 5 results
The minimum weld depth was obtained in the weld without any fluxes. It could be understood that the normal TIG welding deals with Marangoni effect which states that a lower value of surface tension could be achieved with higher temperature area of fusion zone and vice versa. Therefore, surface tension at the center of weld pool is lower than that of edges and molten metal flows from center to edges in case of conventional TIG welding. In this way, conventional TIG welding yields less penetration and more bead width. However, A-TIG welding yields higher bead penetration because of the used flux, which act as surface-active elements and alters the surface tension gradient. The stable oxide fluxes helps to achieve reversal of Marangoni effect by maximizing surface tension gradient at the center of the weld pool [10, 11]. In addition to this, these fluxes constrict the welding arc and the penetration is more in comparison to normal TIG. In addition to this, the arc is focused at the center of the fusion zone as these mixtures of fluxes constrict the current density at outer radius of the arc column. This enhanced current density at center leads to higher magnetic force and achieves strong convective molten fluid flow downwards in the weld pool. Hence, the weld depth is significantly increased [12]. It is also reported in literature that TiO2 also creates anode spot on weld area, which attracts more electrons from the tungsten electrode and helps to achieve deeper penetration [13].It is seen from Table 5 that maximum penetration (4.15 mm) could be achieved with 10 gram of MnO2, 2 gram of SiO2 and 7 gram of TiO2 with highest level of penetration. It is also seen from the table that MnO2 has highest significance in deciding the penetration. In most of the cases, the weld with deeper penetration has higher value of bead width. It is seen from the mean effect plot (refer Fig. 5) for bead penetration that a mid-level of MnO2, SiO2, TiO2 and low level of current is necessary condition for getting deeper penetration in A-TIG welding.Table 5 indicates that a higher beam current always yields shallow bead profile with high level of bead width. So, current has significant contribution in deciding the bead width and content of MnO2 has second highest contribution. A high level of MnO2, low level of SiO2 and mid-level of TiO2 and current are suggested from mean effect plot of bead width (refer Fig. 6) to achieve minimum bead width during welding. A desirability approach is also selected to optimize the process parameters of A-TIG to get maximum penetration in the joint. It is seen from Fig. 7 that 9.9 gram of MnO2, 2.7gram of SiO2, 6.7 gram of TiO2 and 110 amperage of current is suitable for achieving a penetration of 5.3 mm with desirability of one.In the similar manner, desirability approach is also conducted for composite responses of bead penetration and width, where target is set to maximize and minimize penetration and width respectively. A composite desirability of 0.65 could be achieved (refer Fig. 8) with 11.33 gram of MnO2, 2.0 gram of SiO2, 8.12 gram of TiO2 and 122 amperage of current. The composite desirability also refers to a penetration of 3.3 mm and width of 4.74 mm. It is observed from the discussion that although penetration is significantly increasing with activated fluxes, but bead width is not shown any significant reduction. Therefore, A-TIG has great demand in the cladding industries in addition to the welding application.
S. Jaypuria et al. / Materials Today: Proceedings 18 (2019) 4703–4710
MnO2(g)
4709
S iO2(g)
6.5
6.0
5.5 8
10
12
2
TiO2(g)
3
4
Current
6.5
6.0
5.5 4
7
10
110
130
150
Fig. 6. Mean effect plot for bead width based on Table 5 results
Fig. 7. Optimization plot for bead penetration
4. Conclusion The data plots provided in the paper clearly demonstrates that oxides have a greater influence on the penetration. Increased penetration is also noticed with all the constituents. Moreover width also increased with penetration. Therefore this can be used for the purpose of cladding. Also the reinforcement that we achieved is because of rolled sheets gets relieved after heating. This increase in penetration is due to direction of fluid flow as explained by Heiplerepair theory. A significant increase in penetration was obtained in welds done with a TiO2 activating flux. This effect is mainly due to not only the arc constriction produced by the flux and consequent increase in the arc force but the reversal of Marangoni convection. When TIG welding is performed at a temperature higher at center of weld pool in comparison to edge of weld pool, the surface tension gradient is established between the center and edge of weld pool. This results in generation of the centrifugal Marangoni flow in the molten weld pool. The temperature co-efficient of surface tension changed from negative to positive when TIG welding with SiO2 ,TiO2, MoO2 and MnO2 is performed. This gives rise to greater heat input in central region and consequently, the penetration has been increased.
