U.S. patent number 3,891,456 [Application Number 05/407,380] was granted by the patent office on 1975-06-24 for surface treatment of titanium and titanium alloys.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Air. Invention is credited to Alvin E. Hohman, Gerald W. Lively.
United States Patent |
3,891,456 |
Hohman , et al. |
June 24, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Surface treatment of titanium and titanium alloys
Abstract
A process for treating a titanium or titanium alloy surface to
improve its bonding characteristics is provided that comprises
directing a stream of a slurry of aluminum oxide grit in a
hydrofluosilicic acid solution onto the surface for a period of
time sufficient to obtain a uniform, oxide-free surface, washing
the treated surface to remove grit and terminate acid reaction, and
drying the treated surface. The treatment provides a chemically
stable surface that is receptive to adhesives and coatings.
Inventors: |
Hohman; Alvin E. (Dallas,
TX), Lively; Gerald W. (Arlington, TX) |
Assignee: |
The United States of America as
represented by the Secretary of the Air (Washington,
DC)
|
Family
ID: |
23611803 |
Appl.
No.: |
05/407,380 |
Filed: |
October 17, 1973 |
Current U.S.
Class: |
134/3; 134/7;
134/41; 216/92; 216/109; 216/52 |
Current CPC
Class: |
C23G
1/106 (20130101) |
Current International
Class: |
C23G
1/02 (20060101); C23G 1/10 (20060101); B08b
003/02 () |
Field of
Search: |
;134/3,7,41 ;156/6,18
;252/79.3,145 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Metals Handbook, 8th Ed., Vol. 2, 1964, pp. 664-666,
[TA472A3]..
|
Primary Examiner: Bashore; S. Leon
Assistant Examiner: Caroff; Marc L.
Attorney, Agent or Firm: Kuhn; Cedric H.
Government Interests
RIGHTS OF THE GOVERNMENT
The invention described herein may be manufactured and used by or
for the Government of the United States for all governmental
purpose without the payment of any royalty.
Claims
We claim:
1. A process for treating an oxidized surface of a titanium or
titanium alloy part which comprises directing a stream consisting
essentially of a slurry of aluminum oxide grit in an aqueous
hydrofluosilicic acid solution onto the surface for a period of
time sufficient to obtain about 3 to 15 minutes; washing the
surface with water to remove grit and terminate acid reaction; and
drying the part. an oxide-free, uniform, matte surface free from
gloss, provided that the total period of time the surface is
allowed to remain in contact with the slurry is.
2. The process according to claim 1 in which the surface is cleaned
with a solvent prior to directing the stream thereon.
3. The process according to claim 1 in which the stream is in
contact with the surface for about 10 to 60 seconds.
4. The process according to claim 3 in which the part is dried with
filtered dry air.
5. The process according to claim 1 in which the concentration of
acid in the solution ranges from about 1 to 10 weight percent and
the amount of aluminum oxide grit in the solution range from about
15 to 25 volume percent, based upon the volume of solution.
6. The process according to claim 1 in which the part is immersed
in a nitric acid bath after the water washing step; the part is
rinsed with cold tap water after removal from the bath; and the
washed part is dried with filtered dry air.
Description
FIELD OF THE INVENTION
The present invention relates to a process for treating the
surfaces of articles fabricated from titanium and titanium alloys
so as to improve their bonding characteristics.
BACKGROUND OF THE INVENTION
It has been over 15 years since the first announcement of
production bonding of titanium. The bonded assembly was a firewall
sandwhich structure of aluminum honeycomb with titanium and
fiberglass skins. The purpose of this design was to take advantage
of titanium's heat resistance for a relatively short period of
time. However, supersonic aircraft require resistance to elevated
temperatures for prolonged periods. The need for bonded assemblies,
which will remain structurally sound with high strength-to-weight
ratios, has prompted adhesive manufacturers to develop organic
adhesives which retain good bond strength at high temperatures. In
practice, a great deal of difficulty has been experienced in
obtaining consistently durable titanium bonded structures.
