Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3064 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on 316 L Austenitic Stainless Steel Welds Abdeljlil Hdhibi Mechanical Engineering Department College of Engineering Prince Sattam Bin Abdulaziz University Al Kharj, Saudi Arabia and Preparatory Engineering Institute of Nabeul (IPEIN), Tunisia a.hedhibi@psau.edu.sa Kamel Touileb Mechanical Engineering Department College of Engineering Prince Sattam Bin Abdulaziz University Al Kharj, Saudi Arabia k.touileb@psau.edu.sa Rachid Djoudjou Mechanical Engineering Department College of Engineering Prince Sattam Bin Abdulaziz University Al Kharj, Saudi Arabia r.djoudjou@psau.edu.sa Abousoufiane Ouis Mechanical Engineering Department College of Engineering Prince Sattam Bin Abdulaziz University Al Kharj, Saudi Arabia a.ouis@psau.edu.sa Mohamed Lamjed Bouazizi Mechanical Engineering Department College of Engineering Prince Sattam Bin Abdulaziz University Al Kharj, Saudi Arabia my.bouazizi@psau.edu.sa Jamel Chakhari Mechanical Engineering Department College of Engineering Prince Sattam Bin Abdulaziz University Al Kharj, Saudi Arabia j.chakhari@psau.edu.sa Abstract—Tungsten inert gas (TIG) is a wide common process used in fabrication due to its low cost equipment, high quality and accuracy welds but has low productivity related to the low penetration depth in single pass. A new perspective, the Activated Tungsten Inert Gas (ATIG), in which the same equipment as TIG is used, except that a thin layer of activated flux is deposited on a workpiece surface. In this work, eight kinds of oxides were tested on 316L austenitic stainless steel. Three levels of welding current were used to study the effect of different activating fluxes on weld bead geometry and mechanical properties. X-ray Photoelectron Spectroscopy (XPS) was used for the first and the second level energy for different ATIG welds to analyze the relationship between the weld shape and oxygen content in welds. The experimental results showed that the weld profile is related to the thermodynamic stability of selected oxides and in relation to the energy provided. ATIG with TiO2, SiO2, MnO2 oxides presented the deepest welds followed by Cr2O3, Fe2O3, and ZnO. Finally ZrO2, CaO oxides had no effect on the weld depth. The ATIG welded joint showed better tensile strength than TIG. The ATIG hardness measurements carried out showed also better if not the same as TIG weld except for the Silicon oxide weld. Results of the impact test showed that, except for the titanium dioxide TiO2 which has a good benefit, the weldment using the other oxide fluxes exhibits worse withstanding to sudden shock than TIG welding. Keywords-ATIG; austenitic stainless steel; weld shape; mechanical properties I. INTRODUCTION TIG welding uses a non-consumable tungsten electrode and an inert gas. In this process, an electric arc is formed between the tungsten electrode and the base metal. However, there are a few limitations of the TIG welding process, like low penetration depth, and if the workpiece thickness is greater than 3mm joint edge preparation and multiple passes are required to fill the joint completely. TIG welds are also affected by cast-to- cast compositional variation in the base metal being welded [1, 2]. These limitations can be overcome by using ATIG welding. This technic was invented in the 1960’s by researchers at the Paton Electric Welding Institute (PWI) in Ukraine [3, 4]. Few data are available in the open literature about the activating flux composition. Such data are very important to determine the weld penetration improvement. The mechanism related to deep penetration is linked to: A. Arc Constriction [5-7] This phenomenon can occur by two means:  The interaction between the peripheral negative charge (electrons outside the arc) and the neutral particles (atoms and molecules) formed from the dissolution of flux components causes the constriction of the arc, leading to the increase of the welds depth.  