CA1285857C - Method of surface treating aluminum alloy - Google Patents
Method of surface treating aluminum alloyInfo
- Publication number
- CA1285857C CA1285857C CA000519568A CA519568A CA1285857C CA 1285857 C CA1285857 C CA 1285857C CA 000519568 A CA000519568 A CA 000519568A CA 519568 A CA519568 A CA 519568A CA 1285857 C CA1285857 C CA 1285857C
- Authority
- CA
- Canada
- Prior art keywords
- alloy
- zinc
- product
- temperature
- layer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/10—Electroplating with more than one layer of the same or of different metals
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D3/00—Electroplating: Baths therefor
- C25D3/02—Electroplating: Baths therefor from solutions
- C25D3/22—Electroplating: Baths therefor from solutions of zinc
- C25D3/24—Electroplating: Baths therefor from solutions of zinc from cyanide baths
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Health & Medical Sciences (AREA)
- Toxicology (AREA)
- Electroplating Methods And Accessories (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The corrosion fatigue resistance of an aluminum alloy is increased by cleaning the surface of the alloy, zincating such surface, depositing a layer of zinc on the surface of the alloy, heating the resulting product at a first temperature of 350 - 420°C to anneal the product, heating the product at a second temperature of 492 - 495°C in a vacuum to solution anneal the product, quenching the product with water, aging the product, and depositing a second layer of zinc on the surface of the alloy.
The corrosion fatigue resistance of an aluminum alloy is increased by cleaning the surface of the alloy, zincating such surface, depositing a layer of zinc on the surface of the alloy, heating the resulting product at a first temperature of 350 - 420°C to anneal the product, heating the product at a second temperature of 492 - 495°C in a vacuum to solution anneal the product, quenching the product with water, aging the product, and depositing a second layer of zinc on the surface of the alloy.
Description
~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.
:
. : . . . :
5~3Si7 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.
d ,.. ' : ` : . ' :' , . , . '` ' . .
, . ', - : . ' ,. : .' ,' `. . : ' ' .
' ~
'- ' '' ' ~ ' ', ' ' ~ ' ~l~8~ i7 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.
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.
:
. : . . . :
5~3Si7 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.
d ,.. ' : ` : . ' :' , . , . '` ' . .
, . ', - : . ' ,. : .' ,' `. . : ' ' .
' ~
'- ' '' ' ~ ' ', ' ' ~ ' ~l~8~ i7 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 '' - : ' ' ' : ' . -, : - ' : . .
gL2~5~3~7 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 .~ ' .
'. , ' ',: " ' '~ , ' -~2~ 7 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 -,~
.
:- . - ,, . - ~ .
. ~ ~ . . . . : :
~Z~3S8S7 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 .
'~
.~1 . : . -. -.... . . . : ,.
.
:: . .
.. .. . ~ . - ,, . , - , , : ~ ' ' ' . : . .: -~85~7 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.
" , ,:
- . : ' ~ :, , -, . . , ,, ~ . :
, ' , ', ~' ~, .
The cleaning of the alloys includes acid cleaning as outlined in the literature ~Metals Handbook, 9th Ed., Vol. 5 '' - : ' ' ' : ' . -, : - ' : . .
gL2~5~3~7 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 .~ ' .
'. , ' ',: " ' '~ , ' -~2~ 7 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 -,~
.
:- . - ,, . - ~ .
. ~ ~ . . . . : :
~Z~3S8S7 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 .
'~
.~1 . : . -. -.... . . . : ,.
.
:: . .
.. .. . ~ . - ,, . , - , , : ~ ' ' ' . : . .: -~85~7 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.
" , ,:
- . : ' ~ :, , -, . . , ,, ~ . :
, ' , ', ~' ~, .
Claims (6)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. 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 temperature, and (g) depositing a second layer of zinc on the surface of the alloy.
(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 temperature, and (g) depositing a second layer of zinc on the surface of the alloy.
2. A process according to claim 1, wherein the zinc layer is deposited by electroplating the alloy in a zinc cyanide bath containing approximately 51 g/l Zn(CN)2, 42 g/l NaCN, 79 g/l NaOH and 15 g/l Na2CO3.
3. A process according to claim 2, wherein electroplating is continued for up to 5 minutes to produce a zinc coating thickness of up to 0.12 mm.
4. A process according to claim 2, wherein electroplating is continued for approximately 5 minutes to produce a zinc coating thickness of 1.5 to 2.5 µm.
5. A process according to claim 1, wherein the first heating step is effected at a temperature of 350 to 420°C.
6. A process according to claim 1, 2 or 3, wherein the alloy contains approximately 4.5% Cu, 1.5% Mg and 0.6% Mn, the balance being aluminum.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000519568A CA1285857C (en) | 1986-10-01 | 1986-10-01 | Method of surface treating aluminum alloy |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000519568A CA1285857C (en) | 1986-10-01 | 1986-10-01 | Method of surface treating aluminum alloy |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1285857C true CA1285857C (en) | 1991-07-09 |
Family
ID=4134063
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000519568A Expired - Lifetime CA1285857C (en) | 1986-10-01 | 1986-10-01 | Method of surface treating aluminum alloy |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1285857C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1624093A1 (en) * | 2004-08-04 | 2006-02-08 | Aluminal Oberflächentechnik GmbH & Co. KG | Coating of substrates of light metals or light metal alloys |
-
1986
- 1986-10-01 CA CA000519568A patent/CA1285857C/en not_active Expired - Lifetime
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1624093A1 (en) * | 2004-08-04 | 2006-02-08 | Aluminal Oberflächentechnik GmbH & Co. KG | Coating of substrates of light metals or light metal alloys |
| WO2006013184A1 (en) * | 2004-08-04 | 2006-02-09 | Aluminal Oberflächentechnik Gmbh & Co. Kg | Coating of substrates made of light metals or light metal alloys |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed |