Thermal stability of ultrafine grained aluminium alloy prepared by large strain extrusion machining

In the present study, ultrafine grained (UFG) Al alloy chips with average grain sizes of similar to 200 nm were successfully prepared by large strain extrusion machining (LSEM) process using a combined cutting tool with rake angle of 10 degrees and chip compression ratio of 1.0. The tests showed that the Vickers hardness of the UFG Al alloy is significantly improved due to grain size reduction. To understand effect of heat on the microstructure and mechanical properties, the UFG chips were subjected to heat treatment at different temperatures and different annealing time durations. When annealed <100 degrees C, most of fine grains within the UFG chips were found to be replaced by elongated grains whose grain sizes increased with a significant increase in the aspect ratio as the annealing time increased. Despite such increase in grain size, the Vickers hardness was not reduced as expected because of the precipitation of secondary phases. When annealed at temperatures up to 200 degrees C, recrystallisation occurred, along with grain growth, but the Vickers hardness did not deteriorate because of precipitation of secondary phases, as before. However, annealing at temperatures of 300 degrees C and above resulted in significant reduction in hardness of the chips due to dominance of grain growth over secondary precipitation. These results indicated that UFG Al alloy chips have a good thermal stability at temperatures <200 degrees C.