Tool and lubricant choice
Gamma titanium is difficult to machine because of:
- minimum roughness surface,
- high hardness,
- high thermal barrier,
- high thickness,
- minimum friction coefficient,
- high resistance at the mechanism of “abrasive” and “adhesive” wear
Tool wear mechanisms for this material include:
- Abrasive wear
- Adhesive / attritional wear
- Diffusive wear
- Fatigue wear
- Plastic deformation
- Flank wear
- Face wear
- Catastrophic failure
- Surface roughness
- Surface hardness
General solutions for machining gamma titanium are rigid machine set-ups with adequate cutting strategy (feed and tool rotation in the same direction), low cutting velocities, high feed and reduce tool length in order to limit the vibrations. High lubricant pressure needs to be used to evacuate the generated heat and sharp cutting edges and positive geometries need to be used in order to reduce cutting forces. The tool should be hard metal with high strength.
Heating damage and loss of surface integrity of the workpiece must be avoided. A dramatic loss of mechanical behavior such as fatigue can result. On the other side, basic fatigue properties of many titanium alloys rely on a favorable compressive surface stress induced by tool action during machining
A detailed study was performed, taking into account heat transfer distribution modalities during the machining and turning process. Thiswas done to maximize production rates and surface quality. Tools were optimised for geometry and coatings.
The best cutting tools for the selected γ-TiAl alloy machining operations.
Cutting tool material can be micro-grain tungsten carbide, whisker reinforced ceramics, SiAlON (FGM ceramic combirening hardness and toughness) and polycrystalline cubic boron nitride. High hardness coatings, high temperature oxidation resistant with low deposition thickness (to reduce cutting edge roughness) and fanocomposite AlCrTiN4 and nc-AlCrN/a-Si3N4 (Platit), Nanocomposite AlTiN Hyperlox (Cemecon) are the possible coatings.
PVD coatings with a low thermal conductivity and improved surface finish are can counteract machining difficulties, and result in improved frictional characteristics and chip evacuation. The CoroMill Plura AlCrN coating is recommended.
When grinding, fluid application increases grinding wheel longevity and workpiece surface quality.
There are four main categories of cutting fluids, synthetics, semi-synthetics, soluble oil and straight oil. Straight oil in the past has often been the preferred fluid used in grinding applications due to it leaving a superior surface finish. However, recent environmental concerns have meant that it is being phased out of industry. There is also a fire risk associated with straight oil, particularly when grinding titanium alloys. This has become more prominent as industry has moved towards higher speeds and fluid pressures.
Methods of cooling and lubricating in grinding don’t have to be restricted to liquid coolants in flood form. Graphite has been used as a lubricating medium to reduce heat in the grinding zone. This is generally used as a graphite power paste mixed with oil and grease. This method usually gives a better surface roughness and decreased tangential forces when compared with standard flood coolant. On the other hand, using graphite increases normal force during grinding.
There are a variety of different nozzles that can be used in grinding. The shoe nozzle and free jet nozzle are the most common type of nozzles used. One reason why shoe nozzles are very effective is they are able to make the most of cutting fluid passing through the grinding zone however they have to be in very close proximity to the grinding wheel. The shoe nozzle can reduce grinding forces by 40-60%, surface roughness by 10-20% and increase workpiece removal rate before surface burn begins when compared with grinding without the shoe nozzle method. There are numerous other methods to optimise the cutting fluid application for grinding such as jet nozzle placement, jet velocity, radial coolant jets and dual fluid supplies.
For titanium grinding it is a general rule to use a half to one third of conventional operating speeds. The best material to use in grinding titanium is silicon carbide. Using it at 1200-1800m/min should give an optimum surface finish whilst minimising grinding wheel wear. At such high speeds intense sparking can occur. To avoid this, the work piece should be flooded with coolant to reduce sparking.