U.S. patent application number 10/057612 was filed with the patent office on 2003-08-14 for filters for filtering molten metals and alloys field.
Invention is credited to Ault, James E., Barrett, James R., Durbin, Patrick J., Nikolas, Douglas Gene, Springgate, Mark E., Stefansson, Njall, Sturgis, David Howard.
Application Number | 20030150294 10/057612 |
Document ID | / |
Family ID | 27658221 |
Filed Date | 2003-08-14 |
United States Patent
Application |
20030150294 |
Kind Code |
A1 |
Stefansson, Njall ; et
al. |
August 14, 2003 |
Filters for filtering molten metals and alloys field
Abstract
A molten metal or molten metal alloy is filtered to remove
contaminants. Filters may include imaging agents, especially those
useful for N-ray radiography, X-ray analysis, and neutron
activation. Currently preferred imaging agents include gadolinia
and tungsten. Currently preferred refractory materials include
yttria, zirconia, tantalum, tungsten and rhenium. A homogeneous
distribution of refractory and imaging materials may be
accomplished by vacuum impregnation, chemical vapor deposition,
physical vapor deposition, chemical vapor infiltration, fusing,
cocalcining, alloying, and physical mixing. Filters substantially
resistant to chemical and mechanical degradation may be used. For
example, filters may be constructed with at least an outer layer of
the filter comprising yttria. Filters may define a porosity and
mesh size for efficient filtration and to exclude the maximum
allowed size of defects for the particular application. Castings
may be analyzed for inclusions by X-ray analysis, neutron
activation, and N-ray radiography. Preferred methods for filtering
melts include providing a filter including an imaging agent and
filtering the melt into a mold to cast a metal article. Filtration
may include heating the filter during filtration. The melt may
include any molten metal or metal alloy, particularly aluminum,
steel, alloys of titanium, chromium, nickel, cobalt and alloys
thereof.
Inventors: |
Stefansson, Njall;
(Portland, OR) ; Springgate, Mark E.; (Portland,
OR) ; Barrett, James R.; (Milwaukie, OR) ;
Sturgis, David Howard; (Boring, OR) ; Ault, James
E.; (Portland, OR) ; Durbin, Patrick J.;
(Portland, OR) ; Nikolas, Douglas Gene;
(Battleground, WA) |
Correspondence
Address: |
KLARQUIST SPARKMAN, LLP
One World Trade Center
Suite 1600
121 S.W. Salmon Street
Portland
OR
97204
US
|
Family ID: |
27658221 |
Appl. No.: |
10/057612 |
Filed: |
January 25, 2002 |
Current U.S.
Class: |
75/407 |
Current CPC
Class: |
B22C 9/086 20130101;
C22B 9/023 20130101; Y02P 10/234 20151101; Y02P 10/20 20151101;
B22D 43/004 20130101 |
Class at
Publication: |
75/407 |
International
Class: |
C21C 007/00 |
Claims
We claim:
1. A filter for filtering a molten metal or molten metal alloy,
comprising an N-ray imaging agent other than borate or titanium
boride.
2. A filter for filtering a molten metal or molten metal alloy,
comprising a refractory material and an N-ray imaging agent.
3. The filter according to claim 2, where the refractory material
comprises a refractory metal oxide.
4. A filter for filtering a molten metal or molten metal alloy,
comprising a refractory material and an imaging agent selected from
the group consisting of dysprosium, erbium, europium, gadolinium,
holmium, iridium, lutetium, neodymium, osmium, praseodymium,
rhenium, samarium, tantalum, tungsten, ytterbium, isotopes thereof,
physical mixtures thereof and chemical mixtures thereof.
5. The filter according to claim 4 where the imaging agent is a
metal, a metal oxide, an intermetallic, a boride, a carbide, a
nitride, metal halide, physical mixtures thereof or chemical
mixtures thereof.
6. The filter according to claim 4 where the refractory material is
a ceramic material, a refractory metal or combinations thereof.
7. The filter according to claim 6 where the ceramic material is a
metal oxide.
8. The filter according to claim 4 where the refractory material is
selected from the group consisting of yttria, zirconia, tantalum,
tungsten, rhenium, physical mixtures thereof and chemical mixtures
thereof.
9. The filter according to claim 4 where the imaging agent
comprises gadolinia.
10. The filter according to claim 4 where the filter has a porosity
of from about 10 to about 30 pores per inch.
11. The filter according to claim 4 where the filter defines
apertures of a maximum dimension of from about 0.005 inches to
about 0.05 inches.
12. The filter according to claim 4 where the filter defines
apertures of a maximum dimension of from about 0.01 inches to about
0.03 inches.
13. The filter according to claim 4 having a first layer comprising
the refractory material and a second layer comprising the imaging
agent.
14. The filter according to claim 4 where the refractory material
is substantially impregnated with the imaging agent.
15. The filter according to claim 4 where the refractory material
and the imaging agent are mixed by chemical vapor deposition,
physical vapor deposition, chemical vapor infiltration, fusing,
cocalcining, alloying, and combinations thereof.
16. The filter according to claim 4 where the refractory material
is mixed substantially homogeneously with the imaging agent.
17. The filter of claim 4 where the refractory material comprises
yttria and zirconia.
18. The filter of claim 4 where the refractory material comprises
yttria and zirconia and the imaging agent comprises gadolinia.
19. The filter of claim 13 where the second layer comprises yttria
and gadolinia.
20. A method for filtering molten metals or molten metal alloys,
comprising: providing a filter comprising a material selected from
the group consisting of dysprosium, erbium, europium, gadolinium,
holmium, iridium, lutetium, neodymium, osmium, praseodymium,
rhenium, samarium, tantalum, tungsten, ytterbium, isotopes thereof,
physical mixtures thereof, and chemical mixtures thereof; providing
a mold; heating the filter; and filtering a molten metal into the
mold to cast a metal article.
21. The method of claim 20 where the molten metal comprises
titanium or titanium alloys.
22. The method of claim 20 where the filter comprises a material
selected from the group consisting of yttria, zirconia, tantalum,
tungsten, rhenium, physical mixtures thereof and chemical mixtures
thereof.
23. The method of claim 20 where the imaging agent comprises
gadolinia.
24. The method of claim 20 where the filter has a porosity of from
about 10 to about 30 pores per inch.
25. The method of claim 20 where the filter defines apertures of
from about 0.005 to about 0.05 inches.
26. The method of claim 20 where the filter defines apertures of
from about 0.01 to about 0.03 inches.
27. The method of claim 20 further comprising analyzing the metal
article for inclusions by radiography.
28. The method according to claim 20 further comprising analyzing
the metal article by X-ray analysis.
29. The method according to claim 20 further comprising analyzing
the metal article by neutron activation.
30. The method according to claim 27 where analyzing the metal
article comprises N-ray radiography.
31. A method for filtering molten metals, comprising: providing a
filter comprising an N-ray imaging agent other than borate or
titanium boride; providing a mold; and filtering a molten metal
into the mold to cast a metal article.
32. The method according to claim 31, further comprising heating
the filter before filtering the molten metal.
