Extruded Titanium (used in the Aerospace Industry) Near-net extrusion provides advantages over other manufacturing processes
Submitted by Tony Esposito
Titanium
has long been the material
of choice for aerospace
manufacturers seeking
materials to withstand the
high-temperatures, chemical
corrosion, and mechanical
stress occurring within
military and commercial
aircraft. Typical
applications for titanium
include helicopter parts,
high- temperature engine
components, airframe and
structural parts such as
engine pylons, pylon attach
beams, floor beams,
longerons, vertical
stabilizer spars, and bomb
and missile racks. In a new
aerospace application,
titanium is being used to
produce aircraft seat tracks
by the near-net extrusion
process and finish
machining. This trend away
from aluminum has been
driven by major airlines,
aerospace manufacturers, and
the FAA, concerned with
eliminating corrosion-prone
materials in “wet areas.”
They are discovering that
the higher cost of titanium
can more than offset by
reduced maintenance and
replacement costs because
using titanium makes tack
replacement unnecessary.
Using the near-net extrusion
process also provides
advantages over alternative
manufacturing processes-low
tooling costs, higher
surface quality, the ability
to produce longer parts,
reduced material usage, and
less downstream labor and
finishing expense.
Titanium’s advantages
Maintenance is the primary reason some manufacturers are selecting titanium instead of aluminum for wet-area seat tracks. Despite titanium’s initial higher cost, the material provides excellent corrosion resistance over the aircraft’s life, which eliminates seat track-related maintenance downtime. The material offers other cost-saving advantages, such as high strength-to-weight ratio. Its strength is nearly three times that of aluminum. Consequently, a smaller quantity of titanium is required to achieve the specified strength.
Titanium is available in various grades and alloys to meet demanding aerospace, biomedical, industrial, chemical, and marine applications. It has a high melting point, low density, and low coefficient of thermal expansion-about one-half that of stainless steel and one-third that of aluminum. Additionally, it can withstand constant exposure to salt water, body fluids, fruit juices, and a range of acids, alkalis, and industrial chemicals.
The extrusion process
Complex metal shapes have traditionally been produced using a variety of metal-working processes, including welding, forging, casting, fabricating, brake forming, rolling, or machining from bar or plate stock. However, the near-net extrusion is not a new concept in metal conversion, the manner in which it is applied has advanced with the development of powerful, automated extrusion presses and highly sophisticated control systems.
Typically, metal shapes for extrusion are custom designed to achieve the final profile-within close tolerances-in relatively few operations, resulting in optimum material use, with less waste and less need to downstream machining, welding, or fabrication. In addition, because of extrusion’s faster cycle times, lead times can be shorter, even for high volumes. Tooling for extrusion is relatively inexpensive, unlike some other production processes such as forging, making short runs both feasible and economical.
Near-net extrusion of titanium enables engineers to design solid and hollow metal shapes in a range of lengths and cross sections.
Complex shapes emerge from the extrusion process with superior strength, dimensional control, and a matte surface finish free of alpha case.
The titanium used in the near-net extrusion includes pure and alloyed bar or billets in chemistries designed to deliver the physical and mechanical properties required by the application. A solid round bar, measuring 5,6,7, or 8 inches in diameter, is precision-cut into shorter billets measuring 20 to 28 inches in length. The billets are induction heated to a specific extrusion temperature, dependent upon the alloy. The heated billets are rolled down a conveyor chute and, as they roll towards the press, each billet is coated with a powered glass compound that both lubricates and protects the surface of the material.
At the extrusion press, each billet is hydraulically pushed through a die at pressure of up to 2000 tons, forcing the metal to configure to the die’s profile. The press operates at high speeds, with each extrusion taking just 1.5 to 5 seconds.
The key to close-tolerance extrusions is the quality of the die.
Extrusion dies are severely affected by the intense heat and pressure, resulting in die wash, a kind of interior erosion that can cause dimensional tolerances to shift. This problem is avoided and tolerances are maintained by inserting a new die for each billet.
After the shapes are extruded, downstream operations vary with the material. For example, to resist sensitizing, stainless steel parts require rapid water quenching immediately after extrusion. Carbon steels and alloys are air-cooled, while titanium parts require heat treating to refine their grain structure. Titanium extruded parts are hot stretch-straightened, using air cooled at tension in the stretcher. Other materials are cold stretch-straightened. The stretching process is crucial to minimize the bow and twist of the finished product so it can next properly in a machining fixture.
After stretching, the extrusions are “chem-milled” by immersion in a chemical bath to remove any remains of the glass lubricant as well as the titanium’s alpha case-a thin but tough exterior layer formed during high-temperature heat treating. Removing the alpha case enhances machinability and prolongs tool life.
Before the extruded parts are packaged and shipped, extensive metallurgical testing and dimensional inspection methods are employed to verify that the specified physical, metallurgical, and mechanical properties have been attained, and that the extrusion meets the performance and quality standards of the customer.
Generally, the near-net extrusion process is suitable for shapes that fit within a 6-inch circle and lengths ranging from 30 in. to 35 ft. Minimum possible part thickness is typically 0.188 in; and maximum thickness depends on the alloy and part configuration.
One manufacturer that uses the process is Heizer Aerospace, Inc. The company supplies a range of seat tracks and other components to military and commercial aircraft procedures. More than 60 part configurations, ranging in lengths from 2 ½ in. to over 28 ft, are manufactured from seven near-net extruded shapes in 6A1-4V titanium. To date, the firm has machined more than 60,000 ft of titanium seat tracks from near-net extrusions supplied by Plymouth Extruded Shapes.
When compared with bar or plate stock, near-net extrusion has saved the company significantly on raw material and machining.
The typical seat track cross section currently manufactured for Heizer by Plymouth weights 5.9 lb per linear ft. With machining and other bar stock or plate methods, the cross section of the raw stock would weigh about 13.4 lb-more than twice as much. One of Heizer’s customers saved nearly 4,000 lb of titanium per aircraft on one model alone by using near-net extrusions. Substantial machining savings are also realized, as metal removal requirements are reduced by 60% over the bar or plate stock methods.
The length of the tracks also presents a manufacturing challenge. Alternative manufacturing methods are typically limited to producing lengths to 20 ft. With relatively narrow cross sections and 90% material removal during machining, the results are often excessive bow, twist, and warpage. Near-net extrusions yield straight and flat tracks 35 ft in length or longer, depending on the size and configuration of the cross section. The high quality level results from the stretch-straightening process, proper machining, and the greatly reduced stock removal requirement of near-net extrusions. An additional benefit to the customer is a reduced parts count and consequently, lower number of splices required at installation.
