Segregation in titanium alloy ingots

Abstract

In this study We report an experimental determination of the segregation coefficients of some elements in commercial titanium alloys. The methods used are point analysis of as-cast alloys and analysis of zone-melted samples. The as-cast samples for point analysis consisted of laboratory arc-cast ingots and also of sections of as-cast vacuum-arc remelted ingots of conventional industrial sizes. The zone-melted samples were prepared from either an optical furnace system or from an induction zone refiner. In both these cases the melting was carried out under argon of 35 kPa pressure to prevent volatilization of alloy elements. We find that the values of segregation coefficients determined differ significantly from those derived from binary phase diagrams, due to the strong inter-element interactions in these systems, but follow the general trends to be expected from the reported liquidus/solidus relationships. The implications of the results in terms of the segregation to be expected in commercial ingots prepared by either vacuum arc remelting or by hearth remelting is discussed and it is concluded that proposed new alloy compositions containing substantial amounts of Fe and Cr will prove difficult to manufacture with adequate composition control as a result of the segregation of these elements during solidification.