A Review Paper on A-Tig Welding Process

IJSTE - International Journal of Science Technology & Engineering | Volume 3 | Issue 09 | March 2017 ISSN (online): 2349-784X

A Review Paper on A-TIG Welding Process Bhavin Shah PG Student Department of Production Technology (Mechanical) Parul Institute of Technology, Parul University

Bhavesh Madhvani Assistant Professor Department of Mechanical Engineering Parul Institute of Technology, Parul University

Abstract This paper presents a review on Activated Flux Welding Process (ATIG). In present age of competition different types of organizations are striving hard to control costs, maintain high levels of productivity, meet changing expectations of the customers and attain quality. Gas tungsten arc welding is fundamental in those industries where it is important to control the weld bead shape and its metallurgical characteristics. However, compared to the other arc welding process, the shallow penetration of the TIG welding restricts its ability to weld thick structures in a single pass thus its productivity is relativity low.

The use of activated flux in conventional GTAW process is one of the most significant advancements for overcoming the shortcomings of TIG welding, which helps in increasing the depth of penetration and depth to width ratio of the weld pool, thereby increasing the productivity of the process and also it helps in achieving better mechanical properties.

Keywords: GTAW, A-TIG, Activated Flux, Penetration, Reverse of Marangoni Effect ________________________________________________________________________________________________________ I.

INTRODUCTION

Active tungsten inert gas (A-TIG) welding was invented in the 60s by researchers at the Paton Electric Welding Institute (PWI) in Ukraine. By applying a thin coating of an activating flux to the surface of the material before welding, the A-TIG process can produce a drastic increase in weld bead penetration. In A-TIG, flux is pasted onto the surface of the base material and filler wire is used. To improve the penetration of TIG welding, thorough analysis has been done A-TIG welding (Active flux TIG welding) because the weld shape is sensitive to the free oxygen content in the weld pool. A-GTAW process can achieve, in a single pass, a full penetration weld in stainless steel up to 10mm thickness without the use of bevel preparation and the addition of filler wire.

The weldment aesthetics are observed to be unaffected. Although the A-TIG welding is still under consideration, the effects of the oxide flux quantity on the weld penetration showed that the arc constriction and the reversed Marangoni convection on the top surface of the weld pool were the main mechanism for changing the weld shape according to the experimental investigation.

Comparison has been made between mechanical and metallurgical properties of welds with flux and without flux. The A-GTAW process has been exploited to improve production efficiencies in a wide of industries, including power generation, chemical, aerospace and marine manufacturing and in nuclear power plants.

Activated flux is used in the A-TIG welding, which is the only difference from the conventional TIG welding. Activated flux can be prepared by using different kind of component oxides packed in the powdered form with about 30-60 Îźm particle size.

These powders mixed with acetone, methanol, ethanol etc to produce a paint-like consistency. Before welding, a thin layer of the flux, brushed on to the surface of the joint to be welded. The coating density of the flux should be about 5-6 mg/cm². Activated TIG welding can increase the joint penetration and weld depth-to-width ratio, thereby reducing the angular distortion of the weldment.

Fig. 1: Schematic diagram of TIG welding with activating flux.

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A Review Paper on A-TIG Welding Process (IJSTE/ Volume 3 / Issue 09 / 065)

II. MECHANISMS OF A-TIG WELDING PROCESS According to literature there are two proposed mechanisms associated with the working of Activated flux in GTAW process.

Both of them are explained below.

Reverse of Marangoni Effect The ability of flux to wet the surface of the molten pool effects on the composition which in turn modifies the surface tension.

Change in fluid flow is related to Thermal Coefficient of Surface Tension (TCST) of the molten pool: If the TCST is negative, the cooler peripheral regions of pool will be having a higher surface tension than the centre of the weld pool and the flow of the molten metal will be directed outwards creating a wide shallow weld pool.

Heiple and Roper proposed that surface active elements such as oxygen, sulfur and selenium could change the temperature coefficient of the surface tension for iron alloys from negative to positive and further influence the direction of the fluid flow in the weld pool.

