Texture Characterization of Autogenous Nd:YAG Laser Welds in AA5182-O and AA6111-T4 Aluminum Alloys

Transcription

1 Texture Characterization of Autogenous Nd:YAG Laser Welds in AA5182-O and AA6111-T4 Aluminum Alloys L.G. Hector, Jr. and Yen-Lung Chen Materials and Processes Lab GM Research Center Mound Road Warren, MI S. Agarwal and C.L. Briant Division of Engineering Brown University Providence, RI The reduced weight and high strength-to-weight ratio of aluminum alloys makes them desirable alternatives for automotive structural materials. However, some significant challenges remain in the large scale implementation of aluminum alloys in the automotive industry. One of these pertains to the selection of joining methodologies and how these methodologies impact the mechanical properties of aluminum joints. The possibility of using laser welding is very attractive because of its speed, and a number of investigations have examined its application to aluminum alloys [1-8]. However, there are some concerns with this process. For example, aluminum alloy weld strength may decrease, with respect to the parent metal. This effect is especially pronounced with heat-treatable aluminum alloys due to the loss of the T-temper treatment. Attempts to improve aluminum weld strength may therefore involve alteration of welding process parameters and post-weld heat treatment. Most published work on the weld microstructure of aluminum alloys has focused on linking the weld microstructure as observed with optical microscopy or SEM to the measured mechanical properties of the weld and to the compositional changes in the weld zone. Given that texture often causes anisotropic mechanical properties, its presence in the weld zones of aluminum alloys could be quite significant. An important technique for texture characterization in metals is electron backscattered diffraction (EBSD), and reference 9 describes both the technique and a number of its applications. While the EBSD technique has been widely used to investigate texture in aluminum alloys, it has only recently been used to investigate texture in aluminum welds. To date, these studies have focused on friction stir welds [10,11], and no studies appear to have examined texture development in laser welds in aluminum alloys. The purpose of the 1

2 present work is to characterize the textures at selected depths of autogenous Nd:YAG laser welds in AA5182-O and AA6111-T4 alloys. The materials examined in this work were AA5182-O and AA6111-T4 aluminum alloys. AA5182-O is a solid solution strengthened alloy that is commonly used for automotive structural components. AA6111-T4 is a solution treated and naturally aged alloy that is usually used for automotive body panels due to its relatively high strength and good dent resistance. Typical chemical compositions of these alloys are listed in Table I and typical tensile properties are listed in Table II. Autogenous, bead-on-plate welds were produced with a flash lamp-pumped, 3 kw CW Nd:YAG laser on 2 mm thick sheets with mill finishes; the sheets were not exposed to any cleaning processes prior to the welding tests and no filler materials were used. A 20 /min He assist gas was used and the focal length of the delivery optic was 200 mm. The weld speed was held fixed at 6 m/min for all tests. These conditions produced an approximately 2 mm wide fusion zone on the top surface of each sheet with partial penetration. Using the thermophysical data in Table III, we estimated that the cooling 6 rates after welding were on the order of 10 K s for both alloys. A schematic of the samples used for EBSD analysis is shown in Figure 1. The weld runs down the center of the 2 mm thick sample. Three 0.5-mm thick coupons from each sample were sliced parallel to the welding direction and the top and bottom surfaces of the sample by using electro-discharge machining so that we could examine internal surfaces within the weld zone. We numbered the surfaces as shown in Table IV, with surface 1 being the top surface of the weld (i.e. the surface directly exposed to the laser beam), surfaces 3 and 4 associated with the middle slice, and surfaces 5 and 6 associated with the bottom slice (i.e. the slice on the side opposite that exposed to the laser beam). In this report we will use surfaces 1 and 3 to describe the texture in the welds, as they can be used to make all necessary points. For EBSD characterization, each surface was first mechanically polished with 0.05 µm polishing alumina and then electropolished with a solution containing 250 ml ethanol and 50 ml perchloric acid at 15 V, 0 o C, for 5 minutes. Texture maps were obtained by EBSD on a selected region from each of these sectioned surfaces. In collecting EBSD data, a coordinate system was chosen to describe the sample geometry. In all of our work, the samples were arranged so that the x-direction was parallel to the welding direction, the y-direction was in the plane of the sample but perpendicular to the welding direction, and the z-direction was normal to the sample surface. On each surface, the EBSD runs covered a 1.5 mm length parallel to the x-direction and the width of the weld in the y-direction. Channel 5 software, developed by HKL, Inc. was used to analyze the 2