While many of the inconsistencies were originally attributed to
adhesive problems, a recent study suggests that bonding differences
with variously treated titanium alloys were caused by variations in
the interfacial structure of the treated adherend. Titanium and its
alloys are so reactive that even at room temperature metal oxides
are formed. The problem is not just one of providing a clean
titanium surface but rather of providing a chemically stable
surface receptive to bonding and resistant to degradation. The
surface obtained must also promote adhesion without forming a weak
surface layer or altering the metallurgical characteristics of the
alloy.
A process widely used in industry to treat surfaces of titanium
articles involves a multi-step procedure. Briefly, the process
includes the steps of (1) vapor degreasing, (2) alkaline cleaning,
(3) rinsing with water, (4) scale conditioning by contacting with
saturated sodium hydroxide at 250.degree.F, (5) rinsing with water,
(6) removing oxide by contacting with a mixture of nitric and
hydrofluoric acids, (7) rinsing with water, (8) conversion coating,
(9) rinsing with water, and (10) drying. An obvious disadvantage of
this conventional process is the many steps that are involved in
the treatment. A less obvious but greater disadvantage from a
quality control standpoint lies in the fact that there is a break
in the processing between steps (6) and (8), i.e., between oxide
removal and conversion coating by application of a reactive
solution. As a result, the freshly cleaned metal rapidly becomes
reoxidized before the reactive solution can be applied. When
titanium surfaces so treated are bonded to one another,
inconsistencies in adhesive reliability are experienced.
It is an object of this invention, therefore, to provide an
improved and simplified process for treating the surfaces of
titanium and titanium alloy articles.
Another object of the invention is to provide a process for
treating titanium and titanium alloy surfaces in which cleaned
surfaces are not subject to reoxidation prior to application of the
conversion coating.
A further object of the invention is to provide a process for
treating titanium and titanium alloy parts so as to provide
chemically stable surfaces which are receptive to bonding and
resistant to degradation.
Still another object of the invention is to provide a surface
treatment process for titanium alloy adherends that makes it
possible to fabricate adhesive-bonded titanium structures having
high reliability, high ambient and elevated temperature strengths,
and high resistance to corrosive environments and weathering.
Other objects and advantages of the invention will become apparent
to those skilled in the art upon consideration of the accompanying
disclosure and the drawing in which the sole FIGURE shows a
schematic representation of apparatus, partly in perspective, that
can be used in carrying out the process of this invention.
SUMMARY OF THE INVENTION
The present invention resides in a process for treating parts or
articles fabricated from titanium or titanium alloy so as to
provide surfaces that are receptive to adhesive bonding. Broadly
speaking, the process comprises the step of directing a stream of a
slurry of aluminum oxide grit in a hydrofluosilicic acid solution
onto a surface of a titanium or titanium alloy part for a period of
time sufficient to obtain a uniform, oxide-free surface.
Thereafter, the treated surface is washed to remove grit and
terminate the acid reaction and then dried.
Prior to contacting the surface with the slurry, the surface is
generally cleaned with a solvent to remove oil, grease and other
foreign material. This can be conveniently accomplished by wiping
the surface with cheese cloth saturated with methyl ethyl ketone or
other suitable solvent. In cases where the parts are heavily
soiled, they are immersed for a minimum of 5 minutes in an alkaline
cleaning solution. The actual time of immersion will depend upon
the condition of the parts. Any of the commerically available
alkaline cleaners can be utilized in forming the solution. An
alkaline cleaner sold under the trade mark Ridoline No. 53 by
Amchem Products, Inc., Ambler, Pa., has been found to be effective.
The solution is formed by adding the cleaner to a tank of hot water
(180.degree.F .+-. 5.degree.F) while agitating the contents to
dissolve the solids. After removal from the alkaline cleaning
solution, the parts are rinsed with hot water, e.g., by suspending
the parts in a hot spray rinse (130.degree. to 140.degree.F) for
about 2 minutes.
After completion of the cleaning operation, the bond surface of the
titanium or titanium alloy part is vapor blasted with the slurry of
aluminum oxide in the aqueous hydrofluosilicic acid solution. The
concentration of acid in the solution generally ranges from about 1
to 10 weight percent. However, it has been found that a solution
concentration of about 2 percent gives optimum results, being
sufficiently strong to etch the titanium yet dilute enough to be
easily controlled at minimal material cost. During impingment of
the slurry, the treated surface is continuously flooded with the
reactive solution under pressure. As a result it is unnecessary to
include a wetting agent in the solution.
