Effects of Tensile and Compressive Stresses on Stress Relaxation Behavior and Mechanical Properties in an Al-Cu Alloy

The influences of tensile and compressive stresses on stress relaxation behavior and mechanical properties of 2219 aluminum alloy have been systematically studied with different initial stress levels at 165℃. Under the same initial stress level and ageing time, tensile stress relaxation is greater than that in compression. The stress relaxation difference between them mainly originates from the primary relaxation stage, while the steady-state relaxation stage is basically identical. With the increase of stress level, the yield strength of tensile-stress-relaxation aged sample augments gradually, and the compressive stress sample fluctuates slightly. The corresponding deformation and strengthening mechanisms are revealed through transmission electron microscopy observation and analysis. When the initial stress is 100MPa, the sample in compression has larger diameter and volume fraction of the mixed GP I and GP II precipitates with unobvious stress- oriented effect. These microstructural features in compression lead to higher yield strength and smaller stress relaxation rate than the tensile sample. Nevertheless, at the high initial stress of 170MPa, massive dislocations and main strengthening θ’precipitates in tensile sample result in stronger yield strength and rapider stress relaxation as compared with compressive sample.