Machining Titanium:
Titan Engineering Pte Ltd Singapore offer
the following technical information on the machinability of Titanium. This
information is derived from the ASTM
Technical guide to Titanium and should be used for reference knowledge only.
Introduction:
Titanium can be economically machined on a routine production basis if shop
procedures are set up to allow for the physical characteristics common to the
metal. The factors which must be given consideration are not complex, but
they are vital to successful handling of titanium.
Most important is that different grades of titanium, i.e., commercially pure titanium and various titanium alloys, will not all have identical
machining characteristics, any more than all steels, or all aluminum grades
have identical characteristics. Like stainless steel, the low thermal
conductivity of titanium inhibits dissipation of heat within the workplace
itself, thus requiring proper application of coolants.
Generally, good tool life and work quality can be assured by rigid machine
set-ups, use of a good coolant, sharp and proper tools, slower speeds, and
heavier feeds. Use of sharp tools is vital, because dull tools will
accentuate heat build-up, to cause undue galling and seizing, leading to
premature tool failure.
The machinability of commercially pure grades or titanium has been compared by
veteran shop men to that of 18-8 stainless steel, with the alloy grades being
somewhat harder to machine.
Characteristics Influencing
Machinability.
The
fact that titanium
sometimes is classified as difficult to machine by traditional methods in part
can be explained by the physical, chemical, and mechanical properties of the
metal. For example:
Titanium is a poor conductor of heat. Heat, generated by the cutting action, does not dissipate quickly. Therefore, most of the heat is concentrated on the cutting edge and the tool face.
Titanium has a strong alloying tendency or chemical reactivity with materials in the cutting tools at tool operating temperatures. This causes galling, welding, and smearing along with rapid destruction of the cutting tool.
Titanium has a relatively low modulus of elasticity, thereby having more “springiness” than steel. Work has a tendency to move away from the cutting tool unless heavy cuts are maintained or proper backup is employed. Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing, and tolerance problems. Rigidity of the entire system is consequently very important, as is the use of sharp, properly shaped cutting tools.
Titanium’s fatigue properties are strongly influenced by a tendency to surface damage if certain machining techniques are used. Care must be exercised to avoid the loss of surface integrity, especially during grinding.
Titanium’s work-hardening characteristics are such that titanium alloys demonstrate a complete absence of built-up edge. Because of the lack of a stationary mass of metal (built-up edge) ahead of the cutting tool, a high shearing angle is formed. This causes a thin chip to contact a relatively small area on the cutting tool face and results in high bearing loads per unit area. The high bearing force, combined with the friction developed by the chip as it rushes over the bearing area, results in a great increase in heat on a very localized portion of the cutting tool. Furthermore, the combination of high bearing forces and heat produces cratering action close to the cutting edge, resulting in rapid tool breakdown.
With respect to titanium’s fatigue properties, briefly noted in the above list, the following details are of interest.
As stated, loss of surface integrity must be avoided. If this precaution is not observed, a dramatic loss of mechanical behavior (such as fatigue) can result. Even proper grinding practices using conventional parameters (wheel speed, downfeed, etc.) may result in appreciably lower fatigue strength due to surface damage. The basic fatigue properties of many titanium alloys rely on a favorable compressive surface stress induced by tool action during machining. Electro-mechanical removal of material, producing a stress-free surface, can cause a debit from the customary design fatigue strength properties. (These results are similar when mechanical processes such as grinding are involved, although the reasons are different.)
Titanium is a poor conductor of heat. Heat, generated by the cutting action, does not dissipate quickly. Therefore, most of the heat is concentrated on the cutting edge and the tool face.
Titanium has a strong alloying tendency or chemical reactivity with materials in the cutting tools at tool operating temperatures. This causes galling, welding, and smearing along with rapid destruction of the cutting tool.
Titanium has a relatively low modulus of elasticity, thereby having more “springiness” than steel. Work has a tendency to move away from the cutting tool unless heavy cuts are maintained or proper backup is employed. Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing, and tolerance problems. Rigidity of the entire system is consequently very important, as is the use of sharp, properly shaped cutting tools.
Titanium’s fatigue properties are strongly influenced by a tendency to surface damage if certain machining techniques are used. Care must be exercised to avoid the loss of surface integrity, especially during grinding.
Titanium’s work-hardening characteristics are such that titanium alloys demonstrate a complete absence of built-up edge. Because of the lack of a stationary mass of metal (built-up edge) ahead of the cutting tool, a high shearing angle is formed. This causes a thin chip to contact a relatively small area on the cutting tool face and results in high bearing loads per unit area. The high bearing force, combined with the friction developed by the chip as it rushes over the bearing area, results in a great increase in heat on a very localized portion of the cutting tool. Furthermore, the combination of high bearing forces and heat produces cratering action close to the cutting edge, resulting in rapid tool breakdown.
With respect to titanium’s fatigue properties, briefly noted in the above list, the following details are of interest.
As stated, loss of surface integrity must be avoided. If this precaution is not observed, a dramatic loss of mechanical behavior (such as fatigue) can result. Even proper grinding practices using conventional parameters (wheel speed, downfeed, etc.) may result in appreciably lower fatigue strength due to surface damage. The basic fatigue properties of many titanium alloys rely on a favorable compressive surface stress induced by tool action during machining. Electro-mechanical removal of material, producing a stress-free surface, can cause a debit from the customary design fatigue strength properties. (These results are similar when mechanical processes such as grinding are involved, although the reasons are different.)
For further information, please contact Titan Engineering Pte Ltd, Singapore.
