Metallographic Preparation of Copper and Its Alloys

Copper is soft and ductile: easy to work with, yet prone to surface damage during preparation. Its unique combination of properties, especially its exceptional electrical and thermal conductivity, makes it the material of choice in applications such as electrical wiring, telecommunications, heat exchangers, and kitchenware. Copper also plays a crucial role in the energy transition, being indispensable in technologies like wind turbines, solar panels, and electric vehicles. Its warm, attractive appearance further makes it popular for decorative and functional items like handles, doorknobs, countertops, and tables.

To accurately assess the microstructure of copper, careful metallographic preparation is essential, as improper handling can easily introduce artifacts like smearing or scratches caused by oxide pull-out. This guide walks you through each step in the metallographic preparation of copper and its alloys, from cutting and mounting to grinding, polishing, and etching.

Metallographic Preparation of Copper and Its Alloys

Cutting and Mounting of Cu and Cu-alloys

Copper and its alloys are relatively soft and ductile materials, which allows for easy sectioning—but also increases the risk of smearing, deformation, and thermal damage if the cutting process is not properly controlled. The cutting method must therefore be carefully adapted to the material type and geometry:

• Pure copper is especially prone to smearing and surface deformation during sectioning. To minimize these effects, use a resin-bonded silicon carbide (SiC) cut-off wheel. For normal and floor-standing cut-off machines we recommend using our NF-A cut-off wheel, which is ideally suited for soft, non-ferrous metals with hardness values up to 300 HV, ensuring clean and low-distortion cuts.
• Copper alloys, such as brasses, bronzes, or Cu-Ni systems, can generally be cut using the same parameters as pure copper. However, their slightly higher hardness and reduced ductility mean they often tolerate higher feed rates or mechanical loads without compromising cut quality.

To ensure precise and damage-free sectioning, it is essential to use a precision cut-off machine with sufficient coolant flow. Low cutting speed is recommended to reduce thermal effects and smearing. The sample must be clamped securely but gently to prevent vibration or distortion, especially for thin-walled or tubular specimens, which may deform due to internal stresses when cut longitudinally. For extreme thin and pressure sensitive samples like copper tubes or ammunition we recommend mounting the sample before clamping and sectioning.

Careful cutting is a critical first step in metallographic preparation and ensures that subsequent grinding and polishing can reveal the true microstructure without interference from preparation artifacts. For this reason, wet cutting with precise cut-off machines is the best method for cutting copper samples with minimal deformation.

Mounting provides mechanical support during grinding and polishing and is particularly important for small, irregular, or edge-sensitive copper specimens. It ensures stable handling, protects the sample edges, and improves preparation consistency.
The preferred mounting method for copper samples due to the low annealing temperature of these samples, is cold mounting but hot mounting is also possible.
For hot mounting, we recommend using Bakelite, available in red, black, or green, for routine applications. If transparent preparation is desired (e.g., for target preparation) using THERMOPLAST is an alternative. Here the heating temperature should be below 190°C.

Cold mounting is recommended when thermal exposure must be avoided, such as in failure analysis, heat or pressure-sensitive structures, or the preparation of complex geometries. Commonly used systems include the PMMA-based resins KEM 20 and KEM 30, as well as the epoxy resin Qpox 93, which provides excellent edge retention and minimal shrinkage.

Etching of Cu and Cu-alloys

Etching is an essential step in the metallographic preparation of copper and its alloys. It allows microstructural features such as grain boundaries, twins, and second-phase particles to become visible under the microscope. In many cases, especially with cast alloys, etching is straightforward. However, finding the optimal etchant can be more challenging for wrought copper alloys, particularly those that have undergone extensive cold deformation. In such cases, colour etching can offer additional contrast and clarity.

Common etchants can be prepared in the laboratory using standard chemical reagents. The following table summarizes typical formulations and usage conditions:

Safety Notice: Acids must be used with caution. Wear protective equipment and follow lab safety guidelines.