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MnO2(g) 12.0 [11.3535] 8.0
SiO2(g) 4.0 [2.0] 2.0
TiO2(g) 10.0 [8.1212] 4.0
Current 150.0 [122.1212] 110.0
Composite Desirability 0.65336
P Maximum y = 3.3144 d = 0.67692
B Minimum y = 4.7426 d = 0.63063
Fig. 8. Optimization plot for bead penetration and width
References [1] Howse, D. S., Lucas, W., Science and Technology of Welding and Joining, 5(3) (2000), 189-193. [2] Loureiro, A. R., Costa, B. F. O., Batista, A. C., Rodrigues, A., Science and Technology of Welding and Joining, 14(4) (2009) 315-320. [3] Gurevich, S. M., Zamkov, V. N. and Kushmienko, N. A., Avtomaticheskaya Svarka, 9 (1965), 1-4. [4] Mills, K. C., Keene, B. J., Brooks, R. F. Shirali, A., 1998. Philosophical Transactions-Royal Society of London Series A Mathematical Physical and Engineering Sciences, pp.911-926. [5] Tseng, K. H., Hsu, C. Y., Journal of Materials Processing Technology, 211(3) (2011).503-512. [6] Huang, H. Y., et al., Science and Technology of Welding and Joining, 10(5) (2005), 566-573. [7] Tseng, K. H., Powder technology, 233 (2013), 72-79. [8] Anderson, P. C. J., Wiktorowicz, R., Welding and metal fabrication, 64(3) (1996) 108-109. [9] Lucas, W., Howse, D., Welding and metal fabrication, 64(3) (1996) 11-17. [10] Datta, S., Bandyopadhyay, A. Pal, P. K., The International Journal of Advanced Manufacturing Technology, 39(11-12) (2008) 1136-1143. [11] Kou, S., New Jersey, USA, (2003) 431-446. [12] Dhandha, K. H., Badheka, V. J., Journal of Manufacturing Processes, 17 (2015) 48-57. [13] Zhang, R. H., Pan, J. L., Katayama, S., Frontiers of Materials Science, 5(2) (2011) 109.
ScienceDirect Materials Today: Proceedings 18 (2019) 4703–4710
www.materialstoday.com/proceedings
ICMPC-2019
Development of Activated Flux for Deep Penetration in GTAW Sanjib Jaypuriaa, Swagatika Khandaib, Trupti Ranjan Mahapatrab*, Amresh Singhc a
b
Mechanical Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur – 721302, India Department of Production Engineering, Veer Surendra Sai University of Technology (VSSUT), Burla – 768018, India c Indian Ordnance Factory Board, Kolkata -700001, India
Abstract Gas metal tungsten (TIG) welding is one of the best suited fabrication methods in industries due to lower capital cost, contamination free and sound quality of weld joints. However, there are few drawbacks like the limited penetration capacity and limited productivity in TIG welding. That is why to improve the productivity attempts are made every now and then through improving the penetration of the TIG welding. In this regard, the Activated Tungsten Inert Gas (ATIG) welding is a unique method, where developed flux combination is pasted onto the surface of the base metal. The flux formulations have been done essentially based on empirical data as well as the practical experience. The in house developed activating flux is applied as a thin coating to the metal surface before welding. It was observed that there was a significant increment in bead penetration and quality through ATIG. Here efforts were made to increase the weld penetration by applying the active flux and to optimize the process parameters. An optimized mixture of MnO2, SiO2 and TiO2 was found to be effective for enhancement of penetration. Content of MnO2 and beam current have significantly contributed for enhancement of penetration and width of bead, respectively. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: TIG; GTAW; Flux; Weld Penetration; ATIG; Marangoni; Surface Tension.
