Microstructural studies of friction stir welds in 2024 ... - KUNDOC.COM

Materials Science and Engineering A323 (2002) 160– 166 www.elsevier.com/locate/msea

Microstructural studies of friction stir welds in 2024-T3 aluminum M.A. Sutton *, B. Yang, A.P. Reynolds, R. Taylor Department of Mechanical Engineering, Uni6ersity of South Carolina, 300 Main Street, Columbia, SC 29208, USA Received 18 January 2001; received in revised form 9 March 2001

Abstract Friction stir welds in 7 mm thick, 2024-T351 aluminum rolled sheet material have been completed. Metallurgical, hardness and quantitative energy dispersive X-ray measurements have been performed which demonstrate that a segregated, banded, microstructure consisting of alternating hard particle rich and hard particle poor regions is developed. Mixed-mode I/II monotonic fracture experiments confirm that the observed banded microstructure affects the macroscopic fracture process. Since the band spacing is directly correlated with the welding tool advance per revolution, our results indicated that the opportunity exists to manipulate the friction stir weld process parameters in order to modify the weld microstructure and improve a range of material properties, including fracture resistance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Friction stir weld; 2024-T351 aluminum; Banded microstructure; Mixed-mode fracture

1. Introduction The difficulty of making high-strength, fatigue and fracture resistant welds in aerospace aluminum alloys (e.g. highly alloyed 2XXX and 7XXX series aluminum) has long inhibited the use of welding processes for joining aerospace structures. Typically, mechanical fastening has been the joining method of choice except in production of pressure vessels for rocket propellant and oxidizer tanks. Even in pressure vessel applications, heavy weld lands must be incorporated to account for the severe property knock-down resulting from the fusion welding processes. Many of the problems with welds in aerospace aluminum stem from the unfavorable distribution of brittle solidification products and porosity in the weld region. Friction stir welding (FSW) is a relatively new technique for making solid-state welds in aluminum alloys. Encouraging results have been obtained when FSW has been used on highstrength, aerospace, aluminum alloys that are typically difficult to weld [1]. Friction stir welding was invented at The Welding Institute (UK) in 1991 [2]. Since that time, it has been introduced into commercial practice in a number of * Corresponding author. Tel.: + 1-803-7777158; Fax: +1-8037770106.

applications. Although aluminum alloys may be joined using conventional, fusion welding techniques, certain aspects of friction stir welds make FSW extremely attractive for the joining of aerospace alloys. Of greatest importance is the fact that the process occurs in the solid state and, therefore, the formation of brittle solidification products is minimized and grain boundary liquation cracking does not occur. Heat input is, in general, significantly lower than in fusion welding processes [3]. One area of interest to structural analysts is the response of cracks in friction stir welded structure to various loading conditions. For example, if a flaw in a friction stir weld is subjected to mixed-mode loading, the relationship between the fracture resistance of the base material and the FSW joint is needed to properly design such joints for optimal performance. In addition, it may be possible to tailor the FSW process to optimize the fracture resistance of the joint rather than, for example, the ultimate strength. In this regard, there have been several recent studies regarding the fracture performance of the aluminum alloy base materials [4– 9] under mixed-mode loading conditions. Results from this previous work suggest that: (a) crack opening displacement (COD) at a fixed distance behind the crack tip is a viable parameter to predict both the direction and occurrence of stable crack growth in

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M.A. Sutton et al. / Materials Science and Engineering A323 (2002) 160–166

thin-sheet, homogeneous materials; and (b) the effect of crack orientation in a non-isotropic microstructure, such as that resulting from a rolling operation, can alter the critical COD by up to 25% for tensile (pure mode I) loading cases [10]. Recently, a series of mixed-mode I/II fracture tests were performed on a 2024-T3 FSW by the authors. The resulting crack paths load– crack extension data were quite different from those measured in the base material [9]. Examination of the fracture surfaces at low magnification strongly suggests that a complex, banded microstructure within the FSW is a major factor in determining weld fracture response. In this paper, a detailed study of the variations in microstructure within a 2024-T3 aluminum FSW joint is presented. Both fractured and as-welded FSW material were studied, with emphasis on: (a) grain size variations; (b) characteristics of the banded microstructure; and (c) micro-hardness variations within the FSW. Since the crack growth process appears to be a function of the microstructure, emphasis is placed on assessing the relationship of local microstructure to the observed direction of crack growth in the FSW region of the material.

Fig. 1. Crown-side view of the FSW process.

Fig. 2. Schematic diagram of a weld, with definitions for cross-sections and the axis system.

