Brazing
element role description
Silver structural , wetting Enhances capillary flow , improves corrosion resistance of less-noble alloys , worsens corrosion resistance of gold and palladium . Relatively expensive . High vapor pressure , problematic in vacuum brazing . Wets copper . Does not wet nickel and iron . Reduces melting point of many alloys , including gold-copper
Copper structural Good mechanical properties . Often used with silver . Dissolves and wets nickel . Somewhat dissolves and wets iron . Copper-rich alloys sensitive to stress cracking in presence of ammonia
Zinc structural , melting , wetting Lowers melting point . Often used with copper . Susceptible to corrosion . Improves wetting on ferrous metals and on nickel alloys . Compatible with aluminium . High vapor tension , produces somewhat toxic fumes , requires ventilation ; highly volatile above 500 °C . At high temperatures may boil and create voids . Prone to selective leaching in some environments , which may cause joint failure . Traces of bismuth and beryllium together with tin or zinc in aluminium-based braze destabilize oxide film on aluminium , facilitating its wetting . High affinity to oxygen , promotes wetting of copper in air by reduction of the cuprous oxide surface film . Less such benefit in furnace brazing with controlled atmosphere . Embrittles nickel . High levels of zinc may result in a brittle alloy . Prone to interfacial corrosion in contact with stainless steel in wet and humid environments . Unsuitable for furnace brazing due to volatility
Aluminium structural , active Usual base for brazing aluminium and its alloys . Embrittles ferrous alloys
Gold structural , wetting Excellent corrosion resistance . Very expensive . Wets most metals
Palladium structural Excellent corrosion resistance , though less than gold . Higher mechanical strength than gold . Good high-temperature strength . Very expensive , though less than gold . Makes the joint less prone to fail due to intergranular penetration when brazing alloys of nickel , molybdenum , or tungsten . Increases high-temperature strength of gold-based alloys . Improves high-temperature strength and corrosion resistance of gold-copper alloys . Forms solid solutions with most engineering metals , does not form brittle intermetallics . High oxidation resistance at high temperatures , especially Pd-Ni alloys
Cadmium structural , wetting , melting Lowers melting point , improves fluidity . Toxic . Produces toxic fumes , requires ventilation . High affinity to oxygen , promotes wetting of copper in air by reduction of the cuprous oxide surface film . Less such benefit in furnace brazing with controlled atmosphere . Allows reducing silver content of Ag-Cu-Zn alloys . Replaced by tin in more modern alloys . In EU since December 2011 allowed only for aerospace and military use
Lead structural , melting Lowers melting point . Toxic . Produces toxic fumes , requires ventilation
Tin structural , melting , wetting Lowers melting point , improves fluidity . Broadens melting range . Can be used with copper , with which it forms bronze . Improves wetting of many difficult-to-wet metals , e.g . stainless steels and tungsten carbide . Traces of bismuth and beryllium together with tin or zinc in aluminium-based braze destabilize oxide film on aluminium , facilitating its wetting . Low solubility in zinc , which limits its content in zinc-bearing alloys
Bismuth trace additive Lowers melting point . May disrupt surface oxides . Traces of bismuth and beryllium together with tin or zinc in aluminium-based braze destabilize oxide film on aluminium , facilitating its wetting
Beryllium trace additive Traces of bismuth and beryllium together with tin or zinc in aluminium-based braze destabilize oxide film on aluminium , facilitating its wetting
Nickel structural , wetting Strong , corrosion-resistant . Impedes flow of the melt . Addition to gold-copper alloys improves ductility and resistance to creep at high temperatures . Addition to silver allows wetting of silver-tungsten alloys and improves bond strength . Improves wetting of copper-based brazes . Improves ductility of gold-copper brazes . Improves mechanical properties and corrosion resistance of silver-copper-zinc brazes . Nickel content offsets brittleness induced by diffusion of aluminium when brazing aluminium-containing alloys , e.g . aluminium bronzes . In some alloys increases mechanical properties and corrosion resistance , by a combination of solid solution strengthening , grain refinement , and segregation on fillet surface and in grain boundaries , where it forms a corrosion-resistant layer . Extensive intersolubility with iron , chromium , manganese , and others ; can severely erode such alloys . Embrittled by zinc , many other low melting point metals , and sulfur
Chromium structural Corrosion-resistant . Increases high-temperature corrosion resistance and strength of gold-based alloys . Added to copper and nickel to increase corrosion resistance of them and their alloys . Wets oxides , carbides , and graphite ; frequently a major alloy component for high-temperature brazing of such materials . Impairs wetting by gold-nickel alloys , which can be compensated for by addition of boron
Manganese structural High vapor pressure , unsuitable for vacuum brazing . In gold-based alloys increases ductility . Increases corrosion resistance of copper and nickel alloys . Improves high-temperature strength and corrosion resistance of gold-copper alloys . Higher manganese content may aggravate tendency to liquation . Manganese in some alloys may tend to cause porosity in fillets . Tends to react with graphite molds and jigs . Oxidizes easily , requires flux . Lowers melting point of high-copper brazes . Improves mechanical properties and corrosion resistance of silver-copper-zinc brazes . Cheap , even less expensive than zinc . Part of the Cu-Zn-Mn system is brittle , some ratios can not be used . In some alloys increases mechanical properties and corrosion resistance , by a combination of solid solution strengthening , grain refinement , and segregation on fillet surface and in grain boundaries , where it forms a corrosion-resistant layer . Facilitates wetting of cast iron due to its ability to dissolve carbon . Improves conditions for brazing of carbides
Molybdenum structural Increases high-temperature corrosion and strength of gold-based alloys . Increases ductility of gold-based alloys , promotes their wetting of refractory materials , namely carbides and graphite . When present in alloys being joined , may destabilize the surface oxide layer ( by oxidizing and then volatilizing ) and facilitate wetting
Cobalt structural Good high-temperature properties and corrosion resistance . In nuclear applications can absorb neutrons and build up cobalt-60 , a potent gamma radiation emitter
Magnesium volatile O 2 getter Addition to aluminium makes the alloy suitable for vacuum brazing . Volatile , though less than zinc . Vaporization promotes wetting by removing oxides from the surface , vapors act as getter for oxygen in the furnace atmosphere
Indium melting , wetting Lowers melting point . Improves wetting of ferrous alloys by copper-silver alloys . Suitable for joining parts that will be later coated by titanium nitride
Carbon melting Lowers melting point . Can form carbides . Can diffuse to the base metal , resulting in higher remelt temperature , potentially allowing step-brazing with the same alloy . At above 0.1% worsens corrosion resistance of nickel alloys . Trace amounts present in stainless steel may facilitate reduction of surface chromium ( III ) oxide in vacuum and allow fluxless brazing . Diffusion away from the braze increases its remelt temperature ; exploited in diffusion brazing
Silicon melting , wetting Lowers melting point . Can form silicides . Improves wetting of copper-based brazes . Promotes flow . Causes intergranular embrittlement of nickel alloys . Rapidly diffuses into the base metals . Diffusion away from the braze increases its remelt temperature ; exploited in diffusion brazing
Filler metals -- Role of elements
Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal. Brazing differs from welding in that it does not involve melting the work pieces and from soldering in using higher temperatures for a similar process, while also requiring much more closely fitted parts than when soldering. The filler metal flows into the gap between close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal (known as wetting) and is then cooled to join the work pieces together. A major advantage of brazing is the ability to join the same or different metals with considerable strength.