Experimental Mechanics DOI 10.1007/s11340-014-9939-2
Ultrasonic Measurements of Residual Stresses Caused by Severe Thermomechanical Deformation during FSW H. Papahn & P. Bahemmat & M. Haghpanahi & A. Valipour
Received: 20 March 2014 / Accepted: 27 July 2014 # Society for Experimental Mechanics 2014
Abstract In engineering processes, residual stresses can be intense once high plastic deformation and temperature gradient are involved. This is exactly the case for friction stir welding (FSW) in which both rotational and translational movements of the tool induce extreme temperature gradient and plastic deformation. In this research, the extents of longitudinal and transverse residual stresses are measured within the AA7075-T6 plates welded through FSW process using ultrasonic method. According to the obtained results, it can be found that the residual stress is of the tensile type adjacent to the welding line whereas it is of the compressive type far from the welding line. Another observation is that the longitudinal residual stresses are considerably greater than the transverse residual stresses. Furthermore, with the aim of investigating the effects of rotation and traverse velocities of the tool on residual stress, experiments are carried out at three different rotation and traverse velocities. Based on the acquired results, it is observed that upon increasing the rotation and traverse velocities, the longitudinal and transverse residual stresses decrease and increase, respectively.
Keywords Residual stress . Ultrasonic . Thermomechanical deformation . FSW . Rotation velocity . Traverse velocity
Electronic supplementary material The online version of this article (doi:10.1007/s11340-014-9939-2) contains supplementary material, which is available to authorized users. H. Papahn : P. Bahemmat (*) : M. Haghpanahi : A. Valipour Department of Mechanical Engineering, Iran University of Science and Technology, NarmakHangam Street, Tehran, Iran e-mail:
[email protected]
Introduction In all engineering processes including cutting, welding and different metal shaping processes, and in the absence of external force or temperature gradient after the process, the residual stress could be formed due to the presence of deformation or change in thermal and material behavior in the course of the process [1, 2]. Over the years, different techniques have been developed to acquire residual stress for various types of processes with different levels of residual stress in order to reach reliable assessment. Among them, ultrasonic, as one of the recent residual stress measurement techniques, offers the following advantages over the other measurement methods: non-destructivity, lower equipment costs compared to the other non-destructive methods such as X-ray diffraction (XRD); shorter time required to obtain the results and portable measurement equipment. The scientific concept of this method is relatively simple and is based on the acousto-elastic phenomena allowing one to acquire the velocity variation of the ultrasonic wave, which is consistent with the stress state. Subsequently, the correlation between the stress and acoustic velocity can be found via a calibration test. In general, residual stresses are inherently more severe when both extremely high plastic deformation and elevated temperature are involved (thermomechanical residual stress), which is exactly the case for friction stir welding (FSW). In FSW, an inconsumable rotating pin is plunged into the adjoining edges of two pieces of plate material, and then, it is fed through the joint. The material of the plates is extremely plasticized by frictional heat arising from the rotational tool without reaching the melting point, and stirred by the tool pin rotation. Figure 1 shows a schematic diagram of the FSW process. While FSW offers numerous advantages for joining lightweight alloys, which are not weldable using conventional techniques, significant residual stresses are inevitable.
