TIG Welding (GTAW)

TIG welding uses a tungsten electrode that does not melt, plus a filler rod that you feed by hand. Its formal name is gas tungsten arc welding (GTAW). It is the cleanest and most precise arc process, and it can weld almost any metal. It is also the slowest and needs the most coordination, because both of your hands and often one foot are doing different jobs. You will see it on stainless tube, aluminum, thin parts, pipe roots and anything that needs a clean, high-quality weld.

What You Will Be Able to Do

• Choose and grind a tungsten for the job.
• Set polarity, amperage and gas for steel, stainless and aluminum.
• Coordinate the torch, filler and foot pedal.
• Protect the weld and the tungsten with gas coverage.
• Read weld color and fix common TIG problems.

How the Equipment Works

Air-cooled torches handle lighter work, and water-cooled torches are smaller and handle more amps. A gas lens lets you stick the tungsten out farther for tight joints, and it improves coverage, so use one where you can.

Setting Polarity

DCEN means direct current, electrode negative. AC is alternating current. On AC, the half of the cycle when the electrode is positive breaks up aluminum oxide, which is why aluminum uses it. Modern machines have an AC balance control, which sets how much of the cycle goes to cleaning and how much to penetration.

Choosing Tungsten

Two percent lanthanated is the best all-purpose choice for both AC and DC, so many shops use it for everything.

Warning: Thoriated tungsten contains thorium, which is slightly radioactive. The main concern is breathing the dust when you grind it. Use a lanthanated or ceriated tungsten if you can. If you must use thoriated, grind it with dust control and do not breathe the dust.

Grinding a Tungsten

1. Use a grinder or wheel kept only for tungsten. A diamond wheel works best. A wheel used for steel will contaminate it.
2. Hold the tungsten so the grinding marks run lengthwise, along the axis from tip to back.
3. Grind a taper about 2 to 2.5 times the electrode diameter long. A 3/32 inch tungsten gets a taper of about 3/16 to 1/4 inch.
4. Blunt the very tip slightly for higher amps, so it does not break off into the weld.

Lengthwise marks let the arc leave the point in a straight, stable line. Marks that go around the electrode make the arc wander.

For aluminum on an older transformer machine, the tip forms a small ball on its own. On a modern inverter, you can use a pointed or slightly blunted tip with the AC balance set for more electrode-negative time.

Choosing Gas and Filler

Use pure argon for most TIG work, at about 15 to 25 CFH indoors. Argon-helium blends give extra heat on thick aluminum and copper. Never use CO2 or argon-CO2 mixes with TIG, because oxygen-bearing gas will destroy the tungsten.

Match the rod diameter to the joint and keep rods clean and dry. Moisture and oil cause porosity. Don't grab a rod with oily gloves.

Setting Amperage

A handy starting rule on steel is 1 amp for each 0.001 inch of thickness. For a 1/8 inch (0.125 inch) plate, start at about 125 amps. Set the machine a bit above what you need and use the pedal to come down. Thicker metal and metals that pull heat away, such as aluminum and copper, need more.

Running a Bead

1. Prepare. Clean the metal and the rod. TIG has no flux, so it shows every bit of dirt.
2. Set the torch. Hold it like a pencil, tilted about 10 to 20 degrees back from straight up, pointing in the direction of travel. Keep the tungsten about one tungsten diameter off the metal, roughly 1/8 inch or less.
3. Start the arc. Use high-frequency or lift-arc starting. Scratch-starting tends to contaminate the tungsten.
4. Form the puddle. Hold the arc until a small, shiny puddle forms.
5. Add filler. Hold the rod low, about 15 to 20 degrees from the work. Dip it into the front edge of the puddle and pull it back. Let the puddle melt the rod, not the arc.
6. Move ahead. Dip, pull back, move the torch, dip again. This rhythm sets your ripple spacing.
7. Keep the rod in the gas. Pull it back only as far as the edge of the gas shield. If you pull the hot tip out into the air, it oxidizes and you carry that into the next dip.
8. Use the pedal. Back off near thin edges, and add heat on tacks and heavy sections.
9. End cleanly. Slowly ease off the pedal while you add a last bit of filler to fill the crater.
10. Hold the torch in place. Let post-flow gas cover the weld and the tungsten for about 5 to 10 seconds, or until the tungsten stops glowing.

Walking the Cup

On groove welds and pipe, some welders rest the cup in the bevel and rock it from side to side, moving forward a little with each rock. This gives a very consistent bead. A gas cup made for this work and a steady wrist make it easier.

Reading Weld Color

On stainless, the color tells you how well the gas protected the cooling weld.

Black, crusty oxidation on the back side of a stainless weld is called sugaring. It means air reached the root. On tube and pipe, you stop it with a back purge: argon inside the joint. On open plate, use a backing bar or purge box.

Warning: Back-purging fills pipes and vessels with argon. Inert gas pushes out oxygen, and you can pass out with no warning. Never put your head into a purged space, and ventilate it before anyone enters.

Welding Aluminum

Aluminum needs special handling:

• Clean with solvent, then a stainless steel brush kept only for aluminum.
• Weld soon after brushing, because aluminum oxide melts at about 3,700 F, far above the 1,220 F melting point of aluminum. The skin can block fusion.
• Use AC. Pre-heat large parts if the procedure allows.
• Aluminum gives little color warning. It goes from solid to liquid with no glow, so watch for the shine of a wet puddle.
• Add filler sooner than on steel, and move a little faster as the part heats up.

Troubleshooting

If you dip the tungsten, stop right away. Grind it, and grind out the spot in the weld. A contaminated tip leaves tungsten bits that show up on X-rays.

Common Mistakes

• Pulling the rod out of the gas between dips. The tip oxidizes and contaminates the next dip.
• Melting the rod with the arc. The filler balls up and drops onto the weld.
• Skipping the post-flow. The hot tungsten oxidizes and turns dark.
• Using one wire brush for all metals. Carbon steel on stainless or aluminum causes contamination.
• Using a wheel shared with steel. It puts steel on your tungsten.

Check Your Understanding

You are TIG welding 304 stainless and the bead comes out dull gray. The arc felt normal. What do you check?

Gray means weak gas coverage on the cooling weld. Check flow, leaks, cup size, post-flow time and drafts.

You need a TIG setting for 1/8 inch carbon steel. Where do you start, and what polarity do you use?

About 125 amps, using the 1 amp per 0.001 inch rule, with DCEN.

You touch the tungsten into the puddle. Can you keep going?

No. Stop, regrind the tungsten, and grind out the contaminated part of the weld. Otherwise tungsten bits stay in the weld.

Why does aluminum use AC and not DCEN?

The electrode-positive part of the AC cycle breaks up aluminum oxide, which melts far above the aluminum and blocks fusion.

Key Takeaways

• TIG uses a non-consumable tungsten and a separate filler rod.
• Use DCEN for steel and stainless, and AC for aluminum.
• Blue-band 2% lanthanated tungsten is a good all-purpose choice.
• Grind tungsten lengthwise on a dedicated wheel.
• Use pure argon at about 15 to 25 CFH, and keep the hot rod tip in the gas shield.
• Weld color shows how good your gas coverage was.