BACKGROUND OF THE INVENTION
This invention relates to the preparation of coated aluminum-alloy
articles, and, more particularly, to the preparation of coated aluminum rivets.
Fasteners are used to mechanically join the various structural elements
and subassemblies of aircraft. For example, a large transport aircraft typically
includes over one million fasteners such as bolts, screws, and rivets. The fasteners
are formed of strong alloys such as titanium alloys, steel, and aluminum alloys.
In some cases, the fasteners are heat-treated, as by a precipitation-hardening
aging treatment, to achieve as high a strength, in combination with other desirable
properties, as is reasonably possible for that particular alloy. Heat-treating
usually involves a sequence of one or more steps of controlled heating in a controlled
atmosphere, maintenance at temperature for a period of time, and controlled cooling.
These steps are selected for each particular material in order to achieve its desired
physical and mechanical properties. In other cases, the fastener is used in an
It has been the practice to coat some types of fasteners with organic
coatings to protect the base metal of the fasteners against corrosion damage.
In the usual approach, the fastener is first fabricated and then heat-treated to
its required strength. After heat-treatment, the fastener is etched with a caustic
soda bath to remove the scale produced in the heat-treatment. Optionally, the
fastener is alodined or anodized. The coating material, dissolved in a volatile
carrier liquid, is applied to the fastener by spraying, dipping, or the like. The
carrier liquid is evaporated. The coated fastener is heated to elevated temperature
for a period of time to cure the coating. The finished fastener is used in the
fabrication of the structure.
This coating approach works well with fasteners made of a base metal
having a high melting point, such as fasteners made of steel or titanium alloys.
Such fasteners are heat-treated at temperatures well above the curing temperature
of the coating. Consequently, the curing of the coating, conducted after heat-treating
of the fastener is complete, does not adversely affect the properties of the already-treated
On the other hand, aluminum alloys have a much lower melting point,
and thence a generally much lower heat-treatment temperature, than steel and titanium
alloys. It has not been the practice to coat high-strength aluminum-alloy fasteners
with curable coatings, because it is observed that the curing treatment for the
coating can adversely affect the strength of the fastener. The aluminum-alloy fasteners
are therefore more susceptible to corrosion than would otherwise be the case. Additionally,
the presence of the organic coating aids in the installation of the fastener for
titanium alloys and steel. The absence of the coating means that aluminum fasteners
such as rivets must be installed using a wet sealant compound for purposes of corrosion
protection. The wet sealant compound typically contains toxic components and therefore
requires precautions for the protection of the personnel using it and for environmental
protection. It is also messy and difficult to work with, and may require extensive
cleanup of the area around the fastener using caustic chemical solutions.
There exists a need for an improved approach to the protection of
aluminum-based fasteners such as rivets. The present invention fulfills this need,
and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a method for preparing an aluminum-alloy
article such as a fastener, and more specifically a rivet. For a heat-treatable
article, the article is heat-treated to have good mechanical properties and also
is protected by a cured organic coating. For a cold-worked article, the coating
is applied and cured while still achieving the desired deformation state in the
article. The application of the coating does not adversely affect the desired final
properties of the article. The present approach is accomplished at an additional
cost of much less than one cent per fastener above its unprotected cost.
In accordance with the invention, a method for preparing an aluminum-alloy
article such as a rivet or other fastener comprises the steps of providing an aluminum-alloy
article precursor that is not in its final required heat-treatment and mechanical
state, and providing a curable organic coating material. The coating material has
a non-volatile portion that is predominantly organic and is curable at about a
heat-treatment temperature of the aluminum-alloy article precursor. The method
further includes applying the organic coating material to the aluminum-alloy article
precursor, and heat-treating the coated aluminum article precursor to its final
heat-treated state at the heat-treatment temperature and for a time sufficient
to heat-treat the aluminum to its final required heat-treatment and mechanical
state, and simultaneously cure the organic coating, forming the article.
