Hardness-tensile property relationships for HAZ in 6061-T651 alum.pdf

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Introduction

Alloy 6061 belongs to the 6XXX se-ries of wrought heat-treatablealuminum alloys. These are of the (Al-Mg-Si) ternary system, in which mag-nesium and silicon are the major alloy-

ing constituents; they can also be ther-modynamically approximated as apseudo-binary (Al-Mg2Si) system.Alloy 6061-T651 is solution heat-treated, quenched, stress-relieved bystretching, and then artificially agedto peak hardness. By solution heat-

treating and quenching the alloy, itsubsequently produces asupersaturated solid solution, arelatively soft structure that exists in athermodynamically metastable state.If this is followed by an aging heattreatment, it induces precipitation ofthe excess solute atoms and,ultimately, the dissociation of the su-persaturated solid solution into theequilibrium phase. It is the precipita-tion of these excess solute atoms thatprovides the alloy with enhancedstrengthening properties, by acting asbarriers to dislocation movement.Peak hardness is associated with thecombination of the intermediatephases " and . However, alloys agedbeyond peak hardness experience a de-crease in hardness associated with anincrease in the interparticle spacingbetween precipitates, which makesdislocation bowing much easier. Thisis commonly referred to as theoveraged condition. Porter and Easter-ling (Ref. 1) have reported thatincreasing the time and/ortemperature used for heat treatmentof a system subsequently induces anincrease in the driving force for precip-itation and precipitate coarsening. Du-molt et al. (Ref. 2) as well as Enjo andKuroda (Ref. 3) associate the softeningof the alloy with the transformation of phase into the phase withincreasing aging time and/or tempera-ture. The Ostwald ripening of precipi-tates increases the interparticle spac-ing, giving a more coarse dispersion

HardnessTensile Property Relationships for HAZ in6061T651 Aluminum

Hardness is a reliable method of characterizing the tensile strength of the HAZ;relationships between hardness and tensile properties were established

P. A. STATHERS, A. K. HELLIER, R. P. HARRISON, M. I. RIPLEY, AND J. NORRISH

ABSTRACT Highstrength aluminum is used extensively in industry, with welding being awidely used fabrication method. This work focuses on welding of 6061T651aluminum and establishment of the hardnesstensile properties relationship in theheataffected zone (HAZ) of a gas metal arc weld using 4043 filler material. Test weldswere prepared from 12.7mmthick plate with a singleV weld preparation. Base platetemperatures were measured with an array of eight embedded thermocouples during welding, relating temperature to properties at intervals from the weld. Throughthickness slices 1.7 mm thick were removed, by electric discharge machining, fromthe plate parallel to the weld at 2mm intervals and extending from the weld centerline to 40 mm into the HAZ and base plate. Tensile samples were prepared fromthese slices, and tensile properties and hardness values measured to establish a relationship between these two parameters. Both EQUOTIP (portable hardness tester)and Vickers microhardness measurements were conducted and related to tensileproperties. Although a significant body of work exists relating tensile properties tohardness, no previous study was found that used this approach. Most work appearedto use crossweld tensile tests, which only give the point of lowest strength. Sectionsof base plate material having a different thickness (31.75 mm) from that of thewelded samples, and from a different source, were thermally aged to four hardnessvalues and the hardnesstensile relationship was also established for this material.These results were compared with those of the HAZ samples; the results were foundto fall within the scatter band of HAZ results.

KEYWORDS6061T651 Aluminum Alloy 4043 Filler Metal Gas Metal Arc Welding (GMAW) Gas Tungsten Arc Welding (GTAW ) HeatAffected Zone (HAZ) Weld Metal Vickers Microhardness Test EQUOTIP Portable Hardness Tester TensileProperties

P. A. STATHERS was materials engineer, R. P. HARRISON is program leader, Structural Integrity, and M. I. RIPLEY is visiting fellow at the Institute of MaterialsEngineering, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia. A. K. HELLIER is visiting fellow, School of Mechanical andManufacturing Engineering, The University of New South Wales, Sydney, Australia. J. NORRISH is professor, Materials Welding and Joining in the Faculty ofEngineering, University of Wollongong, Australia.

