r
AD-766 328
COMPARISON OF ALUMINUM ALLOY 7050, 7049, MA52,
AND 7175-T736 DIE FORGINGS
ALUMINUM COMPANY OF AMERICA
PREPARED FOR
AIR FORCE MATERIALS LABORATORY
MAY 1973
DISTRIBUTED BY:
National Technical Informatimn ServiceU. S. DEPARTMENT OF COMMERCE
U
AFML-TR-73-34
COMPARISON OF ALUMINUM AI,LOY 7050,
7049. MA52. AND 7175-T736DIE FORGINGS
J. T. StaleyAlcoa/Alcoa Laboratories
TECHNICAL REPORT AFML-TR-73-34
May !973
7
Air Force Materials LaboratoryAir Force Systems Command
Wright-Patterson Air Force Base. Ohio 45433
Rtproduced by
NATIONAL TECHNICALINFORMATION SERVICE
UJ S Dertlent of CommerceSpr;".fiel, VA 22151
APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED
NOTICE
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or permission to manufacture, use, or sell any patented invention that may in any way be
related thereto.
A
Copies of this report should not be returned unless returr is required by security considerations,
contractual obligations, or notice on a specific document.
-u)
.:-o-
Secunty Classificstion .n, ,i, a .a io
DOCUMENT CONTROL DATA -R & D_($vcuriry Classification of title, body of abst ract and Indoringt .,nora;on must beentered when the overal!l report is clssl(lod
ORIGINATING ACTIVITY (Corpore author) 20. REPORT SECURITY CLASSIFICATION
Aluminum Company of America UAlcoa Laboratories
1ce
Alcoa Center, Pa. 150693 REPORT TITLE j
COMPARISON OF ALUMINUM ALLOY 7050, 7049, MA52, AND 7175-T736 A
DIE YIRGINGS 2
4 DESCRIPTIVE NOTES (ryp* olteport and Inclusive date.)
Final Technical Report June 1, 1971 - December 31. 1972S AU THORIS) (First name, middle initial, last name)
James T. Staley
6. REPORT DATE 7s. TOTAL NO. OF PAGES 0b. NO.OF REFS
May 1973 115 1 11C8 CONTRACT OR GP.ANT NO. 9a. ORIGINATOR'S REPORT NUMBERISI
F33615-69-C-1644b. PROJECT NO.
C. Ob. OTHER REPORT NO(SI (Any other numbers that may be &esttedthis teport)
d. AFML-TR-73-3410 DISTRIBUTION STATEMENT
Approved for public release; distribution unlimited
It. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY
Air Force Materials LaboratoryWright-Patterson Air Force Base,Ohio 45433
13. ABSTRACT
Die forgings in aluminum alloys 7050, 7049, and MA52 were fabricatedand evaluated for resistance to stress-corrosion cracking, quenchsensitivity, and fracture toughness. In addition, all Alcoa data on7050, 7049, and special process 7175-T7X die forgings were examined andthe properties were collated. Stress-corrosion resistances wereevaluated using the severest combinations of forging type and testconditions.
All of these newer alloys were less quench sensitive than alloy 7075,and all developed better combinations of resistance to stress-corrosion.cracking and fracture toughness than 7075-T6 and 7079-T6 at equalsti'engths. Because it developed the best combination 'of properties,alloy 7050 is a preferred selection for use as die forgings ofrelatively heavy section thickness for the aerospace industry. Thisalloy also can be supplied as hand forgings, plate, extrusions, andsheet. Special process 7175 is an equally good selection for dieforgings of thin to moderate section thickness.
D D , N O V. 5 1 4 7 3 _ _ _ _ _ _ _ l a s i f c a i o
Security Classification
LEY INK A LINK I LINK C
Alloy
7049 '7175Stress-corrosion
KcIC
Die Forging
c i
Security Classification
AFML-TR-73-34
COMPARISON OF ALUMINUM ALLOY 7050,7049, MA52, AND 7175-T736
DIE FORGINGS
J. T. Staley
A
Approved for public release;distribution unlimited
AIR FORCE MATERIALS LABORATORYAIR FORCE SYSTEMS COMMAND
WRIGHT-PATTERSON AIR FORCE BASE, OHIO 45433
%,'1
FORE WORDA
This report was prepared by the Physical Metallurgy A
')ivision, Alcoa Research Laboratorics, New Kensington,
Pennsylvania under contract F3361S-69-C-1644. The contract
was initiated under Project No. 7351, "Ietallic ;Iaterials",
Tas. No. 735105, "High Strength Metallic Haterials'". The
program is monitored by the Air Force Materials Laboratory,
vith Dr. T. 1. F. Ronald, AFII,/LLS, as Project Engineer.
This report covers the period June 1, 1971 through December 31,
1972 and was released by the author in February 1973.
This technical report has been reviewed and is approved.
c. :I. PIERCEActg. ChiefMetal and Ceramic Synthesis BranchMetals and Ceramics DivisionAir Force Materials Laboratory
xI
ABSTRACT
Die forgings in aluminum alloys 7050, 7049, and MA52were fabricated and evaluated for resistance to stress-corrosioncracking, quench sensitivity, and fracture toughness. Inaddition, all Alcoa data on 7050, 7049, and special process7175-T7X die forgings were examined and the properties werecollated. Stress-corrosion resistances were evaluated using theseverest combinations of forging type and test conditions.
All of these newer alloys were less quench sensitivethan alloy 7075, and all developed better combinations of resist-ance to stress-corrosion cracking and fracture toughness than7075-T6 and 7079-T6 at equal strengths. They ranked as followson the basis of these criteria.
Criteria Rank
Resistance to SCC, 1-7050 and 7175; 2-7049 and MA52365-500 daysnatural environment
Resistance to SCC, 1-7050; 2-7049; 3-7175 and MA5284 days 3.5% NaCl
Resistance to SCC, 1-7175; 2-7050 and 7049; 3-MA52
30 days 3.5% NaCl
Low Quench Sensitivity l-MA52; 2-7050; 3-7175 and 7049
Fracture Toughness All equal and greater than 7075-T6and 7079-T6
This analysis indicates that alloy 7050 is a preferredselection for use as die forgings of relatively heavy sectionthickness for the aerospace industry. This alloy alsn can besupplied as hand forgings, plate, extrusions, and sheet. Specialprocess 7175 is an equally good selection for die forgings ofthin to moderate section thickness.
~iii
TABLE OF CONTENTS
SectionNo. Page
I. INTRODUCTION 1
II. MATERIAL 3
1. CURRENT WORK 3
2. OTHER INVESTIGATIONS 4
a. 7050 4
b. 7049 4
c. 7175 5 1
III. TEST PROCEDURES 6
1. FRACTURE TOUGHNESS 6
2. STRESS CORROSION 6
IV. RESULTS AND ANALYSES 9
1. TENSILE PROPERTIES 9
2. FRACTURE TOUGHNESS 9
3. STRESS CORROSION 10
a. Factors Affecting SCC TestPerformance 10
b. mean Critical Yield Strength 11
c. Mean Critical Stress 11
d. Effects of Strength and Stress 12
e. Results of SCC Analyses 13
V. SUMMARY 17
VI. CONCLUSIONS 18
VII. RECOMMENDATION 19
VIII. REFERENCES 20
iv j ----
TABLE OF CONTENTS (CONTINUED)
SectionNo. Page
IX. APPENDIX I 81Properties and Heat Treating Conditionsof Die Forgings Not Produced for thisContract
X. APPENDIX II 113Probit Analysis
A
v
LIST OF TABLES
No.
I. Melt Analyses
Ii. Results of Metallographic Examination of Ingots
III. Transverse Tensile Properties of Alloy 7049 inDie No. 15093
IV. Transverse Tensile Properties of Alloy MA52 inDie No. 15093
V. Transverse Tensile Properties of Alloy 7050 inDie No. 15093
VI. Results of Corrosion Tests on 7175-T7X DieForgings Exposed 84 Days to 3.5% NaCl A.I.
VII. Results of Corrosion Tests on 7175-T7X DieForgings Exposed 84 Days to Controlled A.I.Federal Test Method 823
VIII. Effect of Specimen Location and Size on Stress-Corrosion Performance of 7050 Die ForgingsExposed 84 Day-; to A.I., Federal Test Method 823
IX. Results of Corrosion Tests on 7049-T73 and 7050Die Forgings Die 9619
X. Properties of 7050 Alloy Die No. 9078 Forgings
XI. Properties of 7049 Alloy Die No. 9078 Forgings
XII. Properties of MA52 Alloy Die No. 9078 Forgings
XIII. Tensile Pr'perties of Forgings Alcoa Die No. 9078
XIV. Tensile Properties of 7050 Forgings in AlcoaDie 15093
XV. Tensile Properties of 7049 Forgings in AlcoaDie 15093
XVI. Tensile Properties of MA52 Forgings in AlcoaDie 15093
XVII. Results of Fracture Toughness Tests on Forgings
vi
LIST OF TABLES (CON"INUED)
NO.