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2. Specimen and the FSW process In this work, a 1.22 m× 2.443 m× 7 mm thick sheet of 2024-T3 aluminum was cut into two square pieces. These were friction stir welded together using a tool with a 23.0 mm diameter shoulder and a 8.2 mm diameter pin. The measured process parameters were a rotational speed of 360 r.p.m. and a travel speed of 3.3 mm s − 1 perpendicular to the rolling direction of the sheet. Fig. 1 shows a crown-side view of a typical FSW used in this work. As shown in Fig. 1, the tool contacts the ‘crown side’ of a FSW during processing, while the ‘root side’ is in direct contact with a backing plate and opposite to the crown side. The ‘advancing and retreating sides’ of the weld correspond to locations where the maximum and minimum relative velocities between the rotating tool and the work-piece are observed.

3. Microstructure measurements To acquire high-contrast images of each FSW microstructure, different etching processes were used. These included Keller’s etch, Barker’s etch and HF etch. All photomicrographs are taken using either a light microscope or a scanning electron microscope (SEM), depending on the size of the features to be characterized. In addition, energy dispersive X-ray spectroscopy (EDX) is used to analyze the chemical composition of the material. Fig. 2 presents a schematic diagram of a typical FSW specimen, along with definitions for both the Cartesian axes and section planes used in this work. As shown in Fig. 2, the longitudinal (L) axis coincides with the direction of tool travel. The depth (Z) axis is in the specimen thickness direction and directed towards the weld crown. The transverse (T) axis is perpendicular to the direction of tool travel and is directed towards the advancing side of the weld; depending upon the direction of tool rotation, the L– T–Z coordinate will either be a right-handed (CW tool rotation) or left-handed (CCW tool rotation) system. The origin of the L–T–Z coordinate system is at mid-thickness at the weld centerline, where the longitudinal location of the origin within the specimen is arbitrary Each ‘vertical transverse cross-section’ is parallel to the T –Z plane and perpendicular to the longitudinal axis. A ‘vertical longitudinal cross-section’ is parallel to the L– Z plane and located at a pre-selected T=T0 location. Here, positive and negative values for T0 correspond to cross-sections on the advancing side and retreating side of the weld, respectively. A horizontal cross-section is parallel to the L–T plane and located at a depth position Z =Z0. Here, positive and negative values for Z0 correspond to cross-sections on the crown side and root side of the weld, respectively.

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3.1. Microstructural studies Figs. 3–5 present micrographs showing the evolution of 2024-T3 microstructure in the FSW region. Fig. 3 shows how the microstructure evolved within a vertical transverse section as a function of transverse position at the mid-thickness of the welded plate (i.e. Z=0). The central portion of the friction stir weld, or ‘nugget’ (locations 1 and 2 in Fig. 3), has a refined microstructure that is consistent in general character with previous observations [11–13]. At location 3, corresponding approximately to the edge of the welding tool’s pin, an abrupt transition from the highly refined, equiaxed, grains comprising the nugget to deformed base metal grains occurs. At a location corresponding to approximately the tool shoulder radius on the advancing side, location 4, a relatively coarse, recrystallized grain structure is observed. At 4.5 mm from the edge of the shoulder, the microstructure appears similar to the original, elongated grain structure present in the rolled, 2024-T351 base material. Fig. 4 portrays the change in microstructure with through-thickness position in a vertical longitudinal cross-section at T= 0. As one travels from the crown to Fig. 3. Microstructure as a function of transverse location in the vertical transverse cross-section at mid-thickness of the specimen (Z= 0).

Fig. 4. Microstructure as a function of through-thickness location in the vertical longitudinal section along the weld centerline (T= 0).

Fig. 5. Microstructure in the horizontal cross-sections at different depths near the shoulder region of the friction stir weld (T : 4 mm); HF etching.


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3.2. Banded microstructure

Fig. 6. SEM image of the banded microstructure in the horizontal cross-section, Z = −1.0 mm; HF etching. Fig. 7. High-magnification micrograph of the banded microstructure.

the root of the FSW, the grain size decreases, most likely due to the higher heat input near the crown that causes additional grain growth in this region. As the trailing edge of the shoulder passes over the welded material, a substantial amount of heat is generated causing grain coarsening in the crown relative to the root of the weld. In addition, some mechanical fibering, related to the complex flow accompanying the FSW process, can be observed in the grain structure shown in Fig. 4 [14]. Fig. 5 shows a notable feature of the FSW microstructure. For all horizontal cross-sections from − 2.5 mm5 Z5 2.5 mm, the microstructure exhibits banding that correlates with the primary material flow observed in friction stir welding. The banded structure is most pronounced near the mid-thickness of the welded plate [14]. Since our preliminary fracture studies suggested that fracture tends to occur along weak microstructural features, a detailed study of the banded structure was initiated.