The flux acts as insulating layer reducing the current density of the outer arc radius and increases the current density in the center as well as magnetic force (Lorentz Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3065 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … force) in the weld pool which lead to the increasing of weld depth. B. Marangoni Convection [8-10] During TIG welding, without surfactants elements like sulfur, oxygen, selenium, and tellurium, the surface tension gradient is negative and the convection movement is centrifugal leading to a shallow penetration. The addition of activated flux induces an inversion of the convection current and changes the sign of the surface tension resulting in a centripetal convection movement, to the increase of penetration depth. High sulfur percentage weld beads are sensitive to centerline cracking and heat affected zone (HAZ) cracking. On the other hand, very low sulfur content can lead to irregular weld penetration. For acceptable penetration without a risk of crack, the sulfur range should vary from 0.005% to 0.030% for 316 stainless steel with the suggested Creq/Nieq ratio range from 1.43 to 1.9 [11]. This element can be provided as pasted flux on the workpiece to improve the depth of welds [12]. On the other hand, the addition of 500ppm of SO2 in shield gas improves the D/W ratio by 80% [13]. As for selenium, an addition of 0.1% by weight of Se in steel reduces its superficial tension by 30% [14]. While the variations of O2 range from 600ppm to 1200ppm, can lead to an increase of penetration from 15% to 45% for the 1010 steel [15]. The activation of the cited elements is limited by a reversal temperature beyond which they don’t affect the weld morphology [14]. Authors in [16] showed that oxygen as active element is active in pure liquid iron in the range of 150-300ppm. Above this range the temperature coefficient of surface tension becomes zero or negative leading to shallow and wide weld bead. According to a study [17], the oxygen content in the weld metal in the range of 70ppm-300ppm, can increase the geometric parameter D/W from 1.5 to 2 times. Many works have been conducted using single flux (oxide or halide type) to study their individual effect on the morphology and the mechanical properties [12, 18-25]. Due to the complexity of the reactions in the weld pool and interconnection with the welding arc, a single component flux cannot meet a good morphology and mechanical properties requirements. On the other hand, several studies were conducted in order to find the suitable flux compositions. Binary fluxes have been developed in order to get optimum percentages of binary flux components that can improve the aspect and the shape as well as the mechanical properties of welds [1, 27-29]. In addition, other studies were devoted to ternary flux where statistical programs were used to develop a formulation on the percentages of the ternary flux components to enhance depth and mechanical properties of welds [30] as well as the effect of the welding parameters and the flux on weld bead geometry, angular distortion and mechanical properties. In this study, eight different kinds of oxide fluxes (Cr2O3, Fe2O3, CaO, ZnO, ZrO2, SiO2, MnO2 and TiO2) were used to investigate the effect of single component flux on the morphology, and mechanical properties of 316L stainless steel TIG welds. This study contributes to the understanding of the phenomena that occur in the weld pool. II. EXPERIMENTAL PROCEDURE This work was conducted on 316 L stainless steel. The tensile strength of the material is 618MPa, Vickers hardness is 180HV and melting point is about 1440oC. Table I lists the chemical compositions of this steel. Plates 6mm in thickness were cut into 204mmx204mm to have test samples for mechanical testing and weld morphology (Figure 1). Acetone was used to clean and remove dust and contamination. TABLE I. CHEMICAL COMPOSITION OF 316L STAINLESS STEEL SS 316 L percentage C % 0.018 Mn % 1.30 Si % 0.43 P % 0.034 S % 0.0029 Cr % 16.78 Ni % 10.00 Mo % 2.005 N ppm 321 Ti % 0.0042 Cu % 0.2082 Co % 0.226 Fig. 1. Different test specimens on welded plate Eight different kinds of oxide fluxes were used. Table II lists the melting and boiling temperature of the different oxides. Thermodynamic stability of selected oxides is showed in Table III where the stability decreases from left to right. Products have been heated separately in furnace at 150ºC for 1 hour to eliminate humidity. The powder has been mixed with acetone (1:1 ratio) forming a paste. A brush was used to apply the mixture on the plain surface to be welded. The acetone was allowed to evaporate, so only the flux powder was left on the metal surface. The coating density of flux was about 7- 8mg/cm2, and the flux layer thickness about 0.2mm. A TIG welding machine was used. A water-cooled torch with a standard 2% thoriated tungsten electrode rod having diameter of 3.2mm has been used for the experiments. The torch was mounted on a motorized carriage as shown in Figure 2. Three series of tests have been carried out with three levels