Fatigue Properties of 6061-T6 Aluminum Alloy T-Joints Processedby Vacuum Brazing and TIG Welding

Huei Lin1, Jiun-Ren Hwang1,+ and Chin-Ping Fung2

1Department of Mechanical Engineering, National Central University, Taoyuan 32001, Taiwan2Department of Mechanical Engineering, Oriental Institute of Technology, New Taipei 22061, Taiwan

The increasing use of aluminum alloys in transportation including railways, shipbuilding and aeronautics demands more efficient andreliable welding processes, which requires sufficient understanding of fatigue failure. Tungsten inert gas (TIG) welding and vacuum brazing(VB) T-joints of Al­Mg­Si alloy 6061 in the artificially aged condition T6 were studied. This work focuses on the contrasting difference offatigue behavior of T-joints made from both the traditional process of TIG welding, and the emerging process of vacuum brazing. The fatigueproperties of AA 6061-T6 welding under constant amplitude (CA) and variable amplitude (VA) loading were studied. The aim of the CA part inthis paper was to identify the differences between brazing and welding on fatigue performance and size effect. The fatigue experiments of TIGwelding and vacuum brazing in 6061-T6 aluminum alloys were performed to investigate fatigue strengths. The test results were compared withthe results suggested by the International Institute of Welding (IIW), British Standard (BS) and Eurocode 9 recommendations. Meanwhile, interms of size effect, the thickness correction exponents were compared with the thickness correction exponents suggested by the IIW. The VApart of the work was examined to identify the effects of the mean stress which might increase fatigue lives less than predicted by linear damagesummation models. The effects of the mean stress in different correction methods were evaluated. [doi:10.2320/matertrans.M2015343]

(Received September 1, 2015; Accepted November 20, 2015; Published January 18, 2016)

Keywords: vacuum brazing, tungsten inert gas (TIG) welding, aluminum alloy, fatigue life, mean stress effect, size effect

1. Introduction

Recently, environmental problems and concerns aboutenergy consumption have resulted in the increasing use ofaluminum alloys in the automobile, aerospace and otherindustries. In particular, 6000 series aluminum alloys havebeen extensively studied because they have better strength,weldability, corrosion resistance, and cost than otheraluminum alloys.1,2) AA 6061 is one of the most versatileof the heat-treatable alloys and is popular for use in medium-to-high strength applications; it also has favorable toughnesscharacteristics. Aluminum alloy weldments with T-config-urations are becoming increasingly important in the transportsector, especially in aerospace and airplane manufacturing,shipbuilding, car body manufacturing and other areas.3) T-joints are generally fabricated by fusion welding, extrusionand rivet connection.

However, its porosities, cracks, high residual stresses anddistortions, are well known seriously to degrade the me-chanical properties of aluminum alloys T-joints, and cannotbe prevented by traditional fusion welding. This fact isconsistent with some results that have been obtained inscientific research into the fatigue resistance of welded T-joints. These investigations studied fusion welding processes.The effects of localized heating and subsequent rapid coolingon residual stresses and distortions have been studied usingvarious methods.4,5)

The frequently preferred process for welding aluminumalloy is tungsten inert gas (TIG) welding, which is relativelysimple and not too costly. However, this process causesgrains to coarsen in the fusion zone, distortion, an increasedtendency to undergo hot cracking and residual stresses.Vacuum brazing (VB) is used because it has none of thedisadvantages of TIG welding. Vacuum brazing is carried outin an especially designed furnace in the absence of air and it

has many advantages; it is conducted at 1.3332 © 10¹1 ³1.3332 © 10¹4 Pa, and so no oxidation problems arise andproduces flux-free braze joints with high integrity andsuperior strength. Vacuum brazing improves the uniformityof the temperature of the base metal and reduces residualstresses by using a slow heating and rapid cooling cycle,resulting in drastically improved mechanical and thermalproperties of the material to which it is applied. The processis utilized in various industries, including the automotive,aerospace, medical, defense industries and many others.

Welding is the primary joining method and fatigue is animportant design criterion for welded structures subjected tocyclic loading.6) However, as is well known, welded jointscan exhibit poor fatigue properties. In welding techniques,failure is a key problem that is related to the stability andsafety of the welded structure.7) The fatigue properties ofwelded structures are influenced by the welding material, theshape of the welded structure, the radius of the weld toe, theweld angle, the height and width of the reinforcement and thewelding quality.8­10) Most literature in fatigue of aluminumalloy welding were discussed the molten welding methods,such as inert-gas metal-arc welding,11) tungsten inert gaswelding12­14) and friction stir welding.15­19) Few studies wereinvestigated the diffusion bonding methods.

