I NBSIR 78-1587 (NASC)

Stress Corrosion Behavior of

7049-T76510, -T73510 and

-T7351 1 Aluminum Alloy

Extrusions

B. T. Sanderson and W. F. Gerhold

1 2I£S£ COPY

m NOT BEBfiOVE

Chemical Stability and Corrosion Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

January 1979

Prepared for Naval Air Systems Command

Department of the Navy

Washington, D.C. 20360

NBSIR 78-1587 (NASC)

STRESS CORROSION BEHAVIOR OF

7049-T76510, -T73510 AND

-T73511 ALUMINUM ALLOY

EXTRUSTIONS

B. T. Sanderson and W. F. Gerhold

Chemical Stability and Corrosion Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

January 1 979

For Official Distribution Only

Prepared for Naval Air Systems Command

Department of the Navy

Washington, D.C. 20360

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
_

'
Stress Corrosion Behavior of

7049—T7 6510, -T73510 and -T73511

Aluminum Alloy Extrusions

B. T. Sanderson and W. F. Gerhold

National Measurement Laboratory

Center for Materials Science

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D.C. 20234

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

request by AIR 52031G.

Introduction

Previous studies performed by NBS on the stress corrosion behavior

of a 7049-T73 aluminum alloy die forging concluded that the alloy had

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 a marine atmosphere after exposure for 798

days. C-ring specimens machined from similar areas of the forging had not

failed after 62 days intermittent immersion (once in every 45 minutes) in an artificial laboratory environment (3.5% sodium chloride solution). Flat

transverse tensile specimens 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 the marine atmosphere.

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

corrosion behavior of 7049-T76510, -T73510 and -T73511 aluminum alloy

extrusions in a marine environment.

Material

Three extruded 7049 aluminum alloy sections, each having a different

temper (T76510, T73510 and T73511) were submitted to NBS. Specimens in

the form of flat tensile bars were machined from the two extrusions with

the -T76510 and -T73510 tempers and round tensile bars were machined from

the 7049 -T73511 aluminum alloy extrusion. The flat specimens were approximately 6 in. (15.2 cm) long x 1.0 in. (2.54 cm) wide x 0.125 in. (3.2 mm)

thick with a 2 in. long (5.1 cm) uniform reduced section. The round

specimens having a temper of -T73511 were approximately 2 in. (5.1 cm) long

with a reduced section of 0.125 in. (3.2 mm) diameter. All specimens were machined from the extrusions so that their principal axes were in the trans- verse direction with respect to the direction of extrusion of the alloy

(long transverse direction for the flat specimens and short transverse

direction for the round specimens).

*NBSIR #74-559, Stress Corrosion Behavior of Die Forged 7049-T73 Aluminum

Alloy, E. Escalante and W. F. Gerhold, August 1974.

Metallography

The microstructures of the three different heat treated 7049

aluminum alloy materials display similar grain size and structure.

There is some evidence of recrystallization having occurred. There also

appears to be some intermetallic precipitation at the grain boundaries.

Photomicrographs of the materials are shown in figures 1, 2 and 3.

Mechanical Properties

Tests were performed by NBS to determine the transverse mechanical

properties of the three extrusions. The results of these tests are shown in Table I.

Stress Corrosion Tests

Stressed and unstressed specimens were exposed in the marine

atmosphere at Kure Beach, NC [80 ft.(24 m) lot]. A system of weights

and levers was used to obtain the desired stress on the flat test

specimens having the T76510 and T73510 tempers. A constant strain

fixture was utilized to apply the desired stress on the round specimens.

For the stressed specimens, the applied stress was equivalent to

approximately 50 and 75% of the yield strength of the alloys as determined

by NBS [67.1 Ksi (463 MPa) for the -T76510 temper, 70.2 Ksi (484 MPa)

for the T73510 temper, and 61.5 Ksi (424 MPa) for the-T735ll temper].

Results

Test results show that none of the flat 7049-T76510 nor 7049-

T73510 specimens had failed after exposures of 798 days in the marine

environment. However, one of the -T73510 tempered specimens which had

been exposed for 798 days in the marine atmosphere, while stressed at 75%

of its yield strength, developed a crack at the bolt hole where the load was applied to the specimen. The specimen fractured at this crack

during the test to determine the tensile properties after exposure to the

environment. Attempts were made to determine the cause of the crack at

its origin. Metallographio examination and microhardness tests were employed but were inconclusive due to the extent of corrosion.

Two of the three round 7049-T73511 aluminum alloy tensile specimens

which had been exposed at 75% of their yield strength for 797 days had

failed in threaded areas during exposure. Another 7049 T73511 specimen

which had also been exposed for 797 days at 50% of its yield strength

failed in the threaded area during the test to determine its tensile

strength after exposure. These failures were attributed to crevice

corrosion. None of the round 7049 -^73511 test samples had failed at the

reduced section after exposure in the marine atmosphere for 797 days.

