On the Impact Properties and Fractography of Al-11%Si Casting Alloy.

This study focused on investigating the effects of Sr-modification and solidification rate as well as the aging treatment conditions on the impact toughness behavior of Al-11%Si near-eutectic alloys with the objective of fully evaluating its behavior under high rates of loading. The Al-11%Si-based alloys, however, display lower impact energy values than those obtained for hypoeutectic 356 and 319 alloys. The impact behavior of aged Al-11%Si-based alloys is similar under both low and high solidification rate conditions; the air-cooled alloys (high solidification rate), however, display higher impact energy values than do furnace-cooled alloys (low solidification rate). Artificial aging at 240 °C for 44 h produces a significant improvement in the impact energy values as a result of softening effect, regardless of alloy composition. The presence of undissolved intermetallic phases, mainly AlCu and α-Al(Mn,Fe)Si, increases alloy brittleness and thus also reduces alloy toughness. The segregation of AlCu caused by Sr addition is a further parameter to be considered, especially for furnace-cooled alloys. The impact energy values of air-cooled alloys are significantly higher than those obtained for furnace-cooled alloys based on the microstructural differences between the two categories of alloys, regardless of alloy conditions. Crack propagation occurs mainly in the near-eutectic alloys by passing through the AlCu and/or α-iron intermetallic phases, shattering them into fragments. In numerous cases, the morphology of the intermetallic phase is retained, indicating that the fracture is of the transverse type rather than a particle/matrix fracture. [ABSTRACT FROM AUTHOR]