XVIII. Results of Corrosion Tests of Die 9078 Forgings
Exposed in A.I. Federal Test Method 823
XIX. Results of Corrosion Tests of Die 9078 ForgingsExposed in New Kensington Atmpsphere
XX. Stress-Corrosion Performance of Alloy 7049 inAlcoa Die 15093
XXI. Stress-Corrosion Performance of Al?.oy fMIA52 inAlcoa Die 15093
XXII. Stress-Corrosion Performance of Alloy 7050 in
Alcoa Die 15093
XXIII. Results of Stress-Corrosion Tests, Exposure of
7175-T7X Die Forgings to New Kensington
Atmosphere
XXIV. Results of Corrosion Tests of 7050 Die Forgings
Exposed in A.I. Federal Test Method 823
XXV. Results of Corrosion Tests on 7050 Die ForgingsExposed at Least One Year in New KensingtonAtmosphere
XXVI. Results of Corrosion Tests of 7049-T7X Die ForgingsExposed 84 Days to A.I. Federal Test Method 823
XXVII. Results of Stress-Corrosion Tests on 7049-T7X DieForgings Die No. 9078
XXVIII. Results of Stress-Corrosion Tests on 7C4<-!X DieForgings S-399301, Die No. 9078 Processed bySpecial Fabricating Practice
XXIX. Results of Corrosion Tests on 7049-T7X Die ForgingsDie 9078
XXX. Effect of Forging Type and Test Conditions on MeanCritical Yield Strength of 7050 Die Forgings
XXXI. Effect of Forging Type and Test Conditions on Me-inCritical Yield 3trength of 7049 Die Forgings
XXXII. Effect of Forging Type and Test Conditions on MeanCritical Yield Strength of MIA52 Die Forgings
vii
L j
LIST OF TABLES (CONTINUED) - APPENDIX I
No.
XXXIII. Mechanical Properties of 7075-T7X in Die No. 9619
XXXIV. Tensile Properties of Plant Aged 7050 Die ForgingNo. 15789
XXXV, Tensile Properties and Electrical Conductivities ofARL Aged 7050 Die Forging No. 15789
XXXVI. Tensile Properties and Electrical Conductivity ofPlant Aged 7050 Die Forgings 8457
XXXVII. Tensile Properties of 7050 Die Forgings No. 8457Aged at ARL
XXXVIII. Tensile Properties and Electrical Conductivities of7050 Forgings Die No. 10853
XXXIX. Tensile Properties of 7050 Die Forging No. 15093
XL. Tensile Properties of Plant Aged 7050 Die
Forgings 9078
XLI. Tensile Properties of ARL Aged 7050 Die Forging 9078
XLII. Tensile Properties of a Plant Aged 7050 ForgingAlcoa Die No. 15789
XLIII. Mechanical Property Tests 7049-T7X Die ForgingsDie No. 9078
XLIV. Tensile Tests on 7049-T7X Die Forgings Die No. 9078
XLV. Mechanical Properties of 7049-T7X Die ForgingDie No. 15621
XLVI. Mechanical Properties 7049-T7X Die ForgingsDie No. 16347
XLVII. Tensile Properties of 7175-T7X Die Forgings
viii
LIST OF ILLUSTRATIONS
No.
1. Boeing Rib Forging Hinge Support Elevator StationAlcoa Die 9078
2. McDonnell-Douglas Nose Landing Gear CylinderAlcoa Die 15093
3. 7050 Ingot - 15" Diameter
4. 7049 Ingot - 16" Diameter
5. MA52 Ingot - 16" Diameter
6. Macrostructure of 7050 Ingot
7. Macrostructure of 7050 Ingot
1 8. Macrostructure of MA52 Ingot
9. Macrostructure of MA52 Ingot
10. Macrostructure of 7049 Ingot10. Macrostructure of 7049 Ingot
12. Compact Tension Fracture Toughness Specimen
13. Measured Percent Survival
14. Specimen Location in Alcoa Die Nos. 9078 and 8457J
15. Quench Sensitivity Curves
16. Fracture Toughness of Die Forgings
17. Days to Failure vs Short-Transverse Yield Strength
18. Percent Survival vs Short-Transverse Yield Strength
19. Graphical Means of Determining Mean Critical Stress
20. Percent Survival vs Yield Strength, Web-FlangeDie Forgings
21. Stress vs Critical Strength, Web-Flange Die Forgings
22. Stress vs Critical Strength, 30 Days 3.5% NaCl, A.I.
ix
LIST OF ILLUSTRATIONS (CONTINUED)
No.
23. Stress vs Critical Strength, 84 Days 3.5% NaCi, A.I.
24. Stress vs Critical Strength, Web-Flange Die Forgings
25. Percent Survival vs Time in Industrial Atmosphere
26. Percent Survival vs Time in Alternate Immersion Test
LIST OF ILLUSTRATIONS - APPENDIX I
27. Landing Gear Part - Die No. 9619
28. Landing Gear Part - Die No. 9619
29. Web-Flange Die Forging - Die No. 9078
30. Web-Flange Die Forging - Die No. 9078
31. Landing Gear Cylinder - Die 15621
32. Landing Gear Cylinder - Die 15621
33. Landing Gear Cylinder - Die 16347
34. Landing Gear Cylinder - Die 163 7
35. 7050 Alloy Test Locations - Die No. 10853
36. Test Bar Locations, Alcoa Die 15093
37. Location of Short-Transverse SCC Specimens in DieForging 8457
38. Die No. 15789
39. SCC Specimen Location - Die No. 15789
40. Test Bar Locations, Alcoa Die 15093
LIST OF ILLUSTRATIONS - APPENDIX II
41. Method of Calculating Overall Probability ofPassing SCC Test
x
oi
SECTION I
INTRODUCTION
Four aluminum alloys have emerged as potential solutionsto the need for aluminum-base material with a combination ofstrength, fracture toughness and resistance to stress-corrosioncracking superio: to combinations provided by 7075. These are7175-T736, 7049-T73, 7050, and an alloy (designated MA52 in thisreport) representing the compositions selected in two independentinvestigations under U. S. Air Force contracts.
The four alloys are all of the Al-Zn-Mg-Cu type. Alloy7175-T736 forgings employ the higher purity 7175 modification of7075 coupled with special processing conditions from ingot tofinal heat treated product. Alloy 7049-T73 is a variant withhigher Zn content and decreased Cu, Cr, Fe and Si contents relativeto 7075.1 Alloy 7050, developed under U. S. Navy contracts
2 ,3 ,4
and the first phase of this contract,5 has increased Zn and Cucontents relative to 7075 with Zr in place of Cr and low impurities,Fe and Si, specified. The MA52 composition, representing theselections of two contract programs, has increased Zn and decreasedCu contents relative to 7075 and Zr plus Mn in place of Cr.6 ,7
These alloys are in various stages of development andapplication. Alloy 7175-T736 forgings were developed first andare being used in many applications. Guaranteed mechanicalproperties are higher than those of any other stress-corrosionresistant aluminum alloy forged material. The stress-corrosionacceptance test criterion is a 30-day 3.5% NaCl alternate immersiontest at a stress level of 35 ksi. Alloy 7049-T73 forgings weredeveloped later and are being used in some applications. Mechanicalproperties are substantially higher than those of 7075-T73 forgings.The stress-corrosion acceptance test criterion is a 30-day 3.5%NaCl alternate immersion test at a stress level of approximately45 ksi (75% of the minimum, guaranteed longitudinal yield strength).Alloys 7050 and MA52 are being evaluated as forgings by severalproducers. Guaranteed mechanical properties and stress-corrosionacceptance criteria have not been established.
These alloys were developed using different productsand were initially evaluated using different stress-corrosiontests. Alloy 7175-T736 was developed using rapidly quenched dieforgings of three-inch maximum section thickness, most of whichhave a pronounced grain directionality; resistance to SCC wasinitially established using test conditions less severe than thosespecified later by Federal Test Method 823. Alloy 7050 wasdeveloped primarily using plate and all SCC tests were performed
1 .!1
according to Federal Test Method 823. Alloy 7049 was developedusing hot water quenched hdnd forgings and die forgings whichdid not have pronounced grain directionality, and SCC testsoften included C-rings and 0.225" diameter tension specimenswhich are less sensitive than the 0.125" diameter specimensused in developing 7175 and 7050. Alloy MA52 was developed usingthin plate and various die forgings. SCC test specimens wereused which are less critical than the 0.125" diameter tensionspecimens. In addition, and perhaps most significantly, atmosphericexposure data were unavailable. Consequently, initial comparisonof the relative merits of these alloys was highly speculative.
The purpose of Phase II of this contract was to produce7050, 7049,and MA52 in the same die forgings, to evaluate themunder identical test conditions, and to compare properties withthose of commercially established aluminum alloys 7075 and 7079.In addition, all Alcoa data from other die forgings in thesealloys were analyzed and results are compared with those ofspecial processed 7175 forgings used in the development of7175-T736.
To permit evaluation of the stress-corrosion testperformance of the forgings which were not always tested at thesame strength and stress levels, a new analytical method wasdeveloped.