of welding currents (150amp, 540kJ/m-180amp, 680kJ/m, 200amp, 800kJ/m).The welding parameters selected for TIG welding are listed in Table IV. After welding, the samples were cut as shown in Figure 1, far from the welding starting point to be sure that the arc welding was stabilized. Etching operation is necessary to reveal the morphology of the welds and for hardness measurements. Solution of one volume of water and 2 volumes of Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3066 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … Hydrochloric acid (HCL) and one volume of nitric acid (HNO3) was used for etching. The tensile tests were conducted at room temperature. The tests were carried out with a computer control electro-hydraulic servo universal testing machine model WAW-300E and the impact tests with the impact testing machine model JBS-500. Vickers hardness measurement was performed with a digital hardness tester model HVS-50. The tensile, impact, and hardness test were conducted only with 180amp welds (second energy level). Fig. 2. Experimental setup of TIG welding with flux paste (A motorized carriage, torch–workpiece). TABLE II. MELTING AND BOILING TEMPERATURE OF USED OXIDES Oxides Tm(ͦc ) Tb (ͦ C) Cr2O3 2435 3000 Fe2O3 1540 1987 CaO 2613 2850 ZnO 1975 1975* ZrO2 2715 4300 SiO2 1713 2950 MnO2 535 540* TiO2 1843 2972 *Decomposition temperature of oxides TABLE III. THERMODYNAMIC STABILITY OF SELECTED OXIDES IN DECREASING ORDER CaO ZrO2 TiO2 SiO2 Cr2O3 ZnO Fe2O3 MnO2 TABLE IV. WELDING PARAMETERS Welding speed 15cm/min Welding current (150-180-200)Amp Arc Length 2mm Electrode Tip Angle 60 ͦ Shield gas on work piece Argon with flow rate 12l/min Shield gas Back side Argon with flow rate 5l/min Welding mode Direct current electrode negative III. RESULTS AND DISCUSSION A. Oxygen Content XPS was used to analyze the surface chemistry of sectional weld bead, especially the percentages of oxygen remaining in weld bead. The result obtained with the flux SiO2 is reported in Figure 3. Results for two energy levels series for different fluxes are reported in Table V. Fig. 3. Oxygen spectrum in ATIG welded bead with SiO2 flux TABLE V. PERCENTAGES OF O IN ATIG WELD BEAD Flux E=540kJ/m E=680kJ/m SiO2 4.99 6.29 TiO2 10.19 9.57 Fe2O3 11.07 7.33 MnO2 10.9 8.84 Cr2O3 23.95 11.89 ZnO 11.8 10.32 ZrO2 16.85 8.53 CaO 28.52 10.44 B. Effect of the Energy Level E=540kJ/m Regarding the fluxes of CaO and ZrO2, it can be seen that the quantity of oxygen remaining in weld bead is relatively high. This is due to their high thermodynamic stability. With this level of energy there is no or very small quantity liberation of oxygen in weld pool. The melting point of those oxides is very high comparatively to parent metal (Tm316L =1440oC) the gradient of temperature is high, so a high energy is needed to melt them: ΔT (ZrO2-316L)=2715-1440=1275oC ΔT (CaO-316L)=2613-1440=1173oC As for the flux of Cr2O3 it is characterized by its high melting and boiling temperature, a lot of energy is necessary to melt or vaporize it, that’s why a quantity of oxygen remains in the weld bead. Cr2O3 has moderate thermodynamic stability. ΔT (Cr2O3-316L)=2435-1440=995oC As for the fluxes of ZnO, MnO2, and Fe2O3, they have low stability, but manganese dioxide, with its low melting point of 535oC, can liberate easily its oxygen. ΔT (ZnO-316L)=1975-1440=535oC (less energy required to melt it) ΔT (Fe2O3-316L)=1540-1440=100oC (easy to melt) Finally, the fluxes SiO2, TiO2: Their standard Gibb's energies of formation as well as their boiling point are close. ΔT (SiO2-316L)=1713-1440=273oC 0.00E+00 1.00E+05 2.00E+05 3.00E+05 4.00E+05 5.00E+05 020040060080010001200 C ou nt s / s Binding Energy (eV) Survey 5 Scans, 2 m 16.1 s, 200µm, CAE 200.0, 1.00 eV Fe2p Cr2p Ni2p3 Mo3dO1s Ar2pC1s Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3067 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … ΔT (TiO2-316L)=1843-1440=403oC, so less energy is required to melt them. C. Effect of the Energy Level E=680kJ/m In this case, the low level of oxygen remaining in weld bead means that the oxygen was liberated. This element contributes to having centripetal convection of weld bead leading to full penetrated welds. Maybe there is some oxygen trapped in the weld bead due to the lack of protection effectiveness of the melt. IV. WELD MORPHOLOGY: The weld morphology is characterized by the penetration depth D and the bead width W. A. Effect of Different Fluxes on Weld Morphology with a Current of 150amp - E=540kJ/m Figure 4 shows the morphology of TIG