3 data. Figures 2 and 3 show optical micrographs of surface 2 of the AA5182-O and AA6111-T4 specimens, respectively. As can be seen in these micrographs, the microstructure near the fusion boundary consisted mainly of a fine cellular-dendritic microstructure. As shown below, more equiaxed grains were found near the weld centerline. For the AA6111-T4 weld, grain boundary liquation (i.e., localized melting along the grain boundaries) was found next to the fusion zone, and this liquation led to fine cracks that can be seen in Figure 3. Figure 4 shows the results of microhardness tests across the weld zones. Figure 4a shows that there is no significant variation in hardness across the AA5182-O weld zone, while Figure 4b shows that there was a hardness drop in the AA6111-T4 weld zone compared with the base material. This difference in hardness was not explored in this research. The purpose of the EBSD experiments was to determine whether or not any significant texture resulted from the rapid solidification of the fusion zone material. Figure 5 shows EBSD maps of surfaces 1 and 3 for both alloys AA5182-O and AA6111-T4. The color code for each of these maps is the same. Those grains that are colored blue have a <100> direction within 20 o of being parallel to the y-axis, which is the vertical axis of the figures. Those grains colored red have a <110> direction within 20 o of the y-axis, and those grains colored yellow have a <111> direction within 20 o of the y-axis. The color bars above Figure 5d show how the intensity of the color varies from the deep colors for grains that have these directions almost perfectly parallel to the y-axis, to the pale colors for grains that have these directions almost 20 o away from the axis. The primary purpose of using these color-coded maps was to determine visually if there were areas of the samples where a significant number of grains had similar orientations. A number of important observations can be made from from the results shown in Figure 5. The weld appears to consist of two distinct zones. Down the centerline of the weld the grains are equiaxed, but on either side there are columnar grains that appear to be a transition between these equiaxed grains and the parent metal. These columnar grains grew from the parent metal into the weld pool during cooling and are the cellular dendritic grains shown in Figures 2 and 3. The size of the grains in the weld of alloy AA6111-T4 appeared to be larger than those in AA5182-O. These maps also indicate that in all four surfaces that we examined, there is a tendency for the long axis of the columnar grains to be near <100> (i.e. a cube texture). However, the coloring of grains depends on the orientation of the grains relative to some defined axis, and the texture could exist in some other direction relative to this axis. Thus, to gain more information about the texture, we now examine the pole figures taken from different regions on these 3

