Residual stresses in friction stir dissimilar welding of aluminum alloys

Accepted Manuscript Title: Residual stresses in friction stir dissimilar welding of Aluminum alloys Author: J. Zapata M. Toro D. L´opez PII: DOI: Reference:

S0924-0136(15)30103-5 http://dx.doi.org/doi:10.1016/j.jmatprotec.2015.08.026 PROTEC 14530

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

22-4-2015 22-8-2015 24-8-2015

Please cite this article as: Zapata, J., Toro, M., L´opez, D., Residual stresses in friction stir dissimilar welding of Aluminum alloys.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2015.08.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Residual stresses in friction stir dissimilar welding of Aluminum alloys J. Zapataa,M.Toroa, D. Lópeza* a

Tribology and Surfaces Group, Mechanical Engineering Department, Universidad Nacional de Colombia, Medellín, Colombia. *corresponding author: tel: +57-4-4309269, fax: +57-4-4255000, email

addresses [email protected], [email protected] (J. Zapata), [email protected] (M.Toro).

Highlights 

A detailed methodology for residual stresses measurement of FSW joints by XRD is presented.



The effect of FSW parameters on residual stresses profiles of dissimilar aluminum joints was analyzed.



Rotational speed was the most significant parameter in residual stresses profiles.



M-shape profiles had peak values near the edges of the tool shoulder.



The maximum value of stress was MPa, 15% of the yield strength of the material.

Abstract The effect of the rotational and welding speed on the residual stresses of dissimilar friction stir welds of aluminum alloys AA2024-T3 and AA6061-T6 were studied using X-ray Diffraction. The methodology used to obtain the residual stresses is presented in detail, including the measuring parameters and software corrections

1

used. The proposed methodology allowed toobtained low scatter in the residual stress measurements aspresented in the results. The most significant variations in the residual stresses behavior and magnitude were relatedwith the modification of the rotational speed. The effect of increasing the rotational speed is a reduction in the magnitude of the longitudinal residual stresses.This was directly associated with the increase of heat input and the reduction of thermal mismatch between the different zones of the weld.The influence of the welding speed on the residual stress was not significant when compared with the effect of the rotational speed, generating only a slightly increment in the profile of the retreating side when it was increased.

Keywords Friction stir welding (FSW), residual stress measurement methodology, dissimilar aluminum alloys, X-ray diffraction.

1. Introduction Friction stir welding,as other manufacturingprocesses, generates residual stresses caused by thermal cycles, the plastic deformation and/or the mechanical constraints used in the process. These residual stresses can affect significantly the performance of a component in service.Therefore, several studies on residual stresses ofaluminum alloys friction stir welds through diffraction techniques have been done. Sutton et al. employed neutron diffraction technique to determine the variation of residual stresses along the cross section of butt joints of AA2024-

2

T3.They found that the longitudinal component of stresses were higher than the transverse component, with a maximum magnitudeof 105 MPa(Sutton et al, 2000). Later, Peel et al.analyzed the effect of different welding speeds on the properties and residual stresses of AA5083 joints.Thisstudyshowed that the peak of longitudinal stresses increased as the welding speed increased,up to values around 60 MPa(Peel et al, 2003). In 2006, the same groupcharacterized the residual stresses of dissimilar friction stir welds of AA5083 and AA6082 using techniques such as neutron and synchrotron X-ray diffraction.It was found that the rotational speed had a stronger influence in the residual stress profile than the welding speed, and its variations were related to microstructural and hardness changes during the welding process. Moreover, the maximum residual stress found on the study had a magnitude around150 MPa. In both works, the authors reported an “M” shape profile for the longitudinal residual stresses withpeaks of maximum tensile stresses in regions near to the edge of the tool shoulder(Peel et al, 2006).

