Traditional Manufacturing Processes

Casting

Forming

Sheet metal processing

Cutting

Joining

Powder- and Ceramics Processing

Plastics processing

Surface treatment

FUNDAMENTALS OF METAL

FORMING

• Overview of Metal Forming

• Material Behavior in Metal Forming

• Temperature in Metal Forming

• Strain Rate Sensitivity

• Friction and Lubrication in Metal Forming

Metal Forming

Large group of manufacturing processes in which

plastic deformation is used to change the shape

of metal workpieces

• The tool, usually called a die, applies stresses

that exceed yield strength of metal

• The metal takes a shape determined by the

geometry of the die

Stresses in Metal Forming

• Stresses to plastically deform the metal are usually

compressive

– Examples: rolling, forging, extrusion

• However, some forming processes

– Stretch the metal (tensile stresses)

– Others bend the metal (tensile and compressive)

– Still others apply shear stresses

Material Properties in Metal

Forming

• Desirable material properties:

– Low yield strength and high ductility

• These properties are affected by temperature:

– Ductility increases and yield strength

decreases when work temperature is raised

• Other factors:

– Strain rate and friction

Bulk Deformation Processes

• Characterized by significant deformations and

massive shape changes

• "Bulk" refers to workparts with relatively low

surface area-to-volume ratios

• Starting work shapes include cylindrical billets

and rectangular bars

Basic bulk deformation processes: (a) rolling

Basic bulk deformation processes: (b) forging

Basic bulk deformation processes: (c) extrusion

Basic bulk deformation processes: (d) drawing

Material Behavior in Metal

Forming

• Plastic region of stress-strain curve is primary interest because material is plastically deformed

• In plastic region, metal's behavior is expressed by the flow curve:

nK

where K = strength coefficient; and n = strain hardening exponent

• Stress and strain in flow curve are true stress and true strain

Flow Stress

• For most metals at room temperature, strength increases when deformed due to strain hardening

• Flow stress = instantaneous value of stress required to continue deforming the material

where Yf = flow stress, that is, the yield strength as a function of strain

nf KY

Temperature in Metal Forming

• For any metal, K and n in the flow curve

depend on temperature

– Both strength and strain hardening are

reduced at higher temperatures

– In addition, ductility is increased at higher

temperatures

Cold working is metal forming performed at room temperature.

Advantages: better accuracy, better surface finish, high strength and hardness of the part, no

heating is required.

Disadvantages: higher forces and power, limitations to the amount of forming, additional

annealing for some material is required, and some material are not capable of cold working.

Warm working is metal forming at temperatures above the room temperature but bellow the

recrystallization one.

Advantages: lower forces and power, more complex part shapes, no annealing is required.

Disadvantages: some investment in furnaces is needed.

Hot working involves deformation of preheated material at temperatures above the re

crystallization temperature.

Advantages: big amount of forming is possible, lower forces and power are required, forming

of materials with low ductility, no work hardening and therefore, no additional annealing is

required.

Disadvantages: lower accuracy and surface finish, higher production cost, and shorter tool life.

Friction in Metal Forming

• In most metal forming processes, friction is

undesirable:

– Metal flow is retarded

– Forces and power are increased

– Wears tooling faster

• Friction and tool wear are more severe in hot

working

Lubrication in Metal Forming

• Metalworking lubricants are applied to tool-work

interface in many forming operations to reduce

harmful effects of friction

• Benefits:

– Reduced sticking, forces, power, tool wear

– Better surface finish

– Removes heat from the tooling

Considerations in Choosing a

Lubricant

• Type of forming process (rolling, forging, sheet

metal drawing, etc.)

• Hot working or cold working

• Work material

• Chemical reactivity with tool and work metals

• Ease of application

• Cost

Definitions

Plastic Deformation Processes

Operations that induce shape changes on the work piece by plastic deformation under forces applied

by various tools and dies.

Bulk Deformation Processes

These processes involve large amount of plastic deformation. The cross-section of workpiece

changes without volume change. The ratio cross-section area/volume is small. For most

operations, hot or warm working conditions are preferred although some operations are carried

out at room temperature.

Sheet-Forming Processes

In sheet metalworking operations, the cross-section of work piece does not change—the material

is only subjected to shape changes. The ratio cross-section area/volume is very high.

