Vacuum melting/casting method to reduce inclusions

In the melting of one or successive alloy charges including one or more volatile alloying elements, each alloy charge is melted in a melting vessel under an inert gas partial pressure effective to reduce volatilization, migration and condensation of the volatile alloying elements and build-up of condensate deposits of the volatile elements on cool regions of the melting vessel and melting chamber where the deposits constitute inclusion precursors that can eventually enter successive charges melted in the vessel. Wetting of the crucible by the melt is also reduced by the gas partial pressure. The incidence of inclusions found in castings produced from the successive melts is thereby reduced.

FIELD OF THE INVENTION 
The present invention relates to a method for reducing inclusion levels in 
the vacuum melting of successive alloy charges containing one or more 
volatile alloying elements. 
BACKGROUND OF THE INVENTION 
The presence of inclusions in superalloy investment castings is a concern 
of gas turbine engine manufacturers as the engine operating conditions, 
especially temperature, have become more severe. As is known, these 
inclusions can adversely affect the mechanical properties of the casting 
and, if present at a critical size, can cause catastrophic failure of the 
castings under the high temperature and stress conditions of engine 
service. Moreover, the adverse effect of inclusions on the casting 
properties may be exacerbated as the section size (e.g., wall thickness) 
of the castings is reduced for weight savings purposes. The presence of 
unacceptable levels and/or sizes of inclusions in investment castings 
results in excessive scrap rates that increase the overall cost of 
producing castings for use in gas turbine engines. The inclusion problem 
worsened as remelt vacuum levels and leak rates of casting equipment were 
improved. 
Non-metallic and dross type inclusions can originate from several sources 
during the investment casting operation. For example, inclusions can be 
introduced by the master alloys used, by remelting in a ceramic crucible, 
by the remelting environment, and by mold/melt reactions that can occur 
during solidification, especially during the relatively long mold/melt 
contact periods required in the production of directionally solidified and 
single crystal castings. 
In efforts to produce cleaner castings (i.e., castings with lower 
non-metallic inclusion levels), superalloys are typically remelted and 
cast under relatively high vacuum conditions (e.g., &lt;10.sup.-3 torr) to 
reduce the presence of residual oxygen and other gases in the 
remelting/casting atmosphere and/or in the melt. Typical remelting 
procedures employed include vacuum induction melting in a refractory 
crucible and "cold hearth" melting including vacuum arc, induction skull 
and electron beam melting in a water cooled metallic (e.g., copper) 
crucible. Despite this effort, unacceptably high inclusion levels may 
still be experienced in the production of certain superalloy investment 
castings that include one or more relatively volatile alloying elements, 
such as Cr and Al, especially during the melting of successive alloy 
charges in the same crucible. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved method for reducing 
inclusion levels attributable to the vacuum melting in a melting vessel of 
one or successive alloy charges including one or more volatile alloying 
elements. 
It is another object of the invention to provide an improved method for 
reducing inclusion levels attributable to formation of inclusion 
precursors at cool regions of the melting vessel during the melting of one 
or successive alloy charges therein. 
It is still another object of the invention to provide an improved method 
for reducing inclusion levels attributable to wetting of the melting 
vessel by the melt during the melting of one or successive alloy charges 
therein. 
The present invention contemplates the melting of one or successive alloy 
charges including one or more volatile constituents in a manner to reduce 
inclusions attributable to the melting operation. The present invention is 
based on the discovery that the problem of unacceptably high inclusion 
levels in vacuum melted alloys including one or more volatile 
constituents, (e.g., volatile alloying elements including Al, Cr, Mg, 
etc.) melted in a melting vessel arises from volatilization of such 
constituents under the temperature and pressure conditions of melting and 
the condensation of the volatized constituent as condensate deposits on 
cool regions of the melting vessel and associated melting chamber where 
the deposits constitute inclusion precursors that can enter the melt 
during the melting and/or casting operations to provide the observed high 
inclusion levels in the resultant casting. The condensate deposits may 
comprise metal oxides as a result of reaction of the volatile constituent 
with residual gas, such as oxygen in the melting chamber, during 
constituent migration from the melt and/or after constituent condensation 
on the cool regions. The condensate deposits build-up on the melting 
vessel and associated melting chamber as alloy charges are melted and cast 
and eventually reach a condition where entry of the deposits into the melt 
occurs. 