4 maps. Figure 6 shows the (111), (110), and (100) pole figures for a subset of columnar grains from surface 1 of alloy AA5182-O. These were taken from the lower part of the region shown in Figure 5a near the right hand side. The x and y directions are indicated on the (111) pole figure and z is normal to the plane of the figure. One can see that there is evidence of a strong texture in the (100) pole figure. By comparing the micrograph and the location of the high intensity region in the pole figure, one can observe that this intensity must arise from the fact that the long axis of the columnar grains is near <100>. The other two {100} poles which must lie 90 o to this pole appear to be randomly oriented, as indicated by the band of intensity across an equator of the pole figure. The bands of intensity in the (110) and (111) pole figures are those that must result from this orientation of the {100} poles. We note that in this pole figure the red color indicates an intensity of approximately fifteen times above the random background. A similar result was obtained for the columnar grains on the opposite side of the weld centerline for this sample. Figure 7 shows the same type of data for the equiaxed region from surface 1 that is near the weld centerline. In this region there are no particular regions of strong texture and the red regions only indicate an intensity five times above the random background. Figure 8 shows the data from a subset of columnar grains from surface 1 in the AA6111-T4 material. This subset was taken from the upper part of Figure 5c, and from an area where almost all of the grains were colored blue in the map. We note several important differences between these results and those obtained for the AA5182-O material. We see that all three {100} poles are strongly aligned. One of them is parallel to the growth direction of the columnar grains, one is parallel to the weld direction and one is normal to the surface of the sample. This strong alignment of all three poles then leads to the texture observed in the (111) and (110) pole figures. We note that this strong alignment was observed in both sets of columnar grains on either side of the weld centerline, although the texture was weaker in the grains in the lower part of Figure 5c. The red color in the contour plots in Figure 8 now corresponds to a value that is 25 times the random background. For the lower part of Figure 5c, the corresponding maximum intensity was approximately 14 times the random background. One can also observe in Figure 5c that there are regions where the grains could be described as columnar but were not colored blue in this figure. This change could result from the fact that these grains had their long axis far enough from the y-axis that even if they were closely parallel to <100> they would not take on this color. Figure 9 shows the pole figures for these grains. Although the texture is not as strong as that 4

5 shown in Figure 8, there appears to be an alignment of the <100> direction with the long axis of the grains. Surface 3 of alloy AA6111-T4, even deeper into the weld material, showed similar results. From these results we can conclude that a strong cube texture forms in laser welded AA5182-O and AA6111-T4 alloys and that the strength of the texture depends on the particular alloy and the depth through the weld zone. In particular, we note that the columnar grains that form on either side of the weld center lines and appear to grow out from the parent metal into the liquid are highly textured, with a <001> direction parallel to the growth direction. Given that mechanical properties are strongly dependent on texture one would expect this texture could have a strong effect on the local mechanical properties of the welds. ACKNOWLEDGMENTS The authors would like to thank R. Mishra for his critical review of the manuscript, J. Cross for his assistance in metallographic sample preparation and microhardness measurements. It is a pleasure to acknowledge stimulating discussions with P. Krajewski and M. Verbrugge. The work at Brown University was supported by GM through the GM Collaborative Research Lab at Brown University. 5

6 REFERENCES 1. D.W. Moon and E.A. Metzbower, Welding Journal, 1983, vol. 62, pp. 53-s-58-s. 2. R. Irving, Welding Journal, 1991, vol. 71 (9), pp S. Venkat, C.E. Albright, S, Ramasamy, and J.P. Hurley, Welding Journal, 1997, vol. 76, pp. 275-s-282-s. 4. Wei Tong and Xiquan Jiang, Research Reports in Mechanics of Solids and Materials Science, Yale University, March M.J. Cieslak and P.W. Fuerschbach, Metall. Trans. B, 1988, vol. 19B, pp S. Ramasamy, Nd:YAG Laser Beam Welding of 6111-T4 and 5754-O Aluminum Alloys for Automotive Applications, Ph.D. Thesis, Ohio State University, Columbus Ohio, P.A.Friedman and G.T. Kridli, Journal of Materials Engineering and Performance, 2000, vol. 9, pp D. Fabregue and A. Deschamps, Materials Science Forum, 2002, vols pp Electron Backscatter Diffraction in Materials Science, A. J. Schwartz, M.Kumar, B.L. Adams, eds., Kluwer Academic, New York, A.F. Norman, I Brough, P.B. Prangnell, Proceedings of the 7th International Conference on Aluminium Alloys (ICAA-7),Trans Tech Publications, Virginia, USA, pp (2000) W.Y. Chien, J.Pan and P.A. Friedman, Failure of Laser Welds in Aluminum Sheets, SAE Technical Paper Series, J.O. Milewski, G.K. Lewis, and J.E. Witting, Welding Journal, 1993, vol. 73, pp 314- s-346-s. 14. Aluminum and Aluminum Alloys, ed. J.R. Davis, ASM International, Materials Park, OH, (1993) M.G. Strout and U.F. Kocks, in Texture and Anisotropy, U.F. Kocks, C.N. Tome, and H.-R. Wenk, eds., Cambridge University Press, Cambridge (1998), pp W. Tong, L.G. Hector, Jr., H. Weiland, and L. Wieserman, 1997 Scripta Mater., Vol. 36, No. 11, pp

