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

Copper-nickel alloys, commonly referred to as CuNi or cupronickel, are the backbone of marine engineering and desalination industries due to their exceptional resistance to saltwater corrosion and biofouling. For the professional welder, mastering the art of TIG welding (Gas Tungsten Arc Welding or GTAW) these alloys is a valuable skill set that opens doors to high-level industrial projects. While copper-nickel shares some characteristics with stainless steel and pure copper, it possesses unique thermal and metallurgical properties that require specific techniques and stringent preparation. This guide provides a comprehensive technical overview of how to successfully TIG weld copper-nickel alloys, ensuring high-quality, X-ray-grade welds.

Understanding Copper-Nickel Metallurgy and Grades

Before striking an arc, it is crucial to understand what distinguishes copper-nickel from other conductive metals. Unlike pure copper, which acts as a massive heat sink requiring immense amperage, the addition of nickel significantly lowers the thermal conductivity of the alloy. This makes CuNi behave more like austenitic stainless steel under the arc, although the weld pool is generally more fluid and sluggish. The material is renowned for its ductility and moderate strength, but it is highly sensitive to contamination from elements like lead, sulfur, and phosphorus, which can cause immediate cracking.

There are two primary grades of copper-nickel that welders will encounter in industrial settings: 90/10 (C70600) and 70/30 (C71500). The 90/10 grade contains roughly 90% copper and 10% nickel, offering a cost-effective solution for seawater piping systems, while the 70/30 grade provides superior strength and corrosion resistance for high-pressure heat exchangers and submarine components. Regardless of the specific grade, both alloys are welded using Direct Current Electrode Negative (DCEN) and require similar handling practices. Understanding the specific grade is vital because it dictates the choice of filler metal and slight variations in heat input management.

Essential Surface Preparation and Joint Design

The success of a copper-nickel TIG weld is determined long before the torch is turned on; it is decided during the preparation phase. Copper alloys are notoriously unforgiving of surface oxides, grease, oil, and paint, all of which can lead to porosity and lack of fusion. The oxide layer on copper-nickel is refractory, meaning it melts at a much higher temperature than the base metal, potentially trapping inclusions if not removed. Therefore, mechanical cleaning must be performed immediately prior to welding using clean, dedicated tools.

• Degreasing: thorough cleaning with an approved solvent (such as acetone or alcohol) is required to remove all cutting fluids, oils, and marking crayons.
• Oxide Removal: Use a dedicated stainless steel wire brush or a carbide burr to remove surface oxides within one inch of the weld joint on both the face and root side.
• Tool Segregation: Never use grinding discs or brushes that have been used on carbon steel or aluminum, as iron contamination will ruin the corrosion resistance of the CuNi.
• Edge Preparation: For material thicker than 3mm (1/8 inch), a V-groove with a 60 to 75-degree included angle is typically required to ensure full penetration.

Proper fit-up is equally critical to prevent burn-through and control distortion. Because copper-nickel has a coefficient of thermal expansion similar to stainless steel, tack welds should be substantial and spaced frequently. When fitting pipes, ensure the root gap is consistent—typically 1.6mm to 3.2mm depending on the welding procedure specification (WPS)—to allow for proper filler metal consumption and root reinforcement. Tacks should be feather-ground before being incorporated into the final weld bead to ensure seamless fusion.

Selecting the Correct Filler Metal and Tungsten

Choosing the correct filler metal is vital to maintaining the mechanical properties and corrosion resistance of the finished weldment. Generally, the filler metal should have a slightly higher nickel content than the base metal to compensate for dilution and segregation during solidification. For almost all copper-nickel welding applications, the industry standard is ERCuNi (often referred to as AWS A5.7), which contains approximately 30% nickel and small additions of titanium to act as a deoxidizer.

Filler Metal 67 (ERCuNi)

This 70/30 composition filler wire is used for welding both 70/30 and 90/10 base metals. Using a 70/30 filler on 90/10 pipe is standard practice because the higher nickel content ensures the weld bead is anodic to the base metal, or at least galvanically compatible, preventing preferential corrosion of the weld seam. The titanium content in the wire is essential as it reacts with nitrogen and oxygen in the weld pool, helping to suppress porosity, which is the most common defect in CuNi welding.

Tungsten Electrode Selection

For the electrode, a 2% Lanthanated (blue or dark blue tip) or 2% Ceriated (grey tip) tungsten is recommended. Pure tungsten (green) is not suitable for DCEN welding, and Thoriated (red) is increasingly being phased out due to radiation concerns, though it performs well. The tungsten should be ground to a sharp point with a slight land (flat spot) at the tip to stabilize the arc. A sharp electrode helps concentrate the heat, which is necessary to penetrate the copper alloy without putting excessive heat into the surrounding heat-affected zone (HAZ).