In accordance with the philosophy at Alcoa, the severestcriteria were imposed when rating resistance to stress-corrosioncracking. The most critical stress-corrosion cracking tests ofdie forgings which exhibited pronounced grain directionality wereused in the analysis.
In addition to resistance to stress-corrosion cracking,the alloys are ranked on the basis of quench sensitivity andfracture toughness.
2
SECTION II
MATERIAL
1. CURRENT WORK
For this contract, 7049, 7050,and MA52 were evaluatedin two shapes. Alcoa die 9078, Figure 1, is a Boeing rib forginghinge support elevator station. Alcoa Die 15093, Figure 2, isa McDonnell-Douglas nose landing gear cylinder. This forgingis normally rough ma-hined by boring out the cylinder before heattreatment, but in this experiment it was not machined so thattensile properties in the center could be determined.
Alloys 7049 and MA52 were cast as 16" diameter D.C. ingotsand were homogenized 36 hours at 870-880 F and ultrasonicallyinspected. A 15" diameter homogenized ingot of 7050 that wasavailable from plant stock was applied to this project. Chemicalanalyses of the melts are presented in Table I, along withcomposition limits. Analyses of 7050 and 7049 were well withinthe limits, and analysis of alloy MA52 was within limits ofboth Boeing Alloy 21,6 and the composition recommended hyReynolds.
7
Ingot macrostructures are presented in Figures 3, 4 and5, and results of metallographic examinations to determinedendrite cell sizes and amount of porosity are presented inTable II. Grain morphology of the 7050 and MA52 inoots wascompletely equiaxed, but some twin columnar grains were apparentin the 7049 ingot. Dendrite cell sizes of 7049 and the M)52ingots were about .002", while cell size of the 7050 ingotwas about .001". No significant amount of porosity was observed.
The forgings were fabricated using practices standardfor high strength 7XXX alloy forgings. Ingots that were forgedin die 9078 were extruded prior to forging; ingots that wereforged in die 15093 were preforged in blocker dies.
All forgings produced in die 9078 were sound and developedhigh tensile elongation values, but alloys 7049 and MA52 forgingsin die 15093 developed low transverse elongation values (TablesIII, IV, V), and metallographic examination disclosed porosity.Consequently, additional ingots of 7050, 7049, and MA52 werecast and new forgings were fabricated.
Structures of the preheated (52 hours at 880 F) 15"diameter ingots were examined at the top and bottom of theingot. Etched slices, Figures 6 through 11, revealed twincolumnar grains extending from the cast surface of slices from
3
both ends of the 7050 and MA52 ingots and if a slice from oneend of the 7049 ingot. Equiaxed grain size near the center ofthe ingots ranged from microscopic to 1/4" in diameter. Dendritearm spacings were measured near the center of the ingot and atmidradius. Spacings were about 1-2 mils. Some porosity upto .003" in the longest dimension was observed in each ingot.
Solution heat treatment practices of the forgings in die9078 and die 15093 are described in the tables giving tensileproperties. Forgings in die 9078 were quenched in water-at atemperature of either 150 F or 212 F, while forgings in die 15093were quenched in water at 150 F. After 4-5 days at room tempera-ture, they were aged 24 hours at 250 F.
The second-step aging practice was applied in the laboratory.Sections from the web of die 9078 forging were aged 4 to 45 hoursat 340 F. Using longitudinal tensile properties as a guide,half-forgings were subsequently aged various times at 340 F inan attempt to provide equal strength in the three alloys. Two ofthe 7050 forgings did not attain target strength, so reservesections were used to provide additional material. Whole die15093 forgings were aged either 10 to 60 hours at 340 F (firettrial) or 3 to 12 hours at 350 F (second trial).
2. OTHER INVESTIGATIONS
Photographs or sketches of the forgings and mechanical prop-erties are presented in Appendix I.
a. 7050
Independently of this contract, alloy 7050 was evaluated inweb-flange type forgings having highly directional grain flow atthe test specimen location (Die Nos. 9078, 8457, 15789); inlanding gear type forgings (Die Nos. 9619 and 15093); in a thick,bulky die forging (Die No. 10853); and in a laboratory-fabricateddie forging (Die No. 783). Forgings were prepared in Die No.9078 using proprietary Alcoa practices and forgings in Die No.8457 were fabricated using both conventional and proprietaryAlcoa practices, but the conventionally fabricated materials wereproduced about two years after the specially processed materials.The other forgings were fabricated using conventional practices.
b. 7049
Alloy 7049 has been evaluated at Alcoa in web-flange typeforgings having highly directional grain flow at the test specimenlocation (Die No. 9078) and in landing gear type forgings (DieNos. 9619, 15621, 16347). Forgings were prepared in Die No.9078 using both conventional and proprietary Alcoa practices,and the other forgings were fabricated using conventionalpractices.
4
c. 7175
The 7175-T7X data were obtained from production 7175-T736forgings as well as from forgings used in the development ofthis material. Most of the data is from forgings that receivedless aging than is used for production forgings. Therefore,these test results rhould not be considered representative ofproduction 7175-T736 forgings. The data do, however, providea basis for comparing the characteristics of the newer alloyswith special process 7175 forgings at various strength levels.
5
SECTION III
TEST PROCEDURES
1. FRACTURE TOUGHNESS
Fracture toughness was evaluated using compact tensionspecimens of the type illustrated in Figure 12. The largestspecimen obtainable was used in all cases and specimens weremachined from cylinder, strut, web and flange areas in thevarious die forgings. Values of Ko that were not strictlyvalid KIc were used in the analysis because the KQ values wereconsidered to be good estimates of KIc.
2. STRESS CORROSION
Because alternate immersion test conditions; forging type;and specimen type, size and location in the forging were notconstant in all investigations, effects of these factors wereexamined before analysis of the data was begun. The goal wasto select data for analysis which provided the most criticaltest of susceptibility to stress-corrosion cracking.
Past experience indicated, and results in this contractconfirmed, that stress-corrosion test performance of materialsevaluated using C-rings was substantially higher than testperformance of the same material using tension specimens.Consequently, results of C-ring tests were not considered inthe analysis.
Preliminary inspection of the data indicated that stress-corrosion performance of 7175-T7X forgings tested by alternateimmersion (10 minutes in solution, 50 minutes drying) using ahigh purity solution with controlled humidity and room temper-ature according to Federal Test Method 823 differed from per-formance of similar forgings tested using New Kensington tapwater (<200 ppm total solids) and 99.7% pure NaCl (0.1 max. Nal)and ambient temperature and humidity. (The pH of the lowerpurity solution was 6.4 to 7.2 and evaporation losses weremade up once a month.) Survival rates in both environments weresimilar for test periods of about 30 days, but surrival ratesin the controlled test were substantially lower after periodsapproaching 84 days. This phenomenon is illustrated in Figure 13,using data from Tables VI and VI. Only data that were determinedunder the controlled conditions were used in the analysis.Consequently, any susceptibility to stress-corrosion crackingwas emphasized.
6
Inspection of the data also revealed that specimen locationhad an effect on stress-corrosion performance of web-flangeforgings which had a high degree of grain directionality inthe flange near the flash. The data in Table VIII reveal that100% of 1/8" diameter 7050 specimens removed immediately adjacentto the flash (Figure 14) of Die Nos. 9078 and 8457 forgingsfailed in less than 30 days in the alternate immersion testat a stress of 30 ksi. In contrast, 100% of the specimens takenimmediately behind this location passed the same test. Becauseof this strong effect, only results of tests of specimens takenadjacent to the flash were used in the analyses of the stress-corrosion resistance of web-flange forgings. Omitting resultsof tests of specimens in other locations decreased the proportionof specimens that survived the test.
Contrary to the effect in web-flange forgings, specimenlocation in landing gear cylinder forgings had no apparent effecton SCC test results. Performances of specimens from the strut,trunnion or cylinder of 7049 and 7050 landing gear cylinders,Die No. 9619 (Table IX),were similar so test results of specimenstaken from all locations of this type forgings were used in theanalyses.
Increasing specimen diameter also significantly affectsstress-corrosion performance. Fourteen 1/4" diameter 7050 tensionspecimens were removed adjacent to the flash of two Die No. 9078forgings (Figure 14) and were stressed at 25 to 45 ksi. All butone of these specimens passed the 30-day alternate immersionstress-corrosion test (Table VII). In contrast, 100% of the 1/8"diameter specimens removed adjacent to the flash of the sameforgings failed in less than 30 days. Because of the effect ofspecimen diameter, only the results of tests of 1/8" diameterspecimens were analyzed. Limiting the analysis to results oftests of the smaller specimens also decreased the proportion ofspecimens surviving the test.