33. A method for filtering molten metals, comprising: providing a
filter comprising a refractory metal oxide and an N-ray imaging
agent; providing a mold; and filtering a molten metal into the mold
to cast a metal article.
34. A method for filtering molten metals, comprising: providing a
filter comprising a first refractory material and a second imaging
material, where the second imaging material comprises an imaging
material selected from the group consisting of dysprosium, erbium,
europium, gadolinium, holmium, iridium, lutetium, neodymium,
osmium, praseodymium, rhenium, samarium, tantalum, tungsten,
ytterbium, isotopes thereof, physical mixtures thereof, and
chemical mixtures thereof; providing a mold; and filtering the
molten metal into the mold to cast a metal article.
35. The method of claim 34, where the molten metal comprises
titanium or a titanium alloy.
36. The method of claim 34 where the filter comprises yttria and
zirconia.
37. The method of claim 34 where the imaging material comprises
gadolinia.
38. The method of claim 34 where the filter comprises yttria,
zirconia, gadolinia, physical mixtures thereof and chemical
mixtures thereof.
39. The method according to claim 34 further comprising analyzing
the metal article for inclusions by radiography.
40. The method according to claim 34 further comprising analyzing
the metal article by X-ray analysis.
41. The method according to claim 34 further comprising analyzing
the metal article by neutron activation.
42. The method according to claim 39 where analyzing the metal
article comprises N-ray radiography.
43. A method for filtering molten metals, comprising: providing a
filter comprising a metal or metal alloy having a melting point
substantially the same as or greater than the molten metal;
providing a mold; heating the filter; filtering molten metal
through the filter; and introducing the molten metal into the mold,
thereby casting a metal article.
44. The method according to claim 43 where the filter comprises a
metal selected from the group consisting of iridium, hafnium,
osmium, rhenium, tantalum, tungsten, and alloys thereof.
45. The method according to claim 43 where the molten metal
comprises titanium or a titanium alloy.
46. The method according to claim 43 where the molten metal is
selected from the group consisting of aluminum, steel, nickel-based
superalloys, cobalt-based superalloys and chromium-based
superalloys.
47. The method according to claim 43 further comprising heating the
filter to greater than about 2,400.degree. F. (1,300.degree. C.)
before filtering molten metal.
48. The method according to claim 43 where the filter is heated by
a method selected from the group consisting of resistive,
conductive, inductive, convective, radiative heating and
combinations thereof.
49. The method according to claim 43 where the filter is heated by
striking an arc from the filter.
50. The method according to claim 43 further comprising analyzing
the metal article for inclusions by radiography.
51. The method according to claim 43 further comprising providing
the metal article to a third party to analyze the article for
inclusions.
52. The method according to claim 43 further comprising analyzing
the metal article for inclusions by X-ray analysis.
53. The method according to claim 43 further comprising analyzing
the metal article for inclusions by neutron activation.
54. The method according to claim 50 where analyzing the metal
article for inclusions comprises N-ray radiography.
55. The method according to claim 50 where analyzing the metal
article for inclusions comprises producing a three-dimensional
image of the article.
56. The method according to claim 50 where analyzing the metal
article comprises analyzing the article in real time.
57. A method for making a filter for filtering molten metals,
comprising: providing a filter pattern; and applying an aqueous or
non-aqueous slurry comprising an N-ray imaging agent other than
borate or titanium boride to the filter pattern.
58. A method for making a filter for filtering molten metals,
comprising: providing a filter pattern; and applying an aqueous or
non-aqueous slurry comprising a refractory metal oxide and an N-ray
imaging agent to the filter pattern.
59. A method for making a filter for filtering molten metals,
comprising: providing a filter pattern comprising a ceramic
material; providing an aqueous or non-aqueous slurry comprising an
imaging agent where the imaging agent is selected from the group
consisting of dysprosium, erbium, europium, gadolinium, holmium,
iridium, lutetium, neodymium, osmium, praseodymium, rhenium,
samarium, tantalum, tungsten, ytterbium, isotopes thereof, physical
mixtures thereof, and chemical mixtures thereof; applying the
slurry to the filter pattern; and firing the filter pattern at a
temperature sufficient to sinter the imaging agent and the ceramic
material.
60. The method according to claim 59 where firing the filter
pattern comprises firing at a temperature of greater than about
2,000.degree. F.
61. The method according to claim 59 where applying the slurry to
the filter comprises depositing the imaging agent in a quantity
sufficient for detection of inclusions.
62. The method according to claim 59 where the imaging agent is
deposited substantially homogeneously in at least an outer layer on
the filter.
63. The method according to claim 59 where the filter pattern
comprises partially stabilized zirconia.
64. The method according to claim 59 where applying the slurry
comprises vacuum impregnating the filter pattern.
65. The method according to claim 59 where the slurry comprises
yttria.
66. The method according to claim 59 where the slurry comprises
yttria and gadolinia.
67. A system for filtering and casting molten metals, comprising: a
crucible for pouring molten metal; a filter comprising an N-ray
imaging agent other than borate or titanium boride positioned in a
flow path of the molten metal; and a mold positioned to receive
filtered molten metal.
68. The system according to claim 67 further comprising heating
means for heating the filter.
69. A system for filtering and casting molten metals, comprising: a
crucible for pouring molten metal; a filter comprising a refractory
metal oxide and an N-ray imaging agent positioned in a flow path of
the molten metal; and a mold positioned to receive filtered molten
metal.
70. The system according to claim 69 further comprising heating
means for heating the filter.
71. A system for filtering and casting molten metals, comprising: a
crucible for pouring molten metal; a filter comprising an imaging
agent selected from the group consisting of dysprosium, erbium,
europium, gadolinium, holmium, iridium, lutetium, neodymium,
osmium, praseodymium, rhenium, samarium, tantalum, tungsten,
ytterbium, physical mixtures thereof and chemical mixtures thereof
positioned in a flow path of the molten metal; and a mold
positioned to receive filtered molten metal.
72. The system according to claim 71 where the filter is heated
with a heater.
73. The system according to claim 72 where the filter is connected
to the heater.
74. A filter for filtering a molten metal or molten metal alloy,
comprising a refractory material with at least an outer layer that
includes yttria.
75. The filter of claim 74, where the filter comprises an inner
layer comprising zirconia.
76. A metal alloy comprising titanium and an N-ray imaging agent
selected from the group consisting of gadolinium, samarium,
europium, and mixtures thereof.
77. The metal alloy of claim 76 where the N-ray imaging agent
comprises less than ten percent gadolinium.
78. The metal alloy of claim 76 comprising about 6% by weight
gadolinium.
79. A method for filtering a first metal or metal alloy comprising
a solid fraction that includes an imaging agent, comprising:
forming a mixture comprising a metal or a first metal alloy and a
second imaging alloy comprising titanium and an N-ray imaging agent
selected from the group consisting of gadolinium, samarium,
europium, and mixtures thereof; providing a filter; and providing a
molten material comprising the metal or first metal alloy and a
solid fraction comprising the second imaging alloy; and filtering
the molten material comprising the solid fraction.
80. The method according to claim 79 further comprising determining
if the metal alloy is retained on the filter by N-ray
radiography.
