Thermomechanical processing route to induce ultrafine grain structure by continuous recrystallisation in aluminium and its alloys

The role of transition element impurity Fe in commercial aluminium alloys during deformation, recovery, and recrystallisation was investigated. When the Fe solute concentration is reduced to atomic ppb levels, continuous recrystallisation may occur concurrently with cold deformation or during subsequent anneals, resulting in a uniform ultrafine grain structure with 1-2 mum mean grain size. A thermomechanical processing route to produce ultrafine grained aluminium sheets is proposed. It consists of a prestrain rolling to 20%., reduction, an intermediate heat treatment at 230-320degreesC, continued cold rolling to final gauge, leading to a total reduction of about 85-95%, and subsequent recrystallisation anneal at a temperature below 320degreesC. The formation of ultrafine grain structure is attributed to the progressive increase of misorientation angles among grain subdivisions during plastic flow. This progress is enhanced whenever negligible solute pinning permits rapid dislocation climb and cross-slip. The microstructural evolution in the solute free matrix suppresses the discontinuous recrystallisation partly due to the evenly distributed fine particles. Redundant shear strain due to friction facilitates continuous recrystallisation by increasing the effective strain and generating more point defects to promote the aforementioned mechanisms.