Effect of TIG Welding Process Parameters Tensile behavior of 5XXX and 6XXX series Aluminium Alloys: A Review

 

Abhi Bansal¹, B.S Pabla¹ , S.C Vettivel²

¹Department of Mechanical Engineering, National Institute of Technical Teachers Training and Research (NITTTR), Chandigarh, India

²Department of Mechanical Engineering, Chandigarh College of Engineering and Technology (CCET) Degree Wing, Chandigarh, India

 

ABSTRACT:

Aluminium and its alloys have a wide range of application in the manufacturing industries. Welding process is essential for manufacturing the different varieties of products such as frames, building structures, pressure vessels, automotive components etc. Metal Inert Gas and Tungsten Inert Gas (TIG) welding are commonly used fusion welding processes for joining Aluminium Alloys (AA). It is difficult to weld AA as compared to steel due to its high thermal, electrical conductivity and high expansion coefficient. TIG welding has restricted its ability to weld thick structure in single pass. A new advancement of TIG welding namely Activated-TIG has been developed. In this method a ultra-deep penetration is obtained in a single pass. This paper presents the state-of-the-art review on recent development in TIG welding process for joining 5xxx and 6xxx AA. The effects of various welding parameters such as welding speed, welding current and shielding gas flow rate on the yield and tensile strength have also been explored.

 

 

 

1. INTRODUCTION:

Aluminium Alloys (AA) are widely used in aerospace, defense and marine applications. Welding of AA is of great importance in many sectors. Tungsten Inert Gas (TIG) welding is a fusion welding process for welding AA. TIG welding is also known as Gas Tungsten Arc Welding (GTAW) [1]. It utilizes non-consumable tungsten electrode for producing high quality of welds. Arc is shielded by inert gas like argon, helium, nitrogen or their combination [2].TIG welding process is most preferable joining process in manufacturing industry. The manufacturing industries are being forced continuously to optimize their process parameters [3].

 

The quality of welded joints depends upon the geometry of weld, melt pool behavior during welding, the metallurgy of welded zone, heat affected zone and welding defects. Around 45% of welding imperfections are occurred due to poor process conditions [4]. It has been reported that there are lot of difficulties in obtaining satisfactory properties in welded structure due to low dissolution of base metal in the weld [5]. Due to poor weld quality in ordinary TIG welding; various researchers have used different techniques to improve the existing TIG welding process. Liu et al. [6] carried out keyhole mode welding process. It is also known as Keyhole Tungsten Inert Gas (K-TIG) welding. It was concluded that compared to ordinary TIG, K-TIG has very good penetration ability with improved process efficiency and weld quality.

 

A new advancement of TIG welding process is the use of activated flux also knows as Activated-Tungsten Inert Gas (A-TIG) welding which enhances the penetration capabilities of ordinary TIG in single pass square butt weld[7]. It was found that the use of flux like SiO₂ and TiO₂ affect the weld morphology significantly and hence results in better mechanical properties[8]. The use of activated flux in TIG welding influences the performance to a greater extent and hence the optimization of welding parameters is necessary to get better mechanical properties of the welded joints [9].The systematic arrangement of TIG welding process is shown in Fig. 1.

 

 

Fig. 1 Systematic arrangement of TIG welding process [10]

 

