Alu Tig 6061

1 Copyright © 2012 by ASME

Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition

IMECE 2012 November 9-15, 2012, Houston, Texas, USA

Paper Number: IMECE2012-85889

EFFECTS OF RESIDUAL STRESSES AND THE POST WELD HEAT TREATMENTS OF TIG WELDED ALUMINUM ALLOY AA6061-T651

Mohammad W. Dewan Department of Mechanical Engineering

Louisiana State University Baton Rouge, Louisiana 70803, USA

Jiandong Liang Department of Mechanical Engineering


M. A. Wahab Department of Mechanical Engineering


Ayman M. Okeil Department of Civil and Environmental Engineering


ABSTRACT Heat treatable AA-6061 T651 Aluminum alloys (Al-Mg-

Si) have found considerable importance in various structural

applications for their high strength to weight ratio and corrosion

resistance properties. Weld defects, residual stresses, and

microstructural changes are the key factors for the performance

reduction as well as failure of welded structures. Tungsten inert

gas (TIG/GTAW) welding was carried out on AA-6061 T651

Aluminum Alloy plates using Argon/Helium (50/50) as the

shielding gas. Non-destructive phased array ultrasonic testing

(PAUT) was applied for the detection and characterization of

weld defects and characterization of the mechanical

performances. In this study, ultrasonic technique was also used

for the evaluation of post-weld residual stresses in welded

components. The approach is based on the acoustoelastic effect,

in which ultrasonic wave propagation speed is related to the

magnitude of stresses present in the materials. To verify the

estimated residual stresses by ultrasonic testing, hole-drilling

technique was carried out and observed analogous results. The

effects of post weld heat treatment (PWHT) on the residual

stresses, grain size, micro hardness, and tensile properties were

also studied. The grain size and micro hardness were studied

through Heyn’s method and Vickers hardness test, respectively.

Lower residual stresses were observed in post-weld heat-treated

specimens, which also experienced from microstructure and

micro hardness studies. The PWHT also resulted enhanced

tensile properties for the redistribution of microstructures and

residual stresses.

INTRODUCTION AA-6061 T651 Aluminum Alloy is a heat treatable alloy,

has high strength and corrosion resistance properties. It is used

in various structural applications. Magnesium and silicon are

added either balanced amounts to form quasi-binary Al-Mg2Si

or with an excess of silicon, needed to form Mg2Si precipitate

(Lakshminarayanan et al., 2009). Al-Mg-Si alloys find wide

applications for its weldability advantages over other high

strength aluminum alloys (Dudas and Collins, 1966; Metzger,

1967). It is widely used in the aircraft industry, and has

gathered wide acceptance in the fabrication of lightweight

structures. The increased use of aluminum alloy calls for more

efficient and reliable welding processes which has always

represented a great challenge for designers and technologists.

For aluminum alloy, generally Friction -Stir Welding (FSW)

and fusion welding are used to make a joint. Two of the most

common fusion welding practices are tungsten inert gas (TIG)

and metal inert gas (MIG) welding. TIG welding is a high

quality weld that uses a non-consumable electrode and smaller

current compared to MIG welding. Kumar and Sundarrajan,

2006 studied the effects of welding parameters on the

mechanical properties of the as-welded condition for aluminum

alloy AA6061-T6. High coefficient of thermal expansion of

aluminum, solidification shrinkage, and high solubility of

hydrogen during its molten state creates problem during fusion

welding of aluminum alloys (Lakshminarayanan et al., 2009).

All of these factors can have variable degrees of decrease in

strength along the weld and its surrounding area.

During the welding process, the exposure to high

temperature followed by cooling near the weld causes the

grains to coarsen in the heat- affected- zone (HAZ) and induce

residual stresses along the weld line and in the HAZ (Leggatt,

2008). The materials in the HAZ effectively becomes softer

and more susceptible to failure (Malin, 1995). The material on


the surface or nearest to the weld is the last to cool; and the rest

of the material causes this portion of the weld plate to form a

tensile residual stress. In some materials, the maximum tensile

residual stress is equal to that of the yield strength of the

material. The resistant of the welded joint to expand and

contract has an effect on the various residual stresses in each

direction; transversely, longitudinally, and in the direction

normal to the plane of welding. Different factors play a role in

the magnitude of stresses that accumulate along the weld. The

geometry of the weld, the pass sequence (single or multi-pass

welds), or the use of fabrication aids, such as jigs, tacks, or

cleats may have the direct effect on the development of

residual stresses on the welded joint. During the in-service

operation of welded parts residual stresses can cause harmful

damages. Therefore, measuring of the amount of residual

stresses in a welded structure has a great importance. Over the

last few decades various residual stress measurement

techniques have been developed. In general, these techniques

are qualified as destructive and non-destructive techniques.

