Microtexture Analysis of Friction Stir Welded Al 6061-T651 Plates

Transcription

1 Microtexture Analysis of Friction Stir Welded Al 6061-T651 Plates Suk Hoon Kang 1, a, Woong Ho Bang 1, b, Jae Hyung Cho 2, c Heung Nam Han 1, d, Kyu Hwan Oh 1, e Chang Gil Lee 3, f, Sung-Joon Kim 3, g 1 School of Materials Science and Engineering, Seoul National University, San 56-1 Shin-Lim 9 dong, Kwank-Ak gu, Seoul, South Korea Sibley School of Mechanical and Aerospace Engineering, 287 Grumman Hall, Cornell University, Ithaca, NY High Strength Materials Group, Korea Institute of Machinery & Materials, 66 Sangnam-dong, Changwon-city, Kyeongnam, a sangle77@snu.ac.kr, b usk2@snu.ac.kr, c jc464@cornell.edu, d hnhan@snu.ac.kr e kyuhwan@snu.ac.kr, f cglee@kmail.kimm.re.kr, g sjkim@kmail.kimm.re.kr Keywords : Friction Stir Welding, Al 6061-T651, Microtexture, EBSD Abstract. Microstructural characteristics of friction-stir-welded Al 6061-T651 with varying rotating and advancing speed were examined by the electron backscattering diffraction (EBSD) installed in field emission-scanning electron microscopy (FE-SEM). It was found that FSW produced an equiaxed fine-grained microstructure in weld zone and the grain size in weld zone decreased up to about 4~6 µm with decreasing rotating speed. The primary textures developed in weld zone were {100}<001>, {110}<001> and {111}<110>. In thermo-mechanical affected zone, the change in grain size was not significant, however, large number of low angle grain boundaries were observed, which seems to be concerned with the formation of subgrains due to the development of dislocation cells. Introduction Friction stir welding (FSW) is a solid-state joining process using frictional and adiabatic heat generated by a rotating and traversing cylindrical tool with a profiled pin along a square butt weld joint. The advantages of the solid-state FSW process also encompass better mechanical properties, low residual stress and deformation, weightsavings, and reduced occurrence of defects [1,2]. The FSW was first developed in 1991 by The Welding Institute (TWI), and ever since, the FSW studies have been mainly focused on the joining of Al alloy systems [3,4], which has the greatest demand in various industries over conventional welding processes. In this study, the microstructure of Al 6061-T651 alloy welded by FSW method was investigated. The main focus was to examine the texture evolution, grain size change and grain boundary characteristics with respect to the welding parameters of advancing and rotating speed. Experimental Procedures

2 Butt joint was made between 4-mm-thick Al 6061-T651 plates, which do not have the preferred orientation at as-received state, by using an automated FSW machine. The welding and rotating speed were varied from 200 to 500 mm/sec and from 1600 to 2000 rpm, respectively. Microstructure evolution was observed at the three positions of weld zone (WZ), thermomechanically affected zone (TMAZ) and base metal (BM) as shown in Fig. 1. Cross-section perpendicular to welding direction was analyzed by an EBSD system (Oxford, INCA) installed in high resolution FE-SEM (JEOL 6500F). Orientation data obtained from EBSD system were reprocessed by an in-house program called as REDS, which had been developed for the quantitative characterization of the texture distribution and the misorientation angle of grain boundary. Results and Discussion 1) Microstructure in TMAZ The typical cross-sectional views and microstructures of friction-stir-welded Al 6061-T651 alloy plate are given in Fig. 1. It is shown that the FSW process produces the reduction of grain size at WZ, whereas the change in grain diameter at TMAZ is not remarkable. Compared with the BM, TMAZ exhibits the high fraction of low angle grain boundaries, which is including the misorientation angle between neighboring grains less than 5 o. These low angle boundaries in TMAZ seem to be concerned with the plastic deformation without recrystallization Fraction [%] Fraction [%] Fraction [%] Misorientation [degree] Misorientation [degree] Misorientation [degree] Fig. 1. The macroscopic view, microstructure and grain boundary misorientation of the friction-stir-welded Al 6061-T651 at the locations of: (1) WZ (weld zone) - viscous material flow occurs; (2) TMAZ (thermo-mechanically affected zone) - region affected from the thermo-mechanical reaction in weld zone; (3) BM (base metal) - substrate with as-received condition.

