Metal binder in compaction of metal powders

A method for improving the compaction characteristics of a substantially noncompactable metal powder comprising preparing a superalloy, for example, a nickel base alloy, minus a portion of at least one metal (i.e., 5 weight percent); atomizing the melt and milling it to a fine powder (i.e., about average Fisher size of 9.0 microns); blending an equal portion (i.e., about 5 weight percent) of, for example, carbonyl nickel into the milled powder; sinterbonding the mixture into a "cake" and then further processing as may be required to obtain the desired article. It is believed the "soft" carbonyl nickel acts as a binder for the prealloyed nickel-base alloy powder.

This invention relates to the manufacture of powder metallurgy articles, 
and, more specifically, to a method of producing finished powder 
metallurgy articles without the use of organic binders in normally 
noncompactable alloy powders. Metal powder prepared by the method of this 
invention has unique engineering properties. 
In the art of powder metallurgy relating to this invention, there are three 
distinct methods of producing alloys and composite materials into powder 
metallurgy parts: METHOD I blending elemental metal powders to produce a 
final alloy; METHOD II mixing metal powders and metal compounds to produce 
bonded composites and METHOD III preparing a prealloyed powder to be 
processed into a finished alloy article. METHOD I is especially suited for 
relatively simple binary and ternary alloys, i.e., Ni-Cu and Ti-Al-V. 
METHOD II is especially suited for metal-ceramics and metal-bonded 
compounds, i.e., thoriated tungsten and cobalt-bonded tungsten carbide. 
METHOD III is especially suited for complex alloys (superalloys) for use 
in severe service conditions. 
Each of these methods, as noted above, is especially suited for a specific 
application and/or alloy system. METHOD I and METHOD II, described above, 
generally require no special efforts to make the powders compactable when 
the powders are blended together. METHOD III, relating to prealloyed 
superalloys, is generally more difficult because each particle of the 
prealloyed powder is actually a miniature superalloy casting. The hardness 
and other inherent mechanical and physical properties of cast superalloys 
are especially resistant to the deformation and agglomeration 
characteristics as are required for metal powders to become readily 
compacted into articles. Because of this, prealloyed superalloys generally 
require additional complex processing together with the use of organic 
binders to effectively compact the powder into an article with sufficient 
green strength. Such binders include resins and waxes such as polyvinyl 
alcohol, cellulose, and similar organic materials. 
This invention is principally concerned with METHOD III relating to the 
compaction of superalloy powders by an improved process and the metal 
powder made by the process of this invention. 
The prior art provides a variety of methods to produce powder metallurgy 
articles. Many of the steps in the overall processing steps, as mentioned 
above, are found in prior art methods. 
U.S. Pat. Nos. 3,914,507; 3,734,713 and 3,741,748 describe a process 
similar to Method II described above wherein platelets of metals are 
coated with disperoids by an attrition milling process. 
U.S. Pat. No. 3,779,717 describes a method of mixing nickel carbonyl with 
tantalum scrap to obtain a master alloy having a high rate of solution in 
molten nickel. 
U.S. Pat. No. 3,171,739 describes a method of adding carbonyl nickel into a 
melt of nickel-tungsten-chromium alloy to obtain a casting with improved 
resistance to lead oxide corrosion. 
U.S. Pat. No. 2,936,229 discloses spray-welding alloy powders containing 
aluminum powder to improve the self-fluxing characteristics of the 
spray-welding alloy powders. 
U.S. Pat. No. 3,723,092 discloses a process for making thoriated nickel by 
mixing thoria and carbonyl nickel powders and mechanically "alloy" the 
mixture in an attritor mill. Examples of more complex alloys are also 
discussed. 
The prior art patents described above disclose various methods of making 
elemental metal additions to metal products. These methods do not provide 
a solution to the problem of compaction of superalloys. 
All compositions, herein, are given in weight percent (w/o) unless 
otherwise stated. 
The term "superalloy" as used herein may be defined as an alloy for use in 
severe service conditions, for example, comprising a nickel, iron or 
cobalt base and may also contain chromium, tungsten, molybdenum, and/or 
other elements, as exemplified by the alloys listed in Table 2. 
The term "sinterbonding" as used herein describes the metallurgical bonding 
of a "soft" metal-bearing powder to a substantially noncompactable metal 
powder. 
It is a principal object of this invention to provide a method of 
compaction of superalloy powders that simplifies processing and eliminates 
the need for organic binders. 
It is another principal object of this invention to provide a metal powder 
with physical and/or mechanical properties equal or exceeding properties 
of organically bindered powders. 
