This article is an investigation of the aluminum extrusion process and its basic elements.
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Extruded aluminum is a metal shaping process that involves pushing a preheated or cold aluminum billet through a die with a specific cross-sectional shape. This method manufactures aluminum components starting from a two-dimensional profile with required width, height, and initial thickness. The profile's third dimension is determined during post-extrusion processing, which includes pulling the profile onto a runout table and cutting it into desired lengths.
The two ways that aluminum can be extruded are hot heading and cold heading. With hot heading, the billet is heated to approximately 25% below its melting temperature. With cold heading, the billet is extruded at room temperature. Both methods of extrusion have their benefits. Hot heading is a faster process, while cold heading produces more durable and long-lasting products.
Extruded aluminum is known for its cost-effectiveness in manufacturing processes. It can produce products with exceptional tolerances, precision, and accuracy. Each extruded profile maintains consistent dimensions from the first part to the last, ensuring uniformity in length and measurements.
Aluminum is valued for its lightweight nature, high strength, and resistance to corrosion. In its pure form, aluminum is soft and relatively weak. To enhance its mechanical properties, it is commonly alloyed with elements like copper, magnesium, manganese, and silicon. Additionally, aluminum can undergo heat treatment to achieve optimal strength and ductility.
The development of metal extrusion dates back to the late 1700s when the first patent for the process was filed by Joseph Bramah, an English inventor and locksmith. Initially used to create lead pipes and sheathing for cables, early extrusion processes were suitable for soft metals.
However, the extrusion of harder and stronger metals, such as aluminum, required higher temperatures and pressures. It wasn't until Alexander Dick introduced the hot extrusion process in 1894 that aluminum became a viable material for extrusion.
Today, aluminum extrusion enjoys widespread use, with a global market size exceeding $67 billion. The market is projected to grow at an annual rate of 3.8% from 2020 to 2027. Extruded aluminum finds primary applications in building and construction, automotive and transportation, consumer goods, and electrical and energy sectors.
Aluminum is the most widely used metal for extrusion forming due to its unique combination of mechanical properties, including high strength, low density, and excellent workability. These qualities remain consistent across various temperatures. Additionally, desirable properties like electrical conductivity, reflectance, and paramagnetism further expand its range of applications.
High Strength-to-Weight Ratio: Aluminum is widely popular due to its light weight and high strength. Its density is about one-third that of steel. Depending on the grade, aluminum alloys are stronger than steel by up to a factor of five. Because of this property, aluminum is widely used in aerospace and automotive applications.
Corrosion Resistance: Aluminum has an inherently high corrosion resistance compared to most metals. This is attributed to its tendency to form compact layers of oxides on its surface. This makes aluminum suitable for outdoor applications after a coating has been added.
Electrical Conductivity: The electrical conductivity of aluminum is around 61% that of copper. It is preferred over copper for certain applications due to its lower density and cheaper cost. An application that takes advantage of these properties is low-cost power transmission lines.
Thermal Conductivity: Aluminum conducts heat twice that of brass and four times that of steel. This makes aluminum extensively used in heat sink applications in electronics and electrical components.
Resilience and Impact Strength: Aluminum has high resilience and impact strength because of its natural toughness. Aluminum parts can absorb sudden forces or shocks and can elastically flex from dynamic loads.
Reflectance: Aluminum has the highest reflectance ofany metal in the 200 to 400 nm range, much better than gold and silver. Aluminum film coating is commonly applied on the glass to make mirrors instead of silver. Depending on its finish, aluminum can reflect about 90% of light across the visible spectrum wavelengths.
The extrusion process, when combined with the properties of aluminum, offers distinct advantages that benefit both manufacturers and end-users. This process excels in producing parts with intricate cross-sections and can handle brittle materials that are challenging for other forming processes.
Ease of creating complex cross-sections. Complex parts can be made provided that it has the same cross-section throughout its length. The increment in operating cost for producing more complex parts is minimal compared to with other forming processes.
Seamless hollow parts can be formed: Hollow profiles can be made by extruding the aluminum through combinations of dies and mandrels. These profiles do not require mechanical joints or welded seams that are potential weak points for the product.
Good surface finish. Secondary operations can be integrated easily to create various kinds of finishes. The product's surface can be buffed or polished to achieve a mirror-like surface or brushed for a matte finish. Aside from polishing, aluminum extrusions can also be anodized, painted, powder-coated, electroplated, or laminated.
The standard finish is mill finishing, where the product is deburred and cleaned and made ready for anodizing, powder coating, or other secondary surface finishing. Sandblasting is a common surface treatment completed before anodizing. The types of anodizing finishes include clear and black, which are the most common anodizing finishes, with custom colors also available.
Anodizing is a finish that combines to the underlying aluminum with unmatched adhesion. The finishes are chemically stable, non-toxic; and heat-resistant. The thickness of anodizing is 6 μm to 18 μm and comes in black or clear, with customized colors available. The limitation of the anodizing process is the length of the extrusion since long extruded aluminum will not fit in the anodizing tank. Electrostatic spray coatings are also used to coat extruded aluminum and produce the highest quality products in a variety of colors.
