How To TIG Weld Copper-Nickel Alloys

How to TIG Weld Copper-Nickel Alloys: A Professional Guide

Copper-nickel alloys, often called CuNi or cupronickel, are widely used in marine engineering and desalination because they resist saltwater corrosion and biofouling. For a professional welder, learning to TIG weld these alloys can open the door to demanding industrial work. CuNi shares some traits with stainless steel and pure copper, but its thermal and metallurgical behavior is different. That means you need careful preparation, controlled heat, and clean shielding. This guide covers the main steps for producing sound, high-quality joints, including those subject to X-ray inspection.

Understanding Copper-Nickel Metallurgy and Grades

Before you strike an arc, it helps to understand how copper-nickel behaves. Pure copper conducts heat extremely well, so it acts as a large heat sink and often requires high amperage. Adding nickel lowers that thermal conductivity. CuNi therefore behaves more like austenitic stainless steel under the torch, although its molten pool can be more fluid and sluggish. The alloy is ductile and moderately strong, but contamination from lead, sulfur, or phosphorus can cause cracking.

Two grades are common in industrial work: 90/10 (C70600) and 70/30 (C71500). The 90/10 grade contains about 90% copper and 10% nickel and is widely used in seawater piping. The 70/30 grade offers greater strength and corrosion resistance for applications such as high-pressure heat exchangers and submarine components. Both grades are normally joined with Direct Current Electrode Negative (DCEN), and their preparation practices are similar. Still, you need to confirm the grade because it affects filler selection and heat-input control.

Essential Surface Preparation and Joint Design

With copper-nickel, the result is often decided before the torch is turned on. Oxides, grease, oil, paint, and marking residue can cause porosity or lack of fusion. The oxide layer is refractory, so it melts at a much higher temperature than the base metal and can become trapped in the pool. Remove it immediately before starting with clean, dedicated tools.

• Degreasing: Clean the joint thoroughly with an approved solvent, such as acetone or alcohol, to remove cutting fluids, oil, and marking crayons.
• Oxide Removal: Use a dedicated stainless steel wire brush or carbide burr to clean within one inch of the joint on both the face and root sides.
• Tool Segregation: Never use grinding discs or brushes that have touched carbon steel or aluminum. Iron contamination can damage the CuNi surface and reduce its corrosion resistance.
• Edge Preparation: Material thicker than 3mm (1/8 inch) typically needs a V-groove with a 60 to 75-degree included angle for full penetration.

Fit-up matters just as much because it helps control burn-through and distortion. Copper-nickel expands in a way that is similar to stainless steel, so use substantial tack welds and space them closely enough to hold the joint. On pipe, keep the root gap consistent—typically 1.6mm to 3.2mm, depending on the welding procedure specification (WPS). This leaves room for filler metal and root reinforcement. Feather-grind the tacks before incorporating them into the final bead so the fusion remains smooth.

Selecting the Correct Filler Metal and Tungsten

Filler selection affects the finished joint's mechanical properties and corrosion resistance. In general, the filler should contain slightly more nickel than the base metal to account for dilution and segregation during solidification. For most copper-nickel applications, the standard choice is ERCuNi, commonly identified under AWS A5.7. This filler contains about 30% nickel, along with small additions of titanium that help deoxidize the molten metal.

Filler Metal 67 (ERCuNi)

This 70/30 filler is used with both 70/30 and 90/10 base metals. Applying 70/30 filler to 90/10 pipe is standard practice because its higher nickel content helps keep the bead anodic to the base metal, or at least galvanically compatible. That reduces the risk of preferential corrosion along the joint. Titanium in the wire also reacts with nitrogen and oxygen in the pool, which helps limit porosity, one of the most common defects in CuNi work.

Tungsten Electrode Selection

A 2% lanthanated tungsten, with a blue or dark-blue tip, or a 2% ceriated tungsten, with a grey tip, is a suitable choice. Pure tungsten (green) is not suitable for DCEN. Thoriated tungsten (red) performs well, but many shops are moving away from it because of concerns about radioactive material. Grind the electrode to a sharp point and leave a small land, or flat, at the tip to stabilize the arc. The focused arc helps drive heat into the joint without spreading unnecessary heat through the surrounding heat-affected zone (HAZ).