81. The method according to claim 79 and further comprising:
forming a cast article from the molten material after filtering;
and determining whether the article has inclusions by at least
N-ray radiography.
Description
FIELD
[0001] This application concerns filters useful for filtering
molten metals and metal alloys and methods for filtering metals and
metal alloys using such filters.
BACKGROUND
[0002] Investment casting is a process for forming metal or metal
alloy articles (also referred to as castings) by solidifying molten
metal or alloys in a mold having the shape of a desired article.
Investment casting is an important method for producing quality
metal articles for a variety of industries that rely upon high
integrity precision castings for critical applications. Castings
often include undesirable material, referred to as "inclusions".
Inclusions can result from the incorporation of slag or dross
material from the melt or from foreign material derived from the
casting mold or from contamination from the foundry environment,
such as dust and dirt. Inclusions in cast metal articles are
detrimental to the mechanical properties of the metal and can lead
to catastrophic failure under stress. Therefore, many industries,
particularly the aerospace industry, have developed stringent
specifications governing the presence and acceptable size of
inclusions in cast parts, such as turbine blades and airframe
components.
[0003] Filtration is one method for removing impurities, such as
slag and dross, from molten metals. References that disclose
filters for filtering molten metals include U.S. Pat. Nos.
3,981,352; 4,671,498; 4,719,013; 5,045,511; 5,177,035 and
5,369,063. For example, Nurminen et al.'s U.S. Pat. No. 3,983,352
discloses a filter constructed "of substantially spherical
refractory particles. A ceramic binder is utilized for securing the
particles in a bonded assembly, the binder composition
substantially completely coating the particles." Column 2, lines
13-17. Nurminen states that, "[t]he binder is characterized by an
affinity for dross and slag constituents in the molten metal so
that all or substantially all of such constituents can be removed."
Column 2, lines 20-23. Gee et al.'s U.S. Pat. Nos. 5,177,035 and
5,369,063 teach "a porous ceramic body" made from various ceramic
powders for filtering molten metals. The '035 patent states that
"it is desirable to separate exogenous intermetallic inclusions
from the molten metal. Such inclusions result in molten metals from
impurities included in the raw materials used to form the melt,
from slag, dross and oxides which form on the surface of the melt
and from small fragments of the refractory materials . . . " Column
1, lines 18-24. Sane et al.'s U.S. Pat. No. 5,045,111 describes "a
porous ceramic filter preferably comprising particles of alumina
and partially stabilized zirconia . . . for separating non-metallic
inclusions and contaminants from molten ferrous metal as it is
flowed." Column 1, lines 17-19.
[0004] Despite the teachings of these prior patents, certain
problems associated with filtering molten metals have not been
adequately addressed. For example, one problem is the premature
solidification of the molten metal being filtered. Solidification
occurs when the filter acts as a heat sink to cool the metal or
unduly restricts the flow of the melt, allowing the melt to
solidify on the filter. Efforts to address this premature
solidification have met with little success. The Sane patent
states, for example that "[p]reheating a filter while it is in the
casting mold is impractical." Column 2, lines 30, 31.
[0005] Another problem is that there are few materials suitable for
filtering reactive, high melting point metals and superalloys.
Highly reactive metals and metal alloys, such as titanium, titanium
alloys, and superalloys, are incompatible with most conventional
filter media. For example, typical refractory ceramic filter
materials, such as metal oxides, are reduced by molten titanium and
titanium alloys, resulting in the incorporation of brittle
oxygen-enriched metal in the castings. In addition, filters used
for filtering molten metals are subjected to considerable
mechanical stress. Therefore, it is possible for filter material to
flake, fragment or otherwise become incorporated into the melt. The
result is contamination of the casting by the material used to form
the filter.
[0006] Nondestructive evaluation (NDE) of castings can reveal
inclusions in cast articles. Some inclusions, if detected, can be
removed from the metal article, and the article repaired, without
compromising its structural integrity. However, it is difficult,
and often impossible to detect, locate and repair flaws using
conventional techniques.
[0007] Various techniques for NDE have been used by the investment
casting industry. For example, some tungsten and thorium oxide
inclusions can be detected by X-ray analysis of titanium castings
because there is a sufficient difference in the density of tungsten
and thorium oxide and that of titanium. ASTM (American Society for
Testing and Materials) publication No. E 1320-90 describes X-ray
reference radiographs for analyzing titanium cast articles of less
than about two inches thick. Generally, X-ray analysis has proved
useful for detecting inclusions in titanium and titanium alloy
articles having a maximum thickness of only about two inches. To
date, titanium has been used by the investment casting industry
primarily for casting articles having relatively small cross
sections. However, investment casting is now being considered for
producing structural components of aircraft having significantly
larger cross sections than articles cast previously. Therefore,
N-ray analysis may provide a more general solution to detection and
imaging of inclusions in investment castings.
[0008] N-ray imaging agents have previously been used in the
investment casting industry. For example, ASTM publication No. E
748-95 states that "[c]ontrast agents can help show materials such
as ceramic residues in investment-cast turbine blades." ASTM E
748-95, page 5, beginning at about line 46. This quote refers to
detecting ceramic residues by N-ray on metal articles having an
internal cavity produced by initially casting metal about a ceramic
core. The ceramic core is removed to form the cavity, and
thereafter a solution of gadolinium nitrate is placed in the
cavity. The gadolinium nitrate solution is left in the cavity long
enough to infiltrate porous ceramic core residue that is on the
surface of the article. The residue can then be imaged by
N-ray.
[0009] Methods for incorporating imaging materials into
refractories are known in the art. For example, Sturgis et al.'s
U.S. Pat. No. 6,102,099 teaches "the incorporation of an imaging
agent into the investment casting mold, particularly in the
facecoat of the mold, prior to casting so that inclusions can be
imaged in the cast article." However, this method does not provide
means for detecting filter-derived inclusions.
SUMMARY
[0010] The present method concerns preventing inclusions and
detecting inclusions in metal articles if they are present. An
article or a casting may be an ingot or a processed ingot,
processed by forging to form an item such as a billet or a sheet.
One disclosed feature includes filters for filtering a molten metal
or molten metal alloy. The filters may include one or more N-ray
imaging agents. N-ray imaging agents may be selected such that the
difference between the linear attenuation coefficient of the
imaging agent and the cast metal article sufficient to allow
detection of a filter-derived inclusion in the article. Gadolinia
is an example of a useful N-ray imaging agent.
[0011] Other types of imaging agents, especially those useful for
X-ray spectroscopy, X-ray radiography, and neutron activation may
be included in the filters. Imaging agents of particular utility as
filter materials may be selected from the group consisting of
dysprosium, erbium, europium, gadolinium, holmium, iridium,
lutetium, neodymium, osmium, praseodymium, rhenium, samarium,
tantalum, tungsten, ytterbium, isotopes thereof, physical mixtures
thereof and chemical mixtures thereof. Examples of suitable imaging
agent materials used to construct the filters may include metals,
metal oxides, intermetallics, borides, carbides, nitrides, metal
halides, physical mixtures thereof, and chemical mixtures thereof.