In the case of the exposure in the New Kensington atmosphere,no interpretation of the failures was required because pastexperienced indicated, and spot checking confirmed, that fracturedspecimens would exhibit evidence of the characteristic inter-granular secondary cracks typical of clazsical stress-corrosioncracking. Metallographic examination of specimens exposed inthe 3.5% NaCl alternate immersion test revealed in some casesdeep pitting associated with secondary cracks that did notresemble the cracks of specimens exposed in the natural environ-ment. In small experiments consisting of relatively few specimens,such failures are sometimes regarded as "no-test." All of theapproximately 1,000 specimens which fractured during the stress-corrosion tests in this summary, however, were not examinedmetallographically. Moreover, of those that were examined,
7
interpretation of the nature of the secondary cracks in manyof the specimens was not straightforward. Consequently, all.specimens that fractured were considered as failures for thisanalysis. Because stress-corrosion cracking may not have beenresponsible for all of the failures, the percentage of specimensthat failed the accelerated test may be exaggerated.
8
;iU
SECTION VJ
RESULTS AND ANALYSES
1. TENSILE PROPERTIES
Tensile properties of the die 9078 forgings that were quenchedin water at either 150 F or 212 F, Tables X-XII, revealed thatalloy MA52 was the least sensitive to quench rate and alloy 7050was intermediate. Quench sensitivity of alloy 7049 was so highthat tensile properties of the forgings quenched in boiling waterwere too low to be useful. Tensile properties of the die 9078forgings that were used in the stress-corrosion evaluation arepresented in Table XIII.
A wide range of strengths could be developed in the die15093 forgings depending on aging conditions, Tables III through Vand XIV through XVI. Elongation and reduction in area valuesof the 7049 and MA52 forgings in the first trial (Tables III and IV)and those of the 7050 and MA52 forgings in the second trial(Tables XIV and XV3 were low and erratic. Quality differencesand structural factors were the principal reasons for the erraticresults rather than alloy composition effects. Porosity wasresponsible for the low values in the first trial, and grainstructure was responsible for the erratic results in the secondtrial.
To summarize effects of quench rate on strength, quenchrates at the location of the tension test specimens in the webof forgings in Alcoa Die No. 9078 were estimated. Maximumlongitudinal yield strengths that can be developed in die forgingsof these alloys are presented as a function of average quenchrate in Figure 15. This figure illustrates that quenchsensitivity of all of the newer alloys was lower than that of7075. Quench sensitivity of alloy MA52 was the lowest of thenewer alloys, that of 7050 was next, and quench sensitivitiesof special process 7175 and 7049 were equal.
Depending on section thickness and need for low residualstress, 7175 forgings are quenched in either cold or hot water.Forgings of 7050 and 7049 are regularly quenched in 140 F or150 F water, and thin forgings of 7050 and thicker forgings ofMA52 can be quenched in boiling water.
2. FRACTURE TOUGHNESS
The results of Alcoa fracture toughness tests on alloyMA52, 7175-T7X, 7049, and 7050 die forgings are summarized inTable XVII.
9
Fracture toughness, like resistance to stress-corrosioncracking, should be compared at equal strength levels. K0 valueswere plotted versus yield strength to determine the fracturetoughness-yield strength relationship for the four alloys. Fracturetoughness of the 7050 forgings that were quenched !n boiling waterfell below the bulk of the data, so these results were not includedin the plots.
Figure 16 compares the toughness of the new alloys relativeto that of 7075-T6 and 7079-T6 forqings. All of the neweralloys developed an equal combinat-on of strength and toughnesswhich was higher than that of the established alloys.
3. STRESS-CORROSION
Although the new alloys develop strengths substantiallyhigher than those of either 7075-T6 or 7079-T6 in their higheststrength temper, all of them must be overaged to provide accept-able resistance to stress-corrosion cracking. The minimumamount of overaging needed to promote the development of adequateresistance to stress-corrobion cracking is desired so that lossin strength will be minimal. Because overaging can produce largechanges in resistance to stress-corrosion cracking with relativelysmall changes in strength, stress-corrosion performances of alloysshould be compared using products overaged to the same strengthlevel.
a. Fectors Affecting SCC Test Performances
Stress-corrosion performances of alloys in tempers havingeither very low resistance or very high resistance are relativelyinsensitive to factors such as specimen size and type, testenvironment, product, grain structure, test stress and strength.Stress-corrosion performances of alloys in intermediate tempers,however, are very sensitive to these factors, particr'larlystrength and stress. Moreover, alloys such as MA52, 7050,7049 and 7175 can develop resistances approaching those ofeither 7075-T6 or 7075-T73 depending on the degree of aging.
Because of the influence of these factors, the ideal wayto compare the stress-corrosion resistance of these alloyswould be to compare test performances of similar specimens fromidentical die forgings tested simultaneously under the sameconditions. To determine the relationship among stress-corrosionresistance, stress and strength, the alloys should be comparedat a number of equal stress and strength levels. Furthermore,the experiment should be repeated to determine the variance inresults, and the performance of a number of different die forgingtypes should be evaluated. Although MA52, 7050 and 7049 dieforgings have never been evaluated under these conditions, enoughtesting has been performed to permit useful comparisons of therelationship between strength and resistance to stress-corrosioncracking.
10
b. Mean Critical Yield Strength
Critical yield strength is defined as the strength above
which stress-corrosion failure occurs in a particular stress-
corrosion test and below which stress-corrosion failure does
not occur. It is a material property, but depends on test
stress, specimen size and type, grain structure, test environrent
and exposure time. To understand this concept, consider a stress-
corrosion test specimen from a particular Al-Zn-Ma-Cu alloy
product aged to peak strength. At this strenoth level, itwould fail by stress-corrosion cracking at some stress level.
But, when overaged below its critical yield strength, it would
not fail at this stress. Because of the nature of criticalyield strength, it can be estimated by overaging a series of
replicate specimens various degrees, determining their strengths
and subjecting then to an appropriate stress-corrosion test.
Precision in estimating critical strength depends on the
nature of the data generated. When a series of specimens taken
from products having similar grain structures has been exposed
for a sufficient time in an environment which induces stress-
corrosion cracking in susceptible materials without causing
specimen failure dun to localized corrosion and mechanical
overload, critical yield strength can easily be estimated within
one ksi (Figures 17a and 18a). When exposure time is insufficient
to induce failure in specimens of susceptible material (Figure 17b)
or is so long that it causes specimens of nonsusceptible material
to fail by localized corrosion and mechanical overload, (Ficure 18b)
precision decreases. Because of the large zone of mixed results
in such cases, a mcan critical yield strenqth must be estimated.
One way is to plot the data as in Figure 13b arid draw a curve
through the points. Mean critical yield strength iq the midpoint
of the curve. The method of analysis to be described fits a
curve to such data.
c. Mean Critical Stress
Critical stress-corrosion test stress, abbreviated as
critical stress, is analogous to critical strenoth. Critical
stress of an alloy is the applied stress above which stress-
corrosion failure occurs in a particular stress-corrosion test
and below which stress-corrosion failure does not occur. Like
critical strength, it is a material property and depends on the
strength, specimen size and type, grain structure, test environ-
ment and exposure time. To understand this concept, consider a
stress-corrosion test specimen of an Al-Zn-Mg-Cu alloy product
aged to a particular strength level. When exosed in a stress-
corrosion test below the critical stress it will pass, and
when exposed above this level it will fail. Critical stress
can be estimated by obtaining a series of replicate specimens
from material having the same strength, stressing these at anumber of stress levels and exposing them in a corrosive environ-ment for an appropriate time.
Because the error in applying the stress is greater thanthe error in determining the strength, precision in estimatingcritical stress is generally lower than precision in measuringcritical yield strength. Moreover, to obtain a sufficintnumber of samples for analysis, data from material having slightlydifferent yield strengths are often pooled. Consequently, a
mean critical stress must be estimated. An example of a procedureto estimate mean critical stress of three 7079-T6 die forgingsis illustrated in Figure 19. Test results from all of the dieforgings were pooled and percent survival was plotted versusapplied stress. Mean critical stress is the midpoint of thec'rve. The method of analysis to be described eliminates theneed to pool data from materials having different strengths.
d. Effects of Strength and Stress
When attempts are made to compare materials and when results
from a number of separate investigations are pooled, it isfrequently found that the strengths of the materials are notthe same and that the applied test stresses are different.Consequently, comparisons of resistance to stress-corrosioncracking are difficult. One way is to determine mean criticalyield strengths at several stresses and to compare relationshipsbetween mean critical yield strengths of several alloys as afunction of applied stress. Another way is to compare meancritical stresses of alloys determined by pooling data frommaterial having strengths within some arbitrary limits. Amuch better way, however, is to analyse all of the data todetermine concomitantly the relationship among stress-corrosionperformance, stress and strength.
Probit analysis (Appendix II) was used in this work todetermine these relationships. 9 It is hypothesized in thisanalysis that the probability of passing a stress-corrosion testdecreases with increasing strength of the material and withincreasing applied stress, and that the proportion of specimensthat survived the test is an estimate of the probability ofpassing the test. It is further hypothesized that a function ofthe proportion survived has a linear relationship with the yieldstrength and the applied stress. Weighted multiple regressionanalysis is used to estimate the constants a, b and c in theequation:
f(PS) = a + b(AS) + c(YS)
where PS = percent survived, AS = applied stress, and YS = yieldstrength.
