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Peening effects on mechanical properties in friction stir welded AA 2195 at elevated and cryogenic temperatures
Materials and Design 30 (2009) 3165–3173
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Peening effects on mechanical properties in friction stir welded AA 2195 at elevated and cryogenic temperatures Omar Hatamleh a,*, Rajiv S. Mishra b, Ovidio Oliveras c a
Structures Branch, NASA – Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, United States Department of Material Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, United States c Jacobs Engineering, Houston, TX 77058, United States b
a r t i c l e
i n f o
Article history: Received 21 September 2008 Accepted 17 November 2008 Available online 27 November 2008 Keywords: Friction stir welding Laser peening Shot peening Mechanical properties Digital image correlation AA 2195
a b s t r a c t The shot peening and laser peening effects on the mechanical properties of friction stir welded 2195 aluminum alloy were investigated at elevated and cryogenic temperatures. The tensile properties were evaluated at different regions of the weld using a digital image correlation (DIC) system and mini-tensile testing samples. The surface and through thickness residual stresses were also obtained by using X-ray diffraction and the contour method. The specimens processed using laser peening exhibited superior mechanical properties for both elevated and cryogenic temperatures. Moreover, an electron backscattered Kikuchi diffractometry (EBSD) technique indicated a decrease in surface grain size for the laser peened samples when compared to the as welded condition. Published by Elsevier Ltd.
1. Introduction Increasing operating expenses are driving manufacturers to reduce weight in aeronautical and aerospace manufacturing applications. The goal is to reduce the costs associated with manufacturing techniques to result in considerable cost and weight savings by reducing riveted/fastened joints and part count. One way to achieve that is by utilizing friction stir welding (FSW). Since its invention by the Welding Institute in 1991 [1], friction stir welding (FSW) has emerged as a promising solid state process with encouraging results in several industrial fields. Friction stir welding uses a non-consumable cylindrical probe that rotates at high speeds, and plunges into butting edges of the work pieces to be joined [2]. This process transforms the metal into a plastic state at a temperature below the melting temperature of the material [3], and then mechanically stirs the material together under pressure to form a welded joint. Because FSW is considered a solid state welding process, significant differences may be expected in terms of microstructure, and residual stress fields around the weld [4]. In addition, many of the disadvantages associated with fusion welds are reduced. However, tensile residual stresses developed during welding can still impact the service performance of the welds [5]. A reduction in mechanical properties may also oc-
* Corresponding author. Tel.: +1 281 483 0286; fax: +1 281 244 5918. E-mail address: [email protected] (O. Hatamleh). 0261-3069/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.matdes.2008.11.010
cur due to the coarsening and/or dissolution of the strengthening precipitates. Therefore, different peening techniques like shot and laser peening were used in this investigation as potential means for improving the mechanical properties of friction stir welded AA 2195 joints under elevated and cryogenic temperatures. Shot peening is an established method in which the surface of a part is deformed plastically by multiple overlapping impacts using glass, ceramic, or metal spheres. Laser peening is a surface treatment technique with the capability of introducing deep compressive residual stresses. These compressive stresses after laser peening can be significantly deeper than that of conventional shot peening [6,7]. In this study, the surface and through thickness residual stresses were also assessed, and the peening influence on the mechanical properties was characterized globally and locally throughout different weld regions. 2. Experimental set-up Aluminum alloy (AA) 2195 was used in this investigation. This type of alloy is well suited for many aerospace applications due to its low density, high strength, and corrosion resistance. The base material was obtained as a plate with a 1.25 cm thickness and mechanical properties as outlined in Table 1. Before the aluminium plates were joined by friction stir welding (FSW), the top surfaces of all the plates were cleaned, and the butt surfaces were milled to obtain uniform and clean butting surfaces. Standard fixtures with
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clamps were used to firmly press the parts against the back plate/ anvil and prevent the parts from separating during FSW. The friction stir welds were made with a 3.3 cm diameter shoulder tool
Table 1 Tensile properties for the as received AA 2195-T8. Material
0.2% Yield stress (MPa)
Ultimate strength (MPa)
2195-T8
503
537
Fig. 1. FSW specimen used for tensile testing.
using a threaded triple flutes pin with a diameter of 0.914 and 1.21 cm long. The FSW took place using a rotational speed of 300 rpm in the counterclockwise direction, and a translation speed of 15 cm/min. The force in the Z direction (downward direction) was 55.6 kN, and the force in the X direction (translation direction) was 11.6 kN. Several transverse tensile specimens were then sectioned from the welded plates. The specimens consisted of conventional dog bone geometry with 20 cm long, a gage length of 8.5 cm, and a gage width of 1.25 cm (Fig. 1). The coupons were oriented such that the weld was in the center of the specimen and the load was applied perpendicular to the weld direction. The shot peening process was applied using an Almen intensity of 0.008–0.012 A with a 200% coverage, while the laser peening samples were processed with a laser power density of 5 GW/cm2 and 18 ns long. Laser peened specimen were processed using single and multiple layers of peening. The number of layers is the number of times the peening was applied over a specific area. The peening was applied uniformly on all sides of the gage section of the tensile specimens. The microstructure of the weld zone was evaluated using optical, scanning electron microscopes, and an electron backscatter diffraction. The surface residual stresses in the weld were acquired by means of the X-ray diffraction technique while the through thickness residual stresses were measured using the contour method [8]. The local tensile properties across the weld region were acquired using a digital image correlation (DIC) technique through
Fig. 2. Metallographic cross-section of AA 2195 friction stir weld.
