Note: Descriptions are shown in the official language in which they were submitted.
3~ .
The present invention relates to the diffusion coating
of the interior of a hollow workpiece where that interior is
accessible only through a restricted passageway.
Such coating is highly desirable for example to increase
the resistance of the workpiece to attack. Thus, as described in
U. S. Patents 4,132,816 and 4,148,275, jet engine blades and vanes
that have internal cooling passages frequently require the
diffusion coating of the surfaces of those passageways to increase
their resistance to attack by the hot combustion products to
which they are subjected. These patents suggest that such
coating be effected by forcing a gaseous specially formulated
diffusion coating composition through the passageways to be
coated while the workpieces are heated to diffusion coating
temperature.
Among the objects of the present invention is ~he
provision of a novel techniyue for diffusion coating through
restricted passageways
Additional objects of the present invention include the
provision of a diffusion coating process which very uniformly
coats through restricted passageways without requiring forced
circulation through the passageways.
The foregoing as well as additional objects of the
present inven~ion will be more fully appreciated from a considera-
tion of the following description of several of its exemplifica-
tions, reference being made to the accompanying drawings, wherein:
Il . '
1 2. ~ '
1~5~iL636
~¦ Fig. 1 is a vertical sectional view of a diffusi~n
coating set-up for practicing the present invention; and
Fig. 2 is a similar view of a modified set-up
pursuant to the present invention.
¦ According to the present invention diffusion coating
I of the internal surface of a hollow in a metal workpiece where
jl that hollow is accessible only through a passageway less than
, about 5 millimeters wide, is readily effected by applying over
that internal surface an essentially uniform layer of particles
consisting essentially of all the metal to be diffused into that
;I surface, and the workpiece so treated is subjected to diffusion
li I I i'
¦ coating temperature while the hollow is exposed through the I -
¦~' passa~eway to a diffusion coating atmosphere.
!~ The layer of particles is converiently applied as a
¦, layer of a dispersion of the particles in a binder that is
driven off at diffusion coating temperatures. A water dispersion
¦ of alùminum particles, such as is described in U. S. Patent
3,318,716, can be used, but it is preferred to use dispersion ~
Il vehicles in which heavier metals such as chromium can also be ~¦
j; ~airly uniformly dispersed. A 1 to 10% by weight solution of
an acrylic resin such as ethyl methacrylate in methylchloroform
ma~es a very desirable dispersion medium in which powdered
chromium, powdered aluminum, mixtures of these powders, and other
metals like powdered cobalt, in particular sizes up to about 150
microns are easily suspended to make a fairly uniform mobile
suspension that does not settle out appreciably for the minute
¦ or so needed to apply the suspension and then distribute it as a
uniform coating.
3.
1~546;3~i
Settling can be slowed by dissolving in the suspension
vehicl~ a long-chain acid such as C12 to C50 aliphatic acid, or
a copolymer of ethylene and acrylic acid, as described in U. S.
patent 4,208,357.
Only about 0.3% to about 0.5% by weight of such additive is very
helpful. Low-foaming non-ionic surface active agents such as
polyethoxy ethers of linear alcohols like cetyl alcohol or of an
alkyl phenol, in amounts as low as 0.1% to 0.3% by weight can
also be used to slow the settling of the suspended particles.
With the very narrow passageways involved in the pre-
sent invention, the mobile dispersion coatings do not spread into
uniform layers~ but build up in excessive thicknesses by reason
of surface efects. Thus a passageway about 1 millimeter in
diameter will generally be completely filled with the mobile
dispersion. It is accordingly necessary to expel the excess
dispersion as by applying suction to the passa~eway opening to
suck out gas as a rapid stream that carries along with it all
but a residual thin and quite uniform layer of the dispersion.
Suction from a simple water-pump suction generator or from a
suction pump ~hat applies a suction of about l/10 atmospheric
pressure or less, as measured on a pressure gauge, is adequate.
Where the passageway whose coating is to be le~Telled
has separate outlets at its opposite ends, the redistribution }
is easily effected by directing a stream of compressed air into
one of the outlets. A stream propelled by a 15 pounds per square
inch ~auge source ~f air is guite effective.
4.
. .
i
1~5~636
The excess dispersion can also be expelled by centri-
fugal force. Spinning the heavily coated workpiece in a centri-
uge at about 10 to 20 times gravity for a few seconds does a
good job of levelling where the centrifugal force is directed
longitudinally of a filled passageway, for example. For com-
. plicated passageways it may be necessary to spin the workpiece , ~,
I in steps, each s~ep with a different orientation. - ;
¦~ The following examples illustrate the present invention -;
more fully.
EXA~LE 1
Into a short retort box 10 as in Fig. 1, a half-inch
layer 12 of a diffusion aluminizing powder mix is poured,
following which a perforated flushing tube 14 is placed ovex the
mix and then another two-inch layer 16 of ~he mix covers the
perforated tube.