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as a result of the bigger, more stableprecipitates consuming the smaller,less stable precipitates surroundingthem. Welding is a popular method ofjoining 6061 aluminum (Ref. 4) and ithas been demonstrated (Refs. 5-7)that the heat-affected zone (HAZ)and the fusion zone metal have lowerstrengths (softening) resulting fromthe welding process, if welded in theT6 condition. This process of overag-ing occurs because the microstructurehas been modified by exposure to atemperature/time regime that

produces lessthan optimalstrength.These weld-inducedchanges inproperties in-fluence thedesign ofstructures:the designer

must take into account themagnitude of the change inproperties, and the location and ori-entation of the weld with respect tothe direction of applied stress.Although hardness measurementshave long been used to indicate theextent of the HAZ in welds (Ref. 8),no systematic study could be foundthat attempts to relate hardness tomeasured properties on a single weldmetal and accompanying HAZ. Therelationship between hardness andyield stress for all alloy systems ingeneral is well established (Ref. 9),

but that between hardness and othertensile properties is less so. The pres-ent paper is taken from theexperimental work in the dissertationof Ref. 10 and is aimed at providing amodel relating tensile properties tohardness in the 6061-T651 baseplate4043 filler metal combinationby sectioning thin zones of the weldand HAZ. Although taking out microtensilespecimens at different locations fromthe welded plate is an originalapproach, mention should be made ofan alternative method using Gleeblemachine experiments in which it ispossible to reproduce the thermal cy-cles at any location in the HAZ,thereby producing a tensile specimenwith more or less homogeneous mate-rial properties. These types of experi-ments are used to verify numericalmodels and the results from thisactivity are extensively published(Refs. 1122). Such models can becoupled with a finite element code,e.g., WELDSIM (Refs. 23, 24), to cal-culate material properties at any loca-tion in the HAZ as a function of weld-ing parameters and geometry of thewelded parts. These calculations aremore or less standard procedure atHydro Aluminium Structures, N-2831, Raufoss in Norway, for weldedautomotive components. However,from a practical standpoint, hardnessmeasurements are quicker and morestraightforward than numerical calcu-lations of the HAZ properties. Previous workers, e.g., Malin (Ref.25), used cross-weld test samples asthe basis for the hardnesstensile re-lationship, the position of the breakon the tensile sample being related toits location with respect to the weld.The present work examines the ten-sile properties at 2-mm (0.079-in.) in-tervals from the center of the weld to

Fig. 1 Weld preparations for test plates.

Fig. 3 Tensile sample design. Overaged base plate (Plate 1). Dimensions are in mm.

Fig. 2 Macrographs of welded plates. Scales are in mm. A Plate 2A (singleV GMAweld); B Plate 2B (singleV GTAweld); C Plate 2C (doubleV GMA weld).

Table 1 Conversions from SI Units to U.S. Customary Units

SI Unit Conversion Factor U.S. Customary Unit

1 millimeter (mm) 0.0393701 inch (in.)1 meter (m) 3.28084 foot (ft)1 milliliter (mL) 0.0338140 uid ounce (U.S.)1 liter (L) 0.264172 gallon (U.S.)1 kilogram (kg) 2.20462 pound (lb)1 kilonewton (kN) 0.224809 kilopoundforce (kip)1 megapascal (MPa) 0.145038 kilopoundforce per square inch (ksi)1 deg Celsius (C) = 9/5 deg Fahrenheit (F)Temperature in deg Fahrenheit (F) = 9/5 x temperature in degrees Celsius (C) + 32

Table 2 Test Plate Material Specication

Plate No. Alloy Thickness Specication Lot No. AsSupplied Hardness(mm) EQUOTIP Vickers

LD HV(20)

1A 6061T651 31.75 ASM QQA250/11 Lot 304831 456 112

2 6061T651 12.7 ASM QQA250/11 #681417 465 114Type 200 Alcoa

A B C


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the unaffected base metal. Table 1contains all necessary conversionsfrom SI units to U.S. customary units.

Experimental Materials

Specification Two thicknesses of plate were used:31.75 mm (1.25 in.) and 12.7 mm (0.5

in.). The 31.75-mm plate is referred toas Plate 1 and the 12.7-mm plate,Plate 2. Both plates were supplied tothe same specification (ASM-QQA-250/11) and were provided in theT651 temper. Table 2 gives details ofthe plate and initial hardness readingfrom each plate. Table 3 gives thechemical analysis of the two test platesas well as the nominal composition of

the alloy for comparison. The Zn con-tent of Plate 1 was found to be out ofspecification; however, the small devi-ation is not expected to affect the me-chanical properties to any significantextent. Both plates supplied were factory-marked at regular intervals along theirlength with alloy, temper,specification, and lot number for secu-

Fig. 4 Overaged base plate (Plates 1A1 to A4) sample removal locations. Note that only the tensile samples were used inthe present work reported in this paper.

Fig. 6 HAZ tensile sample design. Dimensions are in mm.

Fig. 5 Extraction layout for HAZ samples removed from Plate2A. Dimensions are in mm.

Fig. 7 Overaged base plate (Plate 1). Longitudinal 0.2% yieldstress and tensile strength as a function of EQUOTIP and Vickershardness.

