Painstaking selection of materials, processes is necessary for aircraft applications
June 13, 2006
Senior Editor Eric Lundin visited a fabricator that specializes in aircraft components, M-DOT Aerospace, to learn how the company uses warm-forming of titanium to manufacture a cradle for an auxiliary power unit, or APU. Understanding titanium's characteristics is the key in forming this durable, corrosion-resistant, tough material.
|Although an auxiliary power unit cradle looks simple to manufacture, it is a complex component. The base is made of titanium parts heated to more than 800 degrees F before they are formed, and assembly requires 284 holes for the rivets.|
There is no question that steel is king in the metal fabrication industry. Fabricators know its characteristics and how to work with it—how to cut it, bend it, punch it, weld it, and coat it. Strong and durable, steel is the benchmark against which other metals are measured.
One of steel's drawbacks is its density. At 7.8 grams per cubic centimeter (g/cm3), it is not well-suited for many applications in the aerospace industry, where every ounce counts.
A chief alternative to the king, aluminum is favored for aerospace applications for its corrosion resistance, favorable strength-to-weight ratio, and its density. At 2.7 g/cm3, it has just 34 percent the density of steel.
It's main drawback is its strength. For instance, AISI 1080 carbon steel has a tensile strength of 140,000 pounds per square inch (PSI) in the as-rolled condition. The tensile strength of 2024, an aluminum alloy used for many structural applications in aircraft, is approximately half that at 72,000 PSI, depending on the temper.
Enter the 22nd element on the periodic table, titanium. Titanium has many of the same characteristics that make aluminum attractive: corrosion resistance, low density, and high strength-to-weight ratio. Like aluminum, it is less dense than steel at 4.5 (g/cm3). The big difference, however, is in its strength. The tensile strength of titanium alloy Ti-6Al-4V, when annealed, is better than that of 1080 steel—144,000 PSI.
The main drawback with all of these metals is their limited elongation, which determines the amount the material can bend before it cracks. AISI 1080 carbon steel has approximately 12 percent elongation at room temperature; 2024 has 13 percent; annealed Ti-6Al-4V is only slightly better with 14 percent elongation.
The main tool for working around this drawback is heat. For instance, elevating 2024 to 400 degrees F increases its elongation to 23 percent to 27 percent, depending on the temper. It doesn't stop there—heating it to 700 degrees F boosts the elongation to 100 percent, regardless of the temper. Elevating Ti-6Al-4V also increases its elongation. At 800 degrees F, it can elongate approximately 22 percent.
While titanium tubing gets much of the attention, titanium is used for many other aircraft parts. One such component is a cradle that holds an aircraft's auxiliary power unit, or APU. Powered by a small turbine, an APU provides compressed air to start the aircraft's engines. It also provides electrical power and air conditioning while the aircraft is on the ground; depending on the design, an APU can provide hydraulic power also.
M-DOT Aerospace Inc., Phoenix, makes an APU pallet cradle, a frame that secures the APU to the aircraft. It manufactures the main components from Ti-6Al-4V in sheet and plate forms. Sheet components make up the cradle's base; plate makes up the arms (see lead photo).
Although it doesn't look all that complicated—it consists of just a few main components and some fasteners—manufacturing the parts and assembling them are intensely painstaking processes.
The main challenge in manufacturing APU cradles is bending the titanium sheet components to tight-radius, 90-degree bends. To increase the alloy's elongation, M-DOT heats the titanium to more than 800 degrees F and, while the workpiece is red-hot, removes it from the oven and bends it in a press brake.
After being descaled, all the sections require holes, and lots of them. Each cradle requires 284 holes for 284 rivets that hold all the pieces together. Hole diameter and location are critical; the hole tolerance is ± 0.010 in. Six other holes require even tighter tolerances at ± 0.002 in. These holes are made in two operations to meet this specification. The holes are drilled undersized, then reamed to the correct diameter.
Finally, a sealant is applied to each of the rivets and the 284 rivet holes to prevent galvanic corrosion, and the entire cradle is assembled.
Manufacturing an APU cradle, just one small aircraft component, is a meticulous and thorough process and illustrates the many difficulties aircraft manufacturers and their suppliers face.
Looking at material costs reveals another reason that working with titanium is not for the faint-hearted. Unlike steel, which is priced by the ton, titanium is priced by the pound. Although the price of cold-rolled steel coil has come down from its peak of $800 per ton in November 2004, it was still pricey at around $650 per ton early in 2006. The cost of titanium is something else altogether. M-DOT pays approximately $600 for a 17-pound billet of Ti-6Al-4V titanium. This is the equivalent of about $70,600 per ton.
Machining one billet yields the two upper components of the APU cradle (the arms that span it), which weigh 1 lb. apiece. The rest of the billet, or 88 percent of it, becomes scrap.
Michael Bauccio, ed., ASM Metals Reference Book, Third Edition (Materials Park, Ohio: ASM International), pp. 306, 414, 512.
George S. Brady, Henry R. Clauser, and John A. Vaccari, Materials Handbook, 14th Edition (New York City: McGraw-Hill, 1997), p. 905.
J.R. Davis, ed., ASM Specialty Handbook®, Aluminum and Aluminum Alloys (Materials Park, Ohio: ASM International), p. 78.
Matthew J. Donachie Jr., ed., Titanium, a Technical Guide (Metals Park, Ohio: ASM International), p. 186.