Materials Science and Engineering A289 (2000) 54–59

Aluminum alloy 6069 part II: fracture toughness of 6061-T6 and6069-T6

F.J. MacMaster a, K.S. Chan a, S.C. Bergsma b, M.E. Kassner c,*a Southwest Research Institute, San Antonio, TX 78224, USA

b Northwest Aluminum Company, The Dalles, OR 97058, USAc Department of Mechanical Engineering, Oregon State Uni6ersity, Cor6allis, OR 97331, USA

Received 9 February 2000; received in revised form 27 March 2000

Abstract

This work investigates the fracture toughness of a new high strength aluminum alloy AA 6069. Earlier work reports that thisalloy has higher tensile and fatigue strength and corrosion properties than AA 6061. The results of this research reports that thefracture toughness of 6069-T6, similarly, is comparable or superior to that of 6061-T6 with identical ingot preparation andsubsequent forming procedures. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Aluminum alloy 6069; Fracture toughness; 6061-T6; 6069-T6

www.elsevier.com/locate/msea

1. Introduction

6069 Alloy was developed for application in hot andcold extrusion and forging. The composition of thisalloy is listed in Table 1. The alloy has favorableformability with nominal tensile properties listed inTable 2. The 6069-T6 properties after hot or coldextrusion range from about 350 to 450 MPa yieldstrength and 400 to 475 MPa UTS and 14–20% elonga-tion. This represents a 10–20% increase in strengthover 6061-T6 properties (although a few percent de-crease in ductility is sometimes observed [1]). Thestrengthening over conventional 6061-T6 appears tobe mostly due to increased b%% [1–3] precipitates.Chakrabarti et al. [2] characterized both the b%% phaseand the Q phase in aged Al-Mg–Si–Cu alloys. 6069-T6had similar b%% and Q phase morphology and dimen-sions [1,4], though the latter may not be important forthe alloy strengthening [3]. Thus the strength of the twoalloys may have identical bases, although the concen-tration of the precipitates may be different. Also, theconcentration of the larger, non-strengthening precipi-tates is higher in the 6069 due to an increased concen-

tration of elements (such as Mg and Si) exceeding thesolubility in Al. The mechanical properties listed inTable 2 compare ‘Air Slip’™ direct chill (ASDC) cast6069 and 6061-T6. (The properties of conventional(typical non-ASDC) 6061-T6 extrusions are, of course,even lower than ASDC 6061-T6 at 310 UTS, 275 YSand 12% elongation [5].) Particular interest in fracturetoughness emerged as the 6069-T6, though having bet-ter strength, had only comparable or somewhat lowerductility to 6061-T6 with similar ingot preparation andsubsequent extrusion processing.

This report summarizes the results of the investiga-tion of the plane-strain fracture toughness of AA6069-T6 and AA6061-T6. Standard ASTM KIC and JIC testswere performed on these alloys in both the longitudinaland transverse directions. The materials and the testmethods, as well as the fracture toughness results, willbe discussed. Limited fractography was performed onthe fracture surfaces and used to correlate with thefracture toughness data.

2. Experimental procedure

Nominal compositions of AA6069-T6 and AA6061-T6 are given in Table 1. All fracture toughness testingwas performed by the Southwest Research Institute,

* Corresponding author. Tel.: +1-541-7377023; fax: +1-541-7372600.

E-mail address: kassner@engr.orst.edu (M.E. Kassner)

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.PII: S0921 -5093 (00 )00918 -7

F.J. MacMaster et al. / Materials Science and Engineering A289 (2000) 54–59 55

Table 1Alloy compositions for this study

6069a6061a 6069b

0.790.66 0.6–1.2Si0.28Fe 0.4 max0.20.580.24 0.55–1.00Cu

0.01Mn 0.02 0.05 max1.320.93 1.2–1.6Mg0.22Cr 0.05–0.300.050.0480.013 0.10 maxTi

0.01V 0.13 0.10–0.300.020.01 –Ga

–Sr – 0.05 max–Be ––

a Mechanical tests, this study.b Specification that includes changes accepted by The Aluminum

Association as of 30 January 1997.

toughness tests were performed. Initially, only plane-strain KIC tests were to be performed on the twoaluminum alloys. However, the results obtained fromthe initial tests necessitated the addition of limitedelastic-plastic JIC testing.

Fracture testing was conducted in the Solid andFracture Mechanics Laboratory at SwRI using a servo-hydraulic testing workstation. The fracture toughnesstest procedures used conformed to the relevant ASTMtest specifications for both plane-strain KIC [6] andelastic-plastic JIC fracture testing [7]. Fracture tough-ness testing was performed using compact-tension(C(T)) specimens, with the maximum possible size spec-imen extracted from the extruded alloy. Schematics ofthe C(T) specimens are illustrated in Ref. [8] along witha description of the orientation to the extrusions.

