THE CAST STRUCTURE OF A 7075 ALLOY PRODUCED BY A WATER-COOLING CENTRIFUGAL CASTING METHOD

A water-cooling centrifugal casting method was applied to cast the 707 5 Al alloy to generate a much finer cast structure than that produced by conventional ingot casting methods. The effects of casting paramete rs, i.e., rotation speed, pouring temperature, water flow, and grain r efiner, on casting structure were systematically studied so that the o ptimum casting condition and the solidification mechanism could be est ablished. The typical cast structure along the thickness direction of a cast ring could be divided into four equiaxed zones, including the c hill zone which is in contact with the mold wall. All zones have their characteristic grain size, morphology, and relative thickness, which are all dependent on the casting condition. The optimum casting condit ion yielding the finest structure available was found to be 3000 rpm, 650-degrees-C, and sufficient water cooling. A uniform portion occupyi ng 90 pct of the whole thickness and having a grain size of 17 mum cou ld be achieved under such a casting condition. When a grain refiner wa s added, the whole ring became further concentrated with grains of fin e structure. A mechanism concerning the overall effects of rapid solid ification, turbulent flow, and centrifugal force has been proposed for the present casting method and might explain the zone-structure forma tion and the effects of the casting parameters on microstructural feat ures.