Effect of Heat Exposure on the Fatigue Properties of AA7050 Friction Stir Welds
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This work examines the effect of heat exposure on the subsequent monotonic and fatigue properties of friction stir-welded AA7050. Mechanical characterization tests were conducted on friction stir-welded specimens as-welded (AW) and specimens heated to 315 °C in air for 20 min. Monotonic testing revealed high joint efficiencies of 98% (UTS) in the AW specimens and 60% in the heat-damaged (HD) specimens. Experimental results of strain-controlled fatigue testing revealed shorter fatigue lives for the HD coupons by nearly a factor of four, except for the highest strain amplitude tested. Postmortem fractography analysis found similar crack initiation or propagation behavior between the AW and HD specimens; however, the failure locations for the AW were predominantly in the heat-affected zone, while the HD specimens also failed in the stir zone. Microhardness measurements revealed a relatively uniform strength profile in the HD group, accounting for the variety of failure locations observed. The differences in both monotonic and cyclic properties observed between the AW and HD specimens support the conclusion that the heat damage (315 °C at 20 min) acts as an over-aging and a quasi-annealing treatment.
Keywordsaluminum fatigue fracture friction stir welding heat damage
A portion of this work was performed under the auspices of the U.S. Army Engineer Research and Development Center, administered by SOSSEC, INC, Subcontract No. 1006-14-1-1. The authors would like to recognize the efforts of the Edison Welding Institute in conducting the friction stir welding, as well as the microscopy resources provided by the Central Analytical Facility and its staff at the University of Alabama. Permission to publish was granted by Director, Geotechnical and Structures Laboratory.
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