April AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 1 9 1 6 Figure 2 shows a cross section through a developmental AA5754 weld containing an oxide line and voids. Figure 2(a), using fully cross-polarized light and the lambda plate inserted, provides grain flow and grain structure/ size information, and clearly delineates the oxide stir line. Reducing the level of polarization below fully cross polarized makes the oxide line stand out, and it is clearly observed in the microscope (Fig. 2b). This technique enables col- lecting a significant amount of infor- mation from one sample, which can be used to make process modifications and quickly optimize the process to eliminate voids and oxide lines. The etching technique is also use- ful during FSW process development to characterize the failure of samples subjected to mechan- ical testing. Mechani- cal testing was per- formed using a limit- ing dome height (LDH) tester. When a sample with dissimilar thick- ness blanks was test- ed, an aluminum shim was used in the LDH tester to prevent un- even loading. A fluoro- polymer film was used for lubricity between the punch and the wel- ded blank in all LDH testing. A micrograph of a friction stir weld in AA5754 that has undergone LDH me- chanical testing is shown in Fig. 3. The test was conducted with the bottom (root) of the weld up, which is similar to the intended forming direction on this part, to determine if voids or the oxide line influences the location of the fracture. The image was taken with polarization and color tint reduced to highlight the oxide line. While voids are present near the weld root at the peak of the dome, the oxide stir line is the lo- cation where fracture initiated. Figure 4 shows a friction stir butt weld made using two pieces of AA6111 of the same thickness. Excessive dry film lubricant was applied to the sur- faces of the Al alloy sheets to determine whether it would reduce friction-gen- erated heating or have any other neg- ative influences on the weld. The oxide line at the interface of the two alumi- num sheets was not broken up well, but a substantial amount of the lubri- cant was incorporated on the advanc- ing side (right side) of the FSW stir zone (Fig. 4a). Figure 4(b) shows the lubricant in the stir zone at higher magnifica- tion. The etching technique is useful in Fig. 2 — Optical micrographs of an etched friction stir butt-weld joint of AA5754 of dissimilar gauges: (a) image collected with cross-polarized light and lambda plate shows grain flow and grain structure/size information, and clearly delineates the oxide stir line, magnification 25x; and (b) same region with polarization moved slightly away from the fully cross-polarized condition and lambda filter slightly reduced clearly shows oxide line. Fig. 3 — Micrograph of a friction stir welded AA5754 sample containing voids and an oxide line subjected to mechanical testing using the limiting dome height (LDH) method with root side up (away from the ball). Etching shows that the oxide line was the origin of the fracture, magnification 25x. Fig. 4 — Micrographs of a sample of friction stir welded AA6111 sheets of same thickness made with excessive dry film lubricant on their surfaces: (a) oxide line present at the center of the joint and lubricant incorporated into the joint; and (b) lubricant in the stir zone shown at higher magnification, 100x. 1 mm 1 mm a b 1 mm 2 mm