This approach yields surprising and unexpected technical and cost
advantages when used in conjunction with high-strength aluminum fasteners such
as rivets. The aluminum-alloy fasteners exhibit their full required strength produced
by the heat-treatment used by itself or the required deformation state. The achieving
of a specified strength level is important, because users of the rivets, such as
the customers of aircraft, will not permit a sacrifice of mechanical performance
to achieve improved corrosion resistance. Instead, in the past they have required
both acceptable mechanical performance and also the use of wet sealants to achieve
acceptable corrosion resistance. In the present approach, on the other hand, the
article has both acceptable mechanical performance and a coating for acceptable
corrosion protection. Therefore, during installation of a fastener made by the
present approach, wet sealants need not be applied to the fastener and faying surfaces
of the hole into which the fastener is inserted just before upsetting the fastener.
The elimination of the requirement for the wet sealant installation
approach for the over-700,000 rivets in a large cargo aircraft offers a cost savings
of several million dollars per aircraft. The elimination of the use of wet sealants
also improves the workmanship in the fastener installation, as there is no possibility
of missing some of the fasteners as the wet sealant is applied. The coated fasteners
are more resistant to corrosion during service than are uncoated fasteners.
Other features and advantages of the present invention will be apparent
from the following more detailed description of the preferred embodiment, taken
in conjunction with the accompanying drawings, which illustrate, by way of example,
the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE INVENTION
- Figure 1 is a process flow diagram for a first embodiment of the method of
- Figure 2A is a process flow diagram for one form of a second embodiment of
the method of the invention;
- Figure 2B is a process flow diagram for another form of a second embodiment
of the method of the invention;
- Figure 3 is a process flow diagram for a second embodiment of the method of
- Figure 4 is a schematic sectional view of a protruding-head rivet fastener
used to join two pieces, prior to upsetting;
- Figure 5 is a schematic sectional view of a slug rivet fastener used to join
two pieces, prior to upsetting;
- Figure 6 is a schematic sectional view of a flush-head rivet fastener used
to join two pieces, prior to upsetting; and
- Figure 7 is a schematic sectional view of the flush-head rivet fastener of
Figure 5, after upsetting.
As depicted in Figure 1, an untreated (i.e., uncoated and annealed)
article is first provided. The preferred embodiment of the invention relates to
the preparation of fasteners such as rivets, and the following discussion will
emphasize such articles. The use of the invention is not limited to fasteners
and rivets, and instead is more broadly applicable. However, its use in fasteners
offers particular advantages that will be discussed.
A rivet 40 is provided, numeral 20. The present invention is used
with a rivet, fastener, or other article manufactured to its conventional shape
and size. Figures 4-6 illustrate three types of rivets 40, at an intermediate stage
of their installation to join a first piece 42 to a second piece 44, after installation
to the first and second pieces but before upsetting. The rivet 40 of Figure 4
has a premanufactured protruding head 46 on one end. The rivet 40' of Figure 5,
a slug rivet, has no preformed head on either end. The rivet 40" of Figure 6 has
a premanufactured flush head 46" on one end, that resides in a countersink in the
piece 42. The present invention may be used with these and other types of rivets.
The rivet 40 is manufactured of an aluminum-base alloy. As used herein,
"aluminum-alloy" or "aluminum-base" means that the alloy has more than 50 percent
by weight aluminum but less than 100 percent by weight of aluminum. Typically,
the aluminum-base alloy has about 85-98 percent by weight of aluminum, with the
balance being alloying elements and a minor amount of impurity. Alloying elements
are added in precisely controlled amounts to modify the properties of the aluminum
alloy as desired. Alloying elements that are added to aluminum in combination to
modify its properties include, for example, magnesium, copper, and zinc, as well
as other elements.
In one case of interest, the aluminum alloy is heat-treatable. The
article is first fabricated to a desired shape, in this case a fastener such as
a rivet. The alloying elements are selected such that the fabricated shape may
be processed to have a relatively soft state, preferably by heating it to elevated
temperature for a period of time and thereafter quenching it to lower temperature,
a process termed solution treating/annealing. In the solution treating/annealing
process, solute elements are dissolved into the alloy matrix (i.e., solution treating)
and retained in solution by the rapid quenching, and the matrix itself is simultaneously
annealed (i.e., annealing).
