Formation of Fine Spheroidal Microstructure of Semi-Solid Al-Zn-Mg-Cu Alloy by Hyperthermally and Subsequent Isothermally Reheating

The high strength 7xxx series aluminium alloys are usually difficult to be prepared as semi-solid feedstock, because some dispersoid particles (soluble only in liquid) could pin at grain and subgrain boundary to make the alloy difficultly recrystallise. In this work, a novel multistep reheating regime is developed for recrystallisation and partial melting (RAP) route to prepare fine spheroidal microstructure of semi-solid Al-Zn-Mg-Cu alloy. After reheating to 665 degrees C, holding for 4 min and subsequent isothermally holding at 620 degrees C with total heating time of 15 min, fine spheroidal microstructures with grain size of similar to 40 mu m were prepared without remained unrecrystallised grains and directionality, which are much finer than the conventionally obtained microstructures (similar to 100 mu m).