On the solidification response of single-crystal-melt pools to gravitational, thermocapillary, and electromagnetic forces

Abstract

4. The gravitational body force, whose magnitude and direction govern buoyancy driven convection and the equilibrium free surface curvature, has a profound effect on the amplitude of microripples and the overall morphological nature of the as-solidified free surface, as determined through extensive ground based experiments.

6. Inherently metastable solidification conditions have been observed which provide insight to the nature of preferred growth orientations and their relation to the heat flow conditions present within the liquid. The bistability of the system due to dendritic growth competition manifests itself within the internal microstructure as intense side branching in the <100> directions defined by the single crystal substrate, ultimately resulting in the fonnation of microstructural patches.

In an attempt to understand the convoluted interaction of physical forces that affect basic solidification phenomena, a series of experiments have been devised and implemented. More specifically, stationary melt pools formed on single-crystal stainless steel (70% Fe-l5% Cr-l5% Ni) were subjected to various physical conditions during the melting and solidification phases and subsequently analyzed to reveal the solidification response to changes in these parameters. Responses of the melt pool in terms of mass transport/convection, surface morphological instabilities and features, cellular microstructure, and growth velocities were characterized correspondingly with controlled changes in the gravitational, thermocapillary, electromagnetic, and mechanical forces acting upon the fluid body. Furthermore, time dependent perturbative forces such as mechanical vibration, rotation, and pressure waves were introduced to the solidifying system in order to study the dynamics controlling the morphology of the free surface and its evolution during solidification. Additionally, the geometrical relationship between the crystallographic orientation of the single crystal and the heat flow conditions have been explored, revealing conditions for microstructural bistability. The data collected from these various experiments have offered a great wealth of knowledge in terms of understanding the relative roles of the governing forces and heat flow conditions in melt pool solidification dynamics. These studies have revealed, but are not limited to, the following:

1. Palladium has been determined through numerous experiments to be an ideal dopant material for mapping the fluid motion inherent during melting and solidification in stainless steel. This novel technique, referred to as tracer mapping, has allowed, for the first time, qualitative information on the general path for fluid transport within the molten region to be obtained.

2. Perturbative forces acting upon the free surface can excite liquid undulations, referred to as capillary waves, whose period and amplitude can be described quantitatively by the pool radius, surface tension, gravitational force, and the velocity of the moving interface. Furthennore, the periodic motion of the free surface is effectively frozen into the exposed surface of the melt pool resulting in concentric ripples approximately 1-2 um's in amplitude and 40-100 um's in wavelength. These frozen artifacts provide a timeline of the wetting angle between the liquid, solid, and atmosphere as the liquid/solid interface advances towards the center of the molten fluid. A mathematical analysis of this phenomena is developed and compared with experimental profilometery data. The mathematical analysis suggests that through quantification of the radial wavelength dependence of the sinusoidal ripples, the radial velocity dependence of the solid/liquid interface can be determined. This new tool has revealed that, in general, the velocity of the solid-liquid interface in stationary melt pools increases with time during the solidification process.

3. The microripples existing on the free surface are superimposed on a larger macro-profile whose shape is strongly correlated with the gravitational vector, temperature coefficient of surface tension, convective fluid transport, nature of mechanical perturbations, arc pressure, and electromagnetic stirring in the fluid. The reaction of the overall shape defining the top surface to each one of these forces is studied in some detail. A strong linear correlation exists between the ripple amplitude and the overall height variation of the gross profile suggesting that a complicated interplay exists between the mechanisms involved in the formation of these motphologies whose characteristic length scales are two orders of magnitude apart. Analogous to the microripple analysis, the shape of the macroprofile provides information on the history of the time averaged wetting angle during solidification. The shape information can be used as a tool in drawing conclusions about the nature of fluid motion beneath the free surface as a function of time.

5. Data obtained from low gravity DC-9 flights suggest that gravity does have a pronounced and definitive influence on the velocity of the solid-liquid interface, and on the melt pool diameter; however, a consistent correlation between the gravitational level and the sub-scaling of the solidified microstructure is not found in the data.