Prior to fracture testing, the specimens were metallo-graphically polished to a mirror finish to aid in visuallymeasuring the crack length during precracking. Cracklength was measured on both sides of the specimen witha traveling microscope mounted on precision microme-ter stages capable of distinguishing crack length mea-surements to the nearest 0.025 mm. Precracking wasperformed at a load ratio of R=0.1 and at a cyclicfrequency of between 2 and 20 Hz. Load shedding wasemployed during precracking to insure that the cracktip was sufficiently sharpened to provide suitable dataduring fracture testing. The load during precrackingwas kept below approximately 60% of the anticipatedfracture load. The ratio of the initial crack length, a, tospecimen width, W, was about 0.5.

The fracture toughness tests were conducted at ambi-ent temperature in air under a constant displacementrate of 3.2×10−2 mm s−1. The ASTM E399 testprocedure [6] was followed for KIC testing, while theASTM E813 test procedure was followed for JIC testing[7]. The crack mouth opening was measured using aclip gauge. The applied loads and crack mouth opening

San Antonio, TX (SwRI). Two sections each of boththe 6061-T6 and 6069-T6 hot extrusions were suppliedto SwRI by the Northwest Aluminum Company, TheDalles, OR. Extrusions were performed by AnodizingIncorporated, Portland, OR, from NWA ‘Air Slip’™direct chill (ASDC) ingot. For the 6061-T6 rectangularextrusion, the axial length of the sections was approxi-mately 280 mm with a width of 64 mm and a thicknessof 50.8 mm. The 6069-T6 circular extrusion was ap-proximately 280 mm long with a diameter of 61 mm.6061-T6 consisted of 1 h solution anneal at 527°Cfollowed by a water quench and 8 h age at 177°C.6069-T6 consisted of 1.5 h solution anneal followed bya water quench and 16 h age at 171°C.

Fracture toughness tests were performed on speci-mens in the L-T (L-R) orientation, as well as the T-L(R-L) orientation for both materials. Following theASTM E399 Standard [6], this nomenclature defines theapplied loading axis by the first letter (L= longitudinal,T= transverse, R=radial) and the direction of crackadvance by the second letter. Both KIC and JIC fracture

Table 26069-T6 and 6061-T6 tensile properties [1]

Extrusion Yield stress UTS (MPa) El (%)direction

ASDC ingot408373NA 11.76069-T6

6061-T6 322NA 365 16.5

Summary of selected 6069-T6 properties of round bar, bar, and flat bar hotextrusions using ASDC ingot

Longitudinal 435 465 15

Summary of selected properties for cold impact extrusions using ASDC ingot

Longitudinal 3956069-T6 18450Transverse 385 435 16Longitudinal 350 395 21.06061-T6Transverse 350 390 23

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Fig. 1. J-resistance curves of 6069-T6 and 6061-T6 in the L-Torientation.

rized in Table 3. Valid plane-strain KIC results wereobtained for both aluminum alloys in the T-L orienta-tion. The KIC for AA6069-T6 is 36.5 MPa m−2, com-pared to 33.3 MPa m−2 for AA6061-T6. For the L-Torientation, KQ values are reported for both alloysbecause the fracture toughness levels are high and thethickness requirement for plane-strain fracture was notsatisfied. Invalid plane-strain fracture toughness (KQ)results were also obtained on three of the T-L speci-mens. The primary reasons that these test results areinvalid include: (1) insufficient specimen thickness, t, astB2.5 (KQ/sy)2 (where sy is the yield stress) and (2) theratio of the maximum load, Pmax, to conditional ‘frac-ture load,’ PQ, exceeded the required value of 1.1. Table3 indicates that the L-T specimen exceeded both thethickness and load ratio requirements, while two of theT-L specimens exceeded the required load ratio bysmall amounts (:0.9%).

JIC fracture toughness tests were performed on theremaining L-T (L-R) specimens in order to meet theplane-strain requirements. Both aluminum alloys dis-played nonlinear load–displacement behavior [8],which is indicative of crack-tip plasticity and ductile,slow-stable crack growth. The J resistance curves,shown in Fig. 1, illustrate that both materials exhibitductile tearing and stable crack growth as the fractureresistance increases with increasing crack extension [8].The J integral values were measured at the blunting lineand are reported as JQ values in Table 3. The slope ofthe JR curves was determined and used to determine thetearing modulus, TR. The results of TR and KQ are alsoindicated in Table 3. All four J tests satisfied thethickness and ligament requirements for a valid plane-strain test as both the specimen thickness and initialligaments exceeded 25JQ/sy [7]. Unfortunately, thecracks in all four J test specimens exhibited an unevencrack front that violated the straight-crack requirementspecified in the ASTM E813 standard [7]. As a result,all four J tests were judged to be invalid JIC test, asindicated in Table 3.

