Triple nickel primer

  • 18-Jul-2011 02:52 EDT
Landing Gear.jpg

Triple nickel forgings are used in highly loaded parts such as landing gear.

Traditional aluminum alloys are being replaced by titanium and composites in many aerospace applications because when aluminum is bonded to composites, aluminum begins to corrode, according to a PowerPoint presentation supplied to Aerospace Engineering by Methods Machine Tools Inc., to which all the information in this article is attributable.

The aerospace industry consumes almost 42% of all titanium produced globally, with double-digit growth expected in the next 10 years. Demand is being driven by commercial and military markets for new-generation aircraft.

A new titanium alloy penetrating the market is Ti-5Al-5V-5Mo-3Cr, or Ti-5-5-5-3 for short (nicknamed “triple nickel” and “triple five”). This is a near-beta titanium alloy offering fatigue and fracture toughness in aircraft structural applications requiring superior tensile strength compared to beta-processed alpha beta titanium alloys.

The ease of forging this material into complex shapes and the ability to heat-treat to above 180 ksi (1240 MPa) ultimate tensile strength makes Ti-5-5-5-3 a logical choice for advanced structural applications vs. the traditional titanium alloys such as Ti-6-4 and Ti-10-2-3.

Low-frequency machining is required due to the nature of the low surface footages that are required. As an example, a 20-mm (0.8-in) end mill would run about 112 rpm. When the tool cuts the material at that rpm, a low frequency is emitted into the machine. Traditional machines do not handle induced low frequency very well; a machine has to be specially designed for these conditions to allow low-frequency machining while eliminating induced vibration to reduce chatter and promote longer tool life.

What is the force that will pull the taper out of the spindle? On a standard machine with a BT/Cat 50 taper, it is 960.4 N·m (708.4 lb·ft). A 178-mm (7-in) long, 25.4-mm (1-in) diameter four-flute cutter will remove about 19.7 cm3 (1.2 in3) of material per minute from 5-5-5-3 titanium before the tool separates from the spindle taper. If you increase your stock removal by increasing your speed, you will exceed 960.4 N·m.

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