One embodiment of the present invention involves establishing a gas partial 
pressure on the melted alloy charge effective to reduce mobility of the 
volatile constituent and formation of the inclusion precursors at cool 
regions of the melting vessel and chamber. Preferably, the gas partial 
pressure is established by introduction of an inert gas to the melting 
chamber prior to melting of the alloy by, for example, vacuum induction 
melting or "cold hearth" melting, and after evacuation of the vessel to a 
subambient pressure. The gas partial pressure is effective to minimize 
volatilization, migration and deposition of the volatile constituents 
while avoiding trapping harmful amounts of the gas in the casting produced 
from the melt. 
In a particular embodiment of the invention, the alloy charge is melted in 
a ceramic crucible in a manner that a cool region lies between the melt 
line and an upper lip of the melting vessel (e.g., crucible). The melted 
charge may be tilt poured from the vessel into a casting mold such that 
the melted alloy flows over the cool region of the crucible where 
formation of the inclusion precursors is minimized or reduced by the 
presence of the gas partial pressure. 
In melting one or successive charges of a nickel, cobalt or iron base 
superalloy including one of Al and Cr as an alloying element, the 
superalloy is melted in a ceramic crucible under an inert gas partial 
pressure effective to reduce volatilization of the Al and/or Cr alloying 
elements and formation/build-up of the inclusion precursor deposits on the 
cool regions of the crucible and melting chamber where they might enter 
the melt. An argon partial pressure in the range of about 50 microns and 
above is preferably established on the superalloy melt to this end. In 
melting superalloys for directional solidification, an argon partial 
pressure of about 5000 microns is used since the long residence time of 
the melt in the mold allows any entrapped argon gas to escape. 
Another embodiment of the present invention involves establishing a gas 
partial pressure on the melt effective to reduce wetting of the melting 
vessel by the melt during the melting operation. This embodiment reduces 
the extent of wetting between the melt and the vessel and resultant 
erosion of the vessel over time so as to reduce inclusion levels in the 
melts attributable to the melting operation. 
In the melting of successive nickel, cobalt, or iron base superalloy 
charges, this embodiment of the invention involves melting the superalloy 
charges in a melting vessel including a working ceramic (e.g., silica, 
zirconia, etc.) surface contacting the melt and establishing an inert gas 
partial pressure on the melt effective to reduce wetting of the inner 
surface of the melting vessel by the melt. 
The present invention may be better understood when considered in light of 
the following detailed description of certain specific embodiments thereof 
which are set forth hereafter in conjunction with the following drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is illustrated hereinbelow with respect to the vacuum 
induction remelting of successive charges of an alloy including one or 
more volatile constituents in a refractory melting vessel (e.g., ceramic 
crucible) in a manner to reduce formation/build-up of inclusion precursors 
on cool regions of the vessel and melting chamber. However, the invention 
is not limited to vacuum induction melting and can be practiced using 
other vacuum melting techniques such as the so-called "cold hearth" 
techniques of induction skull, electron beam and vacuum arc melting where 
the alloy charge is melted in a water-cooled metal (e.g., copper) melting 
vessel. Each charge is remelted and cast into a suitable refractory 
casting mold (e.g., a conventional ceramic investment mold). The remelt 
charge cast into the mold may be solidified by known techniques to produce 
equiaxed, directionally solidified, and single crystal castings. The 
invention may also be used in the alloy manufacture process where 
individual alloy constituents are blended and remelted by one of the 
aforementioned processes and cast into ingot molds. 
Referring to FIG. 1, a remelting/casting apparatus 10 is illustrated for 
vacuum induction remelting successive charges of a superalloy, which may 
include a nickel, cobalt, or iron base superalloys such as IN713LC, 
MAR-M-427, MAR-M-509, etc., in a ceramic crucible 14 enclosed in a melting 
chamber 16 having water cooled walls 16a. The ceramic crucible 14 is 
enclosed and supported by a ceramic member 15. An induction coil 18 is 
disposed about the crucible 14 in the chamber 16 and is energized by 
suitable electrical power in conventional manner to inductively heat and 
melt each charge in the crucible 14 to a desired casting superheat 
temperature (i.e., a temperature above the melting point of the 
superalloy). The crucible 14 is charged with the alloy through a vacuum 
charge interlock 13 in conventional manner. 
Each superheated melt 12 (melted charge) is cast into a ceramic investment 
mold 20 which is positioned in the melting chamber 16 through a 
conventional vacuum interlock chamber 22 communicating to the melting 
chamber 16. The mold 20 can be positioned in the melting chamber before or 
after the charge is melted via the interlock chamber 22. The superheated 
melt 12 is cast into the mold 20 by tilting the crucible 14 using an 
associated conventional tilting device (not shown) in the melting chamber 
so as to pour the melt over a crucible pour lip 14b and into the mold 20. 