,~ i
, The retort box and the tube are made of Inconel 6~0
,j and the mix has the following formulation by weight:
! Aluminum powder about 40
micron particles 15 %
Alumina powder about 200 to
300 micron particles 85 %
NH4Cl powder 3/4~l. based on the Al
plus A12O3 total
i The tube 14 runs to and fro the length of the box, with
each run about 1 1/2 inches from the next, and its perforations
¦ are 1/16 inch holes. It is connected to an unperforated supply
~ k
! 5.
I i
i .,
1154~36
exten ion 18 that le~ds o~t of the retort box to a source of
I argon. The retort wall opening through which extension 18 passes,
¦ can be sealed as by welding or filled with tamped powder or fiber
to permit a pressure build-up within the retort. Very fine
alumina or ceramic fibers is suitable.
Suspended by sturdy nickel wires 22 hooked over the
tops of the side walls of the box, are a series of blocks 20 of
nickel-base alloy having 7% aluminum, 14.5% molybdenum and 7%
1 tungsten, the balance being essentially nickel. Each block is
I about an inch high and has a central cylindrical bore 24 about 23
; ¦! mils in diameter penetrating its entire hei~ht.
I' Also placed in the retort is a thermocouple 30 inserted
; , in a thimble 32 welded to an inside wa:Ll and opening to the
exterior through a perforation in the wall.
.
Before the blocks 20 are placed in the retort, they
first have their passageways 24 filled with a dispersion of 30
"
: 1 grams 325 mesh alu~inum powder in 40 cc. of a 5% by weight solu- '
li tion of poly(ethyl acrylate) resin in methyl chloroform. A
j, suction hose is then promptly applied to one end of aperture 24
~ ,~ to suck out excess dispersion. The blocks so treated are per-
I ,i mitted to stand a few minutes to set the residual coating.
l .
The exteriors of the blocks are then painted with a 10
j milligram per square centimeter layer of the masking slurry of
¦ Ni3Al powder as described in U. S. Patent 3,801,357, and the
slurry coating permitted to dry.
I After being loaded, the retort box 10 is covered with a lid
¦ 36 which can also be made semi-tight by asbestos fibers tamped around
I its periphery. The covered box is placed inside an outer retort
!~ I
ll l
..
~: .
which is then covered by a furnace as sho~ ln U. S. Patent
3,801,357, and heated to 1900~ where it is held for nine hours
while argon is fed into the perforated tube at a rate that takes
about one hour to supply an argon volume equal to the box volume.
- The heat is then turned offJ the furnace lifted off the
outer retort, and the retorts permitted to cool. The retort box
10 is opened when sufficiently cool, and the blocks 20 removed
and cleaned of the masking layer. They then show a very uniform
aluminized case about 2 mils thick over the entire internal sur-
face of passageway 24. No cleaning is needed in that passageway, -
.~ other than blowing air through it to clear out any ash, and the
rinsing off of residual halide with water.
The same results are obtained when the blocks are held
.
1/16 inch or 2 inches from the top of layer 16, and when the
blocks are positioned in the retort box 1~ with their passageways
horizontally oriented. ~ith such orientation the blocks can
simply be laid on top of layer 16 so that no special work-support-
ing equipment is needed.
It is not necessary to force the energizer-containing
atmosphere through the narrow passage 24 as described in U.S.
Patent 4114g,275, nor is it necessary to use complex energizers
with special throwing power, as described in U.S. Patent 4,132,~16.
However those complex energizers as well as fluoride energlzers
in general do a very effective job in the present invention.
E~AMPLE 2
In this example a group of jet engine blades with
internal cooling passages have the walls of the passages heavily
chromaluminized while the airfoil surfaces are lightly chrom-
aluminized and the roots are given little or no external coating.
l!
5~636
I :
Such a blade is schematically illus~rated at 120 in Fig. 2 and
has a number of passages 124 extending the entire length of its
airfoil section 120 from the airfoil tip 123 to the opposite face
of the mounting flange 125. At their extreme ends the passages
are about 1 mil by 2 mils in cross section, and in their inter-
mediate portions their cross section is a little larger.
The blades, which ~re made of B-l900 alloy, are cleaned
by mild blasting with fine aluminum grit,followed by degreasing.
There is then introduced into the cooling passages, with the help
of a medicine dropper, a suspension of 40 grams 325 mesh aluminum
powder and 5 grams 325 mesh chromium powder in 53 cc. of a 7% by
weight solution of poly(methyl ethacrylate) resin and a 0.5 by
weig~ht solution of stearic acid in methyl chloroform. Suction
from a water-pump is then promptly applied to each end of each
passage in the airfoil for a few seconds, and the blade permitted
to stand to cause the suspension remaining in the passages to dry.'
