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Applications: Friction Stir Weld

 
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Residual Stress
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(preprint) Prime, M. B., Gnäupel-Herold, T., Baumann, J. A., Lederich, R. J., Bowden, D. M., Sebring, R.J., “Residual Stress Measurements in a Thick, Dissimilar Aluminum Alloy Friction Stir Weld,” Acta Materialia, 54(15), 4013-4021, 2006. (LA-UR-06-0512) *** DOI link if you have access to this journal and want the final article instead of a preprint.

paper on the effect of removing the test specimen from a larger plate. "Contour-Method Determination of Parent-Part Residual Stresses Using a Partially Relaxed FSW Test Specimen," (LA-UR-04-2131) For more on residual stress in friction stir welds, see also the papers by Dalle Donna here.

Results (see below for details):

measures residual stresses in friction stir weld

  • This is map of the longitudinal residual stress over the cross section of a friction stir weld between 2024 aluminum and 7050 aluminum. Two tensile peaks on either side of the weld are commonly observed for friction stir welds.
  • Although fairly low, these stresses are very important. For example, such residual stresses in test coupons affect measurements of fatigue crack growth rates
  • Contour results compare very well with neutron diffraction measurements on same part:

  • These are line scans at 3 different positions in the above stress map
  • Measurements done by Thomas Gnaupel-Herold at NIST
  • Considering low stress magnitudes, agreement is great
  • Agreement is better in weld and poorer in base material, probably because of microstructural changes effecting neutron results
Highlights of this measurement and specimen: (back to top)
Specimen:
  • Friction stir weld between 25.4 mm thick plates of aluminum alloys 7050-T7451 and 2024-T351
  • A one-pass single-sided joint was made with the 2024 on the advancing side
  • After welding the panel was aged at 121°C for 24 hours to stabilize the weld nugget
  • Because of other testing requirements, a small test specimen was removed for residual stress measurement (see below for effect of small test specimen)
friction stir welded plate

EDM cutting:
  • Picture shows test piece being cut by wire EDM.
  • Cuts were made using 100 µm brass wire and "skim cut" settings
friction sir weld being cut by wire EDM

Metallography :
  • This is a micrograph of the weld cross section
  • As in the results at the top of the page, the left side is the 7050, the right side is the 2024, which is the advancing side of the weld

Measure contour :
  • After EDM cut but before metallography, measure surface contours
  • Surfaces scanned using confocal laser ranging probe
  • Peak-to-valley is about 20 µm
  • The low spots correspond to tensile stress regions

FEM stress calculation:
  • To calculate stress, elastically forced FEM model into opposite of measured contour (shown just above this)
  • Displacements exaggerated to show shape
  • Performed using ABAQUS

Change in stresses from removing small test specimen: The residual stresses were measured in a 54 mm long test specimen that had been removed from a 457 mm long plate. Because of the short length of the test specimen, the stresses relaxed somewhat when the specimen was removed. In the paper (pdf) listed at the top of the page, a method for calculating how much the stresses relaxed is described. The results of that estimation are below, showing that the peak tensile stresses were relaxed by about 25%. (back to top)

Stresses measured in test specimen (same as results on top of page, just color scale changed)

Estimated stress in parent plate prior to stress relaxation caused by removing specimen

 

 

 

 

 

 

 

 

 

 

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Operated by Los Alamos National Security, LLC for the U.S. Department of Energy's NNSA
contact: Mike Prime at prime@lanl.gov | Copyright & Disclaimer
U.S. patent 6,470,756 | Last Modified: November 23, 2009 }

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