After the article is solution treated/annealed, it may be further
processed to increase its strength several fold to have desired high-strength properties
for service. Such further processing, typically by a precipitation-hardening aging
process, may be accomplished either by heating to an elevated temperature for
a period of time, termed artificial aging, or by holding at room temperature for
a longer period of time, termed natural aging. In conventional Aluminum Association
terminology, different artificial aging precipitation treatments, some in combination
with intermediate deformation, produce the T6, T7, T8, or T9 conditions, and a
natural aging precipitation treatment produces the T4 condition. (Aluminum Association
terminology for heat treatments, alloy types, and the like are accepted throughout
the art, and will be used herein.) Some alloys require artificial aging and other
alloys may be aged in either fashion. Rivets are commonly made of both types of
In both types of aging, strengthening occurs as a result of the formation
of second-phase particles, typically termed precipitates, in the aluminum-alloy
matrix. Collectively, all of the processing steps leading to their strengthening
is generally termed "heat-treating", wherein the article is subjected to one or
more periods of exposure to an elevated temperature for a duration of time, with
heating and cooling rates selected to aid in producing the desired final properties.
The temperatures, times, and other parameters required to achieve particular properties
are known and are available in reference documents for standard aluminum-base alloys.
A specific artificially aged aluminum-base alloy of most interest
for rivet applications is the 7050 alloy, which has a composition of about 2.3
percent by weight copper, 2.2 percent by weight magnesium, 6.2 percent by weight
zinc, 0.12 percent by weight zirconium, balance aluminum plus minor impurities.
(Other suitable alloys include, but are not limited to, 2000, 4000, 6000, and
7000 series heat-treatable aluminum alloys.) This alloy is available commercially
from several aluminum companies, including ALCOA, Reynolds, and Kaiser. After fabrication
to the desired shape such as one of those shown in Figures 4-6, the 7050 alloy
may be fully solution treated/annealed to have an ultimate shear strength of about
234,430 - 241,325 kilopascals (kPa) (34,000-35,000 pounds per square inch (psi)).
This state is usually obtained following the fastener's fabrication processing
including machining, forging, or otherwise forming into the desired shape. This
condition is termed the "untreated state" herein, as it precedes the final aging
heat-treatment cycle required to optimize the strength and other properties of
the material. The article may be subjected to multiple forming operations and periodically
re-annealed as needed, prior to the strengthening precipitation heat-treatment
After forming (and optionally re-annealing), the 7050 alloy may be
heat-treated at a temperature of about 121°C (250°F) for 4-6 hours. The temperature
is thereafter increased from 121°C (250°F) directly to about 179°C (355°F) for
a period of 8-12 hours, followed by an ambient air cool. This final state of heat-treatment,
termed T73 condition, produces a strength of about 282,695 - 317,170 kPa (41,000-46,000
psi) in the 7050 alloy, which is suitable for fastener applications. (This precipitation-treatment
aging step is subsequently performed in step 26 of Figure 1.)
Returning to the discussion of the method of Figure 1, the untreated
fastener is optionally chemically etched, grit blasted or otherwise processed to
roughen its surface, and thereafter anodized in chromic acid solution, numeral
30. Chromic acid solution is available commercially or prepared by dissolving
chromium trioxide in water. The chromic acid solution is preferably of a concentration
of about 4 percent chromate in water, and at a temperature of from about 32°C (90°F)
to about 38°C (100°F). The article to be anodized is made the anode in the mildly
agitated chromic acid solution at an applied DC voltage of about 18-22 volts. Anodizing
is preferably continued for 30-40 minutes, but shorter times were also found operable.
The anodizing operation produces a strongly adherent oxide surface layer about
0.000254-0.000762 cm (0.0001-0.0003 inch) thick on the aluminum alloy article,
which surface layer promotes the adherence of the subsequently applied organic
coating. Anodizing can also be used to chemically seal the surface of the aluminum
article. In this case, it was found that it is not as desirable to chemically seal
the surface in this manner, as the chemical sealing tends to inhibit the strong
bonding of the subsequently applied coating to the aluminum alloy article.
