Diffusion welding is a solid state welding process that joins materials together without melting through the application of pressure and heat. It is commonly used to join refractory metals at temperatures just over half their normal melting point. Close tolerances and a protective atmosphere are required to accomplish the welding. The process is considered diffusion brazing when a filler material is placed between surfaces before joining. Diffusion bonding results from atomic diffusion and plastic deformation at the interface between surfaces. It is used for joining dissimilar metals and composites in applications like aerospace and nuclear industries.

1. 1
Diffusion Welding (DFW)
Diffusion welding is a solid state welding process which produces coalescence
of the faying surfaces by the application of pressure and elevated temperatures. The
process does not involve microscopic deformation melting or relative motion of the
parts. Filler metal may or may not be used. This may be in the form of electroplated
surfaces.
The process is used for joining refractory metals at temperatures that do not
affect their metallurgical properties. Heating is usually accomplished by induction,
resistance, or furnace. Atmosphere and vacuum furnaces are used and for most
refractory metals a protective inert atmosphere is desirable.
Successful welds have been made on refractory metals at temperatures slightly
over half the normal melting temperature of the metal. To accomplish this type of
joining extremely close tolerance joint preparation is required and a vacuum or inert
atmosphere is used. The process is used quite extensively for joining dissimilar
metals. The process is considered diffusion brazing when a layer of filler material is
placed between the faying surfaces of the parts being joined. These processes are
used primarily by the aircraft and aerospace industries.

2. 2
Diffusion Bonding
• A process in which the strength of the joint results from
– diffusion (primarily), and
– plastic deformation of the faying surfaces
• Diffusion is the movement of atoms across the interfaces
• Temperatures about 0.5 Tm (absolute)
• The bonded interface has the same physical & mechanical
properties as the base metal
• Strength of bond depends on:
– Pressure
– Temperature
– Time (duration) of contact
– Cleanliness of faying surfaces
• Bonding may be facilitated by use of a filler metal at the
interface
• For some materials,
at interface
brittle intermetallic compounds may form
– Prevented by electroplating the surfaces

3. matrix composite materials
Diffusion Bonding
• Use high pressure autoclaves for complex parts
• Suitable for joining
– Dissimilar metals (most common)
– Reactive metals (e.g. Titanium, Beryllium)
– Metal-matrix composite materials
• An important PM sintering mechanism
• Relatively slow process
– To allow time for diffusion
• Automation enables economic production in
moderate volumes;
– Aerospace, nuclear, electronics
• Requires skilled operator

4. FIGURE 31.17 Aerospace diffusion bonding applications.

5. FIGURE 31.18 The sequence of operations in the fabrication of a
structure by the diffusion bonding and superplastic forming of three
originally flat sheets. See also Fig. 16.48. Sources: (a) After D. Stephen
and S.J. Swadling. (b) and (c) Courtesy of Rockwell International Corp.

6. FIGURE 31.19 (a) Cutaway view of the piston, showing the oil gallery
and friction-welded sections; (b) detail of
the friction welds before the external flash is removed by machining;
note that this photo is a reverse of the one on the left.

7. FIGURE 31.19 (a) Cutaway view of the piston, showing the oil gallery
and friction-welded sections; (b) detail of
the friction welds before the external flash is removed by machining;
note that this photo is a reverse of the one on the left.