Shielding Gas and Back Purging Strategies

Atmospheric contamination is a major problem while copper-nickel is molten. The pool can absorb hydrogen and oxygen, which may contribute to voids and embrittlement. For most manual TIG work, 100% argon is the standard shielding gas because it provides a stable arc and reliable coverage.

For sections thicker than 6mm (1/4 inch), a helium-argon blend, such as 75% argon and 25% helium, can help. Helium produces a hotter, more penetrating arc and can improve wetting at the toes of the bead. The trade-off is higher gas consumption and a slightly more difficult arc start.

Back purging is especially important on copper-nickel pipe. The inside of an open-root joint needs protection from air, or the root can sugar and oxidize heavily. That rough, oxidized surface can become a corrosion site and restrict flow. Use 100% argon for the purge, bring the oxygen level in the chamber below 1% (1000 ppm) before starting, and leave the purge dams in place until the joint is cool to the touch.

Machine Settings and Welding Technique

Machine setup connects the procedure to the work in front of you. Select DCEN and use remote amperage control, such as a foot pedal or hand control, when possible. Being able to taper the current is important when filling the crater because it helps prevent crater cracks. A high-frequency start is also preferred because it avoids touching the tungsten to the pool.

Amperage and Pulse Settings

As a starting point, allow about 10 to 12 amps for every thousandth of an inch of material thickness, similar to the starting point used for steel. Then adjust for joint design, fit-up, and actual puddle behavior. Continuous high-frequency AC is not used here as it is with aluminum, but DC pulse can be useful. A pulse rate of 1.5 to 2.5 pulses per second (PPS) can agitate the pool and help gas bubbles escape. Higher settings, above 100 PPS, can stiffen the arc and improve directional control.

Torch Manipulation

The molten pool is relatively sluggish, so keep the arc short—ideally about the same length as the electrode diameter. That maintains shielding and keeps the heat where you need it. A forehand technique, pushing the pool, with a torch angle of 10 to 15 degrees is recommended. Keep weaving to a minimum because stringer beads usually provide better mechanical properties and lower heat input. If you need a weave for vertical-up work, keep it tight.

Troubleshooting Common Defects

Even with careful preparation, defects can still show up. Porosity and hot cracking are two of the most common problems when TIG welding copper-nickel. Find the cause early, then correct the setup before continuing through the joint.

Porosity

Porosity appears as pinholes on the face or as subsurface voids on an X-ray. In CuNi, gas entrapment from hydrogen or nitrogen is usually responsible. Check the torch and hose connections for leaks, and make sure the gas flow is not turbulent; 15 to 20 CFH is often sufficient. The base metal must also be completely dry. Moisture that condenses on the pipe is a common source of hydrogen.

Hot Cracking

Hot cracks often form in the center of the bead or at the crater. The risk increases when the profile is too concave or the heat input is too high. Aim for a slightly convex bead. At the end of the pass, taper the current down gradually and add a final dab of filler to build up the crater. Do not snap the arc off abruptly.

Post-Weld Cleaning and Safety

After the joint cools, clean the area with a stainless steel wire brush to remove heat tint and oxidation. If the component will receive nondestructive testing (NDT), such as dye penetrant or X-ray inspection, the surface needs to be exceptionally clean. Copper-nickel does not normally need pickling to the same extent as stainless steel, but removing heat tint makes visual inspection easier.

Safety still comes first. Fumes from copper-nickel can contain copper, nickel, and manganese. Copper fumes may cause metal fume fever, a temporary flu-like illness, while long-term nickel exposure is a known cancer risk. Work with good general ventilation and local exhaust, such as a fume-extraction arm. Wear a respirator rated for welding fumes, such as a P100 or equivalent, when required by your site risk assessment. Clean preparation, controlled heat, sound shielding, and proper respiratory protection are what make this work repeatable and safe.