This article gives industry insights into extruded aluminum. Read further to learn more about:
- Overview of aluminum extrusion
- Properties of aluminum
- Benefits of aluminum extrusion
- And much more…
Chapter 1: Overview of Extruded Aluminum
Extruded aluminum is a continuous piece of aluminum that usually has a constant profile or cross-section throughout its length. It is made by forcing a block of aluminum, called a billet, through a die opening which is smaller than the original cross-sectional area of the billet. The extrusion process can be hot or cold which depends on the type of aluminum alloy and the extrusion method used.
Aluminum is a desirable material due to its light weight, high strength, and corrosion resistance. In its pure form, it is soft and has low strength. To improve its mechanical properties, it is usually alloyed with elements such as copper, magnesium, manganese, and silicon. Furthermore, it can be heat treated to further improve and attain the right balance of strength and ductility.
The first development of metal extrusion happened in the late 1700s when the first patent for the forming process was filed by an English inventor and locksmith, Joseph Bramah. The process is originally made to create lead pipes and lead sheathing of cables. This early process of extrusion can work soft metals.
Harder and stronger metals such as aluminum require higher forming temperatures and pressures. Aluminum was not used as an extruded material until the introduction of the hot extrusion process by Alexander Dick in 1894.
Today, aluminum extrusion is widely used, with a global market size of more than $67 billion that is expected to grow by 3.8% annually from 2020 to 2027. The main applications of extruded aluminum are in the fields of building and construction, automotive and transportation, consumer goods, and electrical and energy.
Chapter 2: Properties of Aluminum
Aluminum is the most popular metal used for extrusion forming. This metal offers distinctive combinations of mechanical properties such as high strength, low density, and good workability. These qualities are almost constant at all temperatures. Moreover, other desirable properties such as electrical conductivity, reflectance, paramagnetism, etc. further widen its areas of application.
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 without the need for protective coatings.
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.
- Ductility and Workability: Aluminum can be easily formed even at room temperatures. Aside from extrusion, aluminum can be formed by rolling, drawing, stamping, and forging.
- Low-temperature Toughness: In contrast with steel, aluminum retains its toughness at low temperatures. Low temperatures typically make metals fail under brittle fracture. The mechanical properties of aluminum are almost constant across all temperatures.
Resilience and Impact Strength: Because of its natural toughness, aluminum has high resilience and impact strength. Aluminum parts can absorb sudden forces or shocks and can elastically flex from dynamic loads.
- Non-magnetic: Unlike steel, aluminum is not ferromagnetic but paramagnetic. This means it does not acquire a magnetic charge when subjected to strong magnetic fields. This makes aluminum suitable for electronic and electrical enclosures and parts that emit high electromagnetic fields. Moreover, along with its electrical conductivity, aluminum can be used to create electromagnetic field shields.
Non-sparking: In its pure or alloyed form, aluminum does not produce sparks. This property is required for making tools used in flammable or explosive environments. A common aluminum alloy used for non-sparking tools is aluminum bronze made by adding copper.
Reflectance: Aluminum has the highest reflectance of any 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 wavelengths of the visible spectrum.
- Recyclability: Aluminum is easily recycled without loss of desirable properties. The energy required to recycle aluminum is only about 5% of the energy consumed to produce a virgin product.
Chapter 3: Benefits of Aluminum Extrusion
The extrusion process has its advantages which, when in tandem with the properties of aluminum, produces products with unique qualities. These qualities are both beneficial for the manufacturer and end-user. The extrusion process is mainly used for producing parts with complex cross-sections. Additionally, it can also work with brittle materials that are difficult 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, in comparison with other forming processes.
- Ability to form brittle materials. Extrusion produces parts without tear and cracks because of the metal’s favorable configuration for plastic flow. This is due to the high compressive stresses produced by the forces acting on the billet against the chamber and die.
- High dimensional accuracy. Aluminum extrusions can be produced with tight tolerances in comparison to casting and rolling. Metal flow can now be modeled using computer numerical simulation to predict the extrudate dimensions and profile.
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.
- Flexibility of operation. It takes small tweaks on process parameters when changing from one extrudate profile to another which requires only minimal or almost no breaks in production.
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.
Chapter 4: Factors to Consider in Aluminum Extrusion
Extrusion is categorized as a bulk-forming process where there is a significant change in the surface-to-volume ratio of the formed metal. This is achieved by subjecting the billet to compressive forces with the help of rams, punches, tools, and dies. To produce the desired profile with a predictable grain structure, the plastic theory is applied to determine the mechanics of the metal’s plastic deformation.
The formed product largely depends on several variables. The main variable is the extrusion pressure which is influenced by other factors such as temperature, extrusion ratio, and extrusion speed. The extrusion variables are enumerated 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.
