Mitigation of Active Corrosion Fatigue and Corrosion Pit Initiated Fatigue in AA7075-T6 with Low Plasticity Burnishing

Corrosion related fatigue failures of aluminum
structural alloys adversely impact the structural
integrity of aging aircraft. A chemically active
environment, susceptible material and alternating
tensile stresses are all required for failure.
Conventional mitigation strategies isolate the
corrosive environment with coatings, change the
alloy or heat treatment, or reduce the applied stress
by redesign, 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 environment, material or
design.
Either 100 hr salt fog pitting or active corrosion in
3.5% NaCl during fatigue reduced the endurance
limit of the baseline machined 7075-T6 surface from
205 MPa (30 ksi) to 103 MPa (15 ksi). 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). The
results support the use of LPB processing of fatigue
critical aircraft structural components as a cost
effective alternative to alloy substitution or
component redesign.