Tungsten metal is an example of an X-ray imaging agent used to
construct filters.
[0012] Filters may include refractory materials in addition to an
imaging agent, and the refractory materials used in the filters may
be or include a ceramic, a refractory metal, or combinations of
those materials. Some useful refractory materials include those
selected from the group consisting of yttria, zirconia, tantalum,
tungsten, rhenium, isotopes thereof, physical mixtures thereof and
chemical mixtures thereof. The refractory material may be or
include a metal oxide, and the refractory materials may be alloyed
with particular elements in order to increase their performance
characteristics, such as resistance to the chemical and mechanical
stress of filtering molten metal. For example, tungsten may be
alloyed with rhenium to give an alloy with a high melting point and
a high tensile strength.
[0013] The refractory material and imaging material used in these
filters may be homogeneously distributed throughout the entire
filter or solely or primarily only in a layer of the filter that is
directly contacted by molten metal or metal alloy. A homogeneous
distribution may be accomplished by vacuum impregnation of one
material in another. Other methods for distributing the materials
include chemical vapor deposition, physical vapor deposition,
chemical vapor infiltration, fusing, cocalcining, alloying, and
physical mixing.
[0014] Another feature of the disclosed embodiments involves
filtering with filters substantially resistant to chemical and
mechanical degradation. For example, filters may be constructed
such that at least an outer layer of the filter comprises a
chemically robust material, such as yttria. The filters may be
constructed by a method such as vacuum impregnation, such that the
filters are resistant to mechanical degradation.
[0015] Filters may be constructed from metals or metal alloys
suitable for imaging and having melting points substantially the
same as or higher than that of the molten metal or metal alloy to
be filtered. Particular useful metals for filter construction
include those selected from the groups described above. Certain of
these materials may be alloyed with other elements to improve their
chemical or mechanical properties, such as high temperature
strength. A particularly useful metal for making filters is
tungsten, and hafnium and platinum are two elements that, when
alloyed with tungsten, confer desirable chemical and mechanical
properties. For example, one application for the disclosed filters
is filtration of titanium and titanium alloys, which have a typical
melting point of about 3,000.degree. F. (1,600.degree. C.). Thus,
the filter material should be chosen such that it has a melting
point sufficiently close to or in excess of that of the metal or
metal alloy being cast.
[0016] A further embodiment of the disclosure includes a method for
filtering impurities from molten metals or molten metal alloys
using filters comprising the imaging agents described above.
Moreover, filter-derived inclusions may be detected and imaged in a
product casting. The filter may be heated to a temperature suitable
to substantially avoid solidification of the molten metal or metal
alloy during filtration. Filtering a molten metal may be followed
by casting the metal to form a metal article. The molten metal or
metal alloy may include any molten metal or metal alloy, including
reactive metals, aluminum, steel, titanium, alloys and superalloys,
particularly alloys of titanium, chromium, nickel, cobalt, and
zirconium. In further embodiments of the present method, cast metal
articles may be analyzed for inclusions by X-ray analysis
(including X-ray radiography and X-ray spectroscopy), neutron
activation, and/or N-ray spectroscopy, and/or N-ray
radiography.
[0017] Further embodiments of the present method include procedures
for making the filters described above. One embodiment includes
providing a filter comprising a refractory material, such as those
described above, and providing an imaging agent by a surface
coating or precipitation method. For example, the filter may
comprise partially-stabilized zirconia and the imaging agent may
comprise gadolinia. The imaging agent may be provided such that at
least an outer surface region of the filter comprises the imaging
agent, and such methods for providing layers of different
refractory materials are known to those of ordinary skill in the
art. The imaging agent may be applied by one of the methods
described above, and may be applied with additional refractory
material. For example, yttria and gadolinia can be applied to a
filter or filter pattern.
[0018] The imaging agent should be supplied in a sufficient amount
to allow detection of filter-derived inclusions. For working
embodiments, the imaging agent used to make the filters typically
comprised from about 0.5 to about 100 weight percent imaging agent,
more typically from about 1 to about 100 weight percent, even more
typically from about 1 to about 65 weight percent, and preferably
from about 2 to about 25 weight percent, imaging agent. Systems for
filtering molten metals may include a crucible for pouring a molten
metal or metal alloy through a filter. The filter may be one of the
filters described above. The filter may be positioned such that the
filtered metal or metal alloy is directed into a mold. Systems also
may include a heater for heating the filter. Filters may be heated
by an electrical arc or by a method selected from the group
consisting of resistive, conductive, inductive, convective and
radiative heating and combinations thereof. Systems may include a
thermocouple for monitoring the filter temperature, and means for
positioning the filter in the melt pour path.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is diagram of a system for filtering molten metal
into a mold.
[0020] FIG. 2 is a plan view of one embodiment of a wire mesh
filter.
[0021] FIG. 3 is a cross-sectional view of filter 30 taken along
line 3-3.
[0022] FIG. 4 is a perspective view of one embodiment of a wire
mesh filter for filtering molten metals including connections for
heating the filter.
[0023] FIG. 5 is an X-ray radiograph of a wire mesh filter after
filtration of a titanium melt containing inclusions showing
inclusions retained on the filter.
[0024] FIG. 6 is an N-ray radiograph of a ceramic filter after
filtration of a titanium melt; inclusions comprising gadolinium are
visible on the filter as light flakes.
[0025] FIG. 7 is an N-ray radiograph of a GdNO.sub.3 infiltrated
filter after filtration of a titanium melt.
DETAILED DESCRIPTION
[0026] Embodiments of the present disclosure concern removing
inclusions from molten metals, particularly titanium and titanium
alloys, via filtration prior to casting. Since filtration can
result in the incorporation of filter-derived material into the
molten metal, and thus the casting, it is desirable to detect this
material in the product casting. To ensure high quality castings,
it may be desirable to use filters containing an imaging agent,
such that filter-derived inclusions can be detected by NDE. If
there are inclusions larger than the allowed size for the
particular application, the article may be repaired in some cases.
To aid accurate repairs, it may be desirable to image inclusions
three-dimensionally in the cast article. Filters have been
constructed to include imaging agents, particularly X-ray and N-ray
detectable materials, such that filter-derived inclusions can be
detected and imaged. The filters and methods of filtering are
applicable to filtering virtually all metals or metal alloys, with
particular examples being titanium and titanium alloys.
I. Introduction
[0027] With reference to FIGS. 1-4, the following paragraphs
describe several working embodiments of the present method and
apparatus. An embodiment of a system 10 for filtering molten metals
is depicted in FIG. 1. Crucible 12 is provided to pour the molten
metal or metal alloy to be cast. The melt is poured into pour cup
16. Path 14 shows the flow of the molten metal through filter 18
and into mold or die 20. A casting is produced upon solidification
of the melt.
[0028] A working embodiment of a filter 30, suitable for use as
part 18 in FIG. 1, is shown in FIG. 2. Wire mesh screen 32 is
placed over a refractory ceramic frame 34. The wire of the mesh
screen may be selected with a wire diameter sufficient to withstand
the mechanical stress of filtration for the particular pour size.
