16 ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2023 has been the most problematic of all alloys during laboratory-based classes. The new etching procedure provides not only an improvement over other etches, but one that is easy to use and train new users for immediate success. Aluminum alloys in the 7000 series are typically etched effectively using Keller’s reagent to reveal grain structure. The Papageorge two-step etch also produced effective grain contrast in AA7475-T61 as shown in Fig. 1d. While there is no advantage of this etchant as compared to Keller’s reagent etching, this information was included in the study as having the ability to use one etching procedure across all alloys is desirable for some laboratories (including a teaching laboratory for undergraduate students). Another application where the Papageorge two-step etching procedure provides advantages for metallographers is for examining joining of dissimilar alloys. Figure 2 presents The etching process begins by lightly swabbing the polished surface of the sample in a circular fashion using the modified Keller’s reagent. The application time for the first etchant varies by alloy series, and the best practices based on results obtained here are listed in Table 1. After the first etchant is rinsed away with water, followed by ethanol, then the sample is submerged in the modified Weck’s reagent, while slowly rotating in a circular motion, for the times indicated in Table 1. The samples were rinsed in flowing water, rinsed with ethanol, dried using warm air, and examined using bright-field optical microscopy. Optical micrographs were collected at magnifications in the range of 100 – 500x. All micrographs were collected in bright-field (BF) mode. Resistance spot weld (RSW) samples between a 6000 and 7000 series alloy were also prepared and etched in this work. Those images were collected using an Olympus DSX510 optical micro- scope with a linear motion x-y stage. RESULTS AND DISCUSSION The etching procedure, which the authors refer to as the Papageorge two-step etch, was found to be effective on multiple aluminum alloys. Table 1 shows the times that were found to be optimal for creating grain contrast without creating pitting on the surface. Figure 1 shows the results from AA2024-T3, AA5754-0, AA6061-T6, and AA7475-T61 in the rolling direction at magnifications of 500x. The images in Fig. 1 were collected in BF mode with no polarizer, quarter wave plate, or lambda plate required. The AA2024 alloy was the only alloy that exhibited a color variation after etching. The AA2024 alloy has at least 4 wt% copper in the alloy that is distributed in the matrix and present in precipitates. The copper content is an order of magnitude (or more) higher than any other alloy investigated in this work. The Weck’s stain etch appears to interact with the copper and form films on the surface of the grains that create the apparent color in this alloy. While the goal of this process is not to create a color etchant, the color helps to delineate the grain boundaries in this alloy and produces visually interesting micrographs of the grains. Additional work is required to understand this effect in more detail and that work is underway. Grain structure was also revealed in AA5754-O as shown in Fig. 1b. The anneal on this sample appears to be incomplete. Aluminum alloy 6061 is regarded as a difficult alloy to etch for grain contrast. While there are suggested etchants in ASM Handbook, Volume 9[2], none are particularly consistent or easy for novice metallographers to use to reveal the grain structure. The Papageorge two-step produced an excellent grain etch on AA6061-T6 shown in Fig. 1c. The etching process creates a high level of contrast at the grains and did not create significant pitting on the polished cross-section. The ability to consistently create high contrast etching at the grain boundaries of AA6061 Fig. 1 — Bright-field optical micrographs of a nominally 3 mm thick sheet of (a) AA2024-T3, (b) AA5754-O, (c) AA6061-T6, and (d) AA5754-T61 in the rolling direction. Micrographs were collected at 500x original magnification. (d) (a) (b) (c)
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