Floating heat insulating baffle for directional solidification apparatus utilizing liquid coolant bath

An improved casting apparatus for directionally solidifying molten metal is disclosed. The apparatus typically includes a furnace for heating a mold containing molten metal, a liquid metal coolant bath positioned below the bottom of the furnace and means for gradually withdrawing the heated mold from the furnace into the coolant bath to effect directional solidification of the molten metal. The apparatus is improved by positioning a heat insulating baffle between the bottom of the furnace and coolant bath, the baffle being constructed to float on the bath surface during the solidification process and minimize heat loss from the mold until it is immersed in the bath. The provision of the floating baffle between the furnace bottom and coolant bath provides several important advantages including increased thermal gradients, shorter casting cycles and improved cast microstructures. In addition, the floating baffle reduces vaporization of the liquid metal coolant and maintains a smooth, ripple-free coolant surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to directional solidification apparatus and, 
more particularly, to high rate directional solidification apparatus 
employing a liquid cooling bath. 
2. Description of the Prior Art 
U.S. Pat. No. 3,763,926, of common assignee herewith, discloses an 
apparatus and process for casting directionally solidified articles at 
high rates. In the process, a mold resting on a chill plate is heated to 
an elevated temperature in the heating zone of a furnace, molten metal is 
then introduced into the heated mold and the mold is gradually withdrawn 
from the heating zone into a liquid cooling bath, such as molten tin at 
500.degree. F. Heat removal through both the chill plate and mold walls 
establishes a steep thermal gradient in the molten metal and results in 
unidirectional solidification. Although the apparatus and process of the 
subject patent have proved highly successful, improvements allowing higher 
thermal gradients to be achieved in the mold, shorter casting cycles and 
improved cast microstructures are nevertheless deemed very desirable. 
SUMMARY OF THE INVENTION 
The present invention has as its primary object the achievement of these 
and other improvements. 
The present invention provides an improved casting apparatus characterized 
by the placement of a heat insulating baffle between the bottom of the 
furnace from which the mold is withdrawn and the liquid coolant bath, the 
baffle being constructed to have a density less than that of the liquid 
coolant so that it floats on the bath surface during the solidification 
process and minimizes heat loss from the mold until the mold is immersed 
in the liquid coolant. The baffle typically includes one or more openings 
which are aligned beneath one or more openings in the furnace bottom to 
permit continuous withdrawal of the molds from the furnace, through the 
baffle and into the liquid coolant bath. Preferably, the baffle openings 
are suitably contoured to closely conform to the outer mold walls during 
withdrawal. 
The provision of the floating heat insulating baffle between the furnace 
and liquid coolant bath results in several important advantages among 
which are an increased thermal gradient in the molten metal and 
corresponding increased solidification rate, reduced casting time and 
improved microstructure, these advantages being available regardless of 
whether a single mold or a multiple mold cluster is utilized. In addition, 
vaporization of the liquid coolant is reduced while, at the same time, a 
smooth, ripple-free coolant bath surface is maintained for uniform cooling 
of the metal. 
These and other objects and advantages of the present invention will become 
apparent from the following drawing and detailed description of the 
preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the FIGURE, the mold 2, which is shown as a gang mold for the 
production of two or more directionally solidified articles at one time, 
is positioned on a chill plate 4 and is located within a heating furnace 
which includes a cylinder in the form of a graphite susceptor 6, the 
latter surrounded in turn by one or more graphite felt sleeves 8 for 
insulation purposes and a surrounding insulating ceramic cylinder 10 which 
may be a quartz fiberboard sleeve. Outside of the cylinder 10 is an 
induction coil 12 for heating the susceptor, the coil preferably being 
more closely spaced at the bottom of the furnace than at the top, as 
shown, to provide more uniform heating along the length of the cylindrical 
chamber defined by the susceptor. A suitable insulating cover 16 may be 
positioned on the upper end of the susceptor overlying the cylindrical 
heating chamber defined thereby. Suitable means are provided for moving 
the chill plate downwardly to withdraw the mold out of the cylindrical 
heating chamber, for example, withdrawal rod 18 in combination with any 
conventional actuating means as, for example, hydraulic devices, may be 
utilized to effect mold withdrawal. 
