Although metallic copper occurs naturally, it is mainly extracted from sulfide ores in a metallurgical smelting process. There are four main steps.
1. An initial smelting process extracts copper concentrates, resulting in copper matte (75 % Cu).
2. In a converter, air is blown into the liquid matte to oxidize the sulfides, resulting in blister copper (96-98 % CU).
3. Blister copper is refined in an anode furnace, resulting in anode copper (99 % Cu).
4. Impurities (including Ni, Pb, Ag, Pd and Au) are removed through electrolytic refining, resulting in copper cathodes (99.99 % Cu).
A small amount of copper is also produced by hydro-metallurgical process.
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Fig. 1: Copper with red copper oxides, dark field, 500x
Fig. 2: Oxygen-free copper, etched with ammonium peroxydisulfate, 100x
Fig. 3: Copper cathode, etched according to Klemm, 100x
By adding aluminum, manganese, iron, nickel, tin or even small amounts of lead, special types of brass with specific chemical or mechanical properties can be produced.
Fig. 4: α- brass, color etched, 200x
Fig. 5: α-β brass cast (CuZn40Pb2) with grey-blue lead inclusions, unetched, 500x
Fig. 6: α-β brass cast, etched according to Klemm, light α- solid solution in dark matrix of β- solid solution, 100x
Bronze (copper-tin alloys)
Bronzes are divided into three types: wrought alloys, cast alloys, and bell casts. Depending on the required properties, small amounts of zinc, phosphorus, lead, nickel or iron can be added.
When working with copper and its alloys, metallography is generally used for grain size measurement, as well as to perform purity checks by qualifying and quantifying the copper oxide content.
In certain brasses, it may be necessary to determine the distribution of lead, as this can influence the machining process.
In cast alloys, metallography is generally used for general structure evaluation, to assess the distribution of eutectic or lead, and to evaluate the presence of shrinkage cavities or porosity.
Fig. 8: Bronze cast (CuSn10), etched with iron (III) chloride, dendritic structure α-δ eutectoid, 200x
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Pure copper is soft and ductile, which makes it easy to deform and prone to scratches. Even bronzes and some of the harder brasses may be susceptible to severe scratching. This creates a problem for the metallographer. However, there are some simple solutions:
- Avoid coarse grinding abrasives
- Give a thorough diamond polishing with soft cloths
- Use chemical-mechanical fine polishing
Read on for a detailed description of how to prepare copper and its alloys for metallographic analysis quickly and accurately, without scratches or deformation.
Fig. 9: Pure copper wire, with final polish of OP-S, DIC, 200x
Fig. 10: Same sample as Fig. 9 with final polish of OP-S-ammonia/water/hydrogen peroxide mixture, DIC, 200x
There are numerous etchants for copper and its alloys that are relatively easy to apply. Most cast alloys are not difficult to etch. However, it can be difficult to find the right etching solution for some wrought alloys, especially if they have been severely cold worked. In these cases, a color etch can be useful.
It is important to note that lead is attacked by the etchants and only black voids will remain after etching. Therefore, micrographs to document the quantity and distribution of lead must be taken before etching. The color of pure lead is grey-blue.
Fig. 13: Bronze cast (CuSn8Pb), unetched, showing large and small blue-grey lead inclusions, pale blue α-δ eutectoid discernable, 500x
Fig. 14: Same specimen as in Fig.13, color etched according to Klemm. The dendritic structure with light blue eutectoid and blue lead inclusions can be seen, but small lead inclusions cannot be clearly differentiated, 500x
Fig. 15: Bronze cast (CuSn10), etched with iron (III) chloride, dendritic structure α-δ eutectoid, 200x
The removal rate in grinding and polishing is closely related to the abrasives used. Diamonds are one of the hardest known materials, as they have a hardness of approximately 8,000 HV. That means it can easily cut through all materials and phases. Different types of diamonds are available. Tests have shown that the high material removal, together with a shallow scratch depth, is obtained because of the many small cutting edges of polycrystalline diamonds. Silicon carbide, SIC, with a hardness of about 2,500 HV, is a widely used abrasive for grinding papers for mainly non-ferrous metals. Aluminium oxide, with a hardness of about 2,000 HV, is primarily used as an abrasive in grinding stones. It is mainly used for the preparation of ferrous metals. It was also extensively used as a polishing medium, but since the introduction of diamond products for this purpose, it has largely lost its usefulness in this application. Colloidal silica is used to produce a scratch-free finish in oxide polishing steps In general, the abrasive must have a hardness of 2.5 to 3.0 times the hardness of the material to be prepared. Never change to softer abrasives - this might lead to preparation artifacts. The amount of abrasive applied depends on the grinding/polishing surface and the hardness of the specimen. The combination of cloths with low resilience and hard specimens requires a larger amount of abrasive than cloths with high resilience and softer specimens, because the abrasive particles wear faster.
The force is expressed in Newton. The figures stated in the preparation methods are typically standardized for six specimens of 30 mm diameter, clamped in a specimen holder. The specimens are mounted, and the specimen area should be approximately 50% of the mount. If the specimens are smaller, or there are fewer specimens in a holder, the force has to be reduced to avoid damage, such as deformations. For larger specimens, the force only needs to be slightly increased. Instead, the preparation time shall be extended. Higher forces increase the temperature because of higher friction, so thermal damage may occur.
Preparation time is the time during which the specimen holder is rotating and pressed against the grinding/polishing disk. The preparation time is stated in minutes. It should be kept as short as possible to avoid artifacts such as relief or edge rounding. Depending on the specimen size, the time may have to be adjusted. For larger specimens, the time shall be extended. With specimens smaller than the standard, the time is kept constant and the force reduced.
This is normally the first step in the grinding process. Plane grinding ensures that the surfaces of all specimens are similar, despite their initial condition and their previous treatment. In addition, when processing several specimens in a holder, care must be taken to make sure they are all at the same level, or "plane," before progressing to the next step, fine grinding. To obtain a high, consistent material removal rate, short grinding times and maximum flatness, totally fixed grains with a relatively large grain size are preferred for plane grinding. Suitable PG surfaces will provide perfectly plane specimens, thus reducing the preparation time on the following fine grinding step. In addition, some surfaces can provide good edge retention. During wear, new abrasive grains are revealed, thus ensuring a consistent material removal.
Fine grinding produces a surface with little deformation that can easily be removed during polishing. Because of the drawbacks with grinding papers, alternative fine grinding composite surfaces are available, in order to improve and facilitate fine grinding, A high material removal rate is obtained by using grain sizes of 15, 9.0 and 6.0 µm. This is done on hard composite disks (rigid disks) with a surface of a special composite material. Thus, the diamond grains, which are continuously supplied, are allowed to embed the surface and provide a fine grinding action. With these disks, a very plane specimen surface is obtained. The use of a diamond abrasive on the fine grinding disks guarantees a uniform removal of material from hard, as well as soft, phases. There is no smearing of soft phases or chipping of brittle phases, and the specimens will maintain a perfect planeness. Subsequent polishing steps can be carried out in a very short time.
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