Trial & Success With Mu-metal For Stanford University
A Unique Challenge
Solar Atmospheres of Western Pennsylvania recently took on an exciting and uncommon project – annealing three 348” long (29’) shields for an atom interferometer apparatus for Stanford University. These were the longest single part components shields Stanford has ever made.
The material requiring annealing added another layer of complexity to this project. The shields were made of Mu-metal (75% Nickel, 15% Iron, balance Copper and Molybdenum) which is known for its high permeability. Anytime Mu-Metal is welded, bent or formed it must be annealed to alter the crystal structure to assure free motion of the magnetic domain boundaries.
Attacking The Unknown
Approaching this interesting project raised many questions before the shield tubes were ever made:
- Mu-Metal, how much distortion can we expect at 2150°F?
- What can we use to support the tubes to minimize distortion? Stainless?
- What are the known eutectics of Mu-metal and stainless, Mu-metal to Carbon (graphite), Molybdenum, etc?
- Is there a chance the fixturing may stick to the Mu-metal?
- How can we prevent the fixturing from bonding with the Mu-metal?
- What is the coefficient of thermal expansion (CTE) of Mu-metal versus the fixturing used?
Testing For Answers
Over a three month period and numerous conference calls, the above questions were answered through trial and error. Mock up scenarios on scrap Mu material was run in smaller vacuum furnaces to test various theories.
This approach is always recommended when dealing with different materials that have to eventually work together to achieve a successful thermal process.
As you can see in the photo, the 300 series stainless gussets worked very well in supporting the tubes throughout the thermal process. These gussets were also coated with milk of magnesia as the Magnesium helps ward off diffusion bonding and sticking during the thermal process. We also utilized hundreds of moly shims to bridge any gaps which may exist between the stainless gussets and our graphite platen. I really believe the hero of this whole job was the graphite base platen. As you know throughout any thermal process, all metals grow while raising temperature and contract once the cooling begins. The graphite allowed the fixture to move free without binding or restricting the material in any way.
Do You Have Our Next Challenge?
The one thing that is paramount throughout this whole process is Solar’s ability to thermally process mocked up scenarios in our smaller vacuum furnaces to truly understand how all these materials will interact with each other once at temperature.