Other anodizing media were also tested for various anodizing times.
Sulfuric acid, phosphoric acid, boric acid, and chemical etch were operable to
varying degrees but not as successful in producing the desired type of oxide surface
that results in strong adherence of the subsequently applied coating.
A coating material is provided, numeral 22, preferably in solution
so that it may be readily and evenly applied. The usual function of the coating
material is to protect the base metal to which it is applied from corrosion, including,
for example, conventional electrolytic corrosion, galvanic corrosion, and stress
corrosion. The coating material is a formulation that is primarily of an organic
composition, but which may contain additives to improve the properties of the final
coating. It is desirably initially dissolved in a carrier liquid so that it can
be applied to a substrate. After application, the coating material is curable to
effect structural changes within the organic component, typically cross linking
of organic molecules to improve the adhesion and cohesion of the coating.
Such a curable coating is distinct from a non-curable coating, which
has different properties and is not as suitable for the present corrosion protection
application. With a non-curable coating such as a lacquer, there is no need to
heat the coated article to elevated temperature for curing. The overaging problems
associated with the use of curable coating materials, and which necessitated the
present invention, simply do not arise.
The anodizing process, preferably in chromic acid, conducted prior
to application of the coating serves to promote strong bonding of the organic
coating to the aluminum alloy article substrate. The bonding is apparently promoted
both by physical locking and chromate activation chemical bonding effects. To achieve
the physical locking effect, as previously discussed the anodized surface is not
chemically sealed against water intrusion in the anodizing process. The subsequently
applied and cured organic coating serves to seal the anodized surface.
A number of curable organic coating materials are available and operable
in the present process. A typical and preferred coating material of this type has
phenolic resin mixed with one or more plasticizers, other organic components such
as polytetrafluoroethylene, and inorganic additives such as aluminum powder and/or
strontium chromate. These coating components are preferably dissolved in a suitable
solvent present in an amount to produce a desired application consistency. For
the coating material just discussed, the solvent is a mixture of ethanol, toluene,
and methyl ethyl ketone. A typical sprayable coating solution has about 30 percent
by weight ethanol, about 7 percent by weight toluene, and about 45 percent by weight
methyl ethyl ketone as the solvent; and about 2 percent by weight strontium chromate,
about 2 percent by weight aluminum powder, with the balance being phenolic resin
and plasticizer. A small amount of polytetrafluoroethylene may optionally be added.
Such a product is available commercially as "Hi-Kote 1" from Hi-Shear Corporation,
Torrance, CA. It has a standard elevated temperature curing treatment of 1 hour
at 218°C-190°C (400°F ± 25°F), as recommended by the manufacturer.
The coating material is applied to the untreated fastener article,
numeral 24. Any suitable approach, such as dipping, spraying, or brushing, can
be used. In the preferred approach, the solution of coating material dissolved
in solvent is sprayed onto the untreated rivets. The solvent is removed from the
as-applied coating by drying, either at room temperature or slightly elevated
temperature, so that the coated article is dry to the touch. Preferably, evaporation
of solvent is accomplished by flash exposure at 93°C (200°F) for about two minutes.
The coated article is not suitable for service at this point, because the coating
is not sufficiently cured and adhered to the aluminum alloy base metal and because
the coating is not sufficiently coherent to resist mechanical damage in service.
In the case of the preferred Hi-Kote 1, the as-sprayed coating was
analyzed by EDS analysis in a scanning electron microscope. The heavier elements
were present in the following amounts by weight: Al, 82.4 percent; Cr, 2.9 percent;
Fe, 0.1 percent; Zn, 0.7 percent; and Sr, 13.9 percent. The lighter elements such
as carbon, oxygen, and hydrogen were detected in the coating but were not reported
because the EDS analysis for such elements is not generally accurate.