Excess suspension on the outside surface of the blade is removed ¦
with the help of a cloth wet with a little methyl chloroform, and
a group of blades so prepared is loaded into previously prepared
retort box 110. This box is similar to box 10, but made of type
~)stainless steel and it has shelves 135 welded onto its end~
walls and carrying spaced rods 137 that span the box length. The
blades are fitted between the bars with their airfoils 121 ex~
tending downwardly and ~heir flanges 125 supported by the bars.
The bars can be plain carbon steel heavily aluminized beforehand,~
and for example have a diffusion-aluminized case at least about
one mil thick with a maximum aluminum content of at least about
35% in the case.
: 1,
~ ~ C5 ~
In addition to the fittin~ of the aluminized rods, box
110 is prepared with layers 112, 116 having the same composition
as layers 12 and 16. After loading the blades, the box is
inserted in an outer retort and heated to 1950~F, while a slow
stream of hydrogen is fed through perforated tube 114 at a rate
that requires about 1/2 hour to supply an amount of hydrogen
I equal to the volume of the box. Before the heating is started ¦
i the hydrogen stream is temporarily speeded up to more effectivelyi
replace the previous atmosphere in the box by hydrogen.
i The 1950F temperature is maintained for 8 1/2 hours,
!~ and the box then cooled. After sufficient cooling the hydrogen
~`
atmosphere is replaced by argon, and t~e box opened. The internal
i surfaces of the passages in the blade show an extremely uniform
aluminized case about 2 to about 2.3 mils thick. The airfoil
! surfaces have an aluminized case about half as thick, and the
blade root 126 has a less than 0.4 mil thick case.
The flange 125 has its lower face aluminized to about
j the same extent as the airfoil surface, and has its upper face
¦ aluminized to a~out the same extent as the root. The aluminized
; lower face does not show a drop in aluminum content where that
face rested on the bars 137. Quite the contrary it appears that
the heavily aluminized bar surfaces help to aluminize the upper
portions of the airfoil as well as the flange, and thus compen-
¦ sate for the greater distance of these surfaces from the powder
11.
~ 9
I! !
-
~ i3
I
I .
The varying dis~ance of the internal passageway por-
tions from the powder 116 seems to have no significant effect
inasmuch as the metal being diffused into the passageway surfaces
is located at those surfaces. Such diffusion takes place rela-
ti~ely rapidly when a diffusion atmosphere reaches those surfaces
after travelling 4 to 6 inches or more. Such an atmosphere need
only be a vaporized tiffusion energizer, such as a halogen or
halogen compound, but the action of such an atmosphere is im- i
proved if it also contains a halide of the metal being diffused. ,
Such an improved atmosphere is the usual atmosphere produced
during diffusion coating, and powders 12, 16, 112, 116 are usual
prior art diffusion coating powders.
The chromium present with the aluminum in the disper- I
sion applied to the internal passages, diffuses into the passage-,
way surfaces along with the aluminum and ~urther improves the
resistance of those surfaces to attack. The proportion of
chromium can be increased and the aluminum completely eliminated
to provide a chromized surface rather than an aluminized or
chromaluminized surface. The chromium and aluminum particles can
be pre-alloyed together if desired, or they can be mixtures of
the separate metals.
For diffusion coating nickel-base superal].oyswith aluminu~
it Lspreferred that the aluminum content of aluminum-chromium
dispersions be greater than twice the weight of the chromium.
The metal particles in the metal dispersions should be
not over about 3 mils in size, preferably not over 2 mîls, where
the passageway ~alls they are difused into are to remain very
smooth.
1 10.,
1,i .
1~5463~
¦ The diffusion coating he~t should ùe maintained at least
as long as needed to cause all of the dispersion metal particles
to diffuse into the passageway surfaces. This leaves those sur- ',
faces clean and ready for service without further treatment. When
the workpiece being coated is a nickel-based superalloy and the
, metal being diffused in is aluminum or chromium or mixtures of the
¦ two, at least about two hours is needed for every 0.1 mil of dis-
persed metal when the diffusion is effected at 1800F, although
~¦ somewhat shorter times can be used when the diffusing metal is
aluminum alone. Silicon, cobalt, iron and other metals used to
make diffusion coatings can be used in addition to or in place of
the aluminum and/or chromium. Some combinations of metals are
known not to coat very well if at all
1 Cobalt-based superalloy workpi.eces require about twice ,
I the diffusion time that nickel-based superalloys take, but iron-
base alloys such as RA 330 and Incolo~; 800 take less time than the
' nickel-based superalloys. A cobalt-base superalloy MAR M 509 vane
also with cooling passages 7 when subjected to treatment as in
I Example 2 but at 2000F for 20 hours provides excellent results.
