Effects of Rare Earth Er Additions on Microstructure and Mechanical Properties of an Al-Zn-Mg-Cu Alloy

The effects of Er additions on the microstructure and tensile properties of cast Al-15Zn-2.5Mg-2.5Cu aluminum alloy have been investigated. The results show that by adding 1 wt% Er grain refiner in the cast alloy, the grains can be refined to a fine degree. The microstructures and fracture surfaces of cast aluminum alloy samples were examined by SEM. In addition, the Er modified the eutectic structure from a coarse plate-like and acicular structure to a fine branched and fibrous one. The tensile properties were improved by the addition of Er, and good ultimate tensile strength (325 MPa) but poor elongation (6%) were obtained when the Er addition was 1 wt%. Furthermore, fractographic examinations revealed that refined pore and spheroidized a-Al dendrite were responsible for the high ultimate tensile strength. At higher magnifications, unrefined specimens showed cracking along the grains, whereas Er-refined specimens showed cracks in individual intermetallic compounds.