Note: Descriptions are shown in the official language in which they were submitted.
~2858~7
~his invention relates to the sur~ace treatment of an
aluminum alloy, and in particular to a process ~or treating the
surface of an aluminum alloy to increase the corrosion ~atigue
reistance of the alloy.
The effect on fatigue properties by the sur~ace treating
of steels has been extensively studied. More recently, similar
studies have been made with respect to titanium alloys. We
are aware of no such studies with respect to aluminum alloys.
The yield strength of commercial aluminum alloys has
been greatly increased by alloying and heat treating the alloys.
However, such methods do not increase the fatigue or corrosion
fatigue strength of the alloys.
Several attempts have been made to eliminate the effects
of corrosion by applying protective coatings to aluminum alloys.
For example, porous anodic coatings of alu~inum alloys have been
impregnated with palmitic acid and other organic polar molecules.
Such a treatment increases the fatigue strength at 107 cycles in
air by 38 to 45%. -The effect of coatings on aluminum alloys
depends on the intrinsic fatigue strength o~ the coatings, the
thickness of the coatings and the interaction of the coatings
with the environment. It has been found that soft aluminum
cladding reduces the fatigue strength, while polymeric coatings
effectively increase the strength and life of the alloy in
corrosive environments. It is also known that anodic layers
improv~ the fatigue strength in corrosive environments.
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The fatigue s~rength of aluminum alloys in the absence
of corrosion can be increased by the time honoured process
of shot-peening. Shot-peening rai.ses the stress threshold
at which reversed plastic deformation begins by superficially
workhardening and/or introducing residual stresses, An increase
of the threshold stress in neutral environment is also possible
by surface alloying such as with a thin layer of copper on
commercially pure aluminum.
The object of the present invention is to overcome
the problems mentioned above by providing a relatively simple
method of surface treating an aluminum alloy to increase the
corrosion fatigue resistance of such alloy.
Accordingly, the present invention relates to a process
for treating an aluminum alloy to increase the corrosion fatigue .
strength thereof comprising the steps of:
(a) zincating the surface of the alloy;
(b) depositing a first layer of zinc on the surface
of the alloy;
(c) heating the resulting product at a first temperature
to anneal such product;
(d) heating the product at a second temperature
higher than said first temperature to solution anneal the product;
(e) quenching the product;
(f) allowing the product to stand for aging or aging
at an elevated temeprature, and
(f) depositing a second layer of zinc on the surface
of the alloy.
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ALLOYS
The method of tne present invention was tested usiny
two different alLoys, namely (1) a first aluminum alloy containing
4.5% Cu, 1.5% Mg, and 0.6~ Mn, the balance being aluminum,
and (2) a second alloy containing 5.5~ Zn, 2.5~ Mg, 1.5~ Cu
and 0.33 Cr, the balance being aluminum. The first alloy does
not contain zinc, and thus zinc is being added as an additional
alloy element. The second alloy contains zinc, and thus a
determination has been made concerning the effect on the fatigue
strength of increasing the zinc concentration of the surface
of the alloy.
METHOD
A variety of methods for surface treating the alloys
were attempted. The optimum method proved to include the steps
15 of:
1. Cleaning.
2. Zincating.
3. Electroplating.
4. Prediffusion heating.
5. Solution annealing.
6. Water quenching.
7. Aging at room temperature and by heating.
8. Electroplating.
The cleaning of the alloys includes acid cleaning
as outlined in the literature ~Metals Handbook, 9th Ed., Vol. 5
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p. 604). Zincating is also effected in the manner set out
in the literature (Metals Handbook, 8th Ed. Vol. 2, p. 628-631).
Zincated aluminum specimens are electroplated in
a zinc cyanide bath at 0.3 A/cm2 requiring 1 V a-t 1 cm electrode
spacing. The bath contains 61 g/1 Zn(CN)2, 42 g/1 NaCN, 7g g/1
NaOH and 15 g/1 Na2CO3. The coating time should no-t exceed
five minutes to provide coating thickness of up to 0.12 mm.
Diffusion of the zinc into the surface of the aluminum
alloy is effected by heating to diffusion temperatures of 350
to 420C for three to six hours. The preferred diffusion temperature
is 415C in a vacuum for three hours which results in an acceptable
coating of the alloy.