In order to obtain some indication of the effect of corrosion attack

on these extrusions, a comparison was made of the tensile strength of

-2 -

unexposed specimens vs. that of stressed and unstressed specimens exposed

to the marine 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 test results

given in Table II indicate a small average loss in tensile strength

4.5 to 5.3% for the-T76510 temper, 3.3 to 5.0% for the -T73510 temper and

0.5 to 3.4% for the-T73511 tempered alloy.

Conclusions

The results obtained from stress corrosion tests on 7049-T76510,

7049-T73510 and 7049-T73511 aluminum alloy extrusions indicate that the

alloy is resistant to stress corrosion cracking in a marine atmosphere

environment. There were no failures attributed to stress corrosion on any of the specimens after exposure periods up to 798 days. However,

cracks or failures did develop in crevice areas where the specimens were secured to the stressing fixture. The alloy also exhibited a considerable

amount of shallow surface pitting accompanied by heavy gray corrosion

products. This surface pitting and corrosion resulted in an average

loss in strength of 5% or less.

- 3 -

TABLE I

Transverse Mechanical Properties of 7049

Extruded Aluminum Alloy

Alloy and

Treatment

Tensile Strength (1)

Ksi (2)

Yield Strength (1)

(0.2% offset) Ksi (2)

Percent

Elongation

in 2 in. (5.08 cm)

1

7049—T76510 74.9 67.1 10

7049-T73510 78.0 70.2 10

7049—T73511 73.6 61.5

(1) average of three specimens

(2) 1 Ksi = 6.8948 MPa

Table II. Average Loss in Tensile Strength of

Transverse Specimens From 7049 Aluminum Alloy Extrusions

Alloy

and

Treatment

Exposure

Percent of

Yield

Strength

Stress

Ksi

(1)

Days

Exposed

(2)

Percent loss in

Tensile Strength

Individual Average

Specimen (3)

Standard

Deviation

7049-T76510 0 0 798 NF 4.1

(long trans- 0 0 798 NF 4.0 verse

)

0 0 798 NF 5.3 4.5 + 0.4

50 33.7 798 NF 6.0

50 33.3 798 NF 5.1

50 33.6 798 NF 5.1 5.4 + 0.2

75 49.9 798 NF 6.0

75 50.2 798 NF 4.8

75 49.9 798 NF 5.2 5.3 + 0.2

7049-T73510 0 0 798 NF 4.7

(long trans- 0 0 798 NF 5.0 verse

)

0 0 798 NF 4.7 4.8 <+ 0.1

50 35.5 798 NF 3.7

50 35.5 798 NF 4.5

50 35.5 798 NF 1.7 3.3 + 1.4

75 52.2 798 NF 4.9

75 52.7 798 NF 7.6

75 52.2 798 NF 5.0 5.8 (5) + 1.6

7049-T73511 0 0 798 NF + 0.5

(short trans- 0 0 798 NF 2.2 verse

)

0 0 798 NF + 0.5 0.4 + 1.6

50 30.8 798 NF 0.1

50 30.8 798 NF (4)

50 30.8 798 NF 2.1 1.1 (5) + 1.0

75 46.1 798 NF 3.4

75 46.1 798 NF (4)

75 46.1 798 NF (4)

(1) 1 Ksi = 6.8948 MPa.

(2) NF denotes no failure after exposure for number of days shown.

(3) Average for 3 specimens unless noted otherwise.

(4) Specimen failed at thread area during test to determine tensile properties after

exposure in the marine atmosphere.

(5) Average for 2 specimens.

Figure 1 Microstructure of 7049-T76510 aluminum extrusion (long transverse

' direction). Etched, Keller's etch, X 50.

Figure 2 Microstructure of 7049-T73510 aluminum extrusion (long transverse

direction). Etched, Keller's etch, X 50.

Figure 3. Microstructure of 7049-T73511 aluminum extrusion (short-transverse

direction). Etched, Keller's etch, X 50.

HBS-114A (REV. 1 1-77)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78-1587

2. Gov’t Accession

No.

4. TITLE AND SUBTITLE

Stress Corrosion Behavior of 7049-T76510, -T73510 and -T73511

Aluminum Alloy Extrusions

7. AUTHOR(S)

B.T. Sanderson and W.F. Gerhold

3. Recipient’s Accession No.

5. Publication Date January 1979

6. Performing Organization Code

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OP STANDARDS

DEPARTMENT OP COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 5610410

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 factual summary of moat significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Transverse tensile specimens of extruded 7049 aluminum alloy with tempers of

-T76510, -T73510 and -T73511 were stressed at zero, fifty and seventy-five

percent of the alloy's yield strength and exposed in a marine environment. After

798 days, there were no failures due to stress corrosion. The extruded 7049

material studied appears to be resistant to stress corrosion cracking in this

environment.

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

Alloy; aluminum; extrusion; heat treatment; marine; mechanical properties;

stress corrosion

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