Lecture 4 Diffusion Bonding. 3.371 Fabrication Technology / Prof. Eagar / Copyright 2010 2 Review...

Lecture 4

Diffusion Bonding

3.371 Fabrication Technology / Prof. Eagar / Copyright 2010 2

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


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)


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)


• 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

• Excellent process, makes perfect joints (if you can afford it)

Diffusion Bonding, continued

Mo 2500C6*10-6 / C

900-1300 C

15*10-6 / C

Thermal Expansion to Force Parts Together


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

• Tendency of grains to grow across interface depends on temperature– If the temperature is too high, process moves to stage two too quickly, and we

get a bad joint full of porosity– At lower temperatures, the process just takes too long


Diffusion Bonding, OverheadThree Stage Mechanistic Model of Diffusion Bonding


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

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

Hipping Press


50 T

200 T

60”

Hipping Press (300 T)

Ni Can

11”

17”

Ar20,000 Psi

1000-2000 C

50 T

Transient Liquid Phase (TLP)


Reduces fixturing pressure to ~10psiShortens diffusion time to a few hoursLiquid provides 100% contact area

Must have a non-harmful melting point depressant

Ni - BoronFe - CAl - Sn, ZnTi - Cu, Ni, AgCu - Zn, Sn, Ag, Au, InAu - Sn

Isothermal Solidification During TLP Bonding


Phase Diagram for Solidification of Ni-B Alloys


Temp

% Boron

IsothermalSolidification

AthermalSolidification

NiB3

FCC

S + L

L

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

Al2O3 – Mn

Si

Al

TLP Bond: Silicon Chip to Carrier


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


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



Diffusion Bonding ExampleAEGIS Electronic Warfare SystemCross 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


Diffusion Bonding Example, overhead

AEGIS Electronic Warfare System


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


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


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

Supplemental Info


http://eagar.mit.edu/3.37/H-337-18.pdf






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Lecture 4 Diffusion Bonding. 3.371 Fabrication Technology / Prof. Eagar / Copyright 2010 2 Review...

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