_{Note: If the sample contains lead, most etchants will attack the inclusions, leaving black voids. For accurate documentation of lead distribution, images should be taken before etching.}

Copper is characterized by its exceptionally high thermal and electrical conductivity, outstanding corrosion resistance, moderate strength, and good formability. The physical properties of copper can be seen in Table 1. As one of humanity’s oldest metals, copper has been used for thousands of years and remains indispensable in modern technology and industry. The study of copper and its alloys explores their structure, properties, processing, and applications, offering insights into how these versatile materials can be engineered for specific uses. There are currently more than 400 copper alloys, each offering distinct properties, and new alloys continue to be developed. These materials are engineered to meet the highest technical standards and are customized for a broad spectrum of uses. Copper can be alloyed with a variety of different elements. Some of the most widely recognized copper alloys include various types of brass, which contain different amounts of zinc, and bronze, which is made from copper and tin. Other familiar copper alloys include Copper-Nickel alloys, Copper-Aluminum alloys, gunmetal (Cu-Sn-Zn alloy), Copper-Manganese alloys and Copper-Lead-Tin alloys. The copper family tree, designed in 2013 by the German Copper Institute (DKI), is the best representative image for illustrating copper and its alloys.

The main objectives of metallographic investigation of copper and its alloys are:

1. Microstructural Characterization:
   To observe and analyze the grain structure, phase distribution, and microstructure, which influence mechanical, electrical, and corrosion properties.
    
2. Phase Identification:
   To identify the different phases present (such as solid solution, intermetallic compounds, or precipitates) resulting from alloying elements or specific thermal treatments.
    
3. Quality Control:
   To detect and evaluate defects such as porosity, inclusions, cracks, segregation, and non-metallic impurities that can affect performance and reliability.
    
4. Assessment of Processing Effects:
   To study the effects of manufacturing and metal forming processes (casting, forging, rolling, annealing, etc.) on the microstructure and, consequently, on the properties of the material.
    
5. Corrosion and Degradation Analysis:
   To investigate corrosion mechanisms, evaluate the extent of degradation, and identify areas susceptible to corrosion or other forms of environmental attack.
    
6. Failure Analysis:
   To determine the causes of failure in service by examining the microstructure at and around the fracture or failure site.
    
7. Homogeneity and Alloy Distribution:
   To assess the uniformity of alloying elements and detect any segregation or uneven distribution that could affect material performance.

Metallographic investigation provides critical insights for optimizing processing parameters, ensuring quality, and improving the performance and longevity of copper and its alloys in various applications.

Mounting

Mounting is required for:

1. Unwieldy samples with small or complicated dimensions
2. Sensitive sample material, for instance soft, brittle, fragile, porous.
3. Combination of cuts samples in one sample.
4. For the preservation of marginalized layers and coatings such as nitrided steels, plasma sprayed coatings or paint coating layers.
5. To allow further processing, it is necessary for the samples to have a constant size.

Cold mounting, and UV mounting are the preferable mounting processes for copper samples. For hot mounting, it is possible to be performed but the following pointe should be considered accurately. The temperature of hot mounting should be below 190 °C, as the annealing temperature for most copper alloys, as already mentioned, is between 200 - 400 °C, and temperatures above 190 °C during hot mounting can impair the microstructure. Below are some examples the mounting process.

UV Mounting

Cold mounting

Copper alloys

The properties of pure copper are not suitable for plenty of applications, therefore adding the alloying element can broad the application of copper widely. Each alloying element can significantly improve one or more properties of pure copper, such as strength, corrosion resistance, wear properties and machinability. Some alloys can also change the color of pure copper such as Ni, Sn or Zn. Below you will find two images which summarize the effect of various alloying elements in copper alloys.

Vibration polishing of electronic samples

To prepare the electronic components free of any deformation and scratches, sometimes the short final polishing is not enough. In these cases, a special technique named vibration polishing comes up. This part of the preparation is done very gentle for achieving a polished surface without deformation. A special mechanism for the generation of the vibrations yields optimal energy transmission onto the specimen. As deformation and its associated layers underneath the surface distort the microstructure a preparation serving a polished surface without artefacts is of high importance. The vibratory polishing device Qpol Vibro is designed for the virtually deformation-free preparation of specimen surfaces. (Link to QPOL Vibro)

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