Fractographic analysis of the AA 7075-T6 alloy subjected to fatigue in the RRA treatment condition and plasma nitride

ABSTRACT

This work assessed the fracture behavior of the AA7075-T6 aluminum alloy subjected to fatigue loading. Both the notched and unnotched specimens underwent a retrogression heat treatment and re-aging (RRA) and plasma nitriding before the mechanical tests. Plasma nitriding, which has been a method chosen to increase the surface hardness and the fatigue, wear and corrosion resistances of various alloys was carried out with an active screen with a 20 Hz DC source and a 40 Hz duty cycle. The RRA treatment, which is used to increase the strength and hardness of AA7075 aluminum alloy and obtain improvements in the fatigue and corrosion behaviors, was performed at 200 °C for 20 minutes with water cooling (resolubilization), followed by 24 hours at 120 °C (re-aging). The axial fatigue tests of notched and unnotched specimens were carried out with R = 0.1, frequency of 14 Hz, and at stress level of 150 MPa for notched specimens, and 500 MPa for unnotched specimens. The fatigue fracture was then performed via stereomicroscopy and scanning electron microscopy. For the unnotched specimens, AA7075-T6 and AA7075-RRA conditions presented similar fracture features, with good definition of the three stages of fatigue failure; the AA7075 nitrited condition presented no evidence of typical fatigue fracture. The notched specimens presented fractographies similar to the severe stress concentration condition, with surface marks convex to the nucleation region.

Keywords
Fractography; Fatigue; Plasma Nitriding; RRA; 7075-T6