Extrusion and Drawing of Metals Chapter 15 Introduction The Extrusion Process Extrusion Practice Hot Extrusion Cold Extrusion Impact Extrusion Hydrostatic Extrusion Extrusion Defects Extrusion Equipment The Drawing Process Drawing Practice Defects and Residual Stresses
Extrusion – It is a process where a billet is forced through a die.
Drawing – It is a process where a cross-section of solid rod, wire, or tubing is reduced or changed in shape by pulling it through a die.
Fig : Extrusions and examples of products made by sectioning off extrusions.
Types of Extrusion :
Direct Extrusion (or) Forward Extrusion – Billet is placed in a chamber and forced through a die opening by a hydraulically-driven ram or pressing stem.
Indirect Extrusion – Die moves towards the billet.
Hydrostatic Extrusion – The billet is smaller in diameter that the chamber, which is filled with a fluid, and the pressure is transmitted to the billet by a ram.
Extrusion Ratio = Ao/Af
Ao – cross-sectional area of the billet
Af - cross-sectional area of extruded product
Fig : Schematic illustration of direct extrusion process.
Fig : Types of Extrusion (a) indirect (b) hydrostatic (c) lateral
Fig : Process variables in direct extrusion. The die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure.
Fig : Method of determining the circumscribing-circle diameter (CCD) of an extruded cross-section.
F = AokIn(Ao/Af)
Ao, Af billet and extruded product areas
Fig : Extrusion constant k for various metals at different temperatures
Fig : Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction, or in indirect extrusion. (b) Pattern obtained with high friction at the billet-chamber interfaces, (c) Pattern obtained at high friction, or with cooling of the outer regions of the billet in the chamber. This type of pattern, observed in metals whose strength increases rapidly with decreasing temperature, leads to defect known as pipe, or extrusion defect.
Fig : (a) An extruded 6063-T6 aluminum ladder lock for aluminum extension ladders. This parts is 8mm thick and is sawed from the extrusion, (b)-(d) Components of various dies for extruding intricate hollow shapes.
Fig : Poor and good examples of cross-sections to be extruded. Note the importance of eliminating sharp corners and of keeping section thickness uniform.
Improved mechanical properties
Good control of dimensional tolerances
Improved surface finish
Elimination of the need for billet heating;
Fig : Two examples of cold extrusion. Thin arrows indicate the direction of metal flow during extrusion.Cold Extrusion
Fig : (a) Two examples of products made by impact extrusion. These parts may also be made by casting, by forging, or by machining; the choice of process depends on the dimensions and the materials involved and on the properties desires. Economic considerations are also important in final process selection. (b) and (c) Impact extrusion of a collapsible tube by the Hooker process.
Fig : General view of a 9-MN (1000-ton) hydraulic-extrusion press.
Fig : (a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are inspected, such internal defects may remain undetected, and later cause failure of the part in service. This defect can also develop in the drawing of rod, of wire, and of tubes. (b) Schematic illustration of rigid and plastic zones in extrusion. The tendency toward chevron cracking increases if the two plastic zones do not meet. Note that the plastic zone can be made larger either by decreasing the die angel or by increasing the reduction in cross-section (or both).Extrusion Defects
Die angle has great influence on the drawing force and the quality of the drawn product
Drawing FORCE :
F = Yavg Af In (Ao/Af) ; Yavg – average true stress of the material in the die gap
Fig : Process variables in wire drawing. The die angle, the reduction in cross-sectional area per pass, the speed of drawing, the temperature, and the lubrication all affect the drawing force, F.Drawing Process
Fig : Examples of tube-drawing operations, with and without internal mandrel. Note that a variety of diameters and wall thickness can be produced from the same initial tube stock (which had been made by other processes).
Characteristic features of a typical die design for drawing
Fig : Terminology of a typical die used for drawing round rod or wire.
Fig : Tungsten-carbide die insert in a steel casing. Diamond dies, used in drawing thin wire, are encased in a similar manner.
Proper lubrication is essential in drawing, in order to improve die life, reduce drawing forces and temperature, and improve surface finish.
Types of Lubrication
Typical defects are
Fig : Schematic illustration of roll straightening of a drawn round rod.
Fig : Two views of multistage wire-drawing machine that is typically used in the making of copper wire for electrical wiring.