Fig. 6 is a high-contrast, SEM view of the banded microstructure. Here, the banding is shown to result from alternating regions of high and low particle density. Further confirmation of this is shown in Fig. 7, where a high-magnification micrograph shows two zones, one with high particle concentration (zone B) and one with reduced density of particles (zone A). Of particular interest is the observation that the band spacing corresponds very closely to the welding tool advance per revolution. This finding indicates that the homogeneity of the weld microstructure may be modified by changing the welding parameters. For example, one expects that band spacing will decrease as the tool advance per revolution is reduced. A smaller band spacing should result in a more homogeneous structure within the weld; however, changes in the tool advance per revolution have ancillary effects on weld energy and hence local material properties within the weld. To determine the difference between particles and the surrounding FSW material, both microhardness tests and EDX analysis of the chemical composition were performed. Fig. 8 shows a high-magnification view of the region where EDX measurements were made, as well as the EDX data for four regions surrounding points in zone A and zone B, respectively. In Fig. 8, positions 1 and 3 correspond to the matrix material, and positions 2 and 4 correspond to particle positions. As shown in Fig. 8, the EDX results demonstrate that the particles contain higher concentrations of Cu, Fe, Mg and Mn than the surrounding material. The composition trends are consistent with identification of the particles as those typical of constituent particles in 2XXX series aluminum alloys [15]. To determine the effect of this variation on material properties, a series of microhardness tests were performed across the banded microstructure. Fig. 9 shows the results of these tests. The Knoop Hardness data indicates that the hardness is higher in zone B than that measured in zone A, indicating that the particles are acting as discontinuous reinforcement to the aluminum alloy matrix. 4. Relationship between fracture and banded microstructure As noted in Section 1, our study of the microstructure was motivated by a series of preliminary fracture experiments. Specifically, a series of mixed-mode I/II experiments were performed with through-thickness fatigue cracks located in the L–Z plane along the centerline of the FSW (T =0). Since the general trends were independent of mixed-mode loading, Fig. 10 presents a representative fracture path for a loading angle = 60°.

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As shown in Fig. 10, the crack grows almost vertically towards the advancing side of the FSW for a distance of 5–6 mm from the weld centerline to a position near the edge of the welding pin. As the banded microstructure turns toward the load direction, the fracture path continues along the general direction of the bands until the crack direction is aligned with the loading direction. At this point, several small cracks initiated ahead of the main crack tip and coalesced along a path that approximates the L direction of the weld. Once the crack has turned, it grows stably near the edge of the FSW along the advancing side. However, as shown in Fig. 10, separation continues to occur along bands in the microstructure at angles that are

almost perpendicular to the overall direction of crack growth. Therefore, the banded microstructure has a relatively strong effect on the fracture path, especially during the early stages of crack growth.

5. Discussion of results The presence of clearly defined bands in the 2024T351 microstructure appears to be due to the parameters chosen for the FSW joining process employed. The FSW process requires rotational extrusion of material around the front of the tool. This extrusion takes place in a very thin layer of material surrounding the tool,

Fig. 8. Chemical composition of particles and matrix material in the banded microstructure.


Fig. 9. Variations in micro-hardness across the banded microstructure .

Fig. 10. FSW crack growth path under mixed-mode I/II loading, = 60°.

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and large strain and strain rate gradients are predicted [16]. It is conjectured that the segregation of particles results from the entrainment of the particles in the region corresponding to high strain rate gradient; if the particles are large relative to the distance over which the strain rate changes appreciably, then the particles may tend to ‘roll’ toward the low strain rate region, resulting in clearly defined bands of high and low particle density. However, the reason for the segregation is not critical to understanding the results of the segregation. Specifically, the high density bands of particles in the microstructure are clearly locations for initiation of fracture within the material, with separation apparently occurring due to the presence of sufficiently high local tensile stresses in the region. This is consistent with effects typically observed in regions of high particle density, especially when the particles are hard relative to the matrix material. Thus, it is expected that, in a material with regions of high and low particle density (especially in the size range of the particles in question here), the fracture strain will be significantly reduced in the high particle density regions relative to the low density regions, while the deleterious effects of the particles will be exacerbated under conditions of high hydrostatic stress [15,17]. As noted previously, the fracture path shown in Fig. 10 is strongly influenced by the FSW microstructure. During the early stages of crack growth under far-field mixed-mode loading, fracture propagates along particle-rich bands. As the cracks grow along the bands and the direction of the band changes, it is conjectured that the reduction in tensile stress perpendicular to the bands eventually becomes inconsistent with crack growth in the bands under the applied loading. At that point, the fracture path transitions and the fracture occurs along a path that stays within the FSW and near the advancing side of the FSW. It is worth noting that the base metal material used in this study, 2024-T351, is a relatively tough alloy that is used in applications where damage tolerance is of the utmost importance (e.g. airplane fuselages). The FSW process has been shown to reduce the fracture toughness in 2024-T351. However, welds with modified processing have not yet been examined. For example, if the particle band spacing can always be correlated with the welding tool advance per revolution, then performing the weld at a higher ratio of rotation rate to welding speed should result in a more homogeneous structure (smaller, perhaps vanishing, band spacing). This parameter modification is expected to result in a hotter weld as well, possibly reducing the weld strength. A substantial amount of work remains to be done to optimize the FSW process for particular alloys and applications.

