NBSIR 74-559

Stress Corrosion Behavior of Die

Forged 7049-T73 Aluminum Alloy

E. Escalante and W. F. Gerhold

Corrosion and Electrodeposition Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

August 1974

Final Report

Prepared for Naval Air Systems Command

Department of the Navy

Washington^ D. C. 20360

NBSIR 74-559

STRESS CORROSION BEHAVIOR OF DIE

FORGED 7049-T73 ALUMINUM ALLOY

E. Escalante and W. F. Gerhold

Corrosion and Electrodeposition Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Wastiington, D. C. 20234

August 1974

Final Report

Prepared for Naval Air Systems Command

Department of the Navy

Washington, D. C. 20 360

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

Stress- Corrosion

Die Forged 7049-T73

Behavior

Aluminum

of

Alloy

Reference: (a) Naval Air Systems Command, Department of the Navy

Request by AIR-52031A, March 31, 1969.

Introduction : A recently developed aluminum alloy designated 7049-T73

is reported to have relatively low quench rate sensitivity, strength in

the range of that for 7075-T6 and 7079-T6 aluminum alloys and stress

corrosion resistance equal to that of 7075-T73 aluminum alloy. Reference

(a) requested that NBS conduct tests to determine the stress corrosion

behavior in artificial laboratory and marine atmosphere environments.

NBS Report No. 10 694, "Progress Report on the Stress-Corrosion

Behavior of a Die Forged 7049-T73 Aluminum Alloy", dated February 1,

1972 contained the results obtained from stress corrosion tests con- ducted in artificial laboratory environments. This report is the final

report on the stress-corrosion behavior of the alloy and sumarizes the

results obtained from tests conducted in the marine atmosphere as well

as the previously reported tests conducted in the laboratory.

Material ; A 7049-T73 aluminum alloy die forging (DF 5532) was submitted

for test purposes. Specimens in the form of flat tensile bars were

machined from this forging so that their principal axes were in the

transverse direction with respect to the direction of working of the

forging. These specimens were approximately 4 in. long x 0.1875 in.

wide X 0.127 in. thick. Additional specimens in the form of C-rings

(1 in. wide x 0.75 in. dia. x 0.060 in. thick) were machined from the

die forging so that the area of maximum stress when applied to the Cring would be in an area adjacent to a parting line of the forging.

Mechanical Properties : The results of tests conducted by NBS to

determine the transverse mechanical properties of the alloy are given

in Table 1.
-2-

Table 1. Transverse Mechanical Properties of Die

Forged 7049-T73 Aluminum Alloy.

Tensile Strength, ksi 72.9 + 0.3

Yield Strength (0.2% Offset), ksi 65.3 + 0.3

Percent Elongation in 2 in. 5.8 + 0,6

(a) Average for six specimens with standard deviations.

Stress-Corrosion Tests ; Stress-corrosion tests were conducted in a marine environment and in an artificial laboratory environment. For

the flat tensile bar specimens a system of weights and levers was used to obtain the desired stress. The C-rings were stressed prior to

the tests using strain gages to measure the applied stress on the outer

fibers of the specimens

.

In the laboratory tests, the specimens were encased in polyethylene

cells which were used to contain the corroding media. The specimens

were immersed intermittently, once in every 45 minutes, by pumping the

corrodent into the cell and then allowing it to drain back into a reservoir. The corrodent was a straight 3.5% sodium chloride solution.

For the marine environment, the specimens were exposed in the atmosphere

at Kure Beach, North Carolina, 80-ft. lot.

Stressed and unstressed specimens were exposed in all of the

environments. In the laboratory, the applied stress for the flat

tensile bars was equivalent to 75% of the yield strength of the alloy,

while the stress applied to the C-rings was equivalent to 50 or 75%

of the yield strength of the alloy. For specimens exposed in the marine

atmosphere the exposure stress for both flat tensile bars and C-ring

specimens was equivalent to 50 or 75% of the yield strength of the alloy.

