チクソキャスティング法により製造したAl–7%Si–0.5%Mg合金の組織と機械的性質

Abstract

Microstructure and mechanical properties of Al–7 mass%Si–0.5 mass%Mg 357 alloys produced by thixocasting and permanent mold casting process were investigated. The microstructure of the thixocast specimen consists of the spherical α–Al phase particles and fine eutectic Si phase, and cross section macrograph of the thixocast specimen exhibits no segregation and defects. The average size and volume fraction of the α–Al phase particles are about 58 μm and 64%, respectively. The tensile strength, 0.2% proof stress and elongation of T6–treated thixocast specimen aged at 443 K for 18 ks are 363 MPa, 313 MPa and 12%, respectively. These values are about 30 MPa in tensile strength, 40 MPa in 0.2% proof stress, and 6% in elongation higher than those of permanent mold cast specimen. The absorbed energy of the as-cast and T6–treated thixocast specimens are 102 and 53 kJ/m2, respectively. These values are far superior to those of T6–treated permanent mold cast specimen. The fatigue strength of the T6–treated thixocast specimen at the 107 cycles is 140 MPa. Such an good tensile properties, impact value and fatigue strength of thixocast specimen may be caused by high cooling rate during solidification, resulting in effective refining of eutectic Si phases and Al–Fe system intermetallic compounds, compared with permanent mold casting process.