The base metal of the rivet article and the applied coating are together
heated to a suitable elevated temperature, numeral 26, to achieve two results
simultaneously. In this single step, the aluminum alloy is precipitation heat treated
by artificial aging to its final desired strength state, and the coating is cured
to its final desired bonded state. Preferably, the temperature and time treatment
of step 26 is selected to be that required to achieve the desired properties of
the aluminum alloy base metal, as provided in the industry-accepted and proven
process standards for that particular aluminum-base alloy. This treatment is typically
not that specified by the coating manufacturer and may not produce the most optimal
cure state for the coating, but it has been determined that the heat-treatment
of the metal is less forgiving of slight variations from the optimal treatment
than is the curing treatment of the organic coating. That is, the inventor has
demonstrated that the curing of the coating can sustain larger variations in time
and temperature with acceptable results than can the heat-treatment of the metal.
Contrary to expectations and manufacturer's specifications, the coating cured by
the non-recommended procedures exhibits satisfactory adhesion to the aluminum-alloy
substrate and other properties during service. Thus, the use of the recommended
heat-treatment of the metal yields the optimal physical properties of the metal,
and extremely good properties of the coating.
In the case of the preferred 7050 aluminum-base alloy and Hi-Kote
1 coating discussed above, the preferred heat-treatment is the T73 precipitation
treatment aging process of 7050 alloy of 4-6 hours at 121°C (250°F), followed by
a ramping up from 121°C to 179°C (250°F to 355°F) and maintaining the temperature
at 179°C (355°F) for 8-12 hours, and an ambient air cool to room temperature.
Thus, the precipitation treatment artificial aging procedure 26 involves
significantly longer times at temperature and different temperatures than is recommended
by the manufacturer for the organic coating. There was initially a concern that
the higher temperatures and longer times, beyond those required for the standard
curing of the coating, would degrade the coating and its properties during service.
This concern proved to be unfounded. The final coating 48, shown schematically
in Figures 4-7, is strongly adherent to the base metal aluminum alloy and is also
strongly internally coherent. (In Figures 4-7, the thickness of the coating 48
is exaggerated so that it is visible. In reality, the coating 48 is typically about
0.000762-0.00127cm (0.0003-0.0005 inch) thick after treating in step 26.)
The coated and treated rivet 40 is ready for installation, numeral
28. The fastener is installed in the manner appropriate to its type. In the case
of the rivet 40, the rivet is placed through aligned bores in the two mating pieces
42 and 44 placed into faying contact, as shown in Figure 4. The protruding remote
end 50 of the rivet 40 is upset (plastically deformed) so that the pieces 42 and
44 are mechanically captured between the premanufactured head 46 and a formed head
52 of the rivet. Figure 7 illustrates the upset rivet 40" for the case of the flush
head rivet of Figure 6, and the general form of the upset rivets of the other types
of rivets is similar. The coating 48 is retained on the rivet even after upsetting,
as shown in Figure 7.
The installation step reflects one of the advantages of the present
invention. If the coating were not applied to the fastener, it would be necessary
to place a viscous wet-sealant material into the bores and onto the faying surfaces
as the rivet was upset, to coat the contacting surfaces. The wet-sealant material
is potentially toxic to workers, messy and difficult to work with, and necessitates
extensive cleanup of tools and the exposed surfaces of the pieces 42 and 44 with
caustic chemical solutions after installation of the rivet. Moreover, it has been
observed that the presence of residual wet sealant inhibits the adhesion of later-applied
paint top coats over the rivet heads. Prior to the present invention, the wet sealant
approach was the only viable technique for achieving sufficient corrosion resistance,
even thought there had been efforts to replace it for many years. The present coating
approach overcomes these problems of wet sealants. Wet sealant is not needed or
used during installation. Additionally, the later-applied paint top coats adhere
well over the coated rivet heads, an important advantage. The use of wet sealants
sometimes makes overpainting of the rivet heads difficult because the paint does
not adhere well.
