Selection of Joining Metals

Brazing is the most efficient metal joining process. It produces bonds, which are stronger than the metals joined. The metals to be joined are called base metals. The metals to be joined can be same or different. The most frequent base metals are copper, nickel, silver, aluminum, gold, etc. Not only pure metals are joined by the brazing, their alloys are used as well.

1) Aluminum & Aluminum Alloys
A number of variations of aluminium and their alloys are used for brazing. The most commonly brazed alloys are given in the table below. Those aluminium alloys, which contain higher percentage of Mg are more difficult to braze because of poor wetting by filler metal and excessive penetration.

 

Aluminum Alloys
Al 0.25% Cu 0.6% Si 1.0% Mg 0.25% Cr	Al 0.25% Cu 0.35% Si 0.6% Mg 0.25% Cr
Al 1% Zn    Al 1.2% Mn

 

2) Copper & Copper Alloys
Copper and its alloys are readily joined by the brazing processes. Oxygen containing coppers are not usually fit for brazing owing to loss of ductility and strength, and due to oxide migration. Atmosphere containing hydrogen also embrittles this material. However oxygen free copper is widely used and suitable for brazing components.

 

Copper Alloys	Comments
Cu-Zn alloys	The loss of zinc should be minimized by fluxing the components because it results in reduction of hardness and tensile strength.
Cu-Si alloys	These alloys should be stress relieved because in stressed condition they are prone to inter-granular penetration of filler metal with a continuous loss in mechanical properties.
Cu-Al alloys	Special fluxes are required for brazing Cu-Al alloys because refractory aluminum oxides are formed at brazing temperature.
Cu-Be alloys	For brazing copper-beryllium alloys, silver brazing rods using flux are employed.
Cu-Ni alloys	Cu-Ni alloys are susceptible to inter-granular penetration and cracking if stressed. It can be brazed readily with silver brazing rods.
Cu-Sn alloys (Phosphor Bronze)	These alloys may be brazed using silver, copper-phosphorus or copper-zinc filler rods.

 

3) Steels

 

Steels	Comments
Low Carbon and low alloy steels	Low carbon steels are brazed with ease. Low alloy steels tend to harden. It should be brazed below the critical temperature.
Stainless and heat-resisting steels	Due to high Cr content, special fluxes are required for refractory oxides and difficult wetting. Extra low carbon stainless steels or stabilized (Ti or Nb) steels are suitable for brazing by the nickel group fillers.

 

4) Cast-Iron
Silver brazing filler rods are perfect for brazing malleable and ductile cast-irons. The silver filler reduces the oxidation effect and make wetting easier.

5) Nickel and Nickel Alloys
Traces of S, Pb, Bi or Sb make nickel and high nickel alloys prone to embrittle. These alloys should be cleaned properly prior to brazing to assure the removal of any substance containing them. The parts should be stress relieved to avoid stress corrosion cracking.

6) Beryllium
Beryllium and its alloys has been brazed for use in missiles, nuclear reactors and aircraft.

7) Other Brazeable Materials

• Ceramics
• Graphite
• W, Mo, Ta and Cb
• Carbides and cermets
• Gold, Platinum and Palladium.
• Titanium and zirconium base metals
• High carbon and high speed tool steels