Effect of Heat Exposure on the Fatigue Properties of AA7050 Friction Stir Welds
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Effect of Heat Exposure on the Fatigue Properties of AA7050 Friction Stir Welds B.C. White, R.I. Rodriguez, A. Cisko, J.B. Jordon, P.G. Allison, T. Rushing, and L. Garcia (Submitted December 1, 2017; in revised form February 16, 2018) 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. Keywords
aluminum, fatigue, fracture, friction stir welding, heat damage
1. Introduction Aluminum alloys have been extensively used as structural materials in the transportation industries because of their high strength-to-weight ratio, ease of machining and forming and corrosion resistance. However, difficulties in joining highstrength aluminum alloys cause inefficiencies in structural design and increased manufacturing costs (Ref 1). For example, in 2xxx and 7xxx series aluminum alloys, traditional fusion welding is problematic due to the formation of brittle secondary phases, and solidification and liquation cracking (Ref 2-4). Because of this limitation, significant effort has been undertaken in developing friction stir welding (FSW) for joining hard to weld alloys (Ref 5-10). Since FSW is a solid-state welding technique, many of the problems associated with the fusion zone in traditional welding are eliminated, though it does have drawbacks—such as ineffective material mixing, lack of penetration and excessive flash (Ref 2). In particular, multiple studies have shown the potential for FSW joining of AA7050, where yield strength joint efficiencies of 55-62% and ultimate strength joint efficiencies of 77-92% are reported (Ref 7-10). Because of the recent use of AA7050 as a possible replacement for AA7075 due to improved stress corrosion cracking and fatigue resistance, the fatigue performance of FSW AA7050 has been investigated in only a limited scope (Ref 9-11). Additionally, B. C. White, R. I. Rodriguez, A. Cisko, J. B. Jordon, and P. G. Allison, Department
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