1:
EXAMPLE 3
First stage hot section ~et engine vanes made of the
nickel -based IN 100 alloy and with cooling passageways about 30
¦ mils in diameter, are treated in the manner described in Exam?le ;
2, but ~i~h the following specific differences:
(a) The powder on ~he floor of the retort is
I a chromizing powder mixture of 20% ultrafine chromium
I powder (particles less than 20 microns in size), 80%
325 mesh alumina, and 1% NH4Br based on the total
weight of the chromi~m and alumina.
~J~ ~a~k
11 .
1~5~636
(b) The coating slurry is a dispersion of 15
grams of the ultrafine chromium in 20 cc. of the binder
solution of Example 1.
~c) The vanes are held in their horizontal
position about 1 inch above the powder on the floor.
! (d) The inner retort box has its cover loosely
¦l applied without any attempt to seal its edges.
I ~ (e) The rods 137 are chromized Inconel 600.
¦! (f) The diffusion coating was maintained at
-2000F for 15 hoursl the flow of hydrogen into the ' `
inner retort box is stopped when that temperature
,, was reached and a flow of argon is started when the
i temperature reaches 300F in the cool-down. I
" (g) A slow flow of hydrogen is maintained
through the outer retort throughout the heat, but is
' stopped when the argon flow starts in the inner
retort. ' `
¦! Both the external surfaces of the vanes and the sur-
¦~ faces of its cooling passageways are very effectively and uni-
I formly chromized.
The powder on the retort floor need not have the same
metal components as the powder in the passageways. Thus by
having chromium as the only metal in the passageway powder and
¦ aluminum as the only metal in the powder on the retort floor,
¦ the passageways can be chromized while the exterior of the
I ~ ~ rk
I 1,
1 12.
: -.. ~
~ 636
I
workpiece treated are aluminized. A little aluminum may appear
in the chromi~ed case on the passageway surfaces, particularly
if the difusion treatment is prolonged. The Al and Cr can be
reversed in position.
Omitting all metal particles from the powder on the
retort floor seems to slow down the diffusion case formation on
the passageway surfaces, but good alwminizing, chromizing, and
chromaluminizing is still obtained.
It is not essential that the process of the present
invention be carried out with a flushing gas fed through the
perforated tube 14 or 114. A little flushing does help flush
away any vapors formed by the decomposition of whatever binder is
used to hold the dispersed metal layer :in place, but by the time !
the retort box reaches about 600F during heat-up, the flushing
can be stopped. Where the retort box is surrounded by another
retort having a clo~ely controlled atmosphere, such as is
maintained when a stream of hydrogen, argon or other inert gas
is flushed through the outer retort chamber only, flushing of the
inner retort box need not be resumed except when the outer retort
atmosphere is hydrogen or other combustible gas~ In that event
it is helpful to flush an inert gas through the inner and outer
retorts to sweep away combustible gas before the re~orts are
opened.
During the dwell at diffusion-coating temperature, any
1ushing of the inner retort should not be so rapid as to sweep
out too much activator from its atmosphere. The activator present
in the powder on the retort floor is all converted ~o vapor by
.~
13. I
~15~636
the time the heat-up brings the powder to about 700F, and after
such vaporization the flushing gas should not be supplied any
faster than re~uired to equal the volume in the inner retort
space when flowing for a time corresponding to about one twentieth
the diffusion-coating time~ The flushing action is not complete,
particularly with a light gas such as hydrogen, so tha~ with such
maximum flow there is still some activator present at the end of
¦ ~he diffusion-coating heat. I ~,
Any halogen or halogen compound vaporized at diffusion-
coating temperature can be used as an energizer. Where the ~,
i diffusion is effected at relatively low temperatures, such as 160~F or
below aluminum chloride is a very desirable energizer, parti-
Il cularly when aluminum is being diffused into a workpiece. Other I -
¦' energizers (sometimes called activators) are llsted in U. S.
¦ Patent 3,7~4,371.
I`l The methyl chloroform solvent of Examples 1 and 2 can ,
., I
i~ be replaced by other solvents such as methyl ethyl ketone,
~ chloroform, toluene, isopropyl alcohol and the like. However,
¦ methyl chloroform is a particularly safe material to work with
i because it does not burn and its hazard to health is extremely
low. Water can also be used as a solvent with water-soluble
binders, but it is generally not desirable to keep finely divided
metal particles in contac~ with water for a long period of time.
Other acrylic resins that make effective binders
include poly(methyl methacrylate) and the various polymeric ¦
acrylic and methacrylic esters of Cl to C8 alcohols, as well as
'.
1~ i
1 ~5 ~J;~
polyacrylic acid and mixtures or copolymers of the monomers from
which these are made. Other binders that can be used include
rosin, polyethylene, polystyrene, methyl cellulose and even
dimethyl silicone oils. The acrylic resins are driven off quite~
cleanly during the diffusion heat, but some binders might leave
a little carbon behind and this would also diffuse into the
workpiece surface.