1.1  Activated- Tungsten Inert Gas Welding:

A-TIG welding is one of the new fusion welding techniques that utilize a flux layer, made by mixture of inorganic material suspended in volatile material. It has been reported in literature that A-TIG welding increases the performance of conventional TIG welding [11]. Fujii et al. [12] studied that A-TIG has involved various physical mechanisms; the flux used in A-TIG lowering the surface tension of molten weld pool which results in increased depression, and also the arc constriction due to vaporized flux molecule.  It was found that conventional TIG welding has relatively low productivity because it has inability to weld thick structures in single pass. Vasudevan et al. [13] studied the effect of flux for A-TIG welding of Low Activation Ferritic–Martensitic steels (LAFM) plates of 10 mm thickness and indentified the flux which gave maximum depth of penetration. Shyu et al. [14] examined the effects of various oxide fluxes like Al₂O_3, Cr₂O_3, TiO_2, SiO₂ and CaO on stainless steel 304 plate using A-TIG welding process. It was revealed that Cr₂O_3, TiO_2, SiO₂ had significantly effect on the depth of penetration. Yang et al. [15] reported that SiO₂ flux constricts the arc plasma as well as changes the surface tension gradient towards the weld center  and TiO₂ flux increases the penetration due to change in surface tension gradient. Huang et al. [16] investigated the heat transfer and fluid flow in A-TIG welding by numerical modeling. Fig. 2 shows schematically differences between the TIG and A-TIG welding processes.

 

 

                                                                                                (a)                                            (b)

Fig.2 Schematic Sketch of (a) TIG (b) A-TIG

 

 

1.2 TIG Welding Parameters:

Many researchers investigated the effects of various TIG welding parameters on the weld properties and reported that welding parameters are important factors for weld strength and weld morphology. To predict the optimal welding parameters various optimization methods are available like Taguchi [17], Response Surface Methodology (RSM) [18], and Factorial design approach[19].

 

Shyu [20] reported that the welding parameters influence the performance of weld quality to a great extent and hence optimization of welding parameter is important in order to achieve superior mechanical properties joint. The various process parameters in TIG welding process are welding current, shielding gas flow gate, arc voltage, filler material, filler diameter, tungsten electrode diameter, plate thickness and root gap [21].  Root gap plays an important role in order to achieve better mechanical properties [22]. The Selection of filler rod material is mainly depending upon parent material composition and plate thickness respectively [23, 24]. Uniform rate of shielding gas is required because it provides shielding to the weld surface [25]. Welding current is related to welding speed and voltage [26]. Many researchers used different techniques to optimize these parameters to achieve better mechanical properties with improved weld quality.

 

Patil et al. [27] optimized the TIG welding parameters like shielding gas flow rate, filler diameter, electrode diameter, welding current, plate thickness, root gap, arc travel speed by multi response simulation using Definitive Screening Design (DSD) and Central Composite Design (CCD) to obtain better tensile strength and hardness. It was observed that at high value of root gap and filler rod diameter yield maximum tensile strength but low value of hardness. It was also concluded that by increasing gas flow gate increased brittleness of weldment. Prabaharan et al. [28] optimized the TIG welding parameters like welding voltage, welding current, gas flow rate nozzle to plate distance and torch angle by using factorial design approach and then analyzing the effect of said parameters on weld deposit area. They developed a mathematical model followed by sixteen trail experiments as per factorial design approach. Depending upon the experiment it has been identified the most influencing parameter on weld deposit area. It was revealed that welding current affect weld deposition area. It is more significant compared to another parameter. Increasing the current weld deposition area also increased. It was also concluded that factorial design approach is very effective tool for the parametric optimization.

Nandagopal et al. [29] optimized the TIG welding parameter like arc current, arc travel speed and arc voltage on joining of dissimilar metal titanium (6Al-4V) and AA 7075 by Taguchi using L₂₅ orthogonal array and different experiment was conducted to get the better mechanical properties. It was concluded that welding speed is the most significant process parameter which influence joint strength. Ishikawa diagram for TIG welding process is shown in Fig. 3.

 

Fig. 3 Ishikawa diagram for A-TIG welding parameters

 

2.  ALUMINIUM ALLOYS:

AA has great application in the fabrication of light weight structures that requiring a high strength to weight ratio. Depending upon the application, AA are classified as wrought and cast alloys [30]. AA is most commonly used in welding application from transportation packaging construction, marine, automotive component and aerospace [31]. These alloys are further classified as heat treatable (2xxx, 6xxx, 7xxx) series and non-heat treatable (1xxx, 3xxx, 4xxx and 5xxx) series. Most of the heat treatable AA is strengthening by work hardening and precipitation hardening depending upon the alloying composition [32]. The precipitation hardening involves solutionzing followed by quenching [33]. Most of the non-heat treatable AA is work hardenable i.e. strength can be improved by strain hardening. The strength of these alloys mainly depending upon hardening effect of alloying element depending upon the alloying composition [34]. The chemical composition of various AA is shown in Table 1.

