Why Copper Tube Is Brazed Rather Than Welded
Copper has a melting point of about 1083 C and very high thermal conductivity, so fusion welding draws heat away from the joint almost as fast as a torch can add it. The consequences in practice are incomplete penetration, oxide entrapment, porosity and cracks in the heat affected zone, and the problem is worst on thin wall tube where there is little metal to absorb the heat. Brazing avoids all of this by keeping the tube wall below its melting point and filling the joint with a molten filler metal that is drawn in by capillary action.
The method is suited to the service conditions of copper tube as well: heat exchange equipment and condensers, low-temperature pipelines in gas separation plants, lubricating and hydraulic lines carrying pressurised fluid, and domestic hot and cold water and refrigeration piping. Copper tube itself is specified by ASTM B280 for air conditioning and refrigeration, EN 12735-1 for refrigeration tube, ASTM B88 for water tube and GB/T 17791 for air conditioner tube, and the brazed joint has to satisfy the same pressure and tightness requirements as the tube.
Filler Metals and Flux Selection
| Joint | Typical filler metal | Flux requirement |
|---|---|---|
| Copper to copper | copper-phosphorus, or copper-phosphorus-silver | none; the phosphorus content acts as a flux on copper |
| Copper to brass or bronze | silver-phosphorus or silver brazing alloy | required, unless the process uses a reducing atmosphere |
| Copper to steel or stainless steel | silver brazing alloy | required |
| Aluminium or aluminium alloy to copper | not recommended as a brazed joint | use mechanical or clad transition joints instead |
Filler metals are classified internationally by ISO 17672 and in China by GB/T 6418, and the copper-phosphorus family is the workhorse for refrigeration and water tube because it flows well on copper and needs no separate flux. Silver-bearing versions improve flow and lower the working temperature, which matters when the joint is close to a valve seat, an elastomer seal or a heat-sensitive fitting. Flux residues on refrigeration piping are a real hazard, so the flux must be selected for the service and removed after brazing; nitrogen or another inert gas is purged through the tube during heating for air conditioning circuits to prevent internal oxide scale.
Joint Design and Heating Practice
Clearance: aim for a capillary gap of roughly 0.05 to 0.15 mm on tube diameters in normal use. Too tight and the filler cannot penetrate; too loose and capillary action fails and the joint is bridged rather than brazed.
Insertion depth: use a depth of at least the tube diameter in a socket joint, and keep the tube square cut and burr free so the gap stays uniform around the circumference.
Cleaning: degrease and abrade the joint zone to bare metal immediately before assembly; oxide is the single most common cause of an incomplete joint.
Heating: heat the socket, not the tube end, and sweep the torch so both parts come up to temperature together. The filler should be drawn in by the joint rather than melted onto the surface by the torch.
Temperature: bring the joint uniformly above the flow range of the filler metal but well below the melting point of copper. Uneven heating leaves one side cold and the filler solidifies before it reaches the far side of the joint.
Support and alignment: because copper expands noticeably when heated, adequate support and alignment fixtures keep the assembly within the flatness and pitch tolerances required for U-bend and manifold work.
Process Problems and How to Control Them
Overheating is the most damaging fault. Copper oxidises heavily when the surface is held at high temperature for too long, forming loose scale that prevents the filler from wetting the metal; overheating also erodes the tube wall and can produce local melting at a thin section. Black, sooty deposits and a rough joint surface are typical evidence that the joint was overheated or starved of shielding gas.
The opposite fault is underheating, which shows up as incomplete penetration, a partially filled joint or a filler bead sitting on the surface instead of inside the gap. Poor fit-up, an oversized clearance or a joint that was not cleaned produce similar symptoms, so the order of diagnosis is to check clearance and cleanliness before increasing the heat input.
Contamination is the third common cause of trouble. Traces of cutting oil, drawing lubricant or flux residue can decompose at brazing temperature and open a leak path, and chloride-bearing residues left in a refrigeration or drinking water circuit are a corrosion risk in their own right.
Inspection and Supply of Brazed Tube Assemblies
Brazed joints are inspected by visual examination for fillet formation and joint filling, by dimensional check of assembly geometry, and by a pressure or leak test: hydrostatic proof of the assembly, or a pneumatic test with immersion or an electronic leak detector for refrigeration circuits. Destructive sectioning of sample joints during process qualification shows whether the filler has penetrated the joint fully and whether voids or inclusions are present, and it is the fastest way to confirm that clearance and heating practice are correct before a large order is run.
Tube for brazed assemblies is supplied cut to length, cleaned, capped and, where the drawing requires it, in the annealed temper suitable for bending. Dimensional control of the tube ends matters more than usual in brazing work, because ovality and burrs directly change the capillary gap. For export orders a typical documentation set includes the tube material certificate with heat number, a dimensional record, and the pressure and leak test records for the finished assembly.
Frequently Asked Questions
Q: Why is brazing preferred over welding for copper tube?
A: Copper conducts heat away from the joint very quickly, so fusion welding of thin wall tube tends to produce incomplete penetration, cracks and porosity. Brazing keeps the tube wall below its melting point and fills the joint by capillary action, giving a sound joint at much lower risk.
Q: Which filler metal is used for copper to copper joints?
A: Copper-phosphorus or copper-phosphorus-silver filler metal, classified by ISO 17672 and GB/T 6418. The phosphorus content self-fluxes the joint on copper, although silver-bearing grades are used where a lower working temperature is needed.
Q: What joint clearance should be used?
A: Roughly 0.05 to 0.15 mm capillary gap for normal tube sizes. A tighter gap prevents penetration and a wider gap defeats capillary action, so clearance is one of the first things to verify when a joint leaks.
Q: Why is nitrogen purged through refrigeration tube during brazing?
A: To keep oxygen out of the tube interior. Without purging, heating forms internal oxide scale that later blocks filters, damages compressor valves and contaminates the refrigerant circuit.
Q: How are brazed copper tube joints tested?
A: By visual inspection of fillet and joint fill, dimensional checks, and a hydrostatic or pneumatic leak test, with destructive sectioning of sample joints during process qualification to confirm full penetration.
Q: What causes a leak in an otherwise well made joint?
A: Most leaks come from oil or drawing lubricant left in the joint, oxide from overheated metal, or flux residue that has not been removed. Cleaning before assembly and correct heat control prevent nearly all of them.
