抄録
The solidification structures and segregation of the VAR cast ingots of alpha+beta titanium alloys were investigated by using a new etching technique and CMA analysis.
Main results were summarized as follows:
(1) The fine structures of the solidification were revealed by immersing the specimens in an advanced hydrofluoric acid etchant.
(2) The metallographic structure consist of the chilled layer, the columnar grain structure in its inner zone and the equi-axis grain structure in the central zone.
(3) The etch pit and shrinkage cavity lines intersect perpendicularly to the growing directions of the columnar grains. And these lines correspond to the interface of solid-liquid phases under solidification. This parallel lines show intermitted solidification.
(4) In Ti-6Al-6V-2Sn alloy with 0.7% Fe and 0.7% Cu, the segregation of these elements was observed in the final solidification zone, which is referred to so called beta flecks.
(5) To prevent segregation, the authors proposed a melting method in which faster solidification is maintained by making the edge diameter of the consumable electrodes smaller, as the hot-topping of VAR.
