THERMOMECHANICAL FATIGUE BEHAVIOR OF BURNISHED 7075–T6 ALUMINUM ALLOY

Abstract

This paper presents the results of series of isothermal and thermomechanical fatigue(TMF) tests of unburnished and burnished 7075-T6 Al specimens. A designed rollerburnishing tool was employed to improve the strength of the surface layer of thefatigue specimens. The fatigue stresses were developed in the specimens bycombining constant amplitude rotating bending stresses along with constanttemperature variation. Isothermal (ITF) and TMF tests were conducted on a rotarybending fatigue testing machine. In ITF tests, two different constant temperatureswere used namely: 523 and 623 K. For TMF tests, a constant temperature variationbetween 523 and 623 K was applied. All these tests were conducted at a constantoperating speed of 1200 rpm.The present results revealed that roller burnishing processes have played asignificant role in increasing the fatigue lifetimes for both ITF and TMF specimens.The enhanced fatigue strength of the burnished specimens was attributed to theoverall increase in the surface layer strength which may delay fatigue crack growthfrom the surface. Two distinct fatigue fracture regions were observed: region I andregion II. In region I, the fracture surface is associated with the formation of fatiguestriations. In region II, the fracture surface is covered with surface dimples. Thisindicates that local strain softening mechanism has dominated the final stage offatigue failure. Extensive effort has been paid at investigating the fracture surface ofITF and TMF specimens.