 

Table 1 Composition of Aluminium Alloys

 

 

 

The important factors in selecting aluminum and its alloys in most of the engineering applications are their high strength-to-weight ratio [35], resistance to corrosion by many chemicals [36], non-toxicity, reflectivity, appearance, ease of formability, high electrical and thermal conductivity, machinability and its non-magnetic nature [37]. Due to these properties aluminium is most suitable material for all types of engineering applications. Hence to make a complete assembly welding of these AA is required [38].

 

2.1  Welding of Aluminium Alloys

Fauzi et al. [39] performed the microstructure analysis on AA6082-T6 and compared the results of TIG and Metal Inert Gas (MIG) welded joints. It was observed that TIG welded joints welded joints are better than MIG. The joint produced by TIG is 25 % more reliable than MIG. Chen et al. [40] analyzed the effect of low current auxiliary TIG arc on the microstructure of the weldment in TIG-MIG hybrid welding. It was reviewed that TIG-MIG hybrid welding process are more effective for producing high quality weld. It was found that the microstructure of the weld zone obtained by TIG-MIG hybrid welding is better than the conventional MIG weld. Jesus et al. [41] studied the influence of Friction Stir Processing (FSP) on the fatigue behavior of GMAW T-welds in AA5083. The results of GMAW and GMAW+FSP were compared using metallographic on optical microscope. The fractured strength was analyzed with Scanning Electron Microscope (SEM). It was concluded that FSP of the GMAW specimens slightly increases the hardness near the weld toes and has lower number of crack initiation sites hence defects such as porosity, hot cracking can be reduced.

 

Wu et al. [42] used fiber laser Variable Polarity Tungsten Inert Gas (VPTIG) hybrid welding with filler wire on the weld samples of 7N01P-T4 AA and analyzed the microstructure and fatigue properties of hybrid welding joints by using SEM and Energy Dispersive Spectroscopy (EDS). The effect of reinforcement on the fatigue behavior was investigated. It was reviewed that presently GMAW is most commonly used in joining AA but due to large heat input, welding defects such as pores, cracks and deformation are generated during GMAW of AA. It was investigated that laser arc hybrid welding provides a feasible approach to overcome these difficulties in AA welding. It was also found that fatigue performance of laser MIG hybrid welding joints exceeded that of the single MIG welded on the joint of 4mm thick 5083 AA. A defect free hybrid weld joint in 4mm thick plate on A7N01 AA and concluded that base metal weld zone has fastest fatigue cracks growth then followed by heat affected zone.

 

2.2  Selection of Filler for Welding Aluminium Alloys

AA has a potential to use for high strength alloy in automotive, aerospace, structural and marine applications due to its light weight [43]. Welding leads to vaporization of alloying element which decreases the mechanical properties of welded joints and also affecting the chemistry of weld pool. TIG welding process is preferred for these alloys because it produces high quality of weld [44]. In older to optimize the quality characteristics of weld proper selection of base material and filler selection is required. The various AA which are commonly used as weld filler rod material in TIG welding include ER 1100, ER4043, ER4047 and ER5356. These fillers can be used to join various grades of AA. The chemical composition of the filler rods is shown in Table 2.