Several institutions divide aluminum alloy weldments intovarious categories, based on joint geometry and load type.Numerous relevant experiments are performed. The exper-imental data thus obtained and their analysis yield the stress-life relationship of each category, enabling design specifica-tions to be established. Research into fatigue has led tothe establishment of such design specifications as BritishStandard (BS) 8118, Eurocode 9, recommendations by theInternational Institute of Welding (IIW).

Owing to the complexity of the materials used and the localgeometry of welded structures, current fatigue-related spec-ifications of welded structures of aluminum alloy are some-times not met. This study considers the following problems.+Corresponding author, E-mail: jrhwang@ncu.edu.tw

Materials Transactions, Vol. 57, No. 2 (2016) pp. 127 to 134©2016 The Japan Institute of Metals and Materials

(1) Distinguishing between brazing and welding;(2) The effect of weldment thickness and the variable

amplitude loading on the fatigue life;This study examines the fatigue behavior of 6061-T6

aluminum alloy that has been T-welded by TIG welding andvacuum brazing. Fatigue tests were conducted using loadingsof constant and variable amplitudes. The results of the fatiguetests were compared with the fatigue design specifications ofEurocode 9, BS 8118, and IIW, and are found to providedesigners with improvements to fatigue design. With respectto weldment thickness, Goodman’s mean stress correctionmethod and Gerber’s mean stress correction method wereused to determine the effect of variable amplitude loadingon the fatigue life of TIG welding and vacuum brazingweldments.

2. Experimental Procedure

The materials that were used herein were 4mm-thick and10mm-thick sheets of Al-Mg-Si alloy (AA6061-T6). Thesheets were cut into rectangular plates with dimensions of500mm © 290mm and from which stringer plates with theuniform dimensions of 500mm © 50mm were prepared.Tungsten inert gas-welded joints were fabricated in a single-bevel T-joints configuration, as displayed in Fig. 1. AnER5356 grade filler rod was used in the TIG welding process.The shielding gas was highly pure argon. Multi-pass weldingwas performed to fabricate the joints. Figure 2 presents theconfiguration of the T-joints formed by vacuum brazing. Thecooling gas was nitrogen. The direction of welding wasnormal to the direction in which the rectangular plates wererolled. All necessary care was taken to prevent joint distortionand the joints were formed after the sheets were clamped.Table 1 presents the chemical compositions of the base metaland the weld metals. The 6061-T6 aluminum alloy in thevacuum brazing process was optimized as specified by thebrazing parameters, using the method of Lin.20) Table 2presents the process parameters of vacuum brazing.

The preparation of the test specimens followed theguidelines of the American Society for the Testing ofMaterials (ASTM). Figure 3 shows the geometries anddimensions of the tensile and fatigue specimens. The tensileproperties of the rectangular AA 6061-T6 plates wereevaluated in the transverse direction using a universal testingmachine, MTS 810. The deformation of the tensile specimenswas measured using an extensometer. The specimen finallyfailed after necking and the load was recorded as a functionof displacement.

Fatigue test specimens were prepared from welded jointsin the transverse direction of rectangular plates. Fatigue testswere conducted at room temperature using a servo-hydraulicmachine with a frequency of 10Hz and a stress ratio ofR = ¹1. Before the fatigue tests were carried out, thesurfaces of the specimens were polished in the loadingdirection using emery paper with a grade of up to 1000.

To characterize the welded joints, Vickers hardness wasmeasured using a microhardness tester with an indentationload of 0.3 kgf, consistent with the ASTM E384 standard.The microhardness was measured from the center to bothsides thereof until it was stabilized. The interval of measure-

ment of the vacuum brazing and TIG welding specimens was1mm.

Optical microscopic images of cross-sections of theweldments were captured. TIG welding and vacuum brazingT-joints were prepared for optical microscopic analysis. Afterthe specimens were cut, they were cold-mounted in resinpowder and liquid, and then mechanically ground andpolished. Then, they were etched using Keller’s reagent.De-ionized water and ethanol were subsequently used to

(a) (b)

Fig. 1 Configuration of T-joint for TIG welding: (a) 4mm; (b) 10mm (alldimensions in mm).

(a) (b)

Fig. 2 Configuration of T-joint for vacuum brazing: (a) 4mm; (b) 10mm(all dimensions in mm).