Table 3 Chemical Analysis of Test Plates (wt%)

Test Plate Si Cu Fe Mg Zn Cr Mn Ti Ni Sr Zr Al

1 0.67 0.30 0.61 1.05 0.42 0.20 0.10

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rity of identification. Subdivisions ofthe original 31.75-mm plate were as-signed an alphabetical postscript (Aand B); further subdivisions wereassigned a further numerical subdivi-sion (Plate 1 A1, A2, A3, A4) andtest samples produced from theseplates were assigned individual testcode numbers (TXXXX).

Filler Metal

A new spool of 4043 filler metal wasused to weld the gas metal arc welded(GMAW) test plates. The spool of 1.6-mm wire had been sealed until use. Gastungsten arc welds (GTAW) wereprepared using 2.5-mm-diameter 4043filler metal.

Experimental Procedure

Overaging of Test Plates

The objective was first to producefour plates with different hardnessesby overaging heat treatments.Hardness vs. tensile properties werethen obtained using full-size tensilesamples. These results were comparedwith values obtained from the subsizetensile test pieces extracted from thewelded plates, to ensure that therewere no sample size effects. Plate 1A was cut into four pieces,each measuring approximately 500 250 mm. Three of these were heattreated to overage the material to vari-ous hardnesses. Type K thermocouples

were attached to the plates to monitortheir surface temperatures during heattreatment. Heating times and tempera-tures are shown in Table 4 together withthe hardnesses measured for each plate.

Preparation of Welded Test Plates

Three welded test plates were pre-pared in accordance with therecommendations in AS 1664 (Ref.26); Table 5 summarizes the prepara-tion and welding method used in eachcase. Details of the weld preparationfor both types of test plate (single- anddouble-V) are shown on the weld pro-cedure sheets for each weld Fig. 1.

Welding Procedure

Gas metal arc welding wasperformed using a Synchro-pulseCDT Model No. CP 34 welding unitmanufactured by Welding Industriesof Australia (WIA). This unit uses asquare wave pulsed waveform. A cop-per backing bar, 100 mm wide by 6mm thick, was fitted under thesingle-V weld preparation. The weldpreparations and interpass regionswere thoroughly cleaned withacetone and stainless steel wirebrushing immediately prior to weld-ing. The shielding gas was weldinggrade argon at a flow rate of 26L/min. The single-pass GMA weldwas mechanically back-machined to adepth of 4 mm to remove any defectsin the root run; this groove was thenfilled with a single weld pass.

Fig. 8 Overaged base plate (Plate 1). Transverse 0.2% yieldstress and tensile strength as a function of EQUOTIP and Vickershardness.

Fig. 9 HAZ throughthickness tensile sample results vs. EQUOTIPhardness and Vickers microhardness, GMAweld (Plate 2A).

Table 4 Base Plate Heat Treatments and Hardnesses

Plate Number Temperature Time EQUOTIP Vickers(C) (h) Hardness Hardness

LD HV(20)

1A1 Asreceived material 456 1101A2 247 0.7 413 911A3 260 9.5 354 711A4 260 9.5

270 65300 2.5 313 57

Table 5 Summary of Welded Test Plates

Plate Thickness Weld Preparation Welding Method Weld Procedure Sheet No.(mm) (as per Fig. 1) (see Appendix 1 in Ref. 10)

2A 12.7 Single Vee GMAW Pulsed WP12B 12.7 Single Vee GTAW AC WP22C 12.7 Double Vee GMAW Pulsed WP3


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Gas tungsten arc welding was con-ducted using a Hitachi inverter GTAWmachine 300GP, model 300A AD-GPVE used in AC square wave mode.The shielding gas employed was weld-ing grade argon at a flow rate of 5L/min. The single-V GTAW plate wasback-machined to a depth of 4 mmand three weld passes were then usedto fill the weld preparation groove. Ineffect, this weld emerged as an unbal-anced double-V weld. Prior to welding, a distance scalewas marked on the surface of theplates to measure welding travel speedin order to estimate the heat input tothe work. Waveforms of the GMAWvoltage were recorded at regular inter-vals during the welding procedure, inorder to store the waveform andobtain the RMS value of the weldingvoltage. Welding currents andvoltages were recorded manually dur-ing the procedures as summarized inTable 6. Measurements of the wirefeed speed, travel speed, and other

variables wererecorded on thewelding proceduresheet.

Radiography ofTest Plate

Test plates wereradiographed tochoose defect-freetensile samples.Minor porosity wasobserved and waslocated principallyin the weldreinforcement.