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220
Microhardness reading (Knoop 300g)
210 200 190 180 170 160 150 140 130 120 110 100 90 80
-37 -33 -29 -25 -21 -17 -13 -9 -5 -1 3 7 11 15 19 23 27 31 35 Distance from weld centerline (mm) Fig. 3. The microhardness profile across the weld region for FSW AA 2195.
an ARAMIS system. ARAMIS is a powerful system for inspecting complex materials and geometries for their deformation and strain during loading [9,10]. By using that system, the intrinsic tensile properties for various locations across the weld zone were characterized by a tensile test. The local strain data was mapped to the corresponding global stress levels by assuming that the transversely loaded FSW specimens were considered a composite material loaded in an iso-stress configuration [11]. The iso-stress assumption suggests that the different regions of the weld are arranged in series and that the crosssection at any location in the specimen is homogeneous [12]. Consequently, this assumption implies that the stress is uniform over the cross-section and is equal to the average stress. The accuracy of the measured tensile properties is therefore determined by the degree of non-homogeneity at all cross-sections to which the load is applied. Using this assumption, local constitutive stress–strain relationships were obtained.
The tensile testing was performed at temperatures equivalent to 182 and 100 °C on a 200 kN servo-hydraulic universal testing machine using a constant crosshead speed of 0.25 mm/min. The specimens were exposed to the specified temperatures for 15 min before the testing took place in an environmental chamber. Heating elements were used for the hot temperatures, while liquid nitrogen was used to cool the specimen to the desired temperatures. Three samples were used for each testing condition. 3. Results and discussion 3.1. Weld microstructure and microhardness A micrograph showing the cross-section for different regions of the weld in the FSW AA 2195 is illustrated in Fig. 2. The higher temperature and extensive plastic deformation in the weld nugget resulted in fine and equiaxed grain sizes significantly smaller than
No Peening Laser Peening (6 layers)
Laser Peening (1 layer) Shot Peening
0
Residual Stress (MPa)
-20 -40 -60 -80 -100 -120 -140 -160
-4
-2
0
2
Distance from Weld Centerline (mm) Fig. 4. Residual stresses in the weld region.
4
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160 120
Residual Stress (MPa)
80 40 0 -40 -80
No Peening Shot Peening Laser Peening (1 layer) Laser Peening (6 layers)
-120 -160 -200 -240
mid-width 0
2
4
6
8
10
12
Distance from surface (mm) Fig. 5. Line plots of the residual stresses through the thickness in the weld region.
the parent material grain. The grain structure in the thermomechanical affected zone (TMAZ) region was elongated and distorted due to the mechanical action from the welding tool. The heat affected zone (HAZ) region resembles the parent material grain structure, where the grain is elongated as a result of the rolling process. The EBSD provided a complete misorientation analysis of the grain boundaries. The resultant color coded orientation map is shown in Fig. 2. The results show a highly shear banded structure in the vicinity of the weld and a highly random microstructure in the stir welded region. The hardness measurements across the weld region are shown in Fig. 3. A softened region correspondent to the weld zone is evident in the figure, and is mainly attributed to the coarsening/dissolution of strengthening precipitates induced by the thermal cycle of the FSW. Strengthening precipitates like T1 were found to be
missing in the weld nugget in FSW AA 2195 [13,14]. The T1 (Al2CuLi) precipitate is considered to be the primary strengthening precipitate in the 2195 alloy [15]. 3.2. Residual stress The residual stresses (RS) on the surface of the specimens were measured using the X-ray diffraction (XRD) technique. The surface residual stresses at the center of the weld are shown in Fig. 4. The residual stresses for the unpeened specimens were slightly compressive in this region of the weld. It is expected that the tensile residual stresses present around the weld nugget will relax as the tensile specimen are sectioned from the large welded plates. The highest measured compressive residual stresses on the surface corresponded to shot peening due to the high degree of cold work on the surface.
400 350
Stress (MPa)
300 250 200 150 HAZ (Advancing Side)
100
TMAZ (Advancing Side) Weld Nugget TMAZ (Retreating Side)
50 0
0.0
HAZ (Retreating Side)
2.0
4.0
6.0
8.0
10.0
12.0
Strain (%) Fig. 6. Tensile properties at different regions of the weld for a FSW AA 2195 at room temperature.
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No Peening
Shot Peening
Laser Peening (3 layers)
350 300
Stress (MPa)
250 200 150 100 50 0 0
5
10
15
Strain (%) Fig. 7. Tensile properties at the weld centerline for FSW AA 2195 at 182 °C.
The through thickness residual stresses profiles are shown in Fig. 5. The line plots represent the acquired RS through the thickness at the mid-width of each sample. It is clear that the highest subsurface residual stresses were obtained through laser peening, specifically the ones peened with six layers. The depth of penetration for the laser peened samples processed with six layers was substantially higher than samples processed with shot peening. The line plots also indicate a small amount of tensile residual stresses (15 MPa) for the shot peened specimen, while the corresponding values for the laser peened samples exhibited a high level of tensile residual stresses (96 MPa). These tensile residual stresses are generated in order to compensate for the high compressive residual stresses measured on the surface/subsurface.
No Peening
The contour method generally lack accuracy when used to predict stresses at the surface, but through thickness measurements tends to be fairly accurate. Near the surface, there is additional noise in the data. The noise may be caused by machining irregularities on the edge or by the coordinate measuring machine spherical tip going slightly past the actual edge of the part as indicated by Prime [8]. Therefore, the stress gradient at the surface tends to produce a large displacement gradient which is difficult to distinguish from the noise in the data (surface roughness from cutting and measurement noise). To produce a reasonably smooth stress map, the noise must be filtered out by eliminating the variations in the displacements below some prescribed length scale. The net form of the surface is then used to calculate the residual stress.
