A fast, universally applicable metallographic specimen-preparation method produces consistent results in aluminum alloys including high edge retention.

A fast, universally applicable method of mechanical specimen preparation based on a Struers MD (magnetic disc) system and its MD-Largo disc was developed for fine grinding soft materials (HV 40-150 hardness). This technique enables preparation of aluminum alloys specimens with excellent edge retention. The method can be applied to almost all types of cast and wrought aluminum alloys by making minor adjustments to the force and duration of the preparation process, and has proved to be particularly beneficial when preparing specimens of aluminum materials having significantly harder surface coatings.

Fig 1 "Grasshopper" intermetallic segregation in a cast aluminum alloy. 500x; Fig 2 "Dinosaur" intermetallic segregation in a cast aluminum alloy. 100x; Fig 3 "Ugly-duckling" intermetalllic segregation in a cast aluminum alloy. 500x

Etching difficulties

Many etchants for use on aluminum materials are described in the literature [1-3]. Unfortunately, information about the etchants, such as "?uitable for use with most types of aluminum..." [1], have driven some metallographers and students to despair in their unsuccessful attempts to etch aluminum alloys with these so-called "universal" etchants.

Metallographers working with aluminum alloys have learned by experience that most of these etchants are only suitable for use on specific alloys or a narrowly defined groups of alloys. Investigating a new material that may belong to the same group of alloys as the alloy usually prepared, but differing only slightly in chemical composition can produce entirely different results than that normally expected. In the worst case, etching may cause severe pitting rather than revealing the desired grain boundaries or surfaces.

A case in point is Bohner's etchant, which produces excellent results when used to etch the grain boundaries of alloy EN AW 6060 (approx. 0.5% Si). However, when this etchant is used on alloy EN AW 6005 (approx. 0.7% Si), the grain boundaries are barely revealed, and etching alloy EN AW 6082 (approx. 1% Si) causes significant pitting.

Barker's method of electrolytic etching (with subsequent optical microstructural contrasting in polarized light) is the only really universal etchant for aluminum materials. However, this method requires electrical contact with the specimen, which is not always easy to achieve.

The aluminum color etchant developed by Weck [4] is very useful, particularly for etching casting alloys. Advantages of this etchant are that chemically, it is relatively innocuous, and it does not attack intermetallic phases and precipitates, which are often etched away using many well-known etchants. One disadvantage of the etchant is its extremely sensitive reaction to remnant deformation layers on the surface of the prepared specimen. Using this etchant on mechanically prepared specimens of many aluminum materials frequently produces poor results. A simple double etching technique that enables achieving the required level of microstructural contrast using Weck's etchant on mechanically prepared specimens is discussed below.

Fig 4 Intercrystalline corrosion in a cast aluminum alloy. 1,000x; Fig 5 Microstructure of EN AW 7175 aluminum alloy wire

Specimen preparation

Specimen preparation and etching methods were applied to a large number of different cast (gravity die, investment and pressure die cast) and wrought aluminum alloys including 2000, 3000, 5000, 6000 and 7000 series, as well as CrN-coated and Al2O3/TiO-plasma or flame-sprayed coated and lacquered castings and coated (anodized, hard anodic coating with PTFE, lacquered and electroless plated nickel) wrought alloys.

Mounting: Specimens can be cold or hot mounted. When selecting a mounting resin, it is important to ensure that the hardness of the resin is comparable to, or slightly greater, than that of the hardest constituent in the surface of the specimen (e.g., intermetallic phases and edge layers). When investigating surface coatings, it is recommended to hot-mount the specimen after tightly wrapping it in thin metal foil (e.g., household aluminum foil or nickel foil), which separates the coating from the mounting resin. The hardnesses of the foil and coating should be comparable.

Grinding was performed on individual specimens using a Struers RotoSystem, a semiautomatic preparation system with a disc diameter of 250 mm (10 in.). Grinding parameters are shown in Table 1.

Polishing parameters used are summarized in Table 2; polishing forces and times are approximate values, which must be adjusted to suit actual conditions. When polishing higher strength aluminum materials, it is often possible to eliminate the second stage of the polishing process (diamond, 1 Km). However, the quality of the surface should be checked after completing the first polishing stage.

If etching is done immediately after specimen preparation, it is recommended to perform final polishing using OP-S suspension. OP-U suspension is preferred if precipitates will be investigated, because OP-S attacks the precipitates, thereby altering their appearance.

The SP-PoliCel polishing cloth (listed in Table 2 as an alternative to MD-Chem) is particularly well-suited for final polishing of very soft aluminum alloys.

Etching: Excellent results can be achieved when specimens prepared using the technique described above are electrolytically etched using Barker's etchant, and then viewed under polarized-light illumination. However, this preparation method requires making electrical contact with the specimen, which often presents difficulties. Because our experience with commercially available electrically conductive hot mounting resins have been poor, an alternative etchant was sought.

Weck's etchant is based on an alkaline potassium permanganate solution and is very well-suited for etching cast aluminum alloys. The immediate application of this etchant to specimens of a number of aluminum materials, wrought alloys in particular, produces unsatisfactory results as expected. Despite final polishing using an oxide suspension, specimens still are not completely deformation-free, and, thus, unsuitable for use with this etchant. Longer final polishing using a variety of oxide suspensions did not significantly improve the result, but led only to a reduction in the high level of edge retention achieved in the preceding preparation stages.

Fig 6 Beginning of liquid-metal damage at the surface of EN AW 3103 aluminum alloy heat exchanger. Damage is from solder metal penetration at grain boundaries (particles above the surface are residues of the solder-metal eutectic).; Fig 7 "Aluminum wave" burr at solder connection of a heat exchanger

Solving the problem

A series of experiments was conducted in which specimens were pre-etched with the aim of chemically removing residual deformation from the surface of the specimen. The best results from the range of different etching solutions and variety of concentrations were achieved by pre-etching the specimen with a 2% aqueous solution of sodium hydroxide.

Results show that simple pre-etching with dilute sodium hydroxide solution enables achieving excellent microstructural contrast using Weck's etchant on nearly all the aluminum materials examined, without impairing surface quality (edge retention or contrasting of precipitates).

A limitation of the method is that it in the case of a few wrought alloys in a cold work-hardened condition (e.g., EN AW 5083, or AlMg4.5Mn0.7), it was not possible to achieve a satisfactory level of microstructural contrast using the procedure. The parameters used in a double etching procedure are presented in Table 3. A particular benefit of the double etching technique is that polarized light can be used for additional optical microstructure contrast; an advantage that is not generally realizable when etching is performed using potassium permanganate alone.

Fig 8 Intermetallic segregation in a cast aluminum alloy. 250x; Fig 9 Intercrystalline pitting corrosion in EN AW 6060 aluminum alloy. 200x

Summary

A universal method of mechanically preparing aluminum-alloy specimens has been developed, in which the usual multistage grinding procedure using silicon-carbide paper is replaced by a diamond fine-grinding stage using a Struers MD-Largo grinding disc. The method is characterized by the very high level of edge retention that can be achieved-a feature that makes the technique particularly well-suited for preparing surface-coated specimens.

A double-etching procedure also was developed for specimens prepared using the discussed method, which involves a pre-etch using an aqueous 2% sodium-hydroxide solution and subsequent color etching using Weck's alkaline potassium-permanganate solution. Results demonstrate that the double-etching procedure is almost as universal in its applicability as the well-known Barker's electrolytic etching technique, but without requiring electrical contact to be made with the specimen.

This article is reprinted from Struers Journal of Materialography, Stucture 38 (first published in Praktische Metallographie, Vol. 38, No. 2, 2001), with permission.