Table 1 Chemical compositions of the base metal and weld metals(mass%).

Type of Material Si Fe Cu Mn Mg Cr Zn Al

6061-T6 0.48 0.4 0.27 0.12 1.00 0.19 0.2 Bal

ER5356 0.25 ® 0.1 0.05 4.5 0.2 0.1 Bal

BAlSi-4 12 0.8 0.3 0.15 0.1 0.2 0.1 Bal

Table 2 Process parameters of vacuum brazing.

Soak temperature(°C)

Soak time(min)

Brazing temperature(°C)

Brazing time(min)

590 50 600 30

(a) (b)

Fig. 3 Dimensions of fatigue and tensile specimens: (a) 4mm; (b) 10mm(all dimensions in mm).

H. Lin, J.-R. Hwang and C.-P. Fung128

neutralize the specimens. All samples were then cleaned for20 seconds in ethanol in an ultrasonic bath, dried and placedin a desiccator until they were analyzed under a microscope.

To discuss the effect of thickness correction and meanstress on weldments, constant and variable amplitude fatiguetests were performed on weldments with thicknesses of 4mmand 10mm. In the variable amplitude tests, the transmission(TRN) and bracket (BRK) history that was developed by theSociety of Automotive Engineers (SAE) was utilized. Thetransmission history is the tensile mean stress and the brackethistory is the slight compression mean stress. As mosthistorical signals have small amplitudes, they have littleeffect on the assessed fatigue life. Accordingly, the signalswith small amplitudes are neglected to reduce markedly thelength of the history. The trajectory tracking method is ahistory-editing technique that ignores small amplitudes forsimplification and compression. The transmission historycomprised 1709 points, and was reduced to 103 points by thetrajectory tracing method, as shown in Fig. 4(a). The brackethistory had 5937 points, and was reduced to 414 points bythe trajectory tracing method, as shown in Fig. 4(b).

3. Results and Discussions

3.1 MicrostructuresSince TIG welding requires a large local heat input and

provides a low welding rate, it easily generates hightemperature in the welding bead and expands the HAZ.During welding, the temperatures of the weld bead and heat-affected zone differed, so the produced microstructures alsodiffered. Figure 5(a) displays the microstructure of the 6061-T6 welded zone; the high temperatures and rapid cooling inthe weld bead caused dendritic grains to be generated with anaverage grain size of 50 µm. Figure 5(b) shows the micro-structure of the heat-affected zone: the high temperature andslower cooling than that of the weld bead caused a columnargrain structure to be formed with gradually coarsened grainsthat had an average grain size of 67 µm, degrading themechanical properties. The temperature gradients dominatedthe formation of the different grain structures.

Vacuum brazing is a heating method that utilizes radiationand causes uniform heating and automatic temperaturecontrol. It can prevent local overheating and the formationof a HAZ. In this method, the deformation of the work pieceand the residual stress are minimal. Figures 6(a) and 6(b)

show the microstructures of a brazing bead and the basemetal, respectively, after vacuum brazing. No obviousdendritic or columnar grains are observed. The average grainsizes of the bead and post-brazing base material were 35 µmand 44 µm, respectively. In Fig. 6(a), many inter-metalliccompounds surround the diffusion layer of the brazing beadafter the metal elements in the filler diffuse toward the basemetal. According to Fig. 6(b), since the temperature invacuum brazing process was high and maintained for morethan an hour, most of the precipitates were solid-dissolved.After quenching with nitrogen gas, the solid solution formeda supersaturated phase, and then, a small quantity ofprecipitates was generated. Unlike TIG welding, vacuumbrazing causes no obvious variations among the brazingbead, the HAZ and the base metal.

3.2 MicrohardnessThe main testing surface in the microhardness test was the

side surface of the weldment. Figures 7(a) and 7(b) plotthe microhardness distributions that were achieved by TIGwelding and vacuum brazing, respectively.

(a) (b)

Fig. 4 Condensed SAE history: (a) transmission; (b) bracket.

(a) (b)

Fig. 5 Optical micrographs of TIG weldment: (a) welding bead; (b) heat-affected zone.

(a) (b)

Fig. 6 Optical micrographs of vacuum brazing specimen: (a) brazing bead;(b) base metal.