Metallographic Examination ofWelds The test plate was sectioned toprovide a range of other samplesincluding some for metallographic ex-amination of the welds. Transverse

through-section macrosections wereprepared and the samples etched inKellers reagent (2.0 mL of HNO3, 1.5mL of HCl, 1.0 mL of HF, and 95 mLof H2O). This etchant revealed the

Fig. 11 Plot of actual and predicted tensile results based onthe hardness model. The hardness data for the fitted resultswere obtained from an independent hardness traverse of a different section of the sample, GMAweld (Plate 2A).

Fig. 10 HAZthroughthickness tensile sample results vs. distance from weld centerline, GMAweld (Plate 2A).

Table 6 Summary of Welding Conditions

Plate Weld Process Filler Wire No. of Passes Arc RMS Current Travel Wire Feed Power Drawn Linear EnergyID Prep Voltage, I (A) Speed, S Rate P = I V Q = P/S(kJ/mm)

V (volts) (mm/s) (mm/s) (kW) (see note)

2A Single GMAW 4043, 5 + 1 3.22* 185 8.19.7 38.7 0.596 (3.22V) 0.07 (3.22 V)Vee Pulsed 1.6mm pass on back nominal avg. 8.9 3.700 (20 V) 0.42 (20 V)

diam. wire 20 V2B Single GTAW 4043, 4 + 3 22** 230295 1.451.95 N/A 5.786 3.4

Vee AC 2.5mm passes avg. 263 avg. 1.7diam. on backrod

2C Double GMAW 4043, 3 per side 3.22* 185 1018.3 38.7 0.596 (3.22V) 0.07 (3.22 V)Vee Pulsed 1.6mm nominal avg. 14.2 3.7 (20 V) 0.26 (20 V)

diam. 20 Vwire

* Voltage measured between electrode and work.** Nominal operating voltage only.Note Both the measured and nominal voltages have been used to calculate the linear energy (Q). The value of 3.22 V appears to be very low compared with the nominal operatingvoltage.

Fig. 12 Vickers microhardness results vs. EQUOTIP results,GMA weld (Plate 2A).


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macrostructure of the weld metal andHAZ Fig. 2.

Estimation of Weld Metal DilutionFactor

The weld filler metal (4043) will bediluted by the 6061 base material andthis dilution affects the mechanicalproperties of the weld. This dilutionfactor has been estimated from themacrosections of the weld and theoriginal weld preparation profile. Theaverage dilution factor is 18.6% of thebase plate in the weld metal. The dilu-tion is not constant over the wholeweld section; the root runs will have ahigher dilution factor and theinterpass runs will have a lower

dilution factor due to the fact thatthey will be diluted by weld materialthat has already been diluted by thebase plate; the weld reinforcement willhave lowest dilution. The dilution factor that best appliesto the tensile sample test sectionremoved from the weld metal is13.9%.

Measurement of Temperature Profileduring Welding

The temperature profileexperienced by the welded test plateswas measured by instrumenting thetest plates with eight 0.5-mm-diame-ter type K thermocouples. Thethermocouples were embedded to a

depth of 6.35 mm (midthickness); asthe two half test plates were of thesame dimensions and the welding pro-cedure was symmetrical, only one sideof the assembly was instrumented.Temperature was continuouslyrecorded during welding by acomputer-based monitoring andlogging system (Ref. 10).

Preparation of Tensile Samples

Two tensile testing programs wereconducted: Tensile testing of overaged plate inboth the transverse and longitudinaldirections. These results were used tocheck the hardnesstensilerelationship established by the HAZtensile samples. Testing of through-thicknesstensile samples that were cut (usingEDM) parallel to the direction of weld-ing. These samples were prepared inorder to test thin zones of HAZ mate-

Fig. 13 Hardness profile of midthickness singleV GMA weld (Plate 2A).

Fig. 15 This work compared with thework of Ref. 25. (Reproduced from Welding Journal, September 1995, p. 313s,by V. Malin.)

Fig. 16 Comparison of aswelded results from The Aluminum Association (Ref.8) with this work. (Reproduced courtesy ofThe Aluminum Association, Inc.)

Fig. 17 Comparison of results fromthis work with TWIreference (Ref. 29).(Reproduced courtesy of The Welding Institute Ltd.)

Fig. 14 Hardness profile of single and doubleV GMAwelds plus singleV GTA weld.


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rial and establish the hardnesstensileproperty relationships.

Tensile Samples from Overaged Base Plate

Tensile samples were prepared fromthe four overaged 6061 plates. Thesesamples were prepared in accordancewith Australian Standard AS 1391-1991 (Ref. 27). The sample design isshown in Fig. 3 and the location fromwhich the samples were cut is shownin Fig. 4. The samples were tested onan INSTRON 8501 servo-hydraulictesting machine at a strain rate of 8 10-5/s and in accordance with AS1391-1991.