Exp Mech
Tilt Angle
Axial Force
Rotation velocity
Traverse velocity Translational Force
Pin
Shoulder Back Plate
Fig. 1 Schematic illustration of the FSW process
Furthermore, in FSW, both thermal and mechanical residual stresses are simultaneously formed which could be either tensile or compressive [3, 4]. The tensile residual stress is mainly destructive while the compressive residual stress is commonly advantageous. The tensile residual stress is responsible for the crack growth and could be considered as the major reason for fracture. On the other hand, the compressive residual stress precludes the crack growth and extends the structure life-time [1]. Therefore, measurement of residual stresses in the FSW is of great importance because once the weld is subjected to the static and cycling loads, it would be vital to determine the extent of destructive stresses and predict the object life-time. Several researchers have calculated and measured the extent of residual stresses induced by various mechanical processes using commercial software, numerical modeling as well as experimental methods including destructive and non-destructive methods [5–11]. For instance, Kartal et al. [5] introduced a method for developing the capability of the inverse problem of Eigenstrain to determine the varying multi-axial residual stresses at the micro-scale level. In addition, Kartal [6] recently employed the contour method to establish analytical solutions for residual stresses in two-dimensional domains. Up to that time, the contour method had involved the use of numerical models to evaluate the residual stresses from the experimental measurements. Very lately, Coules et al. [7] presented a method which enables one to evaluate the state of residual stress from a limited set of experimental acquisitions. Furthermore, Prime et al. [8] also proposed a method for measuring the geometric mismatch between the mating fracture surfaces allowing a posteriori determination of the residual stresses. Besides, there are some other variable destructive and non-destructive methods with their own pros and cons for measuring multiple residual stress
components, some of which are classified in either destructive or non-destructive methods including holedrilling and cross-slitting [9–11]. In the case of FSW process, the residual stresses have been experimentally obtained by measuring in longitudinal and transverse states through destructive methods [12] and nondestructive methods such as ultrasonic as well as X-ray diffraction [13–22]. One of the destructive methods, which can be employed to measure the residual stress field, is hole-drilling strain-gauge. Using this method, Xu et al. [9] have measured the extent of longitudinal residual stress in AA2219 subjected to FSW at both top and bottom of the sheet. They observed that the extent of residual stress at the top of the sheet is larger than that at the bottom of the sheet, the extent of residual stress in the heat affected zone (HAZ) is greater at the advancing side and the longitudinal residual stress is reduced upon increasing the rotation velocity. Beside the destructive methods, various researches have been reported regarding residual stress measurement using XRD method, as a non-destructive method, in the FSW process. For instance, Peel et al. [13] have demonstrated that, for AA5083, the longitudinal residual stress peak increases upon increasing the traverse velocity. They attributed this observation to the generated heat and reduction in the release time. Reynolds et al. [14] have observed that the residual stress profiles were similar for fusion and FSW process. On the other hand, Chen and Kovacevic [15] found that the longitudinal residual stress increases upon the increase in traverse velocity. Staron et al. [16] have proved that the compressive residual stress could be formed via applying tension during the welding process. It was also shown by Hatamleh et al. [17] that the maximum extent and location of the residual stress depend on the welding support. They have concluded that such location is within the thermo-mechanically affected zone (TMAZ). Furthermore, Altenkirch et al. [18, 19] have demonstrated that global tensioning is required to reduce the residual stress and a reverse linear relationship holds between the residual stress and the applied force in the case of thin sheets. Furthermore, despite the aforesaid advantages of ultrasonic method, just a few studies have been carried out so far on measurement of the residual stress field induced by the FSW process, as a solid state joining technique, by using the ultrasonic method. However, in addition to the other non-destructive methods, ultrasonic method has been widely employed for residual stress measurements in elements joined through the conventional welding methods. For instance, Liu et al. [20] have measured the extent of residual stress using the acoustic theory in the AA2219 plates welded by a conventional welding technique via ultrasonic method. It was resulted that the
Exp Mech
residual stress exhibits a rational relationship with the first critical longitudinal wave. In their work, the residual stress has been obtained in both longitudinal and transverse directions. Eventually, instability acousto-elasticity effect was observed in the elastic residual stress results. On account of a change in acousto-elastic equation, not taking the microstructural properties into account for measuring the residual stress and environment noise, the results were different as compared to those obtained by the methods which are able to measure the microstructural residual stresses. In the studies done by Gachi et al. [21, 22], the residual stress was obtained under FSW of AA7108 and the following results were reported: the residual stress is dependent on the acousto-elastic effect which itself is originated from the variation of ultrasonic wave velocity within the material. The maximum longitudinal and compressive residual stresses have occurred within the HAZ and parent material (PM), respectively. Therefore, according to the literature reviewed above, it is obvious that despite the need for measuring the residual stresses in FSW, the effects of welding parameters like rotation and traverse velocities on the transverse residual stresses have not been investigated yet. The reported researches have focused on the longitudinal stresses whereas the transverse residual stresses could significantly influence the mechanical properties of the obtained welds. Furthermore, only a few studies have been done regarding the feasibility as well as scientific and technical aspects of using ultrasonic method for measuring the residual stress field in FSW. Thus, there are abundant ambiguities regarding scientific jargon of this method for measuring the residual stresses caused by a severe thermomechanical deformation such as FSW process, which should be scrutinized, accentuated and delineated. To this end, in the current work, the effects of rotation and traverse velocities on the extent of both longitudinal and transverse residual stresses are studied for aluminum AA7075-T6, which has widespread applications in aerospace industry, using a relatively modern technique based on ultrasonic method. The current paper is comprised of two main sections. In the experimental procedure and theory, a brief explanation is initially provided regarding the welding procedure and the needed fixture and instruments. In the meantime, the data acquisition and microstructural test preparations are explained. Afterwards, the test setup for measuring the residual stress through the ultrasonic method is described, and the corresponding key technical jargons are accentuated. In the results and discussion section, the obtained results are illustrated and the effect of the welding parameters on the extent of both longitudinal and transverse residual stresses is scrutinized. Likewise, the possible measuring error sources are discussed.