1. Introduction Two or more work pieces are welded through an arc welding method using a non-consumable tungsten electrode in Tungsten Inert Gas Welding (TIG), which is also regarded as the Gas Tungsten Arc Welding (GTAW). To a great extent it is analogous to Metal Inert Gas (MIG) Welding. TIG welding is used for welding a variety of metals and broadly used in all types of manufacturing industry due to good weld surface finish. A variety of materials including Stainless Steel, Aluminum, Copper, Alloy steel, Titanium, Nickel and Magnesium alloys can be welded using TIG
* Corresponding author. Tel.: +91-9861425597; fax: +91-6632430204 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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S. Jaypuria et al. / Materials Today: Proceedings 18 (2019) 4703–4710
Welding. However, at the same time Conventional Tungsten Inert Gas (TIG) arc welding suffers from the following disadvantages: (a) low productivity; (b) High skill requirement (c) Expensive setup as compared to other arc welding process (d) relative shallow penetration. To obtain main objective of company regards quality and productivity, there are continuous attempts to improve the productivity and penetration of the TIG welding. One of the emerging methods is Active tungsten inert gas (A-TIG). The A-GTAW process uses activating fluxes to increase weld depth [1, 2]. Researchers developed this welding process in the 60s at the Paton Electric Welding Institute (PWI) in Ukraine [3]. Activated flux is a form of flux in powder form which is mixed with acetone to make paint like constituent and then applied to joint with brush to certain thickness to achieve enhanced penetration with reduced heat input in TIG Welding. A-TIG process involves application of thin coating of an activating flux to the surface of the material before welding that can bring out a significant increase in weld bead penetration. The methodologies adopted to improve the penetration of conventional TIG welds by using active fluxes are reviewed and studied from open literature. Tithering of the electric arc originated due to the presence of some components (mostly oxygen and fluorine) of the flux in the arc is the most prominent mechanism for improved penetration. As a result of this effect the penetration of the weld bead increases because of increasing anode current density as well as the arc force acting on the welding pool [4]. It is also reported that the surface tension of the weld pool decrease by the activating fluxes and a deeper invading in the pool is caused by the arc pressure. Subsequently the arc pressure reaches a deeper penetration with the help of this invading [4]. Therefore, Marangoni effect is playing a significant role in enhancing penetration. Some researchers reported that use of MnO2, TiO2, MoO3, and SiO2 fluxes in TIG welding leads to variation of the temperature coefficient of surface tension on the molten pool from a negative to a positive value. This indicates that the surface tension at the pool edge is lower than at the pool center and therefore, centripetal Marangoni convection is introduced by the surface tension gradient in the molten pool. This change in surface tension gradient with temperature from negative to positive provides the desired flow of molten pool to enhance heat input in central region of molten pool [5]. The chemical composition of the weld metals does not undergo any significant change in comparison to that of the base metals due to the use of activated flux. Scholars also observed that during the welding of structural components made of 304LN and 316LN stainless steels, the activated flux developed has greater potential for use. This is because 316 LN A-TIG weld joints showed no degradation in the microstructure and mechanical properties and 75% enhancement of creep rupture life is noticed [6]. Oxide powders (Al2O3, Cr2O3, TiO2, SiO2 and CaO) were applied through a thin layer of the flux and a bead on plate welds was produced. The increase in the penetration is found to be more significant with the use of Cr2O3, TiO2, and SiO2 [6]. The researchers applied, oxide powders (Al2O3, Cr2O3, TiO2, SiO2and CaO) was applied on a type 304 stainless steel through a thin layer of the flux to produce a bead on plate welds. The experimental results indicated that the increase in the penetration is significant with the use of Cr2O3, TiO2, and SiO2 [6]. Some researchers observe that mixture of SiO2TiO2 is used as an activated flux and welding of type 316L stainless steel was investigated. The 80% SiO2+ 20%TiO2 mixture can produce