Extrusion is classified as a bulk-forming process that involves a substantial change in the surface-to-volume ratio of the metal being formed. This process uses compressive forces applied to the billet using rams, punches, tools, and dies. The plastic theory is employed to ensure the production of the desired profile with a predictable grain structure, governing the mechanics of the metal's plastic deformation.
The characteristics of the formed product depend significantly on several variables, with extrusion pressure being a primary factor. This pressure is influenced by other parameters including temperature, extrusion ratio, and extrusion speed. The key extrusion variables are listed below:
Type of extrusion process: The two major categories are direct and indirect. Direct extrusion is a process where the ram travel and metal flow are in the same direction, while indirect extrusion is the opposite. Each process has its advantages and disadvantages. Other types of aluminum extrusion technologies have been developed such as hydrostatic and impact extrusion.
Die Type and Design: Dies are the components that deform the metal. Die design determines the mechanical working of the metal as it is being extruded. Extrusion dies can be solid, semi-hollow, or hollow dies.
The most widely used alloy for extrusion is the 6xxx series alloy.
Temperature: Aluminum extrusion is usually carried out in elevated temperatures, known as hot extrusion. High temperatures enhance metal flow producing extrudates without defects. However, the downside of using high temperatures is the increased rate of oxidation. Aluminum hot extrusion can range from 705 ° F to 932 ° F (375°C to 500°C).
Extrusion Ratio: This is defined as the ratio between the billet and the die opening cross-sectional areas. A larger extrusion ratio means larger deformation. This requires higher extrusion pressures. Moreover, higher deformation results in higher exit temperatures.
The manufacturing of aluminum extrusions begins with the production of aluminum billets, which are fed into the extrusion machine. Aluminum is refined from bauxite to produce alumina, or aluminum oxide. Oxygen is then separated from the aluminum through a reduction process to obtain pure aluminum. The virgin aluminum is smelted into ingots, which are used to create billets along with recycled aluminum.
Aluminum billets are supplied to manufacturing plants for creating end-use parts through various metal forming processes. In aluminum extrusion, the billet is typically heated to increase the metal's plasticity. The heated billet is then placed into a cylindrical chamber equipped with a ram on one end and a die on the other. The ram, powered either mechanically or hydraulically, applies sufficient compressive force to plastically deform the billet, forcing it to flow through the die. The setup may vary depending on the specific type of extrusion process employed.
Aluminum extrusions can be produced through different processes. These can be categorized according to the method of applying pressure to the billet, as summarized below.
As described by the graph, at the start of the extrusion, the required pressure starts to increase rapidly to its peak value known as the breakthrough pressure. Once the flow is initiated, the pressure decreases, and steady-state extrusion proceeds. When the loaded billet is almost consumed, the extrusion pressure reaches a minimum value, followed by a sharp rise as the remaining is compacted. The remaining billet that is not extruded is called the butt or discard, which is 5 to 15% of the billet.
Indirect Extrusion: In contrast with the direct extrusion process, instead of pressing the billet against the die, the die is pressed against the billet. A hollow ram is attached to the die, which compresses the aluminum billet, forcing it to flow. The direction of metal flow is opposite to the direction of ram travel. Regarding the generated frictional force, since there is no relative displacement between the billet and the chamber, there is no friction between the billet and the extruder chamber. The effect of the absence of this initial frictional force is described by the pressure-displacement curve shown in the image below.
As seen in the graph, the required pressure only rises to the steady-state extrusion pressure. Indirect extrusion proves to be a more energy-efficient process than direct extrusion. Despite this advantage, indirect extrusion fails to be a replacement for direct extrusion. This is because of the requirement to use a hollow ram which is weaker compared to a solid press. This limits the loads that can be applied to compress the billet. Hence, this process is only applicable for producing extrudates with small cross-sections.
Hydrostatic Extrusion: This process involves the use of a working fluid to force the billet through the die. In this process, the working fluid is compressed inside a sealed chamber that completely surrounds the billet, except at the tapered end, which is initially fitted to the die opening. The pressurization can be achieved by either pressing the fluid with a ram or plunger or by pumping more fluid inside the chamber. The former is known as constant-rate extrusion, while the latter is constant-pressure extrusion. Oil is typically used as the working fluid, with modified properties to resist degradation from high temperatures due to the heat from forming and compression.
Hydrostatic extrusion offers the best of both worlds from direct and indirect extrusion. This process solves the problem of the high frictional forces experienced in direct extrusion and the limitation on the cross-sectional area of the indirect process. However, they also suffer from disadvantages such as lower throughput due to the longer preparation per extrusion cycle and sealing difficulties at high pressures. Lower throughput is the consequence of the additional billet tapering process and the necessary injection and removal of fluid for every cycle. In some setups, instead of removing the fluid, the discard is retained to prevent the sudden release of the extrusion fluid. This discard is usually tougher due to cold working and will require additional compression to extrude. Sealing difficulties are from the tighter seal between the chamber and ram and the seal between the billet and die.