These and other objects and benefits are provided by this invention as 
described in this specification and claims. It was discovered that the 
objectives are obtained when producing an article by the following steps: 
(1) Melt the basic alloy composition minus a portion (for example 5%) of at 
least one relatively soft element as required in the final alloy: 
(2) Make powder from the melt and, if required, mill the powder to desired 
particle size: 
(3) Add the withheld portion (for example 5%) in the form of a "soft" pure 
metal (i.e., metal carbonyl and blend): 
(4) Sinterbond the blend (preferably in vacuum and about 2000.degree. F. 
for 2 hours) into a cake: 
(5) Crush cake to a convenient particle agglomerate size (i.e., -60 mesh): 
(6) Add lubricant, if required, (for example 0.5% Acrawax C) and blend: 
(7) Fashion the crushed powder into desired shape (i.e., cold pressing, 
etc.): 
(8) Further process as may be required for desired article. Benefits of 
this invention are obtained in steps (1) and (3). The withholding of a 
portion of at least one relatively soft element during melting and the 
provision and metallurgical bonding of that portion (as "soft" metal) 
before compaction constitutes the gist of this invention. The sinterbonded 
powder, step 4 above, constitutes an article of this invention.

EXAMPLE I 
An alloy was melted having an aim composition of 9 to 11% cobalt, 11.5 to 
13.5% iron, 25 to 27% chromium, 2.1 to 2.7% carbon, 9 to 11% each of 
molybdenum and tungsten, up to 1% each of silicon and boron, up to 0.75% 
manganese and the balance nickel. Said melt composition was calculated to 
have 5% less nickel than required in the final alloy. The melt was 
atomized by an inert gas and screened to minus 30 mesh and then ball 
milled to an average Fisher size of 9.0 microns. The milled powder was 
thoroughly blended with 5% carbonyl nickel powder then sinterbonded into a 
"cake" in vacuum at 1950.degree. F. for 2 hours. After cooling, the 
sinterbonded cake was crushed to minus 60 mesh agglomerates. The powder 
was then thoroughly blended with 0.5% atomized grade ACRAWAX C dry 
lubricant. The powder was then compacted in the form of test specimens for 
testing. The product of this example is identified as No. 208 powder. 
An alloy identical in final composition to No. 208 powder was prepared as 
powder and processed by methods known in the art. The powder was 
organically bindered with polyvinyl alcohol. This powder was also 
similarly compacted in the form of test specimens and is identified as No. 
208P powder. 
Table 1 presents a comparison between No. 208 powder produced by this 
invention and No. 208P powder made by prior art method. 
Table 1 shows the improved compactability of No. 208 powder compared to No. 
208P powder. Note that the compactability of No. 208P powder at 50 Tsi 
(100,000 psi) is almost identical to the compactability of No. 208 powder 
at only 30 Tsi (60,000 psi). 
The standard Hall Flow test shows that the flow characteristic of No. 208P 
is nil while the flow characteristic of No. 208 powder is within an 
acceptable working image. This feature improves the reproducibility of 
part size through more uniform die fill. 
The transverse rupture green strength of 208 powder far exceeds the 
strength of 208P powder. Increases in the green strength and 
compactability of the process of this invention constitute a major 
improvement in the art of superalloy powder metallurgy. These major 
improvements in the art are realized without an anticipated reduction in 
sinterability characteristics. It would be expected that the substitution 
of a metal binder to replace an organic binder would increase the lower 
limit of sinterability range. However, test results shown in Table 1 show 
an unexpected improvement. The lower limit of sinterability (2170.degree. 
F.) remains constant. This improvement is realized whether the powder is 
sintered in vacuum or hydrogen atmosphere. 
Test results of sintered properties on No. 208 and No. 208P powders 
indicate both powders yield sintered products with practically identical 
physical properties. However, sintered products of No. 208 have much 
higher mechanical strengths as noted in Table 1. 
Other advantages of the process yielding No. 208 powder over prior art No. 
208P powder include: 
(1) The cost of bindering No. 208 is about 40% less than the cost of 
bindering No. 208P. 
(2) The rejection rate of scrap material was higher for No. 208P, probably 
because of the higher green strength of No. 208 powder. 
(3) The handling of No. 208 is less dusty than the handling of No. 208P. 
This feature is helpful in meeting certain OSHA requirements. 
(4) Segregation is no problem in No. 208 because the particles are 
metallurgically bonded and exist as uniformly blended agglomerates. 
(5) The process of this invention appears to produce products essentially 
identical to prior art products in final form. The microstructure and 
X-Ray analysis indicated no difference between the two products. 
