Extrusion

Metal Extrusion Definition Extrusion is the conversion of an ingot or billet into lengths of uniform cross section by forcing m etal 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 m etal 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: Im pact extrusion is a special process to form hollow shapes with short lengths. Typical product is collapsible toothpaste alum inum tube. As shown in figure, the die has an accurate outside diam eter 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 sides 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 m andrel, the clearance between the m andrel 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 comm only used for non-ferrous materials. The cross section explains the entrance angle, die land and relief angle.

This figure shows the steps of m aking 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 im proper metal flow during extrusion can produce various defects. In extrusion m etal 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 im proper 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 tem peratures 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 deform ation, as m uch as one third of the extruded product length m ay 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 rem ove 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 im purities. 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 tem perature at which extrusion is performed depends on the material being extruded. Hot extrusion is used to produce long, straight m etal 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 alum inum 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.