Fatigue Properties of 6061-T6 Aluminum Alloy T-Joints Processed by Vacuum Brazing and TIG Welding 129

Figure 7(a) indicates that, in TIG welding, under theinfluence of a local high temperature, the HAZ had the lowestmicrohardness with a microhardness value of HV66. Themicrohardness of the base metal was HV107. The HAZextended to approximately 22mm to the left and right of thewelding bead. In Fig. 7(b), no clear HAZ is observed becausethe weldment was heated to a uniform temperature in avacuum, preventing local overheating. The rapid cooling andpartial coarsening of precipitates reduced the microhardnessof the base metal from HV107 to HV75, which is still higherthan the microhardness of the HAZ in the TIG weldment.

3.3 Tensile propertiesTable 3 shows the tensile properties of two types of 6061-

T6 weldment. The tensile strength of the base metal was306.7MPa, and that of the TIG welding joint was 186.1MPa,which was 60.6% of the strength of the base metal, whereasthe tensile strength that was achieved by vacuum brazing was190.7MPa, which was 62.1% of the strength of the basemetal. The experimental results reveal that the tensilestrength achieved using vacuum brazing exceeds that ofTIG welding. When the vacuum brazing temperature iselevated to 600°C and then rapidly reduced using nitrogengas, precipitation hardening improves the mechanical proper-ties of the weldment.

In TIG welding, local-high-temperature molten weldingminimizes the tensile strength of the HAZ, causing neckingthere, and the consequently formed fracture surface isclassified as ductile, as shown in Fig. 8(a). In vacuumbrazing, the specimen fractures at the brazing toe and thefracture surface exhibits necking, so the fracture surface isclassified as ductile, as shown in Fig. 8(b).

3.4 Fatigue propertiesThe fatigue properties of aluminum T-joints are studied.

The fatigue design curve of aluminum T-joints is classified asIIW FAT 28, BS 8118 class 29 and Eurocode 9 category 31.

When Nf < 2 © 106, the S-N equation can be written as:

LogNf ¼ LogK1 �mLogSn � Z· ð1Þwhere Nf represents the fatigue life in cycles; K1 is theempirical constant; m is the slope coefficient of the S-N

curve; Sn is the nominal stress; Z is the number of standarddeviations; · is the standard deviation of the LogNf

distribution. Table 4 lists the parameters for that are obtainedfrom the S-N curve of T-joints of AA 6061-T6.

Figure 9 plots the S-N curves of AA 6061-T6 T-joints thatwere formed by TIG welding and vacuum brazing, and therecommended design curves for three specifications. Thestress amplitude in the fatigue experiments was 35% to 54%of the tensile strength. The 50% survival probability wasobtained from experimental data by linear regression. Theexperimental S-N curve with 95% survival probability wasobtained from the standard deviation shift with Z95.

Figure 9 indicates that the S-N curves of both TIG weldingand vacuum brazing T-weldments were higher than the

(a) (b)

Fig. 7 Schematic diagram of microhardness tests: (a) TIG welding; (b) vacuum brazing.

Table 3 Tensile properties of different AA 6061-T6 weldments.

Welding methodYield strength

(MPa)Tensile strength

(MPa)Elongation

(%)

TIG welding 148 186.1 10.9

Vacuum brazing 122.8 190.7 11.7

(a) (b)

Fig. 8 Fracture surfaces for tensile specimens: (a) TIG welding;(b) vacuum brazing.

Table 4 Parameters for the S-N curve of T-joints of AA 6061-T6.

Welding method K1 m · Z95

TIG welding 3.3 © 1020 6.75 0.09 3.07

Vacuum brazing 4.21 © 1034 13.15 0.04 3.07

H. Lin, J.-R. Hwang and C.-P. Fung130

recommended design curves of IIW FAT 28, BS 8118 class29 and Eurocode 9 category 31. These three specifications areconservative. The slope of the S-N curve of the TIGweldment exceeded that of the vacuum brazing weldment.Regarding to the influence of welding and brazing process onfatigue strength, TIG welding was superior to vacuumbrazing. This discrepancy in the welding process producesdifferences in fatigue strength and the slope of the S-N curve.

Safety factors are obtained from experimental and designcurves.

f�· ¼ �·ExpðNrÞ=�·codeðNrÞ ð2Þwhere ¦·Exp and ¦·code are the stress range relative toexperimental results and that indicated in the design fatiguecurve for the same number of cycles to failure, respectively.

Safety factors on stress range are listed in Table 5 andplotted in Figs. 10(a) and 10(b) of TIG welding and vacuumbrazing respectively. The following observations are made.(1) Safety factors vary and become larger as life duration

increases. The fact that life durations of greater than theendurance limit of 105 cycles are expected from classIIW FAT 28 was taken into account.