Most common destructive techniques are the hole-drilling

method, the ring core technique, the bending deflection method,

and the sectioning method (Ajovalasit et al., 1996; Rossini et al.

2012). These methods are widely used in industry and they are

sensitive to the macroscopic residual stress levels. Non-

destructive methods are developed on the basis of the

relationship between residual stress and the physical or

crystallographic parameters. Different non-destructive

techniques are developed such as the X-ray diffraction method,

the neutron diffraction method, the ultrasonic method, and the

magnetic method. X-Ray diffraction method is used for

measurement of surface and subsurface stresses. It can be

defined as a surface method. On the other hand, neutron

diffraction method allows measurement up to the depth of 50

mm. X-Ray and neutron diffraction methods are expensive and

cannot be carried out in-situ and requires the removal of

components (Rossini et al., 2012). Non-destructive ultrasonic

testing can be used in most materials to measure residual

stresses. Variations in the velocity of the ultrasonic waves can

be related to the residual stress state (Sanderson and Shen,

2010). Ultrasonic waves and acoustoelasticity allows

measurement of surface and subsurface residual stresses.

Surface and subsurface stresses can be determined by using

shear waves or longitudinal waves. Many attempts have been

proposed for this purpose. Recent studies are mostly focused on

critically refracted longitudinal (LCR) wave method (Clark and

Moulder, 1985; Bray, 2001; Uzun and Bilge, 2011). This

technique allows measurement of in-plane stresses. Surface

stresses, as well as bulk stresses can be determined by using

ultrasonic longitudinal waves. Longitudinal waves polarize in

the same direction that it propagates. Anisotropy in the material

caused by stress, affect the propagation velocity of longitudinal

waves. Stresses normal to the wave propagation direction can

be measured using the longitudinal waves.

During the welding process, microstructure of the material

changes and this causes the variations of wave velocities within

the Heat-Affected-Zone (HAZ). Effect of stress on wave

propagation was investigated by Hughes and Kelly in their

study entitled “Second- Order Elastic Deformation of Solids”,

in 1953. They have determined the velocities of longitudinal

and shear waves as a function of applied stress by subjecting

the material to hydrostatic pressure which is defined as

compression. The expression relating to the velocity of a wave

propagating in the longitudinal direction to an internal stressed

field can be written as:

𝑣−𝑣

𝑣 = 𝐾1𝜎1 + 𝐾2( 𝜎2 + 𝜎3) (1)

Where, 𝑣0 in the wave speeds in an unstressed medium, 𝑣 is

the velocity of an ultrasonic wave propagating in an stressed

medium, 𝜎1, 𝜎2, and 𝜎3 are principal stresses, and 𝐾1, 𝐾2 are

the acoustoelastic constants. If the measurement is made in a

single propagation direction (for instance direction-1), the

above equation can be simplified and expressed as follows:

𝑣−𝑣

𝑣 = 𝐾1𝜎1 + 𝐾2 𝜎2 (2)

For the majority of materials studied, 𝐾1 ≫ 𝐾2 (Thompson,

1996), so the above equation can be reduced and the residual

stress component can be calculated by following relationship:

𝜎1 =𝑣−𝑣

(𝐾 ×𝑣 ) (3)

The acoustoelastic constant 𝐾1 relates to the ultrasonic velocity

to the stress, and can be obtained experimentally.

Acoustoelastic constant is determined as the relation between

the total residual stresses normal to the wave propagation and

ultrasonic wave velocity variation. This constant is calculated

by observing wave velocity variations due to applied stress.

From the slope of the wave velocity change vs. stress,

acoustoelastic constant is determined. In this study we have

used acoustoelastic constant 𝐾1 = 5.05 × 10−6(𝑀𝑃𝑎)−1.

Ultrasonic longitudinal waves are propagated through the

thickness of the material and wave transit time is measured.