the greatest improvement in TIG penetration [7]. It is observed from the literature that there are few established studies on dependency of flux and bead parameters. It is also observed from the literature that most of the studies are focused on single element of flux and their effect on penetration. Most of reported literature also focused on flux not on activated flux. Therefore, in this study an attempt has been made to study the effect of different activated flux separately in enhancement of bead penetration. This study also focuses on performance comparison of different activated flux and to design different flux combinations for steel. In addition to this, optimized process parameters have been suggested for enhancing bead penetration and minimizing bead width. 2. Experimentation The material chosen in the present analysis for the experimentation purpose were 304L Austenitic stainless steel of size (150mm x 50mm x 6mm) and chemical composition as shown in Table 1. Table 1. Chemical Composition of 304 Stainless Steel (wt %) Type
Carbon
Manganese
Phosphorus
Sulfur
Silicon
Chromium
Nickel
Nitrogen
304
0.07
2
0.045
0.03
0.75
17.5–19.5
8.0–10.5
0.1
S. Jaypuria et al. / Materials Today: Proceedings 18 (2019) 4703–4710
4705
As the texts about activated fluxes available are few, hit and trial method is adopted for analyzing the effect on weld bead shape with single component flux or examining the effect of individual materials, which is already being used as flux component. Here some process parameters were kept constant and it is tabulated in Table 2. Based on design of experiments (DOE), parameters like SiO2, MnO2, TiO2 and MoO2 are varied. Here, process parameter is current which varies from 90 ampere to 150 ampere by keeping electrode diameter and arc gap constant of 3mm and 3.5mm, respectively. First of all, welding was done without flux and then single component flux to observe the effect of different flux components on penetration. After welding, the sample were prepared and molded with Bakelite powder. Then, after polishing, samples were etched with etchant prepared from HCL, glycerol and HNO3 and penetration was analyzed from image analyzer. The obtained result is tabulated in Table 3. The effect of different activated flux on bead parameters is given in Fig. 1. Table 2. Process parameters Process parameter
Description
Process parameter
Description
Current
90 Amps. - 150 Amps.
Welding speed
0.4 m/min
Electrode diameter
3 mm
Shielding gas
Argon
Work piece to electrode gap
3.5 mm
Gas flow rate
10 lit/min
Table 3. Preliminary weld bead parameters Sample Number
Sample Description
Penetration (mm)
Width(mm)
Height(mm)
1
Without flux
0.56
1.85
0.1
2
With TiO2
1.36
3.33
0.13
3
With SiO2
1.36
3.43
0.06
4
With MnO2
1.85
5.34
0.46
5
With MoO2
2.43
5.68
0.38
Fig.1. Effect of different flux constituents on weld bead geometry.
Although MoO2 had positive effect on the penetration, but later it was decided to eliminate MoO2 due to health hazard and formulations were prepared with rest of the materials. Three different levels of weight taken for each constituent and experiments were conducted as per the design matrix using L9 of Taguchi technique (Table 4). The experiments were conducted according to the design matrix at random order to avoid systematic errors infiltrating
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the system. Here, potassium silicate, water as solvent media and acetone is used as binder in the proportion of binder: water: acetone as 0.8-1.2gm: 1gm: 5-10 drops for each gram of activated flux. After making these 9 formulations, the activated flux with binder, water and acetone was made and applied at center of the plate for bead on plate welding of 304stainless steel. During welding the arc stability was the problem that lead to introduce iron powder to increase its conductivity. Iron powder was also kept constant in the ratio of Fe: activated flux as 1:15. After applying the flux to the plate, it was dried in electric oven to remove wetness/moisture at temperature of 120°C for 10-15 minutes. During Experimentation some process parameters were kept constant which was explained in Table 2 expect current, which was varied with three levels (110A, 130A, 150A). After welding all the nine beads were molded and etched in solution of hydrochloric acid, glycerol and nitric acid in the ratio of 3:2:1. The experimental combination and results of weld bead geometry is shown in Table.4. Main effect plot for bead penetration is shown in Fig. 2. Table 4. Flux formulations based on Taguchi L9 with corresponding responses Sl. No.