Electric Discharge Machined SamplesExtracted from the HAZ Samples were extracted from thesingle-V GMA weld plate (Plate 2A).Through-thickness tensile sampleblanks were electric dischargemachined (EDM) from the plate, start-ing from the center of the weld metaland progressing away from the weld at2-mm intervals, to a distance of 40mm from the weld. A series of 20 sam-ples at 2-mm intervals from the

centerline of the weld was prepared,with particular care taken to ensurethere were no sequencing ororientation errors, nor any bending ordeformation. Figure 5 shows theextraction procedure and cutting lay-out for the welded test plate. The testsection is small and the thermal differ-ence across the test section is also be-lieved to be small. The above process produced sampleblanks that were approximately 1.7mm thick, 65 mm long, and 12.7 mmwide. The width of the EDM wire cutwas approximately 0.3 mm. Electricdischarge machining is performed atroom temperature; however, there is a

damaged layer resulting from the elec-tric discharge. These damaged layerswere removed by hand grinding bothsides of each specimen blank on wetsilicon carbide papers. Figure 6 showsa drawing of the HAZ tensile samples.

Hardness Testing of All Samples Vickers microhardness andEQUOTIP hardness testing were per-formed on all samples and heat-treated test plates. Overaged baseplate material was hardness tested intwo locations on each sample usingboth a Leco model M-400-H2 Vickersmicrohardness testing machine and an

Table 7 Summary of HAZ Tensile and Hardness Results

Table 8 Summary of Fitted Equations for Tensile EQUOTIP Hardness Relationship (Results ShouldBe Rounded to Nearest MPa)

Data Group Regression Fitted Equation to Data Coecient of Determination (R2)

HAZ 0.2% YS Y = 1.035x 194.79 0.9927HAZ TS Y = 0.8424x 73.529 0.98

Parent 0.2% YS Y = 1.2687x 282.05 0.9978TransverseParent TS Y = 0.9621x 115.46 0.9993

TransverseParent 0.2% YS Y = 1.4111x 335.48 0.999

LongitudinalParent TS Y = 0.9614x 118.8 0.9961

Longitudinal


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EQUOTIP hardness tester. Loads of 1and 2 kg were used for the Vickersmeasurements and the average of allfour readings taken; individualreadings varied by no more than 2Vickers microhardness units for bothloads and all readings. Hardness read-ings were also taken using the full-sizeVickers diamond indentation methodand these readings were also within 2Vickers units of the micro-Vickersmeasurements. An average of 12 read-ings, 6 on each side, was taken usingthe EQUOTIP hardness tester with theD indenter. Hardness measurements were alsomade using the Leco microhardnesstester and the EQUOTIP hardnesstester on all 20 through-section HAZtensile samples. Average readings fromtwo sites on each side of each samplewere taken. The hardness sites were atapproximately the midthickness of theplate in order to obtain hardness read-ings that were as close to either end ofthe tensile test section as possible, sothat the indentations did notinfluence the tensile properties.

Tensile Testing of ThroughThicknessHAZ Samples These samples were tested on an IN-STRON 8501 servo-hydraulic testingmachine fitted with a 5-kN calibratedload cell and the data logged usingpurpose-written software. A 10-mmgauge length extensometer with a 1-mm range was attached to the sample,allowing sample strains up to 10% tobe measured. The extensometer wascalibrated prior to use and was foundto have a linear error of 2.25%, thisbeing corrected in software to betterthan 0.5% accuracy. Data logging

included the time, extensometer read-ing, load, and cross-head position. Thetensile testing conditions were thesame as those used for testing theoveraged 6061 tensile samples. A 0.2%offset strain was used to determinethe yield strength. The tests were conducted in accor-dance with AS 1391-1991. The 0.2%yield stress (YS), tensile strength(TS), and elongation were measuredon each sample. The samples weretested in strain control up to approxi-mately 2% strain at an extension rateof 0.001 mm/s, which gave a strainrate of 1 10-4/s (AS 1391 L1 strainrate range). After 2% strain themachine was switched to positioncontrol and run at 0.05 mm/s untilsample fracture.

Results

Overaged Base Plate Tensile and Hardness Test Results Tensile and hardness results forsamples extracted from the overagedbase plates are shown in Figs. 7 and 8.Results for longitudinal 0.2% yield andtensile strengths as a function ofmeasured EQUOTIP and Vickers hard-nesses are shown in Fig. 7. Results fortransverse 0.2% yield and tensilestrengths as a function of measuredEQUOTIP and Vickers hardnesses areshown in Fig. 8. Longitudinal andtransverse elongation, as well as reduc-tion of area, were obtained as a func-tion of measured EQUOTIP and Vick-ers hardnesses. These results are con-sidered to be outside the scope of thispaper, but are contained in Ref. 10(this dissertation may be borrowed oninterlibrary loan from the ANSTO or

University of Wollongong Libraries).

