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ADVANCES IN GAS TUNGSTEN ARC WELDING PROCESSES
Author: A. Santhakumari, Addl. General Manager, Welding Research Institute, Bharat Heavy Electricals Ltd, Tiruchirappalli.
The gas shielded welding processes viz. Gas Metal Arc Welding (GMAW), Gas Tungsten Arc Welding (GTAW) and Plasma Arc Welding (PAW) are widely used in all fabrication industries. These processes are the work horses in power, energy, automotive and other strategic sectors because of high quality welds produced by these processes.
GTAW is observed generally as high quality but low productive process due to the slow travel speed and lower deposition rate with the filler additions. Hence it has been considered only for root pass welding and thin sheet welding in the past.
The choice of one would be TIG when there is chance of choosing the welding process for fabrication of consumer products because of its high quality and pleasing appearance. For the products that call for code requirements including strategic applications always select TIG as an effective process for root pass application. And in some cases which require high quality bonding for multipass application TIG process alone is being adapted for reasons of precision than the faster MIG welding. In the past, pulsed GTAW, Orbital GTAW, etc. have been developed to make the process as semi-automatic process, but the lower deposition rate and speed restricts its applications.
Significant improvements have been made in the recent past to improve the deposition rate to match with GMAW process by means of hot wire addition and twin or tandem wire techniques and also by means of automation efforts. This article intends to highlight few of the recent advances in GTAW process.
Fusion welding processes can be broadly classified as Flux Shielded Welding Processes, Gas Shielded Welding Processes and Beam Welding Processes. Each process is unique and finds wider application across industry.
The Gas shielded welding processes – Gas Tungsten Arc Welding (GTAW), Gas Metal Arc Welding (GMAW) and Plasma Arc Welding (PAW) are widely used in almost all fabrication industry.
Of the three, Gas tungsten arc welding is used in industries where uniform root penetration is desired. GTAW process can be used to weld almost all metals. It is especially useful for joining aluminum and magnesium, which form refractory oxides, and also for joining reactive metals titanium and zirconium. This process is extensively used to join stainless steels, copper, alloy steel and carbon steels.
In carbon steels, it is primarily used for root pass welding with the application of consumable inserts or open root techniques on pipes. This process finds applications where quality and reliability of welded joint is more important than only cost considerations. TIG welding is widely used to produce high quality joints required in the aerospace and nuclear industries.
More advanced ferritic- martensitic Chromium Molybdenum steels and Nickel based alloys are being developed to increase the efficiency of the product. As more and more newer materials have been developed there is a necessity arises to improve joining technologies or introduce advance techniques in the current welding processes. With the advent of power electronics and controlling devices, number of welding power source manufacturers have started developing various techniques to meet the demanding requirements of joining of these newer materials. Development of new variants like hot wire GTAW, activated TIG, continuous wire feed GTAW, etc. leads to higher productivity and enhanced the scope for its application in wide range of fields.
Gas Tungsten Arc Welding Process
|Figure 01: GTAW Process Set up.|
For at least six decades, traditional gas tungsten arc welding (GTAW or TIG) has been considered the process of choice for attaining high quality welds in any metal application. However, this process has certain drawbacks, such as the weld energy limitation influenced by the weld pool dynamics and typically slow manual wire feed rates. Manual GTAW requires highly skilled operators who possess the dexterity necessary to feed the wire. Manual GTAW techniques vary, and the weld-wire-to-arc and weld puddle placement are inconsistent.
The recent advances in GTAW which are resulting in enhanced productivity without compromising the quality are being discussed below.
Recent Advances in GTAW Process
Hot Wire GTAW & Narrow Gap Hot Wire GTAW
Historically, the advantages of the high-quality welds possible with GTAW have been offset by the process’s low deposition rates. But the hot wire method speeds up those rates, making GTAW feasible in many applications where it wasn’t before.
Unlike traditional GTAW, in which the filler wire is added cold and arc energy heats it, hot wire GTAW systems use a second power source to resistance-heat the wire so that it’s already near the melting point when it’s added to the weld puddle. This results in a much faster travel speed, compared to the cold wire method. Increased wire feed rates enable higher weld current per application, which reduces the likelihood of internal weld defects and weld fusion. The typical arrangement for hot wire GTAW is shown in Figure 02
|Figure 03: Hot Wire TIG Facility at WRI.|
Figure 03 shows the facility at WRI and dissimilar weld made by the process is shown in Figure 04
With increasing applications for newer breed of Creep Strength Enhanced Ferritic Steels and Ni-base alloys for high temperature application, the technology offers the best solution for tube and pipe joints, more so in the case of dissimilar weld joints.