MnO2(g)
SiO2(g)
TiO2(g)
Current (amp)
Bead Penetration (mm)
Bead Width (mm)
1 2 3 4 5 6 7 8 9
2.5 2.5 2.5 5 5 5 7.5 7.5 7.5
2.5 5 7.5 2.5 5 7.5 2.5 5 7.5
2.5 5 7.5 5 7.5 2.5 7.5 2.5 5
110 130 150 150 110 130 130 150 110
0.73 1.12 0.42 1.77 0.46 0.69 1.13 0.87 1.21
2.47 5.28 3.16 4.73 3.48 3.28 3.23 1.99 4.66
MnO2(g)
1.4
SiO2(g)
1.2 1.0 0.8 0.6 2.5
5.0
7.5
2.5
TiO2(g)
1.4
5.0
7.5
Current (amp)
1.2 1.0 0.8 0.6 2.5
5.0
7.5
110
130
150
Fig. 2. Mean effect plot for bead penetration based on results in Table 4
From the observation and graph, it was established that penetration increases with MnO2 concentration and vice versa in case of SiO2. It was also observed the mixed response in case of TiO2. We also note that welding current has a linear relation with penetration. From the analysis it was decided to apply the same set of experimentation with modified flux formulation and designed flux formulation is shown in Table 5 below. Thus for the experimentation purpose SiO2 was varied between 10-20%, MnO2 as 40-50% and TiO2 between 20-50%. The finally decided levels for the fluxes for the best result are shown in Table 5.
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Table 5. Final flux formulations with their responses Sl. No.
MnO2(g)
SiO2(g)
TiO2(g)
Current (amp)
Bead Penetration (mm)
Bead Width (mm)
1 2 3 4 5 6 7 8 9
8 8 10 10 10 12 8 12 12
2 4 2 3 4 2 3 3 4
10 4 7 4 10 4 7 10 7
110 150 150 110 130 130 130 150 110
2.84 1.88 4.15 4.14 3.06 2.09 3.00 3.54 2.53
5.24 7.20 6.50 7.18 5.92 5.07 5.52 6.59 5.42
3. Results and Discussion The observations along with latest combination of flux mixture taken from experiments are shown in Table 5. The welded samples then moulded manually with araldite (an epoxy based polymer) and the final measured welded specimens from image analyzer are shown in Fig. 3.
Fig. 3. Final measured welded specimen from image analyzer
Penetration in TIG welding is significantly controlled by surface tension gradient over the pool and this surface tension is function of melt temperature. Surface tension is also affected by the surface-active elements, which helps to increase the surface tension gradient and induces thermo-capillary flow deep into the weld and deeper penetration can be achieved [8]. A-TIG is such a welding process where surface tension gradient of melt pool is altered by the active elements and helps to achieve deep penetration without filler wire and any edge preparation. Fig.4 shows the change in convection movement of the activated flux in the weld pool to centripetal from centrifugal during A-TIG welding. This is an indication of the occurrence of centripetal Marangoni convection in the fusion pool by the surface tension gradient. The temperature coefficient of surface tension on the molten pool altered from a negative to a positive value in A-TIG. Consequently, the surface tension at the pool center was higher
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as compared to at the pool edge. Corresponding to this condition the flow of the fluid in the molten pool surface transfers easily towards the center from the pool edge and then downwards [9]. Mn O 2(g)
S i O 2(g)
3 .5
3 .0
2 .5 8
10
12
2
Ti O 2(g)
3
4
C u rre n t
3 .5
3 .0
2 .5 4
Fig. 4. Schematic diagram of Marangoni Convection
7
10
110
130
150
Fig. 5. Mean effect plot for bead penetration based on Table 5 results
The minimum weld depth was obtained in the weld without any fluxes. It could be understood that the normal TIG welding deals with Marangoni effect which states that a lower value of surface tension could be achieved with higher temperature area of fusion zone and vice versa. Therefore, surface tension at the center of weld pool is lower than that of edges and molten metal flows from center to edges in case of conventional TIG welding. In this way, conventional TIG welding yields less penetration and more bead width. However, A-TIG welding yields higher bead penetration because of the used flux, which act as surface-active elements and alters the surface tension gradient. The stable oxide fluxes helps to achieve reversal of Marangoni effect by maximizing surface tension gradient at the center of the weld pool [10, 11]. In addition to this, these fluxes constrict the welding arc and the penetration is more in comparison to normal TIG. In addition to this, the arc is focused at the center of the fusion zone