In the same year, Prime et al.studied the residual stresses profile of dissimilar thick AA7075-T7451 and AA2024-T351aluminum alloysusing neutron diffraction and the contour method.The results showed the profile with the “M” shape reported by Peel et al.(Peel et al, 2006), with tensile peak stresses near the edge of the tool shoulder and a maximum magnitude of 43 MPa (Prime et al, 2006). This result was in agreement with those presented by (Lombard et al, 2009), who investigated the effect of the welding parameters on the longitudinal and transverse residual stresses profile on AA5083-H321 friction stir welds using Synchrotron X-Ray Diffraction. The peak of the profile with a magnitude of 100 MPa was related with 3

the welding speed, which was considered the dominant parameter on the magnitude of residual stresses. Later, Xu et al.found an “M” shape profile onthe longitudinal and transverse residual stresses in AA2219 T-62 friction stir welds. However, this study reported stronger variations on the residual stresses with changes in the rotational speed, reaching a magnitude of 243 MPa(Xu et al, 2011).

In spite of the information presented in previous works, a detailed sequence or methodology for the residual stress assessment is not available. This work presents a detailed methodology used by the authors for the measurement of residual stresses using X-ray diffraction in friction stir welds of dissimilar AA 2024T3 and AA 6061-T6 aluminum alloys. Furthermore, the evaluation of the rotational and welding speeds effect on the longitudinal residual stresses distribution is presented.

2. Experimental procedure 2.1 Materials and welding details The materials used in the welds were AA 2024-T3 and AA 6061-T6 rolled plates of 4.8 mm thickness,80 mm width, and 300 mm long.The chemical composition and the mechanical properties for both alloys are shown in Table 1.

The welds were done using a modified milling machine and a tool consisted of a 20 mm diameter concave shoulder with a 4 mm diameter tapered threated pin, which worked tilted 2° against to the advance direction and in a counterclockwise 4

rotation. Moreover, a restrain system that avoids the displacement of the plates during the welding process was used toassure the suitable welding of the plates. On Table 2 is presented the combinationof welding parameters that allowed to obtainsound welds. For all the welds, the AA 6061-T6 alloy was settled in the retreating side and the AA 2024-T3 in the advancing side.

2.2 Microstructural characterization Cross section samples were obtained transverse to the welding direction. The samples were metallography prepared and etched with a solution of 50 mL Poulton + 25mL HNO3 + 12gr of Chromic acid in 40 mL of H2O.

2.3 Residual stresses measurement methodology The methodology for the residual stresses measurement involves the sample preparation, the softwareparameters and corrections based on the equipment used and the literature available. Residual stress characterization samples were obtained from the central region of the welds, aiming to analyze a zone with a steady condition. All samples were cut using waterjet into80 x 80 mmsquares to avoid thermal affectation and changes in the residual stress profile. The samples were electropolished to reduce the average roughness with the aim to increase the number of signals obtained in the stress measurement. The electropolish process consisted on an electrochemical cell with a solution of1000 ml of distilledwater, 14.2 g of Na2SO4, and 0.5 g of NaCl,

5

using a voltage range between 0and 0.5 V, withan Ag/AgCl reference electrode and a platinum counter electrode.

Longitudinal residual stresses were measured on a Bragg-Brentano PANalytical X´Pert Pro MPD diffractometer using an X-ray tube with a Cooper anode and a PIXcel detector, located at the Materials Characterization Laboratory of the Universidad Nacional, Colombia, Medellín Campus.

The points selected for measurements are shown in Figure 1, the point PM is located in the middle of the joint, the points 1, 2, and 3, were separated 3 mm between them. As can be observed in Figure 1, the distribution of points is symmetrical about PM giving a total of 7 points for each sample.

Figure 1. Distribution of the measurement points.

An absolute scan was made on a region of the base materials (AA2024-T3 and AA6061-T6) that was not affected by the welding process, with the aimto select the Bragg angle in which the residual stress measurement would focus. The results obtained were similar for both materials and allowed to define the 2 position of the 6

Bragg angle. The hardware and software parameters employed are presented in Table 3. The results obtained from the absolute scan for both alloys are shown in Figure 2.

