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.

Topics

• Overview
• Metallographic preparation of copper
  • Cutting and mounting
  • Grinding and polishing
  • Etching
• FAQ

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.}

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