Machining and cutting of Titanium
Amongst the clichès that have been associated with titanium since it made its appearance in the technological field more than 40 years ago, one is certainly not true: that it is difficult to be machined.
This reputation is probably due to the incorrect approach of many new users who have ignored the Physical characteristics of titanium.
In fact, if specific machining approach and the minimum experience are required in any job, machining titanium does not present any particular difficulty and can be undertaken by any workshop that works with commitment and professionalism.
This leaflet is certainly not intended to be a manual on machining techniques but merely a starting point for workers. lt provides suggestions, advice and data that equipment designers and machine operators can use as a guide in order to find the best specifications for each equipment and machining tool.
General Principles on Titanium Machining
The term machining refers to all the types of working with removal of chips; it also includes cutting.
The first activities include turning, boring, milling, drilling, tapping, broaching, planning, toothing, grinding and lapping.
Cutting could logically be restricted to sawing or nibbling. However, we shall examine not only mechanical cutting but also heat cutting (oxyacetylene, plasma, laser) and water-jet cutting.
lt is not more difficult to work with titanium than with stainless steel, which is commonly machined today.
Only Ti-alloys, with high number of alloy elements, may present greater difficulties.
Machining Grades 1, 2, 3, 4, 7 and 11 presents no particular difficulties as long as chemical and physical characteristics are understood correctly as we said before.
All these characteristics distinguish Ti from other structural materials and regulate the degree to which it can be worked.
The most important, to be mentioned, are:
- Low heat conductivity; so that the heat, generated by cutting, is not dispersed rapidly by being conduction to the inside of piece, but concentrates on the cutting edge and on the tool face. The high temperatures achieved may lead to tempering and blunting of the cutting parts with consequent further rise in temperature and further shortening of working life of the tool.
- High degree of chemical reactivity with almost all the materials, especially at high temperatures; that may lead to abrasions, microwelds and spreading with the cutting tools.
- Low level of elastic modulus: it is appreciated in the final product but it may initially cause some difficulties with machining. Under tool pressure, the elastic material tends to distance itself from the cutting zone, especially during light passes. The thinner parts are deflected and instead to cut the cutting edge tends to slide along the work piece and to create vibrations with generation of heat.
- Hardening caused by machining means that there is virtually no built-up edge. The lack of a stationary mass of material in front of the tool causes high cutting angles to be formed. This leads to the formation of a thin chip in contact with a relatively small area of the tool face so that high loads are created over a section unit. This fact, together with the use of a tool with an inappropriate geometry and probably not sharp, tends to push the material rather than cutting, to stress and cause plastic deformation. ln turn, plastic deformation tends to harden the material and thus increases hardness and resistance so that cutting speeds correct at the beginning of the task become excessive and the tool wears out excessively.
- Stress in the material may appear and are mainly caused by severe deformation during forging of products. This is particular for alloys with high strength. e.g. titanium grade 5.
- Stress and variation in mechanical values may apply for alloyed titanium with high strength. This can be caused by high forge impact, to low forging temperature or insufficient mix of elements during the meeting of ingots. Titanium has also an tendency to return to original shape. This may cause problems when machining thin wall products with narrow tolerances.
Often, those who tried to work titanium proceeded by trial spending a lot of time looking for tools. They based their calculations on data traditional for other materials, failing to observe the characteristics described, which clearly dictate the work criteria to be adopted and which basically hold true for all types of machine tool and titanium.
These criteria can be summarized as follows:
- Use of low cutting speeds to minimize heat built-up. Excessive cutting speeds are much more harmful with titanium than with other materials: a 30% increase in speed can reduce tool life by 80%.
- Use of relatively high speeds of travel. Temperature is less influenced by speed of travel than by cutting speed. Travel speeds should therefore be as high as compatible with efficient working.
- Use of a large flow of cutting fluid to increase the cooling effect. Also, the fluid must be oriented directly to the cutting point. Because of the low heat conductivity, the refrigerating effect is in fact very reduced unless it is carried directly on the point at which the heat is created.
- Use of tools with controlled sharpening and their replacement as soon as they become worn. When working with a complex tooling machine or machining station due to the amortization costs of the machine, production is much more important than the lifetime of the cutting tool. lt is therefore advisable to use the tool to the limit of its capacity and to replace it as soon as its cutting efficiency starts to reduce.
- Guaranty of high stiffness for the whole machining system (machine, spindle, tool holder and tool) to compensate for the elasticity of the material and to reduce vibrations to a minimum level.
- No stopping on travel whilst the work piece and the tool are in Contact. Leaving the tool in position causes the material to be immediately hardened and over-heated and causes abrasions, seizing up and breakage of the tool.
- Items with thin wall and narrow tolerances machined out of solid bars should be pre-machined close down to final size, and kept a day for stabilization prior to final machining of the item.