Metallurgical and Materials Transactions A

, Volume 31, Issue 9, pp 2181–2192 | Cite as

Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-T7451

  • K. V. Jata
  • K. K. Sankaran
  • J. J. Ruschau
Article

Abstract

Aluminum alloy 7050 was friction-stir welded (FSW) in a T7451 temper to investigate the effects on the microstructure and mechanical properties. Results are discussed for the as-welded condition (as-FSW) and for a postweld heat-treated condition consisting of 121 °C for 24 hours (as-FSW + T6). Optical microscopy and transmission electron microscopy (TEM) examination of the weld-nugget region show that the FS welding process transforms the initial millimeter-sized pancake-shaped grains in the parent material to fine 1 to 5 µm dynamically recrystallized grains; also, the FS welding process redissolves the strengthening precipitates in the weld-nugget region. In the heat-affected zone (HAZ), the initial grain size is retained, while the size of the strengthening precipitates and of the precipitatefree zone (PFZ) is coarsened by a factor of 5. Tensile specimens tested transverse to the weld show that there is a 25 to 30 pct reduction in the strength level, a 60 pct reduction in the elongation in the as-FSW condition, and that the fracture path is in the HAZ. The postweld heat treatment of 121 °C for 24 hours did not result in an improvement either in the strength or the ductility of the welded material. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW + T6 condition, at a stress ratio of R = 0.33, shows that the FCG resistance of the weldnugget region is decreased, while the FCG resistance of the HAZ is increased. Differences in FCGRs, however, are substantially reduced at a stress ratio of R = 0.70. Analysis of residual stresses, fatigue-crack closure, and fatigue fracture surfaces suggests that decrease in fatigue crack growth resistance in the weld-nugget region is due to an intergranular failure mechanism; in the HAZ region, residual stresses are more dominant than the microstructure improving the fatigue crack growth resistance.

Keywords

Residual Stress Material Transaction Crack Closure Postweld Heat Treatment Weld Nugget 
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Copyright information

© ASM International & TMS-The Minerals, Metals and Materials Society 2000

Authors and Affiliations

  • K. V. Jata
    • 1
  • K. K. Sankaran
    • 2
  • J. J. Ruschau
    • 3
  1. 1.the Materials and Manufacturing DirectorateAir Force Research Laboratory, AFRL/MLLMWright-Patterson Air Force Base
  2. 2.the Boeing CompanySt. Louis
  3. 3.the Materials Engineering DivisionUniversity of Dayton Research InstituteDayton

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