- Extrusion Pressure: The extrusion pressure overcomes the required pressure to initiate metal flow and overcome interface friction between the billet and the die and chamber. This ranges from 800 MPa to 1200 MPa.
- Friction between the billet and die, or chamber and die: The former occurs in indirect extrusion, while the latter is in direct extrusion. Friction is eliminated or minimized to control metal flow and reduce the required power for compression.
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.
- Lubrication: Lubrication is required for extruding high-strength aluminum alloys. This is to assist the metal as the billet slides against the chamber and the metal is deformed through the die. Lubrication is usually a proprietary formulation generally made from oil, graphite, or glass powder.
Types of Aluminum Alloy: Different aluminum alloys require different extrusion parameters. Aluminum alloys are designated in series which are classified according to the main alloying element. Aluminum alloys are enumerated below:
- 1xxx: 99% Aluminum
- 2xxx: with Copper
- 3xxx: with Manganese
- 5xxx: with Magnesium
- 6xxx: with Magnesium and Silicon
- 7xxx: with Zinc
The most widely used alloy for extrusion is the 6xxx series alloys.
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 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.
- Extrusion Speed: This is the speed at which the metal flows through the die. Higher extrusion speeds require high extrusion pressures and result in higher exit temperatures. Slower speeds provide ample time for the temperature to flow and dissipate. Extrusion speed is balanced to maintain the right temperature of the metal.
- Length of Billet: For a given billet diameter, billet length limits the length, profile, and extrusion ratio of the extrudate. Moreover, the billet length also affects the required extrusion pressure. The longer the billet, the higher the required extrusion pressure.
Chapter 5: Different Aluminum Extrusion Processes
The manufacturing of aluminum extrusions starts with the production of aluminum billets that is fed to the extrusion machine. Aluminum is refined from bauxite to produce alumina or aluminum oxide. The oxygen is then separated from the aluminum through a reduction process to create pure aluminum. The virgin aluminum is then smelted into ingots which will then be used to create billets along with recycled aluminum.
Aluminum billets are supplied to manufacturing plants to create end-use parts through various metal forming processes. Aluminum extrusion usually involves heating the billet to increase the plasticity of the metal. The heated billet is then loaded into a cylindrical chamber with a ram on one end and a die on the other. The ram is driven either mechanically or hydraulically to produce enough compressive force. The pressure is applied to plastically deform the billet forcing it to flow through a die. The setup can vary depending on the type of extrusion process used.
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.
From 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 for using 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 former 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.
The 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.
Impact Extrusion: This process is typically a cold-forming process that forms a metal slug or blanks into hollow shells or tubes. The process is similar to the indirect extrusion process where the direction of the applied force is opposite the metal flow. The difference is that instead of extruding the metal using a progressive application of pressure, sudden impact or punching is used to deform the blank. The impact is applied using a punch designed to mate with a die. Only one impact is required to form the metal. Once the impact force is applied, the metal is completely formed which is then removed from the die using a counter-punch ejector. A new slug is placed to start a new sequence.
Aluminum extrusion is usually done under elevated pressures to increase the tendency of the metal to plastically flow. However, other technologies enable the process to be done at room temperature.
- Hot Extrusion:Hot extrusion is done above the aluminum’s recrystallization temperature where its microstructure begins to change. This in turn changes its mechanical properties such as strength, ductility, and hardness. Extruding the metal above its recrystallization temperature lowers the required pressure since at this state the material has increased ductility. Moreover, deforming the metal does not result in work hardening. Work hardening further hardens the aluminum making it more difficult to form or extrude. To control the resulting mechanical properties of the product, its rate of cooling must be controlled. For some alloys, secondary heat treatment processes are done to enhance their mechanical properties.
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.
- Forward Extrusion: In this process, the travel of the ram or punch is the same as the direction of metal flow. The ram pushes a billet through a die with a smaller cross-section. Forward extrusion processes include direct and hydrostatic extrusion.
- Backward Extrusion: The backward extrusion process involves a ram that travels in the opposite direction of the metal flow. The billet or metal slug has no displacement relative to each other. Backward extrusion processes are indirect and impact extrusion.
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.
- Extruded aluminum is a continuous piece of aluminum that usually has a constant profile or cross-section throughout its length.
- Aluminum is the most popular metal used for extrusion forming. This metal offers distinctive combinations of mechanical properties such as high strength, low density, and good workability.
- The extrusion process is mainly used for producing parts with complex cross-sections. Additionally, the process is suitable for working brittle materials that are difficult for other forming processes.
- Extrusion processes can be classified according to the method of applying pressure on the billet (direct, indirect, hydrostatic, or impact), the temperature at which the process is carried out (hot or cold), or the direction of metal flow relative to ram travel (forward, backward, or lateral).