Those skilled in the art will appreciate that the melt can be cast in 
other ways; e.g., by "bottom pouring" through a suitable valved opening in 
the bottom of the crucible into a mold located beneath the crucible. 
The melting chamber 16 can be evacuated to a desired subambient pressure by 
a vacuum pump 24 communicated to the chamber 16 via an associated conduit 
and valve V1 as shown. Moreover, for purposes to be described, the melting 
chamber 16 is communicated to a source 26 of inert gas (e.g., a 
conventional cylinder of high purity inert gas such as argon) external of 
the melting chamber via an associated conduit and valve V2 as also shown 
in FIG. 1. 
The present invention involves the discovery that the problem of 
unacceptably high inclusion levels observed heretofore in the vacuum 
melting of successive charges of superalloys including one or more 
volatile constituents (e.g., volatile alloying elements) in a crucible 14 
arises from volatilization of such constituents under the temperature and 
pressure conditions of remelting and the condensation of the volatized 
constituents on cool regions of the crucible and melting chamber walls as 
condensate deposits that constitute inclusion precursors. During migration 
and/or after condensation at the cool regions, the volatized alloying 
elements may react with residual gas, such as oxygen, in the melting 
chamber 16, to form oxide or other inclusion precursors. For example, the 
cool regions on the apparatus 10 include the surface 14c of the crucible 
14 between the melt line ML and the upper peripheral lip 14b. Cool regions 
also include the water-cooled melting chamber walls 16a. The upper melting 
chamber wall 16b is especially important as a source of condensate as a 
result of its position overlying the melt 12 in the crucible 14 and mold 
20. The inclusion precursors gradually build-up over time as successive 
alloy charges are melted in the crucible 14 and cast until at some point 
in time the inclusion precursors begin to find their way into the melt by, 
for example, flaking off the cool regions of the crucible or being washed 
off the cool regions of the crucible during melting/superheating and 
pouring of the melt over the pour lip 14b into the mold 20. Moreover, the 
inclusion precursors may flake off the chamber walls 16a, especially 
chamber wall 16b overlying the melt 12 and mold 20. The inclusion 
precursors thereby enter the melt and become incorporated in the resultant 
casting as inclusions. A typical inclusion originating from such build-up 
and found in an IN713LC nickel base superalloy equiaxed casting is shown 
in FIG. 3 wherein the laminated morphology indicative of layer build-up 
over time is apparent. Each layer of the inclusion was determined to 
primarily comprise Al and/or Cr oxides, the Al and Cr comprising volatile 
alloying elements of that particular nickel base superalloy. 
In accordance with one embodiment of the present invention, a partial 
pressure of an inert gas, such as argon, is established in the melting 
chamber 16 (i.e., on the superalloy melt 12 in the crucible 14) at a 
partial pressure level effective to reduce volatilization of the volatile 
alloying elements and thus formation/build-up of the inclusion precursors 
at the aforementioned cool regions of the crucible and vacuum chamber 
walls as successive alloy charges are melted and cast. The inert gas 
partial pressure is established by introduction of an inert gas, such as 
argon, from the source 26 via the associated valve V2 into the vacuum 
chamber 16 prior to melting of the alloy. Typically, the vacuum chamber 16 
is first evacuated to subambient pressure, e.g., &lt;10.sup.-3 Torr, after 
the alloy charge is introduced into the crucible 14 through the interlock 
13 and then the argon is introduced into the vacuum chamber 16 to the 
aforementioned effective partial pressure level. 
The gas partial pressure is selected to be sufficient to suppress 
volatilization and migration of the volatile alloying elements from the 
melt to the aforementioned cool regions while avoiding trapping harmful 
amounts of the inert gas in the casting solidified from the melt. The gas 
partial pressure inhibits condensation of the volatized alloying elements 
at the cool regions of the crucible as well as at the vacuum chamber 
walls. 
FIGS. 2a-2d illustrate the dependence of condensate build-up on a silica 
crucible liner at a high vacuum level (0.5 micron) and at different argon 
partial pressures (100, 275, and 1000 microns Ar) in the melting chamber 
16. Each Figure was generated by vacuum induction melting 3900 grams of 
MAR-M-247 bar stock (nominal composition Ni-10Co-8Cr-10W-5.5Al-1.5Hf-1Ti 
in weight %) in a virgin silica crucible liner and holding the melt at MP 
plus 110.degree. C. (MP is the melting point of the alloy) for one (1) 
minute. The melt was then allowed to solidify in the liner. A significant 
reduction in condensate build-up on the silica liner is observed as the 
argon partial pressure level is increased. 