Working embodiments of the present method use wire mesh filters
with a wire diameter of about 0.004 inch (0.1 mm). The screen is
held in place by rods 36, which also serve as electrodes for
connecting the filter to a power source (not shown) for resistive
heating or as heat conductors so that the filter may be heated
conductively. FIG. 3 shows a cross-sectional view 40 of the filter
32, held in place by rod 36. The screen may be welded to the
electrode or may be fastened by clamps (not shown).
[0029] FIG. 4 illustrates a further embodiment of a filter
apparatus 50. Wire mesh 52 is clamped by clamps 54 to supports 56.
Wire mesh 52 is wrapped around rods 60 and insulative half-tubes 58
are clamped (clamps not shown) around the wire mesh-wrapped rods.
The filter of FIG. 4 also can be heated, as with the embodiment
illustrated by FIG. 3.
[0030] The following paragraphs concern pertinent details of filter
construction, the use of filters for filtering molten metal,
apparatus for filtering molten metals and the incorporation of
filtering in the casting process, as well as working examples of
such methods of construction and of imaging filter-derived
inclusions in cast metal articles.
II. Filters for Filtering Molten Metals
[0031] Filters must be chemically and mechanically robust to
withstand the considerable stress of filtering molten metals. To
maintain their integrity and capture inclusions, materials used to
make filters must be stable at high temperatures and have only "low
reactivity" with the molten metal or metal alloy being filtered.
"Low reactivity" may as used herein is defined with reference to
Aerospace Material Specification Number 4985B (AMS 4985B),
published by SAE International, January 1997. Pertinent data from
AMS 4985B are provided by Table 1. With reference to Table 1, a
filter's reactivity with molten titanium 6-4 alloy should be such
that the composition of a casting made from filtered metal
comprises no more than approximately the amounts of a contaminant
listed in Table 1. Similarly, the solubility of the filter
materials in the molten metal or metal alloy to be filtered should
be low. Low solubility may be defined, with reference to AMS 4985B,
as solubility that yields no more than about the percentage of a
material listed in Table 1 in the product casting. Reference to the
AMS 4985B requirements is made solely to illustrate the industry
standard for certification of investment castings and not to limit
the scope of the present method.
1TABLE 1 Composition Specification of Titanium 6-4 Alloy Investment
Castings.sup.a Element Minimum (%) Maximum (%) Al 5.50 6.75 V 3.50
4.50 Fe -- 0.30 O -- .20 C -- .10 N -- 0.05 (500 ppm) H -- 0.015
(150 ppm) Y -- 0.005 (50 ppm) Residual Elements, each -- 0.10
Residual Elements, total -- 0.40 Ti remainder .sup.aAMS 4985B, SAE
International, Warrendale, Pa, January 1997.
[0032] High melting point metals that are useful for constructing
filters include, without limitation, those selected from the group
consisting of osmium, rhenium, tantalum, tungsten and their alloys.
The melting points of these metals range from about 5,250.degree.
F. (2,900.degree. C.) to about 6,150.degree. C. (3,400.degree.
C.).
[0033] Alternatively, filters may be constructed from ceramic
materials suitable for filtering molten metals. As discussed in the
Background, molten metals such as titanium and titanium alloys
reduce many refractory ceramic materials, resulting in brittle,
oxygen-enriched metal castings. Therefore, it may be preferable to
use a refractory material on at least an outer surface of a filter
that is substantially resistant to reduction by the molten metal or
metal alloy being filtered. For example, a ceramic foam filter may
be coated or impregnated with additional ceramic refractory
material, which may include an imaging agent. The filters typically
are coated by applying a slurry, by a method such as dip coating,
comprising the desired refractory or imaging material. The material
then can be deposited on the filter or impregnated in the filter
via vacuum impregnation. Other deposition methods include methods
selected from the group consisting of chemical vapor deposition,
physical vapor deposition, chemical vapor infiltration and
combinations thereof.
[0034] Investment castings for different applications have various
requirements for the size of inclusions that may be tolerated.
Thus, the aperture size of filters for filtering molten metals will
depend upon the intended application of the product casting. The
aperture shape and size may be chosen such that the molten metal
flows efficiently while all matter above a certain predetermined
size is retained on the filter. Various different aperture shapes
also may be used, including circular, polyhedral and irregular
shapes. Working embodiments of filters had aperture sizes ranging
from about 0.005 inch (0.13 mm) to about 0.05 inch (1.3 mm).
Additional working embodiments of filters had porosity of from
about 10 pores per inch to about 30 pores per inch.
III. Imaging Agents Useful for Imaging Inclusions
[0035] The choice of imaging agent to be used in a filter depends
upon the type of NDE to be employed. Table 2 provides data
concerning some materials that may be used for N-ray and X-ray
imaging of inclusions in investment castings. Data for titanium is
provided for purposes of comparison.
[0036] If X-ray radiographic analysis is to be performed, the
primary consideration is the density of the imaging agent relative
to the density of the metal article to be analyzed. Other
considerations include the size and orientation of the inclusion
and the thickness of the casting cross-section being analyzed. If
the difference between the density of the cast material and the
inclusion is small, X-ray radiography may yield insufficient image
contrast for thick articles. For example, Sturgis teaches that if
the difference in density between the article and the inclusion is
less than about 0.5 g/cc for titanium or titanium alloy castings,
insufficient image contrast may be obtained for inclusion detection
in articles of about a 1 inch (2.5 cm) thickness or less. Sturgis,
column 5, lines 58-65.
2TABLE 2 Densities and Thermal Neutron Linear Attenuation
Coefficients Using Average Scattering and Thermal Absorption Cross
Sections for the Naturally Occurring Elements.sup.a Density of
Metals or Linear Element Metal Attenuation Atomic Cross Section
(barns).sup.b Oxides Coefficient Technique No. Symbol Scattering
Absorption (g/cc) (cm.sup.-1).sup.c Used 3 Li 0.95 70.6 2.01
(Li.sub.2O) 3.31 N-ray 5 B 4.27 767 2.46 (B.sub.2O.sub.3) 101.79
N-ray 22 Ti 4.09 6.09 4.5 (Ti) 0.58 Reference 41 Nb 6.37 1.15 7.03
(NbO) 0.42 X-ray 49 In 2.45 193.8 6.99 (In.sub.2O) 7.52 Both 7.18
(I.sub.2O.sub.3) 60 Nd 16 60.6 7.24 (Nd.sub.2O.sub.3) 1.89 X-ray 62
Sm 38 5670 8.3 (Sm.sub.2O.sub.3) 171.86 Both Sm.sup.149 41000
Sm.sup.151 15000 63 Eu . . . 4565 7.42 (Eu.sub.2O.sub.3) 94.82 Both
Eu.sup.155 14000 64 Gd 172 48890 7.4 (Gd.sub.2O.sub.3) 1483.88 Both
Gd.sup.155 61000 Gd.sup.157 254000 66 Dy 105.9 940 7.81
(Dy.sub.2O.sub.3) 33.13 Both 67 Ho 8.65 64.7 8.79 (Ho) 2.35 Both 68
Er 9 159.2 8.64 (Er.sub.2O.sub.3) 5.49 Both Er.sup.167 700 70 Yb
23.4 35.5 9.2 (Yb.sub.2O.sub.3) 1.43 X-ray 71 Lu 6.8 76.4 9.4
(Lu.sub.2O.sub.3) 2.82 Both 73 Ta 6.12 20.5 16.6 (Ta) 1.47 X-ray
Ta.sup.82 8200 74 W 4.77 18.4 4.2 (Na.sub.2WO.sub.4) 1.46 X-ray
19.35 (W) 77 Ir 14.2 425.3 22.4 (Ir) 30.86 Both 90 Th 12.97 7.97
11.7 (Th) 0.62 Both 9.86 (ThO.sub.2) .sup.aASTM E 748-95 with
updated data primarily from Neutron Cross Sections: Neutron
Resonance Parameters and Thermal Cross Sections, S. F. Mughabghab,
Academic Press, Inc., San Diego, Ca, 1981; and Handbook of
Chemistry and Physics, CRC Press, Boca Raton, FL, 1973. .sup.bAll
cross-section values are most probable values. .sup.cLinear
attenuation coefficients were calculated using nominal elemental
atomic weights and densities.