Shot Peening
Laser Peening (3 layers)
350 300
Stress (MPa)
250 200 150 100 50 0 0
2
4
6
8
Strain (%) Fig. 8. Tensile properties at the TMAZ (retreating side) for FSW AA 2195 at 182 °C.
10
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Fig. 9. Strain concentration before failure for FSW AA 2195 at 182 °C: (a) no peening; (b) shot peening; (c) laser peening (three layers).
Therefore, surface residual stresses are better characterized by the X-ray diffraction method. 3.3. Mechanical properties The tensile properties at room temperature obtained through DIC for different regions across the weld are illustrated in Fig. 6.
The weld nugget exhibited the lowest tensile properties when compared to other locations across the weld. Due to the heat generated during the welding process, the dislocation density in the nugget is lowered from the parent material. Also, since the strengthening precipitates appear to have solutionized during the welding process [13,14], this indicated that the temperature during joining was above the solution temperature of the hardening precipitates. It is also clear that the tensile properties at the weld interface were lower than their correspondent properties in the HAZ. The properties at the TMAZ on both sides of the weld (advancing and retreating) were similar. Since the yield strength of the transversely loaded FSW specimens are lower than the yield strength of the base metal, the base metal experienced predominantly elastic strain throughout the test [16]. The tensile properties results in the nugget region for different samples at 182 °C are illustrated in Fig. 7. The 0.2% offset yield strength corresponded to 190 MPa for the unpeened, 205 MPa for the shot peened, and 247 MPa for the laser peened sample. The laser peened sample failed at a lower strain as is the case in most strain hardened material. When comparing the 0.2% yield stress at the weld centerline on samples tested at room temperature versus the samples tested at 182 °C. The samples tested at 182 °C resulted in 8% decrease in strength. The exposure of the tensile specimen at the specified high temperature may have resulted in further aging of the strengthening precipitates, resulting in the reduced tensile properties outlined in Fig. 7. The mechanical properties for all the welded specimens where lower than the base material as are the case for other precipitation hardened aluminum alloys [15]. The tensile properties for the shot and laser peened specimen in the TMAZ region at 182 °C are illustrated in Fig. 8. Again for this region of the weld, the laser peened samples resulted in higher tensile properties than specimen that were shot peened. The 0.2% yield stress for the shot and laser peened samples were 214 and 278 MPa, respectively. These values are slightly higher than their correspondent values in the weld nugget region where the all specimens failed. The elongation in the laser peened samples was significantly reduced from the unpeened and shot peened specimens. The strain distribution across the weld region for the unpeened, shot and laser peened samples before failure is illustrated in Fig. 9. The figure demonstrates the highest strain localization taking place in the weld nugget. This region was shown earlier in Fig. 3 to be the
375 Laser Peening (6 layers) Laser Peening (3 layers)
325
Shot Peening No Peening
Stress (MPa)
275 225 175 125 75 25
0
2
4
6
8
10
12
Strain (%) Fig. 10. Tensile properties for FSW 2195 AA at 182 °C.
14
16
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softest area of the weld; therefore, the strain concentration will be highest in this region. The unpeened samples had a strain of 15.2% at failure compared to 14.6% and 11.39% for the shot peened and laser peened, respectively. Fig. 10 illustrates the global tensile testing results for different peened samples at 182 °C. As in the case of the local tensile properties outlined in Figs. 7 and 8. Laser peened samples resulted in superior yield and ultimate strength, while shot peened samples only exhibited modest improvements. The laser peened samples failed at lower strains, and as the number of laser peening layers increased, the strain at failure decreased. That behavior is mainly attributed to the strain hardening resulting from the high energy laser pulses. The global tensile testing for the FSW specimen at 100 °C is shown in Fig. 11. Specimen processed with laser peening resulted
in improved tensile properties, however, the difference between the unpeened, shot peened, and laser peened specimen was reduced when compared to the tests conducted at high temperatures. All the specimens tested at 100 °C failed at higher ultimate strength than the ones tested at 182 °C. That is further illustrated in Fig. 12, where various unpeened FSW samples were tested at different temperature. To further investigate the effects of laser peening as a function of distance from the processed surfaces, mini tensile samples were acquired from the crown, middle, and root sides of the weld nugget. The results are shown in Table 2 and indicate a large variation in mechanical properties at different depths. For example, an increase of 17% and 27% in the yield stress was noted in the crown and the root sides of the peened specimens, respectively, when compared to yield stress at the middle of the sample. The higher
Stress (MPa)
475 425
Laser Peening (6 layers) Laser Peening (3 layers)
375
Shot Peening No Peening
325 275 225 175 125 75 25
0
2
4
6
8 10 Strain (%)
Fig. 11. Tensile properties for FSW AA 2195 at
12
14
16
18
100 °C.
500 RT
450
-100 degrees 182 degrees
400
Stress (MPa)
350 300 250 200 150 100 50 0 0
5
10 Strain (%)
15
Fig. 12. Tensile properties for the unpeened FSW AA 2195 at various temperatures.
20
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Table 2 Tensile properties at different depths in the weld for laser peened (six layers) FSW AA 2195.
Laser peened weld nugget (crown) Laser peened weld nugget (middle) Laser peened weld nugget (root)
Yield stress (MPa)
Ultimate stress (MPa)
Elongation (%)
397
454
22
341.5
426.2
29
433.25
481.5
19
properties exhibited at the root side of the weld compared to the crown side may be attributed to the specimen thermal characteristics that take place during welding. The improvement in tensile strength in the laser peened samples could mainly be attributed to the strain hardening which can be explained by the generation of dislocations under the effect of the plastic deformation from the high energy laser peening. The resulting increase in dislocations tends to increase the flow resistance of the material to plastic deformation.