The rods 137 of Fig. 2 preferably have their surfaces
heavily chromized beforehand when chromizing the workpiece
interiors. Similarly when diffusion coating workpieces with
I zinc, cobalt, or other metal, these rods or other work-engaging
! surfaces are preferably correspondingly precoated.
¦, The powders 12 and 16 are not required to be located
li on the retort floor, but can be held in baskets below or even
above the workpieces. The activating vapors generated by these
I powders have a throwing power of as much as six inches, and when
¦ ~he inner retort is not flushed during the diffusion temperature
dwell good diffusion coatings form even further away from the ! ~-
nearest powder. An easily vaporized metal halide such as
a~uminum chloride can be introduced into the inner retort as a
vapor carried by the flushing gas~ and no powder is needed other
than in the fine passageways.
I
15.
. Il
Il l
Some substrates, such as age-hardenable stainless steels
do not take uniform diffusion coatings, particularly when the
diffusion is conduc~ed at low temperatures. At temperatures of
1200F or below, such coatings tend to form a fairly rough sur-
face. Coating uniformity is improved by pre-plating a nickel
or cobalt flash not over about 0.1 mil thick on the walls to be
coated.
Such improvement in uniformity and smoothness is
obtained with coatings whether or not in narrow passageways.
This is shown in the following examples.
EXAMPLE 4
A group of A*i 355 last stage compressor blades about
9/16 inch wide, 2 inches long, and about 30 mils in thickness,
for a J-85 jet engine, were cleaned by anodic treatment at 50
amperes per suqare foot in a 160-180F water solution of sodium
carbonate (1 oz./gal.) and sodium hydroxide (1 oz./gal.) for one
minute, followed by water rinse and then a dip in 18% HCl.
After cleaning these blades showed a surface roughness
of 17 to 20 micro-inches~ They were given a four minute electro-
plating treatment by applying a long magnet to the roots of a row
of individual blades, immersing the airfoils of the blades so
held in a solution of 426 g. of NiC12 6H2O and 70 cc. concen~rated
HCl in enough water to make one liter, and connecting the magnet
as a cathode with respect to a nickel anode also immersed in the
same solution. The cathode current density was 50 amperes per
square foot, and the bath temperature about 27C.
-16-
rt~
The electrolysis was -then terminated, the
plated blades were rinsed with water, dried and inspected.
A bright coating was observed over the entire airfoil
surfaces of the blades, and one of them on sectioning
showed a nickel plate thickness of about 0.04 to about
0.09 mil. The remaining dried blades were then packed in
a plain carbon steel diffusion-coating retort previously
used for aluminizing. The packing was with a powder pack
having the following composition by weight:
Powdered alu~inum - c~out 10 micron particle size 20 parts
Powdered al~ina - minus 325 ~esh 79.7 parts
Aluminum chloride, anhydrous .3 parts
The aluminum and alumina were in the form of
a mixture that had been previously used as an aluminizing
pack.
The packed retort was then placed in an outer
retort as described in U.S. Patent 3,801,357 and under the
bathing action of hydrogen was heated to bring the pack to
a temperature of 850 to 870F as measured by a -thermocouple
also inserted in the pack. The temperature was then
maintained for 25 hours, after which the retorts were
permitted to cool and the blades unpacked. As removed from
the pack they showed a surface roughness from about 24 to
about 30 micro-inches and presented a very good appearance.
One of the thus-treated~blades was sectioned
and examined microscopically. It showed an average
aluminide case about ~.4 mil thick, the outer layer of the
case having a high
17 -
pc/~
63'~
nickel structure that extended into the case about one-fifth the
case depth. A salt-spray test showed a little better corrosion
resistance for these treated vanes as compared with corres- `
ponding blades aluminized without the nickel plate. The
ductility of the aluminized cases was about the same with the
nickel plate as without it, as indicated by deforming such blades.
Additional AM 355 blades of the same type were sub-
jected to the same sequence of treatment steps except that the
electrolytic plating time was extended to 12 minutes. These
sh~ed that before aluminizing a nickel plate thickness of about
0.2 mil was deposited, and af~er aluminizing the case was much
more brittle than the cases applied over the thinner nickel
plating. Ihis 0.2 mil nickel plate thickness is the minimuml such
thickness suggested in U. S. Patent 3,859,061.
