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.