TITANIUM
CONTROL QUALITY

In general, processing and transformation operations and the various mechanical and thermal treatments influence the properties and microstructures of titanium and its alloys.
All these influences then lead to the realization of metallographic quality controls such as: microstructure examinations, porosity and/or heterogeneity research, inclusion cleanliness, hardness tests, hardening controls, grain size controls, etc.

Obtaining an inspection surface requires a succession of operations, each as important as the next, regardless of the material.

These steps are in the following order:

• The 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”.
• Characterisation of the sample: revealing 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 titanium.
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:

Each of the cutting machines in the range is equipped with the appropriate consumables and accessories. The clamping system and the choice of these consumables are always essential elements for the success of a metallographic cut.

Clamping, i.e. holding the workpiece, is also essential. Indeed, if the workpiece is not well held, the cut can present risks for the consumable, the workpiece and the machine.

In addition, titanium is very sensitive to burning during metallographic cutting, which makes it all the more important to determine the appropriate consumables and parameters.

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 sample 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.

The cold process can be used with:
• 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 re- sin. 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.

As a material, titanium polishes in a very special way, it is relatively soft and prone to incrustations and plastic deformation. As a result, during a polishing process where the material is usually cut/removed by the abrasive, the material will in this case have a tendency to “bend” back on itself. This is known as “buttering”. This phenomenon is undesirable and greatly hinders microscopic observation. Titanium therefore has its own specific polishing ranges in order to guarantee an excellent examination surface.

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 polishing ranges given above are complete and are to be carried out in their entirety. The super-finishing step using colloidal silica solution is compulsory. It delivers an excellent inspection surface free of any scratch or buttering phenomenon.
However, an alternative has been developed in the particular case where colloidal silica is not suitable. The SPM can be replaced by a suspension of Al₂O₃ N°2 with the same parameters.

=> Range N°1 is the most versatile range, is effective on most titanium grades and provides an excellent inspection surface.

=> Range N°2 is the traditional titanium polishing range, its advantage being the absence of polishing steps using diamond suspension. Diamond polishing should be restricted to the bare minimum, because it tends to encrust and “butter” the surface of the specimen.

=> Range N°3 was developed specifically for polishing a very soft titanium grade, i.e. unalloyed titanium (T40 for example). The diamond suspension must not be used in this case.

After this preparation, the polished specimens can be observed directly without metallographic etching. Moreover, titanium’s crystallography allows very good inspection of the structure under an optical microscope, revealing it with the help of a polarised light filter.

Metallographic etching of titanium is commonly done using Kroll’s reagent: a solution of 3mL of hydrofluoric acid and 6mL of nitric acid for 100mL of water. The etching creates differences in background and/or colour between the different constituents, allowing them to be inspected.

All micrographs presented were created using the PRESI VIEW software: