Edited from an article written by Ron Scott, Director of Training at Hobart Institute.

Gas tungsten arc welding is commonly used for welding titanium. Preweld cleaning is essential because even a fingerprint on titanium workpieces can cause embrittlement. Scrubbing with lint-free cloths or cellulose sponges using household detergents will remove the dirt. Acetone, ethanol, toluene, or methyl-ethyl ketone will remove stubborn grease or paint. Follow the solvent cleaning by brushing with a new stainless-steel-wire brush to remove light oxide films that form during heating. Light grinding, draw filing, or acid pickling also removes oxide films. When working with chemical solutions, make sure to wear rubber gloves and take care not to leave the solution on the workpiece for very long: the acid can boil spontaneously and explode. Once cleaned, the material should be handled as little as possible. Cover stored work with paper or plastic. Just prior to welding, brush with the stainless-steel-wire brush and rinse with acetone. Also degrease and clean fixtures.

Current is direct current electrode negative with high frequency arc start. The most commonly used shielding gases are argon and helium. The American Welding Society specifies consumables for titanium welding in A5.16, Specification for Titanium and Titanium Alloy Welding Electrodes and Rods. Welded titanium must be shielded from the atmosphere until the weld metal cools to below 800 degrees F. Above that temperature, titanium is highly sensitive to embrittlement. For critical applications, fabricators weld in a gas-tight chamber purged of air and filled with argon or helium or pumped to a vacuum.

A water-cooled torch is recommended. However, in field-welding situations, a low-amperage gas-cooled torch suffices. Ceramic gas cups, ¾-inch diameter, with gas lenses help to direct shielding gas. In general, use the largest nozzle that allows good visibility and access to the weld area, and use thoriated tungsten electrodes, the smallest diameter that can carry the necessary current. Limit electrode extension to 3/8-inch to avoid weld contamination by atmosphere and for good visibility of the weld pool.

At the end of welding, ensure that shielding gas flows directly over the cooling weld behind the torch by equipping the power supply with a foot control, or use a torch-mounted pushbutton contactor control that allows the welder to break the arc without moving the torch away from the work. Keep the heated end of the rod under the gas nozzle until the rod has cooled. If the end of the rod becomes contaminated, cut it off before welding continues.

Primary shielding gas flows through the torch to protect the molten weld pool and adjacent base metal. To protect the hot, solidified weld metal and the heat-affected zone immediately behind the travelling welding torch, secondary gas flows behind the torch. Backing gas shields the backside of the weld joint during welding. Primary shielding gas is typically argon. A mixture of argon and helium for primary shielding creates a hotter arc and deepens penetration. To direct secondary shielding gas to the cooling weld metal, operators fit a custom-made trailing shield to the torch. Trailing shields consist of a metal chamber held by a clamp to the torch nozzle. The gas flows over the weld area through a porous diffuser screen on the chamber. The diffuser screen must be wide enough to direct gas to the heat-affected zone on both sides of the weld bead and long enough to keep the cooling weld shielded until its temperature drops below 800 degrees F., even at the highest travel speeds used. Set flow rate of trailing gas carefully - too high, gas flow can become turbulent and draw in atmospheric contaminants.

If position of the trailing shield device hinders torch manipulation, try another means of secondary shielding. With slow welding speeds, a larger torch nozzle or an auxiliary annular nozzle can direct secondary shielding gas to the weld.

Backing shielding gas can flow through passages in the fixturing or through a groove in a copper backing bar. When field welding makes fixtures impractical, shop-fabricated bars or tents of copper, aluminum, stainless steel, or plastic taped over welds can direct shielding gas to the backside of the weld. Bars or tents require an inlet opening for the inert gas to flow to the weld and an outlet for the displaced air and gas to escape.

When welding vessels, pipes, and other work within enclosed spaces, all air must be purged from the workpiece prior to welding. As a rule of thumb, the volume of inert gas needed to remove the air from an irregular space is 10 times the volume of the enclosure. Gas flow into a tent should be 5 - 10 ft.3/h during welding, continuing until the weld has cooled. As an alternative to continuous-gas-flow shielding, an airtight chamber filled with inert gas can offer shielding to welds when standard shielding devices are impractical. The chambers are flow-purged or vacuum purged prior to filling with inert gas. Access to work in chambers is through gloved ports.

Joint designs for welding titanium are similar to those used for other alloys, with close attention paid to the need for adequate access for shielding gas, fixture design, the method of welding (manual or machine) and inspectability of both sides of the joint. Uniform joint fitup controls the shape of the root pass and minimizes burnthrough. Joint edges should be smooth, clean, and free of contamination.

After the joint and filler metal rod have been cleaned, purge the torch and trailing and backup shields to flush out air and moisture. Use high-frequency arc start or starting tabs, rather than scratch start, to avoid tungsten contamination.

Before beginning production welding, test to evaluate procedures and techniques. Check effectiveness of gas shielding by welding on a scrap piece of titanium. The color of the resulting weld, which will indicate adequacy of primary shielding, should be silvery and metallic. A bronze, blue or purple weld signals insufficient shielding or contaminated gas. A whitish-gray flaky weld surface also indicates serious contamination. A light straw or bronze color to the weld indicates superficial contamination, easily removed with a stainless-steel-wire brush.

If the weld passes the color test, try a progressive-radius bend test to evaluate ductility. Test the weldment with the weld axis perpendicular to the bend axis, for uniform straining of the weld metal and heat-affected zone. Other weld tests include checks for impact strength, notch toughness, and hardness.

Welded titanium may be found in nuclear plants, power plants, refineries, pulp and paper plants, desalination plants, flue-gas desulfurization units, aircraft and aerospace vehicles, medical devices, architecture, automotive vehicles, and recreation equipment.

Original article printed in Welding Design and Fabrication, 1992 (12) 42-46. Penton Publishing Co., Cleveland, Ohio.