The method of producing the initial prealloyed powder is not limited by the 
examples shown herein. The examples are described as the processes used in 
preparing the powders for the tests. The alloys were melted in an 
induction furnace and atomized in an inert gas atmosphere. Other means for 
preparing the initial powder material may be equally effective. Likewise, 
the initial powder need not be an alloy, and can be any substantially 
noncompactable metallic powder. 
Through experimentation, it was found that crushed metal particles tend to 
compact more effectively than "as atomized" particles. For example, test 
specimens made of atomized -325 mesh metal powder generally will have 
lower strength values than test specimens of the same metal made by powder 
that was crushed to a similar -325 mesh from a larger particle size. To 
obtain optimum benefits from this invention, milled powders are preferred 
as initial material. 
OTHER EXAMPLES 
Table 2 lists the nominal composition of other alloys that were tested as 
examples of the process of this invention. These alloys are typical of 
superalloys that may be produced by the process of this invention. 
The process of this invention was tested with a variety of test conditions. 
Table 3 present data obtained with the processing of Alloy N-6. The 
original melt was controlled to contain 5% less nickel than desired in the 
final alloy. Three batches of prealloyed and milled powders were tested 
(A, B, and C). The three batches were milled to contain -325 mesh 
particles at 51.7%, 69.7% and 83.8% or the equivalent of an average Fisher 
particle size of 11.6.mu. 7.9.mu. and 6.1.mu. respectively. 
Each batch was then blended incorporating 5% elemental nickel powder 
(Carbonyl grade). The average particle size after blending was 10.5.mu., 
7.4.mu. and 5.7.mu. respectively. 
Each of the batches was subsequently sinterbonded for two hours at three 
temperatures 1800.degree. F., 1900.degree. F., and 2000.degree. F. The 
effect of the sinterbonding at various temperature is noted by the change 
in average particle size. For example, Batch A powder blended with 5% 
elemental powder had an average Fisher particle size of 10.5.mu.. After 
sinterbonding at 1800.degree. F. for 2 hours the average Fisher particle 
size was 12.2.mu. with an apparent particle growth of 1.7.mu.. 
The sinterbonded and crushed powders were pressed into test samples at 50 
tons per square inch (100,000 psi). The test samples had green density 
values, in percent of theoretical density, as indicated in Table 3. The 
test samples were tested for green strength by means of the standard ASTM 
B528-76 Transverse Rupture Test. Testing was conducted at a load rate of 
0.05 inch per inch. 
Tables 3 through 7 contain data obtained from experimental testing of 
alloys listed in Table 2. Tables 4 through 7 present data obtained by 
similar testing as described above relating to Table 3. 
It will be noted in the data presented in Example 1 and other examples, 
herein presented, that as a given powder is milled finer, the green 
strength of the compacted powder increases. It will also be noted, that as 
the sinterbonding temperature is increased, the green strength increases 
up to a temperature at which the "soft" metal is sufficiently alloyed to 
lose its ductility. 
The significance of the "apparent particle growth" as shown in these data, 
is primarily to judge the degree of sinterbonding with any given alloy 
composition, milled size and elemental metal addition. Although an 
empirical number, it has been found that a given alloy milled to the same 
size and sinterbonded the same, will exhibit reasonably reproducible 
particle growth and green strength. It is, therefore, a useful process 
control data point. 
It will be obvious to those skilled in the art, that the selection of 
powder processing parameters must include the desired sintering 
characteristics of the powder as well as the desired green strength level 
for the handling of the parts produced. The data in the Tables provide a 
basis for such parameters. 
Other modifications within the scope of this invention may include a large 
variety of alloys. For example, copper base alloys or copper containing 
alloys may use copper powder as the "soft" metal. 
Tables 5 and 6 additionally have data obtained from tests wherein 10 and 
15% of the "soft" metal (cobalt) was withheld from the initial powder then 
added at the blending steps. These data tend to show that higher portions 
of "soft" metal blended into the powders provide higher strengths when 
higher strengths are desirable. 
These data further suggest the effective range of "soft" metal portion may 
vary from about 1% up to the maximum content of that metal in the final 
alloy. Because of the higher costs of "pure" metals, economics, of course, 
suggest an upper limit of about 25% as an effective amount. Thus, the 
broad range is about 1% to the maximum content of the "soft" metal. A 
preferred range is about 1% to about 25%. Of course, it is understood that 
the actual effective content depends upon several possible conditions, for 
example, (1) the composition of the alloy, (2) the sinterability of the 
alloy, (3) the effectiveness of the "soft" metal, (4) the choice of "soft" 
metal depending upon availability, costs and other considerations. 
