Brazing at TWI

Copper

Copper and its alloys are used in a wide range of applications because of their excellent formability, corrosion resistance and electrical conductivity.

The ability to braze Cu-Zn (brass), Cu-Si (silicon bronze), Cu-Al (aluminium bronze), Cu-Sn (phosphorus bronze), Cu-Ni, and other Cu alloys are generally rated from good to excellent.

Copper and copper alloys can be brazed using a variety of processes and techniques. The more common processes are: torch, induction, and furnace brazing with controlled atmosphere or vacuum.

When vacuum brazing copper and its alloys care has to be taken to ensure that they are free from elements that have high vapour pressures at the brazing temperatures (lead, zinc, etc). For high-vacuum brazing, the filler metals should be restricted to vacuum grades, containing low amounts of the high vapour pressure elements, such as cadmium.

Filler metals

The most common filler metals are based on silver alloys or copper-phosphorus alloys with brazing temperatures from 620°C to 870°C, and 704°C to 816°C respectively. In the copper-phosphorus filler metals the phosphorus is present to deoxidise the copper, and so this filler metal can be used without flux. Copper-phosphorus filler metals are therefore useful for most Cu based metals; however, Cu-Ni alloys with >10%Ni form brittle intermetallics.

Silver filler metals are based around the Ag-28%Cu eutectic alloy, and can be used on all copper based metals. These filler metals are more expensive and have lower strength than copper based brazes; however, silver filler metals offer better corrosion resistance.

Copper-zinc filler metal alloys can be used to braze Cu, Cu-Ni, Cu-Si and phosphorus bronze. However, care has to be taken to prevent overheating which can vaporise the zinc, leaving voids in the joint.

Joint performance

Softening of the base metal frequently occurs during copper brazing because many copper based alloys derive their properties from heat treatment at relatively low temperatures, and/or cold working. Softening of areas close to the braze can be minimised by use of localised heating. Brazing with a low melting filler metal for a minimum time will also reduce the softening.

Residual stresses arising from cold-working, casting, or machining operations can cause cracking of some copper alloys during brazing. Controlled and uniform heating is particularly important with brasses, cold-worked phosphor bronze and silicon bronze, as they are all susceptible to cracking.