Description of Weld Macrosections Macroetching successfully revealedthe weld runs and HAZ of the weldsection. Some porosity was visible inthe weld, this being consistent withthe radiographs taken of the test plate.As mentioned previously, the porositywas mainly confined to the reinforce-ment. Some misalignment of the runon the reverse side exists and this isplainly visible in the macrograph. Mis-alignment of the two plates, due todistortion, occurred and was in theorder of 1 mm; this was notconsidered important for the remain-der of the experiment. A high level ofporosity was evident in the reinforce-ment of the backing pass. This regionof the weld was subsequentlymachined away.

Tensile and Hardness Results for HAZSamples Extracted from GMA Weld(Plate 2A) A summary of the tensile resultsand hardness measurements is shownin Table 7. The diagram at the bottomof the table indicates the location ofthe hardness tests; suchmeasurements were taken on bothsides of each tensile blank. BothEQUOTIP and Vickers microhardnessmeasurements were taken at sites A,B, C, and D (with sufficient spacing toensure no interaction). Reference 10 contains the tensilecurves for all test samples; the 0.2%yield stress graph and complete tensilegraph are shown as separate graphs.The tensile curves were produced withthe extensometer attached to a strainof 10%, which is its nominal limit.These curves have been retained inthis work because they may be usefulfor future finite element modeling ofthe HAZ behavior; the data can also bemade available in electronic format.Some high-elongation curves will betruncated due to the extensometerreaching its stops before failureoccurred. The cross-head position wasalso logged; however, these data werenot plotted. All results in this sectionare related to the single-V GMA weldtest Plate 2A. All the results shown inFigs. 914 are taken from Ref. 10.

Table 9 Summary of Fitted Equations for Tensile Vickers* Hardness Relationship (Results ShouldBe Rounded to Nearest MPa)

Data Group Regression Fitted Equation to Data Coecient of Determination (R2)

HAZ 0.2% YS Y = 2.9263x 44.289 0.9703HAZ TS Y = 2.4079x + 46.39 0.979

Parent 0.2% YS Y = 3.2637x 65.518 0.99TransverseParent TS Y = 2.4828x + 48.1266 0.9974

TransverseParent 0.2% YS Y = 3.6337x 94.949 0.993

LongitudinalParent TS Y = 2.4848x + 44.32 0.9976

Longitudinal

*Vickers microhardness HV (average of 1000 g and 2000 g).


Discussion

Hardness was found to accuratelypredict the yield strength (0.2% offsetYS) and tensile strength (TS) inwelded 6061-T651 test plates Fig.11. Separate hardness profiles wereconducted on another section of theGMA weld (Plate 2A) and found to bein good agreement with measured val-ues. The test results from Plate 1(Table 2) which had been overaged tofour hardness levels, show that thestandard size tensile and hardness re-sults compared well with those of theHAZ samples. This indicates that theuse of subsize tensile samples fromthe HAZ did not affect the accuracy ofthe results. The agreement betweenPlate 1 results and the HAZ sample re-sults was found to be very good Fig.12. Only the transverse results fromthe base plate have been used for com-parison purposes as this was the sameorientation as the miniature HAZ ten-sile samples. The longitudinal tensileresults from the base plate gaveslightly higher results than the trans-verse results. If hardness results are tobe used to estimate tensile propertiesin both the longitudinal andtransverse directions for the purposesof finite element modeling, then it isworth making the corrections toobtain longitudinal results. Hardnesstest methods used in this work do notdiscriminate between longitudinal andtransverse hardness. For this reason, itis important to establishtransverselongitudinal propertyratios if an additional degree of accu-racy is required. Comparisons between fitted and ac-tual results for the tensilehardnessrelationship showed a linearcorrelation. The correlation was verygood (coefficient of determination>0.99) for the overaged base platesamples, and slightly lower for theHAZ samples (~0.98). The increasedvariation in the HAZ samples is proba-bly a real effect, and may reflect varia-tions in the weld metal and HAZ; ob-servations of the etchedmacrosections indicate thatsubsequent weld passes had modifiedthe structure of underlying weldpasses, and variations in the dilutionfactor may also affect the results. It would appear that the rate ofheating does not influence the

