A A B B C (left) and blue (right) grains. Here, it should be noted that the color of each grain is affected by its crystallographic orientation due to the use of an etching technique and optical microscope with polarized light as described in the metallographic, chemical, and EPMA examination section. It is of interest that such a texture was observed in all Alloy 1050A welds as well as in the coarsegrained Alloy 6082 welds (Fig. 4). The welds made from Alloy 5083, however, did not produce any crystallographic texture at all. The EBSD analysis was performed to determine the exact orientation angle of the yellow and blue grains. The results from this analysis are shown in Fig. 6, which shows etched micrographs from regions A and B (Fig. 5A) at a higher magnification (Fig. 6A and D). Furthermore, Fig. 6B, C, E, and F contain the EBSD results from both regions. Accordingly, the optical images (etched micrographs) show for each region many neighboring grains that have a similar color and hence a similar atomic lattice orientation; only a few grains are oriented completely different. In the EBSD images (Fig. 6B and E), each color reveals how each grain is oriented; the exact orientation can be understood with the color key and FCC aluminum unit cell at the bottom of Fig. 6. The three arrows are surface normals that are perpendicular to the cross-sectional area of the micrographs in Fig. 6A, B, D, and E. Furthermore, the color and position of each arrow in the FCC unit cell indicate how each arrow is located in the FCC atomic lattice of the grains with the corresponding color in Fig. 6B and E. Accordingly, a virtual FCC unit cell that has the same atomic lattice orientation as the red grains in Fig. 6B and E, for instance, stands with one of its cube faces on the crosssectional areas in Fig. 6B and E (because the red <100> arrow is located at the cube edge); the FCC unit cell that represents green grains stands on one of its cube edges, and the unit cell that represents blue grains stands on one of its body diagonals, respectively. The crystallographic orientation of all grains from region A and B is summarized in Fig. 6C and F. These two pole figures reveal the distribution of the <100> direction of all detected lattice orientations as a stereographic projection. Looking at both pole figures (Fig. 6C and F), one can see that 1) they are approximately mirror images of each other (mirror axis z) and 2) there is a frequency maximum close to the point of origin. This maximum represents the predominant lattice orientation (texture). If one compares these results with both typical weld solidification behavior (recall Fig. 1) and the position of regions A and B in the weld metal (recall Fig. 5), the following becomes clear: The crystallographic texture in regions A and B is equal to the local growth direction during solidification of each region. The observed texture is likely related to competitive growth during solidification. Grains with favorable lattice orientation (yellow in Fig. 6A and blue in Fig. 6D) grow with minimum undercooling because their easy growth direction <100> in aluminum FCC crystals (Ref. 53), is similar to the direction of the thermal gradient and hence to the maximum heat extraction (Ref. 54). In contrast, grains with unfavorable lattice orientation grow at higher undercooling and become overgrown by more favorable oriented grains (Fig. 6A and D). Competitive growth is known to form in aluminum fusion welds (Refs. 38, 54) although one would generally expect completely random grain orientation for equiaxed grains that form ahead of the solid-liquid interface. One reason why the texture was observed not only in coarse, columnar, but also in fine, equiaxed grain structure could be repeated epitaxial nucleation. This means that new grains nucleate epitaxially on existing grains (that have recently formed in the fusion zone) resulting in many grains with equal lattice orientations. This nucleation mechanism competes with equiaxed nucleation on particles present such as TiB2 or Al3Ti. In this regard, it is of note that epitaxial nucleation needs much less undercooling than equiaxed nucleation (Ref. 54). Since undercooling is provided particularly by alloying elements, one can conclude the following: For low-alloy contents (Alloy 1050A), the ability to activate nucleating particles was low and thus epitaxial nucleation was dominating. At higher alloy con- WELDING JOURNAL 57-s WELDING RESEARCH Table 3 — Grain Morphology in GTA Weld Metal Dependent upon Welding Speed and Weld Metal Ti Content (C: Predominantly Columnar, E: Predominantly Equiaxed, C/E: Mixture of Both), Determined in Top-Sectional Micrographs Alloy 1050A Alloy 6082 Alloy 5083 Welding Speed ν Ti Content in wt-% Ti Content in wt-% Ti Content in wt-% in mm s-1 0.01 0.02 0.06 0.02 0.04 0.06 0.03 0.05 0.07 2.0 C C E C E E C E E 4.2 C C E C E E C/E E E 6.0 C C E C E E E E E 8.0 C C/E E C/E E E E — — 10.0 C C/E — E E E E — — 11.5 C — — E — — — — — Fig. 7 — Ti distribution in GTA weld metal dependent upon mean Ti content (WDS images). Alloy 6082, plate thickness 3 mm, welding speed 4.2 mm s–1, mean heat input 467 J mm–1. Fig. 8 — GTA weld metal with mean contents of 0.137 wt-% Ti and 0.045 wt-% B revealing the following: A — Ti (black) and B (colored) distribution; B — TiB2 particle covered by a thin, white Al3Ti layer; and C — TiB2 particle adjacent to an intermetallic phase rich in Si and Fe (A is the WDS image while B and C are TEM images). Alloy 6082, plate thickness 3 mm, welding speed 4.2 mm s–1, mean heat input 467 J mm–1. C
Welding Journal | February 2014
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