Aluminum extrusion: temperature control

Heat balance in direct extrusion

Sources of thermal energy for the direct extrusion process are shown in Fig.. 1. Most of the work of deformation is converted into heat. Friction forces, operating in three different locations, affect the overall temperature change in the workpiece, as well as in extruded profiles, which exits the extrusion die.

Fig. 1 – The sources of heat energy to the direct extrusion process.
Most of the work of deformation is transformed into heat.
This temperature rise due to plastic deformation and friction.
Friction forces acting in three different locations:
container-billet interface, dead-metal zone and die bearing [1].

Temperature is one of the most important extrusion parameters. As the temperature rises, the flow stress decreases and, Consequently, plastic deformation becomes easier. In the same time, However, reduced maximum extrusion speed, since local temperature can lead to local sub-melting of a particular alloy. Temperature changes during extrusion depend on many factors, such as [1]:

Starting billet temperature
Alloy flow stress at a given temperature, strain and strain rate
Plastic deformation
Friction between container and billet
Friction during metal flow in the dead zone of the die
Friction during the flow of metal through the bearings of the extrusion die
Heat transfer (both conduction, and convection)

The heat balance of the aluminum extrusion process is shown in Fig.. 2.

Fig. 2 – The heat balance of aluminium extrusion [1]

The scheme of the numerical model of the heat balance in the direct extrusion of aluminum is shown in Fig.. 3 [2]. The thermal balance between the deformable material and the extrusion tools determines the temperature rise in the extrudate. Apply control volumes around the warp zone and other zones, where does the heat flow occur.

Fig. 3 – The heat balance during the extrusion process [2]

Aluminum Alloy Extrusion Series 6000

Aluminum direct extrusion is the process of applying a hydraulic force to a billet in a container through an orifice(s) of a fixed die. An example is the extrusion of aluminum alloy series 6000 (Fig. 4). Aluminum blanks are heated to 450-500°C (depending on the alloy, mold and extrusion ratio) and loaded into a preheated container (420-470°C). The hydraulic piston pushes the workpiece through the hole(s) of the die at a pressure of up to 680 MPa. Hot metal passes through an extrusion die, forming a continuous extrudate with a cross section, identical to the hole of the extrusion die. The cross-sectional shapes of the extrudate can be from complex hollow to simple solid (without cavities).

Fig. 4 – Schematic of direct extrusion process of 6000 aluminium alloys:
temperatures for the billet, container, die and extrusion [3]

The outlet temperature of the matrix

The temperature of the aluminum exiting the die (extrusion temperature) is important for several reasons..

Extrusion temperature affects the quality of the extruded product and the life of the extrusion die, as shown in pic. 5.
Outlet temperature affects heat treatment processes and dimensional stability, and also causes extrusion defects.
Outlet temperature is a critical issue for ‘die life’. The wear of the extrusion die and its productivity depend on the outlet temperature, which, in turn, causes an increase in the temperature of the working belts of the extrusion die.

Fig. 5 – Extrusion temperature (exit temperature) effects
on product quality and die life [1]

What affects the temperature of the profile at the exit of the press

material properties

The mechanical properties of the aluminum alloy billet significantly affect the amount of heat, which is formed by deformation and friction. In case of deformation, heat dissipated current proportional to the voltage at a given temperature, strain and strain rate. In the case of friction, the temperature rises in proportion to the friction shear stress.

Friction

Distribution temperature of aluminum strongly depends on the conditions of its friction on the container, along the boundary of the dead zone and the girdle matrix.

Extrusion speed

The temperature profile of aluminum increases with increasing speed of the ram. This increase is due, that the rate of deformation is directly proportional to the ram speed, and the amount of heat generated in proportion to the deformation speed. The lower ram speed, the longer the time for heat transfer and dissipation.

The ratio of the pressing

The greater the ratio of pressing, the higher the temperature at the outlet of the matrix due to the severe plastic deformation.

Perimeter belt matrix

The temperature of the aluminum profile at the outlet of the matrix increases with the perimeter of the girdle matrix. This is because, that if the length of the belt from increasing its perimeter increases friction over the belt area of the matrix.

Temperature measurement on an extrusion press

Extrusion temperature measurement, emerging from the head, can be done in several ways:

Inserting a thermocouple into an extrusion die
Measurement outside the extrusion die with a contact-type thermocouple
Using an optical pyrometer

Non-contact temperature measurements are made within a few seconds after, how the extruded profile leaves the extrusion die. The sensor continuously monitors the temperature from the start to the end of the extrusion. Continuous monitoring of the extrusion temperature allows the process to be carried out as much as possible at a constant temperature (isothermal extrusion) by controlling the speed of the ram, thereby optimizing performance.

The system can be used to more accurately control and monitor billet and extrusion temperatures., to maintain constant extrusion quality. The installation diagram for aluminum extrusion is shown in fig.. 6.

Fig. 6 – Typical billet and extrusion temperature measurement system [1](Williamson Corp.)

Sources:

Aluminum Extrusion Technology / Saha P.
Extrusion of Aluminium Alloys / T. Sheppard
Basic Metallurgy: 6000 Series Extrusion Alloys, Comalco Publication