For the MAR-M-247 and other nickel, cobalt, and iron base superalloys such 
as IN713LC, MAR-M-509, IN718, a gas (e.g., argon) partial pressure in the 
melting chamber 16 (i.e., applied on the melt 12) generally of about 50 
microns and above approaching atmospheric pressure may be used depending 
on the particular casting procedure employed. However, the gas partial 
pressure should not be so high as to entrap gas in the castings, or to 
otherwise adversely affect a particular casting process, such as direction 
solidification (DS) processes. For melting superalloys for directional 
solidification, an argon partial pressure of about 5000 microns is 
employed since the DS superalloys typically include higher levels of 
volatile alloyants and since the long residence time of the melt in the 
mold allows escape of any entrapped argon gas. 
The present invention also involves the discovery that establishment of the 
inert gas partial pressure in the melting chamber 16 (i.e., on the 
superalloy melt 12 in the crucible 14) is effective to reduce wetting of 
the crucible 14 by the melt during the melting operation. For example, 
FIGS. 4a-4d illustrate a series of ceramic (zirconia) crucibles used to 
melt the highly wetting cobalt base superalloy, MAR-M-509, at MP plus 
150.degree. C. for 2 minutes under a high vacuum (0.5 micron) and at 
different argon partial pressures (100, 275, and 1000 microns). The 
zirconia crucible used to melt the MAR-M-509 superalloy in the high vacuum 
(0.5 microns) shows a significant degree of wetting by the melt. On the 
other hand, the degree of wetting of the zirconia crucibles by the melt is 
progressively reduced as the argon partial pressure is increased to 100, 
275, and 1000 microns. The effect of argon partial pressures to reduce 
crucible wetting is beneficial in that wettability of the crucible for a 
particular alloy system strongly determines the life of the crucible. In 
particular, lower wettability of the crucible by the melt tends to retard 
crucible erosion by the melt, extending the crucible life and reducing 
crucible erosion as a source of inclusions in the melt. The beneficial 
effect of argon partial pressure on reduced wetting of the crucible is 
observed in the same general range of argon partial pressures that 
produces the beneficial reduction in the formation/build-up of the 
inclusion precursors at the cool regions of the crucible and at the vacuum 
chamber walls. 
The following EXAMPLE is offered to illustrate, but not limit, the 
invention. 
EXAMPLE 
A series of 4 charges of IN713C alloy (composition of 
Ni-13.5Cr-4.5Mo-6.0Al-1.0Co-2(Cb+Ta) in weight %) were successively melted 
in a water cooled Cu induction skull crucible. Each charge was in the form 
bar stock weighing 4550 grams. After charging into the crucible, the 
melting chamber was evacuated to 5 microns. Then, high purity argon gas 
was introduced into the melting chamber from a gas cylinder external of 
the melting chamber to an initial Ar partial pressure level of about 
360,000 microns (about 1/2 atmosphere Ar). Each charge was then melted by 
induction skull melting procedures to a superheat temperature of 
MP+30.degree. C. and held at that temperature for 2-3 minutes. Each melted 
charge was cast from the crucible by tilt pouring into a ceramic integral 
rotor investment mold in the melting chamber and solidified in the mold 
inside the melting chamber to form an equiaxed casting. Just prior to tilt 
pouring, the melting chamber was evacuated to about 200 microns Ar to 
minimize gas entrapment in the castings. 
The castings were metallographically analyzed to determine inclusion levels 
therein. The results are presented in the histograms shown in FIGS. 5a-5d. 
The inclusion levels in the four castings made in accordance with the 
invention (FIGS. 5a-5d) were significantly reduced as compared to 
inclusion levels (FIGS. 6a-6d) found in four castings similarly melted and 
cast under a vacuum level of 5 microns (i.e., without an argon gas partial 
pressure); for example, as shown by comparing FIGS. 5a-5d and 6a-6d. 
Although the invention is described hereinabove as being practiced using 
argon or other inert gas, the invention is not so limited and can be 
practiced using other gases which are inert or nondegrading toward the 
melt. Furthermore, although especially useful in the casting of 
superalloys, the invention can be used to cast other alloys such as 
titanium alloys. In the event the alloy to be melted is other than a 
nickel, cobalt, or iron base superalloy, the selection of the particular 
gas as well as gas partial pressure used to practice the invention will be 
tailored to the particular alloy to be melted. 
While certain embodiments of the invention have been described in detail 
hereinabove, those familiar with the art will recognize that various 
modifications and changes can be made therein within the scope of the 
appended claims which are intended to include such modifications and 
changes.