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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
Louisiana State University Baton Rouge, Louisiana 70803, USA
M. A. Wahab Department of Mechanical Engineering
Louisiana State University Baton Rouge, Louisiana 70803, USA
Ayman M. Okeil Department of Civil and Environmental Engineering
Louisiana State University Baton Rouge, Louisiana 70803, USA
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
2 Copyright © 2012 by ASME
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
3 Copyright © 2012 by ASME
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.
4 Copyright © 2012 by ASME
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
5 Copyright © 2012 by ASME
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.
6 Copyright © 2012 by ASME
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
7 Copyright © 2012 by ASME
(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
8 Copyright © 2012 by ASME
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.
9 Copyright © 2012 by ASME
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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