Thermal stability of ultrafine grained AA8090 Al–Li alloy processed by repetitive corrugation and straightening

Abstract

The thermal stability of an ultrafine-grained (UFG) AA8090 Al–Li alloy with an average grain size of 2 μm is reported. The UFG structure was obtained by repetitive corrugation and straightening (RCS) process after 8 passes using optimized processing parameter conditions identified in the previous research, i.e., processing temperature 300 °C; ram velocity 1.5 mm/s using V-Grooved corrugating die profile (pitch 20 mm, corrugating angle 30° and curve radius 2 mm) and straightening by a flat die. The grain size distribution ranged from 200 nm to 8 μm. The average hardness of the RCS processed specimen had increased to a mean value of 104 HV from an average value of 75 HV in the parent material. The RCS processed specimens were annealed at different temperatures (T) for varying time periods (t) to investigate their thermal stability using hardness and microstructure changes as the quantitative measures. The microstructure analysis of the samples was done using electron backscattered diffraction (EBSD) analysis. The study was based on a full factorial design of experiments (DOE) and the results were analyzed using a TOPSIS optimization tool. The results prove that thermal stability is more influenced by temperature than the time of annealing, a known result based on earlier qualitative studies on conventional materials. The study also demonstrates that the material is fairly stable up to 300 °C. The activation energy for grain growth is found to be 76 kJ/mol in a range where the annealing time is 2–6 h and the temperature range is 200–400 °C. Keywords: Severe plastic deformation, Repetitive corrugation and straightening, Al–Li alloys, Thermal stability