Solution annealing of the alloy and coating is effected
at a temperature of 492 to 495C. The preferred temperature
is 492C. Heating above 495C causes excessive zinc penetration
of the grain boundaries of the alloy. The solution treated
alloy is then quenched with water.
Coated and solution treated specimens are naturally
aged at room temperature (22C) for one week, and artificially
aged at 120C for twenty-four hours.
It has been noted that quenching following solution
treatment produces a soft layer near the surface of the product,
the hardness of which does not increase by natural aging for
four days. This indicates a need for artificial aging. Aging
for one hour is sufficient to raise the hardness in the near
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surface layer substantially. However, the hardness of thebase material or core also increases beyond that of the basic
material prior to treatment. Aging for three hours decreases
the core hardness to a value which remains unchanged wlth increased
aging time. Prolonqed aging causes over ayiny and softening
of the surface layer. Aging immediately after quench.ing results
in a shallow hardened layer and a more ra.pid drop of the hardness
to the base value of the zinc free core material than when
artificial aging is preceded by four days of aging at room
temperature. Thus, the preferred method of aging is to age
at room temperature for four days followed by aging at 120C
for three hours.
Specimens are electroplated in a zinc cyanide bath
at 0.3 A/cm requiring 1 V at 1 cm electrode spacing. The
bath contains 61 gtl Zn(CN)2, 42 g/l NaCN, 79 g/l NaOH and
15 g/l Na2CO3. Plating time is chosen to achieve additional .
coating thicknesses in the range of 4.5 ,um to 6.6 ,um.
TESTING ~:
As well as fatigue tests on samples prepared in
accordance with the above described method, tests were conducted
to determine the effect of a variety of surface treatments
and environments on fatigue properties. For this purpose,
torsion fatigue tests were carried out on specimens ~1) as
machined, (2~ machined and mechanically polished, (3) chemically
polished, (4) diffusion coated (mechanically polished, zinc
electroplated, diffusion annealed at 415C for 3 hours, solution -
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annealed at 492C for 20 minutes, water quenched, aged at room
temperature for four days and aged at 120C for 3 hours),
(5) mechanically polished, uncoated and heat treated as for
sample (~), and (6) diffusion coated and electropla-ted to inc~ease
the thickness of -the zinc layer.
The results of the testing indicate that chemical
polising results in apprGximately the same low fatiyue life
as mechanical polising provided it is followed by heat treatment.
This indicates that chemicalpolishing removes the cold worked
layer caused by machining. Reheat treating of mechanically
polished specimens also removes the effects of prior cold work.
Since the fatigue life of mechanically polished and reheat
treated specimens is not shorter than that of chamically polished
specimens, surface roughness plays a minor role compared to
surface work hardening.
At low stress, a 2.5 ~m thick electroplated zinc
coating on a mechanically polished specimen without diffusion
has little effect on fatigue life. Fatigue life in vacuum
is significantly increased if a specimen is ~inc diffusion
coated, rather than electroplated only. Enhanced grain boundary
diffusion of zinc during coating leads to premature failure.
Heat treating of specimens (compared to untreated, as received
specimens) has little effect when tests are performed in a
3.5% NaCl solution. Zinc electroplating alone does not substantially
improve the fatigue life in a 3.5~ NaCl solution of the Al-Cu
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alloy, but does improve that of the Al-Zn alloy, Additional
electroplating following diffusion coating increases the fatigue
life in a salt solution when compared -to diffusion coatiny
only. Electroplating of a zinc diffusion coated specimen increases
the fa-tigue life in a salt solution significantly compared
to an uncoated but identical heat treated specimen, or one
in an as machined condition.
In view of the positive influence of additional electro~
plating on the corrosion fatigue life, this step is added to
the surface treatment of the alloys in order,to improve corrosion
resistance.
SUMMARY
In summary, tests have shown that a properly applied
zinc diffusion layer increases the fatigue strength in neutral
and in salt water corrosion environments. Electrolytic zinc
coatings without diffusion are ineffective for the Al Cu alloys
but increase the corrosion fatigue strengths of Al-Zn alloys.
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