BS in Materials Engineering
Materials Engineering Department
Open die forged 7050 aluminum alloy has residual stresses that can be measured by the hole-drilling method following standard ASTM E837-01. Weber Metals (Paramount, CA) assumes that the stress is uniform throughout the thickness of an open die forged plate. Four different stress relieved 7050 aluminum samples were tested to confirm if the measurements by the hole-drilling method at the surface of a plate is indicative of the stress throughout. The different stress relief methods included: two water quenched samples at temperature ranges of 60°-90°F and 120°-130°F and two samples plastically deformed by forging at room temperature 1% and 3%. For accurate measurements, a strain gauge rosette, CEA-06-062UL-120, was applied to the samples and drilled with a RS-200 Milling Guide by Micro-Measurements. The final three microstrain values at the depth of 0.100 inches were tabulated by the P3 Strain Indicator and Recorder. The strains were then converted into stresses following equations provided by the standard. Before drilling into samples, the method was calibrated with a plate that had a known low level of residual stress between 0-2 ksi. Testing confirmed the residual stress to be 0.65 ksi. Each sample was tested and results confirmed the residual stress to be non-uniform through the cross-section of the forged plates. The two water quenched samples had residual stress in tension at the surface; the two plastically deformed samples had residual stress in compression at the surface. There were no trends for either type of sample for residual stress variation from the surface to the center. Overall, the 3% plastically deformed sample would be the recommended residual stress reducing method before machining.