12
After the constants in the equation relating proportionsurvived with stress and yield strength have been determined,alloys can be compared in several ways. A useful way is tocompare mean critical strengths as a function of stress.
Although critical yield strength could be clearly establishedusing the data for 7175-T7X forgings exposed three years in theNew Kensington atmosphere (Figure 17a), mean critical strength of7175-T7X forgings based on performances after shorter times inthe New Kensington atmosphere or after exposure in the 3.5% NaClalternate immersion test could not be established. Relativestress-corrosion resistance was demonstrated by comparing measuredstress-corrosion performances of 7175-T7X with performance pre-dicted for 7049 and 7075 at the mean short-transverse yieldstrength of the 7175-T7X forgings that were tested.
Short-transverse yield strength was selected as the criterionrather than longitudinal or long-transverse yield strength forseveral reasons:
1. The short-transverse yield strength used in theanalysis was the yield strength of specimensremoved along with the stress-corrosion specimens;consequently, the correlation was between strengthand resistance to stress-corrosion cracking ofspecimens which had the same grain structure.
2. Specimen location for longitudinal and long-transverse specimens varied from forging toforging.
3. Longitudinal or long-transverse specimens were nottested in a number of cases.
e. Results of SCC Analyses
Data generated in the current contract are presented in TablesXVIII through XXII, and additional data used in the analyses arepresented in Tables VII and IX and Tables XXIII through XXIX.
Inspection of the data revealed that stress-corrosion testperformance of alloy 7050, 7049 and MA52 die forgings stronglydepended on forging type. Of a total of 144 short-transversespecimens of 7050 forged in Alcoa die 783 and aged to a strengthof 57 to 75 ksi, only one specimen (stressed at 45 ksi) failedthe alternate immersion test after 82 days. 5 The remaining 143stressed at 25 to 45 ksi survived the 84-day test. Specimensfrom 7050 forged in Alcoa die 10853 also exhibited an outstandingcombination of strength and resistance to stress-corrosioncracking. Metallographic examination of specimens from both ofthese forgings indicates that the grain structure was not highlydirectional, so these data were not included in the final analyses.
13
Stress-corrosion test performances of web-flange and landinggear type forgings in alloys 7050, 7049, and MA52 were sufficientlydifferent to warrant separate analyses. Performances of the7175-T7X die forgings, however, were essentially equal, so alldata were pooled for the analyses.
Quenching practice of 7050 die forgings also had a pronouncedeffect on stress-corrosion test performance. Performance of I7050 forgings quenched in boiling water was lower than performanceof the same forgings quenched in either hot or cold water andaged to the same strengths, so test results of the 7050 forgingsquenched in the boiling water were excluded from the final analyses.Resistance to stress-corrosion cracking of the 7049 forgings thatwere quenched in boiling water was not determined because of thelow strength. Stress-corrosion performance of the MA52 dieforgings was apparently not affected by quenching rate, so thedata were pooled.
The constants in the probit equation
Y a + b-stress + c(S-T YS)
were determined for up to 9 exposure times in the alternateimmersion test and in the New Kensington atmosphere.
Calculated survival percentages of 7050 and 7049 web-flangeforgings are plotted versus short-transverse yield strength for30 and 84 days in the alternate immersion test and 500 days inthe New Kensington atmosphere in Figure 20. Slopes of the curvesfor the two alloys were similar for tests conducted under each ofthe several test conditions analyzed, but exposure time andenvironment influenced both slope and relative displacement forthe two alloys of the percent survived vs yield strength curves.For tests of 30 days duration by alternate immersion the slopewas high indicating only about 11 ksi strength difference between10% and 90% survived. Performance of the two alloys was indicatedas not significantly different under these test conditions. Basedon the data for 84 days exposure, the difference in strength wasgreater for given differences in percent survived (22 ksi between10% and 90% survived) and increased displacement of the curvesfor the two alloys indicates an advantage for 7050, particularlyat the higher applied stresses. The slopes of the curves basedon tests of 500 days in New Kensington atmosphere approximatedthe slopes for the 30-day A.I. testing conditions, but relativealloy performance corresponded more nearly to that indicatedby the analyses of the 84-day A.I. tests.
Mean critical short-transverse yield strengths of the 7050,7049, and MA52 die forgings at 25, 30, 35, 40, and 45 ksi stresslevels are presented in Tables XXX through XXXII. The data indicatethat stress-corrosion resistance of alloy MA52 forgqngs was the
14
lowest of the three and that relative stress-corrosion resistanceof 7049 and 7050 depended on forging type and test conditions.These effects are more clearly illustrated in Figures 21 through23 which present mean critical strengths as a function of appliedstress for four sets of exposure conditions and for web-flangeforgings separated from landing gear forgings. Performance ofweb-flange forgings of 7050 exposed in the natural environmentexceeded performance of similar forgings of 7049 (Figure 21), butrelative performances in the accelerated test depended on forgingtype and test duration.
After 30 days in the alternate immersion test (Figure 22),performance of landing gear forgings of 7049 exceeded performanceof similar forgings of 7050. These data support the claim of highresistance to stress-corrosion cracking of 7049-T73 forgings basedon a 30-day test in the 3.5% NaCl alternate immersion test. Aspecimen from a 7049-T73 landing gear die forging that was stressedat 45 ksi and exposed in the atmosphere, however, failed in lessthan six months (Table IX).
Performance of web-flange forgings in 7049 and 7050 were i
similar and lower than performances of landing gear die forgings.This behavior suggests that stress-corrosion characteristics of7049 are more sensitive to forging type.
Longer exposure time in the accelerated test decreased per-formances of 7049 to a greater extent than it decreased performancesof 7050. After 84 days (Figure 23), performances of landing gearforgings of 7049 and 7050 were similar, while performance of 7050web-flange forgings exceeded performance of 7049.
These data strongly indicate that the 30-day alternateimmersion test is a safe indication of atmospheric performanceof 7050 forgings which exhibit pronounced grain directionality,but that the 30-day test is not severe enough to predict atmos-pheric performance of similar forgings of 7049. This behavior isillustrated in Figure 24 which compares performances of alloys7049 and 7050 web-flange die forgings after 30 days in thealternate immersion test and after 500 days in the New Kensingtonatmosphere.
Stress-corrosion performance of 7175-T7X was compared withperformances of the other alloys using a different method. Survivalpercentages of 7175-T7X and 7079-T6 die forgings having pronouncedgrain directionality were measured after varios exposure periodsin the New Kensington atmosphere and in the 3.5% NaCl alternateimmersion test. These measured percentages were compared withpredicted survival percentages of comparable forgings of the otheralloys aged to the short-transverse yield strength of 7175-T7X(66.5 ksi mean and 3 ksi standard deviation). The proceduresused in predicting are presented in Appendix II.
15
- - -, - 77 V --
Relative performances of the alloys depended on test environ-ment. In the atmosphere (Figure 25) performances of 7049, 7050and 7175-T7X were all higher than the performance of 7079-T6.Performances of 7175-T7X and 705C were comparable and were higherthan the performance of 7049. In the accelerated test (Figure 26),performances of all of the newer alloys were higher than theperformance of 7079-T6. Performance of 7175-T7X was highest forshort exposure times, and performance of 7050 was higher afterlonger exposure times.
1
11I °!=•I
SECTION V
SUMMARY
The data presented in this report evidence the superior
combination of low quench sensitivity, toughness and resistanceto stress-corrosion cracking of the newer alloys 7050, MA52, 7049,and special process 7175 relative to the properties of alloys7079 and 7075. The rank of the newer alloys relative to oneanother, however, is not so clearly demonstrable because rankingvaries with the properties being measured.
Quench sensitivities of MA52 and 7050 were the lowestof the four and those of 7175 and 7049 the highest. Because mostdie forgings either have relatively thin sections or are partiallymachined before solution heat treatment, however, differencesin quench sensitivity may not be practically significant unlessa very slow quench is required to reduce residual stresses.
Fracture toughness of the newer alloys was higher thanthe average toughness of 7075-T6 and 7079-T6, and no clear dis-tinction among the toughnesses of the newer alloys could bedemonstrated. Fracture toughness of 7049-T73 has been reportedpreviously by Kaiser and others to equal the toughness of7075-T6.1,10 Perhaps purity and fabricating procedures areresponsible for the higher toughness observed in the 7049 testedby Alcoa.
Ranking the alloys on the basis of resistance to stress-corrosion cracking required rigorous analysis. Because of theexcellent correlation between strength and resistance to stress-corrosion cracking and the large effect of test conditions, allcomparisons should preferably be made on materials having the samestrength using stress-corrosion data from specimens tested undernear identical conditions. A sufficient number of specimens ofeach alloy having the same strength were not tested, so comparisonswere made using probit analysis. In the natural environment,performances of 7050 and special process 7175 were highest, whileperformance of 7049 was lowest. Performance of MA52 could not bedefinitely established because of the relatively few specimensexposed and the short exposure time, but other work indicates thatatmospheric stress-corrosion resistance of Al-Zn-Mg-Cu-Zr alloyscontaining 1.25% Cu is substantially lower than resistance ofalloyscontaining 2% or more CU.2,3,4,5,11
The results of this analysis strongly indicate that stress-corrosion performance of 7050 and special process 7175 will exceedperformance of the other alloys in service. These alloys, however,did not rank highest in the 3.5% NaCl alternate immersion test.Consequently, the current acceptance test criteria do not reflectthe relative merits of the newer alloys.