as these mixtures of fluxes constrict the current density at outer radius of the arc column. This enhanced current density at center leads to higher magnetic force and achieves strong convective molten fluid flow downwards in the weld pool. Hence, the weld depth is significantly increased [12]. It is also reported in literature that TiO2 also creates anode spot on weld area, which attracts more electrons from the tungsten electrode and helps to achieve deeper penetration [13].It is seen from Table 5 that maximum penetration (4.15 mm) could be achieved with 10 gram of MnO2, 2 gram of SiO2 and 7 gram of TiO2 with highest level of penetration. It is also seen from the table that MnO2 has highest significance in deciding the penetration. In most of the cases, the weld with deeper penetration has higher value of bead width. It is seen from the mean effect plot (refer Fig. 5) for bead penetration that a mid-level of MnO2, SiO2, TiO2 and low level of current is necessary condition for getting deeper penetration in A-TIG welding.Table 5 indicates that a higher beam current always yields shallow bead profile with high level of bead width. So, current has significant contribution in deciding the bead width and content of MnO2 has second highest contribution. A high level of MnO2, low level of SiO2 and mid-level of TiO2 and current are suggested from mean effect plot of bead width (refer Fig. 6) to achieve minimum bead width during welding. A desirability approach is also selected to optimize the process parameters of A-TIG to get maximum penetration in the joint. It is seen from Fig. 7 that 9.9 gram of MnO2, 2.7gram of SiO2, 6.7 gram of TiO2 and 110 amperage of current is suitable for achieving a penetration of 5.3 mm with desirability of one.In the similar manner, desirability approach is also conducted for composite responses of bead penetration and width, where target is set to maximize and minimize penetration and width respectively. A composite desirability of 0.65 could be achieved (refer Fig. 8) with 11.33 gram of MnO2, 2.0 gram of SiO2, 8.12 gram of TiO2 and 122 amperage of current. The composite desirability also refers to a penetration of 3.3 mm and width of 4.74 mm. It is observed from the discussion that although penetration is significantly increasing with activated fluxes, but bead width is not shown any significant reduction. Therefore, A-TIG has great demand in the cladding industries in addition to the welding application.
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MnO2(g)
4709
S iO2(g)
6.5
6.0
5.5 8
10
12
2
TiO2(g)
3
4
Current
6.5
6.0
5.5 4
7
10
110
130
150
Fig. 6. Mean effect plot for bead width based on Table 5 results
Fig. 7. Optimization plot for bead penetration
4. Conclusion The data plots provided in the paper clearly demonstrates that oxides have a greater influence on the penetration. Increased penetration is also noticed with all the constituents. Moreover width also increased with penetration. Therefore this can be used for the purpose of cladding. Also the reinforcement that we achieved is because of rolled sheets gets relieved after heating. This increase in penetration is due to direction of fluid flow as explained by Heiplerepair theory. A significant increase in penetration was obtained in welds done with a TiO2 activating flux. This effect is mainly due to not only the arc constriction produced by the flux and consequent increase in the arc force but the reversal of Marangoni convection. When TIG welding is performed at a temperature higher at center of weld pool in comparison to edge of weld pool, the surface tension gradient is established between the center and edge of weld pool. This results in generation of the centrifugal Marangoni flow in the molten weld pool. The temperature co-efficient of surface tension changed from negative to positive when TIG welding with SiO2 ,TiO2, MoO2 and MnO2 is performed. This gives rise to greater heat input in central region and consequently, the penetration has been increased.
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MnO2(g) 12.0 [11.3535] 8.0
SiO2(g) 4.0 [2.0] 2.0
TiO2(g) 10.0 [8.1212] 4.0
Current 150.0 [122.1212] 110.0
Composite Desirability 0.65336
P Maximum y = 3.3144 d = 0.67692
B Minimum y = 4.7426 d = 0.63063
Fig. 8. Optimization plot for bead penetration and width
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