COPPER ALLOY
QUALITY CONTROL

Obtaining an inspection surface requires a series of operations, each one as important as the next and this regardless of the material.
These operations come in this order:

• Removal of the product to be examined (if necessary), called «CUTTING».
• Standardisation of the geometry of the sample taken (if necessary), called «MOUNTING».
• Improvement of the surface condition of this sample, called «POLISHING».
• Sample characterisation: to reveal the microstructure of the sample by an etching reagent (if necessary) called «METALLOGRAPHIC ETCHING» and microscopic observation (optical or electronic).

=> Each of these steps must be carried out rigorously, otherwise the following steps will not be possible.

The purpose of cutting is to remove a precise section of a product, in order to obtain a suitable surface for inspection, without altering the physico-chemical properties of the copper.
In other words, it is essential to avoid heating or any deformation of the metal that could lead to degradation of the material. Cutting is a fundamental step which conditions the further preparation and inspection of parts.
PRESI’s wide range of medium and large capacity cutting and micro-cutting machines can be adapted to any need with regard to cutting precision, sizing or quantity of products to be cut:

All cutting machines are used with a lubricating/cooling liquid composed of a mixture of water and anti-rust additive in order to obtain a clean cut without overheating. The additive also protects the sample and the machine from corrosion.

=> The choice of the cut-off wheel type has to be adequate, in order to avoid cutting failure, or excessive cut-off wheel wear or even breakage. The hardness of the workpiece determines the wheel selection.

Samples can be difficult to handle due to their complex shape, fragility or small size. Mounting makes them easier to handle by standardising their geometry and dimensions.

=> Achieving good-quality mounting is essential to protect fragile materials and also to achieve good preparation results for polishing and future analysis.
Before mounting, the specimen should be deburred with coarse abrasive paper, for example, to remove any cutting burrs. Cleaning with ethanol (in an ultrasonic tank for even greater efficiency) is also possible. This allows the resin to adhere as well as possible to the sample and thus limits shrinkage (space between the resin and the sample).

If shrinkage persists, it can lead to problems during polishing. Abrasive grains may become lodged in this space and then be released at a later stage, thus creating a risk of pollution for the sample and the polishing surface. In this case, cleaning with an ultrasonic cleaner between each step is recommended.

There are two mounting options:

He is to be preferred for edge inspection purposes or if the metallographic preparation is carried out in preparation for hardness testing. This option requires a hot-mounting machine.

He is to be preferred:
• If the parts to be examined are fragile/sensitive to pressure
• If they have a complex geometry such as a honeycomb structure.
• If a large number of parts are to be mounted in series.

The cold process can be used with:

Cold resins do not always provide a flat mounting «back» because of the meniscus of the liquid resin. Before any polishing operation, a brief step using abrasive paper will remove this meniscus. The important thing is to ensure that this operation renders the two sides of the mounting parallel.

To meet user needs, PRESI offers a full range of cold mounting moulds:

The cold process has different mounting moulds with diameters from 20-50mm. These are divided into several types: optimised moulds called «KM2.0», rubber, Teflon or polyethylene moulds.
Cold mounting is also more flexible, hence the existence of rectangular moulds for more specific needs.

The last and crucial phase in the sample preparation process is polishing. The principle is simple, each step uses a finer abrasive than the previous one. The aim is to obtain a flat surface and to eliminate scratches and residual defects that would hinder the performance of metallographic control examinations such as microscopic analysis, hardness tests, microstructure or dimensional inspections.

PRESI offers a wide range of manual and automatic polishing machines, with a wide choice of accessories, to cover all needs, from pre-polishing to super-finishing and polishing of single or series samples.

The MINITECH range of manual polishers incorporates the most advanced technologies.
User-friendly, reliable and robust, they provide a simple answer to all needs.

The MECATECH range of automatic polishers allows both manual and automatic polishing. With its advanced technologies, motor power from 750-1500 W, all the PRESI experience is concentrated in this very complete range. Whatever the sample number or size, MECATECH guarantees optimal polishing.

All the polishing ranges below are given for automatic sample preparation (for manual polishing: do not take into account the parameters at the top). They are the most commonly used and are given for information and advice.

All the first steps of each range are called «levelling» and consist of removing material quickly to level the surface of the sample (and resin). Those given below are standard and can therefore be modified as required.

Applied pressures vary according to sample size, but in general the following applies: 1daN per 10mm mounting diameter for the pre-polishing steps (ex: Ø40mm = 4 daN) then reduce force by 0.5daN at each polishing step with an abrasive suspension.

The following is a general polishing range for copper and its alloys:

NB: Levelling with P320 abrasive paper is enough for a sample from a metallographic cut. If additional material removal is required, a larger grit-size should be used.

For pre-polishing, the head and platen rotation direction should not be reversed, as this can detrimentally affect flatness. However, reversing rotation direction can help if a large amount of material has to be removed

The important thing in copper polishing is the use of mono-crystalline diamond suspensions (LDM or Gel2+ mono) with a more rounded diamond shape (compared to polycrystalline diamond) to limit encrustations in soft materials such as copper or its alloys.

The LDM suspension is used with the Reflex LUB lubricant, which allows the fabric to be sufficiently moistened without overloading it only with diamond suspension. In any case, care must be taken not to wet the fabric too much with suspension or lubricant, so as not to «aquaplane» with the fabric, which would make it ineffective.

The second important consideration when polishing copper is to adapt the pressure applied (manually or in semi-automatic polishing) so as not to encrust grains of abrasive paper, for example.

Finally, superfinishing is advised, using a colloidal silica suspension. This SPM suspension can be diluted up to 7 times in water.
During this stage, the rotation of the head is reversed in relation to the platen in order to keep the suspension on the polishing cloth as much as possible.

Depending on the result obtained, this stage can be replaced by the use of a PRESI n°2 alumina suspension.

Figures 9-14 show the result on copper and brass after applying the above-mentioned polishing range.

Sometimes it is necessary to adapt the polishing range according to the sensitivity of the material. Below is an example of a range for a bronze coating with lead

The structure of the copper is easily revealed by using suitable etching reagents.
The use of etching reagents reveals grain boundaries, phases, etc…
The main reagents are:
– alcoholic solution of Fe III acid chloride (figure 17)
– potassium dichromate (Figure 18)
These reagents are available in the PRESI catalogue and other specific reagents are available on request.

All of the micrographs presented were done using the PRESI VIEW software: