The process of pushing metal through a shaped opening, known as extrusion, is a fundamental manufacturing technique. This method relies on the metal’s ability to undergo permanent shape change without cracking or breaking. The success of extrusion depends on intrinsic material properties that allow the metal to flow like a highly viscous solid under immense pressure. These properties ensure the starting material undergoes the necessary plastic deformation to take on the precise form of the die opening.
Material Property for Shape Change: Ductility and Plasticity
The primary characteristic that permits extrusion is plasticity, the ability of a material to undergo permanent shape change when sufficient force is applied. Ductility is a specific measure of plasticity, describing how much a material can be stretched or drawn out under tensile stress before fracturing. A metal must possess a high degree of ductility to be successfully pushed through a die, preventing catastrophic failure as it is compressed into a new cross-section.
This capacity for permanent deformation is rooted in the metal’s crystalline structure at the microscopic level. Metals are composed of atoms arranged in repeating patterns known as crystal lattices, and within these lattices are linear defects called dislocations. When the metal is subjected to the high pressure of the extrusion ram, these dislocations move or “glide” along specific crystallographic planes, referred to as slip planes.
The movement of these dislocations allows atomic planes to slide past one another, enabling the macroscopic change in shape without breaking atomic bonds. Materials with crystal structures that offer many easy slip planes, such as face-centered cubic (FCC) metals like aluminum and copper, exhibit high ductility. The density of available slip systems allows the metal to accommodate the massive strains required to pass through the die.
Overcoming Internal Resistance: Yield Strength and Flow Stress
While ductility determines a metal’s ability to be reshaped, the force required to initiate and maintain that reshaping is governed by its strength properties. The applied pressure must first overcome the material’s yield strength, the stress point where the metal transitions from elastic to permanent plastic deformation. Beyond this point, the permanent shape change necessary for extrusion begins, as the metal would otherwise return to its original shape if the pressure were removed.
Once the metal begins to deform plastically, the resistance it offers to continued flow is described by its flow stress. Flow stress is the instantaneous value of stress needed to keep the material deforming at a given strain, strain rate, and temperature. In an extrusion process, the force applied by the ram must continuously exceed this flow stress throughout the entire procedure to maintain the metal’s movement through the die opening.
As the metal deforms, it undergoes work hardening, or strain hardening, which causes its flow stress to increase. This occurs because the movement of dislocations creates new ones and causes them to tangle, increasing the metal’s internal resistance. The extrusion press must generate a consistently high and often increasing force to overcome this rising resistance and achieve the desired final shape.
Modifying the Material State: The Role of Temperature
For most industrial extrusion applications, the properties of the metal are intentionally modified by increasing its temperature. Heat is a powerful tool to manage the internal resistance and ductility of the material, making the process more efficient and achievable. Elevated temperatures significantly increase a metal’s ductility by promoting the movement of dislocations and activating more slip systems within the crystal structure.
More importantly, increasing the temperature dramatically lowers the metal’s yield strength and flow stress. This reduction in internal resistance means that a much lower force is required from the press to push the material through the die, which makes the process viable for large cross-sections and stronger alloys. Extrusion performed at high temperatures, often above half the metal’s melting point, is known as hot extrusion.
Some specialized processes, known as cold extrusion, are performed at or near room temperature when exceptionally tight dimensional tolerances are required. However, hot extrusion is the most common method, partly because of dynamic recrystallization (DRX). In hot working conditions, DRX is a softening mechanism where new, strain-free grains spontaneously nucleate and grow during the deformation process itself.
Dynamic recrystallization effectively counteracts the strengthening effect of work hardening by replacing the strained, tangled microstructure with a fresh, softer one. This balance between the hardening caused by deformation and the softening from DRX results in a stable, lower flow stress, allowing the metal to be continuously pushed through the die without requiring an ever-increasing amount of force. The temperature and strain rate must be carefully controlled to ensure this dynamic equilibrium is maintained for a smooth and efficient operation.