The present invention has been reduced to practice with rivets made
of 7050 alloy. The rivets, initially in the untreated state, were coated with
Hi-Kote 1 and another, but chromium-free, coating material, Alumazite ZY-138. (Alumazite
ZY-138 is a sprayable coating available from Tiodize Co., Huntington Beach, CA.
Its composition includes 2-butanone solvent, organic resin, and aluminum powder.)
The coated rivets were precipitation heat-treated to T73 condition with the artificial
aging treatment of 4-6 hours at 121°C (250°F), followed by a ramping up from 121°C
to 179°C (250°F to 355°F) and maintaining the temperature at 179°C (355°F) for
8-12 hours, followed by an ambient air cool to room temperature.
The coated rivets were mechanically tested in accordance with MIL-R-5674
to verify that they meet the required ultimate double shear strength requirements
of 282,695-317,170 kPa (41,000-46,000 pounds per square inch) achieved by uncoated
rivets. In the testing, the ultimate double shear strength was 293,037-299,933
kPa (42,500-43,500 pounds per square inch), within the permitted range. Cylindrical
lengths of each type of coated rivet were upset to a diameter 1.6 times their initial
diameter to evaluate driveability. No cracking or spalling of the coatings was
noticed even on the periphery of the upset region, which is the area that experiences
the greatest deformation. Rivets were also installed and subsequently removed to
evaluate coating integrity using a scanning electron microscope. The coatings exhibited
no signs of cracking, spalling, or any other unacceptable conditions or abnormalities.
This latter result is particularly important and surprising. The coatings were
retained on the rivets even after the severe deformation resulting from the upsetting
process. Thus, the coatings remained in place to protect the rivet against corrosion
after installation, obviating any need for the use of wet sealants.
When aluminum alloys are treated to natural-aging tempers by the
approach illustrated in relation to Figure 1, the aluminum alloy will be overaged
due to the heating step 26 required to cure the organic coating. For some fastener
applications, overaging of the aluminum alloy is acceptable. In other applications,
overaging results in unacceptable properties and must be avoided. Figures 2A and
2B depict procedures for obtaining the benefits of a curable organic coating applied
to alloys treated to natural-aged tempers.
In one approach, depicted in Figure 2A, the aluminum alloy rivet stock
selected for precipitation heat treating to a naturally aging temper is furnished,
numeral 32. The rivet stock is supplied slightly oversize (i.e., larger diameter),
as compared with the size furnished for conventional processing in which no curable
coating is used. The preferred aluminum alloy for precipitation treatment by natural
aging to the T4 condition is 2117 alloy having a nominal composition of 0.4-0.8
percent by weight magnesium, 3.5-4.5 percent by weight copper, 0.4-1.0 percent
by weight manganese, 0.10 percent by weight chromium, 0.2-0.8 percent by weight
silicon, 0.7 percent by weight iron, 0.25 percent by weight zinc, 0.15 percent
by weight titanium, 0.05 percent by weight maximum of other elements, with a total
of other elements of no more than 0.15 percent by weight, with the balance aluminum.
The 2117 alloy is available commercially from several aluminum companies, including
Alcoa, Reynolds, and Kaiser. This alloy may be precipitation hardened by natural
aging to the T4 condition at room temperature for at least about 96 hours, developing
a shear strength of about 179,270-208,850 kPa (26,000-30,000 psi). (This natural
aging heat-treatment step is subsequently performed in step 37 of Figure 2A and
2B.) The approach is also operable with other alloys that may be aged with a precipitation
heat treatment of natural aging, such as, for example, 2017, 2024, and 6061 alloys.