Fig. 14. Grain size histogram: (A) laser peened specimen; (B) unpeened specimen.
The effects of grain size on the tensile properties were also investigated using an electron backscattered Kikuchi diffractometry (EBSD) technique. The analysis was performed for the unpeened and laser peened (six layers) friction stir welded conditions. The analysis was performed on an area measuring 100 lm from the surface of the specimens in the weld nugget region. The resultant color coded orientation map show a highly shear banded structure in the stir welded region and a highly random microstructure in the vicinity of the weld. The comparison of the pole figure analysis (Fig. 13) shows that the maximum texture intensity does not change quite drastically as a result of laser peening. The local texture is however different qualitatively between the two. Results of the inverse pole figures show that there seems to be a (0 0 1) texture oriented towards the normal to the sample in the case of laser peened specimen. Grain size distribution analysis for high angle boundaries grain structure (Fig. 14) shows that there is a decrease in grain size for samples that have been laser peened. That decrease in grain size in the laser peened samples may have also contributed to the increase in tensile properties demonstrated in Table 2. 4. Summary and conclusions
Fig. 13. Inverse pole figure plots: (A) laser peened specimen; (B) unpeened specimen.
The effects from laser peening and shot peening on the mechanical properties of FSW AA 2195-T8 were investigated. The peening effects on the global and local mechanical properties through the different regions of the weld were characterized at elevated and cryogenic temperatures. The tensile coupons were machined such as the loading was applied in a direction perpendicular to the weld
O. Hatamleh et al. / Materials and Design 30 (2009) 3165–3173
direction. The local strains and equivalent tensile properties were evaluated at different regions of the weld using a digital image correlation system and mini-tensile testing samples. To help interpret the results, the surface and through thickness residual stresses were also obtained by using X-ray diffraction and the contour method. Laser peened samples showed an average increase of 32% to the 0.2% offset yield strength of the material in the weld nugget region when tested at the high temperature (182 °C). In contrast, shot peening exhibited only modest improvement on the yield strength (8%). In the TMAZ region, the laser peened samples exhibited yield strength 28% higher than their corresponding values in the shot peened samples in the high temperature tests. The specimens tested at cryogenic temperature ( 100 °C) also resulted in an increase to the tensile properties; however, the difference between peened and unpeened samples was diminished. The increase in mechanical properties from the laser peening was mainly attributed to the strain hardening which can be explained by the generation of dislocations under the effect of the plastic deformation from the high energy laser peening. The resulting increase in dislocations tends to increase the flow resistance of the material to plastic deformation. The increase in mechanical properties may also have been resulted from the decrease in grain size on the surfaces due to the laser peening process. References [1] Thomas WM, Nicholas ED, Needham JC, Murch MG, Temple-Smith P, Dawes CJ. Friction stir butt welding. International Patent Application PCT/GB92/02203, and GB Patent Application 9125978.8, 297 US Patent No. 5,460,317, October 1995; 1991.
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[2] Pao P, Gill S, Feng C, Sankaran K. Corrosion-fatigue crack growth in friction stir welded Al 7075. Scripta Mater 2001;45:605–12. [3] Staron P, Kocak M, Williams S, Wescott A. Residual stress in friction stirwelded A1 sheets. Phys B: Condens Matt 2004;350(1–3 Suppl. 1):e491–3. [4] Bussu G, Irving PE. The role of residual stress and heat affected zone properties on fatigue crack propagation in friction stir welded 2024-T351 aluminum joints. Int J Fatigue 2003;25:77–88. [5] Sutton MA, Yang B, Reynolds AP, Taylor R. Preliminary studies of mixed mode fracture in 2024-T3 friction stir welds. In: Best of aeromat session, ASM materials solutions conference and exhibition; 2000. [6] Montross C, Wei T, Ye L, Clark G, Mai Y. Laser shock processing and its effects on microstructure and properties of metal alloys: a review. Int J Fatigue 2002;24:1021–36. [7] Peyre P, Fabbro R, Merrien P, Lieurade HP. Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour. Mater Sci Eng A 1996;210:102–13. [8] Prime MB. Cross-sectional mapping of residual stresses by measuring the surface contour after a cut. J Eng Mater Technol 2001;123:162–8. [9] Tyson J, Schmidt T, Galanulis K. Advanced photogrammetry for robust deformation and strain measurement. In: Proceedings of SEM 2002 annual conference; 2002. [10] Schmidt T, Tyson J, Galanulis K. Full-field dynamic displacement and strain measurement using advanced 3D image correlation photogrammetry. Exp Tech 2003;27(3):41–4. [11] Lockwood WD, Tomas Borislav, Reynolds AP. Mechanical response of friction stir welded AA 2024: experiment and modeling. Mater Sci Eng A 2002;323(1– 2):349–54. [12] Lockwood WD, Reynolds AP. Simulation of the global response of a friction stir weld using local constitutive behavior. Mater Sci Eng A 2003;339(1–2):35–42. [13] Li Z, Arbegast J, Hartley P, Meletis E. In: Proceedings of the fifth international conference on trends in welding research. Materials Park, OH: ASM International; 1999. [14] Oertelt G, Babu S, David S, Kenik E. J Weld 2001;80:71–9. [15] Kroninger H, Reynolds A. R-Curve behaviour of friction stir welds in aluminum–lithium alloy 2195. Fatigue Fract Eng Mater Struct 2002;25:283–90. [16] Peel M et al. Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminum AA5083 friction stir welds. Acta Mater 2003;51:4791–801.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Peening effects on mechanical properties in friction stir welded AA 2195 at elevated and cryogenic temperatures Omar Hatamleh a,*, Rajiv S. Mishra b, Ovidio Oliveras c a
Structures Branch, NASA – Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, United States Department of Material Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, United States c Jacobs Engineering, Houston, TX 77058, United States b
a r t i c l e
i n f o
Article history: Received 21 September 2008 Accepted 17 November 2008 Available online 27 November 2008 Keywords: Friction stir welding Laser peening Shot peening Mechanical properties Digital image correlation AA 2195
a b s t r a c t The shot peening and laser peening effects on the mechanical properties of friction stir welded 2195 aluminum alloy were investigated at elevated and cryogenic temperatures. The tensile properties were evaluated at different regions of the weld using a digital image correlation (DIC) system and mini-tensile testing samples. The surface and through thickness residual stresses were also obtained by using X-ray diffraction and the contour method. The specimens processed using laser peening exhibited superior mechanical properties for both elevated and cryogenic temperatures. Moreover, an electron backscattered Kikuchi diffractometry (EBSD) technique indicated a decrease in surface grain size for the laser peened samples when compared to the as welded condition. Published by Elsevier Ltd.