Experimental Procedure and Theory FSW, Force and Temperature Acquisition The translational and axial forces were obtained using a novel and especially designed fixture which allows concurrent measurement of the longitudinal and axial forces, and temperature (Fig. 2). In this fixture, two s-type and bending dynamometers are used for force measurement. In order to use these dynamometers, one requires a two-channeled 24 bit data logger with a microprocessor, which is of AD9000LT type and has the ability to convert analog signals into digital ones. This microprocessor could be connected to the computer through a USB cable and could be observed via Lab View software. Additionally, in order to measure the temperature, a fourchanneled thermometer was used along with a k-type thermocouple. In order to locate the thermocouples in the workpiece, two grooved fixtures were designed where the thermocouples were equipped. Moreover, in order to maintain the workpiece, four fish-type fixtures were used to handle the workpiece, two fixtures were used to keep the metal sheets joint together and two fixtures with a 90° angle were used to prevent the torsion and translational movement of the workpiece. Four thermocouples were used for temperature measurement within the translational region. Thermocouples (T.1) were installed at a distance of 5-mm from the welding line within the workpiece backside. Moreover, thermocouples (T.2, T.3 and T.4) were installed at 20, 35 and 50-mm distances from the welding line but on the workpiece, respectively (Fig. 3). The used samples are AA7075-T6 with a 5-mm thickness, 200-mm length, 80-mm width, Poisson’s ratio of 0.33, yield stress of 505 MPa and ultimate stress of 570 MPa. To join the plates using FSW process, the FSW tool was gradually plunged into the plates’ material at the butt line until the tool shoulder forcibly contacts the upper surface. A downward force is applied from the milling machine to maintain the contact, and a short dwell time is considered to allow development of the thermal fields for preheating and softening the material along the joint line. Subsequently, a force is applied at the direction of welding (longitudinal direction), and the FSW tool is forcibly moved along the butt line until it reaches the end of the weld line. The experiments were designed at three rotation velocities of 585, 800 and 1100 rpm and three traverse velocities of 15, 32 and 60 mm/min. Table 1 presents the performed experiments in this work. After the welding process, the specimens were cut in a transverse position to the weld line and etched in a modified Keller reagent after being polished by a diamond paste. Subsequently, microstructural textures were taken from the cross-section of welds using optical microscopy. The mentioned utilized milling machine has the traverse and rotation powers of 0.5 and 3 HP, respectively. The
Exp Mech 1 2 3 4 5 6 7 8 9
Fig. 2 A representation of the measurement fixture
10 11 12 13
employed tool is made of H13 on which the hardening thermal treatment was previously applied and has the following characteristics: threaded tapered with the pin taper angle of 73°, the thread step of 1 mm, large diameter of 6 mm, small diameter of 3.06 mm, pin height of 4.8 mm, shoulder diameter of 19 mm and shoulder taper angle of 2°. Ultrasonic Method As earlier mentioned, the ultrasonic method is used to measure the extent of residual stress. Hughes and Kelly [23] demonstrated that the residual stress caused some changes in the ultrasonic wave speed which could be considered as a result of the acoustic non-linear effects. All the ultrasonic waves are converted to longitudinal and transverse waves upon contacting a surface. The variations of ultrasonic speed caused by the non-linear acoustic effect can be summarized as follows [24, 25]: ρ0 V 211 ¼ λ þ 2μ þ ð2l þ λÞθ þ ð4m þ 4λ þ 10μÞε1
ð1aÞ
ρ0 V 212 ¼ μ þ ðλ þ mÞθ þ 4με1 þ 2με2 −
nε3 2
ð1bÞ
ρ0 V 213 ¼ μ þ ðλ þ mÞθ þ 4με1 þ 2με3 −
nε2 2
ð1cÞ
Fig. 3 Plat dimensions and locations thermocouples
FSW Machine and Tool Fixture of workpiece Pool (in the case of underwater FSW) Upper Plate T-Joint Fixture Bending Fixture Bending Load Cell S -Shape Load Cell S –Shape Load Cell Fixture and two Balls Transfers Middle Plate L-Fixture and two Ball Transfers Ball Transfers Bottom Plate