17
I I FASECTION VI
CONCLUSIONS
1. Relatively thin, rapidly quenched die forgings of 7050, MA52,7049, and special proceess 7175 overaged to the strength levelsof 7075-T6 and 7079-T6 developed substantially higher resistancqto stress-corrosion cracking and higher fracture toughness.
2. Alloy 7050 die forgings developed the best combination of lowquench sensitivity, resistance to stress-corrosion cracking,and fracture toughness.
3. Stress-corrosion test performances of the new alloys dependedon forging type, environment and exposure time. Atmospherictest performances of 7050 and special process 7175 die forgingswere higher than performance of similar forgings of 7049. Per-formance of 7050 was superior based on 84 days in the 3.5%NaCl alternate immersion test, while performance of 7175 wassuperior based on the 30-day test.
4. The good combination of strength and resistance to stress-corrosion cracking in the 30-day 3.5% NaC1 alternate immersiontest of 7049 landing gear type forgings was confirmed. Per-formance of weL-flange forgings having pronounced graindirectionality, however, was markedly lower in the acceleratedtest and in a natural environment.
5. Stress-corrosion test performance of alloy 7050 was higherafter 500 days in a natural environment than after 30 daysin the 3.5% NaCl alternate immersion test vihile the reversewas true for allay 7049.
6. Performance in the alternate immersion test can be sub-stantially increased by increasing specimen size andchanging test bar location.
18
.]i
SECTION VII
[ RECOMMENDATION
Continae the use of 7175-T736 die forgings for aero-space applications and consider the use of 7050 die forgings[where the lower quench sensitivity of 7050 makes it attractive.
19
SECTION VIII
REFERENCES
1. T. V. Summerson and J. V. I-uhan, "Development of 7049-T73High Strength, Stress-Corrosion Resistant Aluminum AlloyForgings," presented at 1970 WESTEC, March 11, 1970.
2. J. T. Staley, "Investigation to Develop a High-StrengthStress-Corrosion Resistant Aluminum Aircraft Alloy,"Final Report under Naval Air Systems Command ContractN00019-69-C-0292, January 1970.
3. J. T. Staley, "Investigation to Develop a High-StrengthStress-Corrosion Resistant Naval Aircraft Alloy,"Final Report under Naval Air Systems Command ContractN00019-70-C-0118, November 1970.
4. J. T. Staley, "Further Development of Aluminum Alloy X7050,"Final Report under Naval Air Systems Command ContractN00019-71-C-0131, May 1972.
5. J. T. Staley and H. Y. Hunsicker, "Exploratory Developmentof High Strength, Stress-Corrosion Resistant Aluminum Alloyfor Use in Thick Section Applications," Technical ReportAFML-TR-70-256, November 1970.
6. M. V. Hyatt and H. W. Schimelbusch, "Development of aHigh-Strength, Stress-Corrosion Resistant Aluminum Alloyfor Use in Thick Sections," Technical Report AFML-TR-70-109,May 1970.
7. D. S. Thompson and S. A. Levy, "High-Strength Aluminum AlloyDevelopment," Technical Report AFML-TR-70-171, August 1970.
8. B. W. Lifka and D. 0. Sprowls, "Stress-Corrosion Testing of7079-T6 Aluminum Alloy in Various Environments," in ASTMSTP #425, Stress Corrosion Testing, p. 342, December 1967.
9. D. J. Finney, Probit Analysis, 3rd Edition, Cambridge UniversityPress, 1971.
10. K. J. Os :alt, "Northrup Corporation Air Craft Divieion ReportNOR 71-52, entitled "Engineering Property Evaluation of7049-T73 Die Forged Material," March 5, 1971.
11. J. T. Staley, "Investigation to Improve the Stress-CorrosionResistance of Aluminum Aircraft Alloys Through Alloy Additionsand Specialized Heat Treatment," Final Report under Naval AirSystems Command Contract N00019-68-C-0146, February 1969.
20
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FJGURE 6 MACROSTRUCTURE OF 7050 INGOT
60
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FIGURE 8 MACROSTRUCTURE OF MA52 INGOT
62
FIGURE 9 MACROSTRUCTURE OF MA52 INGOT
63
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2 H
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-- -- SECTION 88
.. ENLARGED
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NOTCH ROOTRADIUS .010" MAX. PROPORTIONS
8 THICKNESS
A 1.18,/W- 2B ; Wl 2.5B
S= O.IBF = 2E %i.IO8
H 2.48
-o
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9o~
NOTCH
ENLARGED VIEW
FIG. 12 COMPACT TENSION FRACTURE TOUGHNESS SPECIMEN
66
1003 5 % N l A 1
CO TRO L 3 5% N aC I 'A
l
50 CONTROLLt D
'FEDERAL TEST ;.UTHOD 823
30 lcs; STRESS
0 - I I I
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3.5% NaCt AlI
~50CONTROLLED
C.3 FEDERAL TEST METHOD 823~25
35 ksi STRESS
0 20 40 60n 80
100
75 TOL[ 3.5% NaCI A.1
FEt)ERAL TEST-.50 METHOD 8-23
2542 ksi STRESS
0'0 20 40 60 80
DAYS OF EXPOSURE
Fig. 13 MEASURED PERCENT SURVIVAL vs DAYS TO FAILURE FOR 1115-T736DIE FORGINGS EXPOSED 84 DAYS TO ALTERNATE IMMERSION
67
12 3
II
SCALE-1X
1 - ~/ INCH DIAMETER CLOSE AS POSSIBLE TO FORGED SURFACE2 - Ye INCH DIAMETER FROM SECOND ROW
3 - 1/ INCH DIAMETER CLOSE AS POSSIBLE TO FORGED SUhiACE
SPECIMENS FROM DIE NUMBER 8457 WERE TAKEN IN A SIMILAR MANNER
Fig. 14 SKETCH OF A CROSS-SECTION THROUGH DIE NUMBER9078 SHOWING LOCATIONS AT WHICH THE SCC
:K SPECIMENS WFRE TAKEN
68
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S I -I I '1"II I I
50 LONGITUDINAL45 7050 L-T
40
35 7049
'u.j 30
25 7079-T6~25
, 7075-T6
011.1
20 -
70790 SHRT6RASESC4 5
50 60 70 80 9C
YIELD STRENGTH, ksi
Fig. 16 FRACTURE TOUGHN4ESS OF DIE FORGINGS
7175
07
7079-T6-E
EXPOSED FOR 1168 DAYS ( a)
1200
f 1000CRITICAL SHORTTRANSVERSE -
800 YIELD STRENGTH
600
400 LEGEND
200 t 0crINTACT FAILED S
o0
cn SHORT TRANSVERSE YIELD STRENGTH - ksi
EXPOSED FOR 365 DAYS b)
400 4 '4 A
INCOMPLETE ZONEOF MIXED RES'LTS
200
060 65 70
SIHORT TRANSVERSE YIELD STPENGTH - ksi
Fig. 17 DAYS TO FAILURE vs SHORT TRANSVERSE YIELD S0P1ENGTHFOR 7175-T736 DIE FOR0tG4S EXPOSED TO NEW KEIISINGTONATMOSPHERE AT A STRESS OF 35 ksi
71
EXPOSED FOR 30 DAYS (a J100
80 ZONE OF MIXED RESULTS
60
40
20
040 50 60 70 80
_ SHORT TRANSVERSE YIELD STRENGTH- ksi
I-I~i.IEXPOSED FOR 84 DAYS lb j
100 w.AAZ ONE OF N "
80 MIXED RESULTS
60
40
20
040 50 60 70 80
SHORT TRANSVERSE YIELD STRENGTH - ksi
Fig. 18 PERCENT SURVIVAL vs SHORT TRANSVERSE YIELDSTRENGTH FOR 7049 -T7X DIE FORGINGS EXPOSEDTO A.I. AT A S.",SS OF 25 ksi
72
100 7079-T6 DIE FORGINGS80 30 DAYS 3.5 % NaCIALTENATEIMMESIO80 ALTRERNE 6ESO
~40
20 ME-ANCRITICAL "iRESS
0 --
10 20
STRESS, ksi
Fig. 19 GRAPHICAL MEANS OF DETERMININGMEAN CRITICAL STRESS
73
30 DAYS 3.5% NaCI A I
25 ksi 30 ksi 35 ksi 40 ksi 45 ksi
7550 -25 -"
60 70 80 60 70 80 60 70 80 60 70 80 60 70
84 DAYS 3.5% NaCI A I100 ', , , ', , = = =
7 5
0 25 - "
C. ""'"
0 S i I ,i f
M1 70 50 60 70 50 60 70 50 60 70 50 6 0 70500 DAYS NEW KENSINGTON ATMOSPHERE
100 "l I , iT miI Ii n i l
75
50
060 70 80 60 70 80 60 70 80 60 70 80 60 70
SHORT -TRANSVERSE YIELD STRENGTH, ksi
7050 '.7049
Fig. 20 PERCENT SURVIVED vs YIELD STRENGTH,WEB-FLANGE DIE FORGINGS
74
* S St
45 45-2 - -7050
35 365 DAYS
NEW KENSINGTON ,7049-\ ""
25 m40 50 60 70 80
I
45 I \
7049 \ 7050
t 35 500 DAYS \
NEW KENSINGTON
25 _ _ _ _ _ _"
40 50 60 70 80
MEAN CRITICAL SHORT - TRANSVERSEYIELD STRENGTH, ksi
Fig. 21 STRESS vs CRITICAL STRENGTHWEB - FLANGE FORGINGS
75
4545 - MA52"---w4,0 \.&--7049
- \r740
35 %%\\
FORGINGS ** 7050
25
50 60 70 80 90
p 4 5 -* S
MA52-- A 0, 7050
35 WEB FLANGE 7050
FORGINGS
7049
25 -
40 50 60 70 80
MEAN CRITICAL SHORT-TRANSVERSEYIELD STRENGTH, ksi
Fig. 22 STRESS vs CRITICAL STRENGTH30 DAYS 3.5% NaCI A.I.