[0037] Imaging agents that may be used to construct filters and
detect filter-derived inclusions in investment castings using X-ray
radiography include materials comprising metals selected from the
group consisting of dysprosium, erbium, europium, gadolinium,
hafnium, holmium, iridium, lutetium, neodymium, osmium,
praseodymium, rhenium, samarium, tantalum, tungsten, ytterbium,
isotopes thereof, physical mixtures thereof and chemical mixtures
thereof. The oxides of these metals (e.g., Dy.sub.2O.sub.3,
Er.sub.2O.sub.3, Gd.sub.2O.sub.3) and the salts of such metals
(e.g., GdNO.sub.3, YNO.sub.3) are examples of types of compounds
that may be employed. Materials that form these compounds upon
further treatment, such as heating, also can be used for filter
construction.
[0038] Tungsten is an excellent imaging agent for X-ray
radiography, and is sufficiently chemically robust to withstand
filtration of reactive molten metals such as titanium and titanium
alloys. Furthermore, tungsten has the physical properties to
withstand the mechanical stress of filtration. For example,
tungsten has the highest melting point and lowest vapor pressure of
all metals, and at temperatures over 3,000.degree. F.
(1,650.degree. C.) the highest tensile strength. Handbook of
Chemistry and Physics, p. B-35, CRC Press, Boca Raton, Fla.,
1973.
[0039] However, it is possible that other materials with the
desired density may be used to practice the present invention.
Indeed, additional X-ray imaging agents can be selected by choosing
materials with a density sufficiently different from the density of
the metal or alloy being cast. Generally, the imaging agents
described above have sufficiently different densities from the
metals and/or alloys used to produce investment castings, such as
stainless steel, titanium and titanium alloys, and the chromium,
cobalt and nickel based superalloys.
[0040] N-ray imaging is discussed in ASTM E 748-95, entitled
Standard Practices for Thermal Neutron Radiography of Materials,
which is incorporated herein by reference. N-ray imaging is a
process whereby radiation beam intensity modulation by an object is
used to image certain macroscopic details of the object. N-ray uses
neutrons as penetrating radiation for imaging inclusions. The basic
components required for N-ray imaging include a source of fast
neutrons, a moderator, a gamma filter, a collimator, a conversion
screen, a film image recorder or other imaging system, a cassette,
and adequate biological shielding and interlock systems. See, ASTM
E 748-95.
[0041] Suitable imaging agents for N-ray imaging of inclusions may
be selected by the linear attenuation coefficient or the thermal
neutron absorption cross section of the material relative to that
of the metal or metal alloy being cast. N-ray imaging agents
currently considered most useful for filtering and detecting
filter-derived inclusions in investment castings include those
materials comprising metals selected from the group consisting of
boron, dysprosium, erbium, europium, gadolinium, holmium, iridium,
lutetium, neodymium, osmium, praseodymium, rhenium, samarium,
tantalum, tungsten, ytterbium, isotopes thereof, physical mixtures
thereof, and chemical mixtures thereof. The oxides of the above
metals (e.g., Dy.sub.2O.sub.3, Er.sub.2O.sub.3, Gd.sub.2O.sub.3)
and the salts of such metals (e.g., GdNO.sub.3, YNO.sub.3) are
particularly useful. Gadolinium, for example, has one of the
highest linear attenuation coefficients of any element (about
1483.88 cm.sup.-1), whereas the linear attenuation coefficient of
titanium is about 0.68 cm.sup.-1. The relatively large difference
between the linear attenuation coefficient of titanium or titanium
alloys and the linear attenuation coefficient of gadolinium makes
gadolinia particularly suitable for N-ray imaging. However, other
materials with the desired linear attenuation coefficient or
thermal neutron absorption cross section can be used to practice
the present method. Finally, certain isotopes of the elements above
have superior neutron attenuation properties. For example,
gadolinium 157 has a thermal neutron absorption coefficient of
254,000 barns. Additional isotopes that may be useful for N-ray
imaging are included in Table 2. Other materials suitable for N-ray
imaging may be identified by comparison of the linear attenuation
coefficient or the thermal neutron absorption cross section of the
material to the metal or metal alloy being cast.
IV. Construction of Filters
[0042] It is possible to construct filters comprising substantially
solely an imaging agent or imaging agents. Preferred imaging agents
for constructing filters often are expensive, and hence it may be
preferable to use a mixture of imaging agent or agents with another
material suitable for filter construction. It is possible to
distribute the imaging agent substantially uniformly throughout the
filter material. Alternatively to use the imaging agent most
efficiently, the imaging agent can be distributed substantially
uniformly on the outer surface of the filter, and also perhaps in
one or more of the outer layers of the filter material. If the
imaging agent is not distributed substantially uniformly within the
desired layer, it is possible that an inclusion could comprise
solely undetectable material.
[0043] The outer regions or layers of filters for filtering molten
metals contact the metal or metal alloy in its molten state during
filtration. Most metals and metal alloys used for investment
casting are highly reactive, particularly at elevated temperature,
so that at least the outer layers of the filter preferably should
be substantially non-reactive with the molten metal or alloy being
filtered.
[0044] A list of materials useful for filtering molten metals
includes those selected from the group consisting of zirconia,
zircon, yttria, tantalum, titania, tungsten, hafnium, iridium,
osmium, rhenium, isotopes thereof, physical mixtures thereof and
chemical mixtures thereof. The choice of the filter material
depends on the metal being filtered. For example, yttria is a
useful component of filters, especially outer regions of filters,
for molten titanium and titanium alloys, primarily because it is
less reactive with molten titanium and titanium alloys than many
other refractory materials.
[0045] Simple physical mixtures of refractory materials for filter
construction and imaging materials generally work to practice the
present invention. Alternatively, "intimate mixtures" formed
between the imaging agent and other components of the filter may be
used. Intimate mixtures are discussed in detail in U.S. Pat. No.