where, Vij is the wave speed propagating along i-axis and polarized in j-axis, ρ0 is the initial density of propagation medium, λ and μ are second-order Lame coefficients, θ is the trace of deformation matrix and L, m and n are third-order Murnaghan constants. Belahcene and Lu [24, 25] indicated that such speed variations appear in the longitudinal critically reflected wave (LCR), in which the critical wave propagates on the surface. Moreover, Hughes and Kelly proved that the following relationship holds between the mentioned wave speed and the major stresses (σ33, σ22 and σ11) [23]. V 11 −V 0L ¼ K 1 σ11 þ K 2 ðσ22 þ σ33 Þ V 0L
ð2Þ
where V11 and VL0 are the speeds of sound for surface longitudinal wave in the stressed and stress-free samples, respectively, and K1 and K2 are the linear acousto-elastic coefficients which can be obtained experimentally. In the case of metallic materials, K2 could be ignored in the above equation due to the considerably high ratio of K1 to K2 [25] which simplifies equation (1a), (1b) and (1c) to the following equation: V 11 −V 0L ¼ K 1 σ11 V 0L
ð3Þ
Exp Mech Table 1 The experiment testes setup Section
Test NO
v (mm/min)
ω (rpm)
1
1 2 3 4 5 6
32 32 32 15 32 60
588 800 1100 800 800 800
2
Furthermore, if the distance between the input and output probes in the stressed and stress-free samples remains unchanged, equation (3) would be simplified to: t0 − t ¼ K 1 σ11 t
ð4Þ
where t and t0 are the needed times for passing the wave between the input and output probes in the welded and stress-free samples, respectively. In other words, the residual stress field is reported with respect to the stress-free specimen in which the residual stress is considered to be zero. In this work, for transmitting and receiving the waves, a transmitter and a receiver were used which have been installed based on two hoofs under the angle of 50° relative to the samples surface (Fig. 4). As can be detected in this figure, after transmitting the wave by the transmitter (input probe), the transmitted wave is converted into two waves, one of which moves into the object’s depth and the other moves on the object surface. The second wave is the aforementioned LCR wave. When the ultrasonic waves pass through a metal, the wave penetration depth is dependent on the waves’ frequency. Waves with lower frequency penetrate deeper as compared to the waves with higher frequency. Nevertheless, there is no reliable equation for the precise determination of the penetration depth of LCR wave but some experimental studies have shown that the penetration depth is approximately as short as the wave length [26]. The center frequency of UT probe used in this study is 12.5 MHz, and according to the material, the speed of sound is 6,000 m/s. In addition, the relation of the wave length and the sound speed can be obtained as follows: C m =s ¼ λðmÞ: f ðHzÞ ð5Þ Thus according to above equation, it is obvious that the wave length is equal to 0.48 mm. Consequently, the penetration depth is approximately equal to 0.48 mm. The utilized equipment and the calibration experiment are also illustrated in Fig. 5. In this experimental rig, a 12.5 MHz normal probe was used to determine surface longitudinal wave. As shown in this figure and schematically illustrated in Fig. 4, two outputs are transmitted by an echograph, one of
Fig. 4 Schematic representation of experimental setup
which is transmitted to the tensile test and the other is transmitted to the oscilloscope to be evaluated and compared to the output wave. The output waves are also transmitted to the oscilloscope after being received by the output probe which is located 10 mm away from the input probe. Regarding equation (4), t0 is extracted by stress-free samples and t is measured for different seven tensile tests. Subsequently, an attempt is made to achieve an appropriate value for k11 employing a calibration process. It should be noted that in order to reduce the imposition of undesired residual stresses to the least possible extent, all the extracted specimens are cut by means of wire cut method. After this step, it would be clear that upon having a proper value of k11 and extraction of the speed of wave and consequently the transmission and receiving time of the wave in the friction stir welded samples, one can easily obtain the corresponding residual stress for each point of the weld zone.