To further enhance the productivity of Hot Wire GTAW, the Narrow Gap Hot Wire GTAW process has been developed as a high productive technology. In Narrow Gap GTAW, consistent side wall fusion is able to achieve with special torch, power source and controller. Number of techniques such as tilting, twisting the tungsten electrode have been developed to ensure side wall fusion in narrow gap. This can be used for heavy wall thickness jobs of pipes, turbine Rotor Shaft, ship building, etc. Stainless steel, duplex stainless steels, nickel-based alloys, and reactive metals such as titanium can be welded by narrow gap hot wire GTAW.
Hotwire narrow gap GTAW offers excellent mechanical properties, high weld quality and high efficiency. The volume of weld metal deposited and total heat input to the weld are lower than conventional TIG. The process offers better economy because of reduced consumable requirements and shorter welding times. There is low angular distortion because the joint preparation is almost parallel-sided. Figures 05 & 06 show a typical arrangement for hot wire narrow gap GTAW and the process.
Orbital Hot Wire GTAW
Orbital GTAW process became practical for many industries when combination power supply/control system were developed /operated from 110 V AC. A mechanism was developed in which the arc from a tungsten electrode was rotated around the tubing weld joint whereas the work piece is kept constant. An Orbital welding system consists of power supply and an orbital weld head. The Orbital welding heads are of mainly three types – Fully Enclosed Fusion Weld Head; Open Arc weld heads; wire feed weld heads.
Orbital Welding System gives the higher productivity, quality, consistency; lower skill level and lesser space for access are some of the advantages of the process. If you cannot rotate the job, then the next best option is rotating the torch.The space constraint may be an issue in some cases to use the method. The equipment is also expensive. All the same, there are many successful examples of its usage in aircraft & aerospace Industry, nuclear, boiler and high pressure tubes, etc.
The advent of hot wire GTAW process lead to development of orbital hot wire GTAW process and is being used for productive applications. Figure 07 shows a typical set up for orbital hot wire GTAW of pipes.
Activated TIG Welding
Activated TIG or ATIG Welding was first reported by the E.O Paton Welding Institute (PWI), Ukraine in the middle of 1960s and got popularized in the 2000s. More recently the Edison Welding Institute (EWI), US, has developed an alternative range of Active Flux. Recently the Welding Research Institute (WRI) and IGCAR India have developed their indigenous flux for stainless steel grade material. The Activated TIG (A-TIG) welding process involves application of a thin coating of flux material onto the joint surface prior to welding to enhance the weld penetration.
Applying of a thin layer of active flux on the top surface of the weld seam (Figure 07) and then running the TIG arc over that is found to enhance the penetration two to three times. However each material such as carbon steel, stainless steel, aluminum requires active flux of different composition. This process works well in automated TIG welding only as the penetration is the ATIG process is sensitive the arc length variation. The process is found to be beneficial in orbital pipe welding applications.
The activated TIG fluxes are commercially available for C-Mn steel, Low Alloy Steel, Cr-Mo steel, Stainless Steel and Nickel Alloys.
Figure 08 shows the application of the process. The A-TIG process offers increased depth of penetration, up to 12 mm thick stainless steel can be welded in a single pass. This also reduces weld shrinkage and distortion, as filler metal is not needed. There is also reduction in welding time, as welding is done with square butt weld edge preparation.
Automated Wire Feeding in GTAW Process
Emergence of this process changed the thinking of people about GTAW. This welding process, is a manual and automated GTAW wire feed control combined with a hot-wire power source that operates with any GTAW. Suitable for all-position welding on materials of any thickness, the process addresses traditional GTAW limitations and can enhance both manual and automated TIG weld quality and productivity. GTAW with automated wire feeding known by trade names viz. TipTIG / TopTIG has been developed to overcome the directional limitation posed by the cold/hot wire GTAW process and enables the completion of manual, semi-automated and/or automated welds, with traditional GTAW weld quality and productivity levels approaching other wire feed processes like GMAW. TipTIG enable automation of cold/hot wire GTAW process. Additionally this also attracts manual applications of the process for pipe welding and also for repair welding. The advantage is a high-speed linear oscillation of the four-roll drive plate that is super-imposed onto the wire. This dynamic oscillation creates a vibration when filler is introduced to the molten weld pool, defeating surface tension. This enables better fusion, allows impurities and gases to escape more readily, and improves wetting and overall weld-ability resulting in higher deposition rates.