6,102,099, which is incorporated herein by reference. "Intimately
mixed" or "intimate mixture" is used to differentiate binary
mixtures that result simply from the physical combination of two
components. Typically, an "intimate mixture" means that the first
material is atomically dispersed in a second, such as with a solid
solution or as small precipitates in a crystal matrix.
Alternatively, an intimate mixture may refer to compounds that are
fused, such as, fused yttria-alumina or fused yttria-titania. By
way of example and without limitation, intimate mixtures may be
formed in the following ways: (1) finely dispersed in a matrix; (2)
provided as a coating on the surface of particles; or (3) provided
as a diffused surface layer of on the outer surface of particles.
The intimate mixture may be a solid solution, or it may be in the
form of small precipitates in a crystal matrix, or it may be a
coating on the surface of a particle or portions thereof.
[0046] The filter material may be physically mixed with the imaging
material. The filter material also may be fused with the imaging
material. Fused materials may be generated by first forming the
desired weight mixture of filter-forming material and imaging
material. The mixture is then heated until molten and then cooled
to yield the fused material. Fused material can then be used to
construct a filter or a filter layer in the same way as the
physical mixture would be used. For example, the filter can be
constructed without the use of an imaging agent, and can be coated
or impregnated subsequently with one or more imaging agents. The
imaging agent may be supplied as a physical mixture, a solution or
an intimate mixture with other components. For example, a filter
may be impregnated with a slurry comprising an imaging agent or
mixture of imaging agents with or without additional refractory
materials. Similarly a filter may be infiltrated with a solution
comprising one or more imaging agents. The filter may then be fired
at a temperature sufficient to sinter the filter components.
Typical firing temperatures are from about 2,000.degree. F.
(1,090.degree. C.) to about 3,200.degree. F. (1,800.degree.
C.).
V. Methods of Filtering Molten Metals
[0047] One embodiment of the present method includes providing a
filter comprising an imaging agent, providing a molten metal or
metal alloy and filtering the molten metal or metal alloy. The
filter can include any filter comprising an imaging agent or an
imaging agent and additional refractory material. The imaging agent
may be selected such that filter-derived inclusions may be imaged
in a product casting.
[0048] Filters may be heated to a temperature sufficient to avoid
substantial solidification of the melt during filtration. For
example, working embodiments of the present invention involve
heating a tungsten wire mesh filter to above about the melting
point of titanium prior to filtration of a titanium melt.
[0049] Filters may be placed in the molten metal flow at any point
in the flow path. For example the filters may be attached to a
crucible used to pour the molten metal, to the mold gating, or to
any other point in the flow path of the molten metal.
[0050] Filtration of reactive metals and metal alloys including
aluminum, steel, titanium, including superalloys, particularly
alloys of titanium, chromium, nickel, zirconium and cobalt may be
performed under an inert atmosphere or at reduced pressure or both.
Any gas of low reactivity may be used to provide an inert
atmosphere. Typical examples of inert gases include helium,
nitrogen, neon, argon, krypton and xenon.
VI. Alloys Comprising Imaging Agents
[0051] Embodiments of the present method include using alloys
comprising an imaging agent. Alloys comprising any imaging agent
may be used to practice the present method. Typically, embodiments
of the present method include titanium alloys comprising any
imaging agent listed in Table 2. More typically, embodiments of the
present method include titanium alloys comprising an imaging agent
selected from the group consisting of gadolinium, samarium,
europium, and mixtures thereof.
[0052] Generally, the method for making alloys of the present
invention comprise providing desired, predetermined material
amounts, such as by stating weight percents. These predetermined
material amounts are then combined and then heated to a temperature
effective to form the alloy. Working embodiments have heated the
constituent materials to a temperature above the melting point of
the constituent metals. Additional information concerning working
embodiments of making and using such alloys in casting processes.
Additional techniques for processing materials are known to those
skilled in the art, for example, hot isostatic pressing (HIP) and
can be used to make desired alloys.
EXAMPLE 1
[0053] This example describes a tungsten wire mesh filter useful
for filtering molten metals. Tungsten wire cloth was purchased from
Twin City Wire, Inc., Eagan Minn. The wire cloth is available in
various appropriate gauges and aperture sizes to withstand the
physical stresses of melt filtration while capturing any undesired
foreign materials. The wire cloth may be fashioned into a filter
apparatus, such as that illustrated by FIG. 4. A
12-inch.times.12-inch (30 cm.times.30 cm) section of tungsten wire
cloth 52 (aperture size 0.029 inch (0.74 cm), having wire of
diameter 0.004 inch (0.1 mm) was secured to ceramic supports 56 by
clamps 54. Tungsten rod 60 was coupled to the tungsten wire mesh by
clamping (clamps not shown) insulative, alumina half-tube 58 around
the mesh-wrapped rod. Tungsten rods 60 were connected to a power
source, and the filter was heated by application of electrical
current to approximately 3,300.degree. F. (1,800.degree. C.) as
measured by a thermocouple (not shown). Molten titanium 6-4 alloy
(33 lbs., 15 kg) was then poured through the filter.
EXAMPLE 2
[0054] This example describes the preparation of a ceramic filter
useful for filtering molten metals. The preparation includes a
method for making a slurry useful for impregnating a filter
comprising a refractory material with an imaging agent. Amounts
stated in this example are percentages based upon the total weight
of the slurry (weight percents), unless noted otherwise. Moreover,
unless otherwise stated, the materials were combined in the order
given.
[0055] In this particular example, the filter was made from
partially stabilized zirconia impregnated with yttria. Yttria is a
robust material suitable for filtering reactive metals and metal
alloys.
[0056] Deionized water (15.6 weight %) was mixed with 1.00 weight %
tetraethyl ammonium hydroxide (TEAH). To this solution was added
1.2% latex (Dow 460 NA), 0.7 weight % surfactant (NOPCOWET C-50)
and 0.2 weight % of a defoamer (Dow 1410 antifoam). Finally, 81.3
weight % yttria was added to the mixture under continuous stirring
to form a slurry.
[0057] A partially stabilized zirconia filter having 20 pores per
inch, purchased from HiTech Ceramics, Alfred, N.Y. was then
positioned in the yttria slurry and placed under vacuum (28 inches
Hg, 711 mm Hg) to infiltrate the slurry into the pores of the
filter. The filter was removed from the slurry and excess solid
material was removed from the filter. After drying at 160.degree.
F. (71.degree. C.), the filter was fired at 2900.degree. F.
(1,600.degree. C.) to sinter and densify the filter.
EXAMPLE 3
[0058] In this particular example, a partially stabilized zirconia
filter having 10 pores per inch was vacuum infiltrated with a
gadolinium nitrate solution. Gadolinium is particularly useful for
imaging inclusions by N-ray because it has a relatively high linear
attenuation coefficient of 1483.88 cm.sup.-1.
[0059] A partially stabilized zirconia filter having 10 pores per
inch, purchased from HiTech Ceramics, Alfred, N.Y. was placed in a
1.66 M solution of GdNO.sub.3 and placed under vacuum (28 inches
Hg, 711 mm Hg) to infiltrate the solution into the pores of the
filter. The filter was removed from the solution and dried at
160.degree. F. (71.degree. C.). The filter was fired at
2,900.degree. F. (1,600.degree. C.) to sinter and densify the
filter.
