Materials Science and Engineering A 519 (2009) 61–69 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Fatigue crack growth performance of peened friction stir welded 2195 aluminum alloy joints at elevated and cryogenic temperatures Omar Hatamleh a,∗ , Michael Hill b , Scott Forth c , Daniel Garcia d a Structures Branch, NASA – Johnson Space Center, Houston, TX 77058, United States b Mechanical & Aeronautical Engineering, University of California, Davis, CA 95616, United States c Materials & Processes Branch, NASA – Johnson Space Center, Houston, TX 77058, United States d Science Applications International Corporation, Houston, TX 77058, United States article info Article history: Received 3 February 2009 Received in revised form 20 April 2009 Accepted 24 April 2009 Keywords: Friction stir welding Laser peening Shot peening Fatigue crack growth Aluminum–lithium 2195 abstract The effects of various surface treatments on fatigue crack growth and residual stress distributions in friction stir welded 2195 aluminum alloy joints were investigated. The objective was to understand the degree to which residual stress treatments can reduce fatigue crack growth rates, and enhance fatigue life of friction stir welded components. Specimens were fabricated from 12.5mm thick 2195-T8 aluminum plate, with a central friction stir weld along their length. Residual stresses were measured for three spec- imen conditions: as-welded (AW), welded then shot peened (SP), and welded then laser peened (LP). Crack growth rate tests were performed in middle-cracked tension specimens under constant amplitude load for each of the three conditions (AW, SP, LP) and at three temperatures (room, elevated, and cryo- genic). At room and elevated temperature, crack growth rates were similar in the AW and SP conditions and were significantly lower for the LP condition. At cryogenic temperature, it was difficult to discern a trend between residual stress treatment and crack growth rate data. Laser peening over the friction stir welded material resulted in the fatigue crack growth rates being comparable to those for base material. Published by Elsevier B.V. 1. Introduction The aluminum–lithium alloys are of great interest in mod- ern aerospace applications [1] due to their lower density, greater elastic modulus, and higher specific strengths [2] compared to traditional aluminum alloys. Aluminum–lithium alloys have been developed primarily to reduce the weight of aircraft and aerospace structures. Aluminum alloys are generally difficult to fusion weld because mechanical properties can be seriously degraded as a result of heat-induced changes to their precipitate structure and dentritic structures formed during the fusion welding process [3]. Aluminum–lithium alloys do not offer improved weldability over other high strength aluminum alloys. Friction stir welding (FSW) [4] has emerged as a successful alter- native to fusion welding for joining aluminium alloys. During FSW, welded components exhibit temperatures below their melting tem- perature, and the residual stresses generated during welding are generally lower than those in fusion welds. Nonetheless, the heat- ing cycle and rigid clamping arrangement during FSW can generate elevated residual stresses in the weld [5–7]. For example, a recent investigation by Hatamleh et al. [8] on friction stir welded AA 2195 ∗ Corresponding author. Tel.: +1 281 483 0286; fax: +1 281 244 5918. E-mail address: omar.hatamleh-1@nasa.gov (O. Hatamleh). identified residual stress values around 231MPa near the weld region. Tensile residual stresses typically degrade the fatigue prop- erties [9] of joints and the structural integrity of weldments. For that reason, compressive residual stress treatments, through techniques like shot or laser peening, can enhance the mechanical proper- ties of welded components [10,11], and could extended fatigue life [12–14]. Shot peening (SP) is an established surface treatment where the surface of a part is deformed plastically by multiple overlapping impacts of glass or metal spheres. Shot peening creates a shallow layer, typically 0.25–0.5mm deep, of compressive residual stress [15,16] that can improve fatigue performance. Laser peening (LP) is a surface treatment capable of introducing a far deeper layer of compressive residual stress than SP, typically 1–3mm deep [17], and can therefore provide a greater benefit on fatigue performance than provided by SP. LP has proven capable of enhancing the fatigue properties of a number of metallic materials [18–26]. The present paper is intended to complement recent work on FSW joints. A significant amount of recent work [27–34] has inves- tigated the fatigue behavior of FSW aluminum alloys at room temperature in the absence of residual stress treatments. This study evaluates fatigue crack growth rate (FCGR) behavior of FSW joints in AA 2195, and compares FCGR in as-welded (AW) specimens with those treated by SP and LP. Because fatigue FCGR data are affected by residual stresses, we also measure residual stresses in the AW, 0921-5093/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.msea.2009.04.049