The amount of aluminum oxide grit contained in the solution
generally ranges from about 15 to 25 volume percent, based upon the
volume of solution. A ratio of one volume of grit to four volumes
of solution has been found to be particularly effective. However,
it should be understood that the optimum ratio of grit to solution
will depend to a large extent on the equipment used, e.g., its to
propel the slurry against the surface. The aluminum oxide used is
preferably a purified Al.sub.2 O.sub.3 which is sold commerically
as white aluminum oxide abrasive. While it is often preferred to
employ 240 grit aluminum oxide, from 180 to 300 grit can be
used.
A sand blast gun having a line connected to source of slurry and a
line connected to an air supply can be advantageously used in
conducting the process. The air pressure is of a magnitude that is
sufficient to aspirate the slurry and usually falls in the range of
about 70 to 100 psi. However, it is to be understood that the
actual air pressure used will depend to a large extent upon the
equipment employed and may be higher or lower than the specific
pressures indicated. The sand blast gun is provided with a tungsten
carbide nozzle having a bore diameter of a size that allows
sufficient air to flow to aspirate the slurry into the nozzle.
In vapor blasting the surface, the nozzle is kept in constant
motion to prevent excess metal removal. The angle of impingment of
the slurry stream, i.e., the angle between the stream and the
treated surface, is less than 90.degree. and is usually from
15.degree. to 45.degree.. The period during which the slurry stream
is in contact with any one spot on the surface usually falls in the
range of about 10 to 60 seconds. To define the contact time in
another manner, the slurry stream is in contact with the bond
surface until a uniform, matte surface free from gloss is obtained.
In general, the average metal removal rate is about 0.001 inch per
treated surface. After completion of the vapor blasting, the part
is allowed to remain in contact with the reactive solution for a
period that is not in excess of 15 minutes. The contact time
generally ranges from about 3 to 15 minutes. At the end of this
period, the part is spray rinsed in cold water to remove aluminum
oxide grit and stop the acid reaction. Following the water rinse,
the part is dried with filtered dry air. The maximum temperature of
the air is about 150.degree.F.
With some titanium alloys, a smut may be produced on the alloy as a
result of the treatment. This smut can be readily removed by
submerging the part at room temperature in a nitric acid rinse
after the above-mentioned water rinse. An immersion of about one
minute in 5 percent nitric acid readily removes the smut.
Thereafter, the part is spray rinsed in cold water for about 2
minutes. The presence of the smut and the acid treatment for its
removal does not have any adverse effect upon the ability of
surfaces to be adhesively bonded.
A better understanding of the invention can be obtained by
referring to the following illustrative examples which are not
intended, however, to be unduly limitative of the invention.
EXAMPLE I
A series of runs was conducted in which titanium alloy specimens
were surface treated in accordance with the process of this
invention. The apparatus employed in the process was substantially
the same as that illustrated in the drawing. As shown, container 10
comprises an upper portion 11 which can be of any appropriate
shape, e.g., a cube, a parallelpiped or a cylinder. Attached to the
lower end of upper portion 11 is a lower portion 12 which, as
appropriate, is in the form of an inverted pyramid or cone. Resting
on the edge where the upper and lower portions intersect is a grate
13. A closure member 14 provided with a vent pipe 16 is adapted to
cover the opening in the top of the container.
Attached to the vertex of lower portion 12 and communicating with
the interior of the container is one end of a flexible tube 17. The
other end of the flexible tube is connected to sand blast gun 18.
Tube 19, also connected to the gun and to a source of air, provides
means for supplying air to the gun so as to aspirate a slurry 21 of
Al.sub.2 O.sub.3 in hydrofluosilicic acid solution into the nozzle
of the gun. The gun has a tungsten carbide nozzle with a bore
diameter of one-quarter inch. An opening in the side of container
10 having a rubber seal 22 in the shape of an O-ring positioned
therein provides means for inserting the nozzle of gun 18 into the
container. Because of the flexibility of the rubber seal, the
nozzle can be moved therein so as to direct the slurry at a desired
angle onto the titanium or titanium alloy part resting on grate
13.