Figure 2.Absolute scan between 74° and 120°forAA 2024 and AA 6061

The highest Bragg angle obtained in the diffractogram was selected for the stress measurement scan. This is based on the small changes in the d-spacing such as those associated with strain. The small changesrepresent slight variations in the position of the diffraction peak and canbe measureat high anglesmore easily compared to lower angles (Fitzpatrick et al, 2005). Therefore, the selected Bragg angle was 2 = 116.544°.Then, for the stress measurements at this angle the hardware parameters remained constant and new software parameters were defined as shown in Table 4.An example of the stress measurement scan results obtained with those parameters is presented in Figure 3.

7

Figure 3. Stress measurement scan for

=

°

.

For the stress measurements the software used was PANalytical X'Pert Stress. In the software, it is required to define the elastic constants of the material to be analyzed. In the case of the aluminum alloys used, the elastic constants were taken from (Davis, 1990); witha modulus of elasticity

= 70

and Poisson's

ratio = 0.34. Other parameter to be defined must be the diffraction peak adjustment method to determine the

component and filter the

radiation. For

all of the measurementsPearson VII was selected to avoid errors caused by defocusing of the diffraction peaks, in accordance to (Prevéy, 1986). Furthermore, a linear behavior was assigned to the background of the

component of the

diffraction peak with the aim to correct any irregularities on both sides of it and 8

generate one in which the background was close to zero. The purpose of the background correction was to modify the tendency of the scans and adjust them to the linear behavior as presented in 4.

Figure 4. Linear behavior between d-spacing and

( )

3. Results and discussion 3.1 Microstructural characterization Figure 5 shows the transverse cross section of a weld obtained with 650 rpm and 65 mm/min. In this figure, the different regionsgenerated during the Friction Stir Welding process such as the Thermo Mechanical Affected Zone (TMAZ) and Heat Affected Zone (HAZ) are shown. These regions had similar shape, location and size in all the samples. Besides,all the cross sectionspresented a ring flux patternin the nugget regionthatindicates the vertical movement of material.Moreover, the width of the top part of the mixture zone preserved a relation with the tool shoulder diameter. 9

Figure 5. Transverse section macrograph of the bead made with 650 rpm – 65 mm/min.

The base material microstructure of the advancing (AA 2024-T3) and retreating (AA 6061-T6) side are shown in Figure 6 and Figure 7 respectively. The grains have an elongated morphology consistent with the rollingprocess of the plates before welding.

Figure 6. AA 2024-T3 base material microstructure, 200 X. 10

Figure 7. AA 6061-T6 base material microstructure, 200 X.

3.2 Residual stresses characterization Aswas mentioned before, the residual stresses characterization was performed in the longitudinal direction instead of the transversal due to the highest magnitudes and variations found in this direction according with (Sutton et al, 2000, Peel et al, 2003, Peel et al, 2006, and Lombard et al, 2009). This effect is associated to the highest temperatures and deformation presented in the longitudinal direction due to the pass of the welding tool.The longitudinal residual stresses profiles are shown inFigure 8 to Figure 10. Figure 8 shows the residual stresses profiles obtained for similar friction stir welds ofAA 2024-T0 and AA 6061-T0made with 500 rpm and 45 mm/min, aiming to obtain reference values.

11

Figure 8.Residual stress profiles for similar AA6061- AA6061 and AA2024AA2024 friction stir welds made with 500 rpm and 45 mm/min.