The T-L specimens generally evinced very flat frac-ture surfaces, which is consistent with the lower fracturetoughness observed in these materials. The fracturesurfaces for both materials in the L-R or L-T orienta-tion exhibited multiple crack fronts and a tortuouscrack profile. The fatigue crack precracks evinced athumbnail shape that was maintained during stablecrack growth in both materials. The stable crackgrowth region was delineated by marker bands formedduring fatigue precracking and post-test fatigue, priorto final fracture by overload.

The J tests were judged to be invalid because ofexcessive curvature in the thumbnail crack during sta-ble crack growth. It was examined whether the plane-strain J dominance occurs in a thumbnail crack and theobserved crack front unevenness (about 22% variation

displacements were the primary data recorded duringfracture testing. Periodic unloading was applied duringJ testing in order to determine the instantaneous cracklength. A computer-automated data processing pro-gram developed by Fracture Technology Associates, atBethlehem, PA, was used to analyze this load–displace-ment data recorded during testing. The critical loads atthe onset of crack extension were determined and theappropriate KQ and JQ values were evaluated on thebasis of the relevant crack lengths and critical loads.Assessments were made to determine whether the testsresulted in a valid KIC or JIC fracture toughness value,based upon a number of stringent specifications forspecimen thickness, load ratio, and crack-front straight-ness listed in ASTM E399 and E813.

Testing was continued until the specimens broke intotwo halves with the KIC tests. Testing was discontinuedafter the crack had extended by about 1.5–2.5 mm forthe J tests. The test specimens were then fatigued toinduce small amounts of crack extension for the pur-pose of delineating the shape and front of the stablecrack when the J testing was discontinued. Opticalmetallography was used to characterize the fracturesurfaces. The length of the final crack was measured atnine locations equally spaced on the fracture surfacesthrough the thickness of the specimens. These resultswere used to determine the average crack length andthe crack profile at the end of the J tests.

3. Results

The load–displacement curves for the KIC tests ofAA6069-T6 and AA6061-T6 exhibited small amountsof nonlinearity prior to the maximum load [8]. Theamount of nonlinearity was larger in the L-T specimensthan in the T-L specimens. The fracture toughness dataderived from the load–displacement plots are summa-

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Table 3Fracture toughness data for AA6069-T6 and AA6061-T6 aluminum alloys

Orientation Valid KQ (E399) KQ (E813) JQ (kJ m−2) Tearing modulusdThickness (mm) Valid Plane-strain JSpecimen IDMaterial KIC (MPa m−2)(TR) dominancee

48.7 Noa,bL-T6061-T6 25.3 F61e-L144.8 47.4 28.8 9.1 Noc YesF61e-L225.348.9 55.5 39.5 12.5 Noc Yes25.3 F61e-L3

51.9 34.2 10.846.948.7Average �

YesT-L 25.3 F61e-T1 33.1Nob25.3 F61e-T2 33.0Nob33.6F61e-T325.2

Average � 33.3

54.56069-T6 Noa,bF69e-L121.5L-T(L-R) 59.7 66.0 56.0 10.5 Noc Yes21.5 F69e-L2

62.2 70.7 64.1 14.7 Noc Yes21.6 F69e-L368.4 60.1 12.661.054.5Average �

T-L 21.6 F69e-T1 35.8 YesYesF69e-T2 35.1(R-L) 21.6Noa21.5 F69e-T3 38.7

Average � 36.5

a Crack length and thicknessB2.5 (KQ/sy)2.b Pmax/PQ\1.10: F61e-L1 (1.20), F69e-L1 (1.18), F61e-T2 (1.11), F61e-T3 (1.11).c Crack front unevenness\7% of crack length (thumbnail cracks); crack extension variation from edge to center\2% of width.d TR= (E/s0

2) (dJ/da), where so= (1/2)(sy+sUTS) is the flow stress, E is Young’s modulus, and dJ/da is the linear slope of the J resistance curve.e JQ/syoyt510 for plane-strain J dominance [4].