3 During FSW, the severe viscous material flow in WZ gives rise to the plastic deformation at TMAZ adjacent to WZ. As a consequence the microstructure expected in TMAZ is the dislocation cell structure [5], and these cells metastably remain as the subgrains inside grains when there is no the succeeding recrystallization. Then, the low angle boundaries are strongly developed in TMAZ as shown in Fig. 1. The distributions of low angle boundary less than 5 o obtained under the various welding conditions of welding and rotating speed are presented in Fig. 2. These distributions of low angle boundary enable us to give shape to TMAZ. The welding speed seems to be a decisive factor in determining the shape of TMAZ. While the shape change of TMAZ with rotating speed is not noticeable, the angle of inclination between WZ and TMAZ (α) approaches 90 o as welding speed increases from 200mm/min to 500mm/min. This variation in TMAZ morphology is associated with the temperature field in welded material due to the welding speed. As the welding speed increases thermal distribution in WZ becomes concentrated and faster transient. As a consequence the recrystallization in WZ is confined in narrower range across the width direction as the expectation of upright TMAZ configuration [6]. Our future work is a quantitative analysis on the relationship between the recrystallization in WZ, the TMAZ configuration, and the temperature distribution in welded material under the wide range of welding condition. Fig. 2. TMAZ shapes recognized by the distribution of low angle grain boundary less than 5 o. ( α is the angle of inclination between WZ and TMAZ and the boundary between TMAZ and BM can be distinguished from the contrast difference in black and white. ) 2) Texture and Grain Size in WZ Microtexture is measured by EBSD and (111) pole figures are presented for texture analysis in Figs. 3 and 4. The various texture components of {100}<001>, {100}<011>, {110}<001>, {111}<112> and {111}<110> are observed in WZ. Fig. 3. Two major textures in WZ, [(111) polefigures]

4 Especially, the textures in WZ consist of two major orientations, {100}<001> and {111}<110>. Both of them are originated from the shear deformation as observed often in the shear deformation of FCC polycrystalline [7]. If upper center texture in Fig.4 is rotated 75 o around TD, it would become middle center texture in Fig.4 [8]. It indicates that these characteristics of the textures are related to the different deformation modes developed during FSW. Major textures given in Fig. 3 are observed at several locations in WZ as shown in Fig. 4. It can be understood from Fig. 4 that the combination of {100}<001> and {111}<110> are responsible for texture distribution in WZ by the rotation and superposition. During FSW of Al 6061-T651, the plastic deformation and recrystallization takes place simultaneously in WZ by the shearing and pressuring of tool shoulder and probe. With these thermo-mechanical conditions, (1) {100}//ND texture becomes dominant at upper region; (2) {111}//ND texture predominant at middle range; and (3) {100}//ND and {111}//ND dominant at lower part, as shown in Fig. 4. Fig. 4. Texture components in overall WZ. Fig. 5. (111) polefigures at WZ centerline measured under the various welding conditions of welding and rotating speed.