Pulse - echo technique and through transition techniques are

able to measure wave transit time. From the time measurement

sound velocity can be measured by knowing the thickness. As a

result of these measurements average residual stress through

the thickness of the material can be measured.

Post- weld –heat- treatment (PWHT) is an option to

recover strength in HAZ of heat- treatable alloys, caused due to

weld thermal cycle. For AA6061, ageing, or precipitate

hardening, is one form of post weld heat treatment (PWHT).

During the ageing process material is kept to a specified

temperature for an extended period of time, depending on the

type of material being used, and the types of precipitates.

Exposing the material to a temperature for longer than required

for artificial age hardening can cause the precipitates to grow

too large and more widely dispersed in the material (Tan and

Said, 2009). This effect causes the material to become softer

and loses its strength. So, optimum ageing temperature and


time required are necessary to obtain better strength. Closely

packed atoms of the solute form required in the solution first.

The atoms then form Guinier-Preston (GP) zones which are

connected with the solvent matrix (Gao et al., 2002). Recent

studies on the effect of PWHT on AA-2219 joints showed

significant improvement in the mechanical properties of the

weldments (Liu et al., 2006). Mechanical properties of TIG

welded AA-8090 alloys were enhanced by PWHT due to grain

refinement (Ravindra and Dwarakadasa, 1992). Uniformly

distributed Mg2Si precipitates, smaller grain size, and higher

dislocation density have been shown to be the reasons of

enhanced mechanical properties due to PWHT of FSW

AA6061 alloys (Elangovan and Balasubramanian, 2008). In the

literature, it is shown that the slight improvement in yield

strength, tensile strength, and hardness of the welded joints can

be achieved by solution treatment followed by artificially

aging (Metzger, 1967; Periasamy et al., 1995).

The general lack of data on residual stresses and PWHT on

the mechanical performances of TIG welded AA6061-T651

alloys with AA-4043 filler metal has prompted this present

experimental study. This research aimed at conducting a

systematic study to determine weld defects and residual stresses

by using nondestructive ultrasonic testing. The effect of PWHT

on the residual stresses, tensile properties, and micro hardness

were also investigated. The fracture morphology was studied by

using scanning electron microscopy (SEM) micrographs. To

observe the effect of PWHT on grain size optical micrographs

were analyzed for grain size determination by Heyn’s method.

EXPERIMENTAL PROCEDURE

Rolled plates of AA-6061 T651 with 6.35 mm thickness

were TIG welded according to AWS welding codes for

aluminum (AWS Welding Code, 2008). The welding and

testing procedures are shown in table 1. The initial joint

configuration was obtained by securing the plates in position

using precision guided rails and tack welding. The welding

direction was normal to the rolling direction and all necessary

care was taken to avoid joint distortion by clamping the plates

at suitable positions. Multi- pass welding was used on both

sides to fabricate the butt joints. A gas mixture of

Argon/Helium (50/50) was used as shielding gas as this mixture

helps in the constriction of the arc and concentrates the heat

with in a restricted area, thereby reducing the size of the heat-

affected-zone (HAZ) (Howse and Lucas, 2000). Welding was

followed by natural ageing at room temperature for 48 hours.

All the welds were visually and ultrasonically inspected for

defects.

After scanning by phased array ultrasonic testing, the

specimens were cut from defect- free regions according to

ASTM standard for tensile testing (ASTM, 2004). To study the

influence of post weld heat treatment (PWHT) on residual

stresses and mechanical properties the welded joints were

subjected to different heat treatment processes. For Solution

treatment (ST) welded specimens were heated at 530°C for 1 h

followed by quenching in water, and maintained at room

temperature. For solution treated and age hardening (STAH)

specimens were heated at 530°C for 1h and then quenched in

water, maintained at room temperature, followed by aging at

160°C for 18 h. For age hardening (AH) as welded specimens

were artificially aged at 160°C for 2 hours to 24 hours. In

previous study (Kardak and Wahab, 2011), showed that the

artificial age hardening at 160°C for 18 hours offer optimum

tensile and micro hardness properties of TIG welded AA6061-

T651 aluminum alloy. In this study, we have used artificial age

hardening to obtain PWHT specimens. As -welded specimens

were age hardened into a conventional oven at 160 °C for 18

hours and then cooled at room temperature. For comparisons

we have tested as welded specimens (without age hardening)

and PWHT (with age hardening).