It can be observed that the maximum value of the stress profile is located on the retreating side for both materials. However, there is a significant difference between the profiles of AA6061 and AA2024, since the profile for the first one is moresymmetricrespect to the center of the bead, while the profile for the AA2024 alloy shows a lower magnitude of stresses on the advancing side compare to the retreating side.In the advancing side, the direction of the velocity vectors associated with the welding speed and the tangential component of the rotational speed coincide; the opposite happens in the retreating side. The AA 2024 is more sensitive to these differences in the flow than the AA6061-T6. Then, the observed variations in the profiles could be related with the differences in the mechanical properties of both alloys and its 12

susceptibility to the position in the joint (retreating or advancing side), which promoted different behaviors in the materials when they were subjected to the same welding conditions.

Influence of rotational speed The Figure 9 and Figure 10 shows the residual stress profiles of the friction stir welds made with different rotational speeds and welding speed of 45 and 65 mm/min respectively. It canbe seen in Figure 9 that the profiles using 45 mm/min with500 and 650 rpm have asimilar shapeand magnitude of residual stresses. The profile using 840 rpm havea similar tendency but the magnitude of the stresses is lower compare to the other rotational speeds. This behavior is associated with the effect of the rotational speed in the heat input and temperature distribution onthe joint. Therefore, an increase on the rotational speed promoteshigher temperatures and a more uniform thermal distribution, reducing the temperature gradients and thermal mismatchon the plates.As a consequence, lower deformations andlower residual stresses are observed on the plates welded with higher rotational speed. It is also noted that the minimum stress value for all the combinations matches with the middle zone of the weld (PM).

13

Figure 9. Variation of the longitudinal residual stresses with the rotational speed at a welding speed of 45 mm/min.

Figure 10 shows the profiles obtained with a welding speed of65 mm/min. When the profiles shown onFigure 9 andFigure 10 are compared, this reveals that an increment of the welding speed from 45 to 65 mm/min increases the magnitude of the residual stresses and causes a loss of symmetry in the curves. In the advancing side the distribution is smoother, showing a change in the shape of the curve for 500 rpm and a displacement of the minimum value of the profile to the retreating side. There is also a displacement of the minimum value of the profilesfor 650 and 840 rpm, but for both cases is to the advancing side. The higher magnitude of the residual stresses for 65 mm/min is associated with the 14

increase in the welding speed, since the time available to deform and accommodate material in a new position is fewer compared to 45 mm/min. This behavior was significantly higher in the retreating sidedue to the kinematical condition for flow material in that side.Finally, it is clear from these figures that higher residual stress magnitudes were obtained in the retreating side of the welds for all the combinations.

Figure 10. Variation of the longitudinal residual stresses with the rotational speed at a welding speed of 65 mm/min.

Figure 11shows the variation of the maximum tensile residual stress with each combination of parameters. It can be observed that the higher the rotational speed the lower the residual stresses for both welding speeds. It can be also noted that 15

for the same rotational speed, the highest residual stress was obtained with the highest welding speed as discussed previously.Therefore, this difference can be relatedto the lower heat input and significant changes in the temperature along the width of the plates. In addition, as the rotational speed increases, the effect of the welding speed on the residual stresses seems to be less significant. This is supported by the differences between the magnitudes of stresses obtained with each welding speed for the same rotational speed that tended to be lesser as the rotational speed increased. This result indicates that the rotational speed influences the heat input and temperature distribution along the width of the plates, and hence the magnitude of the residual stresses.

Figure 11. Variation of the maximum tensile residual stress with the rotational and welding speed.

16

The behavior of the profiles isin good agreement with those reported in the introduction section of this work. The profiles had an “M” shape, and the maximum tensile residual stresses were located near to the edge of the tool shoulder. Some works like the one done byPeel et al.also concluded that the variation of the rotational speed cause more significant changes in the residual stresses profile(Peel et al.,2003, 2006). Moreover, the maximum residual stresses obtained in this work are closer to those reported by Peel et al. (Peel et al., 2003) with values around 60 MPa, which are close to the yield strengthof these aluminum alloys in the tempered condition (75 MPa for AA 2024-O and 55 MPa for AA 60601-O).