The nickel plating can be
applied by vapor deposition, or by ion deposition as described in
. S. Patent 4,039,416 or in the Society of Automotive Engineers,
Paper No. 730546, by Gerald W. White, en~itled "Applications of
Ion Plating't or by sputterin~ as described in the paper RF
Sputtering by the same author and presented at the 8th Annual
FAA International Aviation Maintenance Symposium, Oklahoma City,
Oklahoma, November 28, 1972. Electroless platlng can also be
used with somewhat poorer results, inasmuch as the electroless
platings contain phosphorus or boron or the like. The minimum
suitable nickel plating thickness is about 0.01 mil. Electro-
platingin narrow passageways is readily accomplished with the
help of an anode in wire form penetrating through the center of
the passageways. /~
:,
~5~3~
The alu~ini~ing can be effected with the workpieces
embedded in a diffusion-coating pack as in Example 4, or with the
workpieces kept out of contact with, but adjacent to the pack as
in Examples 1, 2 and 3. The lowest practical aluminizing temper- ;
ature is about 700F, and other activators can be used in place
of the aluminum chloride.
EXAMPLE 5
The processing of Example 4 is repeated with the follow-
ing changes:
The activator is anhydrous aluminum bromide
instead of the aluminum chloride.
The diffusion-bathing atmosphere is argon
: rather than hydrogen.
The initial cleaning of the blades was by sol-
vent degreasing in place of the anodic
electrolytic cleaning.
The aluminizing is conducted at 880-900F to
yield a case about 0.7 mil thick.
The surface roughness after aluminizing is about 28 to
35 micro-inches. Other cleaning steps such as simple glass
blasting can also be used with similar results.
EX~PLE 6
The processing of Example 4 is repeated but CoC12 6H2O
was substituted for the NiC12-6H~0 of Example 1, the quantity
19 .
~ ~5 ~ J~
being unchancJed~ The resulting alumi.nized vanes have a
surface roughness about the same as the Example 4 products,
and showed even greater resistance to corrosion.
EXAMPLE 7
The processing of Example 4 is repeated but
AM 350 airfoils are used, the nickel chloride is replaced
by a mixture of 107 g. NiC12.6H2O and 107 g. CoC12.6H2O,
the HCl content o-E the electroplating solution is
increased 50%, the cathodic electroplating current density
is 100 amperes per square foot, the electroplating
temperature is 35C, and the electroplating time 2 minutes~
The roughness of the final product is only about 5 to 10
micro-inches more than the untreated airfoils. .
The aluminized blades can be used with or
without the top coatings described iII U.S. Patents 3,859,06].,
3,958,046, 3,948,687, 3,764,371 and 4,141,760. These top
coatings after drying and firing generally provide a , -.
surface somewhat smoother than that of the surface on
which they are applied. Thus a top coating containing
leafing aluminum as described in column 6 of U.S. Patent
3,958,046, applied as a 0.3 milligram per square centimeter
layer over the aluminized product of Example 4 in the
present specification and fired at 700F, improves the
smoothness by about 2 to 5 micro-inches. Such a top
coating over a rougher similarly aluminized workpiece ~-
which did not have the thin nickel electroplate, brought
the top smoothness down to close to 30 micro-inches.
- 20 -
pc/, `t,~
.
~ 3~
Increasing the number of top coating layers on the
workpiece further improves the smoothness, but will generally
not get the smoothness much below about 24 micro-inches. A
series of three layers of the above-noted flake aluminum coating
on the product of Example 4 builds up the total top coating
weight to 0.~ to 0.9 milligrams per square centimeter and shows
a surface roughness as low as about 20 micro-inches.
Some top coating formulations when cured form hydro-
phobic suraces over which it is difficult or impossible to
apply a uniform overlying layer. The teflon-containing formula-
tions of U. S. Patent 3,94~,6~7 are examples of such difficult
materials. However top coatings that contain at least about
5% leafing aluminum by weight, or conta:in at least about 0.1%
by weight wetting agent not destroyed or driven off by a curing
operation, will accept overlying coatings fairly well.
One type of coating seems unique in that when applied
over a top coating containing flake aluminum, has an exce?tional
smoothing effect. Thus an aqueous dispersion of colloidal silica
containing 14% of the silica, and also containing 15% of a
bonding agen~ such as magnesium chromate or mixtures of magnesium
phosphate and magnesium chromate or such mixtures that also
contain a little free phosphoric or chromic acid, when applied
over other top coatings or other layers of the same top coating,
will get the smoothness down to 10 to 15 micro-inches. Such a
smoothness does not appear obtainable from other top coating
layers regardless of how many are applied.
~ ~a~ ?ark
21.
Thus an improvement of 14 micro-inches is
obtained when coating an unaluminized Type 304 stainless
steel compressor blade having an original roughness of
42 micxo-inches after glass bead blastiny to clean it~
using the following coating treatmento
EXAMPLE 8
(a~ Spray on the blade surface a suspension
of the aluminum paste of Example I in U~So Patent 3,318,716
dispersed in 30 times its weight of a 4~ water solution
of MgCrO4, the coating residue after drying weighing about
0.25 milligrams per square centimeterO
(b~ Dry and then bake the coated blade at ; -
700-800F for 10 minutes.