1. Introduction Increasing operating expenses are driving manufacturers to reduce weight in aeronautical and aerospace manufacturing applications. The goal is to reduce the costs associated with manufacturing techniques to result in considerable cost and weight savings by reducing riveted/fastened joints and part count. One way to achieve that is by utilizing friction stir welding (FSW). Since its invention by the Welding Institute in 1991 [1], friction stir welding (FSW) has emerged as a promising solid state process with encouraging results in several industrial fields. Friction stir welding uses a non-consumable cylindrical probe that rotates at high speeds, and plunges into butting edges of the work pieces to be joined [2]. This process transforms the metal into a plastic state at a temperature below the melting temperature of the material [3], and then mechanically stirs the material together under pressure to form a welded joint. Because FSW is considered a solid state welding process, significant differences may be expected in terms of microstructure, and residual stress fields around the weld [4]. In addition, many of the disadvantages associated with fusion welds are reduced. However, tensile residual stresses developed during welding can still impact the service performance of the welds [5]. A reduction in mechanical properties may also oc-
* Corresponding author. Tel.: +1 281 483 0286; fax: +1 281 244 5918. E-mail address: [email protected] (O. Hatamleh). 0261-3069/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.matdes.2008.11.010
cur due to the coarsening and/or dissolution of the strengthening precipitates. Therefore, different peening techniques like shot and laser peening were used in this investigation as potential means for improving the mechanical properties of friction stir welded AA 2195 joints under elevated and cryogenic temperatures. Shot peening is an established method in which the surface of a part is deformed plastically by multiple overlapping impacts using glass, ceramic, or metal spheres. Laser peening is a surface treatment technique with the capability of introducing deep compressive residual stresses. These compressive stresses after laser peening can be significantly deeper than that of conventional shot peening [6,7]. In this study, the surface and through thickness residual stresses were also assessed, and the peening influence on the mechanical properties was characterized globally and locally throughout different weld regions. 2. Experimental set-up Aluminum alloy (AA) 2195 was used in this investigation. This type of alloy is well suited for many aerospace applications due to its low density, high strength, and corrosion resistance. The base material was obtained as a plate with a 1.25 cm thickness and mechanical properties as outlined in Table 1. Before the aluminium plates were joined by friction stir welding (FSW), the top surfaces of all the plates were cleaned, and the butt surfaces were milled to obtain uniform and clean butting surfaces. Standard fixtures with
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O. Hatamleh et al. / Materials and Design 30 (2009) 3165–3173
clamps were used to firmly press the parts against the back plate/ anvil and prevent the parts from separating during FSW. The friction stir welds were made with a 3.3 cm diameter shoulder tool
Table 1 Tensile properties for the as received AA 2195-T8. Material
0.2% Yield stress (MPa)
Ultimate strength (MPa)
2195-T8
503
537
Fig. 1. FSW specimen used for tensile testing.
using a threaded triple flutes pin with a diameter of 0.914 and 1.21 cm long. The FSW took place using a rotational speed of 300 rpm in the counterclockwise direction, and a translation speed of 15 cm/min. The force in the Z direction (downward direction) was 55.6 kN, and the force in the X direction (translation direction) was 11.6 kN. Several transverse tensile specimens were then sectioned from the welded plates. The specimens consisted of conventional dog bone geometry with 20 cm long, a gage length of 8.5 cm, and a gage width of 1.25 cm (Fig. 1). The coupons were oriented such that the weld was in the center of the specimen and the load was applied perpendicular to the weld direction. The shot peening process was applied using an Almen intensity of 0.008–0.012 A with a 200% coverage, while the laser peening samples were processed with a laser power density of 5 GW/cm2 and 18 ns long. Laser peened specimen were processed using single and multiple layers of peening. The number of layers is the number of times the peening was applied over a specific area. The peening was applied uniformly on all sides of the gage section of the tensile specimens. The microstructure of the weld zone was evaluated using optical, scanning electron microscopes, and an electron backscatter diffraction. The surface residual stresses in the weld were acquired by means of the X-ray diffraction technique while the through thickness residual stresses were measured using the contour method [8]. The local tensile properties across the weld region were acquired using a digital image correlation (DIC) technique through
Fig. 2. Metallographic cross-section of AA 2195 friction stir weld.