Tensile tests were carried out at room temperature using an

MTS-Universal Testing Machine. For comparisons we have

tested base materials, weld material with transverse center

weld, weld materials in parallel to weld direction, and HAZ

materials. The tensile properties (0.2% proof strength), ultimate

tensile strength, and %age elongation were evaluated using at

least 10 samples in each condition prepared from same weld

joint. All samples were mechanically polished and

ultrasonically tested before tests to eliminate the effect of any

discontinuities present. The hardness across the weld cross

section was measured using Vickers Micro-hardness testing

machine. The hardness was measured at the center of the cross

section as shown in Fig. 1.

Figure1: Schematic diagram of showing Hardness

measurement position of TIG welded AA 6061 aluminum

alloy.

After the hardness testing, the samples were

metallographically polished according to ASTM standard and

etched with Keller’s reagent to expose the grain boundaries.

Optical micrographs were taken using light optical microscope

(Nikon MM-11) equipped with image analyzing software

(SPOT Software version 4.7) to analyze the variation of grain

size due to heat treatment (HT). SEM and EDAX analysis was

conducted using Hitachi S-3600N system.

Residual stresses of the as-welded (AW) and heat- treated

(HT) specimens were calculated using nondestructive

ultrasonic testing. To compare the ultrasonic testing results

destructive hole-drilling method was used to measure residual

stresses. The hole drilling method for surface residual stress

evaluation was conducted according to ASTM E837-0. Type B

strain gage rosettes were used (Fig. 2). By removing the

material in the hole through drilling, the residual stress is

relaxed and hence the principle in-plane residual stresses are

evaluated through the difference in strain values. Thus, the

stresses in specific directions could also be estimated.


Figure 2: Stain gage rosette and wiring for residual stress

measurement by hole-drill method.

Table 1: Experimental procedures

Welding Process: Tungsten Inert Gas (TIG) welding

Materials: AA6061-T651 aluminum plate, 6.5 mm thickness

(ALCOA MILL PRODUCTS, INC.)

Standard: AWS Welding Code D1.2/D1.2M standard, 2008

Weld Type: Double V, groove angle 45°, root opening 3.5 mm,

and root face 3.5 mm

Electrode: tungsten electrode, diameter 2.38 mm

Shielding gas: Argon/Helium (50/50)

Filler rods: AA-4043 (AlSi5), diameter: 1.6 mm (American

Welding Products, Inc.)

Weld current: 115 -120 amps

Welding speed: 120 – 140 mm/min

Uniaxial tensile test: MTS 810 Servo-hydraulic universal

testing machine

Standard: ASTM E8M-04 standard

Test speed: 0.05 mm/sec

Hardness test: Vickers micro-hardness tester

(SunTech FM-1e)

Load: 100 gf, Indentation period: 15 seconds

Microstructural analysis: Scanning electron microscope

(SEM) and optical microscope (OM)

Etchant: Keller reagent (1% hydrofluoric acid, 1.5%

hydrochloric acid, 2.5% nitric acid and 95% DI water)

Residual stress measurement: Hole-drilling method

Standard: ASTM E837-0 standard

Data acquisition unit: InstruNet100 (Omega)

Strain gage: Strain gage rosette (3 strain gages)

Specific directions: 0°, 45° and 135°

Drilling speed: 4000 rpm

Residual stress measurement: Ultrasonic testing

Ultrasonic pulser/receiver: Panametrics (model: 5900PR,

frequency range: 1 kHz – 200 MHz)

Transducers: Panametrics longitudinal wave fingertip size

transducers (model V112, maximum frequency: 10 MHz)

PCI digitizer board: Acqiris PCI digitizer (maximum sampling

rate: 420 MS/s)

Couplant: Sonotech Inc.’s Ultragel II couplant

Weld flaw detection: Phased array ultrasonic testing

(PAUT)

Equipments: OmniScan MX2, 16 elements phased array

probes, wedges, and a manual encoder (Olympus)

RESULTS AND DISCUSSIONS

Mechanical and morphological analysis

The welded aluminum plate was inspected by using both

visual and ultrasonic inspections for weld defects. Phased array

ultrasonic technique was used to detect weld defect precisely.