(c) Repeat steps (a) and (b) on the baked blade.
(d~ Repeat steps (a~ and (b~ againO
(e) Spray on the resulting coated blade a
5% suspension of colloidal alumina in the teflon*-free
magnesium-phosphate-chromate acid solution of Example II in
U.S. Patent 3,948,637, the alumina particles having a
particle si~e below 10 millimicrons, tb leave a stratum
that after dr~ing weighs about 0O6 milligram per square
centimeter.
(f~ Repeat the drying and baking step (b).
(g~ Repeat step (e) on the thus haked blade.
.
(h~ Repeat the drying and baking.
*Trade Mark - 22 -
pc/~O
.
~ 6~
(i) Repeat step (e) again.
(j) Repeat the drying and baking.
The final coated blade shows a roughness of about 23
micro-inches and makes a very effective compressor blade for
jet engines.
This exceptic,nal top smoothness is provided by dis-
persions containing about 1 to 20% of silica or alumina particles
no larger than about 25 milli~.~.icrons in size and a water-soluble
bonding agent in an amount at least equal to that of the dis-
persed particles. However magnesium chromate is a particularly
desirable bonding agent inasmuch as it has strong corrosion-
inhibiting effects on a metal workpiece it covers. As much as
half the ma~nesium chromate can be replaced by magnesium phos-
phate andlor chromic acid and/or phosphoric acid. The hardness
and mar-resistance of aluminum flake coatings is also ~arkedly
increased by such colloidal over-coatings.
The foregoing smoothing effect of top coatings is
provided on other substrates such as on t~rpe 410 stainless steel
airfoils that have been aluminized without the help of the thin
nickel or cobalt flash electroplate, but such electroplates at
least 0.01 mil thick make for a much smoother product on age-
hardenable stainless steels.
The compositions of AM 355 as well as of other typical
age-hardenable steels suitable for the present inventiOn is
given belo~, taken from ASTM Data Series Publication No. DC 9d,
October 1967.
3~
CiROl!P I FER~lTlC(~ KrENSIl-lC)STF,EL'i
l`.'omin:ll Ch micrl CompoSiliOn, pcrc~
Alloy C Mn Si Cr i~'i Co l~lo W Cb . Ti Al 13 i~r i ~ Olhcr
. _ . . . _ _ . . _ . _ . . .
- A~c-~l~rdt~ S~ c~ Stcc_c
Al-1-35û01~)1.0~ 0.4016.50 4.25--2.75 ~ -- ---- -- -- 11~1. --
AM ~5D-- -- _ _ _ _ _ _ _ _ __ _ _ _ _
A~1-3550.15 1.000.~015.50 4.25--2.75 -- -- ---- -- -- 11~ .ION~ -35S .--
AM-3630.01O.IS O.OS11.00 4.0~-- -- -- -- ~.25 -- -- -- 11~1. --
15-SPil0.0: 0.300.~.0 15.00 4.60-- -- -- 0.25 -- -- -- -- Ull. 3 30C~I
v 17 4~'il 00~0.30 0.60 16.00 ~.25-- -- -- 0.25 -- -- -- -- B;ll. 3 30Cu
1~7~'11 0.07O.SO 0.~0 I~ 7 10-- _ _ _ _ i.10 -- -- E~
Il-;!~IL -- ---.-- -- -- ~ ~ ~ -- -- --
rH13-~ Mo0.0~O.OS O.OS 12.75 8.10-- 2.2 -- ---- 1.10 -- -- li~
l~i.1~8 Mo 0.0~0.300.~ 14J5 8.15-- 2.2 -- -- --1.10 -- -- 11~1 _.
P'115:7 Mo 0.07O.SO0.~0 IS.IO 7.10 -- 2.2 ---- -- I.10 -- -- IJ~I. --
Pgtom~l X 150.03 0.10 0.10 15.QO -- 20 3.0 ---- -- -- -- -- l~ul
AFC-770.15-- --14.50-- 13 5.0-- -- -- -- ---- n~l 0.40V
So~inl~ss W0.12. ---- 17.01) 7.00-- -- -- --1.0~ -- --. Bal 0 2N
IlliurD P0.20 Q750.75 28.C0 8.CO-- 2.25 -- _ _ _ _ _ 56.D 3.i5Cu
Jllium PD0.10 0.750.75 26.00 S.OO6.S 2.25 -- ---- -- -- -- 58.0
~Fol ruplurc in ItO ~nd iCoD hr. Nol ~or dcsigo
~Cul ~lloy"
'b1/.~imum.
dE~pcrin~cn~ llo~c
~AI;D~ ~nown nol lo be in comm~rcbl prDdUCliDn.
. . .
