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Brazeability of copper alloys

Copper ad its alloy is selected for many applications because of their wide range of properties. This whole family has excellent resistance and formability. Electrical and thermal conductivities vary widely depending on composition. Most of them can be brazed easily, observing some precautions and warnings that we will try to explain briefly below.

Compared with steel, copper and its alloys have higher thermal expansion, and we have to consider when brazing dissimilar metals, but this potential problem should not discourage the use of these alloys in brazed assemblies.

Softening of base metals occurs frequently during brazing, because may copper alloys derive their properties form low temperature heat treatment of cold working or both. It is possible to minimize it by cooling the assembly, immersion in water, packing with wet rags. But of course, the choice of the correct brazing filler metal is very important.

Below is a short list of copper alloys with their main characteristics:

Oxygen-bearing coppers

This group includes the fire-refined and electrolytic tough pitch grades of copper and silver-bearing copper. Oxygen-bearing coppers are medium strength and low harness. When the copper oxide is uniformly distributed, these coppers are tough, ductile, and high malleable. Thermal and electrical conductivities are the highest of all copper alloys except for the oxygen-free copper grades.

Oxygen-free coppers

This group includes phosphorus-deoxidized copper- and oxygen-free (OF) copper. phosphorus-deoxidized copper is usually copper which oxygen has been removed by the addiction of phosphorus before casting (0,01% to 0,04%). According to the different P contain they are called DLP or DHP.

Special Coppers

This group includes some alloys that offer high electrical conductivity plus special properties such as machinability. Lead, tellurium, selenium and sulfuret-bearing copper have machinability for time more than ordinary copper alloys. They are widely used in the manufacture of electrical connectors.

High Coppers

The high copper group include those with additional small amount of alloying elements that enhance the mechanical properties. Chromium copper enhance the strength and electrical conductivity; Zirconium-copper develops somewhat lower strength of chromium copper. Copper-beryllium are of two types, according the beryllium contain (0,4% or 1,5%72%). Beryllium copper alloys have higher conductivity, but lower strength. Cobal or nickel addiction to these alloys restrict grain growth during annealing.

Copper-Zinc Alloys

Copper-zinc alloys are produced with varying ratios of these two elements provide the desired properties and casting characteristic. Other elements are added occasionally to enhance particular mechanical or corrosion properties. Addition of Manganese, tin, iron, silicon, nickel, lead and aluminium rarely exceed 4%.

Leaded Brasses

Lead is commonly added to brass in amount of up to 5% to improve machinability. This addition has no effect on tensile strength, particularly of cold worked materials, but it does increase the possibility of cracking in materials that are under tensile strength and subjected to heat.

Copper-Tin Alloys (Phosphor Bronze)

Alloys of copper and tin are termed tin bronzes. During casting, 0,03 to 0,5% phosphorus is added as a deoxidizing agent and may be found as residual into the alloys. They are known as phosphor bronzes. They have moderate high tensile strength, depending on tin content and degree of cold work.

Copper-Aluminium alloys (Aluminium bronzes)

Copper-aluminium alloys are high copper alloys that contain 3 to 13% aluminium and varying amount of iron, nickel, manganese and silicon.

Copper silicon alloys

Copper-silicon alloys commonly called Silicon bronzes, generally contain 1.5 to 3.5% Silicon and 1.25% or less of zinc, tin, manganese or iron. The addition of iron increases the tensile strength and hardness. Toughness and shear strength increase with silicon but electrical and thermal conductivities decrease.

Copper nickel alloys

Nickel is added to copper zinc alloys to make them silvery in appearance for decorative purposes and to increase tensile strength and corrosion resistance. The result in generally called Nickel silvers.

The choice of alloy and flux depends on the characteristics of the base metals and the characteristics of the brazed joint. obviously, the heating method, the brazing technique and the design of the joint to be brazed must also be taken into consideration.

in general, however, we can say silver base filler metals with brazing temperature from 620 to 870°C and CuP filler metals are use most commonly. Class ISO 17672 Au, Ag, CuP, Cu-Zn may be used for brazing most of the copper and copper alloys, provide the brazing temperature are sufficiently lower than the melting range of base metals. Cu Class filler metal can be used for nickel silvers alloys but its liquidus is too high for use with other copper-base metals. Cu-Zn (Weld type) may be used to braze the coppers and copper-nickel, copper-silicon, and copper-tin alloys. The liquidus temperatures of these filler metal are too high for brazing the brasses and nickel silvers; are not useful for brazing aluminium bronzes because the required brazing temperatures destroy the effectiveness of the fluxes on these base metals.

CuP (Fos & Silverfos type) filler metals are useful for most of copper base metal because fluxes are not needed. In brazing of cast high-leaded brass pipe fittings are useful when precautions are taken to flux properly and avoid over-heating.

Silver Brazing alloys (Ternalloy type) may be used on all copper base metals. Class Au in electronical application or aero spatial field. Corrosion resistance is an important factor in selecting silver brazing alloys.