 

 

Table 2 Common Aluminium Weld Filler Rod [45]

 

2.3 Problems in Welding of Aluminium Alloy:

Most common problems arise during welding AA is development of gas porosity [46]. The porosity in the weld increases with increase in welding speed [47] and also number of sensitive hot cracks developed in the weld are due to improper selection of weld parameters [48]. These defects reduce the strength of welded joints results in weld failure [49] and also decrease the mechanical properties like fatigue strength [50]. The main reason of decrease in fatigue strength of weld joint presence of porosity in the weld toes. Fatigue strength of weld can be improved by grinding the weld toes [51]. Welding joints have low fatigue strength [52]. To ensure weld qualities the non-destructive testing is required. Weld porosity can be detected by using X-ray testing [53]. Due to high temperature generated during welding there is decay in mechanical properties due to metallurgical transformation of the material close to the welded joints [54]. TIG welding has inability to weld thicker material in single pass due to its poor penetration. An approach to improve penetration is to add small concentration of active chemical element such as oxygen or sulphur to the molten metal [55]. Hot cracking and solidification cracking tends to deteriorate the weld mechanical properties are the major concerns during welding. It requires proper selection of filler is required [56].

 

Chen et al. [57] studied that porosity is the major defect during welding of AA which can large damage to weld performance. It was reviewed that various non-destructive methods available for detection of porosity include x-ray tests, ultrasonic testing and spectroscopy testing. X-ray and ultrasonic testing are unable to differentiate porosity and slag inclusion. It was investigated that spectroscopy technology based on empirical mode deposition (EMD) are the novel approach for the detection of porosity which aim at solving the existing problem in the conventional detection method of porosity. It was developed a welding monitoring system for porosity defects of pulsed TIG welding of AA which improved weld quality and efficiency of welding operation.

 

3. EFFECTS OF WELDING PARAMETERS:

Balasubramanian et al. [58] analyzed the effects of peak current on the tensile strength, of pulsed TIG welds of AA 6061. It was concluded that pulse current having directly proportional relationship with the tensile properties of welded joints. Kumar et al. [59] analyzed the effect of peak current on tensile strength of pulsed TIG welding of AA 5456. The predicted value of their study is shown in Fig. 4.

 

 

Fig. 4 Effect of peak current on Tensile strength

 

Maisonnette et al. [60] studied the yield strength variation during heating and cooling of the specimen. It was concluded that softening occurs at intermediate temperature due to dissolution or coarsening of the hardening precipitates during the tensile tests. Fig. 5 shows the yield strength variation with respect to the temperature.

 

 

 

Fig. 5 Variation of yield strength with temperature

 

4.  CONCLUSION:

This paper reviews the development of TIG welding process parameters and influence of activating flux on the weld penetration. This study suggested that TIG welding is effective and efficient than MIG to weld AA. But the thick plates of AA require multi-pass to complete the weld joints. It is difficult to joint thick AA plates due to their high thermal conductivity and oxide formation. To achieve complete penetration flux assisted TIG welding is used. It is also called Activated-TIG. It orders to achieve better weld quality and complete weld penetration pulsating current is used rather than continuous current. Porosity is the major problem while joining dissimilar AA. Spectroscopy is the new technology used to detect weld porosity. Another major defect occur during welding of AA is cracking. It can be avoided by selecting a suitable filler.

 

5.  RESEARCH TRENDS:

Future research may focus that it is possible to weld thick plates of AA by using flux assisted TIG (A-TIG).The use of activated flux will not cause any significant change in the chemical composition of weld metal compared to weld metals. There is negligible degradation of microstructure and mechanical properties of the A-TIG welds compared to that of weld produced by conventional TIG welding. A-TIG welding process has been found to overcome the limitation of the conventional TIG for joining AA. This new process requires single pass for welding thick plates with higher welding speed and less heat input. Welding of dissimilar AA can also be possible by using A-TIG. To obtain complete weld penetration with better weld quality in single pass for joining thick AA plates  A-TIG welding with pulsating current can be used in future in place of conventional TIG.

 

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Received on 08.07.2017            Accepted on 08.12.2017       ©A&V Publications all right reserved Research J. Engineering and Tech. 2018;9(1): 01-08 DOI: 10.5958/2321-581X.2018.00001.6