Thermomechanical fatigue behavior of burnished 7075????T6 aluminum alloy

摘要

This paper presents the results of series of isothermal fatigue (ITF) and thermomechanical fatigue (TMF) tests of both unburnished and burnished 7075-T6Al specimens.Adesigned roller burnishing tool was employed to improve the strength of the surface layer of the fatigue specimens. The fatigue stresses were developed in the specimens by combining constant amplitude rotating bending stresses along with constant temperature variation. Both isothermal (ITF) and TMF tests were conducted on a rotary bending fatigue testing machine. The ITF tests were carried out at two different constant temperatures namely 523 and 623 K, while the TMF tests were carried out at the temperature range between 523 and 623 K.All these tests were conducted at a constant operating speed of 1200 rpm. These investigations were performed in order to gain more understanding of the effect of interaction between mechanical and thermal stresses on fatigue resistance and fracture behavior of burnished 7075-T6 Al specimens. The present results revealed that roller burnishing processes have played a significant role in increasing the fatigue lifetimes for both ITF and TMF specimens. The enhanced fatigue strength of the burnished specimens was attributed to the overall increase in the surface layer strength which may delay fatigue crack growth fromthe surface. Two distinct fatigue fracture regions were observed: region I and region II. In region I, the fracture surface is associated with the formation of fatigue striations. In region II, the fracture surface is covered with surface dimples. This indicates that local strain softening mechanism has dominated the final stage of fatigue failure. Extensive effort has been paid at investigating the fracture surface of ITF and TMF specimens.