Let’s have a look on procedure of properties of brazing those copper and its alloy:

Oxygen-bearing coppers & oxygen-free copper

When oxygen bearing coppers are heated above 920°C for prolonged periods during brazing operations, copper oxide concentrates in the grain boundaries and reduce strength and ductility. It should be furnace brazed in an inert atmosphere, nitrogen, or vacuum. Torch brazes should be done with a neutral or slightly oxidizing flame. Oxygen free copper are readily brazed by furnace or torch methods.

Silverfos or Fos alloys are generally used because are self-fluxing. But flux is beneficial, however, in heavy assemblies where prolonged heating would otherwise cause excessive oxidation. Weld are not recommended due their working temperature. The overheating of them since volatilization of zinc causes void in the joint.

Special Coppers

The copper that contains small addiction of silver, lead, tellurium selenium or sulphur (generally no more than 1%) are brazed readily with Silverfos and Fos alloys. Wetting action is improved when a flux is used.

High Coppers

Beryllium copper must be clean before brazing. The high strength beryllium copper alloys (2%) can be furnace brazed. Ternalloy special 7228 is generally used. If needed (in case of torch braze) flux F25 of F16. Of course, if the annealing treatment is requested, brazing process chance accordingly.

High conductivity copper-beryllium alloys (0.5%) can be brazed with Brasalloy 45 or Brasalloy 50, in the aged condition. However, the mechanical properties of hardened base metals but be careful if cadmium bearing alloys are allowed.

Chromium copper and zirconium copper can be brazed with silver brazing alloys and fluoride-contain fluxes, after solution annealing and cold working, but before age hardening.

Copper-Zinc Alloys

All brasses can be brazed with Ternalloy or Fos & Silverfos alloys. The higher melting point brass also with Weld alloys, but be careful. Brasses are subject to cracking and should therefore be heated carefully and uniformly. Sharp corner and change in cross section that concentrate stress and produce thermal strains should be avoided.

Leaded Brasses

Lead is commonly added to brass in amount of up to 5% to improve machinability. This addition has no effect on tensile strength, particularly of cold worked materials, but it does increase the possibility of cracking in materials that are under tensile strength and subjected to heat. Maior brazing difficulties occur when the lead contain is over 2 to 3%. To maintain good flow and wetting during brazing, leaded brasses require complete flux coverage to prevent formation of lex oxide or dross.

Copper-Tin Alloys (Phosphor Bronze)

In a stress condition, these alloys are subjected to cracking. It is a good practice to relieve or anneal the part before brazing. All of this bronze type can be brazed with Ternalloy alloy or Fos and Silverfos. Low tin variety also with Weld alloys.

Copper-Aluminium alloys (Aluminium bronzes)

Aluminium bronze can be brazed with silver bearing filler metals and F20 fluxes. Formation of refractory aluminium oxide at brazing temperature in alloys containing more than 8% aluminium present some difficulties. However, the problem can be avoided by electroplating at least 0.013 mm of copper.

Copper silicon alloys

Copper silicon can be brazed with Ternalloy alloy. But should be cleaned and then flux coated or copper plated before brazing to prevent the formation of refractory silicon oxide.

Silicon bronzes are subject to cracking during brazing, and their crack susceptibility increase as silicon contain increase. Stress relieving of these alloys and uniform heating for brazing are important consideration.

Copper nickel alloys

Brazing characteristic of those alloys are good. The alloys should be stress relieved and heated evenly to brazing temperature. Are generally brazed with Ternalloy alloys. The base metal must be thoroughly evaluated for performance and microstructure before using Silverfos or Fos for joining. Stress relieving of these alloys and uniform heating for brazing are important consideration.

Copper nickel zinc alloys

Nickel silvers can be brazed with the same procedure for brazing brass. When Weld brazing filler metal are used, however, great care is required because of the relatively high brazing temperatures. These alloys are subjected to intergranular penetration by molten filler metals unless they are stress relieved before brazing. The poor thermal conductivity of these alloys tends to lead to local overheating.

Dissimilar metals

Dissimilar combination of copper alloys can be readily brazed. Copper alloys combination with steel, austenitic stainless steel, or nickel also can be brazed using Ternalloy filler metals. Suggested filler metals in the table 1.Brazing copper alloys to aluminium and Magnesium alloys is not practical. In case where base metals have widely dissimilar of thermal expansion characteristics, the design and production of satisfactory brazed joint may be extremely difficult or impractical.

Brazing filler metal selection charts for joining dissimilar metals
 Cu & Cu alloysCarbon and low alloy SteelCast IronStainless steelNi and Ni alloysTi and Ti alloysOther reactive metalsRefractory metalsTool steel
Cu & Cu AlloysAubraze Ternalloy, Fos & Silverfos, WeldAubraze Ternalloy, Fos & Silverfos, WeldAubraze Ternalloy, WeldAubraze TernalloyAubraze Ternalloy WeldTernalloyTernalloyTernalloyAubraze Ternalloy,  Weld Nisald
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