Titanium Welding Purge Procedures: Trailing Shields, Back Purge, and Oxygen Monitoring

Every titanium weld surface that’s above 500F must be covered with inert gas. That means the torch side (primary shielding), the trailing zone behind the torch (trailing shield), and the root side of the joint (back purge). Miss any one of these three zones, and the weld absorbs oxygen and nitrogen from the air, turning brittle. There’s no recovering a contaminated titanium weld with additional passes or heat treatment. You grind it out and start over.

This guide covers the hardware design, gas flow rates, and oxygen verification procedures for all three purge zones.

The Three Purge Zones

For full enclosure situations (glove box or purge chamber), a single argon atmosphere replaces all three zones. Glove boxes are the gold standard for aerospace titanium but aren’t practical for large fabrications or field work.

Primary Shielding: The TIG Cup

The TIG torch cup provides primary shielding over the active weld puddle. Standard practice for titanium:

• Gas lens is mandatory. A gas lens (mesh screen collet body) produces laminar gas flow rather than turbulent flow. Laminar flow stays coherent longer and covers a wider area.
• Large cup diameter. Use a #10 (5/8 inch) to #12 (3/4 inch) cup minimum. Larger Pyrex or ceramic cups (#14 to #20, sometimes called “jazzy” or “Furick” cups) extend coverage significantly.
• Flow rate: 15-25 CFH depending on cup size. Too much flow creates turbulence and pulls air into the gas column. Too little leaves gaps in coverage. Set the flow so you can feel gentle, steady gas on the back of your hand held 1/2 inch from the cup.
• Gas purity: 99.997% argon minimum (industrial grade). Welding-grade argon at 99.998% is preferred. Don’t use argon-CO2 or argon-O2 blends. Even 0.1% oxygen in the shielding gas contaminates titanium.

Check all gas lines for leaks. A fitting that leaks a tiny amount of air into the argon stream may be invisible on steel welding but ruins titanium welds. Soap-bubble test every connection.

Trailing Shield Design

The trailing shield extends argon coverage 4-8 inches behind the TIG torch, protecting the weld bead and HAZ as they cool from welding temperature down through 500F. Without a trailing shield, even a perfect weld puddle turns blue or gray within seconds of the torch passing.

Commercial Trailing Shields

Commercial trailing shields clamp to the TIG torch body and trail behind on a flexible arm or solid bracket. They consist of:

• A gas distribution chamber (usually aluminum or stainless steel)
• A porous diffuser screen (sintered metal or mesh) on the bottom
• An argon inlet fitting connected to a separate gas supply
• Adjustable mounting hardware

Flow rate: 25-45 CFH through the trailing shield, depending on its size and the travel speed. Faster travel speed means the weld bead moves through the shield coverage faster, so you need more flow to maintain protection across the entire cooling zone.

Shop-Built Trailing Shields

A shop-built trailing shield works just as well as commercial ones:

1. Start with a piece of copper or aluminum channel, 2-3 inches wide and 6-8 inches long.
2. Drill or punch small holes (1/16 inch) in the bottom, spaced 1/4 inch apart, for gas diffusion. Alternatively, epoxy a layer of stainless steel mesh across the bottom.
3. Seal one end. Connect an argon fitting at the other end or along the top.
4. Line the inside with stainless steel mesh or copper wool to distribute gas evenly.
5. Attach to the torch body with a flexible arm or clamp.

The key design requirements: even gas distribution across the full width and length, and a gap of 1/8 to 1/4 inch between the diffuser and the workpiece surface. Too tight against the surface, and the gas backs up. Too far away, and air mixes in.

Trailing Shield Sizing

Slower travel speeds (thicker material, more heat input) require longer shields because the weld stays hot longer and needs coverage for a greater distance behind the torch.

Back Purge Procedures

The root side of the joint is just as susceptible to contamination as the face side. Back purging fills the space behind the joint with argon, displacing air before you start welding.

Pipe Purging

Pipe is the most common titanium back-purge application. The standard approach:

1. Install purge dams. Place soluble (water-soluble PVA) or foam dams 3-6 inches from each side of the weld joint. The dams should be snug enough to hold gas pressure but not airtight; a small vent opening on the downstream dam lets displaced air escape.
2. Connect argon supply. Feed argon through one dam (inlet side) at 10-15 CFH. The outlet dam has a small vent hole (1/4 inch diameter for most pipe sizes) to let air escape as argon fills the cavity.
3. Pre-purge. Flow argon for a minimum of 5 volume changes. Calculate the volume between dams:
   • Volume (ft3) = pi x (ID/2)2 x dam spacing, converted to cubic feet
   • Time (min) = (5 x Volume) / Flow rate in CFM
4. Verify oxygen level. Insert the oxygen analyzer probe into the vent hole. Don’t start welding until oxygen reads below your target.
5. Maintain purge during welding. Keep argon flowing throughout the root pass and at least one fill pass. Reduce flow to 5-8 CFH after the root is sealed to avoid excessive pressure that causes root concavity (suck-back).
6. Remove dams after welding. Soluble dams dissolve during hydrotest or service. Foam dams are pushed out or left in place on non-critical pipe.

