Quasi-steady-state characteristics of solidification of alloys made from VT3-1 alloy during vacuum arc remelting

Abstract

Numerical calculations are presented of the depth of a liquid pool, time of local solidification, and temperature gradient in the axial region of an ingot during vacuum arc remelting (VAR) of VT3-1 titanium alloy (Ti-6.5Al-2.5Mo-1.5Cr-0.5Fe-0.3Si) corresponding to quasi-equilibrium conditions. Calculations were carried out for ingots of diameters D = 400, 800, and 1200 mm in the ranges of mass melting rates $\dot M$ = 0.5–12.0, 0.5–35.0, and 1.5–30.0 kg/min, respectively. It is found that the depth of the liquid pool (H, mm) linearly increases with increasing $\dot M$ and is virtually independent of D in the conditions under consideration and, therefore, can be presented as a unique dependence H = 66.63 $\dot M$ + 71.91. A finite depth of the liquid pool at a zero mass melting rate is associated with that the state $\dot M$ = 0 corresponds to a finite current of the arc, which holds a part of metal in the liquid state. It is shown that the time of local solidification depending on $\dot M$ has a minimum associated with various physical processes, which determine the kinetics of the solidification front at small and large solidification rates. In relative units, which correspond to a minimum, these dependences are identical for all considered diameters of the ingot. In addition, based on autoradiographic investigations of solidification of VT3-1 alloy under the VAR conditions, critical values of the temperature gradient (G) and velocity of motion of the crystallization front (v) along the ingot axis, which determine the passage from the column structure to the equiaxial one, are determined. Starting from these results, the plots v(G) are constructed, which turn out to be very useful in the development of remelting modes, excluding the appearance of some type of liquation process. It is revealed that at the specified ingot diameter, the dependence v(G) is decreasing and, at the specified temperature gradient, the velocity of motion of the solidification front decreases as D increases, which indicates an increase in D for the smelting of the ingots of highly-doped alloys.