Metal extrusions are metals or alloys that are extruded into bars, rods, angles, channels, tees, or other profile shapes. Examples of the metals and alloys that can be extruded are aluminum, copper, magnesium, zinc, titanium, lead, tin, and steel.
What is Extrusion?
Extrusion is a process used to create a shape of a fixed cross-sectional profile. Extrusion is done by squeezing metal in a closed cavity through a tool known as an extrusion die using either a mechanical or hydraulic press. Extrusion produces compressive and shear forces in the stock. No tensile is produced which makes high deformation possible without tearing the metal. The cavity in which the raw material is contained is lined with a wear resistant material. This can withstand the high radial loads that are created when the material is pushed through the die.
Types of Extrusion
There are various types of extrusion methods, but all can be classified as being done as either a cold process or hot process.
- Cold extrusion is extrusion done at room temperature or slightly elevated temperatures. This process can be used for most materials, provided the tooling is robust enough to withstand the stresses created by extrusion. Examples of parts that are cold extruded are collapsible tubes, aluminum cans, cylinders, and gear blanks. The advantages of cold extrusion are higher strength due to cold working, closer tolerances, good surface finish, and the lack of oxidation.
- Hot extrusion is done at fairly high temperatures, approximately 50 to 75 % of the melting point of the metal. The pressures can range from 35-700 MPa (5076 - 101,525 psi). Good lubrication is necessary to maintain die life due to temperature and pressure wear on the machinery. Oil and graphite work at lower temperatures, whereas at higher temperatures glass powder is used. The biggest disadvantage of this process is its cost for machinery and its upkeep. Typical parts produced by hot extrusion are trim parts used in automotive and construction applications, window frame members, railings, and aircraft structural parts.
Other specific extrusion processes include hydrostatic extrusion, which incorporates a pressurized liquid surrounding the workpiece to reduce force requirements, and impact extrusion, which uses a high velocity metal slug to force the workpiece into the die.
Advantages of Extrusion
Extrusion is a near-net-shape process, meaning it eliminates scrap and waste metal generated from machine processes which cut material from a larger block. Less waste means lower material costs and no time or money used for machining and disposal of the waste metal.
Extrusion often minimizes the need for secondary machining, but products are not of the same dimensional accuracy or surface finish as machined parts. Surface finish for steel is 3 µm (125 µ in), and is 0.8 µm (30 µ in) for aluminum and magnesium.
However, this process can produce a wide variety of cross-sections that are hard to produce cost-effectively using other methods. The extrusion process is generally economical when producing between several kilograms (pounds) and many tons, depending on the material being extruded. There is a crossover point where roll forming becomes more economical. For instance, some steels become more economical to roll if producing more than 20,000 kg (50,000 lb).
Extrusion provides the ability to make clad materials (multi-layered metals) through co-extrusion of the two layers. Metal matrix composites can also be formed by extruding and compacting or canning powdered metals or ceramic particulates.
The metal extrusion process can be used to make complex profiles for a variety of industries. Metals and alloys can be extruded into round or oval tube shapes, hollow square tube shapes, channel shapes, Zee or Z shapes, Tee or T shapes, H or I beam shapes, L shapes, and other structural shapes. Finned shapes and finned tube shapes are made for heat sink, heat exchanger and other thermal dissipation applications using aluminum, copper, or high thermal conductivity alloys. Aluminum sailboat masts and booms are often made using an aluminum extrusion process from marine grade aluminum alloys.
Selecting an extruded shape can be based on a number of different factors, including design specifications, load requirements, or material types. The GlobalSpec SpecSearch Database allows the user to specify the desired product based on size dimensions such as overall thickness and overall width or outer diameter (OD), mechanical properties such as yield strength and tensile strength, or material types such as aluminum and copper.