Deoxidized copper (DHP and DLP grades) is the only type of pure copper that welds reliably. Standard electrolytic tough pitch (ETP) copper contains dissolved oxygen that causes hydrogen embrittlement and porosity when you hit it with an arc. Deoxidized grades have the oxygen removed with phosphorus, producing a base metal that responds to TIG welding with ERCu filler, heavy preheat, and argon or argon-helium shielding gas.
The biggest challenge with welding copper is its thermal conductivity: 8 times higher than steel. Heat leaves the weld zone as fast as you put it in. On anything thicker than sheet metal, you need aggressive preheat (400-1000F depending on thickness) just to establish and maintain a puddle.
Copper Types and Weldability
Not all copper is the same from a welding standpoint. The oxygen content is the deciding factor.
| Type | UNS | Oxygen Content | Weldability | Common Forms |
|---|---|---|---|---|
| ETP (Electrolytic Tough Pitch) | C11000 | 0.02-0.05% (as Cu2O) | Poor - hydrogen embrittlement | Wire, bus bar, roofing |
| DHP (Phosphorus Deoxidized, High Residual P) | C12200 | Nil (0.015-0.040% P) | Good | Tube, pipe, sheet, plate |
| DLP (Phosphorus Deoxidized, Low Residual P) | C12000 | Nil (0.004-0.012% P) | Good | Tube, pipe |
| OFE (Oxygen-Free Electronic) | C10100 | Nil (no deoxidizer) | Excellent | Electronics, vacuum, superconductor |
| OFHC (Oxygen-Free High Conductivity) | C10200 | Nil (no deoxidizer) | Excellent | Electrical, heat exchangers |
DHP (C12200) is the most common weldable copper. The phosphorus content (0.015-0.040%) serves as a deoxidizer during melting, binding any oxygen into phosphorus pentoxide that floats out as slag. The residual phosphorus slightly reduces electrical conductivity (to about 85% IACS), which is why DHP isn’t used for high-conductivity electrical applications.
DLP (C12000) has lower phosphorus (0.004-0.012%), keeping conductivity higher (about 93% IACS). It welds just as well as DHP.
OFE and OFHC grades are produced in oxygen-free atmospheres and contain no deoxidizer. They have the best electrical conductivity (100-101% IACS) and weld excellently because there’s no oxygen to cause problems. They’re also the most expensive copper grades.
ETP (C11000) is the standard electrical copper. Cheap and everywhere. If someone hands you “copper” to weld without specifying the grade, it’s probably ETP, and it’ll give you porosity and cracking if you try to fusion weld it. If you must join ETP copper, braze it instead.
ERCu Filler Metal
ERCu (AWS A5.7) is the standard TIG filler for deoxidized copper. Its composition is essentially deoxidized copper wire with small additions of silicon, tin, or manganese as deoxidizers.
| Filler | AWS Class | Composition | Use |
|---|---|---|---|
| ERCu | AWS A5.7 | 98% Cu, Si + Sn deoxidizers | Matching filler for DHP, DLP, OFHC copper |
| ERCuSi-A | AWS A5.7 | 97Cu-3Si | Not for pure copper; use for bronze, brass, dissimilar |
| ERCuSn-A | AWS A5.7 | 92Cu-8Sn | Phosphor bronze overlay; not for pure copper matching |
Use ERCu when you want the weld deposit to match the base metal’s thermal and electrical properties. ERCuSi-A (silicon bronze) is sometimes substituted because it’s easier to find and flows better, but the 3% silicon reduces conductivity and changes the deposit’s color and corrosion behavior. For applications where conductivity matters (bus bars, ground straps, heat exchangers), stick with ERCu.
Preheat Requirements
Copper’s thermal conductivity (226 BTU/hr-ft-F at room temperature) means the base metal acts as a massive heat sink. Without preheat, the arc can’t build enough heat in the joint zone to create and maintain a weld puddle. The heavier the copper, the more preheat you need.
| Material Thickness | Preheat Temperature | Heating Method |
|---|---|---|
| Under 1/8 in (sheet) | None to 200F | Torch or ambient |
| 1/8 - 1/4 in | 200-400F | Torch (rosebud) |
| 1/4 - 1/2 in | 400-600F | Rosebud or oven |
| 1/2 - 1 in | 600-800F | Oven preferred |
| Over 1 in | 800-1000F | Oven required |
Preheat tips:
- Use a rosebud tip on an oxy-acetylene rig for localized preheat. Resistance heating blankets work for pipe.
- Verify temperature with a contact pyrometer, not a temp stick. Temp sticks leave residue on copper that contaminates the weld.
- Preheat the entire area around the joint, not just the joint itself. Copper conducts heat so fast that localized preheating produces a steep thermal gradient that collapses the moment you move the heat source.
- On thick copper plate (over 1/2 inch), an oven soak is more effective than torch preheat because it brings the entire piece to a uniform temperature.
TIG Welding Procedure
TIG (GTAW) on DCEN is the standard process for copper. Use 2% thoriated or 2% lanthanated tungsten.