Results and Discussion As mentioned above, upon extracting the time interval for the wave transmitting and reciving for different extents of tensile stress in the tensile machine, an attempt is made to achieve a proper value for k11 using a calibration process. The obtained results are reported in Table 2 in terms of the given forces and corresponding stresses. In this table, for each known value of stress, the obtained results are expressed as the measured time required for transmitting and receiving the wave. These results are used to achieve the calibration curve. To this end, Fig. 6 shows the calibration curve in terms of the time constant and stress. Regarding the Fig. 6, it is seen that the fitted curve passing through the above 16 points is linear and the corresponding equation is: t − t 0 S ¼ −145:04 10−3 −8:64 ð6Þ t
Exp Mech
Fig. 5 A representative of calibration experiment and the corresponding equipment
where S is the measured stress (MPa). According to the above equation, the value of k11 is equal to 145 MPa. Moreover, it is seen that an offset about 8.64 MPa is inevitable due to the error arising from the experiment and the curve fitting method. Then, the extents of longitudinal and transverse stresses are measured at the center (0) and the following 22 points: ±2.5, ±5, ±7.5, ±10, ±12.5, ±15, ±20, ±30, ±40, ±50 and ±60 mm from the plate center in the advancing and retreating sides and at 50 mm away from the end of specimen. The locations for these points are illustrated in Fig. 7. Regrading Table 1, at first, the effect of rotation velocity, and then, the effect of traverse velocity on the longitudinal and transverse residual stresses are investigated. The variations of longitudinal and transverse residual stresses upon a change in velocity from 588 to 800 rpm and from 800 to 588 rpm are depicted in Fig. 8a and b. It is evident that as the rotation velocity increases, the extent of stress is reduced in both longitudinal and transverse states. Moreover, based on Fig. 8a, it can be seen that the extent of longitudinal residual stress is in tensile state adjacent to the weld center otherwise it is in compressive state. This trend is observable for the transverse residual stresses as well. Of important observations to note is the considerable reduction in both longitudinal and transverse residual stresses Table 2 Experimental Measurements of Calibration Test
upon increasing the rotation velocity. The reason for this phenomenon could be realized taking the effect of forces and temperature measurement into account. Axial and translational forces and temperature versus rotation velocity are shown in Fig. 9. As can be seen, with increasing the rotation velocity, the translational and axial forces applied by the tool onto the workpiece are decreased, albeit peak temperature increases. To well clarify this phenomenon, the effect of the rotation velocity on the temperature field is also depicted in Fig. 10. Generally, the temperature field, during the FSW process, not only affects the macro-residual stresses of the welded elements but also could have considerable effects on the microresidual stress through dictating the microstructural texture within the weld zone, which will be discussed. Various mathematical and experimental studies on the thermal history during the FSW process are well-documented in the literature and stipulate the profound effect of the welding parameters including welding speeds on the thermal field [27, 28]. As can be seen in Fig. 10, although the temperature level rises as the rotation velocity increases, the temperature gradient remains almost constant. In that case, the thermo residual stress induced by the temperature gradient would remain almost unchanged whereas the applied forces, which govern the mechanical residual stress, decrease leading to lower residual stress level in both directions. Similarly, the effect of an increase in the traverse velocity from 15 to 60 mm/min on the longitudinal and transverse residual stresses could be observed in Fig. 11a and b. According to Figs. 8 and 11, it is easy to deduce that the longitudinal residual stress is considerably greater than the transverse residual stress. As can be seen in Fig. 11, the increase in the traverse velocity leads to a considerable increase in the longitudinal and transverse residual stresses, which is indicative of the direct relationship between the residual stress and the traverse velocity in the FSW. Similar to the trend in Fig. 8, the measured stresses shown in Fig. 11 are of the tensile type adjacent to the welding line otherwise are of the compressive type. Moreover, considering both Figs. 8 and 11, it can be evidently manifested that, near the welding line, the residual stress is
NO
Force (KN)
Stress (MPa)
Time (μs)
NO
Force (KN)
Stress (MPa)
Time (μs)
1 2 3 4 5 6 7 8
0 5 10 15 20 25 35 45
0 37.74 75.47 113.21 150.94 188.68 264.15 339.62