76
45 I ' !w'
MA52 --- 7050
35LANDING GEAR '.
FORGINGS
25 i! "!.,
40 50 60 70 80
.45
MA52 * 7050
35 -'WEB FLANGE.FORGINGS
704
25
40 50 60 70 80
MEAN CRITICAL SHORT-TRANSVERSEYIELD STRENGTH, ksi
Fig. 23 STRESS vs CRITICAL STRENGTH84 DAYS 3.5% NaCI A.I.
*7
-w
45 1 -500 DAYS
ALLOY 7050 \/NEW KEN.
35 I
30 DAYS
A0
2540 50 60 70 80 *
45 130 DAYS
ALLOY 7049 A.I.
35
500 DAYS "
NEW KEN."
25 L "I40 50 60 70 80
MEAN CRITICAL SHORT-TRANSVERSE YIELD STRENGTH, ksi
Fig. 24 STRESS vs CRITICAL STRENGTHWEB-FLANGE DIE FORGINGS
78
DIE FORGINGS WITH PRONOUNCED GRAIN DIRECTIONALITY
100 050 ~
~75\..
S5070794T6
25 30 ks
IG10 200 300 400 500
100 I. ~. 7175+
...........
50 70490 ~
2535 ksi
0 S10 20C 300I 400I 500
707050
42 Icsi
DA'fS IN NEW KENINTON ATOSPHERE
+ ME A;LiRED
*CALCULATED ASSUM~ING YIELD STRENGTH OF 7175
(MEAN S-T YS =66.5 kii STANDARD DEVIATION - 3 ksi)
Fig. 25 %z SURVIVED Vs TIME IN INDUSTRIAL ATMOSPHERE
79I
DIE FORGiNGS WITH PRONOUNCED GRAIN DIRECTIONALITY
100 .................. 30 ks
50
7049/?%
25%
100 ... ...... 35 ksi
50
7049**25 A2 .-..- --
O 20 40 60 Be
100 42 kic~
075'' ~
25 709--
0 20 40 60 so
DAYS IN 3.5*%, Nail ALERNATE IMMERSION TEST
+CALCULATED ASSUMING YIELD STRENGTH OF 7175 tMEAN54T YS - 66 5 ksf STANDARD DEVIATION -- 3 k%,
Fig. 26 !% SURVIVED vs TIME IN ALTERNATE IMMERSION TESTj
80
SECTION IX
APPENDIX I
Properties and Hieat Treating
Co-ditions of Die Forgings
not Produced for this
Contract
81
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82
APPENDI X I
Table )OIV
INJ.[IE PhOPERTIE, O PIANT AGED 7050 DIE FORGING NO. 25789
S-No 394706 S-No 394708 S-No 394709 SR S-No 39471 SR
4/2501 4/3501 4/250+7/3501 4/250* 4/3501 4/250+7/350!T.s. Y.S. % El T.S. Y.S % El T.S. Y.S. % El T.S. Y.S. %EL
Sre k0i ksi ksi k-i ksi s - ksi ksi
WU,! 88.4 83.5 13.0 84.9 78.7 13.0 94.5 89.1 .1.0 86.2 78.9 14.oWL2 88.5 83.8 12.0 84.1 77.9 14.o 94.5 88.8 10.0 86.1 79.2 13.0W1,3 86.1 80.7 8.5 81.4 74.3 10.3 89.4 85.1 6.0 84.3 78.2 11.5wi4 89.4 84.4 12.0 80.4 73-9 9.5 93.3 89.2 11.0 86.5 80.8 12.0
RL 81.6 74.6 10.0 78.4 71.8 9.5 86.0 80.7 7.0 81.9 75.4 9.0
p - - 81.5 75.6 13.0 - - 82.1 75.1 13.5FLI 84.4 79.2 12.0 79.9 73.8 15.1 88.0 83.3 11.0 82.3 76.2 12.5
FI,2 83.2 77.9 11.5 78.7 71.3 13.5 88.9 83.5 10.5 80.6 73.4 13.5FL3 85.2 79.6 11.0 78.6 71.8 13.5 89.7 83.0 9.0 82.5 74.6 11.5115 83.6 77.8 12.0 78.5 71.3 13.5 89.0 83.4 11.0 83.0 75.8 12.01,L6 8197 76.4 9.5 79.3 72.6 11.5 84.0 77.8 9.5 80.1 72.6 9.0;L7 81.8 74.7 12.0 75.9 66.2 14.0 85.9 80.1 12.0 80.9 73.4 14.01,8 82.1 74.8 14.5 77.4 69.2 13.5 81.8 72.8 15.5 76.7 66.0 14.0
WLTI 86.3 80.8 12.0 81.5 74.3 14.0 90.7 84.7 6.0 82.8 72.1 12.0WLT2 86.0 81.1 I.0 81.3 74.4 14.0 90.4 85.0 10.0 82.6 78.6 13.0WLT3 86.3 80.8 14.0 80.9 73.9 14.0 90.6 85.4 11.0 83.6 79.2 13.0WLT4 83.5 78.5 6.5 83.4 76.7 13.0 84.7 77.6 5.5 80.5 72.3 9.0
ST1 76.8 70.8 5.5 72.8 65.2 7.0 78.9 68.3 3.0 74.9 65.2 3.0ST2 78.1 71.8 5.0 74.0 66.0 7.0 79.9 71.1 4.5 76.0 66.0 4.5
FSTI 78.7 71.6 6.0 74.7 65.7 8.o 79.4 68.2 4.0 75.3 65.4 4.o
Note 1. Hr @ temp OF.
WL = Web longitudinalRL = Rib longitudinalP = Prolongation
FL = Flange longitudinalWLT = Web long transverseST = Short-transverse in bulk section
FST = Flange short-transverseSR = Stressed -'elieved by .ompression
Forgings heat treated 16 hr @ 890 F and cold-water quenced. S-394709 and 39471cold worked 2.5-3.5% after quenching. All forgings artificially aged asindicated.
83
APPENDIX I
Table X)XV
TENSILE PROPERTIES AND ELECTRICAL CONDUCTIVITIES OFARL AGED 7050 DIE FORGING NO. 15789
15789 15789, Stress RelievedS-No. 374707 S-No. 394710
10 hr total aging @ 350 F 12 hr total aging @ 350 FT.s. Y.S. El T.S. Y.S. %E1
Direction tsi s in 4D psi p in4D
eL1 78.6 70.9 12.0 80.4 72.8 14.o
WLT! 73.3 70.0 12.5 76.7 67.8 11.0WLT2 79.5 71.8 9.5 80.1 72.6 12.0
FL1 77.4 69.3 14.o 78.1 69.0 13.076.5 70.6 12.5 78.0 69.8 15.0
L2 75.4 66.7 13.5 75.8 66.0 12.5:15 76.5 68.5 12.5 76.7 67.8 12.01L6 77.2 69.0 13.5 78.5 69.9 13.0
RLI 78.4 70.3 11.0 77.8 68.3 11.0
P 78.8 72.3 :5.0 74.7 63.7 14.5
ST! 71.9 63.4 7.0 71.7 59.9 5.5ST2 73.4 64.1 7.5 73.6 61.6 6.0
FSTi 71.8 63.1 5.0 70.2 57.8 4.oFST2 76.6 68.6 8.0 73.9 60.2 6.0FST3 76.2 67.9 8.0 74.8 61.2 6.0
Electrical Conductivity of both = 41.0% IACS
WL = Web longitudinalWLT = Web long-transverseFL = Flange longitudinalRL = Rib longitudinalP = ProlongationST = Short-transverse in bulky section
FS = Flange short-transverse
Forgings heat treated 16 hr @ 890 F, cold-water quenched and artificially aged4 hr/250 F + indicated time @ 350 F.
