07/11/2007
Titanium – working an intractable material
The new carbide grade CTP5240 from CERATIZIT increases productivity when machining titanium alloys in the aerospace industry. |
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Technical article by Peter Uttenthaler, Business Segment Manager ’Energy & Transport’ / Cutting Tools and Dr. Uwe Schleinkofer, head of the Development Department Cutting Tools at CERATIZIT
We deal with titanium every day. A white painted house wall contains titanium as purified rutile (TiO2). In the field of medical systems titanium has become indispensable for joints and tooth implants. And the aerospace industry now counts on titanium for primary structures which in the past were made of steel. But titanium is an intractable material: it is difficult to machine it efficiently. The new carbide grade CTP5240 from CERATIZIT provides higher productivity combined with lower wear and higher cutting speeds.
Light and shade
Titanium is a chemical element which occurs in the crust of the earth. It has the atomic number 22 and belongs to group 4 of the periodic table.
Titanium alloys have properties that are useful for many applications:
- Strength similar to steel with half the density
- High elasticity and strength
- Low thermal conductivity:
o No embrittlement at extremely low temperatures
o Good creep resistance at high temperatures
These properties however also give some disadvantages when it comes to the machining of titanium alloys:
- Difficult to machine
- High machining and tooling costs
- Extremely expensive material (at the moment at least 25 times more expensive than aluminium)
Titanium is a demanding material
Titanium alloys are very bad thermal conductors: their thermal conductivity is ten times less than steel. For cutting tools this means that 75% of the heat produced during the process goes into the tool and is not evacuated with the swarf. In order to withstand this, a heat resistant carbide substrate and efficient cooling are needed during the machining process. This leads to the use of large quantities of coolant, preferably all with high pressure all through the spindle, directly onto the cutting edges of the tool. For this reason a tool provided with internal cooling is the first choice when machining titanium alloys.
A further consequence of the bad thermal conductivity of titanium alloys is the high temperature at the cutting edge. It leads to chemical reactions such as oxidation and diffusion on the surface of the tool’s cutting edge; problems where CT offers a solution with the new grade CTP5240.
Resistance to machining
Titanium is a polymorphic material. After solidification at 1,668°C it is characterised by a cubic body, a centred lattice and it shears into a hexagonal lattice structure at 882°C. Nevertheless titanium shows very good formability below this temperature as numerous glide and twinning planes are present in the crystal lattice.
In cold forming titanium tends to strong hardening while tensile strength triples and ultimate elongation is reduced up to 90%. The tendency to hardening represents significant resistance to the machining process. The cutting edges easily chip or the cutting material may be shattered. Positive sharp cutting edges help to a certain extent by reducing cutting forces but the cutting edge must not be too positive however as it would then be too weak for this application area.
The high elasticity of the material causes the titanium to ‘relax’ immediately after the cutting operation. This creates particular demands on the geometric position of the cutting edge. Result: it is really not easy to machine titanium!
CERATIZIT develops a new carbide grade for the machining of titanium
Many of the components utilised in the aerospace industry are forged titanium alloys. These components show irregular hardness on the surface and thus provide unpredictable stress for the inserts applied in machining. CERATIZIT solves this complex task with a special heat resistant coating, using its HyperCoat coating technology, which has been especially adapted for this application area, combined with an innovative carbide substrate. This new carbide grade is called CTP5240 and is in ISO classes P and M 35.
Highly efficient: the new grade CTP5240
The new grade CTP5240 is an extremely heat resistant carbide substrate with medium grain size which combines high wear resistance with sufficient toughness and very high heat resistance.
The coating is adapted to titanium alloys and shows a clearly reduced tendency to chemical reactions like oxidation and diffusion with the work piece material. It is characterised by excellent tribological properties and high thermal stability combined with extraordinarily high hardness.
In addition to the very high hardness this coating acts as a highly efficient heat shield which protects the carbide substrate against early wear at high cutting speeds. In addition the coating is subject to a special surface treatment which leads to a very smooth rake face and considerably reduces the friction coefficient during machining. The swarf can glide along the rake face considerably more easily.
Highly positive design of the geometry
The cutting edge geometry of grade CTP5240 from CERATIZIT is provided with a highly positive design which resembles their aluminium geometry -27. This geometry allows machining with very low cutting forces and pressures. In this way the process temperature can be maintained between 200 and 250°C for a longer period of time, which is considerably below the level possible until now for titanium alloys.
Other positive effects of our cutting edge geometry are the very good chip formation and the efficient chip evacuation. Thanks to the good chip formation this cutting edge geometry supports swarf evacuation as the swarf form is more compact. A litre of removed material has now notably less volume than previously.
Cutting speeds and feed rates
With this geometry and grade combination CERATIZIT has already achieved 120 m of tool life in rough machining operations at a feed per tooth of 0.12 mm and 6 mm of axial cutting depth.
Application examples from the aerospace industry
Heavy duty structural component Ti 5.5.5.3 (TiAl5V5Mo5Cr3)
Helical flute end mill ANFT.80.R.05-19-A32-60
Vc = 30 m/min
fz = 0.1 mm/rev
ap = 18 mm
ae = 5 mm
Tool life > 100 min
Airframe component TiAl6V4
Button insert cutter C251.32.R.04-10-AR70
Vc = 65 m/min
fz = 0.21 mm/rev
ap = 1.0 mm
ae = 32 mm
Tool life > 70 min
Quotation:
“Applying CERATIZIT HyperCoat CTP5240 inserts we have been able to considerably increase our productivity and process security."
Michele Casarin, production manager, Tibeni group, Italy.
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Specials