EXAMPLE 4
[0060] In this particular example, a partially stabilized zirconia
filter having 10 pores per inch was vacuum infiltrated with a
yttrium nitrate solution to produce a filter suitable for filtering
molten metals including titanium and titanium alloys.
[0061] A partially stabilized zirconia filter having 10 pores per
inch, purchased from HiTech Ceramics, Alfred, N.Y., was placed in a
2.66 M solution of YNO.sub.3 and placed under vacuum (28 inches Hg,
711 mm Hg) to infiltrate the solution into the pores of the filter.
The filter was removed from the solution and dried at 160.degree.
F. (71.degree. C.). The filter was fired at 2,900.degree. F.
(1,600.degree. C.) to sinter and densify the filter.
EXAMPLE 5
[0062] This example concerns the attempted filtration of a titanium
alloy melt through a non-heated tungsten filter. The filter was
constructed by drilling apertures of three different sizes (0.025,
0.050 and 0.1 inch) in a tungsten plate. Ti 6-4 ally was melted
over the tungsten plate. The molten metal solidified on the filter,
blocking the apertures before any molten metal could flow through
the apertures.
EXAMPLE 6
[0063] This example concerns the filtration of a titanium alloy
melt through a tungsten wire mesh filter. In this example, the
filter apparatus of Example 1, comprising a tungsten wire mesh
approximately 12 inches.times.12 inches (30 cm.times.30 cm), having
wire diameters of about 0.004 inch (0.1 mm) and apertures of about
0.029 inch (0.74 cm), was heated to approximately 2,000.degree. F.
A melt comprising approximately 33 pounds (15 kg) of molten Ti 6-4
alloy was prepared, and the melt poured through the filter into a
mold to form a casting. The molten metal solidified on the filter,
and the filter failed during filtration, fracturing in several
places. This resulted in tungsten inclusions in the casting.
EXAMPLE 7
[0064] This example concerns the filtration of a titanium alloy
melt through a tungsten wire mesh filter, and metal articles cast
using the filtration process.
[0065] In this particular example, the filter apparatus of Example
1, comprising a tungsten wire mesh approximately 12 inches.times.12
inches (30 cm.times.30 cm) having wire of diameter 0.004 inches
(0.1 mm) and apertures of 0.029 inches (0.74 mm), was heated to
approximately 3200.degree. F. (1760.degree. C.). A melt comprising
approximately 33 pounds (15 kg) of molten Ti 6-4 alloy was prepared
and 38 tungsten wire clippings of diameter 0.004 inch (0.1 mm)
length ranging from 0.03 inch (0.76 mm) to 0.28 inch (7.1 mm) were
added to the melt. The melt was filtered through the filter (over 4
seconds) into a mold to form a casting.
[0066] X-ray radiography of the filter (FIG. 5) revealed that the
filter was intact and all of the wire clippings were retained on
the filter. The casting contained no inclusions detectable by X-ray
radiography.
EXAMPLE 8
[0067] This example relates to filtering molten Ti 6-4 alloy
through a filter comprising GdNO.sub.3 as an imaging agent. Such a
filter may be produced by the method of Example 3.
[0068] A filter according to Example 3, comprising gadolinium and
having 10 pores per inch is placed in the gating of a mold. A
titanium 6-4 alloy melt is filtered through the filter and into the
mold to produce, after cooling, a metal article. The article is
subjected to at least N-ray radiography to determine if any
material used to construct the filter flaked, fragmented, leached
or otherwise became incorporated into the casting.
EXAMPLE 9
[0069] This example concerns forming a metal alloy comprising an
imaging agent such that inclusions of the metal alloy may be imaged
in cast metal article. In this example, the alloy comprising an
imaging agent was used to evaluate the effectiveness of a filters
for filtering molten metals.
[0070] In this particular example a titanium alloy comprising 6%
gadolinium (Ti 6-6-4) was produced. To prepare the alloy, a hole
was drilled in an ingot of Ti 6-4 alloy to provide an ingot
weighing 20 lbs (9.1 kg). Gadolinium, 1.2 lbs (0.55 kg) was placed
in the drilled hole. The ingot was melted and the melt was allowed
to solidify in a mold to form an ingot. The solid Ti 6-6-4 ingot
was sliced and the slices were heated in an oxyacetylene flame to
form nitride and oxide slag from the Ti 6-6-4. The slag was then
machined to form roughly spherical shapes having diameters ranging
from about 0.010 inch (0.0254 cm) to about 0.15 inch (0.381 cm).
The Ti 6-6-4 slag spheres were added to a Ti 6-4 melt, and the melt
was filtered through the ceramic filter of Example 4. Following
filtration, the filter was removed and subjected to N-ray
radiography. The radiograph (FIG. 6) shows Ti 6-6-4 inclusions
retained by the filter.
EXAMPLE 10
[0071] This example concerns forming a metal alloy such that
inclusions of the metal alloy may be imaged in a cast metal
article. In this particular example, a titanium alloy comprising 6%
samarium is produced. The alloy may be used to evaluate filters for
filtering molten metals.
[0072] To prepare the alloy, a hole is drilled in an ingot of Ti
6-4 alloy to give an ingot weighing 20 lbs (9.1 kg). Samarium, 1.2
lbs (0.55 kg), is placed in the drilled hole. The ingot is melted
and the melt is allowed to solidify in a mold to form an ingot. The
solid ingot is sliced and the slices are heated in an oxyacetylene
flame to form nitride and oxide slag. The slag is then machined to
a roughly spherical shape with a diameter of from about 0.010 inch
(0.0254 cm) to about 0.15 inch (0.381 cm). The slag spheres were
then added to a Ti 6-4 melt. The melt is filtered through a ceramic
filter of about 20 ppi. Following filtration, the filter is removed
and is subjected to at least N-ray radiography.
EXAMPLE 11
[0073] This example concerns forming a metal alloy such that
inclusions of the metal alloy may be imaged in a cast metal
article. In this particular example, a titanium alloy comprising 6%
Europium is produced. The alloy may be used to evaluate filters for
filtering molten metals.
[0074] To prepare the alloy, a hole is drilled in an ingot of Ti
6-4 alloy to give an ingot weighing 20 lbs (9.1 kg). Europium, 1.2
lbs (0.55 kg) is placed in the drilled hole. The ingot is melted
and the melt is allowed to solidify in a mold to form an ingot. The
solid ingot is sliced and the slices heated in an oxyacetylene
flame to form nitride and oxide slag. The slag is then machined to
form roughly spherical shapes having diameters ranging from about
0.010 inch (0.0254 cm) to about 0.15 inch (0.381 cm). The slag
spheres were then added to a Ti 6-4 melt. The melt is filtered
through a ceramic filter of about 20 ppi. Following filtration, the
filter is removed and is subjected to at least N-ray
radiography.
[0075] The present invention has been described with respect to
certain preferred embodiments. However, the present invention
should not be limited to the particular features described.
Instead, the scope of the invention should be determined by the
following claims.
* * * * *