Lecture 4 Diffusion Bonding

3.371 Fabrication Technology / Prof. Eagar / Copyright 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 T m Al, Mg are difficult to bond Fe, Cu, Ti, and Ni are relatively easy to bond

3.371 Fabrication Technology / Prof. Eagar / Copyright 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)

3.371 Fabrication Technology / Prof. Eagar / Copyright Diffusion Bonding – Problems Metals lose all prior cold work Must be done at 0.6 – 0.8 T m Thermal expansion stresses 500 – 5000 psi bonding pressures require expensive tooling Long diffusion times (24 – 48 hours)

3.371 Fabrication Technology / Prof. Eagar / Copyright 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 2500C 6*10 -6 / C C 15*10 -6 / C Thermal Expansion to Force Parts Together

3.371 Fabrication Technology / Prof. Eagar / Copyright 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

3.371 Fabrication Technology / Prof. Eagar / Copyright Diffusion Bonding, Overhead Three Stage Mechanistic Model of Diffusion Bonding

3.371 Fabrication Technology / Prof. Eagar / Copyright 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 Fabrication Technology / Prof. Eagar / Copyright 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 Fabrication Technology / Prof. Eagar / Copyright T 200 T 60” Hipping Press (300 T) Ni Can 11” 17” Ar 20,000 Psi C 50 T

Transient Liquid Phase (TLP) Fabrication Technology / Prof. Eagar / Copyright 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- Boron Fe- C Al- Sn, Zn Ti- Cu, Ni, Ag Cu- Zn, Sn, Ag, Au, In Au- Sn

Isothermal Solidification During TLP Bonding Fabrication Technology / Prof. Eagar / Copyright

Phase Diagram for Solidification of Ni-B Alloys Fabrication Technology / Prof. Eagar / Copyright Temp % Boron Isothermal Solidification Athermal Solidification NiB 3 FCC S + L L

TLP, cont Fabrication Technology / Prof. Eagar / Copyright 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 Al 2 O 3 – Mn Si Al TLP Bond: Silicon Chip to Carrier

3.371 Fabrication Technology / Prof. Eagar / Copyright 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

3.371 Fabrication Technology / Prof. Eagar / Copyright 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

3.371 Fabrication Technology / Prof. Eagar / Copyright

3.371 Fabrication Technology / Prof. Eagar / Copyright 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 (Ni 3 Mo) – 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

3.371 Fabrication Technology / Prof. Eagar / Copyright Diffusion Bonding Example, overhead AEGIS Electronic Warfare System

3.371 Fabrication Technology / Prof. Eagar / Copyright 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

3.371 Fabrication Technology / Prof. Eagar / Copyright 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

3.371 Fabrication Technology / Prof. Eagar / Copyright 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 Fabrication Technology / Prof. Eagar / Copyright