S-394710 cold worked 2.5-3.5% after quenching.
84
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APPENDIX I
Table XXXIX
TENSILE PROPERTIES OF 7050DIE FORGING NO. 15093
Test T.S. Y.S. % El % R
Location ksi ksi in 4D of A
1 76.3 68.2 14.3 302 78.3 70.6 15.0 413 79.0 71.2 15.5 404 77.7 69.3 14.3 365 75.0 66.0 14.0 126 76.7 67.6 13.0 197 75.9 67.6 15.0 368 76.4 67.8 14.3 369 76.8 68.0 17.1 43
10 77.2 68.9 13.6 3211 75.8 66.6 9.0 912 76.2 66.6 9.0 1213 74.0 66.7 10.7 2614 75.8 66.0 13.0 2415 76.8 67.5 12.0 2016 77.9 69.7 14.3 3517 77.1 69.0 15.0 4018 76.3 68.7 14.3 3419 76.8 67.9 12.0 2920 75.6 67.2 6.0 621 71.2 66.7 3.0 522 72.8 66.5 4.0 423 76.0 66.9 9.0 1424 76.5 68.7 7.1 825 77.2 69.3 6.4 726 77.1 70.0 5.0 8
Forging heat treated 8-3/4 hr @ 880 F, quenched in water@ 150 F and artificially aged 24 hr/250 F + 12 hr/350 F.
See Figure 10, Appendix I, for test bar location.
8! 88
Ko
APPENDIX I
Table XL
TENSILE PROPERTIES OF PLANT AGED 7050 DIE FORGINGS 9078
Special Fabri.cating Process
S.No. 395205 S.No. 3952024/250+7/3501 4/250+9/3501
T.S. Y.S. T.S. 3.S.Specimen ksi ksi % El ksi ksi % El
WLl 83.0 77.4 16.0 83.7 77.8 15.0WLT1 80.2 73.7 16.0 81.9 76.6 18.0
RLl 78.7 71.8 14.0 79.5 72.1 15.0
FLI 82.5 76.3 I6.0 79.9 73.0 15.0FL2 83.1 77.5 16.0 81.8 75.7 15.0
FL3 83.3 77.8 15.0 82.7 76.b 15.0FL4 75.6 67.7 10.0 75.3 66.7 12.0FSTI 75.5 69.2 10.0 76.0 68.2 8.0FST2 77.3 71.5 7.0 80.0 73.7 8.0
WL = Web longitudinalWLT = Web long transverseRL = Rib longitudinalFL = Flange longitudinal
FST = Flange short-transverse
Forgings heat treated 16 hr @ 890 F, cold-water quenchedand artificially aged as indicated.
Note: 1. Hr @ temp OF.
89
APPENDIX I
Table XLI
TENSILE PROPERTIES OF ARL AGED 7050 DIE FORGING 9078
Special Fabricating Process
Electrical T.S. Y.S. ElS. No. Age Di r ection Conductivity ksi ksi in 4D
398887 10 WL 41.2 82.0 75.8 16.0FST 41.2 76.1 67.6 8.0FST 41.2 74.4 65.4 10.0
398888 15 WL 42.1 79.2 71.8 16.0
FST 42.2 72.7 65.1 6.0FST 42.2 74.7 65.4 6.0
398889 23 WL 42.6 76.4 68.4 16.0
FST 42.8 70.7 59.8 10.0FST 42.8 72.2 62.5 8.0
Note: 1. Hr @ 350 F.
Forgings heat treated 16 hr @ 890 F, cold-waterquenched and artificially aged 4 hr/250 F +indicated 2nd-step agings @ 350 F
90
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APPENDIX I
Table XLVII
TENSILE PROPERTIES OF 7175-T7X DIE FORGINGS
Elec. Longitudinal Short-TransverseDie Cond. T.S. Y.S. El T.S. Y.S. El
S. No. No. % IACS ksi ksi % in 4D ksi ksi % in 4D
338106 9078 39.3 77.8 67.6 10.0338113 9078 39.5 78.4 67.6 9.5
366254 9078 39.2 77.6 68.5 9.3366379 9078 39.6 78.4 68.7 1C.0366380 9078 39.7 77.6 67.4 10.0
366381 9078 39.3 77.6 67.8 10.0366382 9078 39.6 77.5 66.6 10.0366383 9078 39.6 74.6 65.8 8.5
366384 9078 40.2 76.2 65.7 10.0366385 9078 39.4 77.8 67.8 8.0
366386 9078 39.8 75.4 64.0 10.0410701 9078 -- 82.1 75.0 13.5 74.4 64.4 10.0
337200 40001 39.0 77.8 68.7 7.5337202 40001 39.9 74.7 65.4 8.0366865 40001 40.2 75.7 64.6 10.0
369311 40001 39.5 77.3 68.7 8.2
377676 40001 39.2 75.6 67.0 8.0377879 40001 38.2 76.2 67.6 8.0377483 40002 39.6 74.6 65.8 8.5
377543 40006 39.6 75.3 65.5 7.5
377544 40007 39.2 74.9 66.2 5.5338075 15633 39.0 78.8 69.9 6.5338076 15633 39.0 78.6 69.1 8.0396354 40001 40.2 78.9 71.9 -- 74.4 66.0 7.0
396355 40001 40.4 76.8 68.0 -- 73.6 64.8 10.0
396358 40001 40.4 78.6 70.2 -- 73.1 65.0 8.0
396359 40001 40.6 77.7 69.4 -- 71.8 64.6 10.0396331 40001 39.2 78.4 70.9 -- 78.6 69.6 10.0410703 40006 -- 78.5 69.3 14.0 73.5 63.5 8.0
Note Data have been included for some 7175 forgings
receiving slightly less aging than is used for production7175-T736 forgings.
98
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APPENDIX I - FIGURE 35
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SECTION X
APPENDIX II
PROBIT ANALYSIS
The use of probit analysis presupposes that for any onesubject (stress-corrosion test specimen) exposed undercontrolled conditions for a definite time, there will be acertain stimulus (test stress) applied to the subject at astated dose (stress level) below which failure does notoccur, and above which failure occurs. This stress level is thecritical stress. In addition, there may be a measurablecharacteristic of the subjects tested (yield strenath of thetest specimen) below which failure does not occur and al ovewhich failure occurs. This characteristic is the criticalyield strength. Because the critical stress level and thecritical yield strength will vary from snecimen to specimenin the population and is likely to vary from one occasion toanother as a result of uncontrolled internal variables (e.a.,grain structure) or external variations (e.q., ambient humidity)the concept of a mean critical stress and a mean critical strengthmust be introduced.
The frequency distribution of critical stress or criticalstrenqth over the population studied must be known before meancritical stress and mean critical strenath can be accuratelyestimated. In this report, it is assumed that the frenuencydistribution is the familiar Gaussian or normal form.
Probit analysis determines estimates of the constantsa, b, and c in the equation:
Y = a + b'AS + cYS. ()
where YS = yield strength and AS = applied stress. The termY is obtained from the relationship:
SY-5
PS = exp (:) dx (2)2 7r 2
where PS = percent survived and x is a dummy variable.
113
APPENDIX II
To determine the constants, data sets consisting of yieldstrength, test stress, number of specimens exposed, and numberof specimens survived are prepared. Using the iterative techniqueof pattern search, the constants are varied until the discrepancybetween the observed proportions surviving and the predictedproportions sur-,iving is minimized. Mean critical strength ata stress AS is determined by solving equation (1) with Y=5.
Figure 41 illustrates the method of determining the overallprobability of passing the SCC test when yield strength is anormally distributed variable. Figure 4la illustrates the effectof yield strength on the probability of passing the stress-corrosion test at a particular stress. while Figure 41b illustratesthe distribution of strength values expected at a standarddeviation, ayS, of 3. To determine the overall probability, Po,that this material will pass the SCC test, the products of theareas under each curve between minimum strength, YSmin, tomaximum strength, YSmax, (YSmin + 5.5 ays) are determined. Thiscalculation can be expressed mathematically as follows:
YSma x YSmaxPo N dYS " PS dYS (3)
- YSmin YSmin
where N = 1 exp - 1 (YS - )
aYS 2Y
= mean YS,
PS = previously described
According to Finney, 9 the solution to Equation 3 can be expressedby Equation 2 where
5+ (a-5 + cp)/(l + c7 a 2) Z + As b/( + c a 2)1/2 (4
The terms in this eauation are as described previously.The survival percentages for the 7050 and 7049 die forgingsplotted in Figures 25 and 26 were estimated by calculatina POas above with i = 66.5 and ays = 3.0, then multiplying Po by100 to get percent.
114
APPENDIX i1
S0.8 -(aC-C.)
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MIN MAX-s dYS 1
50 54 58 62 66 70 74 78 82SHORT-TRANSVERSE YIELD STRENGTH, ksi
Fig. 41 METHOD OF CALCULATING OVERALL PROBABILITYOF PASSING SCC TEST
115