8.184 8.188 8.190 8.192 8.194 8.196 8.200 8.204
9 10 11 12 13 14 15 16
0 5 10 15 20 25 35 45
0 37.74 75.47 113.21 150.94 188.68 264.15 339.62
8.184 8.186 8.188 8.190 8.192 8.196 8.198 8.202
Exp Mech
Fig. 6 The stress calibration curve in terms of time constant
notably changed as the rotation and traverse velocities vary. However, as the distance from the welding center increases, the difference between the measured residual stresses diminshes due to the fact that the force and temperature are less influencing. Figures 8a and 11a show that the average extent of longitudinal residual stress is larger on the advancing side as compared to the retreating side while the transverse residual stresses in the retreating side are larger than those on the advancing side. In other words, according to Fig. 8a, the average of longitudinal residual stress in the advancing side is about 21, 20 and 42 MPa greater than the retreating side for the tests 1, 2 and 3, respectively, whereas in Fig. 8b, the mentioned average stress in transverse direction is approximately 5 MPa less than the retreating side. One of the considerable observations in using the ultrasonic method aiming to find the residual stress field in FSW is the inevitable errors. Four different error sources were distinguished in this research. The first one is the error arising from the calibration test, which is evident regarding equation 6. As can be seen, the
maximum deviation from the calibration line is about 8 MPa. Furthermore, one of the other factors inducing error in the residual stress measurement via ultrasonic method is the nonsmooth surface and the onion ring residing on the workpiece. The third error source is the defects and voids arising from inappropriate mixing and stirring of the plasticized materials beneath the tool shoulder. This type of error was observed as noises in the received reflected waves. Furthermore, the ultrasonic method, espesially with inadequate ultrasonic pulses resolution, cannot inherently provide good accuracy in measuring the micro-residual stresses which are mostly caused by heterogeneity of the microstructure texture of the material. Figure 12 shows the microstructural texture in the weld zone for the test 2. According to the figure, three different zones can be distinguished which are as follows: (1) HAZ wherein the material has undergone a thermal cycle altering the microstructural texture. Nevertheless, no plastic deformation takes place in this zone. (2) TMAZ wherein the material has been plastically deformed by the FSW tool,
(a)
(b)
Fig. 7 Locations of residual stress measuring points
Fig. 8 Measured residual stresses upon varying the rotation velocity from 588 to 1100 rpm as a function of distance from the center line. (a) longitudinal residual stress and (b) transverse residual stress
Exp Mech Fig. 9 Axial and translational forces and temperature versus rotation velocity
and the heat flux can considerably affect the microstructure textures. In the case of aluminium alloys, no recrystallization phenomena are observed in this zone. (3) Stir zone (SZ) in which the original grain and subgrain boundaries are replaced with equiaxed recrystallized grains [29, 30]. These three different microstructral textures, arising from the FSW process, lead to the microstructural inhomogeneity in the weld zone. Thus, the micro-residual stress is an indivisible charateristic of the FSW process. On the other hand, ultrasonic wave velocities depend on such microstructural inhomogeneities making the ultrasonic method applicable for measuring the micro residual stresses throughout the weld zone. To this end, the ultrasonic pulses resolution should increase up to ±1 ns [32]. However, there exist difficulties in separating the effects of multi-axial micro and macro residual stresses. Gachi et al. found that the contribution of the micro-residual stress in the total residual stress is neglegible compared to the macroresidual stresses. They also observed that the surface wave has been less influenced by the microstructural textures [22]. The abovementioned notes are the indispensable parts of the ultrasonic technique, which may cause errors in measuring the residual stress in FSW. Hereof, Withers and Bhadeshia [31] showed that the ultrasonic techniques may produce maximum 10 % error in measuring the residual stress. Nevertheless, this paper is not to quantify the mentioned errors and their corresponding effects on the accuracy of the obtained results. The effect of each type of error on the accuracy of the measurement can be separately scrutinized in the future studies. In conclusion, despite some vulnerability involved in such a process, due to the numerous advantages and the fact that this method is rapid and easy, ultrasonic method could be recommended for measuring residual stresses caused by a severe thermomechanical deformation including FSW.
