Extrusion of Metals

8/6/2019 Extrusion of Metals

1/26

Chapter 15

Extrusion and DrawingExtrusion and Drawing

of Metalsof Metals


2/26

Topics

Intr oduction

The Extrusion Process

Extr u

sion Practice

Hot Extrusion

Cold Extrusion

Impact Extrusion

Hydrostatic Extrusion

Extr usion Defects

Extr usion Equipment

The Drawing Process

Drawing Practice

Defects and Residual

Stresses

Drawing Equipment


3/26

Introduction

Extrusion It is a process where a billet is forcedthrough a die.

Parts have constant cross-section

Typical Products of Extrusion Sliding Doors,tubing having various cross-sections, structural andarchitectural shapes anddoor and window frames.

Drawing It is a process where a cross-section ofsolid rod, wire, or tubing is reducedor changed inshape by pulling it through a die.


4/26

ExtrusionsExtrusions

Fig : Extrusions and

examples of

products made by

sectioning off

extrusions.


5/26


6/26

Direct Extrusion

Fig : Schematic illustration ofdirect extrusion process.


7/26

Types of Extrusion

Fig : Types of Extrusion (a) indirect(b) hydrostatic (c) lateral


8/26


9/26

Extrusion Force :F= AokIn(Ao/Af)

k-extrusion constant

Ao, Afbillet and extruded product areas

Fig : Extrusion constant k for various

metals at different temperatures


10/26

Types of Metal flow in extruding with square dies.

Fig : Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction, or in indirectextrusion. (b) Pattern obtained withhigh friction at the billet-chamber interfaces, (c) Pattern obtained athigh friction, or with cooling of the outer regions of the billet in the chamber. This type of pattern, observedin metals whose strength increases rapidly with decreasing temperature, leads to defect known as pipe, orextrusion defect.


11/26

Hot Extrusion

Extr usion is carriedout at elevated temperatures for metals andalloys that do not have sufficient ductility at room temperature, or

in order to reduce the forces required.


12/26

Die Design and Die Materials

Fig : (a) An extruded 6063-T6 aluminum ladderlock for aluminum extension ladders. This parts is 8mm thick andis sawed from the extrusion, (b)-(d) Components of various dies for extruding intricate hollow shapes.


13/26

Cross-sections to be extruded

Fig : Poor and good examples of cross-sections to be extruded. Note the importance of eliminating sharpcorners and of keeping section thickness uniform.


14/26

Cold Extrusion

Combination ofoperations, such as direct and indirect extrusion and forging.

Advantages : Impr oved mechanical properties

Good controlofdimensional tolerances

Impr oved surface finish

Elimination of the need for billet heating;

Fig : Two examples of coldextrusion. Thin arrowsindicate the directionof metal flow duringextrusion.


15/26

Impact Extrusion

Similar to indirect extrusion

Punch descends rapidly on the blank, which is extruded backward

Fig : Schematic illustration of the impact-extrusion process. The extruded parts arestripped by the use of a stripper plate, because they tend to stick to the punch.


16/26

Examples ofImpact 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 theproperties desires. Economic considerations are also important in final process selection. (b) and (c) Impactextrusion of a collapsible tube by the Hooker process.


17/26

Hydrostatic Extrusion

The pressure required for extrusion is supplied through and incompressible fluid medium

surrounding the billet

Usually carried at room temperature, typically using vegetable oils as the fluid

Brittle materials are extruded generally by this method

It increases ductility of the material

It has complex nature of the tooling

Fig : General view of a 9-

MN (1000-ton) hydraulic-

extrusion press.


18/26

Extrusion Defects

1. Surface cracking.

2. Pipe.3. Internal Cracking

Fig : (a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are inspected, such internaldefects may remain undetected, and later cause failure of the part in service. This defect can also develop in thedrawing of rod, of wire, and of tubes. (b) Schematic illustration of rigid and plastic zones in extrusion. Thetendency 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).


19/26

Drawing Process In this pr ocess the cross section of a round rodor wire is typically reducedor

changed by pulling it through a die

Die angle has great influence on the drawing force and the quality of the drawnproduct

Drawing FORCE :

F= Yavg Af In (Ao/Af) ; Yavg average true stress of the material in the die gap

Fig : Process variables in wiredrawing. The die angle, thereduction in cross-sectionalarea per pass, the speed ofdrawing, the temperature,and the lubrication all affectthe drawing force, F.


20/26

Tube Drawing Operations

Fig : Examples of tube-drawing operations, with and without internal mandrel. Note that a variety ofdiameters and wall thickness can be produced from the same initial tube stock (whichhad been madeby other processes).


21/26

Die Design

Characteristic features of a typicaldie design fordrawing

Fig : Terminology of a typicaldie used fordrawinground rodor wire.

Fig : Tungsten-carbide die insert in asteel casing. Diamonddies, used indrawing thin wire, are encased in asimilar manner.


22/26

Die Materials

Commonly used materials are Tool Steels and Carbides

Diamonddies are used for fine wire.

For improved wear resistance, steeldies may be chromium

plated, and carbide dies may be coated with titanium nitride

For Hot drawing, cast-steeldies are used


23/26

Lubrication

Properlubrication is essential in drawing, in order to improvedie life, reduce drawing forces and temperature, andimprove surface finish.

Types of Lubrication

a) Wet drawing : Dies and Rods are completely immersed inlubricant

b) Dry drawing : Surface of the rod to be drawn is coatedwith a lubricant

c) Coating : Rodor Wire is coated with a soft metal that actsas a solidlubricant

d) Ultrasonic Vibration of the dies and mandrels.


24/26

Defects and Residual Stresses

Typicaldefects are

center cracking seams

Fig : Schematic illustration of roll straightening of a drawn round rod.


25/26

DRAWING EQUIPMENT

Fig : Two views of multistage wire-drawing machine that is typicallyused in the making of copper wire for electrical wiring.


26/26

THE END

Documents

Extrusion of Metals