Effects of Cold Hole Expansion on Microstructure and Fatigue Property of 7A85 Aluminum Alloy Forging

Hole expansion was employed to strengthen 7A85 aluminum alloy forging with holes. The fatigue lives of specimens before and after hole expansion were analyzed by comparison. The fatigue fracture, microstructure and residual stresses before and after hole expansion were studied by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the specimens with the shrink value of 5.3% obtained the best effect of strengthening, whose fatigue life was 11 times as long as that before hole expansion. The residual compressive stress and dislocation cell structure were found in the strengthened layer after hole expansion, which can retard the fatigue crack growth rate, and then prolongs the whole fatigue life of the specimen. The depth of strengthening layer was about 4.7 mm, and the maximum residual compressive stress of -319 MPa was observed where was 1 mm away from the border of holes.