Titanium alloys have been used in aerospace structural applications for many years due to the metal's high strength-to-weight ratio coupled with an excellent combination of high ductility and fracture toughness, as well as the ability to heat treat it to achieve a wide variety of properties. Despite these advantages, to date practical structural applications have been hindered due to problems associated with fabricating titanium alloys into large shapes.
The primary obstacle in using titanium comes from the metal being so reactive; it will react with carbon to form carbides, oxygen to form oxides, nitrogen to form nitrides, hydrogen to form hydrides and boron to form borides. All these reduce the ductility of the alloy. Hence, typically titanium is melted under vacuum, poured into ingots and worked down to billets, which are later machined or forged. The wrought producer would have to keep these elements down to a minimum, while the fabricator would strive to not add any more of these elements.
Carbon and oxygen have a combined effect on ductility and toughness. A low quantity of one allows a greater amount of the other. Oxygen is the most difficult to control and variations from 0.035 to 0.20% are seen. Carbon today can be held to below 0.10%. Nitrogen can usually be held below 0.05%, when it does not severely affect either strength or the ductility. Hydrogen has an embrittling effect and has to be held below 200 ppm (parts per million).
Recent advancements
Recent technological advancements have helped mitigate the problem of the reactivity of these alloys. Vacuum arc and induction remelting processes allow for the casting of large aerospace structural frames. Hot isostatic pressing (HIP) eliminates internal shrinkage cavities and pores within these castings. Electron beam welding is used to join frame parts creating larger compliant structures with reduced volumes due to the absence of fasteners.
Cast frame parts joined by electron beam welding must be stress relieved prior to use. During normal heat treating or fabrication of titanium alloy parts, a scale and a shallow underlying layer with diffused oxygen and nitrogen develops, which results in increased surface hardness and lower ductility. This layer, called the alpha case, is removed before the part may be used, and is an integral part of the descaling operation. The amount of material to be removed depends on the heating and the processing method used.
In 2004, Elnik Systems constructed a furnace engineered specifically for the processing of large titanium components. The furnace chamber is approx. 90 in. in diameter by 144 in. long (2,286 by 3,657 mm), and built in two pieces so it can be disassembled and transported via truck without special road permits (Fig. 1). The furnace has a useable load area of 40 in. wide by 60 in. high, or 1,016 by 1,524 mm (Fig. 2), and its curved roof and bottom heating elements provide temperature uniformities of better than ±12°F (approx. ±6.5°C) at temperatures between 1350 and 1500°F (730 and 815°C).
The inside of the chamber is stainless steel with a mild steel outer shell and includes the necessary water cooling. The hot zone incorporates several layers of stainless steel heat shields with molybdenum heating elements; the hot zone uses six-zone temperature control top and bottom for each section and each door. Computer controls with six individual SCR zone control, mass flow controller with throttling valve for the partial pressure control, excel spreadsheet profile creation and complete data acquisition complete the system. This combination of individual zone control with curved roof and bottom heaters created the tight temperature uniformity.
Today, the furnace is used primarily for stress relieving titanium alloy aft booms (Fig. 3) for U.S. Air Force F22 fighter jets. A custom loader is used to stage a 2,500 lb (1,136 kg) aft boom into opposite sides of the furnace. The processed parts exhibit no scaling and a minimum of alpha case at levels less than 0.005 in. (0.13 mm) thick. The absence of the scale and thin alpha case translate into dollar savings in the alpha case-removal process and improved tolerances in the finished part.
This minimal alpha case formation is obtained by processing either under high vacuum or at a low partial pressure (typically 50 torr) of flowing argon. This know-how has been acquired from the sintering of metal injection-molded titanium alloy parts in Elnik Systems MIM 3000 furnaces. The use of high vacuum and flowing argon make these furnaces flexible for annealing other specialty alloys.