From the phased array ultrasonic testing we obtained A, S, and

C scans to detect defects up to 1mm (Fig. 3). From the A-scan

view prominent sharp peaks indicate the defect locations. The

color change (yellow and red color) in S and C scan indicates

the defects in the welded structure. From the C scan data we

can find the exact position of the defect along the weld

direction. From the S scan view we can get the exact size and

shape of the defects. In this study we have used phased

ultrasonic scans to find defect free tensile test specimens for

better comparisons.

Figure 3: Typical A, S, and C scans display showing a

discontinuity in TIG- welded AA6061 T651 joint.

The longitudinal, HAZ, transverse, and heat treated

transverse tensile properties of TIG welded AA6061 T651

aluminum alloy butt-joints are presented in Fig. 4 below. At

least 10 specimens were tested from each category. HAZ and

parallel to weld (longitudinal) direction tensile tests were

performed to see the effect of weld materials and HAZ area

alone on the tensile properties. The average ultimate tensile and

yield strength of the longitudinal weld was 251 and 167 MPa,

respectively. The average ultimate and yield strength of heat

affected zone was 201 and 162 MPa, respectively. The average

ultimate and yield strength of the center welds were 178 and

153 MPa, respectively; whereas, the average ultimate and yield

strength of base material are 330 and 290 MPa, respectively.

The weld and HAZ areas are more susceptible to failure. The

effect of heat treatment on transverse tensile properties of as-

welded, welded and post weld heat treated (PWHT), and base

materials are shown in Fig. 4(d). These are representative

tensile test curves. As-welded (AW) joints had average yield

strength of 153 MPa and ultimate tensile strength of 178 MPa,

indicating a 45-50% reduction in strength when compared to

the base parent metal. Both welded and heat treated specimens

showed average yield strength 172 MPa and ultimate strength

Weld defects

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197 MPa. The yield strength and the ultimate tensile strength of

PWHT joints were about 15% greater than those of as-welded

(AW) joints. The AW joints showed a joint- efficiency of 54%,

while PWHT joints had a joint- efficiency of 60%.

Figure 4: Stress-Strain diagram along loading direction (a)

parallel to weld center line, (b) heat affected zone , (c)

perpendicular to weld center line, (d) Base metal,

perpendicular to weld center line (without heat treatment

and with heat treatment).

Ahmad, and Bakar in 2011 used GMAW (MIG) process to join

AA6061- T6 aluminum alloy and obtained similar effect of

PWHT. After PWHT, they obtained 3.8% higher tensile

strength compared to untreated samples. They have used

artificial aging at 160°C for 20h. They also showed 25.6%

improvement in Microhardness strength due to PWHT. All the

base material specimens failed in the same manner, 45° shear

plane, whereas for AW joint, the failure occurred in the weld

metal region. However, for HT joints fracture initiated in the

HAZ and then final fracture occurred in the weld metal region.

Microhardness tests were performed to characterize the

Vickers hardness profile along the transverse direction of the

welds. Measurements were performed using a 100 gf load and

the indentation period was 15 seconds. The following Figure 5

illustrates the hardness profile of welded AA-6061 T651

specimens. As expected, for the AW specimen the major

softened area is the weld center area and more so, the adjacent

HAZ (Metzger, 1967; Ren et al., 2007; Elangovan and

Balasubramanian, 2008; Ambriz et al., 2009). The average

hardness values for AW specimens in the weld and HAZ area

are 64 HV and 58 HV, respectively. This clearly shows that the

weakest zone is the HAZ. Figure 5 also shows that heat

treatment (HT) processes are beneficial as the hardness values

for all of the three zones are higher than the corresponding

values of AW specimens. The average hardness value of the

weld zone and the HAZ has increased by 46% and 58% due to

HT processes, respectively. Heat treatment results the grain

refinement in the welded and the HAZ zone; and results higher

hardness values compared to as -welded specimens.


Figure 5: Micro-hardness with measurement position on the

weld section.

Optical micrographs of the weld metal and HAZ metal of

the AW and PWHT samples are shown in Fig. 6. All these

micrographs were taken at 50X magnification. Some amount of

grain-coarsening can be seen in the HAZ area of AW samples;

whereas weld metal in PWHT samples have a fine grain

structure. Figures 6(d) shows grain structure at the transition

between HAZ and filler materials. The dendritic structures in

HAZ are formed during the solidification of weld. The dendrite

boundaries appear to be broken up and precipitate in the grain

boundary by heat treatment. Similar trend has also been

observed in literature (Metzger, 1967; Periasamy et al., 1995).