The apparatus is constructed of polymeric materials because of the
possibility of corrosive attack by the reactive solution. Thus,
container 10 is formed of polyester fiberglass while grate 13 and
closure member 14 are fabricated from acrylic. Flexible tube 17 is
formed form polyvinyl chloride.
The specimens that were treated were formed from titanium alloys,
namely, Ti-6Al-4V and Ti-6Al-6V-2Sn. In general, the dimensions of
the specimens were 1 .times. 6 .times. 0.04 in the case of
Ti-6Al-4V and 1 .times. 6 .times. 0.045 in the case of
Ti-6Al-6V-2Sn.
Initially, the specimens were solvent cleaned to remove oil, grease
and other foreign material. This was accomplished by wiping the
surfaces with cheese cloth saturated with methyl ethyl ketone. The
specimens were then immersed for a minimum of 5 minutes in an
alkaline cleaning solution. (The alkaline cleaning step was not
necessary for all specimens, but was used so that the specimens
might all be similarly treated.) The alkaline cleaning solution was
prepared by filling a thoroughly cleaned tank two-thirds full of
tap water and heating to 175.degree.F .+-. 10.degree.F. There was
then added 3 to 6 ounces of Ridoline No. 53 per gallon of water at
operating level. The tank was next filled to operating level with
tap water and heated to 180.degree.F .+-. 5.degree.F while
agitating the contents to dissolve the solids. The 3-6 oz/gal
concentration was maintained by periodic addition of the alkaline
cleaner as indicated by chemical analysis. Immediately following
immersion in the alkaline bath, the specimens were suspended in a
hot spray rinse (130.degree.-140.degree.F) for 2 minutes.
The lower portion of the container shown in the drawing was filled
with 2 percent hydrofluosilicic acid solution. There was then added
an amount of 240 grit aluminum oxide sufficient to provide a ratio
of one volume of grit to 4 volumes of the acid solution.
Surfaces of the specimens disposed on the grate were vapor blasted
(about 90 psi air pressure) for a minimum of 15 seconds in any one
spot. The nozzle was kept in constant motion to prevent excess
metal removal. The total time that the specimens remained in
contact with the reactive solution did not exceed 15 minutes. The
treated surfaces had a uniform matte surface free from gloss. At
the end of this period the specimens were spray rinsed with cold
tap water to remove abrasive and stop the acid reaction. The
Ti-6Al-6V-2Sn specimens were submerged in 5 percent nitric acid for
one minute to remove smut after which they were spray rinsed in tap
water for 2 minutes. (This step was unnecessary for the Ti-6Al-4V
specimens.) After the water rinse, the specimens were dried with
filtered dry air with a maximum temperature of 150.degree.F.
EXAMPLE II
Utilizing Ti-6Al-4v specimens, treated as described in Example I,
six lap shear specimens were prepared. A primer (EC-2333 primer
sold by 3M Co.) was applied, air dried for 30 minutes, and then
force dried at 160.degree.F for 45 minutes. Using 0.09 lb/ft.sup.2
of epoxy-novolac adhesive (AF-130 adhesive sold by 3M Co.),
one-half inch laps were bonded. The cure cycle was 350.degree.F for
one hour with a heat rise to 350.degree.F of 10.degree.F per
minute. A pressure of 50 psi was maintained throughout the cure.
The individual bonded specimens were tested with a grip separation
of 0.05 inch/minute.
The results of the shear strength tests conducted at room
temperature are shown below in Table I.
TABLE I ______________________________________ Specimen No. Failure
Mode Strength, psi ______________________________________ 1
Cohesive 3257 2 " 3393 3 " 3393 4 " 3411 5 " 3418 6 " 3321
______________________________________
Bonded specimens were also prepared that were tested at a
temperature of 350.degree.F. The specimens were prepared as
described above except for a post cure or aging for 2 hours at
350.degree.F prior to testing. The results of the shear strength
tests are shown below in Table II.
TABLE II ______________________________________ Specimen No.
