Through Thickness Residual Stress and Microstructural Mapping of AA7085-T7452 Die Forging

Abstract

Abstract. Significant weight savings and reductions in manufacturing costs have motivated the drive towards the use of unitised structures such as large die forgings in modern aircraft. However, detrimental bulk tensile residual stresses, present in these thick forgings can impact the durability and structural integrity of the primary aerospace structure. Residual stresses can lead to distortion during component machining, and part rejection. In addition, failure to account for the residual stress effects in the fatigue crack growth characteristics of the component in design and in performance could impact the structural integrity. Therefore, there is a need to measure and analyse the residual stresses present in the forged components, and examine the residual stress-microstructure, including grain structure/flow relationship for the forging, if such a relationship could be developed. The preliminary results of this experimental program have shown a link between the measured residual stress and hardness, along with changes in the grain structures through the thickness. IntroductionStructural unitisation of components is being actively pursued by the aerospace industry with the primary aim of weight reduction and cost savings. Large aluminium forgings and thick section rolled products make a significant contribution towards unitisation of critical, primary airframe components, such as wing-carry-through bulkheads. Inherent with the thick section wrought product forms, and particularly in die forged components, is the built-in bulk residual stresses on a macro scale [1], arising from the thermal gradients in the thermo-mechanical processing and the postsolution treatment rapid quenching. The presence of process related bulk residual stresses results in: a) part distortion during machining which is a cost driver and b) high degree of scatter and/or variability in fatigue crack growth rate [2], which is a performance driver affecting the durability and damage tolerance of the component. The residual stresses in an aluminium 7050-T7452 forged block that caused distortion during machining was reported to be a major cost driver for a commercial aircraft manufacturer, as a result of excess manufacturing time, schedule overruns, and part rejection and scrapped material [3].The surface and subsurface compressive residual stresses in an as-quenched (before stress relief treatment) aluminium alloy forgings have been reported to be in the range -200 MPa -250 MPa [4]. The mechanical stress relief treatment by cold compression (forgings) or stretching (rolled plate) that follows, aims to relieve (or reduce) the severity of the bulk residual stresses existed in the asquenched state in the component. However, cold compression stress relief in die forged components do not always fully relieve the complex 3 D bulk residual stresses, and the unrelieved residual stresses may be of sufficient magnitude to cause distortion in machined components [4][5][6]. Moreover, the three dimensional residual stress distribution in the stress reli...