Lecture 4
Diffusion Bonding
Review
• Adhesive joints require larger surface area for
strength
• Diffusion bonding can produce nearly perfect
joints, but is limited in materials, cost, etc.
• Diffusion bonding uses heat to dissolve,
diffuse or evaporate contamination and soften
asperities. Use at 0.6 – 0.8 Tm
• Al, Mg are difficult to bond
• Fe, Cu, Ti, and Ni are relatively easy to bond
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Diffusion Bonding / Welding
Addition of heat to pressure welding
• Aid in asperity deformation
• Diffuse away surface contamination
• Interlayers
– Avoid intermetallics
– Match thermal expansion coefficients
– Achieve compatible joining temperature
– Sometimes use interlayer to reach comparable
temperatures. Example: Al (lower melting point) to Fe
(higher melting point)
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Diffusion Bonding – Problems
• Metals lose all prior cold work
• Must be done at 0.6 – 0.8 Tm
• Thermal expansion stresses
• 500 – 5000 psi bonding pressures require expensive tooling
• Long diffusion times (24 – 48 hours)
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Diffusion Bonding, continued
• Industrial furnace can cost $1,000 / hr.
Expensive process, need parts with value of a
couple hundred dollars per pound
• Can use thermal expansion to force parts
together
– Example: circular parts in molybdenum mold
Thermal Expansion to Force Parts Together
15*10-6 / C
900-1300 C
Mo 2500C
6*10-6 / C
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Diffusion Bonding, continued
• Three-Stage Mechanistic Model
– First stage: grow grain boundary across interface
(this is when pressure must be applied)
• Stage 2 and Stage 3: once half the surface is grain
boundary, you don’t need pressure any more
– Second stage: grow common grains across the
interface, except for where voids are trapped in
the center
– Third stage: can get rid of original interface
completely, but may have a trapped void
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Diffusion Bonding, Overhead
Three Stage Mechanistic Model of Diffusion Bonding
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Hot Isostatic Pressing – Hipping
• Use a large pressure vessel
• Fill with hot, compressed argon (20 ksi, 1,000 – 1,200 C)
– Argon has density of water in those conditions
• Used for powder compression
– For example, to squeeze powders together and get rid of
porosity
– Use Ni can to keep argon out of parts
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Hipping, cont
• World’s largest diffusion bonding press was 300T, up in North
Andover.
– It blew up recently. Problems with design, steel, cooling
water, and transient thermal stresses
– Now we have two larger ones operating
• Uses: large jet engine parts are hipped; motorcycle’s
aluminum cylinder head is hipped
– Example: Paragreen Falcon aircraft. Building quasi-F-15
aircraft by purchasing engine and building airframe. Test
pilots crashed and died. 1-lb weights were adhesively
bonded to control surfaces and broke off
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Hipping Press
Hipping Press (300 T)
50 T
11”
Ar
20,000 Psi
1000-2000 C
Ni Can
200
T
60”
50 T
17”
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Transient Liquid Phase (TLP)
Reduces fixturing pressure to ~10psi
Shortens diffusion time to a few hours
Liquid provides 100% contact area
Must have a non-harmful melting point depressant
Ni Fe
Al Ti Cu
Au
Boron
- C
Sn, Zn
Cu, Ni, Ag
- Zn, Sn, Ag, Au, In
- Sn
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Isothermal Solidification
During TLP Bonding
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Phase Diagram for
Solidification of Ni-B Alloys
Temp
Isothermal
Solidification
L
S+L
FCC
NiB3
Athermal
Solidification
% Boron
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TLP, cont
•Braze alloy for nickel – boron widens joint, then shrinks
•Used to bond silicon chips to carrier
•Gold to gold diffusion bond
•Excellent bond: diffuse away tin
TLP Bond: Silicon Chip to Carrier
Si
Al
Al2O3 – Mn
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Review
• Diffusion bonding is limited on low side temperature by
kinetics and on high side by grain growth: 0.6 – 0.8 Tm
range
• TLP® (Transient Liquid Phase) diffusion bonding starts as
a braze and ends as a diffusion bond. It reduces the time
and pressure for bonding
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TLP Bonding
• Solidify by going across the phase diagram
rather than down
• Atruscans (200 BC) used diffusion bonding to
attach gold beads to copper pots
• Copper TLP bonds: copper powder, silver films
forms 100% solid bond (no porosity)
• Coat copper powder grains with nickel grains.
Nickel diffuses into copper, then silver will
diffuse into nickel-copper alloy
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Diffusion Bonding Example
AEGIS Electronic Warfare System
Cross field amplifier: wanted to braze copper – molybdenum joints
• Amplify radiation: magnetic field in copper modulates electron beam passing
through center hole
• Need high-temperature alloy (molybdenum) for heat resistance
• Brazing with 82Au 18Ni alloy
– Copper and nickel form a brittle intermetallic (Ni3Mo) – breaks after only 10%
stretch
– They had tried a 37Au – 3Ni – 60Cu braze alloy, which could be bent without brittle
fracture. This solved the problem. Gold and nickel diffused into base material and
formed a perfect TLP bond
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Diffusion Bonding Example, overhead
AEGIS Electronic Warfare System
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Activated Diffusion Bonding
• Surface coating changes difficult to bond
interface into easier to bond interface
M. Obrien et al. Weld J. Jan 1976 p.26
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Activated Diffusion Bonding, continued
• GE uses “activated diffusion bonding” to refer to TLP
diffusion bonding
• Achieve bond between high-strength iron with silver
layer in between
– Plated iron with silver in a chemical process, so this was
just a silver-silver bond. Silver is easy to bond because
silver oxide is not stable at elevated temperature
– Bond strength is 100 ksi, which is higher than 20 ksi yield
strength of silver. Phenomena is called contact
strengthening
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Q&A
• Titanium diffusion bonding for bicycles
– Interfaces need to be machined within 0.001”, and need to
pressure between tubes
– Usually gas tungsten arc welding
– Could use TLP bonding (variant of brazing) with Cu-Ni alloy.
Not a true TLP bond because the joint material remains
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Supplemental Info
http://eagar.mit.edu/3.37/H-337-18.pdf
http://eagar.mit.edu/3.37/H-337-19.pdf
http://eagar.mit.edu/3.37/H-337-20.pdf
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