welds with and without fluxes and Figure 5 shows the width to depth penetration. It can be seen that TiO2, SiO2 and MnO2 oxides permit to get total penetration weld bead followed by Cr2O3, Fe2O3 and ZnO with good penetration of around 5mm. Finally moderate penetration welds occur with ZrO2 and CaO oxides. The depth to width ratio R is greater than 0.7 for TiO2, SiO2, Cr2O3 oxides and close to 0.7 for MnO2. It decreases to 0.55 for Fe2O3 and ZnO. But it goes down to 0.4 for ZrO2 and CaO. ZrO2 and CaO oxides behave as pure metals promoting a wide and shallow weld bead. On the other hand, TiO2 and SiO2, and at a less degree Cr2O3, significantly increase the weld depth to weld bead width ratio. B. Effect of Different Fluxes on Weld Morphology with a Current of 180amp - E=680kJ/m As revealed with XPS analysis, increasing the welding current increases the temperature in weld pool leading to more liberation of oxygen. Four fluxes out of eight (TiO2, SiO2, Fe2O3 and MnO2) presented total penetration of welds as shown in Figure 6. In this case, the positive tension gradient temperature coefficient controls the melt metal movement in the weld. C. Effect of Different Fluxes on Weld Morphology with a Current of 200amp - E=800kJ/m The penetration is total for almost all oxides used except zirconium and calcium oxides as shown in Figure 7. In this case, the movement of liquid is from the center to the edges. Fig. 4. Morphology of TIG and ATIG welds (I=150A, V=15Cm/min) Fig. 5. Width to depth ratio of TIG and ATIG welds (I=150A, V=15Cm/min) Fig. 6. Morphology of TIG and ATIG welds (I=180A, V=15Cm/min) V. OXIDE EFFECTIVENESS WITH PROVIDED ENERGY The efficiency of oxides is represented by the ratio R. The welding current level represents the energy level supplied to the melt. This energy indirectly represents the temperature reached in the melt. Normally, each oxide has an optimum temperature where it completely fulfills its role. Figure 8 shows that the coefficient R is more important with current 180A in the cases of TiO2, SiO2, Fe2O3, MnO2, CaO but ZnO and Cr2O3 oxides are more efficient with a current of 200A, and finally ZrO2 oxide is efficient with a current of 150A. Figures 9 and 10 show micrographs of weld beads. Fig. 7. Morphology of TIG and ATIG welds (I=180A, V=15Cm/min) Fig. 8. Width to depth ratio of TIG and ATIG welds (I=150,180 ,200A, V=15Cm/min) Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3068 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … TIG weld bead ATIG weld with SiO2 flux ATIG weld with TiO2 flux ATIG weld with Fe2O3 flux ATIG weld with MnO2 flux ATIG weld with Cr2O3 flux ATIG weld with ZnO flux ATIG weld with ZrO2 flux ATIG weld with CaO flux Fig. 9. Weld beads using different mono-oxides flux with 180A, 15Cm/min VI. EFFECT OF OXIDE FLUXES ON MECHANICAL PROPERTIES A. Tensile Test The configuration and size of the tensile specimens were prepared according to ASTM E8M-04 for the tensile test as shown in Figure 11. Figure 12 presents the results for the tensile strength of TIG weldment with and without fluxes. It can be clearly seen that the tensile strength obtained by using activating fluxes exhibit are better than those of TIG welding. According to [23, 31, 32], the delta ferrite is formed during ATIG welding. The delta-ferrite has a body-centered cubic (BCC) crystal structure, which has a higher mechanical strength than that of the austenitic FCC structure. It was seen that the content of Ferrite number is greater during the ATIG welding than in TIG welding. It was also observed that during the ATIG the delta-ferrite content in the weld metals was increased, and showing a positive effect in increasing the tensile strength. B. Hardness Test The location of the measurements across the weld cross section is shown in Figure 13. The values used are the average of four measurements.Results showed the same trends as the tensile strength results that the oxide flux did not affect the hardness of welds. The hardness of ATIG welds is equal or above of the TIG welding hardness reading, except ATIG silicon oxide flux weld hardness (165HV) which is less to TIG weld bead hardness (176 HV) as shown in Figure 14. In the other hand HV hardness of MnO2, Cr2O3 and ZrO2 are bigger than the hardness of parent metal (180HV). C. Impact Test Figure 15 presents the results for the experimental results for the impact test at weld zone of TIG weldment with and without fluxes. The specimens were prepared according to ASTM E23 for the impact test as shown in Figure 16. It can be observed