effects of residual stresses and the post weld heat

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 M. A. Wahab Department of Mechanical Engineering Louisiana State University Baton Rouge, Louisiana 70803, USA

ABSTRACT Heat treatable AA-6061 T651 Aluminum alloys (Al-MgSi) 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

Jiandong Liang 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

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 heataffected- 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

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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. Nondestructive 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

= 𝐾1 𝜎1 + 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

= 𝐾1 𝜎1 + 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

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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 heataffected-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 AA6061T651 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.

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RESULTS AND DISCUSSIONS

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)

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.

Weld defects

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 aswelded, 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

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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.

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HTFiller HTHAZ

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.

76

500

50

7.6

2.56

148

59

500

50

5.9

1.83

191

a

b

c

d

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)

Table 2: Grain size calculation using Heyn’s method

Material

AWFiller AWHAZ

NL = Ni/(L /M)

Average Grain size, G= (6.643856 log NL3.288)

Average grain diameter , D (µm)

50

7.1

2.37

158

50

5.4

1.58

208

No. of interc ept (Ni)

length, L (mm)

Magni ficatio n (M)

71

500

54

500

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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)

(b) Figure 7: EDAX results (a) unaffected parent metal in aswelded 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 “lockedin” 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

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 holedrilling 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 holedrill 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

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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 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.

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N. Roberts and Mr. A. Kardak during welding and sample preparation. REFERENCES Ajovalasit, A., Petrucci, G., and Zuccarello, B., 1996, "Determination of non-uniform residual stresses using the ringcore method," Journal of Engineering Materials and TechnologyTransactions, Vol. 118(2), pp. 224-228. Ambriz, R. R., Barrera, G., Garcia, R., and Lopez, V. H., 2009, "A comparative study of the mechanical properties of 6061-T6 GMA welds obtained by the indirect electric arc (IEA) and the modified indirect electric arc (MIEA)," Materials & Design, Vol. 30(7), pp. 2446-2453. Ahmad, R., and Bakar, M.A., 2011, “Effect of a post-weld heat treatment on the mechanical and microstructure properties of AA6061 joints welded by the gas metal arc welding cold metal transfer method,” Materials and Design, Vol. 32, pp. 5120-5126.

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 drillhole 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.

ASTM E8M-04, 2004, “Standard test methods for tension testing of metallic materials,” ASTM International, West Conshohocken, PA, USA. AWS structural welding code, 2008, “Aluminum: AWS D1.2/D1.2M,” American Welding society. Bray, D., and Tang, W., 2001, "Subsurface stress evaluation in steel plates and bars using the LCR ultrasonic wave," Nuclear Engineering and Design, Vol.27, pp. 231–240. Clark, A. V., and Moulder, J. C., 1985, "Residual stress determination in aluminum using electromagnetic acoustic transducers," Ultrasonics, Vol. 23(6), pp. 253-259. Dudas, J. H., and Collins, F. R., 1966, "Preventing weld cracks in high strength aluminum alloys," Welding Journal, Vol. 45(6), pp. 241s-249s. Elangovan, K., and Balasubramanian, V., 2008, "Influences of post-weld heat treatment on tensile properties of friction stirwelded AA6061 aluminum alloy joints," Materials Characterization, Vol. 59(9), pp. 1168-1177. Gao, R. Q., Stiller, K., Hansen, V., Oskarsson, A., and Danoix, F., 2002, "Influence of aging conditions on the microstructure and tensile strength of Aluminium alloy 6063," Materials Science Forum, Vol. 396(402), pp. 1211-1216. Hughes, D. S., and Kelly, J. L., 1953, "Second-Order Elastic Deformation of Solids," Physical Review, Vol. 92(5), pp. 11451149. Kardak, A., and Wahab, M. A., 2011, "Evolution of mehcanical properties, and microstructural characterization of butt welded AA 6061," Proceedings of 2011 ASME International Mechanical

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Engineering Congress and Denver, Colarado, USA.

Exposition,

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