Microstructure Evolution and Fatigue Behaviour of AA2099 Alloy Processed through Hybrid Multi-Axial Forging and Cryo-Rolling

Abstract

The effects of post-deformation annealing and hybrid severe plastic deformation (SPD)—multi-axial forging (MAF) followed by cryorolling (CR)—on the microstructural development and low-cycle fatigue (LCF) behavior of AA2099 Al-Cu-Li alloy were examined.  After six MAF cycles (cumulative strain of 3.6), solution-treated specimens underwent cryo-rolling at −196 °C, leading to an overall thickness reduction of about 90%. Post-deformation heat treatment was then carried out at 150 °C and 200 °C. As a result of this processing route, the material developed an ultrafine-grained microstructure with an average grain size close to 0.42 µm, along with a high density of dislocations and a microstructure dominated by low-angle grain boundaries. were produced by the hybrid MAF+CR method.  Grain size was increased to 0.84 μm after post-annealing at 150°C, which also improved texture intensity (~3.7 m.r.d.) and the high-angle grain boundary fraction. Because of the best precipitation strengthening, the MAF+CR+150°C condition demonstrated the best LCF performance, with the biggest hysteresis loops, maximum energy dissipation, and a peak cycle stress of about ±270 MPa. For MAF+CR+150°C, fracture analysis showed primarily ductile failure with fine equiaxed dimples, whereas MAF+CR+200°C displayed coarser features as a result of overaging. Progressive strain recovery with temperature was verified using Kernel Average Misorientation analysis. In ultrafine-grained Al-Cu-Li alloys intended for aerospace applications, this hybrid processing method successfully modifies microstructure to produce improved LCF resistance.