Metal Extrusion Definition Extrusion is the conversion of an ingot or

billet into lengths of uniform cross section by

forcing metal to flow plastically through a die.

Classification of Extrusion Processes Direct Extrusion [Forward Extrusion]:

The metal billet is placed in a container and driven through the die by the ram

(extrusion ram and metal flow are in the

same direction); high forces are required to overcome the metal deformation and

the friction between the billet and container

during process.

1. Direct Extrusion

2. Indirect Extrusion A. The billet is upset to fill

the container.

B. further compression until the beginning of

metal flow through the

die. C. Extrusion proceeds.

D. Piping defect occurs.

E. Formation of dead metal zone.

Impact extrusion:

Impact extrusion is a special process to

form hollow shapes with short lengths.

Typical product is collapsible toothpaste aluminum tube. As shown in figure, the

die has an accurate outside diameter of

the tube and a punch having an accurate inside diameter of the tube strike the billet

at high velocity. It is a restricted process to soft metals such as aluminum, copper,

tin and lead.

Extrusion Forging:

Extrusion forging is a

combined process of

forging and extrusion. A common example is the

production of engine valves, where the billet is partially

extruded to form the valve

shank, and the rest of the billet is forged to form the

valve head.

Hydrostatic Extrusion:

In hydrostatic extrusion, a pressurized fluid is used instead of extrusion ram to

act on the billet s ides and back face, the

advantages of this process are no frictional forces, and the billet need not to

be of the same shape as the container. Typical products are stepped shafts.

Production of Hollow Shapes and Tubes: Tube Extrusion:

Tubes and hollow sections can be produced by extrusion by attaching a mandrel,

the clearance between the mandrel and die wall determines the wall thickness of the tube.

Porthole or Spider Die:

Porthole dies are suitable for

very long section of different

hollow shapes, and for thin wall thickness or small

dimensions.

The metal (Aluminum) flow through the holes and then

pressure welded during deformation at die outlet.

Extrusion Die Extrusion dies are made from different types

of hot working tool steels. The figure below

shows schematic sketches for (a) Taper die for hot extrusion of steels with molten glass

as a lubricant, and (b) flat die commonly

used for non-ferrous materials. The cross section explains the entrance angle, die land

and relief angle.

This figure shows the steps of making T-section extrusion along the die.

Metal Flow in Extrusion: The flow pattern in extrusion is important because of its effect on the quality and

mechanical properties of the final product, where the improper metal flow during

extrusion can produce various defects. In extrusion metal flow longitudinally like a fluid flow, so the extruded products have an elongated grain structure.

A common technique for

investigating the flow pattern is to section the billet into two longitudinal

halves and mark the sectional face with a square grid pattern, the two

halves are placed in the chamber

together and partially extruded, and then they are taken apart and studied.

Extrusion Defects: Due to improper material conditions and process variables, several types of

defects can be developed in extruded products. There are three principle

extrusion defects:

Surface cracks (also called speed cracks)

Cause: high speed or high speed and friction.

These cracks are intergranular (along the grain boundaries) and are usually caused by hot shortness. Hot shortness is the tendency for some alloys to

separate along grain boundaries when stresses of deformed at high temperatures

due to the low melting point constituents segregated at grain boundaries. Solution: Reduce billet temperature and extrusion speed.

Piping (also called fish tailing)

Cause: Surface oxides and impurities.

According to the metal flow pattern, surface oxides and impurities are drawn

towards the center during deformation, as much as one third of the extruded

product length may contain this type of defect, and have to be cut off as scrap. Solution: Piping can be minimized by modifying the flow pattern and controlling

friction, or by machining the billet outer surface before extrusion to remove surface scales.

Internal cracks Cause: high internal stresses at the centerline.

Internal cracks are due to high internal stresses near the centerline in the

deformation zone of the extrusion die. The tendency y for center cracking:

Increases with the increase of die angle and amount of impurities.

Decreases with the increase of extrusion ratio and friction.

Solution: Select proper die geometry and extrusion variables.

Hot Extrusion & Cold Extrusion

HOT EXTRUSION is the process of forcing a

heated billet to flow through a shaped die opening.

The temperature at which extrusion is performed

depends on the material being extruded. Hot extrusion is used to produce long, straight metal

products of constant cross section, such as bars, solid and hollow sections, tubes, wires, and strips,

from materials that cannot be formed by cold

extrusion.

Cold Extrusion

Cold extrusion is a general term often denoting a combination of

operations, such as direct or indirect extrusion and forging to

produce short solid or hollow

shaped products. Workpieces are often cup-shaped

and have wall thicknesses equal

to the clearance between the punch and die.

Ductility must be restored

between operations by annealing,

and any scale formed during annealing must be removed by blasting or pickling

before subsequent extrusion. Cold-Extruded Metals in the order of decreasing extrudability are Aluminum and aluminum alloys, copper and copper alloys, low

carbon and medium-carbon steels, modified carbon steels, low-alloy steels, and

stainless steels.

Reference Lecture notes of Dr. Ahmed Fareed , Ain shams University, 2007.