(2) Safety factors associated with TIG welding are higherthan those associated with vacuum brazing in the rangeN = 104­105. Safety factors associated with vacuumbrazing are exceeds those associated with TIG welding

in the range N = 105­2 © 106.(3) For short life durations, safety factors of TIG welding

and vacuum brazing are below the value of 2, asindicated by IIW FAT 28, BS 8118 class 29, andEurocode 9 category 31.

The fatigue specimen that was formed by TIG weldingruptured in the HAZ. Since fatigue failure of the weldment isdominated by the crack growth, the cracks in the fatiguespecimens were generated most easily in the HAZ. The HAZwas subjected to the high-temperature effect of TIG welding,so it exhibited grain coarsening and the lowest tensilestrength, causing fatigue failure to occur there. Figure 11(a)shows the fracture surface of TIG welding specimen in,which reveal a large number of smaller dimples distribute onthe bottom side of fracture surface. The fracture surface of theTIG welding specimen shows a ductile fracture dominated bydimples due to microvoid coalescence. The fatigue specimenthat was formed by vacuum brazing ruptured near thewelding bead. Fatigue cracking is easily initiated on surfacedefects or in areas of lower hardness. Figure 11(b) shows thefracture surface of fatigue specimen in vacuum brazing. Thedimples are found on the top side of fracture surface. Thefracture modes are ductile fracture.

3.5 Size effectThe fatigue strengths of welded joints that fail from the

Fig. 9 S-N curves.

Table 5 Safety factors for TIG welding and vacuum brazing.

(a) TIG welding

N (cycles) f¦· IIW f¦· BS 8118 f¦· Eurocode 9

104 1.56 1.49 1.57

105 2.39 2.28 2.28

106 3.65 3.5 3.32

2 © 106 4.14 4 3.74

(b) Vacuum brazing

N (cycles) f¦· IIW f¦· BS 8118 f¦· Eurocode 9

104 1.28 1.22 1.28

105 2.31 2.21 2.2

106 4.16 3.99 3.79

2 © 106 4.96 4.79 4.48

(a) (b)

Fig. 10 Safety factors: (a) TIG welding; (b) vacuum brazing.

Fatigue Properties of 6061-T6 Aluminum Alloy T-Joints Processed by Vacuum Brazing and TIG Welding 131

weld toe are widely thought to decline as the plate thicknessincreases.21) Recent work has shown that the thickness effectdepends on the overall proportions of the welded joint.22,23)

The fatigue rules in Eurocode 9 accounts for this effect. Afurther refinement in the IIW recommendations modifies thethickness correction exponent p for various welds. Valuesrange from 0.3 to 0.1, reflecting the fact that the thicknesscorrection also depends on the degree of concentration ofstress that is induced by the welded joint. The lower fatiguestrength of thicker members is accounted for by multiplyingthe FAT class of the structure by the thickness reductionfactor f(t).

In this study, the experimental S-N curves of the AA6061-T6 T-joints with thicknesses of 4mm and 10mm are used todiscuss the thickness correction factors for different fatiguelife. Figure 12 plots the S-N curves of the AA6061-T6T-joints with thicknesses of 4mm and 10mm.

From experimental results, thickness correction exponentis obtained:

tefftref

� �P

¼ fref

feff

� �ð3Þ

where teff = 4mm; tref = 10mm; fref is the fatigue strengthrelative to experimental 10mm S-N curve; feff is the fatiguestrength relative to experimental 4mm S-N curve.

Table 6 lists the thickness correction exponents. Thefollowing facts are observed.

(1) The thickness correction exponents are never constant.The thickness correction exponents for TIG weldingdecrease as life duration increases. The thicknesscorrection exponents for vacuum brazing becomeslightly larger as life duration increases.

(2) For TIG welding, the thickness correction exponents inthe lower life region are consistent with the P valuerecommended in the IIW specification. TIG welding issuitable for the thickness correction exponents of theIIW specification.

(3) The thickness correction exponents in vacuum brazingdiffer from the P value recommended by the IIWspecification. In the lower life region, vacuum brazingdoes not require thickness correction. However, in thehigher life region, vacuum brazing is required to carryout thickness correction.

3.6 Mean stress effectThe mean stress significantly affects the fatigue strength.

When ·m > 0, ·m increases; whereas Sf decreases. When·m < 0, «·m« increases; whereas Sf increases. In this study, thetransmission and bracket histories developed by the Societyof Automotive Engineers in the U.S. was utilized; the meanstress is tensile and slight compression, respectively. The twomean stress correction equations of Goodman and Gerberwere used in the fatigue life analysis.