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220
Microhardness reading (Knoop 300g)
210 200 190 180 170 160 150 140 130 120 110 100 90 80
-37 -33 -29 -25 -21 -17 -13 -9 -5 -1 3 7 11 15 19 23 27 31 35 Distance from weld centerline (mm) Fig. 3. The microhardness profile across the weld region for FSW AA 2195.
an ARAMIS system. ARAMIS is a powerful system for inspecting complex materials and geometries for their deformation and strain during loading [9,10]. By using that system, the intrinsic tensile properties for various locations across the weld zone were characterized by a tensile test. The local strain data was mapped to the corresponding global stress levels by assuming that the transversely loaded FSW specimens were considered a composite material loaded in an iso-stress configuration [11]. The iso-stress assumption suggests that the different regions of the weld are arranged in series and that the crosssection at any location in the specimen is homogeneous [12]. Consequently, this assumption implies that the stress is uniform over the cross-section and is equal to the average stress. The accuracy of the measured tensile properties is therefore determined by the degree of non-homogeneity at all cross-sections to which the load is applied. Using this assumption, local constitutive stress–strain relationships were obtained.
The tensile testing was performed at temperatures equivalent to 182 and 100 °C on a 200 kN servo-hydraulic universal testing machine using a constant crosshead speed of 0.25 mm/min. The specimens were exposed to the specified temperatures for 15 min before the testing took place in an environmental chamber. Heating elements were used for the hot temperatures, while liquid nitrogen was used to cool the specimen to the desired temperatures. Three samples were used for each testing condition. 3. Results and discussion 3.1. Weld microstructure and microhardness A micrograph showing the cross-section for different regions of the weld in the FSW AA 2195 is illustrated in Fig. 2. The higher temperature and extensive plastic deformation in the weld nugget resulted in fine and equiaxed grain sizes significantly smaller than
No Peening Laser Peening (6 layers)
Laser Peening (1 layer) Shot Peening
0
Residual Stress (MPa)
-20 -40 -60 -80 -100 -120 -140 -160
-4
-2
0
2
Distance from Weld Centerline (mm) Fig. 4. Residual stresses in the weld region.
4
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160 120
Residual Stress (MPa)
80 40 0 -40 -80
No Peening Shot Peening Laser Peening (1 layer) Laser Peening (6 layers)
-120 -160 -200 -240
mid-width 0
2
4
6
8
10
12
Distance from surface (mm) Fig. 5. Line plots of the residual stresses through the thickness in the weld region.
the parent material grain. The grain structure in the thermomechanical affected zone (TMAZ) region was elongated and distorted due to the mechanical action from the welding tool. The heat affected zone (HAZ) region resembles the parent material grain structure, where the grain is elongated as a result of the rolling process. The EBSD provided a complete misorientation analysis of the grain boundaries. The resultant color coded orientation map is shown in Fig. 2. The results show a highly shear banded structure in the vicinity of the weld and a highly random microstructure in the stir welded region. The hardness measurements across the weld region are shown in Fig. 3. A softened region correspondent to the weld zone is evident in the figure, and is mainly attributed to the coarsening/dissolution of strengthening precipitates induced by the thermal cycle of the FSW. Strengthening precipitates like T1 were found to be
missing in the weld nugget in FSW AA 2195 [13,14]. The T1 (Al2CuLi) precipitate is considered to be the primary strengthening precipitate in the 2195 alloy [15]. 3.2. Residual stress The residual stresses (RS) on the surface of the specimens were measured using the X-ray diffraction (XRD) technique. The surface residual stresses at the center of the weld are shown in Fig. 4. The residual stresses for the unpeened specimens were slightly compressive in this region of the weld. It is expected that the tensile residual stresses present around the weld nugget will relax as the tensile specimen are sectioned from the large welded plates. The highest measured compressive residual stresses on the surface corresponded to shot peening due to the high degree of cold work on the surface.
400 350
Stress (MPa)
300 250 200 150 HAZ (Advancing Side)
100
TMAZ (Advancing Side) Weld Nugget TMAZ (Retreating Side)
50 0
0.0
HAZ (Retreating Side)
2.0
4.0
6.0
8.0
10.0
12.0
Strain (%) Fig. 6. Tensile properties at different regions of the weld for a FSW AA 2195 at room temperature.
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No Peening
Shot Peening
Laser Peening (3 layers)
350 300
Stress (MPa)
250 200 150 100 50 0 0
5
10
15
Strain (%) Fig. 7. Tensile properties at the weld centerline for FSW AA 2195 at 182 °C.
The through thickness residual stresses profiles are shown in Fig. 5. The line plots represent the acquired RS through the thickness at the mid-width of each sample. It is clear that the highest subsurface residual stresses were obtained through laser peening, specifically the ones peened with six layers. The depth of penetration for the laser peened samples processed with six layers was substantially higher than samples processed with shot peening. The line plots also indicate a small amount of tensile residual stresses (15 MPa) for the shot peened specimen, while the corresponding values for the laser peened samples exhibited a high level of tensile residual stresses (96 MPa). These tensile residual stresses are generated in order to compensate for the high compressive residual stresses measured on the surface/subsurface.
No Peening
The contour method generally lack accuracy when used to predict stresses at the surface, but through thickness measurements tends to be fairly accurate. Near the surface, there is additional noise in the data. The noise may be caused by machining irregularities on the edge or by the coordinate measuring machine spherical tip going slightly past the actual edge of the part as indicated by Prime [8]. Therefore, the stress gradient at the surface tends to produce a large displacement gradient which is difficult to distinguish from the noise in the data (surface roughness from cutting and measurement noise). To produce a reasonably smooth stress map, the noise must be filtered out by eliminating the variations in the displacements below some prescribed length scale. The net form of the surface is then used to calculate the residual stress.