Due to the heat treatment fine precipitation of Mg2Si was

observed throughout transition zone near the grain boundaries,

which was also confirmed by EDAX as shown in Fig. 7. This

suggests that most of the strengthening precipitates present in

the base metal were dissolved during welding process and,

therefore, a reduced density of these precipitates were observed

after welding. In HT sample the precipitates appear to be fine

and are uniformly distributed throughout the matrix. This could

be the main reason for the enhanced hardness and improved

tensile properties of the PWHT joints.

The grain size was calculated by using Heyn’s interception

method. The average grain diameter of AW filler and HAZ

materials were 158 µm and 208 µm, respectively. Whereas,

welded HT filler and HAZ had average grain diameter 148 µm

and 191µm, respectively (Table 2). Due to heat treatment the

grain size decreases, which is also observed from the optical

micrographs. The grain refinement might have resulted the

improvement of microhardness and tensile properties of the

PWHT specimens.

Table 2: Grain size calculation using Heyn’s method

Material

No.

of interc

ept

(Ni)

length, L

(mm)

Magnificatio

n (M)

NL = Ni/(L

/M)

Average Grain size,

G =

(6.643856 log NL-

3.288)

Average

grain

diameter, D (µm)

AW-Filler

71 500 50 7.1 2.37 158

AW-

HAZ 54 500 50 5.4 1.58 208

HT-

Filler 76 500 50 7.6 2.56 148

HT-

HAZ 59 500 50 5.9 1.83 191

Figure 6: Optical micrographs of (a) as-welded filler

materials, (b) as-welded HAZ materials, (c) Welded and

heat treated filler materials, and (d) welded and heat

treated HAW materials showing filler and HAZ material

interface.

(a)

a

d

b

c


(b)

Figure 7: EDAX results (a) unaffected parent metal in as-

welded sample, (b) Mg2Si precipitates found in the heat

treated samples.

Residual stress and fracture behavior

Residual stresses are a major key part in determining the

overall strength of a component and they cannot be overlooked

in the design process. Residual stresses are essentially “locked-

in” to the material after production and extremely hard to

detect. In this study, non-destructive ultrasonic testing method

was used to measure residual stresses. Welding distortion and

clamping condition have a direct effect on the residual stresses

of welded structures. In present study we did not investigate the

effect clamping on the residual stresses. For better comparison,

all welding were performed on same clamping conditions.

During welding we used four clamps to hold the plate with

tables and to avoid any distortion. We have measured the

transverse and longitudinal residual stresses of the as- welded

(AW) and welded heat treated (HT) specimens using UT testing

(Fig. 8). The changes in sound velocity in longitudinal and

transverse direction found for the residual stresses into the

materials (Fig. 9). In case of transverse residual stress

measurement, the overall variations of sound velocity in 50 mm

long specimens were calculated. In case of longitudinal residual

stress measurement, sound velocity variations at different

distances (5 mm, 10mm, and 15 mm) from the weld center

were calculated. In transverse direction, the sound velocity

increases for the tensile residual stresses (Fig. 9 (a)). To show

the variations in sound velocity and residual stresses, error bars

(standard deviation) are added. Heat treatment showed grain

refinement and removal of locked-in stresses. Thus lower

residual stresses were found in heat treated specimens

compared to AW specimens. In transverse weld direction,

average residual was 54 MPa and 30 MPa for AW and PWHT

specimens, respectively. In longitudinal welding direction,

residual stresses at 5 mm, 10 mm, and 15 mm away from the

weld center were calculated. In longitudinal direction, the

sound velocity decreases due to the presence of compressive

locked-in stresses (Fig. 9 (b)). The compressive residual

stresses were decreased as we moved away from the weld

center line. The maximum compressive residual stress was

obtained 5 mm away from weld center line. Average

compressive stresses were 35 MPa and 28 MPa for AW and HT

specimens, respectively.

Figure 8: Schematic diagram of residual stresses in the

longitudinal direction (σx) and transverse direction (σy)

To compare the residual stress measured from ultrasonic testing

hole-drilling method was used for as- welded (AW) specimens.