Cold Extrusion: In contrast with hot extrusion, cold extrusion is done below recrystallization temperatures, typically at room temperatures. The metal is initially at room temperature. As it is being compressed, heat is generated from the continuous deformation. The advantages of cold extrusion are superior hardness and strength, lower oxidation, better surface finish, and closer tolerances.
Enumerated below are the classifications of the extrusion process according to the direction of metal flow relative to the motion of the ram.
Lateral Extrusion: In a lateral extrusion process, the ram is oriented vertically while the extrudate flows horizontally. This is basically a modification of the forward extrusion process to save space or improve the pressurizing efficiency of the ram.
The aluminum extrusion process is inherently complex. Fortunately, there are numerous manufacturers of extruded aluminum equipment in the United States and Canada who have essentially perfected this process. Below, we highlight several of these manufacturers and their machines.
The SMS SmartExtruder is a technologically advanced extrusion machine manufactured by SMS Group. It is designed to enhance productivity and efficiency in aluminum extrusion. This model features intelligent control systems, energy-efficient operation, precise temperature control, and advanced automation capabilities. Known for its versatility, the SmartExtruder can handle a wide range of extrusion profiles while ensuring consistent quality.
The Presezzi Extrusion Press Series 7 is renowned for its advanced design and high precision in aluminum extrusion. It incorporates state-of-the-art automation, superior control over the extrusion process, and energy-efficient performance. This model offers versatility in handling various extrusion profiles and alloys, enabling reliable and high-quality production of extruded aluminum.
The UBE Aluminum Extrusion Press by UBE Machinery is celebrated for its advanced technology and superior performance in aluminum extrusion. It features precision control systems, efficient energy utilization, and high-speed capabilities. This model offers versatility in handling different aluminum alloys and extrusion profiles, ensuring precision and reliability throughout the extrusion process.
HPM (Hamilton Plastic Molding) manufactures the HPM Aluminum Extrusion Press, designed for high productivity and precision in aluminum extrusion. These presses feature advanced control systems, efficient energy consumption, and the capability to handle a wide range of extrusion profiles and alloys. HPM presses are known for their robust construction and reliability in industrial applications.
SMS Elotherm specializes in induction heating systems for aluminum extrusion processes. Their Induction Billet Heating Systems provide precise and efficient heating of aluminum billets. Incorporating advanced induction technology, these systems offer precise temperature control and energy-efficient operation. SMS Elotherm systems are designed to achieve uniform heating and optimal billet malleability, ensuring high-quality extrusions.
Please note that specific model availability may vary, and for the latest information on models and features, it's recommended to contact the manufacturers directly or consult their product catalogs.
In the design of aluminum products, choosing the right alloy is crucial for creating high-quality extruded aluminum. Alloy selection determines many aspects of the final product, especially its strength and durability. Heat treatment is applicable primarily to heat-treatable alloys, enhancing their mechanical properties.
Tempering is a process that involves mechanical, chemical, or thermal treatment of extruded aluminum. It can include softening or annealing, cold working, or spring tempering. There are five forms of tempering for aluminum, each of which is designated by the letters F, O, H, W, and T, with certain tempers having subdivisions. The various letters of the tempering designations refer to the potential physical properties that can be achieved.
Aluminum alloys in series 1xxx, 3xxx, and 5xxx are not heat treatable, while series 2xxx, 6xxx, and 7xxx are heat treatable. Series 4xxx comprises both heat treatable and non-heat-treatable alloys. The strength of non-heat-treatable alloys depends on their inherent properties and cold working. The chemical composition and metallurgical structure of alloy groups dictate their fabrication characteristics.
All aluminum products are distinguished by their properties, alloy, and temper designation. Understanding these aspects is crucial when selecting the appropriate aluminum alloy for producing an extruded aluminum profile. While various processes play a significant role in the selection process, the type of aluminum alloy and its temper designation are fundamental factors that must be carefully considered.
Alloy designations consist of four-digit numbers that denote the alloy's chemistry. The first digit indicates the primary alloying element, such as copper, manganese, silicon, or zinc. Pure aluminum, which starts the series, is denoted by the number 1. The second digit signifies modifications to one of the alloying elements. The last two digits, numbers 3 and 4, identify the specific alloy within the series, except for the 1xxx series where the last two digits indicate aluminum content between 99% and 100%.
| Aluminum Numbering System as Established by the Aluminum Association | |
|---|---|
| 1xxx | Pure Aluminum |
| 2xxx | Copper |
| 3xxx | Manganese |
| 4xxx | Silicon |
| 5xxx | Magnesium |
| 6xxx | Magnesium and Silicon |
| 7xxx | Zinc |
| 8xxx | Other |
Temper identifications are alphanumeric and are added with a dash after the alloy series number. The letter in the temper designation describes the mechanical and thermal treatments applied to the alloy. Each letter in the temper designation indicates a specific class of treatment.
The main purpose of tempering is to enable designers to achieve desired mechanical properties. The strength of an aluminum alloy can be significantly enhanced from a few thousand to several through the use of tempering. This increased strength is possible using a combination of solution heat treatment and artificial aging. Tempering can also change an alloy's characteristics and how it will react to different fabrication processes.
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