Failure Mode Strength, psi ______________________________________ 7
Cohesive 2261 8 " 2359 9 " 2268 10 " 2241 11 " 2245 12 " 2124
______________________________________
EXAMPLE III
Lap-shear specimens (0.5 inch lap) were prepared with a polyimide
adhesive (FM-34 adhesive sold by American Cyanamide Co.), utilizing
Ti-6Al-4V adherends treated as described in Example I. The treated
specimens were primed with BR-34 primer (a product of American
Cyanamid Co.), dried 30 minutes and then oven dried for 45 minutes
at 240.degree.F. The specimens were assembled with 0.135
lb/ft.sup.2 of FM-34 adhesive and then cured for 2 hours at
350.degree.F under 50 psi. The specimens were then post cured for 2
hours at 550.degree.F followed by 2 hours at 600.degree.F. The
bonded specimens were tested at 600.degree.F. The results of the
shear strength tests at 600.degree.F are shown below in Table
III.
TABLE III ______________________________________ Specimen No.
Failure Mode Strength, psi ______________________________________
13 Cohesive 1715 14 " 1718 15 " 1694 16 " 1633 17 " 1722 18 " 1648
______________________________________
EXAMPLE IV
The susceptibility of titanium to reoxidation after surface
treatment has always hampered production processes for bonding
titanium because of the short period available between cleaning and
bonding. A series of runs was conducted to determine an allowable
"open time" (time between treatment and bonding) for specimens
treated according to the procedure described in Example I. The
treated specimens were aged in such a way that air could circulate
around the specimens but airborne particulates could not settle on
the specimens. Both Ti-6Al-4V and Ti-6Al-6V-2Sn alloys were
included in the runs. After one hour and at daily intervals, six
specimens from each alloy were primed with EC-2333 primer, dried,
and then bonded with Af-130 adhesive. The specimens were cured for
90 minutes at 350.degree.F. under 50 psi.
The bonded specimens were tested at ambient temperature, and the
results of the test are shown below in Table IV.
TABLE IV ______________________________________ Lapse Time Shear
Stength, psi Days T1-6Al-4V Ti-6Al-6V-2Sn
______________________________________ 1/24 3207 3338 1 3144 3208 2
3363 3200 3 3150 3411 4 3239 3228 6 2930 3368 7 3070 3143 8 2991
3226 9 2982 -- 10 2936 -- 11 2787 -- 13 2763 3357 20 -- 3196
______________________________________
The data in the table show a decline in strength for the Ti-6Al-4V
alloy with practically no decline in strength for the Ti-6Al-6V-2Sn
alloy. Good bond strengths were obtained with both alloys after an
"open time" of 9 days.
EXAMPLE V
A series of runs was conducted to test the retention of bond
strength of lap-shear specimens after exposure to hostile
environments. The bonded specimens were prepared with 0.5 inch lap,
utilizing Ti-6Al-4V and Ti-6Al-6V-2Sn adherends which has been
treated as described in Example I. The adherends were bonded with
both AF-130 and FM-34 adhesives with their respective primers. The
bonded specimens were subjected to salt spray and high humidity
environments.
For the salt spray exposure, the 0.5 inch lap specimens were
individually positioned in such a way that salt spray had equal
access to all sides of the exposure section. the procedure and
equipment were in accordance with Federal Test Method No. 151A,
Method No. 811.1 "Salt Spray Test". Specimen sets with the AF-130
adhesive were subjected to 30 days of salt spray and tested at
75.degree.F and 350.degree.F. One specimen set was subjected to 24
hours at 350.degree.F, then 7 days of salt spray, and then tested
at 350.degree.F. Duplicate specimens were fabricated with F-34
adhesive and tested as the AF-130 bonded specimens except that the
elevated temperature was 600.degree.F.
To determine the effect of high humidity on bond strength, bonded
0.5 inch lap specimens as described above were subjected to the
high humidity condition in paragraph 4.3.5 of Federal Specification
MMM-A-132. Each exposure section was individually positioned in
such a way that condensing humidity had equal access to all sides
of the exposure section. Specimen sets with AF-130 adhesive were
subjected to 30 days high humidity and tested at 75.degree.F and
350.degree.F. One specimen set was subjected to 24 hours at
350.degree.F, then 7 days of high humidity, and then tested at
350.degree.F. Duplicate specimens were fabricated with FM-34
adhesive and tested as the AF-130 bonded specimens except that the
elevated temperature was 600.degree.F.