The fastener is deformed to a size different from, and typically larger
than, the desired final size, numeral 34, a state termed by the inventor "oversize
normal". In the case of a cylindrically symmetric rivet, the rivet stock is preferably
drawn to an oversize normal diameter that is typically about 10-15 percent larger
than the desired final size. The oversize normal drawn rivet stock is solution
treated/annealed according to the procedure recommended for the aluminum alloy,
numeral 36. In the case of the preferred 2117 alloy, the solution treatment/aging
is accomplished at 476-510°C (890-950°F) for 1 hour, followed by quenching. The
rivet stock is naturally aged according to recommendations for the alloy being
processed, room temperature for a minimum of about 96 hours in the case of 2117
alloy, numeral 37. The drawn and solution treated/annealed and aged stock is thereafter
deformed by cold working, typically drawing, to its final desired diameter, numeral
38, a step termed redrawing or cold working. (However, equivalently for the present
purposes the step 34 may be used to deform the rivet stock to a smaller size than
the desired final size, and the step 38 may be used to deform the rivet stock to
the larger final size, as by a cold heading operation.) This cold working imparts
a light deformation to the rivet. The cold-worked rivet stock is optionally anodized,
preferably in chromic acid solution, and preferably left unsealed, numeral 30,
using the approach described earlier. The coating material is provided in solution,
numeral 22, and applied to the rivet stock, numeral 24. Steps 30, 22, and 24 are
as described hereinabove in relation to Figure 1, and those descriptions are incorporated
The coated fastener stock is cured, numeral 26. The preferred curing
is that recommended by the manufacturer, most preferably 1 hour at 204°C (400°F)
as described previously. However, a modified curing operation may be employed,
depending upon the level of cold working performed on the fastener in step 38.
The modified curing cycle is 45 minutes at 190°C (375°F) and has been demonstrate
to produce acceptable results consistent with the requirements for coating material.
The curing operation has the effect of tending to overage the aluminum alloy, which
normally requires only natural (room temperature) aging to realize its full strength.
However, most surprisingly, it has been found that the additional cold working
operation of step 38, conducted after the solution treat/anneal of step 36 and
the natural aging of step 37, offsets the overaging effect of step 26 and results
in a final rivet that is coated and aged to acceptable aluminum-alloy properties,
but not overaged.
In a variant of the approach of Figure 2A for heat treating and coating
articles that are to be treated to a natural aging temper, depicted in Figure 2B,
the aluminum alloy rivet stock is supplied in an oversize condition, numeral 32.
The rivet stock is drawn or formed to its final size, numeral 34. (This is distinct
from step 34 of Figure 2A wherein the rivet stock is deformed to the oversize normal
diameter.) The drawn rivet stock is solution treated/annealed, numeral 36, and
naturally aged, numeral 37. No step 38 of drawing to the final diameter is required,
as in the procedure of Figure 2A. The remaining steps 22, 30, 24, 26, and 28 are
as described previously in relation to Figure 2A, which description is incorporated
The approach of Figure 2B has been successfully practiced using 2117
aluminum alloy. Rivet stock was provided in an oversize diameter of about 0.508-0.521
centimeter (0.200-0.205 inch), step 32, as compared with a conventional starting
diameter of 0.469-0.472 centimeter (0.185-0.186 inch). The oversize rivet stock
was drawn to a diameter of 0.469-0.472 centimeter (0.185-0.186 inch) in step 34
and cold headed to a diameter of 0.474-0.478 centimeter (0.187-0.188 inch) in step
34. The other steps of Figure 2B were as described previously for the 2117 aluminum
alloy. The required strength of T4 temper was achieved, and additionally the rivets
were protected by the adherent coating.
In the procedures of Figures 2A and 2B, the extra mechanical working
that results to the rivet stock in deforming in steps 34 and 38 from the initial
oversize diameter of step 32, coupled with the extra heating involved in the curing
step 26, results in a final strength and other mechanical properties that meet
the required standards and specifications for fasteners of this type. The extra
mechanical cold working tends to raise the mechanical properties above the acceptable
limits, while the extra heating during curing reduces the mechanical properties
back to the acceptable range. Exact balancing of these effects even permits the
mechanical properties to be set at the high side or the low side of the range permitted
by most standards. The processing modifications yield the important further benefit
that the fastener is coated with a cured coating that protects the fastener from
Some alloys are not solution treated/annealed and precipitation treated
prior to use, but instead are used in a cold-worked state with a minimum level
of deformation-induced strength. The required deformed state of such alloys would
apparently be incompatible with heating to elevated temperature to cure the coating.