Inter Pulse TIG Welding
Inter Pulse TIG is a patented technology from VBC Group, UK. The specially designed Inter Pulse TIG power source provides a highly constricted, fine welding arc which enables welding of “difficult to weld” materials such as super-alloys of gas turbine components. It also allows out of chamber welding of titanium with minimal trailing shield protection. In Figures 09 and 10, the DC TIG arc and the inter pulse arc are compared. It can be observed that the inter pulse arc is constricted and results in small HAZ.
Figure 10: DC TIG Arc 85 Amps (Average Current)
Note. Large arc and extended HAZ.
K-TIG (Keyhole TIG) Technology a single-pass, full-penetration keyhole welding technologywas originally developed by Dr. Laurie Jarvis in 1990-93 (in Australia). CSIRO, which had quickly recognised the potential of keyhole welding, took steps to patent the system. The first successful generation of a GTA keyhole was achieved on 5 mm duplex stainless steel. The first industrial application began in late 1997 for the welding of rail wagons. The process is now being used by various industries in Australia, USA, Europe and Korea.
The fundamental behind this process is intentional increase in arc pressure to the point where it extends the crater to the bottom of the pool. The surface of the weld pool will become anchored to both top and bottom surfaces to form a stable structure. This stable arc is moved along the weld path as keyhole. Figure 12 shows a typical KTIG arc and the macro of a weld is shown in Figure 13.
Twin Wire / Tandem GTA cladding Process
For cladding applications, GTA process was not recommended widely due to low productivity though we could achieve lower dilution level with this method.
Increasing GTAW performance or weld deposition rate is regularly joined by increasing weld current, rising arc force or arc pressure, respectively. The later again is susceptible to cause weld defects, such as undercut or bead humping. To cope with these limitations Yamada in the late 1990’s developed and patented a novel high-efficiency GTAW method. Both electrodes, independently operated by two power supplies and electrically insulated to each other are paired in one weld torch. Feeding hot-wire to the weld pool allows for increased weld performance; i.e. weld deposition rate e.g. in producing large 9% Ni-steel storage tanks. Electrode geometry and adjustment are stated among the specifics of this method. This method also known as multi-cathode GTA, has early been tested to improve both process efficiency and weld quality. Multi-cathode GTA capable of significantly increasing weld travel speed and, by elongating the weld pool, preventing weld defects such as undercut.
Further research leads to development of tandem GTA cladding process with twin hot wire. Two tungsten cathodes combined with two preheated welding wires, all independently controlled, to create one molten pool allows less penetration while enabling faster welding speeds and a much higher deposition rate. Maximum productivity is guaranteed, especially when welding larger components.
Increased welding speed of about 120 cm / min and deposition rate of 6 kg / hr could be obtained by tis high productive process with gas consumption about 50% comparing to conventional GTA cladding. Typical applications of this process includes Subsea components, Valve components, Bearing seal surfaces, Pump components, Turbine blades, CRA pipes, elbows, Extruders, Mining bits, Forging dies, Rolls, etc. Figure 14 shows the cladding in progress and Figure 15 shows the typical electrode and wire arrangement.
In another related variant as shown in Figure 16, two torches are attached to the same weld head to produce two individual TIG arcs. The two arcs are positioned diametrically opposite and hence have little heating influence on each other. An important requirement in weld cladding is that the two arcs produce weld deposits with similar fusion levels, iron content and heat inputs. This is difficult to achieve as the flow of the weld pool in the vertical (2 G welding position) is governed by gravitational forces and surface tension effects.
Internal TIG welding
There are requirement for fabrication product such as Pipe to stub tubes in which the stubs have to be welded on the pipe with the required geometrical dimensions. To join the stub tube to pipe an Internal TIG welding system (Figure 17) has been developed by WRI with a servo motor drive with operational control governed by a Programmable Logic Controller (PLC) handling all inputs and outputs. The inputs to PLC are welding voltage, welding current and gas flow rate. There is provision in PLC for handling expansion options for communication ports, other analog and digital outputs. All operating parameters are also connected to the remote control module for ease of operation and control. The operations can thus be accessed through either remote pendant or from the PLC main panel of control board. Torches are designed to a specific height with an option of tolerance catering to ± 10 mm. This may be used to adjust the tolerance of stub dimensions and positioning. The stubs with 28 mm inner diameter have been tried out and complete fusion inside have been achieved with this development.
Many of the developments mentioned above are being exploited commercially and the tandem overlay and internal TIG and clad processes are peeping into the shop floors. The above high performance processes will result in enhanced productivity with the highest quality of GTAW process.