Characterization of residual stresses in heavy gauge 7140-T7451 plate and resultant stress profile in complex three dimensional components

Contemporary military and commercial aircraft utilize large monolithic aluminum structures which minimize part count, reduce fabrication/assembly and deliver improved performance. A number of advanced 7XXX and 2XXX aluminum plate alloys were developed to provide balanced and uniform properties in the 4.00-10.00” thickness range. One of the most important attributes of these heavy gauge plates is low level of residual stresses. Plates with uncontrolled level of thru thickness residual stresses result in warpage during machining increasing assembly problems plus high level of these stresses may impact fatigue resistance of finished components.

As a part of the Military Affordability Initiative sponsored by the Wright Patterson AFB, a team consisting of Lockheed Martin Corporation, Northrop Corporation, Hill Engineering and Constellium LLC completed a study aimed at characterization of residual stresses in 7140-T7451 plate with nominal size of 6”X60”X240”. Multiple residual stress coupons were excised at locations representing both longitudinal and long transverse orientation. These specimens provide compressive/tensile profile of residual stresses in the raw plate. Subsequently machining blanks were obtained adjacent to the residual stress coupons. Complex three dimensional shapes were machined by Ultimate Machining. Three discrete geometries of varying complexity were produced emulating typical bulkheads and frames used on the aircraft. Multiple replicates were produced from each of the three geometries. Subsequently, level of residual stresses in the finished components was quantified by Hill Engineering. This allowed for correlation of the residual stresses in the raw plate and final components. Results of the study will be used to further optimize fabrication of the plate stock ensuring input material with minimum residual stresses.