Restoring Fatigue Performance of Corrosion Damaged AA7075-T6 and Fretting

Corrosion related fatigue in aluminum structural alloys and fretting damage in high strength steels are primary failure mechanisms that reduce the structural integrity of aging aircraft. A chemically active environment, susceptible material and static and/or alternating tensile stresses are all required for failure. Conventional approaches to mitigate corrosion and fretting related failure mechanisms address either elimination of the corrosive environment with coatings, substitution or modification of alloys, or changes in design, all expensive solutions. This paper describes an alternate approach, employing surface enhancement by low plasticity burnishing (LPB) to introduce a deep, stable layer of compressive residual stress to eliminate the tensile stresses necessary for failure without altering either material or design. The restoration of fatigue performance by LPB processing of severely salt fog pitted AA7075-T6 was previously described.1 That work has been extended to investigate the effects of introducing a deep layer of compressive residual stress with LPB on the fatigue performance of AA7075-T6 prior to salt fog pitting, and prior to active corrosion fatigue in the absence of pitting, and prior to fretting of 4340 steel. The endurance limit of the baseline machined 7075-T6 surface was reduced from 205 MPa (30 ksi) to 103 MPa (15 ksi) by either 100 hr salt fog pitting or active corrosion in 3.5 % NaCl during fatigue. LPB prior to corrosive exposure increased the endurance limit for 100 hr pitted samples to 310 MPa (45 ksi), and for active corrosion fatigue to 275 MPa (40ksi). LPB processing was performed in a conventional CNC machining center. LPB prior to fretting in 4340 steel increased the HCF strength 25%, eliminating the fretting fatigue debit. Surface enhancement of fatigue critical aircraft structural components with LPB offers a cost effective and practical alternative to alloy substitution or component re-design as a means of improving the structural integrity of aging aircraft