Shot Peening
Laser Peening (3 layers)
350 300
Stress (MPa)
250 200 150 100 50 0 0
2
4
6
8
Strain (%) Fig. 8. Tensile properties at the TMAZ (retreating side) for FSW AA 2195 at 182 °C.
10
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Fig. 9. Strain concentration before failure for FSW AA 2195 at 182 °C: (a) no peening; (b) shot peening; (c) laser peening (three layers).
Therefore, surface residual stresses are better characterized by the X-ray diffraction method. 3.3. Mechanical properties The tensile properties at room temperature obtained through DIC for different regions across the weld are illustrated in Fig. 6.
The weld nugget exhibited the lowest tensile properties when compared to other locations across the weld. Due to the heat generated during the welding process, the dislocation density in the nugget is lowered from the parent material. Also, since the strengthening precipitates appear to have solutionized during the welding process [13,14], this indicated that the temperature during joining was above the solution temperature of the hardening precipitates. It is also clear that the tensile properties at the weld interface were lower than their correspondent properties in the HAZ. The properties at the TMAZ on both sides of the weld (advancing and retreating) were similar. Since the yield strength of the transversely loaded FSW specimens are lower than the yield strength of the base metal, the base metal experienced predominantly elastic strain throughout the test [16]. The tensile properties results in the nugget region for different samples at 182 °C are illustrated in Fig. 7. The 0.2% offset yield strength corresponded to 190 MPa for the unpeened, 205 MPa for the shot peened, and 247 MPa for the laser peened sample. The laser peened sample failed at a lower strain as is the case in most strain hardened material. When comparing the 0.2% yield stress at the weld centerline on samples tested at room temperature versus the samples tested at 182 °C. The samples tested at 182 °C resulted in 8% decrease in strength. The exposure of the tensile specimen at the specified high temperature may have resulted in further aging of the strengthening precipitates, resulting in the reduced tensile properties outlined in Fig. 7. The mechanical properties for all the welded specimens where lower than the base material as are the case for other precipitation hardened aluminum alloys [15]. The tensile properties for the shot and laser peened specimen in the TMAZ region at 182 °C are illustrated in Fig. 8. Again for this region of the weld, the laser peened samples resulted in higher tensile properties than specimen that were shot peened. The 0.2% yield stress for the shot and laser peened samples were 214 and 278 MPa, respectively. These values are slightly higher than their correspondent values in the weld nugget region where the all specimens failed. The elongation in the laser peened samples was significantly reduced from the unpeened and shot peened specimens. The strain distribution across the weld region for the unpeened, shot and laser peened samples before failure is illustrated in Fig. 9. The figure demonstrates the highest strain localization taking place in the weld nugget. This region was shown earlier in Fig. 3 to be the
375 Laser Peening (6 layers) Laser Peening (3 layers)
325
Shot Peening No Peening
Stress (MPa)
275 225 175 125 75 25
0
2
4
6
8
10
12
Strain (%) Fig. 10. Tensile properties for FSW 2195 AA at 182 °C.
14
16
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softest area of the weld; therefore, the strain concentration will be highest in this region. The unpeened samples had a strain of 15.2% at failure compared to 14.6% and 11.39% for the shot peened and laser peened, respectively. Fig. 10 illustrates the global tensile testing results for different peened samples at 182 °C. As in the case of the local tensile properties outlined in Figs. 7 and 8. Laser peened samples resulted in superior yield and ultimate strength, while shot peened samples only exhibited modest improvements. The laser peened samples failed at lower strains, and as the number of laser peening layers increased, the strain at failure decreased. That behavior is mainly attributed to the strain hardening resulting from the high energy laser pulses. The global tensile testing for the FSW specimen at 100 °C is shown in Fig. 11. Specimen processed with laser peening resulted
in improved tensile properties, however, the difference between the unpeened, shot peened, and laser peened specimen was reduced when compared to the tests conducted at high temperatures. All the specimens tested at 100 °C failed at higher ultimate strength than the ones tested at 182 °C. That is further illustrated in Fig. 12, where various unpeened FSW samples were tested at different temperature. To further investigate the effects of laser peening as a function of distance from the processed surfaces, mini tensile samples were acquired from the crown, middle, and root sides of the weld nugget. The results are shown in Table 2 and indicate a large variation in mechanical properties at different depths. For example, an increase of 17% and 27% in the yield stress was noted in the crown and the root sides of the peened specimens, respectively, when compared to yield stress at the middle of the sample. The higher
Stress (MPa)
475 425
Laser Peening (6 layers) Laser Peening (3 layers)
375
Shot Peening No Peening
325 275 225 175 125 75 25
0
2
4
6
8 10 Strain (%)
Fig. 11. Tensile properties for FSW AA 2195 at
12
14
16
18
100 °C.
500 RT
450
-100 degrees 182 degrees
400
Stress (MPa)
350 300 250 200 150 100 50 0 0
5
10 Strain (%)
15
Fig. 12. Tensile properties for the unpeened FSW AA 2195 at various temperatures.
20
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Table 2 Tensile properties at different depths in the weld for laser peened (six layers) FSW AA 2195.
Laser peened weld nugget (crown) Laser peened weld nugget (middle) Laser peened weld nugget (root)
Yield stress (MPa)
Ultimate stress (MPa)
Elongation (%)
397
454
22
341.5
426.2
29
433.25
481.5
19
properties exhibited at the root side of the weld compared to the crown side may be attributed to the specimen thermal characteristics that take place during welding. The improvement in tensile strength in the laser peened samples could mainly be attributed to the strain hardening which can be explained by the generation of dislocations under the effect of the plastic deformation from the high energy laser peening. The resulting increase in dislocations tends to increase the flow resistance of the material to plastic deformation.