Sheet and Plate Purging

For open sheet and plate work where you can’t seal the root side into a pipe-like cavity:

• Backing fixtures: Build a copper or aluminum backing bar with argon channels machined into the surface. The channels distribute gas under the joint line. Cover the channels with porous tape, mesh, or drilled cover plates.
• Backing trays: A simple tray (aluminum angle or channel) under the joint, sealed on the sides, filled with argon from one end.
• Purge chambers: For critical work, build an enclosed purge chamber from aluminum sheet or clear acrylic (for visibility). The entire back side of the workpiece sits in the argon-filled chamber.

Flow rate for backing fixtures: 15-25 CFH, depending on the fixture volume and how well it’s sealed.

Purge Chambers and Glove Boxes

Full enclosure purge chambers surround the entire workpiece in argon. The welder works through glove ports (glove box) or uses remote viewing with a mechanized torch.

This is the aerospace approach for Grade 5 (Ti-6Al-4V) and other alpha-beta titanium alloys. It eliminates the trailing shield and back purge entirely because the entire atmosphere is argon.

Glove box requirements:

• Sealed acrylic or metal enclosure with gas-tight glove ports
• Argon inlet and controlled outlet (maintains slight positive pressure, 0.5-1.0 inches water gauge)
• Oxygen analyzer with continuous readout inside the chamber
• Pre-purge to below 50 ppm oxygen before any welding
• Viewing window or camera for weld monitoring

Oxygen Monitoring Equipment

Weld color inspection tells you about contamination after the fact. Oxygen analyzers tell you about purge quality before you strike the arc. For any serious titanium work, an inline oxygen analyzer is not optional.

Types of Analyzers

For most titanium pipe and sheet welding, an electrochemical cell analyzer in the $500-1,000 range does the job. It reads down to single-digit ppm, which is more than adequate for verifying that the purge is below 50 or 200 ppm. Calibrate per the manufacturer’s schedule (typically every 3-6 months).

Oxygen Thresholds

When in doubt, purge to below 50 ppm. The extra purge time is cheap insurance compared to grinding out and re-welding a contaminated joint.

Gas Purity Requirements

Industrial-grade argon (99.997% purity) is the minimum for titanium welding. This grade contains a maximum of 30 ppm total impurities (oxygen, nitrogen, moisture). Welding-grade argon at 99.998% or higher is preferred.

Gas supply system checks:

• Use dedicated regulators and hoses for titanium work. Regulators previously used on CO2 or mixed-gas systems may have residual contamination.
• Purge the gas line (torch hose, trailing shield hose, back-purge hose) for 30 seconds before connecting to the work fixture. This displaces air trapped in the hoses.
• Check for leaks at every fitting. A soap-bubble test at every connection point is worth the 5 minutes it takes.
• On manifold systems, verify that no other gases (CO2, O2, mixed gas) can cross-contaminate the argon supply.

Moisture in the argon supply is as damaging as oxygen. At welding temperatures, water vapor breaks down into hydrogen and oxygen, both of which contaminate titanium. Gas suppliers certify moisture content on the cylinder analysis. Dew point should be -76F or lower (less than 5 ppm moisture).

Purge Verification Checklist

Before striking the arc on any titanium joint, verify all of the following:

1. Oxygen analyzer reads below target threshold at the back purge outlet
2. Trailing shield is attached, positioned correctly, and flowing argon
3. TIG cup is the right size with gas lens installed
4. Primary argon flow rate is set (15-25 CFH)
5. Trailing shield flow rate is set (25-45 CFH)
6. Back purge flow rate is set (10-20 CFH for pipe, 15-25 CFH for fixtures)
7. All gas connections leak-tested
8. Joint and filler rod are clean (acetone wiped, no fingerprints)
9. Tungsten is properly ground (no contamination from previous steel work)
10. Welding gloves are clean (no oil, grease, or shop grime)

For understanding what weld color tells you about contamination after welding, see the titanium weld color chart. For base metal and filler details on commercially pure grades, see the welding CP titanium guide.

Back to the titanium welding category.