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8 Table I Typical Chemical Compositions (wt.%) of Alloys Used in This Study Data Taken From Ref. 11 Alloy Fe Si Cu Mn Mg max max 0.20 max 0.15 max Table II Typical Mechanical Properties of Materials Used in This Study Data Taken From Ref. 11 Alloy Yield Strength Ultimate Strength Elongation (%) (MPa) (MPa) 6111-T O Table III Thermophysical Properties of Aluminum Data Taken From Ref. 12 Alloy AA5182-O AA6111-T4 Liquidus Temperature (C) o Coherency Temperature (C) o Solidus Temperature (C) o Thermal Conductivity (W/mm K) Volumetric Heat Capacity (W s/mm 3 K)

9 Table IV Surface Identification on the Sliced Weld Samples Slice/Surface Number Top/1 and 2 Middle/3 and 4 Bottom/5 and 6 Identification Top surface (1) and surface next to cut (2) Both surfaces next to cuts; surface 3 opposite surface 2 Surface next to cut (5) and bottom surface (6) 9

10 Figure 1 Dimensions (in mm) of the sample used for EBSD analysis. The shaded region down the middle of the sample indicates the weld. In the EBSD analysis we take the x-axis as parallel to the welding direction, the y-axis as perpendicular to the welding direction but in the plane of the sample, and the z-axis as normal to the sample surface. 10

11 Fusion Zone a b Figure 2 Micrographs showing the structure on surface 2 of the 5182-O weld. Note the fine cellular-dendritic microstructure in the fusion zone next to fusion boundary. 11

12 Fusion Zone a b Figure 3 Micrographs showing the structure on surface 2 of the 6111-T4 weld. Fig. 3a shows the grain boundary liquation next to the fusion boundary. Fig. 3b shows the fine cellular-dendritic microstructure in the fusion zone. 12

13 Microhardness (HV) Fusion Zone a Normalized Position (mm) Microhardness (HV) Fusion Zone b Normalized Position (mm) Figure 4 Microhardness profile across fusion zone on surface 2 of the 5182-O weld (a) and the corresponding profile on surface 2 of the 6111-T4 weld (b). 13

14 (a) y x (b) Figure 5 EBSD maps of the welds. All maps are taken so that the colors indicate orientations in the y-direction. Different maps were stitched together to provide the entire area. (a) Surface 1, AA5182-O, (b) Surface 3, AA5182-O, (c) Surface 1, AA6111-T4, (d) Surface 3, AA6111-T4. The color bars with Figure 5(d) show the degree of alignment. 14

15 (c) y x (d) Figure 5 EBSD maps of the welds. All maps are taken so that the colors indicate orientations in the y-direction. Different maps were stitched together to provide the entire area. (a) Surface 1, AA5182-O, (b) Surface 3, AA5182-O, (c) Surface 1, AA6111-T4, (d) Surface 3, AA6111-T4. The color bars with Figure 5(d) show the degree of alignment. 15

16 Figure 6 The pole figures corresponding to the subset of grains shown in the inset. These were taken from AA5182-O, surface 1. Figure 7 The pole figures for the subset of grains shown in the inset. These were taken from the equiaxed grains in the center line of the weld zone in AA5182-O, surface 1. 16

17 Figure 8 - The pole figures for the subset of grains shown in the inset. These were taken from the columnar grains in the upper part of Figure 5c for AA6111-T4, surface 1. Figure 9 The pole figures for the subset of grains shown in the inset. These were taken from the columnar grains in the lower part of Figure 5c for AA6111-T4, surface 1. 17