Brazing is a group of welding processes which produce coalescence of materials by heating them to a suitable temperature and by using a filler metal having a liquidus above 450oC and below the solidus of the base metals. The solidus is the highest temperature at which the metal is completely solid, that is, the temperature at which melting starts. The liquidus is the lowest temperature at which the metal is completely liquid, the temperature at which freezing starts. The solidus and liquidus for a particular alloy are defined values.

The filler metal is distributed between closely fitted surfaces of the joint by capillary attraction. To avoid confusion it is necessary to explain braze welding which is different since the filler metal is not distributed by capillary attraction.

Filler materials for brazing are covered by an AWS specification. They are classified according to analysis: aluminum-silicon, copper, copper-zinc, copper-phosphorus, copper-gold, heat-resisting materials, magnesium, and silver are the basic groupings. Filler metal selection is based on the metal being brazed.

Certain brazing filler metals contain cadmium in significant amounts. When these are used adequate ventilation is required.

Filler metals are available in many forms, the most common is the wire or rod. Filler metal is also available as thin sheet, powder, paste, or as a clad surface of the part to be brazed.
Aluminum Alloys
Brazing of aluminum alloys was made possible by the development of fluxes that disrupt the oxide film on aluminum without harming the underlying metal and filler metals (aluminum alloys) that have suitable melting ranges and other desirable properties, such as corrosion and mechanical resistance.

The aluminum-base filler metals used for brazing aluminum alloys have liquidus temperatures much closer to the solidus temperature of the base metal than those for brazing most other metals. The non-heat treatable wrought aluminum alloys, such as the 1xxx, 3xxx, and 5xxx (low-magnesium) series, have been brazed successfully. Alloys that contain higher magnesium contents are more difficult to braze by the usual flux methods because of poor wetting and excessive penetration by the filler metal. Filler metals that melt below the solidus temperatures of most commercial, non-heat treatable wrought alloys are available.

The most commonly brazed heat-treatable wrought alloys are those of 6xxx series. The 2xxx and 7xxx series of aluminum alloys have low melting points and therefore are not normally brazeable. Alloys that have solidus temperatures above 595oC are easily brazed with commercially binary aluminum-silicon filler metals. Higher-strength, lower-melting-points alloys can be brazed with proper attention to filler metal selection and temperature control, but the brazing cycle must be short to minimize penetration by the molten filler metal.

Sand and permanent mold casting alloys with high solidus temperatures are brazeable. Commercial filler metals for brazing aluminum are aluminum-silicon alloys containing 7 to 12 wt% Si. Brazing fillers with lower melting points are attained, with some sacrifice in resistance to corrosion, by adding copper and zinc. Filler metals for vacuum brazing of aluminum usually contain magnesium.

Most filler metals are used for any of the common brazing processes and methods. Two alloys, 4004 and 4104, have been developed exclusively for use in fluxless vacuum brazing.

Brazing of aluminum to copper is difficult, because of the low melting temperature, 548oC, of the aluminum-copper eutectic and its extreme brittleness. The eutectic is formed due to dissolution of aluminum during brazing. By heating and cooling rapidly, however, it is possible to make reasonably ductile joints for applications such as copper inserts in aluminum castings for electrical conductors.

Copper and Copper Alloys
Most copper and copper alloys can be brazed satisfactorily using one or more of the conventional brazing processes: furnace, torch, induction, resistance, and dip brazing. Their brazeabilities are rated from good to excellent.

Brazing of Tough Pitch Coppers. Tough pitch coppers are subject to embritllement when heated at temperatures above 480oC in reducing atmospheres containing hydrogen. Phosphorus-deoxidized and oxygen-free coppers can be brazed without flux in hydrogen-containing atmospheres without risk of embritllement, provided self-fluxing filler metals are used.

Brazing of Red and Yellow Brasses. Red and yellow brasses are readily brazed with a variety of filler metals. Flux is normally required for best results, especially when the zinc content is above 15 wt%. Low-melting filler metals should be used to avoid dezinfication of the yellow brasses. If added to red brass or yellow brass, lead forms a dross on heating that can seriously impede wetting and the flow of filler metal. Consequently, in brazing leaded brasses, the use of a flux is mandatory to prevent dross formation in the joint area.

Brazing of Phosphor Bronzes. Phosphor bronzes contain small amounts of phosphorus, up to approximately 0,25 wt%, added as deoxidizer. Although susceptible to hot cracking in the coldworked condition, alloys in this group have good brazeability and are adaptable to brazing with any of the common filler metals that have melting temperatures lower than that of the base metal.

Nickel-Base Alloys
In the selection of a brazing process for a nickel-base alloy, the characteristics of the alloy must be carefully considered. The nickel-base alloy family includes alloys that differ significantly in physical metallurgy (such as precipitation-strengthened versus solid-solution strengthened) and in process history (such as cast versus wrought). These characteristics can have a profound effect on their brazeability.

Precipitation-hardenable alloys present several difficulties not normally encountered with solid-solution alloys. Precipitation-hardenable alloys often contain appreciable (greater than 1 wt%) quantities of aluminum and titanium. The oxides of these elements are almost impossible to reduce in a controlled atmosphere (vacuum, hydrogen). Therefore, nickel plating or the use of a flux is necessary to obtain a surface that allows wetting by the filler metal.

Titanium and Titanium Alloys
Titanium is one of the chemical elements that reacts readily with oxygen to form an adherent and stable oxide. This oxide gives titanium and titanium alloys excellent corrosion resistance. Properties such as corrosion resistance, light weight, and high strength make titanium especially attractive in aerospace and chemical applications.

Since titanium and titanium alloys are very resistant to corrosion, the filler metal should be selected carefully to avoid galvanic corrosion.

Titanium and its alloys are usually brazed by induction or furnace processes with a protective atmosphere. The brazing atmosphere is usually a vacuum less than 13 mPa or an inert atmosphere with dew point lower than -55oC. Vacuum brazing with filler metals that contain silver or gallium must be performed with an argon back pressure to avoid the vaporization loss of these elements.

In induction brazing, the chemical elements of the filler metal must alloy readily with titanium, due to the fast heating cycle. However, furnace brazing requires a filler metal with a chemical composition that does not alloy excessively with titanium, due to longer period at elevated temperatures. Torch brazing is not usually done because special fluxes and a skilled operator are required.

b and a+b titanium alloys can be strengthened by heat treatment. The basic heat treatment consists of heating in single-phase field (b alloys) or in a two-phase field (a+b alloys), followed by a quenching. The aging is performed between 480 and 650oC. During the aging, a or other compounds will precipitate in a b matrix, increasing the strength and toughness of the alloy. Thus, the thermal cycle during brazing may affect the mechanical properties of the base metal. The b alloys should be brazed at a temperature close to the solubility temperature. Brazing at higher temperatures will reduce the ductility of these alloys.