Brazing is defined as a joining process wherein coalescence is produced between the adherents by heating them to a suitable temperature above 450ºc and by using a filler nonferrous alloy having its liquidus temperature above 450ºC and below solidus temperature of used base metals.

Principle

The joint is produced by diffusion of elements of filler metal into the base metal or vice versa. Diffusion of the elements creates bonds, which contributes to joint. Since the filler metal is in liquid state the diffusion rate is faster than in solids. The capillary action plays an important role in holding the liquid filler metal which would otherwise flow out. After soaking the samples for a long time at brazing temperature the samples are quenched to room temperature.

The wetting angle depends upon the free surface energy of liquid-vapor interface, solid-vapor interface and solid-liquid interface. For a good wetting the wetting angle should be less than 90º. So the free surface energy of solid-vapor interface must be greater than solid-liquid interface. The presence of adsorbed molecules on a metal surface markedly decreases the surface energy of solid-vapor interface and thus increasing the contact angle. Therefore the brazing surfaces should be free from any oxide layer or impurity. Good wetting increases the brazing efficiency.

Four Steps in BrazingThe assembly or the region of the parts to be joined is heated to a temperature of at least 450ºC.The assembled parts and brazing filler metal reach a temperature high enough to melt the filler metal but not the parts.The molten filler metal, held in the joint by surface tension, spreads into the joints and wets the base metal surfaces.The parts are cooled or solidify, the filler metal, which is held in the joint by capillary attraction and anchors the parts together by metallurgical reaction and atomic bonding.

Types of Brazed Joints

Advantages

Economical fabrication of complex and multi component assembliesSimple method to obtain extensive joint area or joint lengthJoint temperature capability approaching that of base metalExcellent stress distribution and heat transfer propertiesAbility to preserve protecting metal coating or claddingAbility to join cast materials to wrought metalsAbility to join nonmetals to metals

Ability to join metal thickness that vary widely in size Ability to join dissimilar metals Ability to join porous metal components Ability to fabricate large assemblies in a stress-free condition Ability to preserve special metallurgical characteristics of metals Ability to join fiber- and dispersion-strengthened composites Capability for precision production tolerance Reproducible and reliable quality control techniques

COMPARISION OF JOINING METHODS

Parameter Soldering Brazing WeldingJoint formed Mechanical Metallurgical MetallurgicalFiller metalmelt temp.(ºC)

<450 >450 (less thanm.p. of basemetal)

>450 (lessthan or equalto m.p. ofbase metal)

Base metal Does not melt Does not melt MeltsFluxes Required Optional OptionalHeat sources Soldering iron;

ultra-Sonics;oven

Furnace;torch;Induction; in-Frared

Plasma;laserResistance;Electron beam

Tendency toburn

Atypical Atypical Potentialdistortion

BRAZING PROCESSES

Torch brazingFurnace brazingVacuum brazingDip brazingSalt-bath brazingInfrared brazingElectric blanket brazingInduction brazingResistance brazingExothermal brazing

Vacuum Brazing

Vacuum brazing is done by keeping the components in an evacuated chamber with low pressure and then applying heat.

Vacuum brazing is well suited for heat resistant nickel- and iron based alloys that contain aluminum or titanium, reactive metals, refractory metals and ceramics.

The filler metal can be used as a sheet, wire or powder paste or molten rod in the joint area.

Advantages

Vacuum removes all gases and thus reduces the chance of oxidation. The actual pressure used depends upon the base metal, the filler metal and the degree to which gases are expelled.

Removal of Oxides of most metals increases the brazing efficiency. Oxides are removed by dissociation, diffusion or chemical reaction.

The low pressure around the interface removes volatile gases and impurities from the metals. It improves frequently the properties of metals being brazed.