However, it has been demonstrated that a processing such as that illustrated in
Figure 3 for a third preferred embodiment of the invention permits the alloy to
be used in a strengthened state induced by deformation and also to be coated with
a curable coating. A preferred such alloy is 5056-H32, having a nominal composition
of 4.5-5.6 percent by weight magnesium, 0.10 percent by weight copper, 0.05-0.20
percent by weight manganese, 0.30 percent by weight silicon, 0.40 percent by weight
iron, 0.05-0.20 percent by weight chromium, 0.10 percent by weight zinc, 0.05 percent
by weight maximum of any other element with 0.15 percent by weight total of other
elements, balance aluminum. The 5056 alloy, when deformed by cold working with
about 2-3 percent reduction to reach the H32 state, exhibits 179,270-193,060 kPa
(26,000-28,000 psi) ultimate shear strength. If, however, the 5056 alloy is thereafter
heated for 1 hour at 204°C (400°F), the standard curing treatment for the curable
coating material, the ultimate shear strength is reduced to about 165,480-179,270
kPa (24,000-26,000 psi), which is at the very low side of the range permitted by
the strength specification but which is deemed too low for commercial-scale operations
because of processing variations that may result in strengths below the strength
specification for some treated articles.
Figure 3 illustrates a procedure by which the required mechanical
properties are achieved while also having the advantages of a cured coating, for
the preferred case of the rivet fastener. The 5056 aluminum material is provided
in an initial oversize condition, numeral 70. For example, conventionally a rivet
having a final diameter of 0.474-0.478 cm (0.187-0.188 inch) is drawn from stock
initially having a diameter of about 0.482-0.485 cm (0.190-0.191 inch). In the
preferred embodiment of the method of Figure 3, the precursor stock material is
initially about 4-5 percent oversize (e.g., a diameter of 0.495 cm (0.195 inch)
for the case of a rivet of final diameter about 0.474-0.478 cm (0.187-0.188 inch).
The oversize stock is deformed, preferably by cold working, to the required final
diameter, numeral 72. This rivet precursor, because it has been cold deformed from
a size larger than that required to achieve H32 condition, has a strength greater
than that required in the H32 condition. The coating material is provided, numeral
22, and applied to the as-deformed rivet precursor material, numeral 24. Optionally,
the rivet precursor material may be treated to roughen its surface and preferably
anodized in chromic acid (but preferably not chemically sealed) prior to application
of the coating material, as previously described.
The coated rivet precursor material is heated to accomplish the standard
curing cycle of 1 hour at 204°C (400°F) or the modified curing cycle of 45 minutes
at 190°C (375°F), numeral 74. The curing cycle has two effects. First, the coating
is cured so that it is coherent and adherent to the aluminum rivet. Second, the
aluminum material is partially annealed to soften it. The partial softening treatment
reduces the state of cold-worked deformation in the rivet from that achieved in
the overworking operation (step 72) to that normally achieved by the H32 treatment.
The rivet may therefore be installed by the procedures already known for the 5056-H32
rivet. The rivet differs from conventional 5056-H32 rivets in that it has the coating
The approach of Figure 3 has been practiced using the materials and
sizes discussed previously. The initially oversize aluminum stock provided in
step 70 has an ultimate shear strength of 172,375-179,270 kPa (25,000-26,000 psi).
After drawing in step 72, the stock has an ultimate shear strength of 186,165-193,060
kPa (27,000-28,000 psi). After heating in step 74, the final rivet has an ultimate
shear strength of 179,270-186,165 kPa (26,000 - 27,000 psi), which is comfortably
within the range required by the H32 mechanical property specification. By comparison,
if the aluminum stock is initially not oversize, but has the conventional starting
diameter, the final rivet subjected to the remaining steps 72, 22, 24, and 74 has
an ultimate shear strength of 165,480-179,270 kPa (24,000-26,000 psi), at the very
low end of that required by the H32 specification and which, as discussed earlier,
is too low for commercial operations.