Softening Effects Induced by Shot Peening for an Aluminum-Lithium Alloy
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THE ongoing demand for a reduction in structural airframe weight has inspired the development of aluminum-lithium (Al-Li) alloys.[1] Compared with those of conventional Al alloys, Al-Li alloys have a lower density, higher specific strength, and higher specific stiffness.[2,3] Among Al-Li alloys, AA2060 is a representative example and is considered a promising aeronautical material. Thus, AA2060 has been widely applied in transport aircrafts from wing spars and ribs to other internal structures in the wings and fuselages. However, the quality of their surface properties may determine their application scope. To improve the properties of Al-Li alloys, shot peening has been extensively employed owing to its effectiveness and low cost.[4,5] Shot peening is a typical
XUEFEI TAO and YUKUI GAO are with the School of Aerospace Engineering and Applied Mechanics, Tongji University, 100 Zhangwu Road, Shanghai 200092, China. Contact e-mail: [email protected] JIANMEI KANG is with the National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao City, 066004 Hebei Province, China. YUHUI WANG is with the National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University and also with the Research Center for Light and High-performance Materials, Nanjing 211816, China. Manuscript submitted February 17, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
surface modification technology holding great potential for industrial applications, in which the surface of a component is impacted with small spherical media called shots. To date, considerable researches have emphasized the strengthening factors such as compressive residual stresses,[6] grain refinement,[7] work hardening as well as the subsequent improvement in fatigue strength.[8–10] Although the properties of Al-Li alloys can be improved by appropriate shot peening, relatively high-intensity shot peening of Al-Li alloys can induce softening effects which are detrimental to their mechanical properties, especially those of thin plates. Huang et al.[11] first reported the phenomenon of work softening in pure Al and aroused a widespread interest. Later, many researchers reported that the mechanisms of work softening during hot deformation mainly included dynamic recrystallization (DRX) and the transformation of the deformation mechanisms of ultrafine grains.[12,13] Nageswara et al.[14] investigated the effect of annealing on the mechanical properties of an ultrafine-grained 6061 Al alloy and found that a reduction in hardness and strength and an increase in ductility were due to dynamic recovery (DRV) and a decrease in the dislocation density. Nevertheless, the role of grain orientation in determining mechanical properties was not brought into consideration. Kumar et al.[15] conducted an experimental evaluation of the critical grain size for a change in deformation behavior from work hardening to work softening in an Al-Mg-Sc alloy. They found that when the grain size was larger than
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