The risks of machining titanium and magnesium

Dry machining or machining with oil when milling or grinding titanium and magnesium components can be a high-risk activity.

Approximately 10 per cent of machine tools cooled with non-hydrous coolants catch fire in the first five years.

Kraft and Bauer UK has recently fitted various machining centres, including large Matsura, Mazak, Morei-Seiki and Bridgeport machines, with its range of automatic fire extinguishers.

Louise Boraston, managing director of Kraft and Bauer, said: 'The machining of more exotic materials such as titanium and magnesium is becoming widespread as more subcontractors to the aerospace and medical industries are asked to manufacture parts from these alloys.

'They present real risks of fires occurring on machine tools.' In aerospace applications, titanium is commonly used in aero-engine parts such as rotors, blades, discs, rings and engine casings and airframe components such as nacelle frames, tail sections, landing gears, wing supports and fasteners.

The four engines on the Airbus A380 use about 26 tonnes of titanium.

The Boeing 777 contains 59 tonnes of titanium and the Airbus A380 contains 145 tonnes.

Eighty per cent of titanium produced world-wide is used in the aerospace industry and its use for this purpose is growing.

Titanium is so popular because it is as strong as some steels but is 45 per cent lighter.

It is more than twice as strong as 6061-T6 aluminium alloy.

Titanium's problem is it catches fire easily during machining, causing explosions and damage to expensive production machinery.

During machining, small particles of titanium can be heated to the point of ignition and small piles of these particles will easily burn.

When mixed with oxygen, this burning can quickly extend from one particle to the next and because titanium generates oxygen when burnt, an explosion results.

A tiny amount of titanium powder can cause a catastrophic fire or explosion and any discharge of static electricity will produce a spark that will raise the particles of titanium past their ignition point, causing an explosion.

This is the reason powdered titanium is used in pyrotechnics as a source of bright-burning particles.

Electric switches on machine tools, loose electrical connections and any metal-to-metal contact can produce an explosion-causing spark.

Because titanium is a poor conductor, heat generated when the metal is machined dissipates slowly, meaning the hot spot is usually concentrated right back onto the tool or cutting edge, often resulting in rapid breakdown of the tool.

The use of worn or damaged tooling will quickly generate a great deal of heat and cause the titanium to catch fire.

Because of this, tooling should be watched closely and changed before it starts to wear.

Magnesium is the lightest of the structural metals and is 33 per cent lighter than aluminium and 73 per cent lighter than steel, yet it exhibits the highest strength-to-weight ratio of all structured metals, excluding titanium.

It is used in the aerospace industry and its use is growing in the automotive industry, in parts for steering columns, valve covers and housings and intake blades.

Typical applications in aerospace include main rotor gearbox castings for helicopters, intermediate casings for turbine engines manufactured by Rolls Royce and transmission casings for the F16, F22, Eurofighter and Tornado strike aircraft.

Magnesium alloy forgings are also used in many aerospace applications, including gearbox parts for the Westland Sea King helicopter and various aircraft wheels.

Cutting magnesium alloys is similarly problematic to cutting titanium, as magnesium dust is self-ignitable and the machining processes are so risky that according to EU machine directives, production machines should be equipped with fire detection and protection devices to avoid the risk of damage to machines and injury to operators.

Magnesium powder ignites when heated and burns with an intense white light.

A magnesium fire reacts violently with water, releasing hydrogen which feeds the fire.

It is important chips created during machining are kept as large as possible and are not allowed to build up within the working area: the regular cleaning of machines is crucial, as is the correct storage of magnesium swarf.

Magnesium powder is used in the manufacture of incendiary bombs, fireworks and marine flares where a brilliant white light is needed; this light can be so intense it can permanently damage the retinas of the eye.

Kraft and Bauer sells more than 3,500 machine tool fire prevention systems each year and many of these use argon gas to suppress fires caused by machining titanium or magnesium.

Argon is inert under most conditions, is non-toxic and is environmentally friendly.

Special UV and infra red fire detection systems, with back-up secondary temperature sensors designed by Kraft and Bauer for use on machine tools, can detect fires and heat build-up in less than a tenth of a second.

The argon gas is automatically discharged into the machining zone where it quickly reduces the amount of available oxygen to feed the fire from the ambient atmospheric level of 21 per cent to less than 12 per cent.

At this level there is insufficient oxygen for fires to continue.

The machine tool can be put back into operation as soon as the gas cylinder is changed, which only takes a few minutes.

It is possible to generate some heat and sparks without leading to catastrophic ignition of titanium or magnesium, so Kraft and Bauer has made it possible for end-users to build in a manual delay over-ride period whereby operators can determine if they want the automatic fire suppression system to activate or not.

As argon gas is colourless, odourless and tasteless, if it wasn't for the safety warning lights and sirens, most Kraft and Bauer end users would not even be aware a fire incident had been prevented.

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