that, except for the titanium dioxide TiO2, the weldment using the other oxide fluxes exhibits a worse withstand to sudden shock than those of TIG welding. This may be attributed to two reasons: (a) The level of oxygen trapped in the weld pool while welding or due to decomposition of oxides as revealed by XPS analysis and (b) The formation of the ferrite content in ATIG welds decreases to sudden impact loads. Figure 17 presents the results of the experimental results for the impact test at heat affected zone HAZ of TIG weldment with and without fluxes. It can be Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3069 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … observed that a drastic reduction in the impact toughness, probably due to a formation of sigma phase and coarser ferrite and austenite grains. ATIG with Cr2O3 flux, I=150A ATIG with Cr2O3 flux, I=180A ATIG with Cr2O3 flux, I=200A ATIG with Fe2O3 flux, I=150A ATIG with Fe2O3 flux, I=180A ATIG with Fe2O3 flux, I=200A ATIG with ZnO flux, I=150A ATIG with ZnO flux, I=180A ATIG with ZnO flux,I=200apms Fig. 10. Weld beads using different levels of current intensity (150- 180 -200A), welding speed 15Cm/min Fig. 11. Test specimen for the tensile test Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3070 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … Fig. 12. UTS of TIG and ATIG welds (I=180A, V=15Cm/min) Fig. 13. Hardness test locations on weld bead Fig. 14. Vickers hardness for TIG and ATIG welds at weld zone (I=180A, V=15Cm/min) Fig. 15. Energy absorbed for TIG and ATIG welds at weld zone (I=180A, V=15Cm/min) VII. MICROSTUCTURE During welding, weld metal cools so rapidly that the transformation of delta ferrite to austenite is not complete, so, more delta ferrite is retained in the weld metal after solidification. The arc voltage increases when the activated TIG process was used knowing that the heat provided to weld pool is proportional to the voltage. The longer the element in the flux has the ability to influence the voltage, the higher the energy supplied. The measurement of ferrite content in the austenite matrix was performed using Areas image processing software from Microvision as shown in Figure 18 (a)-(d). The study has been conducted on different ATIG welds and TIG conventional weld. Results show a ferrite content equal to 6.8 for TIG. On the other hand there is an increase in ferrite content when using oxide fluxes which vary from 6.9 for MnO2 to 8.4 for SiO2 as shown in Figure 18. The increase of delta- ferrite content explains the increase of hardness and strength of the ATIG welds. (a) (b) Fig. 16. Test specimen for the impact test (a) weld zone, (b) HAZ Fig. 17. Energy absorbed for TIG and ATIG welds at HAZ (I=18A, V=15Cm/min) VIII. CONCLUSION This study was conducted on ATIG welding of austenitic 316L. Eight oxide fluxes were investigated. The results are summarized as follows:  XPS analysis shows that the percentage of oxygen in the welds decreases with the increase of current welding intensity. This contributes to increase the energy supplied to the melt. This can be explained by the fact that the oxides decompose and the oxygen atoms are transferred to the liquid weld pool allowing to centripetal melt convection.  The Oxides which are characterized by melting and boiling point close to those of the base metal should be chosen, like SiO2, TiO2, Cr2O3, and Fe2O3 whose melting and boiling points are close to the ones of austenite stainless steel 316L. It is not a major criterion but it helps to get better welding without loss of energy.  Oxide fluxes with high thermodynamic stability and high melting points are not advisable to use for austenitic stainless steel (Tm=1440 ͦ c), like ZrO2 and CaO. Engineering, Technology & Applied Science Research Vol. 8, No. 3, 2018, 3064-3072 3071 www.etasr.com Hdhibi et al.: Effect of Single Oxide Fluxes on Morphology and Mechanical Properties of ATIG on … TIG: delta ferrite content FN=6.8 ATIG with flux: MnO2- delta ferrite content FN=6.9 ATIG with flux: TiO2 - delta ferrite content FN=7.1 ATIG with flux: SiO2 - delta ferrite content FN=8.4 Fig. 18. Delta-ferrite content for TIG weld (a) and ATIG welds (b,c,d). SEM. Magnification 2500 X.  The optimal geometric ratio of efficiency R is achieved when I=180A, V=15cm/min and arc length=2mm for the 316L stainless steel using oxide fluxes such as Si2O, TiO2, Fe2O3, and MnO2.  The tensile strength of ATIG welds are above the TIG or parent metal tensile strength (618MPa). 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