, Volume 6, Issue 2, pp 75-87

The Microstructure of Direct Squeeze Cast and Gravity Die Cast 7050 (Al–6.2Zn–2.3Cu–2.3Mg) Wrought Al Alloy

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Abstract

A 7050 (Al–6.2 wt% Zn–2.3 wt% Cu–2.3 wt% Mg) Al alloy, conventionally used for wrought products, has been successfully cast to near-net shape using direct squeeze casting. Squeeze casting with an applied pressure of 50 MPa removes the defects observed in gravity die cast billets, in particular, (1) shrinkage pipe, (2) poor die replication and waisting, and (3) microporosity. Squeeze casting results in considerable refinement of the microstructure due to an increase in cooling rate from ∼0.5°C s−1 for gravity casting to ∼11°C s−1 for squeeze casting in a tool steel die lined with porous insulation, and from ∼2.5 to ∼10°C s−1, respectively, in an uninsulated die. A normal segregation pattern of increasing eutectic toward the center of the billet is found for squeeze casting, compared to an inverse segregation pattern of increasing eutectic toward the edge of the billet for gravity casting. This change in segregation pattern is due to a higher radial temperature gradient and reduced time in the semisolid state for squeeze casting.