When a stainless steel workpiece is to be
aluminized, a very effective pre-cleaning is accomplis.hed
by -the following sequence, or by grit blasting with 220
mesh alumina grit:~
EXAMPLE 9
First subject the workpiece to l/2 minute
cathodic treatment at about 50 amperes per square foot
in a 10% solution of sodium carbona-te in water, -then
anodically treat it in the same solution at about the
same current density for about the same time, after which
the ~workpiece is rinsed with water, dipped in 10% NaOH
solution in water to remove any residual smut,
- 24 -
pc/~?;
1~5~
.
; then in cold 1:1 concentrated HCl diluted with water, followed
¦' by another water rinse.
ji The resulting cleaned workpiece with a surface rough-
I ness of about 18 micro-inches is ready for plating in an acid
nickel salt bath to a pick-up of about 1/2 milligram per square
centimeter producing a nickel flash about 0.07 mil thick. After
rinsing and drying it can then be aluminized in the po~der pack
of Example 4 for 30 hours at 870 to 890F ~o yield an aluminized
case about 0.7 mil thick and having a surface roughness of about
22 to 23 micro-inches.
The aluminizing step in the above examples can be
effected in very short times by heating a workpiece embedded
in an activated powder pack, with a thermal input that brin~s
it to diffusion coating temperature and completes the diffusion
coatin~ all in about 50 minutes or less. During this short
interval the activator present in pack begins to be volatilized
at a relatively rapid rate that persists about 45 minùtes, even
if only present in the pack at a concentration of 0.5% by weight,
and the formation of the diffusion coating case is extremely
rapid. Thus a 2 mil aluminized case is produced only about 30
minutes after starting to heat a workpiece to 1800~ in a pac
of
10 weight percent Aluminum powder about 100
microns in size
45 weight percent Chromium powder about 10
microns in size
50 weight percent A12O3 about 100 microns in
size
6 3
i with 0.5% ~14Cl mixed in based on the weight of the pack, if
the workpiece reaches 1800F in 15 minutes.
It is preferred to have the workpiece covered by no
more than about 1/2 inch of activated pack when it is heated,
- inasmuch as the pack acts as thermal insulation and slows do~n
the penetration o:F the heat to the workpiece from the walls of
the retort in which it is held during the heating. With the
workpiece embedded in a pack held in a cylindrical retort having
a 7 inch length and a 2 inch diameter, so that about 1/2 inch
pack thickness envelopes the workpiece, heat supplied at the
rate of at least about 200,000 BTU per hour per pound of work-
piece will effect the desired heat-up to temperatures as high `
as 1800F. During such heat-up the retort can have one or both
its ends loosely covered to permit escape of gases, and can be
held in a lar&er retort through which hydrogen or argon is flowed
at a slow rate to flush out the escaping gases.
It is not necessary to arrange the workpiece so that
i~ comes to within 1/4 inch of the retort as described in U. S.
Patent 3,824,122. Indeed ~he presence of a 1/2 inch thick pack
covering is~preferred when practicing the rapid diffusicn coatin
of the present invention inasmuch as it assures the presence of
sufficient energizer even when the energizer content of the pack
is only 0.5% or less by weight. The energizer content can be
increased, for example to 1%, or 2%, and energizer can be addi-
tionally or alternatively added to the metal powder deposited on
the wall of a narrow passageway to be diffusion coated.
26.
~ 3
A retort packed in accordance with the rapid diffusion
coating technique of examples 4, 5 and 6 can contain a
number of workpieces and there is no need to position each
workpiece into its own carefully dimensioned closely fitting
retort as in U. S. Patent 3,824,122.
Low temperature diffusion coating, as in Example 4
is even more readily accomplished in short periods of time ~
not over 45 minutes of heating is generally needed to bring the
workpieces to temperature and obtain an aluminized case at least
1 mil thick. Thinner cases require only about 30 minutes or
even less.
To further save time the retort cooling is best
effected by withdrawing it from the furnace in which the heating
is carried out. Exposed to the ambient air and with the help
of the flushing gas stream between the retorts, the cylindrical
retort assembly described above cools in about 15 minutes to the
point that the outer retort can be opened and the inner retort
withdrawn,exposed to the atmosphere and emptied. In this way
the enti`re diffusion coating sequence including the completion
of the cool-down takes only about an hour or 65 minutes. This
compares with the 1 1/2 hours disclosed in U. S. Patent
3,824,122 for just the heating time. The cool-down can also
be accelerated by blowing air over the cooling retort assembly
or by lowering it ;nto a quenching liquid such as wa~er.
~ 7.
5~;3
Obviously many modifications and variations of the
present invention are possible in the light of the above
teachings. It is therefore to be understood that within the ¦ ;
¦ scope of the appended claims the invention may be practiced
¦! otherwise than as specifically described.
.,
. Il . . .
I' . i . .