hardnesstensile relationship. TheHAZ results were obtained with heat-ing rates typical of the welding processand heating took place in the order ofseconds. The hardnesstensilerelationship obtained from theseresults gave very similar results tothose obtained from the overaged baseplate material that was heated to lowertemperatures for longer times. Itwould appear that overaging might bedescribed by a singletimetemperature parameter and anopportunity may exist to study thisphenomenon; however, this is beyondthe scope of the present work. Tables 8 and 9 provide a summaryof the linear regression fitted lines tothe experimental data for theEQUOTIP and Vickers hardnesstensile properties relationship, forboth the HAZ and base plate samples.Hardness values (x) can be substitutedinto these equations to obtain YS orTS. The R-squared value is also shownas an indication of the percentage ofthe response variable variation that isexplained by the linear model, and ascan be seen this is excellent, a value of1 (100%) indicating a perfect fit.

Comments on the Shape of the Curves It will be observed from this workand that of others that as the distancefrom the weld interface of the weld in-creases, the hardness drops, reaches aminimum, and then increases againprogressively to the original base platehardness. Figure 13 shows that thehardness increases at a distance ofabout 5 mm from the centerline of thefusion zone, which corresponds to thelocation of the fusion zone edge. Theexplanation for this characteristicshape is given by Malin (Ref. 25). Atthe weld interface, the precipitates of and are dissolved at the hightemperature and enrich the solid solu-tion with Si and Mg, which results insolid-solution hardening of thematrix. As the distance increases, aminimum hardness is reached andthen the hardness increases again. Inthis area, the HAZ is experiencing pro-gressively lower temperatures as thedistance from the weld increases. Thedegree of hardness reduction isproportional to the distance from theweld; that in turn is proportional totemperature, and this is related to the

coarsening of the precipitates. precipitates increase in size withincreasing temperature, and this weak-ens the structure and reduceshardness. Figure 10 shows themaximum elongation is also obtainedat about 5 mm from the weldcenterline, and this corresponds to theweld interface where the structure isclosest to the fully solution-treatedstate. In Fig. 9, which shows HAZhardness vs. tensile strength, theresults for the HAZ are a good fit for alinear relationship between strengthand hardness; however, the results forthe samples extracted from the weldmetal do not fit the straight line plotfor the HAZ. The hardnessstrengthrelationship shows hardness reachesan approximately minimum value andremains constant. This effect isthought to be the result of a complexinteraction of thermal effects and dilu-tion factor in the weld zone. Figure 10, a plot of strength andelongation as a function of distancefrom the weld centerline, shows thatthe strength returns to base plate val-ues at a distance of approximately 20mm from the weld centerline for Plate2A. The elongation has a minimum ofapproximately 5% near the center ofthe weld and reaches a maximum ofapproximately 16% near the weldinterface; the material properties inthis region are more characteristic ofthe 4043 filler metal modified by thedilution from the base plate. Figure 14, which contains plots ofhardness vs. distance from the weldcenterline, shows the effect of heatinput resulting from different weldpreparations and welding methods.The single-V GTA weld has the highestheat input, resulting in lowerhardnesses for a greater distance fromthe weld centerline compared to thesingle- and double-V GMA welds; thedouble-V GMA weld with the lowesttotal heat input shows the highesthardnesses and least overall reductionin strength in the HAZ.

Comparison with Other Results

No direct comparison with thiswork was found for 6061-T6 or 6061-T651 aluminum. This work involved asystematic study of thin sections ofHAZ material as a function of distance

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from the weld and included the meas-urement of hardness. Malin (Ref. 25),however, has carried out a thoroughand systematic study of the Knoophardness and microstructural proper-ties of welds in 6061 aluminum as afunction of distance from the weld.The study also included cross-weldtensile tests, and compared thefracture location and tensileproperties with the hardness at thatlocation in the weld. From theseresults, conclusions were drawn aboutthe hardnesstensile relationship.This approach, naturally, gives onlyone tensile result per weld. Malinswelds were also 6061-T6 using 4043filler metal and the GMAW method;however, the welds were performed onextruded sections that were thinnerthan those used in the present work. As a comparison with the presentwork, Fig. 15 has been reproducedfrom Malins work with the present re-sults, converted to Knoop hardnessusing the conversion tables in ASTM E140 (Ref. 28), superimposed on the di-agram. Malin used three different heatinput values and the results shown arefor the heat input closest to this work,i.e., corresponding to a current of ap-proximately 185 amps. Also shown on Malins diagram inFig. 15 is the temperature profile as afunction of distance from the weld in-terface. The present temperatureresults, taken from Ref. 10, for theroot run of the GMA weld (Plate 2A)have been superimposed on thisdiagram for comparison. The rootweld run also produced the highesttemperature on that plate. The hardness results are in reason-able agreement with Malins work,with hardnesses extending above andbelow his results. This may beexplained by the difference in sectionthickness used to produce thedifferent welds. The magnitude of thehardness peak close to the weld inter-face was not as high as that observedby Malin. This was possibly due to theinfluence of thermal modification ofthe structure resulting fromsubsequent weld passes; Malins workinvolved a single weld pass. The tem-perature profile results are very closeto those of Malins work. Other work was referenced in Weld-ing Aluminum: Theory and Practice (Ref.8) and gave a relationship between dis-