Conclusion In this research, the longitudinal and transverse residual stresses were measured using the ultrasonic method for AA7075-T6 plates joined by friction stir welding as a thermo-mechanical process, which involves severe plastic deformations in the process zone. Furthermore, for the first time, the effects of the rotation and traverse velocities on the transverse residual stress were scrutinized. The following conclusions could be drawn: & &
The friction stir welding induced severe residual stress even up to 250 MPa in the center of the weld line (stir zone). The ultrasonic method might lead to some errors arising from the calibration test, the non-smooth surface of the workpiece, heterogeneity of the microstructure and the defects generated during the welding process.
Fig. 10 Effect of rotation velocity on the temperature distribution in the advancing side
Exp Mech
& & & & & (a)
recommended to measure the residual stress field in a severe thermo-mechanical deformation such as FSW. The residual stresses are of the tensile type adjacent to the welding line and as the distance from the weld line increases, the residual stresses become compressive. The longitudinal residual stress is almost three times greater than the transverse one. As the rotation velocity increases, the longitudinal and transverse residual stresses diminish. As the traverse velocity increases, the longitudinal and transverse residual stresses increase. The results obtained through the ultrasonic method are in agreement with those acquired through using the other methods further confirming the data available in the literature.
Acknowledgments The Authors would like to appreciate Dr. Mohammad Riahi, head of NDT Lab in Iran University of Science and Technology, due to providing the ultrasonic equipment.
References
(b) Fig. 11 Measured residual stresses upon varying the traverse velocity from 15 to 60 mm/min as a function of distance from the center line: (a) longitudinal residual stress and (b) transverse residual stress
&
The desirable attributes of the ultrasonic method including non-destructivity; lower equipment costs; shorter time required to obtain the results and portable measurement equipment were also observed. Thus, despite the mentioned potential error sources, using this method is
SZ HAZ
Fig. 12 Microstructural texture of the weld zone
1. Kandil FA, Lord JD, Fry AT, Grant PV (2001) A Review of Residual Stress Measurement Methods-A Guide to Technique Selection. National Physical Laboratory, UK 2. Bradley G, James M (2000) Residual Stresses/Strains Induced by the Metal Inert Gas and Friction Stir Welding Processes. University of Plymouth, England 3. Haghpanahi M, Salimi S, Bahemmat P, Sima S (2013) 3-D transient analytical solution based on green’s function to temperature field in friction stir welding. Appl Math Model 37:9865–9884 4. Kumar N, Mishra RS, Baumann JA (2013) Residual stresses in friction stir welding. Butterworth-Heinemann Ltd, United Kingdom 5. Kartal ME, Dunne FPE, Wilkinson AJ (2012) Determination of the complete microscale residual stress tensor at a subsurface carbide particle in a single-crystal superalloy from free-surface EBSD. Acta Mater 60(13):5300–5310 6. Kartal ME (2013) Analytical solutions for determining residual stresses in two-dimensional domains using the contour method. Proc R Soc A 469(2159):20130367 7. Coules HE, Smith DJ, Venkata KA, Truman CE (2014) A method for reconstruction of residual stress fields from measurements made in an incompatible region. Int J Solids Struct 51(10):1980–1990 8. Prime MB, DeWald AT, Hill MR, Clausen B, Tran M (2014) Forensic determination of residual stresses and ki from fracture surface mismatch. Eng Fract Mech 116:158–171 9. Kartal ME, Liljedahl CDM, Gungor S, Edwards L, Fitzpatrick ME (2008) Determination of the profile of the complete residual stress tensor in a VPPA weld using the multi-axial contour method. Acta Mater 56(16):4417–4428 10. Schajerand GS, An Y (2010) Residual stress determination using cross-slitting and dual-axis ESPI. Exp Mech 50(2):169–177 11. Pagliaro P, Prime MB, Robinson JS, Clausen B, Swenson H, Steinzig M, Zuccarello B (2011) Measuring inaccessible residual stresses using multiple methods and superposition. Exp Mech 51(7):1123– 1134 12. Xu W, Liu J, Zhu H (2011) Analysis of Residual Stresses in Thick Aluminum Friction Stir Welded Butt Joints. Mater Des 32:2000–2005