Other modifications and variations may be made within the scope of this 
invention. For example, after the crushing step, the metal powder of this 
invention is suitable as a powder for use in metal coating operations such 
as plasma spray processing. The deposition of the powder on a substrate 
constitutes the compaction step. 
Although specific embodiments of the present invention have been described 
in connection with the above examples, it should be understood that 
various other modifications can be made by those having ordinary skills in 
the metallurgical arts without departing from the spirit of the invention 
taught herein. Therefore, the scope of this invention should be measured 
solely by the appended claims. 
TABLE 1 
__________________________________________________________________________ 
PROPERTY COMISON 
No. 208 and 208P Powders 
No. 208P No. 208 
Powder Powder 
__________________________________________________________________________ 
COMTABILITY: 30 TSI 
59.5 63.4 
(GREEN DENSITY, %) 
50 TSI 
63.6 68.9 
70 TSI 
66.2 72.4 
HALL FLOW, SECONDS/50G 
WNF* 35-38 
GREEN STRENGTH: 50 TSI 
300-800 PSI 
700-1200 PSI 
SINTERABILITY: 2170-2260 
2170-2260 
SINTERED PROPERTIES 
DENSITY, % 97.0-97.5 
97.5-98.5 
HARDNESS, Rc** 48-50 48-50 
R. T. TENSILE, KSI*** 
68.7 87.4 
TRANSVERSE RUPTURE, KSI 
120.8 130.7 
__________________________________________________________________________ 
*WNF--WILL NOT FLOW 
**Rc--ROCKWELL "C" SCALE 
***R. T.--TENSILE, KSI ROOM TEMPERATURE TENSILE STRENGTH, 1000psi 
TABLE 2 
__________________________________________________________________________ 
Composition of Alloys Tested 
in Weight Percent, W/O 
Alloy 
No. Ni Co Fe Si Mn Cr Mo + W 
W C B Cb 
Cu 
__________________________________________________________________________ 
N-6 Bal 
*5 *3 .8-1.2 
-- 27-31 
Mo + W 
5-7 .8-1.4 
.4-.8 
711 Bal 
10-15 
20-25 
.6-1.5 
*.8 
25-30 
Mo + W 
8-16 
2.5-3 
*1 *.5 
*.5 
106 *3 Bal *3 *1.5 
*1 27-31 
*1.5 3.5-5.5 
.9-1.4 
*1 
103 *3 Bal *3 *1 *1 29-33 
-- 11-14 
2-1.7 
*1 
587 *3 *3 Bal .5-1 
*.5 
23-26 
15-17 2.6-3.1 
.5-.75 
208 Bal 
9-11 
11.5-13.5 
*1.0 
*.75 
25-27 
9-11 
9-11 
2.1-2.7 
*1 
__________________________________________________________________________ 
*MAXIMUM 
BALANCE INCLUDES IMPURITIES 
TABLE 3 
__________________________________________________________________________ 
Alloy N-6 Test Data 
__________________________________________________________________________ 
Milled Powder Properties 
A B C 
__________________________________________________________________________ 
-325 Mesh, % 51.7 69.7 83.8 
Fisher Size, .mu. 
11.6 7.9 6.1 
Blended With 5 Wt. % 
Elemental Ni Powder 
Fisher Size, .mu. 
10.5 7.4 5.7 
Sinterbonded Powder Properties 
2 hr. at Sinterbonding Temp., .degree.F. 
1800 
1900 
2000 
1800 
1900 
2000 
1800 
1900 
2000 
Fisher Size, .mu. 
12.2 
15.0 
20.0 
9.0 
9.8 
15.3 
7.4 
8.3 
11.7 
Apparent Particle Growth, .mu. 
1.7 
4.5 
9.5 
1.6 
2.4 
7.9 
1.7 
2.6 
6.0 
50 Tsi Compacted Properties 
Green Density, % 
79.8 
80.3 
79.6 
78.4 
79.7 
78.2 
77.7 
79.1 
78.1 
Green Strength, psi 
720 
910 
2160 
1015 
1440 
3090 
1205 
1770 
3710 
__________________________________________________________________________ 
TABLE 4 
__________________________________________________________________________ 
Alloy 711 Test Data 
__________________________________________________________________________ 
Milled Powder Properties 
A B C 
__________________________________________________________________________ 
-325 Mesh, % 95.7 98.4 99.2 
Fisher Size, .mu. 
10.4 8.7 7.2 
Blended With 5 Wt. % 
Elemental Ni Powder 
Fisher Size, .mu. 
10.1 8.7 7.2 
Sinterbonded Powder Properties 
2 hr. at Sinterbonding Temp., .degree.F. 
