15 ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2023 In the mid-6th century BCE, Plato famously stated that “our need will be the real creator,” which has been loosely translated over time to the more well-known English proverb: necessity is the mother of invention[1]. The development of this etching procedure is an example of this axiom. During classes where the authors train large groups of students (up to twenty per laboratory session) in the art of metallographic sample preparation of aluminum alloys, etching the alloys to reveal grain structures has consistently been a point of failure. Multiple aluminum alloys including 2000, 5000, 6000, and 7000 series are used during processing, characterization, and corrosion laboratory sessions. The diversity of alloys required multiple etchants and techniques that were difficult for inexperienced students to master in the limited time available in the laboratory class period. The goal of this work was to develop a single etch that reveals grain boundaries in multiple aluminum alloy series. Three important criteria were identified for a new etchant. First, the technique must be easy to learn for inexperienced metallographers. Second, the technique must be effective at distinguishing grain boundaries in several alloys of aluminum. Specifically, the 2000, 5000, 6000, and 7000 series alloys were targeted in this work since those are the alloys used primarily in the teaching laboratories. Third, the etch should produce a sample suitable for examination using only bright-field optical microscopy. An etchant that satisfies these criteria should be of interest and utility to industrial and quality control laboratories where limited equipment may be available. It would also be beneficial to laboratories that have a high employee turnover rate or work with student interns regularly. While there are several established etchants for revealing the grain structure in select alloys; Kroll’s reagent, Keller’s reagent, Barker’s reagent, and Weck’s reagent[2], none are versatile enough to reveal grain contrast across all of the aluminum alloy series. There are examples in the technical literature of researchers exploring new and alternate etching chemicals and processes for revealing grain boundaries in aluminum alloys. Mohammadtaheri et al. explored a two- step etching process similar to the method proposed here but with a different first etchant[3]. This work demonstrated effective grain etching for 2000 and 5000 series aluminum alloys; however, no other alloys were explored. Other efforts have focused on finding an improved etchant for creating grain boundary contrast in aluminum alloys joined by welding, and specifically solid state welding processes. Tamadon et al. explored numerous combinations of two-step etching to reveal the grain structure in AA6082 joined by friction stir welding (FSW)[4]. While several processes produced excellent results, these processes were not something that could be easily transferred to a novice metallographer. Many involved three steps with an attack etch, followed by an etch to remove any Al2O3 on the surface, followed by a final etch to either stain or enhance grain contrast. Several procedures involved heat and ultrasonic baths which adds additional variables and creates a difficult process to reproduce each time. Beach et al. previously published work on a modified Barker’s reagent etch for revealing grain contrast and the oxide stir line in FSW joints[5]. This etch showed limited effectiveness on the 6000 series alloys and did not produce uniform grain contrast for dissimilar welds. The research conducted here presents a versatile new etching technique that fills the need for grain etching in multiple aluminum alloy series. The procedure is a two-step etch that combines an attack etchant with a stain etchant to reveal grain structure of 2000, 5000, 6000, and 7000 series aluminum alloys. Details of the etchant chemistry, procedures, and results are presented in a full paper published in Metallography, Microstructure, and Analysis[6]. EXPERIMENTAL PROCEDURES Samples of flat rolled aluminum alloy (AA2024-T3, AA5754-O, AA6061-T6, and AA7075-T6) sheets (approximately 3 mm in thickness) were sectioned and oriented in the mount so the rolling direction was polished and etched. Metallographic sample preparation followed methodology specified in ASTM E3-11(17)[7]. Details of the metallographic process are provided in the full paper published in Metallography, Microstructure, and Analysis[6]. The etching process developed is a two-step etch that requires applying an attack etch first followed by a stain etch. Both formulations are modified versions of established etchants. The first etching solution uses a modified version of Keller’s reagent. The formula for this reagent is: 3 mL HNO3, 2 mL HCl, 2 mL HF, and 93 mL distilled or deionized water. The second etching solution is a modified version of Weck’s reagent. The second etchant mixture was: 2 g sodium hydroxide (NaOH), 3.80 g potassium permanganate (KMnO4), and 100 mL distilled or deionized water. TABLE 1 — ETCHING TIMES FOR STEPS 1 AND 2 Alloy Designation Step 1 Modified Keller’s Reagent Step 2 Modified Weck’s Reagent AA2024-T3 10-15 sec 10-15 sec AA5754-O 15 sec 30 sec AA6061-T6 10-15 sec 15-20 sec AA7075-T6 5-10 sec 5-10 sec AA7475-T61 5-10 sec 5-10 sec
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