Results

:

The results summarized in Table 2, indicate that the alloy

has good resistance to stress-corrosion cracking. However, C-ring

specimens machined from parting line areas of the forging that had been

stressed to 75% of the yield strength of the alloy and exposed in the

marine atmosphere had failed after exposure for 798 days. Metallographic

examination of sections obtained from areas where failure was noted

revealed secondary intergranular cracking adjacent to the main crack.

An example is shown in Figure la. Similar specimens exposed intermittently
-3-

for 62 days in an aqueous 3.5% sodium chloride environment were found to

be severely pitted by corrosion along the grain boundaries (Figure lb),

particularly in areas at and adjacent to the zone of maximum tensile

stress on the specimens.

Transverse specimens, machined from other areas of the forging

and stressed to 75% of the yield strength of the alloy, had not failed

after exposure for 798 days in the marine atmosphere or 62 days in the

aqueous 3.5% sodium chloride solution.

In order to ascertain the severity of corrosion attack on the

alloy a comparison was made of the tensile properties of the flat

tensile bars; i.e., the original tensile strength obtained from similar

specimens not exposed to a corrosive environments vs that obtained from

specimens after exposure to the environment. The values obtained were

then averaged for each condition and stress level and calculated as the percent loss in tensile strength due to exposure in the environment.

These determinations, given in Table 2, show that for the stressed

specimens exposed in the 3,5% sodium chloride solution (laboratory

environment) , the loss in tensile strength was nearly twice that of the

unstressed specimens exposed in the same environment. Examination of

these specimens revealed scattered localized attack in the form of

pitting corrosion. While the frequency of the pitting corrosion was

less on the stressed specimens than on the unstressed specimens the

attack was, in general, more severe. Of the stressed and unstressed

specimens exposed in the marine environment there was little or no difference in the loss in tensile strength nor in the frequency or depth of pitting from corrosion. For specimens exposed in either

environment, the corrosion products formed were quite adherent and

difficult to remove using normal cleaning methods.

Conclusions ; The results obtained from stress-corrosion tests con- ducted on die forged aluminum alloy 7049-T73 indicate that the alloy

has good resistance to stress corrosion cracking except at areas immediately adjacent to the parting line. Transverse C-ring test

specimens machined from the forging at areas adjacent to the parting

line and with the maximum stress (75% of the yield strength of the alloy)

applied at these areas were found to have failed in the marine atmosphere

after exposure for 798 days. Similar specimens machined from similar

areas and with the same applied stress had not failed after exposure

for 62 days in an artificial laboratory environment (3.5% sodium

chloride solution) . Flat transverse tensile bars machined from areas remote from the parting line of the forging, had not failed after

exposure for 62 days in the laboratory environment or 798 days in

in the marine atmosphere. The alloy was found to be prone to. pitting

corrosion in both environments.

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NBS-IUA (REV. 7-73)

U.S. DEPT. OF COMM. 1. PUBLICATION OR RKPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA No. SHEET NBSIR 74-559

3. Recipient's Acces.sion No.

4. TITLE AND SUBTITLE

Stress Corrosion Behavior of Die Forged 7049-T73 Aluminum

Alloy

5. Publication Date August 1974

6. Performing Organization Code

^i^f^fscalante and W. F. Gerhold

8. Performing Organ. Report No. NBSIR VL.-SSQ

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

'

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Naval Air Systems Command

Department of the Navy

Washington, D.C. 20360

13. Type of Report & Period ! Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less tactual summary ol most sigftitlcant informatiort. If document irtclades a si^ilicant

bibliography or literature survey, mention it here.)

Stress-corrosion data are presented for a die forged aluminum alloy 7049-T73. Studies

were conducted in artificial laboratory and marine atmosphere environments. Test

specimens included flat tensile bars and C-rings.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Aluminum; atmosphere; C-rings; die forging; laboratory; marine; stress-corrosion;

tensile bars

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