Shielding Gas Selection
Gas selection has a bigger impact on copper welding than on any other metal. The gas determines how much heat the arc delivers to the joint.
| Gas | Arc Voltage | Heat Input | Best For |
|---|---|---|---|
| 100% Argon | Lower | Moderate | Thin copper (under 1/4 in), general work |
| 75Ar / 25He | Higher | High | 1/4 - 1/2 in copper |
| 50Ar / 50He | Higher | Very high | 1/2 - 1 in copper |
| 25Ar / 75He | Much higher | Very high | Heavy copper (over 1/2 in) |
| 100% Helium | Highest | Maximum | Thick plate, mechanized welding |
Helium increases arc voltage (and therefore heat input) without increasing amperage. At 50% helium, arc voltage increases roughly 30-40% compared to pure argon at the same amperage. This extra heat is exactly what you need to overcome copper’s thermal conductivity.
Gas flow rate: 20-30 CFH for argon, 30-50 CFH for helium-rich blends (helium is lighter and dissipates faster).
Welding Parameters
| Thickness | Filler Dia. | Amps (DCEN) | Tungsten | Gas |
|---|---|---|---|---|
| 0.040 in (sheet) | 1/16 in | 80-120 | 3/32 in | 100% Ar |
| 1/8 in | 3/32 in | 150-200 | 1/8 in | 100% Ar or 75Ar/25He |
| 1/4 in | 1/8 in | 200-300 | 1/8-5/32 in | 75Ar/25He or 50/50 |
| 1/2 in | 5/32 in | 300-400 | 5/32 in | 50Ar/50He or 25/75 |
| 1 in | 3/16 in | 350-450+ | 3/16 in | 25Ar/75He or 100% He |
These amperages may look high for the thicknesses listed. That’s the reality of copper welding. A 1/4 inch copper plate takes as much amperage as 1/2 inch steel because the heat pours out of the joint.
Technique Points
- Travel speed matters. Move steadily and quickly enough to maintain the puddle without overheating. If you linger, the preheat plus arc heat can melt through copper sheet in an instant.
- Push the torch. A 10-15 degree push angle keeps shielding gas coverage ahead of the puddle.
- Feed filler continuously. The puddle is fluid and hot; it eats filler fast. Keep a steady feed rate.
- Watch the puddle color. Molten copper is a dull red (about 1980F). If the puddle turns bright orange or white, you’re overheating. Pull back the amperage.
- No weaving on heavy sections. Use stringer beads, multiple passes. Weaving concentrates heat and increases distortion.
MIG Welding Copper
MIG welding copper is feasible for thicker sections (over 3/16 inch) using ERCu wire with argon or argon-helium blends. Spray transfer is required. Short-circuit MIG doesn’t deliver enough heat for copper.
Run 100% argon or 75Ar/25He at 30-40 CFH. Wire feed speed and voltage need to be set for spray transfer, which means higher voltage and faster wire speed than you’d use on steel of the same thickness.
Preheat requirements are the same as for TIG. MIG is faster than TIG on multi-pass joints but produces more spatter and less control over bead shape.
Common Defects on Copper Welds
Porosity is the most frequent defect. Causes include:
- Welding ETP copper instead of deoxidized (hydrogen embrittlement)
- Moisture on the base metal or filler rod
- Insufficient shielding gas coverage (increase flow rate with helium blends)
- Contaminated joint surfaces
Lack of fusion from insufficient heat input. Increase preheat, switch to a helium-rich gas blend, or increase amperage.
Hot cracking on restrained joints. Copper is ductile and rarely cracks, but heavy, restrained sections can develop solidification cracks. Use a balanced weld sequence and avoid excessive restraint.
Excessive distortion from the high heat input required. Fixture firmly, use skip welding or back-step technique, and consider mechanized welding for long seams.
Identifying Copper Type Before Welding
If you’re handed copper stock without a clear material identification, you need to determine whether it’s deoxidized before committing to fusion welding. A few practical approaches:
- Check markings. DHP tube and pipe are typically stamped or stenciled with “DHP” or “C12200.” ETP is marked “ETP” or “C11000.”
- Look at the application. Plumbing tube is almost always DHP. Electrical bus bar and wire are almost always ETP.
- Trial weld on scrap. Weld a short bead on a scrap piece. ETP copper produces heavy, scattered porosity that’s visible on the surface. DHP copper produces clean beads with little or no porosity. This is a destructive but reliable test.
- Spark test. Copper doesn’t spark on a grinding wheel (unlike steel), so spark testing doesn’t help distinguish grades. Use the trial weld method instead.
If you can’t identify the grade and the joint is structural, assume the worst and braze rather than weld.
When to Braze Instead of Weld
Brazing is often the better choice for copper, especially on:
- ETP copper (can’t fusion weld, but brazes fine)
- Thin-wall tube and fittings (less distortion than welding)
- Joints where full base metal strength isn’t needed
- Field work where preheat to 600-1000F isn’t practical
Silver brazing alloys (BAg-1, BAg-5) produce joints at 1100-1300F with excellent strength and minimal distortion. Copper-phosphorus alloys (BCuP series) are self-fluxing on copper and are the standard for HVAC and refrigeration tube joints.
For welding procedures on copper alloy pipes, see the CuNi pipe welding guide. For brazing brass and bronze components, see the brazing brass fittings guide. For bronze and brass fusion welding, check the bronze and brass welding guide.
Back to the copper and brass welding category.