Fig. 14. Grain size histogram: (A) laser peened specimen; (B) unpeened specimen.
The effects of grain size on the tensile properties were also investigated using an electron backscattered Kikuchi diffractometry (EBSD) technique. The analysis was performed for the unpeened and laser peened (six layers) friction stir welded conditions. The analysis was performed on an area measuring 100 lm from the surface of the specimens in the weld nugget region. The resultant color coded orientation map show a highly shear banded structure in the stir welded region and a highly random microstructure in the vicinity of the weld. The comparison of the pole figure analysis (Fig. 13) shows that the maximum texture intensity does not change quite drastically as a result of laser peening. The local texture is however different qualitatively between the two. Results of the inverse pole figures show that there seems to be a (0 0 1) texture oriented towards the normal to the sample in the case of laser peened specimen. Grain size distribution analysis for high angle boundaries grain structure (Fig. 14) shows that there is a decrease in grain size for samples that have been laser peened. That decrease in grain size in the laser peened samples may have also contributed to the increase in tensile properties demonstrated in Table 2. 4. Summary and conclusions
Fig. 13. Inverse pole figure plots: (A) laser peened specimen; (B) unpeened specimen.
The effects from laser peening and shot peening on the mechanical properties of FSW AA 2195-T8 were investigated. The peening effects on the global and local mechanical properties through the different regions of the weld were characterized at elevated and cryogenic temperatures. The tensile coupons were machined such as the loading was applied in a direction perpendicular to the weld
O. Hatamleh et al. / Materials and Design 30 (2009) 3165–3173
direction. The local strains and equivalent tensile properties were evaluated at different regions of the weld using a digital image correlation system and mini-tensile testing samples. To help interpret the results, the surface and through thickness residual stresses were also obtained by using X-ray diffraction and the contour method. Laser peened samples showed an average increase of 32% to the 0.2% offset yield strength of the material in the weld nugget region when tested at the high temperature (182 °C). In contrast, shot peening exhibited only modest improvement on the yield strength (8%). In the TMAZ region, the laser peened samples exhibited yield strength 28% higher than their corresponding values in the shot peened samples in the high temperature tests. The specimens tested at cryogenic temperature ( 100 °C) also resulted in an increase to the tensile properties; however, the difference between peened and unpeened samples was diminished. The increase in mechanical properties from the laser peening was mainly attributed to the strain hardening which can be explained by the generation of dislocations under the effect of the plastic deformation from the high energy laser peening. The resulting increase in dislocations tends to increase the flow resistance of the material to plastic deformation. The increase in mechanical properties may also have been resulted from the decrease in grain size on the surfaces due to the laser peening process. References [1] Thomas WM, Nicholas ED, Needham JC, Murch MG, Temple-Smith P, Dawes CJ. Friction stir butt welding. International Patent Application PCT/GB92/02203, and GB Patent Application 9125978.8, 297 US Patent No. 5,460,317, October 1995; 1991.
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[2] Pao P, Gill S, Feng C, Sankaran K. Corrosion-fatigue crack growth in friction stir welded Al 7075. Scripta Mater 2001;45:605–12. [3] Staron P, Kocak M, Williams S, Wescott A. Residual stress in friction stirwelded A1 sheets. Phys B: Condens Matt 2004;350(1–3 Suppl. 1):e491–3. [4] Bussu G, Irving PE. The role of residual stress and heat affected zone properties on fatigue crack propagation in friction stir welded 2024-T351 aluminum joints. Int J Fatigue 2003;25:77–88. [5] Sutton MA, Yang B, Reynolds AP, Taylor R. Preliminary studies of mixed mode fracture in 2024-T3 friction stir welds. In: Best of aeromat session, ASM materials solutions conference and exhibition; 2000. [6] Montross C, Wei T, Ye L, Clark G, Mai Y. Laser shock processing and its effects on microstructure and properties of metal alloys: a review. Int J Fatigue 2002;24:1021–36. [7] Peyre P, Fabbro R, Merrien P, Lieurade HP. Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour. Mater Sci Eng A 1996;210:102–13. [8] Prime MB. Cross-sectional mapping of residual stresses by measuring the surface contour after a cut. J Eng Mater Technol 2001;123:162–8. [9] Tyson J, Schmidt T, Galanulis K. Advanced photogrammetry for robust deformation and strain measurement. In: Proceedings of SEM 2002 annual conference; 2002. [10] Schmidt T, Tyson J, Galanulis K. Full-field dynamic displacement and strain measurement using advanced 3D image correlation photogrammetry. Exp Tech 2003;27(3):41–4. [11] Lockwood WD, Tomas Borislav, Reynolds AP. Mechanical response of friction stir welded AA 2024: experiment and modeling. Mater Sci Eng A 2002;323(1– 2):349–54. [12] Lockwood WD, Reynolds AP. Simulation of the global response of a friction stir weld using local constitutive behavior. Mater Sci Eng A 2003;339(1–2):35–42. [13] Li Z, Arbegast J, Hartley P, Meletis E. In: Proceedings of the fifth international conference on trends in welding research. Materials Park, OH: ASM International; 1999. [14] Oertelt G, Babu S, David S, Kenik E. J Weld 2001;80:71–9. [15] Kroninger H, Reynolds A. R-Curve behaviour of friction stir welds in aluminum–lithium alloy 2195. Fatigue Fract Eng Mater Struct 2002;25:283–90. [16] Peel M et al. Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminum AA5083 friction stir welds. Acta Mater 2003;51:4791–801.