So for materials having a positive gradient, the flow of the molten metal will be reversed to the centre of the weld pool and in the centre the molten metal flows down. This creates a narrower deeper weld pool for precisely the same welding conditions. For stainless steels, sulphur can change the TCST and hence alter the penetration depth of the resulting weld. Other elements such as calcium or aluminum, also affect penetration, though these elements probably tie up the elements affecting TCST, such as sulphur, rather than having a primary affect themselves. It is believed that the activating flux had a similar effect on the weld pool.

Fig. 2. Marangoni convection mode in the weld pool a) conventional GTAW and b) A-GTAW

Arc constriction

Fig. 3: Arc constriction

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A Review Paper on A-TIG Welding Process (IJSTE/ Volume 3 / Issue 09 / 065)

As the conductivity of the flux is much lower than that of the metal vapors, and the melting and boiling points of the flux is higher than that of the weld metal, the metal evaporation would be generated only in the central regions of the welding arc, where the temperature is higher than that of the dissociation temperature of the flux compounds, which leads to a reduction in the conductivity area of the anode spot.

III. THEORIES REGARDING HIGH PENETRATION IN A-TIG WELDING PROCESS. There is still not agreement today in the explanation of the mechanism of the activating fluxes. In the latest quarter century four different theories were published, which are made known in short forms in the following.

Theory of Savitskii and Leskov 1980 This theory says that the activating fluxes decrease the surface tension of the weld pool which makes able the arc pressure to cause a deeper invading in the pool. This invading helps the arc pressure to reach a deeper penetration.

Theory of Heiple and Roper 1982 This theory explains the high penetration with the reversed Marangoni-effect . The convective flowing of the molten metal from the centre towards the edge is the phenomenon that is called Marangoni-effect. It is working when the gradient of the surface tension of the weld pool is negative. This theory says that the activating fluxes changes the gradient of the surface tension from negative to positive that results that the flowing of the molten metal turns in the opposite direction and flows towards the centre.

This is how the deep penetration obtained by the reversed Marangoni-effect.

Theory of Simonik 1976 Simonik says that oxide and fluorine molecules are present in the activating fluxes which have affinity to chain the free electrons at the edge of the plasma of the arc. It is well-known that the ions formed this way have substantially lower mobility than the free electrons. This leads to increased current density in the centre of the arc by means of the higher movement of the free electrons.

This results better focusing of the arc which leads to the deeper penetration.

Theory of Lowke, Tanaka and Ushio 2005 They explain the deep penetration by the means of the higher electric insulation of the activating fluxes. Thankful to this the arc is able to break through the surface (and the flux on it) at a narrower area. This means that the focus of the arc increases which leads to higher current density in the arc spot and this causes the deep penetration.

IV. ADVANTAGES OF A-TIG WELDING The A-TIG process is suitable for any position welding. By employing A-TIG process, overall welding costs can be reduced upto 50%. These economics in fabrication costs can be achieved through:  Reduction in bevel preparation requirements  Decrease in number of weld passes  Shortening of welding times  Reduced consumption of welding filler wire  Elimination of back gouging and grinding  Reduced distortion V. DISADVANTAGES OF A-TIG WELDING Despite the productivity benefits of A- GTAW, industry to date has been slow in exploiting this process owing to the following reasons:  The use of the flux is seen as an additional cost and its application an additional operation.  The commercial fluxes tend to produce an inferior surface finish compared to conventional TIG welding in mechanized welding operations but in manual welding operations, the surface roughness is similar.

VI. APPLICATIONS    

Pipes and tubes in nuclear industry.

Fabrication of pressure vessels and tube to tube sheets in heat exchangers Power and chemical industries, Hydraulic cylinders and undercarriage legs in aerospace industry.

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A Review Paper on A-TIG Welding Process (IJSTE/ Volume 3 / Issue 09 / 065)

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