Exp Mech 13. Peel M, Steuwer A, Preuss M, Withers PJ (2003) Microstructure, Mechanical Properties and Residual Stresses as a Function of Welding Speed in Aluminium AA5083 Friction Stir Welds. Acta Mater 51:4791–4801 14. Reynolds AP, Tang W, Gnaupel-Herold T, Prask H (2003) Structure, properties, and residual stress of 304 L stainless steel friction stir welds. Scr Mater 48:1289–1294 15. Chen CM, Kovacevic R (2003) Finite element modeling of friction stir welding-thermal and thermomechanical analysis. Int J Mach Tools Manuf 43:1319–1326 16. Staron P, Kocak M, Williams S, Wescott A (2004) Residual Stress in Friction Stir-Welded Al Sheets. Physica B 350:491–493 17. Hatamleh O, Rivero RV, Maredia A (2008) Residual Stresses in Friction-Stir-Welded 2195 and 7075 Aluminum Alloys. Metall Mater Trans A 39:2867–2874 18. Altenkirch J, Steuwer A, Peel M, Richards DG, Withers PJ (2008) The Effect of Tensioning and Sectioning on Residual Stresses in Aluminium AA7749 Friction Stir Welds. Mater Sci Eng A 488:16–24 19. Altenkirch J, Steuwer A, Peel MJ, Withers PJ, Williams SW, Poad M (2008) Mechanical Tensioning of High-Strength Aluminum Alloy Friction Stir Welds. Metall Mater Trans A 39:3246–3259 20. Lu H, Liu XS, Yang JG, Zhang SP, Fang HY (2008) Ultrasonic Stress Evaluation on Welded Plates with Lcr Wave. Sci Technol Weld Join 13(1):70–74 21. Gachi S, Boubenider F, Belahcene F, No.3 (2009) Residual Stresses in AA7108 Aluminium Alloy Sheets Joined by Friction Stir Welding. Nondestruct Test Eval 24:301–309 22. Gachi S, Boubenider F, Belahcene F (2011) Residual Stress, Microstructure and Microhardness Measurements in AA7075-T6 FSW Welded Sheets. Nondestruct Test Eval 26(1):1–11
23. Hughes DS, Kelly JL (1953) Second-Order Elastic Deformation of Solids. Phys Rev 92:1145–1149 24. Belahcene F, Lu J (2000) Determination of Residual Stress in Z8cdwv12 Steel Using Critically Refracted Longitudinal Waves. JSME Int J Series A 43(4):367–373 25. Belahcene F, Lu J (2002) Determination of Residual Stress Using Critically Refracted Longitudinal Waves and Immersion Mode. J Strain Anal 37(1):13–20 26. Javadi Y, Hloch S (2013) Employing the LCR Waves to Measure Longitudinal Residual Stresses in Different Depths of a Stainless Steel Welded Plate. Adv Mater Sci Eng 2013:1–8 27. Haghpanahi M, Solaleh S, Bahemmat P, Sima S (2013) 3-D transient analytical solution based on Green’s function to temperature field in friction stir welding. Appl Math Model 37(24):9865–9884 28. Yeong-Maw H, Kang Z-W, Chiou Y-C, Hsu H-H (2008) Experimental study on temperature distributions within the workpiece during friction stir welding of aluminum alloys. Int J Mach Tools Manuf 48(7):778–787 29. Bahemmat P, Besharati MK, Haghpanahi M, Rahbari A, Salekrostam R (2010) Mechanical, micro-, and macrostructural analysis of AA7075–T6 fabricated by friction stir butt welding with different rotational speeds and tool pin profiles. Proc IMechE Part B: J Eng Manuf 224:419–433 30. Mishra RS, Mahoney MW (2007) Friction Stir Welding and Processing, 1st edn. International, ASM 31. ASM (1998) ASM Metals Handbook Volume 17 - Nondestructive Evaluation and Quality Control. Method of Nondestrative evaluation-Ultrasonic Inspection. ASM International 32. Withers PJ, Bhadeshia HKDH (2001) Overview Residual stress Part 1-Measurement techniques. Mater Sci Technol 17:355–365
