Brazing

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Brazing

  1. 1. 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.
  2. 2. 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.
  3. 3. 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.
  4. 4. Four Steps in Brazing •The 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.
  5. 5. Types of Brazed Joints
  6. 6. Advantages •Economical fabrication of complex and multi component assemblies •Simple method to obtain extensive joint area or joint length •Joint temperature capability approaching that of base metal •Excellent stress distribution and heat transfer properties •Ability to preserve protecting metal coating or cladding •Ability to join cast materials to wrought metals •Ability to join nonmetals to metals
  7. 7. • 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 stressfree condition • Ability to preserve special metallurgical characteristics of metals • Ability to join fiber- and dispersionstrengthened composites • Capability for precision production tolerance • Reproducible and reliable quality control techniques
  8. 8. COMPARISION OF JOINING METHODS
  9. 9. Parameter Soldering Joint formed Mechanical Filler metal <450 melt temp. (ºC) Brazing Metallurgical >450 (less than m.p. of base metal) Base metal Fluxes Heat sources Does not melt Optional Furnace;torch; Induction; inFrared Atypical Tendency burn Does not melt Required Soldering iron; ultraSonics;oven to Atypical Welding Metallurgical >450 (less than or equal to m.p. of base metal) Melts Optional Plasma;laser Resistance; Electron beam Potential distortion
  10. 10. BRAZING PROCESSES •Torch brazing •Furnace brazing •Vacuum brazing •Dip brazing •Salt-bath brazing •Infrared brazing •Electric blanket brazing •Induction brazing •Resistance brazing •Exothermal brazing
  11. 11. 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.
  12. 12. 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.

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