In this study the residual stress was measured at heat affected

zone (5 mm from center of the weld seam). The average 44

MPa tensile residual stress was found in the transverse welding

direction. Average residual stress in the longitudinal direction

was compressive and was - 6.5MPa. Both ultrasonic and hole-

drilling tested results are comparable, but there are few

differences. In case of ultrasonic testing we have calculated

residual stresses within the bulk materials, whereas, in hole-

drill method, we have drilled upto a certain depth (equivalent to

the diameter of the strain rosette) for the measurement of the

relaxed residual stresses. This might be the reasons for the

variations in the measured results.

Figure 9(a): Transverse sound velocity (Vy) and residual

stresses (σy) measured at by ultrasonic testing


Figure 9(b): Longitudinal sound velocity (Vx) and residual

stresses (σx) measured by ultrasonic testing

In case of ultrasonic testing we have measured average

residual stresses 54 MPa in transverse direction and -35 MPa in

longitudinal direction for as -welded AA6061-T651 aluminum

alloy. Whereas, we have obtained 44 MPa and -6.5 MPa

residual stresses by using hole-drilling techniques. In case of

drill-hole techniques, we calculated the residual stresses upto a

certain depth (2 mm). As we know, the residual stresses depend

on depth of hole. In case of UT, the sound wave passes the

whole depth of the specimens and resulted bulk residual

stresses. That might have caused the variation between the

results. But for comparison the results are in same order of

magnitude and direction (tensile/compressive). Steves in 2010

showed 40 MPa and -16 MPa residual stresses in transverse and

longitudinal direction, respectively. He calculated residual

stresses by using hole-drilling techniques (Steves, 2010), which

is quite close to our calculated values. Karunakaran and

Balasubramanian in 2011 calculated residual stress of TIG

welded AA6351-T6 aluminum alloy using X-ray diffraction

method. They obtained residual stress 74 MPa in transverse

direction, which is also same order of magnitude of our results,

although X-ray diffraction results are generally obtained in the

near-surface condition.

The fracture surfaces of the specimens were characterized

using SEM to understand the failure patterns. The SEM images

(Fig.10 (a, b, c)) were taken at the center of the failure surface.

The micrographs indicate that all the surfaces invariably consist

of dimples, which is a typical indication that most of the failure

occurred due to ductile fracture. During tensile testing of

ductile materials voids are formed prior to necking. If the neck

is formed earlier, the void formation would be much more

prominent; and as result coarse and elongated dimples can be

seen. Fine dimples were found on the fracture surfaces of the

HT joints. A complete characterization of the surface near the

root will be carried out in our future work.


Figure 10: SEM images of the fracture surface of the tensile

tested specimens. (a) Base metal, (b) AW joint, and (c) Heat

treated welded joint.

CONCLUSIONS In this research we have studied the effect of heat treatment

on the residual stresses, microstructure, and mechanical

performances of TIG welded AA6061-T651 aluminum alloy.

The following general observations can be made:

The transverse and longitudinal residual stresses were

measured by nondestructive ultrasonic testing method. To

verify the calculated residual stresses semi-destructive drill-

hole technique was used to measure residual stresses and

similar overall trends were observed. Since sound velocity is

high, time required to pass sound wave in a metal is quite

small. Therefore, the time-variations due to residual stresses are

also very small. To get good results, the equipment used to

measure residual stresses must be of high sensitivity and

accuracy. For larger specimen, time required to travel sound

wave will be larger also and accordingly, we can get significant

change in time variations and probably, a much lesser error in

the results. Very thin and small specimen cannot be used to

measure residual stresses accurately by ultrasonic testing.

Using Heyn’s intercept method the grain size of filler and HAZ

materials were calculated. The grain size of materials decreases

due to PWHT, which also results reduction of the residual

stresses during phase transformations. By lowering the grain

size the inter-granular stresses can be minimized, which

account for the flaws between grain boundaries lowering the

risk of failure. This also results increased tensile strength

properties. The grain refinement and precipitation resulted

improved microhardness value in the welded and HAZ areas.

ACKNOWLEDGMENTS Authors gratefully acknowledge the financial support

received from the U.S. Nuclear Regulatory Commission

(NRC). Authors also appreciate assistances received from Mr.

N. Roberts and Mr. A. Kardak during welding and sample

preparation.

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Alu Tig 6061