(a) (b)

Fig. 11 Fracture surfaces for fatigue specimens: (a) TIG welding; (b) vacuum brazing.

Fig. 12 S-N curves for the AA6061-T6 T-joints with thickness of 4mm and10mm.

Table 6 Thickness correction exponents for TIG welding and vacuumbrazing.

(a) TIG welding

N(cycles)

PTIG

(thickness correction exponent)

110000 0.33

210000 0.27

1300000 0.11

(b) Vacuum brazing

N(cycles)

PVB

(thickness correction exponent)

150000 0.04

260000 0.07

420000 0.09

H. Lin, J.-R. Hwang and C.-P. Fung132

Goodman:·a

Se¼ ·m

Su¼ 1 ð4Þ

Gerber:·a

Seþ ·m

Su

� �2

¼ 1 ð5Þ

The predicted and experimental life of the specimens ofthickness of 4mm were compared, using the S-N curves fromthe experimental TIG welding and the vacuum brazing data,following mean stress correction using eqs. (4) and (5),respectively.

Figure 13(a) plots the predicted and experimental lifeachieved by TIG welding given the transmission history; thatobtained using the Goodman mean stress correction methodagreed closely with the experimental life. Figure 13(b) plotsthe predicted and experimental life achieved using vacuumbrazing given the transmission history. The predicted lifeachieved using vacuum brazing given the specified trans-mission history, obtained using the Goodman mean stresscorrection method, agreed closely with the experimentallife.

Figure 14(a) plots the predicted and experimental lifeachieved suing TIG welding under the bracket history. Thepredicted life achieved using TIG welding under the brackethistory, according to the S-N method agreed closely with theexperimental life. The S-N method predicts life without meanstress correction. Figure 14(b) plots the predicted andexperimental life achieved using vacuum brazing under thebracket history. The predicted life achieved using vacuumbrazing under the bracket history using the Goodman mean

stress correction method agreed closely with the experimentallife.

As demonstrated by the above results, given the trans-mission history, the Goodman mean stress correction methodpredicted the life achieved using both the TIG welding andvacuum brazing specimens with aluminum alloy 6061-T6.The predicted fatigue life achieved using TIG welding andvacuum brazing specimens under the bracket history could beused in the S-N method and the Goodman mean stresscorrection method, respectively.

4. Conclusion

This study discussed the mechanical properties ofAA6061-T6 weldments that were prepared by TIG weldingand vacuum brazing. Fatigue tests were carried out on TIGwelding and vacuum brazing of T-joints. The effects ofspecimen thickness and variable amplitude loading on fatiguelife were presented. The following conclusions are drawn.(1) The experimental results reveal that the tensile strength

achieved using vacuum brazing exceeds that achievedby TIG welding. Since vacuum brazing involved anelevated temperature of 600°C and rapid cooling usingnitrogen gas, precipitation hardening occurs, improvingthe mechanical properties of the weldment.

(2) Experimental S-N curves of TIG welding and vacuumbrazing T-weldments were higher than the recom-mended design curves of class FAT 28 in IIW, class 29in BS 8118 and category 31 in Eurocode 9. These three

(a) (b)

Fig. 13 Comparison between the predicted life and experimental life under TRN history: (a) TIG welding; (b) vacuum brazing.

(a) (b)

Fig. 14 Comparison between the predicted life and experimental life under BRK history: (a) TIG welding; (b) vacuum brazing.

Fatigue Properties of 6061-T6 Aluminum Alloy T-Joints Processed by Vacuum Brazing and TIG Welding 133

specifications are conservative.(3) With respect to the size effect in TIG welding, the

thickness correction exponents in the lower life regionare consistent with the P value recommended in the IIWspecification. The thickness correction exponents invacuum brazing differ from the P value recommendedby the IIW specification.

(4) The fatigue life of the TIG welding and vacuum brazingspecimens of aluminum 6061-T6 under the trans-mission history can be predicted using the Goodmanmean stress correction method.

(5) The fatigue life of the TIG welding specimens given thebracket history can be predicted using the S-N method.The fatigue life of the vacuum brazing specimens underthe bracket history can be predicted using the Goodmanmean stress correction method.

Acknowledgments

The authors would like to thank the Ministry of Scienceand Technology, R.O.C. for their financial support of thisresearch.

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H. Lin, J.-R. Hwang and C.-P. Fung134