Improving Fatigue Life of a High-Strength Al Alloy by Deep Rolling Surface Enhancement

Mechanical surface enhancement provides cost-effective solutions to increase fatigue life of metallic components. Deep rolling surface enhancement is a lower-cost, simpler and environmentally friendly alternative to shot peening in some applications. Deep rolling has several advantages: (1) it introduces beneficial compressive residual stress, (2) its processing parameters can be closely controlled, (3) deep rolling tools can be mounted on computer numerically controlled (CNC) machines, and the process can be integrated into regular machining sequences, and (4) deep rolling improves or at least maintains surface roughness of workpieces. In this presentation, results of deep rolling of 7050 Al are presented. 7050 Al testing coupons were deep rolled under different conditions. Surface roughness, cross-section traverse microhardness and residual stress of deep rolled, machined and shot peened coupons were measured. Residual stress measurements showed deep rolling was very effective in introducing compressive residual stress on/under the surface. Level of the compressive residual stress was modified by varying deep rolling pressure. High-cycle fatigue life of deep rolled, machined and shot peened specimens was tested. Results showed deep rolling substantially improved the fatigue life of 7050 Al. The improvement was attributed to the three beneficial effects of deep rolling – compressive residual stress, low level of surface work hardening and improvement in surface roughness. Fractographic examination of the fatigue specimens was performed by using scanning electron microscope (SEM).