Grain size refinement and enhanced precipitation strengthening in a hot extruded 6xxx Al alloy without homogenization

Long duration homogenization treatment (HT) plays an essential role in the fabrication of wrought Al alloys. Although it consumes huge amount of energy, its necessity has seldom been suspected. In our work, homogenization treatments with various holding times were conducted on an Al-Mg-Si-Cu alloy, followed by hot extrusion, short solution heat treatment and final aging. It is interesting to report the alloy with no HT showed the highest strength. This is partly due to the formation of fine sub-micron α-AlFeMnSi dispersoids during homogenization. The dispersoids occupies the Si content that should be used to form the MgSi-rich nanoprecipitates, resulting in a decrease in the volume fraction of the nanoprecipitates. On the other hand, the transformation of the micro-sized α-AlFeMnSi constituent phase into fine dispersoids during HT reduced the number of nucleation cites for dynamic recrystallization and the pinning effect on grain boundaries, and thus led to a significant coarsening of the extruded grains. The alloy without HT thus contained both fine grains and high-density nanoprecipitates, which provided effective hindrance to dislocation sliding and led to a tensile strength of 434 ± 14 MPa, which is 21% higher than that of same alloy with 24 h HT. Our findings provide some deep insights into simplified processing and energy conservation during fabrication of high-performance Al alloys.