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About Welding Process 45

Diffusion bonding is a solid state welding process that joins materials together without melting them through the application of high pressure and temperature. Bonding occurs due to diffusion and depends on factors like temperature, time, and pressure. An interlayer can improve bonding characteristics by promoting recrystallization and grain boundary migration at the interface. The major advantages of diffusion bonding include joining materials with strengths approaching the base metal and enabling the bonding of dissimilar materials like metals and ceramics.

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29 views1 page

About Welding Process 45

Diffusion bonding is a solid state welding process that joins materials together without melting them through the application of high pressure and temperature. Bonding occurs due to diffusion and depends on factors like temperature, time, and pressure. An interlayer can improve bonding characteristics by promoting recrystallization and grain boundary migration at the interface. The major advantages of diffusion bonding include joining materials with strengths approaching the base metal and enabling the bonding of dissimilar materials like metals and ceramics.

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Welding Processes 45

cause local plastic and creep deformation at the temperature of operation. Bonding
will take place due to diffusion and will depend on temperature, time and the
pressure applied. An interlayer foil or coating may be used to improve the bonding
characteristics. Recrystallization and grain boundary migration at the interface
will occur at the final stages of bonding and these processes are essential for
obtaining high strength joints and to eliminate the planar boundary interface.
The pressure can be applied on the surfaces to be joined through a platten. Gas
pressure can also be used to achieve the same. To preserve the clean surface the
bonding can be carried out in vacuum. If gas pressure is employed for the application
of load, then an argon gas chamber can be used. The major advantages of diffusion
bonding are:
(a) Joint strength can approach that of the base metal.
(b) Sintered products, dissimilar materials including metal and ceramics can
be joined by this process.
(c) Bonding involves minimum deformation and distortion and hence close
dimensional control is possible.
(d) In metal ceramics joints residual stresses can be reduced by multiple
interpayers.
(e) Thin sheets of fine grained superplastic materials can be easily joined and
formed to any desired shape and contour by diffusion bonding.
(f) Large area bonding is possible and thick and thin sections can be easily
joined. Process time is independent of area or number of components.
(g) Machining cost is reduced and no flux or electrode is necessary.
The important variables that will affect the bond quality are (i) surface roughness,
and (ii) surface oxide films. Stable oxide films such as Al2O3 or Cr2O3, nitride or
carbide films make solid state bonding difficult and need to be removed by spatter
cleaning. However, unstable oxides such as Ag2O or oxides soluble in the base
metal such as TiO2, allow good bonds to be made if the operating temperature is
above the oxides' instability temperature.
Some typical examples of diffusion bonding are presented in the following:
Titanium alloys: The operating temperature is kept above 850°C so that the
surface oxide film TiO2 will dissolve in the parent metal. The environment must
be free from oxygen and moisture. For thick sheet a pressure of 14 MPa may be
applied for bonding. Void defects in diffusion bonding joints usually originate
from grinding or machining grooves on the surface. In inert gas environment voids
can be caused by entrapped gas. Good surface finish of the base materials and
bonding in vacuum will reduce the chance of void formation. Typical joint shear
strength for Ti-6Al-4V sheet will be 575 MPa (almost the same as the parent metal
strength), for diffusion bonded joint, as compared to 10 MPa for rivetted joints
and 40 MPa for adhesive bonded joints.
Aluminium alloys : The presence of tenacious surface oxide film makes
diffusion bonding of aluminium alloys very difficult. Introduction of soft interlayers
in the form of cladding coating or foil inserts enables bonds to be made with small
overall deformation. 7475 aluminium alloy sheets can be successfully welded using
5052-Al-Mg alloy as an interlayer and the operating temperature and applied

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