Effect of precipitation of primary intermetallic compounds on tensile properties of cast Al-11.5Si-0.4Mg alloy

摘要

The precipitation of primary intermetallics from aluminum melts is usually viewed as having only a harmful influence on casting quality and production. Such precipitation, however, may provide a new approach to remove harmful intermetallics and oxide films from aluminum melts. This research is an attempt to investigate the possibility of purifying aluminum melts to improve casting quality through precipitation processing. A comparison was made between the tensile properties of Al-11.5Si-0.4Mg alloy in two conditions: (1) untreated and (2) precipitated at 600℃ for 10 minutes. After separation from the precipitate-rich material at the bottom of crucible, the precipitated melt was reheated to 730℃ prior to casting. Liquid metal quality was also evaluated by the pressure-assisted filtration technique. The scatter of tensile properties was studied using Weibull analysis together with fracture examination. The Weibull modulus values were 9.2 in the untreated condition and 19.4 in the precipitated condition. A study of the fracture surfaces by SEM revealed that the defects responsible for failure in the untreated material were mainly oxide films and iron-rich inclusions, and in the precipitated castings were gas porosity but fewer oxide films and iron-rich inclusions. The wetted sides (i.e., the outer interfaces) of double oxide films appear to act as preferred substrates for the nucleation and growth of some intermetallics. The moderate improvement in tensile properties of castings through precipitation processing appears to be related to the reduction of oxide films and iron-rich particles.