Metal Extrusion

Both stock material and custom metal parts come in a very wide range of cross-sections. Metal extrusion is one of the ways used to create these seamless tubes or intricate full profiles.

Of course, the importance of metal extrusion cannot be understated. We find the products made using the process in all sorts of industries like construction, manufacturing, retail, information technology, etc.

Having a basic understanding of all the main manufacturing methods, along with their capabilities and flexibility, can help an engineer a great deal. Which is why we are going to take a brief look into the different aspects of this method.

History

The metal extrusion process was invented by Joseph Bramah in 1797. He pushed soft metal through a die using a hand-driven plunger to form metal pipes.

The same man also developed the first hydraulic press for Thomas Burr in 1820. This hydraulic press formed the first lead pipes that revolutionised the metal forming industry.

What is Metal Extrusion?

Metal extrusion is a forming process in which we force a metal (either hot or cold) through a die. This imparts the die shape to the extruded metal as it passes through the cavity. The material emerging from the die is known as “extrudate”.

The metal endures compressive and shear stress to achieve the die shape. The nature of these forces and the elevated temperatures enable us to form materials with otherwise brittle properties using this process.

Besides metals, nonmetals such as ceramic, plastic, clay, concrete, polymers, etc. are also suitable for extrusion.

Key features

• Extrusion is a low-cost process due to reduced wastage and has a high rate of production.
• It can form brittle materials as it applies only compressive and shear forces on the billet.
• The resultant products have an excellent elongated grain structure in the material direction.
• The products also have a smooth surface finish that reduces the amount of post-treatment.
• Extremely thin wall thickness can be achieved through extrusion (~3mm for steel and ~1mm for aluminium).
• Extrusion can create extremely complex cross-sections with a uniform wall thickness throughout the product.

Metal Extrusion Process

The metal extrusion process has undergone many changes since its inception. Besides direct extrusion of final products, we use this manufacturing process to obtain a uniform material feed for additive manufacturing applications such as 3D printing. This material is then deposited layer-by-layer to create the desired product.

A generic metal extrusion process consists of the following steps.

Hot extrusion process

Preparation of feed metal

The billet or ingot is the feed metal that is used as the raw material in the production process. But this feed metal needs to conform to certain standards prescribed by the designers. It usually has a circular or a square profile but may have other shapes as well.

The feed metal itself is created using methods such as hot rolling or continuous casting.

Preparation for extrusion

The standard raw material is then placed into the extrusion equipment such as a press and heated to the recommended temperature for the hot extrusion method.

Cold extrusion does not require any preheating, as the process takes place at room temperature.

Metal extrusion

Now the extrusion process takes place by applying a compressive force that pushes the material towards the die. The die has a small opening. Due to the high pressures, the metal exits the die through the opening, assuming the die shape in the process.

Once this process is complete, the pressure is released and the product moves onto the next step.

Heat treatment and post-processing

The product now undergoes heat treatment to improve its properties and prepare it for its service conditions. The heat treatment process is different for different metals. 

For instance, in the case of aluminium, we cool the extruded part first, then stretch and cut it to the desired lengths. Then the pieces undergo an ageing process where we heat them to 350 degrees Fahrenheit and hold them for four hours to harden them.

Types of Extrusion Processes

There are quite a few different types of metal extrusion forms to choose from. While the core principles remain invariable, we apply them differently in order to achieve a myriad of different products.

We can classify all the metal extrusion processes into one of the following types.

Hot extrusion

In hot extrusion, we heat the feed metal above its recrystallisation temperature. This softens the metal enabling it to flow through the die opening.

The high working temperature prevents the material from undergoing work hardening. The high pressure means that the use of lubricant is necessary.

The setup for hot extrusion is expensive to buy and maintain. It can only turn in profit when the product size is big.

Cold extrusion

When extrusion is carried out at room temperature, we call it cold extrusion. The method remains the same except the material is not heated at all, or only slightly, prior to extrusion. This manufacturing process has a set of advantages and disadvantages compared to hot extrusion.

The advantages include shorter batch timings, finer tolerances, smoother surface finish, and lack of oxidation.

The disadvantages include the need for more power because the material is not easy to work with. Also, the mechanical properties of the material are subject to change over the course of the cold extrusion process. The extrusion speed is lower at such temperatures.

Warm extrusion

When extrusion is carried out at temperatures between room temperature and recrystallisation temperature of the metal, we call it warm extrusion.

Compared to the aforementioned manufacturing processes, warm extrusion offers benefits such as higher control over extrudate properties such as ductility.