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[8]) renders the JQ measurement meaningless. Previ-ously, Nakamura and Parks [9] examined the condi-tions for J dominance in thin cracked plates usingthree-dimensional finite-element analysis. Their resultsindicate that the local J (Jlocal) along a straight crack ina thin plate is not uniformly distributed along the crackfront. Instead, Jlocal is highest at the center (midsection)of the thin plate and its value is larger than that of thefar field, Jfar. In contrast, Jlocal is the lowest with a valueless than Jfar at the free surfaces of the specimens. Aplot of the ratio of Jlocal/Jfar as a function of relativedistance normalized by specimen thickness, in Fig. 2,shows a profile that is reminiscent of the shape of athumbnail crack. The Jlocal/Jfar value differs by 30% forthe surfaces and the midsection for an elastic crack, andthe difference is increased to as much as 80% for anelastic-plastic crack subjected to a normalized load ofJ/syoyt=2, where oy is the yield strain and t is thespecimen thickness. Additional results including thosefor a fully plastic crack in a thin plate are given byNakamura and Parks [9]. Based on these results, Naka-mura and Parks established that plane-strain J domi-nance in a thin plate requires J/syoyt510 [9].

The initial crack front of the fatigue precrack wasrelatively straight. Upon loading in the J test, themidsection of the specimen is expected to sustain ahigher local J compared to the two surfaces based onthe 3D effect analyzed by Nakamura and Parks [9]. Asa result, the crack front at the midsection is expected toextend before those portions of the crack front near thefree surfaces. The consequence is more crack extensionin the midthickness than near the free surfaces, leadingto a thumbnail crack. On the basis of the 3D FEMresults, an uneven crack front is to be expected in the Jtests, even when plane-strain J dominance is satisfied.

The amount of crack front variation observed in thetest specimens is considerably less than that expected onthe basis of the Jlocal/Jfar distribution observed in the3D FEM results [9], as shown in Fig. 2. In addition,J/syoyt51.3 in the J tests performed in this study,compared to J/syoyt510 required for plane-strain Jdominance. The specimen thickness for the J tests inthis study is more than 7.5 times larger than thatrequired for meeting the plane-strain J dominance crite-rion proposed by Nakamura and Parks [9]. Based onthese considerations, plane-strain J dominance occurredin the J tests of the two aluminum alloys and the JQ

values should be considered valid JIC values despite theuneven crack front. Since small-scale yielding applies tothe J tests, the KQ values calculated from J test resultsin Table 3 should also be valid KIC values and they arein agreement with the KQ values determined using theE399 test procedures.

The JR curves indicate that both aluminum alloysexhibit ductile stable crack growth and the fractureresistance increases with increasing crack extension.The resistance-curve behavior might have contributedto the higher Pmax that invalidates some of the KIC testsfor the T-L specimens. If ductile stable crack growth isconsidered, the Pmax/PQ criterion would be irrelevantand the KQ results for the T-L orientation F61e-T2,F61e-T3 tests (see Table 3) may also be consideredvalid KIC results and justified on the basis of the JR

curves.

4. Conclusions

The fracture toughness of AA6069-T6 and AA6061-T6 alloys hot extruded from Air Slip™ direct chill(ASDC) ingot were determined by performing KIC andJIC tests. The following conclusions were reached: (1)The KIC of AA6069-T6 in the T-L (R-L) orientation is36.5 MPa m−2 compared to 33.3 MPa m−2 forAA6061-T6 in the T-L orientation. (2) The KQ forAA6069-T6 is 54.5 MPa m−2 in the L-T (L-R) orienta-tion compared to 48.7 MPa m−2 for AA6061-T6 in theL-T orientation. Plane-strain conditions were not metfor both alloys in this orientation. (3) The J test resultsfor both AA6069-T6 and AA6061-T6 did not meet theJIC criteria specified in ASTM 813 because of excessivecrack front variation due to a thumbnail crack geome-try. The test results, however, satisfy the criterion forthe plane-strain J dominance criterion established byNakamura and Parks based a 3D finite-element analy-sis. The J tests can be considered valid JIC tests basedon the criterion proposed by Nakamura and Parks. TheKQ calculated from the JQ for AA6069-T6 in the L-T(L-R) orientation is 68.4 MPa m−2 compared to 51.9MPa m−2 for AA6061-T6 in the L-T orientation.Thefracture toughness values of AA6069-T6 are higher

Fig. 2. Ratio of crack length to average crack length, a/aave, com-pared to ratio of local J, Jlocal, to far-field J, Jfar, as a functionrelative distance from the mid-section of the test specimen. Experi-mental results are compared against 3D FEM results of the crackprofile under plane-strain J-dominance conditions in elastic andplastic materials from Nakamura and Parks [9]. Plane-strain J-domi-nance criterion (J/syot t510) is met by the J-test since J/syoyt=1.3).

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than those of AA6061-T6 for both the L-T and T-Lorientations.

Acknowledgements

This work was supported, in part, by funding fromthe Oregon Joint Graduate Schools of Engineeringthrough the Oregon Metals Initiative.

References

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