Which are the principal reasons because is very difficult to machining the titanium and titanium alloys? For which reasons, inclusively working with sophisticated tool materials at low cutting speeds, the tool life is short?
Titanium and its alloys have gained widespread applications in aerospace, biomedical industries due to their following favourable properties: They are light weight, possess high strength, have excellent fatigue performance and offer high resistance to an aggressive environment.
It is unfortunate that the above favourable properties in practical applications, prove to be a curse during machining:
- Titanium alloys due to their high strength, low thermal conductivity and chemical reactivity with tool materials (at elevated temperatures), pose a hazard to the tool and significantly reduce the tool life.
- In addition, a relatively low Young’s modulus of titanium alloys leads to spring-back and chatter leading to poor surface quality of the finished product.
- Finally, during turning and drilling, long continuous chips are produced; causing their entanglement with the cutting tool and making automated machining near impossible.
There are solutions however:
- Application of high-pressure coolant during cutting and drilling operations. This is quite expensive owing to coolant costs.
- "Ultrasonic assisted machining" in which the tool is imparted an ultrasonic vibration with very low amplitude. Instead of continuous contact of tool with work, this method offers an intermittent contact reducing the heat affected zones at chip-tool interface, work-flank interface etc. thus increasing the tool life.
Article 3D finite element analysis of ultrasonically assisted turning
Article Analysis of ultrasonic assisted machining (UAM) on regenerat...
Titanium and Titanium Alloys: Fundamentals and Applications
https://books.google.de/books?isbn=3527605207 - Diese Seite übersetzen
Christoph Leyens, Manfred Peters - 2006 - Technology & Engineering
Dear Daniel, this is an excellant book. You should find an answer to your question. Sorry, I don´t have the time to search for it.
Best regards
Peter
Many thanks Peter. I will search the answer in this book!!!!!
One of the major concerns in machining is the tool life. It has been shown in a worked examples in John A. Schey's book (second edition, McGraw-Hill, page 457, ISBN 0-07-055279-7) it has been calculated that the same rise in tool temperature, the cutting speed in IN 100 (a superalloy) is only 1/8.4 times that for 4140 steel. The main reasons are low values of thermal conductivity and specific heat for IN 100 are lower than those for 4140 steel. Using the same argument it should be possible to show that Titanium and its alloys are also more difficult to machine than 4140 steel. This observation has also been made by Schey. Also on page 450 it has been shown by late Dr. Komanduri that segmented chips form in titanium as opposed to wavy chips in 1015 steel. Summing up inferior thermal properties and undesirable chip geometry. make machining if titanium mush more difficult as compared to steel
Titanium is also quite reactive so it easily destroys DLC coated and in general diamond tools forming titanium carbide. But it really depends on which alloy you need to machine.
This kind of alloys has a high work hardening tendency. Which means during deformation it will harden and this impede the tool movement and therefore you need carbide tools to machine it.
The alloy is sticky and has high adhesive affinity to the tool face and therefore you need coated tools. The tool must be properly designed to facilitate chip breakage.
The high strength of the alloy requires more energy to deform it and even during deformation, alloy work hardening increases the required energy for deformation. This will generate excessive heat and therefore you need special tool coating that can has high hot hardness and high oxidation temperature.
The tendency of work hardening increases with larger feed per tooth, that is why it is always recommended to reduce the feed per tooth. Low cutting speed is usually recommended, even though some studies show some positive effect of high speed on surface integrity, but there are several interdependent factors that determine the surface finish in machining such alloys, therefore many studies are done in this field.