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6. Conclusions A series of controlled friction stir welds have been manufactured using 6.25 mm thick, 2024-T351 aluminum rolled sheet material. Results from metallurgical, hardness and quantitative EDX measurements clearly show that the friction stir welding process can create a segregated banded microstructure consisting of alternating hard particle-rich and hard particle-poor regions. In addition, initial results from mixed-mode I/II monotonic fracture experiments demonstrate that the observed banded microstructure affects the macroscopic fracture process in the welded material, with the crack path tending to follow regions of high particle density. Finally, it is important to note that we have shown conclusively the spacing of the bands is directly correlated with the welding tool advance per revolution. By demonstrating that the bands are a direct result of the welding process, it is clear that the opportunity exists to manipulate the FSW process parameters in order to modify the weld microstructure and improve a range of material properties, including fracture resistance.

Acknowledgements The authors gratefully acknowledge the financial support provided by Dr K.V. Jata at the Air Force Research Laboratory through Contract F33615-96-D-5835, Dr Julius Dasch at NASA HQ through grant NCC 5-174, and Dr Delcie Durham,

NSF/DMI program manager through grant DMI9978611. The technical assistance of Dr Jata during this work is also noted and greatly appreciated. References [1] A.P. Reynolds, W.D. Lockwood, T.U. Seidel, Mater. Sci. Forum 331 – 337 (2000) 1719 – 1724. [2] W.M. Thomas et al., International Patent Application Number PCT/GB92/02203 and GB Application Number 9125978.8, 1991. [3] C.J. Dawes, W.M. Thomas, Welding J. 75 (3) (1996) 41 –45. [4] M.A. Sutton, M.L. Boone, F. Ma, J.D. Helm, Eng. Fracture Mech. 66 (2000) 171 – 185. [5] M.A. Sutton, F. Ma, X. Deng, Int. J. Solids Struct. 37 (2000) 3591 – 3618. [6] F. Ma, X. Deng, M.A. Sutton, J.C. Newman, Jr., ASTM STP 1359 on Mixed Mode Crack Behavior (2000) 86 – 110. [7] F. Ma, X. Deng, M.A. Sutton, S. Fawaz, J. Mech. Phys. Solids (2001) in press. [8] M.A. Sutton, J.D. Helm, M.L. Boone, Int. J. Fracture 109 (2001) 285 – 301. [9] J.D. Helm, M.A. Sutton, M.L. Boone, ASTM STP 1323 on Nontraditional Methods of Sensing Stress, Strain and Damage in Materials and Structures (2001) 3 – 14. [10] B.E. Amstutz, M.A. Sutton, D.S. Dawicke, M.L. Boone, ASTM STP 1296 on Fatigue and Fracture (1997) 105 – 125. [11] K.V. Jata, S.L. Semiatin, Scr. Mater. 43 (2000) 743 –749. [12] C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A. Spurling, C.C. Bampton, Scri. Mater. 36 (1) (1997) 69 – 75. [13] L.E. Murr, G. Liu, J.C. McClure, J. Mater. Sci. Lett. 16 (1997) 1801 – 1803. [14] A.P. Reynolds, Sci. Technol. Welding Joining 5 (2000) 120 –124. [15] J.A. Walsh, K.V. Jata, E.A. Starke Jr, Acta Metall. 37 (11) (1989) 2861 – 2871. [16] X. Deng, S. Xu, Trans of NAMRI/SME 29 (2001) 631 –638. [17] M.J. Haynes, B.P. Somerday, C.L. Lach, R.P. Gangloff, ASTM STP 1297 on Elevated Temperature Effects on Fatigue and Fracture (1997) 165 – 190.