1800 
1900 
2000 
1800 
1900 
2000 
1800 
1900 
2000 
Fisher Size, .mu. 
13.0 
13.6 
20.0 
10.4 
11.5 
13.4 
8.8 
9.6 
11.6 
Apparent Particle Growth, .mu. 
2.9 
3.5 
9.9 
1.7 
2.8 
4.7 
1.6 
2.4 
4.4 
50 Tsi Compacted Properties 
Green Density, % 
69.7 
70.6 
71.7 
69.7 
70.4 
70.4 
69.5 
69.9 
70.0 
Green Strength, psi 
680 
1010 
1280 
815 
1240 
1355 
990 
1360 
1635 
__________________________________________________________________________ 
TABLE 5 
__________________________________________________________________________ 
Alloy 106 Test Data 
__________________________________________________________________________ 
Milled Powder Properties 
A B C 
__________________________________________________________________________ 
-325 Mesh, % 81.7 93.3 97.7 
Fisher Size, .mu. 
10.0 7.8 5.6 
Blended With 5 Wt. % 
Elemental Co Powder 
Fisher Size, .mu. 
8.0 6.7 5.1 
Sinterbonded Powder Properties 
2 hr. at Sinterbonding Temp., .degree.F. 
1800 
1900 
2000 
1800 
1900 
2000 
1800 
1900 
2000 
Fisher Size, .mu. 
10.6 
14.9 
18.0 
8.3 
12.0 
15.0 
6.7 
10.0 
14.0 
Apparent Particle Growth, .mu. 
2.6 
6.9 
10.0 
1.6 
5.3 
8.3 
1.6 
4.9 
8.9 
50 Tsi Compacted Properties 
Green Density, % 
69.1 
69.4 
68.2 
68.2 
68.6 
68.4 
67.6 
68.1 
67.2 
Green Strength, psi 
145 
220 
350 
175 
335 
410 
240 
450 
600 
10 Wt. % Co 455 
15 Wt. % Co 520 
__________________________________________________________________________ 
TABLE 6 
__________________________________________________________________________ 
Alloy 103 Test Data 
__________________________________________________________________________ 
Milled Powder Properties 
A B C 
__________________________________________________________________________ 
-325 Mesh, % 92.8 96.5 98.8 
Fisher Size, .mu. 
10.2 9.6 7.3 
Blended With 5 Wt. % 
Elemental Co Powder 
Fisher Size, .mu. 
8.8 8.1 6.3 
Sinterbonded Powder Properties 
2 hr. at Sinterbonding Temp., .degree.F. 
1800 
1900 
2000 
1800 
1900 
2000 
1800 
1900 
2000 
Fisher Size, .mu. 
11.0 
12.3 
14.2 
9.6 
10.8 
12.3 
8.1 
9.8 
11.1 
Apparent Particle Growth, .mu. 
2.2 
3.5 
5.4 
1.5 
2.7 
4.2 
1.8 
3.5 
4.8 
50 Tsi Compacted Properties 
Green Density, % 
67.6 
67.1 
66.9 
66.4 
66.3 
66.3 
65.7 
65.8 
65.5 
Green Strength, psi 
140 
220 
310 
190 
250 
360 
250 
350 
480 
10 Wt. % Co 460 
15 Wt. % Co 585 
__________________________________________________________________________ 
TABLE 7 
__________________________________________________________________________ 
Alloy 587 Test Data 
__________________________________________________________________________ 
Milled Powder Properties 
A B C 
__________________________________________________________________________ 
-325 Mesh, % 97.8 98.2 98.6 
Fisher Size, .mu. 
7.3 5.4 4.1 
Blended With 5 Wt. % 
Elemental Fe Powder 
Fisher Size, .mu. 
7.0 5.4 4.0 
Sinterbonded Powder Properties 
2 hr. at Sinterbonding Temp., .degree.F. 
1800 
1900 
2000 
1800 
1900 
2000 
1800 
1900 
2000 
Fisher Size, .mu. 
8.1 
8.7 
10.4 
6.5 
7.1 
8.5 
5.2 
5.9 
7.4 
Apparent Particle Growth, .mu. 
1.1 
1.7 
3.4 
1.1 
1.7 
3.1 
1.2 
1.9 
3.4 
50 Tsi Compacted Properties 
Green Density, % 
66.7 
66.7 
66.1 
66.4 
66.4 
65.8 
66.0 
65.7 
65.1 
Green Strength, psi 
400 
490 
500 
540 
650 
570 
630 
840 
830 
__________________________________________________________________________