The role of iron in the formation of porosity in Al-Si-Cu-Based casting alloys: Part II. A phase-diagram approach

The mechanism by which iron causes casting defects in the AA309 (Al-5 pet S i-1.2 pet Cu-0.5 pet Mg) may be related to the solidification sequence of t he alloy. Superimposing calculated segregation lines on the liquidus projec tion of the ternary Al-Si-Fe phase diagram suggests that porosity is minimi zed at a critical iron content when solidification proceeds directly from t he primary field to the ternary Al-Si-beta Al5FeSi eutectic point. Solidifi cation via the binary Al-beta Al5FeSi eutectic is detrimental to casting in tegrity. This hypothesis was tested by comparing the critical iron content observed in the standard AA309 alloy to that of a high-silicon (10 pet Si) variant of this alloy.