5 The tool shoulder may highly influence the texture evolution at upside, whereas the shear texture at middle area is maybe concerned with the shear of rotating probe. It can be observed that the texture at lower part is similar to the mixture of ones at the upper and middle region. The texture evolution at lower region may be associated with the two different loading manners from the probe side and bottom. The shearing motion of probe side generates shearing texture as in middle area while the deformation by tool bottom is analogous to that from tool shoulder. Texture developments along the joint centerline are presented for various welding conditions in Fig. 5. With the increasing probe rotation rate from 1600 to 2000 rpm, {100}//ND texture components become spreading in the middle rigion. However, the increase of welding speed from 200mm/min to 500mm/min leads to the expansion of the area with strong {111}//ND component. The texture evolution in WZ is mainly influenced by the tool shoulder as the rotating speed increases, whereas the shear texture by the rotating probe becomes prevailing as the welding speed increases. The grain size distributions in WZ are given in Fig. 6. With exceptions near the free surface, the grain size increases as the measured positions are closer to upper part. The larger grain size at upper region is associated with higher thermal energy dissipated from the mechanical energy originated from the both of tool shoulder and probe. The decrease in grain size at free surface appears to be resulted from rapid cooling rate by the convective heat transfer with air [9,10]. The increase in tool rotation rate causes the increases in grain size by about 2~3µm, whereas the effect of welding speed is negligible. This result suggests that tool rotation rate is a decisive factor in determining the energy input rate from tool shoulder and probe to substrates. Fig. 6. Grain size distribution along the WZ centerline at the various welding conditions of welding and rotating speed Conclusion We investigated the microstructure of friction-stir-welded Al 6061-T651 joint. The conclusions we have drawn are as follows. 1. The special characteristics of TMAZ were the evolution of low-angle grain boundaries that suggested subgrain formation by the development of dislocation cell structure. However, from the similarity in grain size and morphology at TMAZ to those at base metal region, it appeared that recrystallization process had not occurred significantly. 2. Textures in WZ was made of two basic components of {100}<001>, {111}<110>. The rotation

6 and superposition of these textures generated complicated textures observed in WZ. 3. Texture in the middle of joint was typical of shearing texture {111}//ND while the texture of upper side was primarily composed of {100}//ND. The texture in lower part was {100}//ND and {111}//ND. By the change in welding condition, the predominant texture distribution along the thickness direction varied. Higher probe rotation rate made {100}//ND components, while {111}//ND texture became prominent at faster welding speed. 4. Fine-grained microstructure was developed in WZ by FSW. The grain size was approximately 4~9µm, and it decreased by 2~3µm by increase in probe rotation rate. References [1] C.J. Dawes, An Introduction to Friction Stir Welding and Its Development, Welding & Me&l Fab., January, 1995, p. 12. [2] G. Liu, L. E. Murr, C-S. Niou, J. C. McClure and F. R. Vega, Microstructural Aspects of the Friction Stir Welding of 6061-T6 Aluminum, Scripta Materialia, Volume 37, Issue 3, 1 August 1997, Pages [3] J. A. Esparza, W.C. Davis, E. A. Trillo and L. E. Murr, J. Mater. Sci. Lett. 21, 917 (2002) [4] S.G..Lim, S.S. Kim, C.G. Lee and S.J. Kim, Tensile Behavior of Friction Stir Welded Al T651, Metallurgical and Materials Transactions A, Vol. 35A, (2004), 2829 [5] R.W.Fonda and J.F.Bingert Texture and Microstructure Development in the Heat Affected Zone of Friction Stir Welds, Friction Stir Welding and Processing II, TMS (2003) [6] B. Yang, J. Yan, M. A. Sutton and A. P. Reynolds, Banded Microstructure in AA2024-T351 and AA2524-T351 Aluminum Friction Stir Welds, Part I. Metallurgical Studies, Materials Science and Engineering, Vol. A364, 2004, Pages [7] G.R. Canova, U.F. Kocks and J.J. Jonas, Acta Metall., 32 (1984), 211 [8] Yutaka S. Sato et al Microtexture in the Friction-Stir Weld of an Aluminum Alloy, Metallurgical and Materials Transactions A, Vol. 32A, (2001) [9] Humphreys FJ, Hatherly M., Recrystallization and Related Annealing Phenomena, Oxford: Elsevier Science; p & [10] R. W. Fonda, J. F. Bingert and K. J. Colligan, Development of Grain Structure during Friction Stir Welding, Scripta Materialia, Volume 51, Issue 3, August 2004, Pages