Fundamental issues and their potential impact on cavity fabrication and welding

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Fundamental issues and their potential impact on
cavity fabrication and welding

• Lance Cooley SRF Materials Group Leader
• Acknowledgments
• FNAL Mike Foley, Donna Hicks, Dave Burk, Chad
  Thompson,
• Rob Schuessler, Charlie Cooper, Genfa Wu, and
  Dmitri Sergatskov
• FSU / NHMFL Peter Lee and Zu Hawn Sung
• U Chicago Steven Sibener, Miki Nakayama, and
  Tuo Wang

Industry vendor meeting 6 March 2009
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What are the materials science experiements
telling us?

• Coupons pits appear after EP despite
  beautiful welds
• Anatomy of pits they have more complex
  structure!
• Small dislocation etch pits are observed on the
  fine scale at and nearby big pits
• Carbon seems to be associated with pit locations
• Plausible mechanisms and routes for remediation
• Possible experiments to try

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Pits hot spots
Hayano group, Kyoto Image enhanced by FSU
D. Sergatskov, C. Ginsburg FNAL
Thermometry locates hot spots
Hot spots correlate with location of defects
(pits)

• Common features
• Location at edge of HAZ
• Contour, topography

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Enhanced image of TE9AES001 hot spotsEnhancement
by Peter Lee, FSUBubbles, craters, or more
complex? 20 MV/m? 30 MV/m?
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January 2009
Instrumentation
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Weld couponsFNAL welded sheet corners into
coupons at Sciaky, then EPd at FNAL. Pits
appear after the EP
Pit row
HAZ
Pit 1
Stains
HAZ
Pit 2
Pit 3

• 210 total µm removed

• 110 total µm removed (total)

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Representative cross-section of the HAZ(this
is a 3rd sample, not yet electropolished)
1 mm
Water stain
Area marked HAZ in previous slide
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Pit 1
Pit 1 is a whopper on the weld 50 µm
deep!! Profile is across red line Imaged using
Keyence 3D microscopy
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The topography is more like a volcano or
castle and moat
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Pit 3
6 µm height, 51 µm span
Even the small pits have a characteristic
volcano or castle and moat feature
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Pit 1
ASC/NHMFL/FSU Imaged using Olympus Laser Scanning
Confocal 3D Microscopy (LSCM-LEXT) Obj 50X 2
by 2 tiling image Courtesy Zu-Hawn Sung and
Peter Lee, FSU
Profile is across the red line on the left 3D
surface image
LSCM finds weaker contours
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Whats this junk? Carbon? Smaller pits? Pit
pre-cursors?
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Grain Orientation on PIT1 region
EPed surface
OIM image on mechanically polished surface
Polished surface
Grain orientation Map
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Wide-area view of pits from EP coupons
3-D
Pit row using light microscopy and stitching
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Carbon?? Now being studied
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This sample is the cross-cut of pit 1 examined by
LSCM
This sample is the remaining weld (it contains
the remaining half of pit1)
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Samples have been cleaned and degreased this is
from the metal
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(EP side)
These have well-known characteristics of
dislocation pits
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Subsequent study Many black areas have high
carbon levels
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Do dislocations and interstitials pile up at the
edge of the HAZ? Does this produce abnormal
chemical activity?
1 mm
Rx
Rv
Cold Worked
Resolidified
Possible scenario dislocations are swept to edge
of recovered zone (Rv), where they pile up.
Carbon accumulates at dislocations, blocking
normal EP.
Area marked HAZ in previous slide
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Things to try

• Ensure high pumping conductance from normal zone
  to remove outgassing elements
• Increase grain size (reduce grain-boundary
  density)
• Fully recover, or even re-crystallize, material
  before weld
• Cool part quickly to prevent trace element
  absorption at dislocations
• Pay attention to material texture before and
  after deep drawing
• lt100gt grains will thin 2x faster and tend to
  store dislocations
• Improve material specification low carbon, lt111gt
  fiber texture, relax flatness spec
• Insert more QA at sheet level OIM

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