tance from the weld and the hardness.This has been reproduced in Fig. 16and the present results aresuperimposed on the graph. Atdistances of approximately 512 mmfrom the weld, the present work givesa very close approximation for bothdistance and hardness values. As withthe comparison with Malins work,lower hardnesses were observed closeto the weld and higher hardness wasmeasured close to the weld interface.This weld was produced using theGTAW process with no details of fillermetal given. The higher hardness closeto the weld interface was not observedto the same degree; this was possiblydue to thermal modification of thestructure by subsequent weld passes. The Welding Institute (TWI) hasproduced a graph of hardness vs. yieldstrength for all aluminum alloys from1XXX to 7XXX (Ref. 29). The presentyield strength results for 6061-T651aluminum were superimposed on theTWI graph (Fig. 17), and were found tobe very close to the fitted line for allalloys. This is an interesting result be-cause the HAZ samples from the pres-ent work were a mixture of Al-Si alloy(4043 filler) and 6061 alloy, and thehardnesstensile relationshipappeared to hold reasonably well forall samples. Comparison with the work of oth-ers (Refs. 8, 25, 29) indicates that theresults obtained in this work correlatewell with published data. An extensiveliterature review did not reveal adirectly comparable study performedin a similar manner to this work. Mostof the tensile results uncovered werecross-weld results, and from these re-sults, conclusions were drawn aboutthe hardnesstensile propertyrelationship in 6061 aluminum. Thepresent work has extracted miniaturesamples from the HAZ and madedirect measurements of tensile andhardness properties on small zones ofthe HAZ and weld metal.

Conclusions

Hardness was found to be a reliablemethod of estimating the yield andtensile strength of the heat-affectedzone (HAZ). A relationship betweenhardness and tensile properties wasestablished for both Vickersmicrohardness and EQUOTIP portable

hardness testing. These relationshipshave been expressed mathematicallyas follows:

Vickers Microhardness 0.2% Yield stress = 2.9263 HV 44.289 Tensile strength = 2.4079 HV + 46.39

EQUOTIP Portable Hardness Tester (D indenter) 0.2% Yield stress = 1.035 LD 194.79 Tensile strength = 0.8424 LD 73.529 Notes: Stress values are in MPa(and should be rounded to the nearestMPa). HV Vickers microhardness diamondindenter hardness values. LD is Leeb units produced with theEQUOTIP D indenter. Although the relationships were es-tablished with Vickers microhardnessindenters, comparison between Vick-ers microhardness and full-size hard-ness results indicate that the above re-lationships should hold for full-sizeVickers hardness tests. The tensile data obtained from thesections of HAZ could be useful for fi-nite element modeling of weld-zonebehavior, and all curves have been re-tained. The hardnesstensile model estab-lished for 6061 aluminum welded with4043 will be useful in establishing theextent of the HAZ, and assigning ten-sile properties to regions within thiszone based on hardnessmeasurements. From a practicalstandpoint, hardness measurementsare quicker and more straightforwardthan numerical calculation of the HAZproperties.

The authors wish to thank the Aus-tralian Nuclear Science andTechnology Organisation (ANSTO) forthe provision of project funding, labo-ratory facilities, and developmentworkshop support, as well as theCooperative Research Centre forWelded Structures (CRC-WS) at theUniversity of Wollongong, Australia,

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Acknowledgments


for offering the Master of EngineeringPractice Degree in Materials Weldingand Joining undertaken by P. A.Stathers. Appreciation is alsoexpressed to Associate Professor S. R.Yeomans from the Australian DefenceForce Academy (ADFA) for sharing hisideas for this work.

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References

AWS Expands International Services With international membership on the rise, the American Welding Society(AWS) launched a series of country-specific Web sites known as microsites formembers to access information in their native languages. Multilingual microsites are now live for Mexico at www.aws.org/mexico, China atwww.aws.org/china, and Canada (English/French) at www.aws.org/canada. Theyfeature information on services offered by AWS in each country, membership ben-efits, exposition information, online education, and access to AWS publications andtechnical standards. Other countries will be added later.


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Hardness-tensile property relationships for HAZ in 6061-T651 alum.pdf