It was observed in all profiles that the maximum stress value was in the retreading side, a result that does not agree with the conclusions reported by some authors. To illustrate, the results of Sutton et al. present the maximum residual stresses in the advancing side (Sutton et al., 2000) and M. Peel et al. obtained the maximum longitudinal stresses in the advancing side for the different transverse speed evaluated(Peel et al., 2003). Although, the difference between the magnitude of the residual stresses in the advancing and retreating sides is not great, the results reported by Lombard et al. are in agreement with the maximum values on the advancing side(Lombard et al., 2009). It is important to notice that the differences in the maximum stress values obtained in the measurements canbe related with the properties, chemical composition and heat treatments of the materials used in each study. These differences also showed the asymmetry effect of the friction stir welding process.Moreover, the variations of the maximum residual stresses 17

between theprevious researchcanbe related to differences in the mechanical constrains of each equipment configuration.

4. Conclusions 

A detailed methodology for the residual stresses measurement by means of X-ray diffraction was presented. The results were obtained with low standard deviation and good repeatability.



The increase of the rotational speed caused a decrease on the magnitude of the residual stressesand the expansion of the heat affected zone, this is in accordance with the higher temperatures obtained.



Significant changes in the maximum residual stresses values wereobserved with the variation of the rotational speed.This behavior supports the idea that the rotational speedismore influential compare to the welding speed on the residual stresses distribution.



The behavior of the residual stresses profiles is in good agreement with those presented by other authorsbased on itsshape and thelocation of the higher stresses. In this researchthe maximum residual stressesobtained were around 60 MPa, which are close to the yield strength of the studied aluminum alloys in the tempered condition.

5. Acknowledgements The authors want to acknowledge Colciencias for their financial support, the Brazilian Nanotechnology National Laboratory (LNNano), Campinas, Brasil, and 18

the Manufacturing Engineering Laboratory and Materials Characterization Laboratory at the Universidad Nacional, Colombia, Medellín campus, for providing its facilities for the welding and the characterization tasks.

6. References Davis, J. (ed), Properties and selection Nonferrous alloys and special-purpose materials. Vol 2, ASM Handbook, ASM International, 1990. Steuwer, A. Peel, M. J., and Withers, P. J. Dissimilar friction stir welds in AA5083– AA6082: The effect of process parameters on residual stress,” Materials Science and Engineering: A, vol. 441, no. 1–2, pp. 187–196, Dec. 2006. Buffa, G., Fratini, L. Pasta, S., Shivpuri, R. On the thermo-mechanical loads and the resultant residual stresses in friction stir processing operations, CIRP Annals - Manufacturing Technology, vol. 57, no. 1, pp. 287–290, Jan. 2008. Lombard, H., Hattingh, D. G. , Steuwer, A., James, M. N. Effect of process parameters on the residual stresses in AA5083-H321 friction stir welds, Materials Science and Engineering: A, vol. 501, no. 1–2, pp. 119–124, Feb. 2009. Sutton, M. A. , Reynolds, A. P., Wang, D.-Q., Hubbard, C. R. A Study of Residual Stresses and Microstructure in 2024-T3 Aluminum Friction Stir Butt Welds, Journal of Engineering Materials and Technology, vol. 124, no. 2, p. 215, 2002. Fitzpatrick, M. E., Fry, A. T., Holdway, P., Kandil, F. A., Shackleton, J., Suominen, L. Measurement Good Pracice Guide No. 52. Determination of Residual