The result of the above-described runs are set forth below in
Tables V and VI. The tables also identify the alloys and adhesives
used as well as the cure conditions.
TABLE V
__________________________________________________________________________
Adherends: .040" Titanium 6Al-4V .45" Titanium 6Al-6V-2Sn
(machined) Adhesive: AF130/EC2333 Primer Cure: 90 minutes
350.degree.F autoclave Test Temperature: Specified below Type: .5"
lap Ave. Shear Test Temp. Strength Adherend Exposure Condition
.degree.F (psi)
__________________________________________________________________________
Ti 6-4 Control 75 3158 200 hrs. at 350.degree.F 350 2581 24 hrs. at
350.degree.F; 7 days 95% R.H., 100.degree.F 350 2153 7 days
weatherometer 75 2948 30 days salt spray 75 3290 30 days salt spray
350 1737 30 days 95% R.H., 100.degree.F 75 3071 30 days 95% R.H.,
100.degree.F 350 1885 24 hrs. at 350.degree.F, 7 days salt spray
350 2093 Ti 6-6-2 Control 75 3156 30 days salt spray 75 3150 30
days salt spray 350 1806* 30 days 95% R.H., 100.degree.F 75 3069 30
days 95% R.H., 100.degree.F 350 1901*
__________________________________________________________________________
*Some evidence of adhesive failure between primer and metal
adherend.
TABLE VI
__________________________________________________________________________
Adherends: .040" Titanium 6Al-4V .045" Titanium 6Al-6V-2Sn
(machined) Adhesive: FM-34/BR-34 Primer Cure: Two hours
350.degree.F autoclave, two hours 550.degree.F postcure Test
Temperature: Specified below Type: .5" lap Ave. Shear Test Temp.
Strength Adherend Exposure Condition .degree.F (psi)
__________________________________________________________________________
Ti 6-4 Control 75 3302 150 hrs. at 600.degree.F 600 2138 480 hrs.
at 600.degree.F 600 2054 1000 hrs. at 600.degree.F 600 1848 24 hrs.
at 600.degree.F, 7 days 95% R.H. at 100.degree.F 600 1888 7 days
weatherometer 75 3342 30 days salt spray 75 2065* 30 days salt
spray 600 1367 30 days 95% R.H. at 100.degree.F 75 2284* 30 days
95% R.H. at 100.degree.F 600 1458 24 hrs. at 600.degree.F, 7 days
salt spray 600 1905 Ti 6-6-2 Control 75 3145 30 days salt spray 75
1883* 30 days salt spray 600 1483 30 days 95% R.H. at 100.degree.F
75 1676* 30 days 95% R.H. at 100.degree.F 600 1459
__________________________________________________________________________
*Some evidence of adhesive failure between primer and metal
adherend.
The data in the table for the salt spray and high humidity tests
show that there is no difference in results due to substrate alloy.
The specimens subjected to 30 days of salt spray and then tested at
elevated temperatures experienced the greatest loss in shear
strength. In the high humidity tests, there was no loss of strength
with the AF-130 adhesive, but the specimen bonded with FM-34 did
appear to be degraded with prolonged exposure to moisture.
EXAMPLE VI
Runs were conducted to determine the effect of ultraviolet
radiation on bond strength. Specimens prepared with both AF-130 and
FM-340 adhesives, as described in the preceding examples, were
cycled for 7 days in an Atlas Weatherometer. This test exposes
specimens to 45 minutes of water spray followed by 15 minutes with
the spray off. There is continuous ultraviolet radiation from
carbon arc lamps. This exposure had no apparent effect on bond
strength of either adhesive.
From the foregoing examples, it is seen that the process of this
invention provides a simplified, improved procedure for treating
the surfaces of titanium alloy parts. The treated surfaces are
receptive to adhesive bonding, making possible the fabrication of
titanium structures having high ambient and elevated temperature
strengths and high resistance to hostile environments. Furthermore,
the process eliminates the problem of reoxidation that occurs in
conventional processes between cleaning of the surfaces and
application of a conversion coating.
As will be evident to those skilled in the art, various
modifications of the invention can be made in view of the foregoing
disclosure without departing from the spirit or scope of the
invention
* * * * *