Putting the squeeze on metals

March 6, 1997
Manufacturing metal components by extrusion slashes manufacturing steps and material costs by producing near-net shapes
Engineering Manager
Plymouth Extruded Shapes, div. of Plymouth Tube Co.
Hopkinsville, Ky.

Edited by David S. Hotter

Cut costs and reduce manufacturing time. These production demands are all too familiar to engineers. For manufacturers this means finding ways to reduce material requirements, production time, labor costs, and manufacturing steps.

Material and process selection are key to meeting these demands. With metals, in particular, near-net extrusion technology provides a solution to those specifying highstrength solid or hollow shapes.

Extrusions offer many options to designers. For example, the process can form valve bodies in a wide variety of shapes and sizes, using fewer parts, less material, and minimal machining. And complex architectural shapes such as elevator sills, decorative railings, marble and granite hangers, and window frames are extruded with high strength and a smooth, matte finish.

In short, near-net extrusions maximize product quality, streamline production, and solve the challenges of forming unusually shaped components.

More from less
Applications for near-net shape extrusions have expanded to several industries, including automotive, electronics, and medical instrumentation, as well as construction and machine tools. Although extruding metal is not new, recent developments such as sophisticated controls and more powerful presses boost the precision of parts. Advanced metallurgical test equipment and methods also help parts meet stringent performance and quality standards.

The process works well with a wide range of metals, including carbon steel, alloy steel, stainless steels, nickel alloy, and titanium. Engineers customize dies to final-shape profiles within close tolerances using few operations. This saves costs by optimizing material use and reducing postprocessing operations such as machining, welding, or fabrication.

Unlike other processes, extrusion tooling is relatively inexpensive, making short runs feasible and economical. And product lead times are typically shorter because of faster cycle times.

A closer look
The extrusion process starts with high-quality forged and turned steel bar as raw material, with a chemistry tailored to meet required mechanical properties. The process begins with a solid round bar of steel, 5, 6, 7, or 8 in. in diameter, which is cut it into short billets — 20 to 28 in. long — using precision machines. Billets are then induction heated to temperatures ranging from 2,100 to 2,350°F. Heated billets roll down a conveyor chute through a powdered glass compound that lubricates and protects the surface of the steel.

At the extrusion press, a 2,000- ton hydraulic ram squeezes each billet through a die. The high-speed ram processes billets in just 1.5 to 5 sec, depending on press temperature. With such high forces, die wear can affect product quality and shift tolerances. High temperatures and pressures produce die wash, an interior erosion of the die surface. One way to avoid the effects of die wash is to insert a new die after each extrusion.

Downstream operations vary depending on the material and component. Stainless steels require a rapid water quenching immediately after extrusion. Carbon steels and alloy steels, on the other hand, air cool, while titanium parts require heat treating at 1,350°F to refine the grain structure for optimal strength and durability.

Titanium shapes also require an additional step of stretch-straightening. The process calls for resistance-induction heating to bring profiles up to 1,350°F, straightening the shaped bars, and cooling them in air under tension until the material reaches 200 to 300°F. The procedure controls the bow and twist of the finished components. After stretching, titanium extrusions receive a chemical bath to clean their surface and remove the alpha case — a thin but tough exterior skin that forms at high temperatures. Removing alpha case before downstream machining increases tool life. In contrast, stainless steels and alloys are cold stretch-straightened to control

Flying high on extrusions

Military and commercial aircraft manufacturers were among the first to recognize the value and benefits of near-net extrusions for high-strength structural components. Calcor Space Facility Inc., Whittier, Calif., replaced machining operations with near-net extrusions to cut costs and meet stringent quality and performance requirements. The company manufactures nuclear securitysystem assemblies, space-vehicle components, and structural members such as engine pylons, thrust reversers, and exhaust nozzles.

Calcor’s products include T and L-shaped reinforcing members used throughout the inside of aircraft pylons — assemblies that attach engines to the fuselage. These parts are 2 to 4 in. wide and 5 in. to 6 ft long. When machined, long, narrow rails are prone to twisting and warpage which makes it difficult to perform downstream machining such as drilling holes. In the past, Calcor was often forced to send parts out for straightening. Surface imperfections such as pitting and scale also caused problems.

Since converting to extrusions, engineers increased straightness, dimensional consistency, and surface quality of the pylon reinforcements. Company engineers say converting from machining methods to near-net extrusion for selected parts slashed manufacturing costs by minimizing production time, labor costs, and material waste. Using near-net extrusion cuts machining down to precision milling operations, removing less than 0.25 in. which is significant for expensive materials such as titanium.

Getting the most out of extrusions

Near-net extrusions are suitable for shapes which fit into a 6-in. circle and lengths from 30 in. to 35 ft. Minimum part thickness is typically 0.187 in., and maximum thickness depends on the alloy and part geometry. The following design criteria must also be considered:

• Avoid knife edges to promote uniform flow through the die.
• Typical corner radii measure 0.0625 in. and fillet radii 0.1875 in.
• For grooves, keep the depth-towidth ratio close to 1:1.
• Eliminate interface concerns by designing a shape capable of production in a single extrusion, rather than assembling several components.
• Innovative die designs, such as multiple port dies, expand process capabilities.

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