Positioned below the heating furnace is a tank 20 which holds cooling 
liquid 22, such as molten tin at 500.degree. F. The tank 20 may have 
heating elements 24 surrounding it for raising the temperature of the bath 
to that desired for immersion and cooling of the mold and the tank may 
also include cooling coils 26 therearound near the upper end for the 
purpose of maintaining the desired temperature within the bath of liquid, 
especially as the hot mold is immersed during the solidification process. 
Although not shown, suitable stirring means may be provided to circulate 
the liquid coolant around the mold as it is gradually immersed in the 
bath. Generally, the heating furnace and coolant tank are enclosed within 
a suitable vacuum chamber. 
The heating furnace and related apparatus are supported above the liquid 
coolant bath on posts 28 which may be attached to the tank 20 or other 
external supports. A radiation shield 30 in the form of a disc located at 
the base of the susceptor sleeve 6, insulating sleeve 8 and cylinder 10 
projects inwardly and has openings therein large enough to permit downward 
motion of the mold. This radiation shield may be supported together with 
the susceptor sleeve and other parts of the apparatus by heat resistant 
blocks 32 attached to the posts 28. The shield may be of some refractory 
material such as tantalum, grafoil and the like and serves to prevent 
direct heat radiation to the chill plate and the mold as it is being 
withdrawn. 
According to the present invention, a heat insulating baffle 34 is placed 
between the bottom of the heating furnace and the coolant bath, the baffle 
being constructed of heat insulating material to have a density less than 
that of the liquid coolant so that it floats on the coolant bath surface 
as shown. A disc-shaped, heat insulating baffle constructed of a fibrous 
zirconia insulating core, such as Zircar.RTM. available from Zircar 
Products Inc., bonded in a sandwich type arrangement between heat 
resistant grafoil sheets has been found to provide an extremely buoyant 
baffle in molten tin at 500.degree. F. Of course, other suitable 
insulating materials and construction techniques may be utilized as 
desired. The baffle typically includes one or more openings therein to 
permit passage of the individual article molds 2a and 2b therethrough as 
the gang mold is withdrawn from the bottom of the furnace into the coolant 
bath, the number of openings varying with the number of article molds. As 
shown, the baffle openings are axially aligned beneath the corresponding 
openings in the furnace bottom, that is, the openings in the radiation 
shield 30. Axial alignment of the baffle openings can be readily achieved 
by designing the floating baffle to fit snuggly between heat resistant 
blocks 32. Although the openings in the floating baffle may have any 
simple cross-sectional shape, such as circular, it is oftentimes desirable 
to provide contoured openings which conform relatively closely to the 
outer walls of the article molds. 
In solidifying molten metal simultaneously in a plurality of article molds, 
such as in the gang or cluster mold illustrated in the FIGURE, it has been 
found convenient to provide the floating insulating baffle in two 
cooperating components, namely, an outer floating annular member and an 
inner floating circular member positioned within the central hole of the 
annular member. Of course, one or both of the floating members may define 
the openings through which the article molds are withdrawn toward the 
coolant bath. 
Functionally, the floating heat insulating baffle effectively reduces heat 
loss from the mold until it is immersed in the cooling bath, which 
reduction provides a sharp line of demarcation between heated and cooled 
portions of the mold and increases the thermal gradient therein, thereby 
increasing solidification rates and reducing casting cycle time. In 
effect, the baffle reduces the distance between the hot furnace chamber 
and cooling bath surface. Furthermore, spurious nucleation on the mold 
walls is effectively inhibited by the increased thermal gradient and 
provides improved directionally solidified microstructures. The floating 
baffle also minimizes heat radiation to the coolant surface and thereby 
significantly reduces vaporization of the coolant during the 
solidification process. A smooth, ripple-free coolant surface is 
maintained as a result of the floating baffle stifling any surface 
turbulence generated by immersion of the mold and by circulation of the 
coolant therearound. The result is more uniform cooling around the mold 
periphery and improved cast microstructures. Importantly, all of the 
advantages associated with the invention are available regardless of 
whether a single mold or multiple mold cluster, as illustrated, is used. 
The present invention is especially useful in directional solidification 
processes such as described in VerSnyder, U.S. Pat. No. 3,260,505 and 
Piearcey, U.S. Pat. No. 3,494,709 for columnar and single crystal 
castings, respectively. It also is especially useful in solidifying 
eutectic compositions such as according to the Lemkey, U.S. Pat. No. 
3,793,100. 
Of course, those skilled in the art will recognize that other changes, 
omissions and additions in the form and detail of the preferred embodiment 
may be made without departing from the spirit and scope of the invention.