19

Stresses by X-ray Diffraction - Issue 2, National Physical Laboratory, no. 52, p. 77, 2005. Peel, M., Steuwer, A., Preuss, M.,Withers, P. J. Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminium AA5083 friction stir welds, Acta Materialia, vol. 51, no. 16, pp. 4791–4801, Sep. 2003. Peel, M., Steuwer, A.,Withers, P. J. Dissimilar friction stir welds in AA5083AA6082. Part I: process parameter effects on thermal history and weld properties,” Metallurgical and Materials Transactions A, 37, JULY, pp. 2183– 2193, 2006. Peel, M., Steuwer, A.,Withers, P. J. Dissimilar friction stir welds in AA5083AA6082. Part II: Process parameter effects on microstructure,” Metallurgical and Materials Transactions A, 37, July, 2006. Prime, M., Gnaupelherold, T., Baumann, J., Lederich, R., Bowden, D., Sebring, R. Residual stress measurements in a thick, dissimilar aluminum alloy friction stir weld,Acta Materialia, vol. 54, no. 15, pp. 4013–4021, Sep. 2006. Prevéy, P. S. The use of pearson vii distribution functions in x-ra y diffr action residual stress measurement, 29, 1986. Xu, W., Liu, J., Zhu, H. Analysis of residual stresses in thick aluminum friction stir welded butt joints,Materials & Design, 32, 4, pp. 2000–2005, 2011.

Figure and Table Captions 20

Figure captions Figure 1. Distribution of the measurement points Figure 2. Absolute scan between 74° and 120°forAA 2024 and AA 6061 Figure 3. Stress measurement scan for

=

.

Figure 4. Linear behavior between d-spacing and

° ( )

Figure 5. Transverse section macrograph of the bead made with 650 rpm – 65 mm/min. Figure 6. AA 2024-T3 base material microstructure, 200 X. Figure 7. AA 6061-T6 base material microstructure, 200 X. Figure 8. Residual stress profiles for similar AA 6061 and AA 2024 friction stir welds made with 500 rpm and 45 mm/min. Figure 9. Variation of the longitudinal residual stresses with the rotational speed at a welding speed of 45 mm/min Figure 10. Variation of the longitudinal residual stresses with the rotational speed at a welding speed of 65 mm/min Figure 11. Variation of the maximum tensile residual stress with the rotational and welding speed.

Table 1. Nominal chemical composition and mechanical properties of AA 2024-T3 and AA 6061-T6 Chemical composition (weight %) Element

AA 2024-T3

AA 6061-T6

21

Al

94 (balance)

97-99 (balance)

Si

0.5 (maximum)

0.4-0.8

Fe

0.5 (maximum)

0.7 (maximum)

Cu

3.8-4.9

0.15-0.40

Mn

0.3-0.9

0.15 (maximum

Mg

1.2-1.8

0.8-1.2

Cr

0.1 (maximum)

0.04-0.35

Mechanical properties Yield strength (MPa)

345

276

Ultimate strength (MPa)

485

310

Hardness (HV)

125

110

Table 2.Welding parameters N°

Rotational speed (rpm)

1 2

Welding speed (mm/min) 45

500

3

65 45

650 4

65

5

45

6

840

65

Table 3. Hardware and software parameters employed for absolute scan

22

Hardware parameters

Software parameters (Instrument settings)

Incident beam area: 1.5mm x 1.5mm

Type of scan: Absolute scan

Filter: Nickel

Initial Bragg angle: 74°

Radiation: Cupper tube (CuKα)

Final Bragg angle: 130°

Soller slit: 0.04 rad

Step size: 0.10°

Mask: 5mm

Time per step: 150 s

Divergence slit: 1° (1.5mm)

Scan speed: 0.26 (°/s)

Anti-scatter slit: 1° (1.5mm)

Total time: 190 s

Beam attenuator

None

Collimator

None

Receiving slit

None

Table 4. Software parameters for stress measurement scan Software parameters (Instrument settings) Type of scan: stress measurement Tilt axis: Omega Tilt range: Only positive Tilt limit-maximum: 20° 2 step size: 0.04 Number of Phi ( ) steps: 1 Scan axis: 2Theta – Omega (2 − ) Range: 3.007° 23

Software parameters (Instrument settings) Step size: 0.013° Time per step: 150 s Scan speed: 0.025 (°/s) Number of scans: 9 Total time: 43 min

24