Microstructure and stability of nanocrystalline aluminum 6061 created by large strain machining

We present the properties of aluminum 6061 alloys of various tempers, severely deformed in plane strain at room temperature, by machining. Various values of strain are introduced into the chip, in a single pass, by varying the rake angle of the tool. Chips cut from peak-aged 6061 (T6 condition) are composed of finer microstructures and possess higher hardness than peak-aged 6061 subjected to equal channel angular pressing at elevated temperatures. Thermal stability of the chips with different levels of strain is analyzed by studying evolution of Vickers micro-hardness and microstructure after different heat treatments. Chips produced from the peak-aged temper and overaged temper soften following heat treatment while those from the solution-treated state first gain strength before softening. The results are rationalized based on prior studies of the characteristics and kinetics of precipitation and coarsening in Al-Mg-Si systems. The observations also suggest processing routes for consolidation of the chips into bulk forms while retaining the UFG microstructure with enhanced mechanical properties.