THE IMPROVED MICROSTRUCTURES AND PROPERTIES OF 7075-ALLOYS PRODUCED BY A WATER-COOLING CENTRIFUGAL CASTING METHOD

A centrifugal water-cooling casting method was used to cast a 7075 all oy with the aim of refining the grain and inclusion size and improving mechanical properties in the wrought condition. Conventional ingot ca sting methods were also used and investigated for comparison with the centrifugal casting method. The results show that by the centrifugal c asting method, a small equiaxed grain size, 17 mu m, is obtained in as -cast condition. Only 50 minutes are required for material homogenizat ion. After rolling to obtain sheet, a grain size of 15 x 8 x 6 mu m an d an inclusion size of 2 to 3 mu m are achieved. Fine-grained centrifu gal-cast 7075 alloy exhibits higher strength than the ingot-cast one i n the early stages of aging but poorer in the latter stages. However, its ductility and combination of strength and ductility is superior to the ingot-cast ones at all aging times. The reduction in strength in the latter aging stages for the fine-grained structure arises from its higher volume fraction of soft precipitate free zones. The improved d uctility is attributed to the higher fraction of transgranular fractur e, higher transgranular fracture strain, and intergranular fracture st rain. Fine-grained 7075 alloy also displays significant improvements i n the exfoliation corrosion resistance. These improvements are related to the increased density of attacking sites on the surface and the in creased turns for crack propagation along grain boundaries.