Shielding Gas and Back Purging Strategies

Atmospheric contamination is the primary enemy of copper-nickel alloys when they are in a molten state. The molten pool readily absorbs hydrogen and oxygen, leading to embrittlement and voids. Therefore, 100% Argon is the standard shielding gas for most manual TIG applications involving copper-nickel. It provides excellent arc stability and cleaning action.

For thicker sections (above 6mm or 1/4 inch), a helium-argon mixture (such as 75% Argon / 25% Helium) can be beneficial. Helium increases the ionization potential of the arc, resulting in a hotter, more penetrating arc profile. This can help overcome the thermal conductivity of the material and improve wetting at the toes of the weld. However, helium mixtures require higher flow rates and can make arc starting slightly more difficult.

Perhaps the most overlooked aspect of welding copper-nickel piping is the requirement for back purging. Just as with stainless steel, the inside of the weld joint must be protected from oxidation. The root pass will "sugar" or oxidize heavily if exposed to air, creating a site for corrosion and flow restriction. Purging with 100% Argon is mandatory for open-root pipe welds. The oxygen content in the purge chamber should be reduced to below 1% (1000 ppm) before welding commences, and purge dams should remain in place until the weld is cool to the touch.

Machine Settings and Welding Technique

Setting up your TIG machine correctly is the bridge between theory and application. Set the machine to DCEN (Direct Current Electrode Negative) and consider using a remote amperage control (foot pedal or hand control). The ability to taper amperage is critical for crater filling to prevent crater cracks. High-frequency start is preferred to avoid tungsten contamination in the weld pool.

Amperage and Pulse Settings

As a general rule of thumb, start with 10-12 amps for every thousandth of an inch of thickness, similar to steel, but be prepared to adjust based on the joint geometry. Unlike aluminum, continuous high-frequency AC is not used. However, DC pulsing can be highly effective for CuNi. Setting a pulse rate of 1.5 to 2.5 pulses per second (PPS) can help agitate the puddle, helping to float out gas bubbles and refine the grain structure, while higher pulse settings (100+ PPS) can stiffen the arc for better directional control.

Torch Manipulation

The molten puddle of copper-nickel is relatively sluggish and does not flow as freely as carbon steel. The welder must maintain a short arc length—ideally equal to the electrode diameter—to maintain gas shielding coverage. A forehand technique (pushing the puddle) with a torch angle of 10 to 15 degrees is recommended. Weaving should be kept to a minimum; stringer beads generally produce better mechanical properties and lower heat input. If a weave is necessary for vertical-up welding, keep it tight.

Troubleshooting Common Defects

Even with proper preparation, defects can occur. The two most prevalent issues in TIG welding copper-nickel are porosity and hot cracking. Understanding the root causes of these defects allows for quick troubleshooting and correction during the fabrication process.

Porosity

Porosity appears as small pinholes in the weld face or subsurface voids visible on X-ray. In CuNi, this is almost always caused by gas entrapment (hydrogen or nitrogen). If you encounter porosity, check for gas leaks in your torch lines, ensure your gas flow rate is not creating turbulence (usually 15-20 CFH is sufficient), and verify the base metal is completely dry. Moisture from the air condensing on the pipe is a common source of hydrogen.

Hot Cracking

Hot cracking usually occurs in the center of the bead or at the crater. Copper-nickel is susceptible to this if the weld profile is too concave or if the heat input is excessive. To prevent cracking, ensure a slightly convex bead profile. Always use a distinct crater-fill sequence: taper the amperage down slowly at the end of the weld and add a final dab of filler metal to build up the crater. Never terminate the arc abruptly.

Post-Weld Cleaning and Safety

Once the welding is complete, proper post-weld treatment ensures the longevity of the component. The weld area should be cleaned with a stainless steel wire brush to remove any heat tint or oxidation. If the weld requires non-destructive testing (NDT), such as dye penetrant or X-ray inspection, the surface must be impeccably clean. While copper-nickel does not require pickling to the same extent as stainless steel to restore corrosion resistance, removing the heat tint helps in visual inspection.

Safety is paramount when working with copper alloys. Welding copper-nickel generates fumes that can contain copper, nickel, and manganese. Inhalation of copper fumes can lead to "metal fume fever," a temporary but unpleasant flu-like condition. Long-term exposure to nickel fumes is a known carcinogen. Always weld in a well-ventilated area, use local exhaust ventilation (fume extraction arms), and wear a respirator specifically rated for welding fumes (P100 or equivalent). By combining rigorous safety protocols with precise technical execution, you can master the challenges of welding this premium alloy.