Chapter 7: Gas Tungsten Arc Welding

Chapter 7

Gas Tungsten Arc Welding

Have no fear of perfection.You will never achieve it.
—Salvador Dali

Section I – Process Overview

Introduction

Russell Meredith at Northrop Corporation in California refined tungsten arc welding (GTAW) into a workable process early in WWII. It replaced oxyacetylene welding of aircraft aluminum and magnesium, but it wasn’t until the 1950s that commercially available equipment reached the market.

Although gas tungsten arc welding accounts for just a few percent of filler metal consumed, it is an important, and in many applications, an irreplaceable process. Most welds made by GTAW are on steel, stainless steel, aluminum and magnesium under 1/8" thick, but it can weld all pure metals and most alloys. A few alloys, such as brass, cannot be welded because one alloy component is vaporized before another melts.

GTAW differs from other welding processes because:

  • Its very high temperature arc is concentrated in a small area, so the heat-affected zone is smaller when compared with other welding processes.
  • Because the welder positions both the filler metal and the torch independently, and can adjust the torch heat during the welding process, GTAW provides more control over the weld pool than any other process.
  • The welder can adjust torch heat with his foot pedal or torch-mounted control. This ability to add heat when starting a weld and reduce it as the bead progresses gives GTAW unique capabilities. In some ways the GTAW process can be thought of as a high-temperature oxyacetylene torch with a heat-volume control.
  • A sustained electrical discharge through a plasma arc between the work and a non-consumable tungsten electrode provides the welding heat. No filler metal is transferred across this arc.
  • In addition to providing heat, the arc removes oxides from the workpiece through ionic bombardment and electron flow out of the work. This oxide removal is essential to the welding of aluminum and magnesium.

Although the majority of GTAW welding is performed manually, it has been automated too. Some processes use filler metal and some are autogenous—the parts to be joined are fused together without adding filler metal. Thin-walled stainless steel tubing for food processing and semiconductor manufacturing is usually joined this way under automation.

GTAW is commonly called TIG welding from Tungsten Inert Gas welding. Early in its development, it was called Heliarc from its use of helium shielding gas. In Europe it is called WIG for Wolfram Inert Gas after the German name for the element tungsten, which is used for electrodes.

How GTAW Works

Figure 7-1 shows an air-cooled torch performing the GTAW process:

  • A low-voltage, high-current continuous arc between a tungsten electrode and the workpiece flowing through an inert shielding gas produces the welding heat. Temperatures in the arc can exceed 35,000ºF. Although the electrode is called non-consumable, it does gradually wear away over time in a process called burn-off, but it does this very slowly so the tungsten in the electrode is not a factor in the weld metallurgy.
  • The welding arc has a point-to-plane geometry between the arcing end of the electrode and the weld pool. Regardless of electrode polarity, the arc is restricted at the electrode end and widened out at the weld pool. When the arc length becomes too long, the arc is extinguished. The temperature and diameter of the arc depend on the arc-current level, the shielding gas, the electrode tip shape, and the alloy.
  • An inert gas, usually argon or helium from a high-pressure cylinder, flows out and surrounds the tungsten electrode forming a gaseous shield over the work. This shielding gas:
  • Prevents atmospheric contamination of the molten weld metal, but does not react with the weld metal.
  • Provides an easily ionized and stable arc path between the electrode and the workpiece.
  • Cools the electrode, the torch, and the collet body so they do not melt.
  • On thicker materials, the welder adds filler metal to the weld pool as the weld progresses. The filler metal can be in the form of a rod, a wire or a strip of metal. To add very small quantities of filler metal to parts being repaired without disturbing the surrounding structures, electrode wire as small as 0.010" can be used. For the repair and refurbishment of injection plastic molds, jet engine components, and tool steel parts, GTAW is the only choice.

Figure 7-1. An air-cooled GTAW (TIG) torch.

Metals Welded

GTAW welds nearly all metals: ferrous, non-ferrous and precious. The notable exceptions are brass, pot metals, and the leaded steels. Brass and pot metals contain zinc, which comes out of the alloy and vaporizes before the alloy’s other constituents melt. Leaded steels, like 12L14, are chosen for their excellent machining properties, but because they contain a mere fraction of one percent of lead, these steels have a similar problem—the lead vaporizes before the steel melts. For this reason leaded steels cannot be TIG welded.

Although there are a few exceptions, most members of the Series 2000 and Series 7000 aluminum alloys cannot be TIG welded because the zinc content creates pinholes in the weld, but most other aluminum alloys weld without problems.

Many aluminum alloys develop their strength from heat treatments and complex alloys. Welding heat usually removes the positive effects of this heat treatment by annealing the aluminum. Even if the aluminum welds well, remember that welding heat can severely weaken the base metal.

Advantages of GTAW (TIG)

  • Because the heat source and filler metal are controlled independently as the weld progresses, welds which require different heat inputs at different points can be made without burn-through or inadequate penetration—a common problem in repair work and on complex parts.
  • GTAW welds are of high quality, free of spatter, and usually are free of inclusions and defects.
  • GTAW can weld nearly all metals and most alloys and is especially effective in joining refractive metals such as aluminum, magnesium and titanium.
  • All weld positions are possible.
  • GTAW allows precise control of the welding variables.
  • Excellent welder visibility of the arc and the weld pool because there is no smoke.
  • On thinner work, autogenous welds—those without additional filler metal—can be used.
  • GTAW provides excellent welder control of root-pass weld penetration, particularly on critical pipeline and structural welds.
  • Joins dissimilar metals.
  • TIG welds on autobody steel are softer and more malleable than GMAW welds, making them more suited to forming and shaping without cracking.

Disadvantages of GTAW

  • Higher skill—hand dexterity and coordination—are required than for other welding processes. The welder must control both the torch and the filler metal position. With complex and demanding work, the welder must also control the arc current with a foot pedal or a fingertip control as the weld bead progresses.
  • GTAW has lower deposition rates and productivity compared with other processes.
  • Equipment is more complex and more expensive than other processes.
  • Workpiece metal must be clean because GTAW has a low tolerance for contaminants in filler or base metals. Unlike SMAW and FCAW processes, GTAW will not burn through paint, dirt and rust. Even very small amounts of surface contaminants can cause weld defects.
  • Shielding gas can be blown away in drafty environments.
  • Less economical than consumable-electrode arc processes for workpieces greater than 3/8" thick.
  • Tungsten inclusions can occur if the electrode touches the workpiece, which is common with less experienced welders.
  • Water-cooled torches can leak and contaminate the welds, and the water leakage can create dangerous shock hazards.

 

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