Anyone who has machined the super-material titanium knows it can be something of a trouble-maker if not handled properly. Chips do not break, heat does not dissipate, edges build up – these are the difficulties that titanium creates when machined. On the uptick, titanium has outstanding properties that make it a hot favourite in aviation, motor racing and medical engineering, so it is well worthwhile amassing some know-how beforehand. Article by ARNO Werkzeuge.
The history books make no mention whether the chemist Heinrich Klapproth named the element titanium after the deity of Greek mythology because of its divine properties. The fact is, however, its properties make it into a super-material. Titanium combines properties such as an extremely high tensile strength, light weight and outstanding corrosion resistance – but these cause conflicts with other materials or alloys. As titanium is also anti-magnetic, biocompatible and resistant even to the most aggressive media, the expensive material is gaining favour in an increasingly greater number of sectors and applications. Engineers at Bugatti know this very well since they use a lot of titanium in their supercars.
Titanium is Expensive So Scrap Must Be Avoided
Anyone wanting to machine titanium must first invest a lot of money as it costs about three to five times more than tool steel. So, it is obvious you would want to avoid scrap. But the choice of material alone is not enough. The proper tools are needed to machine the precision turned parts made of titanium required in the aerospace industry, chemical industry, vehicle construction or medical technology. This is the only way to bring even obstinate titanium alloys into the desired shape.
These are the special attributes of titanium that make life hard for tools:
- Extremely poor thermal conductivity
- Non breaking chips
- Extreme tendency to stick to the flute
- Low modulus of elasticity
(Ti6Al4V = 110 kN/mm2, steel Ck45 = 210 kN/mm2)
As only the very few are likely to find themselves in the awkward situation of producing titanium screws for the 1500 hp Bugatti Chiron super sports car, let’s first look at the production of a threaded shaft with recess made of the common titanium alloy Ti6Al4V Grade 5/23 as used in medical technology. Its tensile strength of Rm = 990 N/mm2, yield stress of Re = 880 N/mm2, hardness HV between 330 and 380 and elongation factor A5D of about 18 percent make it ideal for use in implants in medical technology and for applications in aviation (3.7164) or industry (3.7165). The alloy contains six percent aluminium, four percent vanadium and ELI (extra low interstitials), giving it very good biocompatibility and practically no known allergic reactions.
Heat Must Be Extracted From the Cutting Zone
The requirements call for a high surface quality, reproducible process reliability and controlled chip evacuation – all this including short process times and possibly a high chip removal rate. If you expect most of the heat generated during turning is normally dissipated through the chip, you are in for your first big surprise: titanium is a very poor conductor of heat and heat is not dissipated when the chip is removed from the cutting zone. In addition, at temperatures of over 1200 deg C prevailing in the cutting zone, the cutting tool is very quick to “burn”. Immediate help is provided by introducing measures such as feeding coolant directly to the cutting zone, reducing cutting force by using a sharp flute and adapting the cutting speed to the process.
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