The temperatures for warm extrusion range from 424°C to 975°C. The temperature never exceeds the critical melting point at any stage of the metal forming process.

Friction extrusion

Friction extrusion is a somewhat overlooked process that has been recently gaining a lot more attention due to its advantages in additive manufacturing.

In friction extrusion, the heat generated from friction between the die and the feed metal is used to heat the feed metal. Thus, feed metal does not require any preheating but uses the internal energy to lift the extrusion temperature.

The application of load pushes the feed metal through the die opening and the extruded part comes out the top in the shape of the die. The advantage of friction extrusion over others is that recyclable metal can be added as a feed directly. Waste such as small scraps, blocks, machining chips, and powder can be used to manufacture finished products without any pretreatment.

Microextrusion

The microextrusion process creates products at the submillimeter range for special applications. The process remains the same but it requires small dies and rams which is a challenge with tight accuracy requirements. The products of microextrusion fit within a 1 mm square.

Several other issues also come into play when dealing with small products such as grain boundaries and structure, deformation defects, and forming stability.

Extrusion Equipment

There is various equipment available for extrusion. We can classify them in different ways depending on the equipment’s characteristics. Some of these are the direction of force application, medium for force application, etc.

Some popular types of extrusion equipment are as follows.

Direct extrusion

Direct extrusion uses a stationary die while the ram creates the pressure on the fluid metal. Due to this pressure, the fluid escapes through the die opening. The die opening is in the container on the opposite side of the punch. Thus, the extruded part comes out in the direction of the ram motion. This is why it is also known as forward extrusion.

Direct extrusion is the most popular extrusion process. It has advantages such as:

• Simple tooling
• Compatible with hot as well as cold extrusion
• No modification in billet

Some disadvantages of this process are:

• High friction force between the billet and the container and so, high forces are necessary for extrusion
• Variable force during the course of extrusion
• Butt end remains inside

Indirect extrusion

In indirect extrusion, the die opening is on the ram. As the punch compresses the fluid metal, it flows through the die opening in the opposite direction of the ram. This is why it is also known as backward extrusion.

Indirect extrusion consumes less power due to the reduced friction between the billet and the container. Similar to direct extrusion, indirect extrusion is used for hot as well as cold extrusion.

A drawback for this process is that it is difficult to support the extrudate. The ram is hollow which limits the applicable load.

Hydrostatic extrusion

In the above methods, the ram was in direct contact with the billet. But in hydrostatic extrusion, the billet is pressurized by means of a fluid. The billet is surrounded by fluid in all directions except where it touches the die.

This eliminates the friction between the billet and the container which, in turn, reduces the amount of force required for extrusion. When force is applied, the die opening is the only release point and extrusion occurs.

The advantages of using hydrostatic extrusion are

• Lower forces for extrusion
• An even flow of material
• Lower temperature of the billet
• This setup eliminates any billet residues in the container.

There are certain drawbacks of hydrostatic extrusion that we need to be aware of:

• Preparation of billet necessary for extrusion
• Can only be used for cold extrusion
• Holding high pressures is a challenge

Impact extrusion

In impact extrusion, the punch is brought in contact with the billet at a high speed. The resultant pressure pushes the extrudate out through the clearance between the punch and the die. This die cavity gives the extrudate the required shape.

Once the punch is lifted the extrudate rises with it and is stripped from it using a stripper plate.

Impact extrusion is based on cold extrusion and usually forms softer metals such as aluminium, copper, and lead.

Lateral extrusion

In lateral extrusion, the die opening is on the side of the container. As the punch pushes down on the billet, the extrudate comes out the side of the container. There may be more than one die openings. In lateral extrusion, the flow of metal is perpendicular to the punch direction.

Lateral extrusion is a special subdivision of closed die forging. We usually use it as an alternative to heading as it is capable of creating products of great length in a single operation.

Common Defects

Defects are an inevitable byproduct of any process. But they can always be reduced in size and number by using correct precautions and post-treatment. There are three main types of defects found in extrudates that can significantly hamper the quality of the final product. 

Surface cracking

The main cause for surface cracking is excessive stresses on the material surface. If there is higher friction present during the extrusion process, surface cracking occurs.

Internal cracking

When cracks form in the centre of the extruded product, it is known as internal cracking. Low friction and low extrusion ratios in the deformation zone lead to this defect. We can control it by increasing the friction at the tool-billet interface.

Piping

Piping refers to a funnel-shaped void in the end product. When there are impurities or oxides present in the stock, it causes a piping defect.