Sheet metalworking operations are performed on thin (less than 6 mm) sheets, strips or coils of

metal by means of a set of tools called punch and die on machine tools called stamping presses.

They are always performed as cold working operations.

Bulk Deformation Processes

Rolling: Compressive deformation process in which the thickness of a plate is

reduced by squeezing it through two rotating cylindrical rolls.

Forging: The workpiece is compressed between two opposing dies so that the die

shapes are imparted to the work.

Extrusion: The work material is forced to flow through a die opening taking its

shape

Drawing: The diameter of a wire or bar is reduced by pulling it through a die

opening (bar drawing) or a series of die openings (wire drawing)

Rolling

Definition

Rolling is a Bulk Deformation

Process in which the thickness

of the work is reduced by

compressive forces exerted by

two opposing rolls

22

Rolling

Rolling

Important Applications:

Steel Plants,

Raw stock production (sheets, tubes, Rods, etc.)

Screw manufacture

Rolling Basics

Sheets are rolled in multiple stages (why ?)

Vo

Vfto tf

Vo

Vfto tfVo

Vfto tf

Vo

Vfto tf

thread rolling machine

stationary die

rolling diethread rolling machine

stationary die

rolling die

Reciprocating flat thread-rolling diesReciprocating flat thread-rolling dies

Screw manufacture:

Forging

Definition

Forging is a Bulk Deformation Process

in which the work is compressed

between two dies. According to the

degree to which the flow of the metal

is constrained by the dies there are

three types of forging:

ΠOpen-die forging

• Impression-die forging

Ž Flash less forging

Forging

Stages in Open-Die Forging

(a) forge hot billet to max diameter

(b) “fuller: tool to mark step-locations

(c) forge right side

(d) reverse part, forge left side

(e) finish (dimension control)

[source:www.scotforge.com]

Stages in Impression-die (Closed-Die) Forging

[source:Kalpakjian & Schmid]

Stages in Impression-die (Closed-Die) Forging

Flash less forging

Forging grain flow

Quality of forged parts

Stronger/tougher than cast/machined parts of same material

Surface finish/Dimensional control:

Better than casting (typically)

[source:www.scotforge.com]

A material is pushed or drawn through a die of the desired cross-

section .Any solid or hollow cross-section may be produced by

extrusion, which can create essentially semi-finished parts. The

metal can forcing through a die in the same direction or opposite

direction.

Extrusion

Typical use: ductile metals (Cu, Steel, Al, Mg),

Plastics, Rubbers

Common products:

Al frames of white-boards, doors, windows, …

hydraulic

piston

chamber

chamber

stock

die

extruded shape

hydraulic

piston

chamber

chamber

stock

die

extruded shape

hydraulic

piston

chamber

chamber

stock

die

extruded shape

Extrusion: Schematic, Dies

Exercise: how can we get hollow parts?

• The cross-sections that can be produced vary from solid round, rectangular, to L shapes, T shapes.

• Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). Extrusions can be done with the material hot or cold.

• Commonly extruded materials include metals, polymers, ceramics, and foodstuffs.

Extruded products

• Typical products made by extrusion are railings for sliding doors,

tubing having carious cross-sections, structural and architectural

shapes, and door and windows frames.

Extruded products

• Direct extrusion: A metal billet is located into a container, and a ram compresses the material, forcing it to flow through one or more openings in a die at the opposite end of the container.

• Indirect extrusion: The die is mounted to the ram rather than at the opposite end of the container. One advantage of the indirect extrusion process is that there is no friction, during the process, between the billet and the container

liner.

Drawing

Commonly used to make wires from round bars

stock (bar)

F (pulling force)

wirediestock (bar)

F (pulling force)

wiredie

Similar to extrusion, except: pulling force is applied

WHAT is DRAWING?

Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die.

The principle of this procedure consist of reducing the thickness of a pointed ,tapered wire by drawing it through a conical opening in a tool made of a hard material.The wire will take shape of the hole.

• Drawing improves strength and hardness when these properties

are to be developed by cold work and not by subsequent heat

treatment• Where is it used?

This process is widely used for the production of thicker walled

seamless tubes and cylinders therefore; shafts, spindles, and small

pistons and as the raw material for fasteners such as rivets, bolts,

screws.

DRAWING TOOLS

• The most important tool in the drawing process is without

doubt the drawplate.This consist of a plate of high grade steel

into which similar shaped holes have been placed whose size

in evenly reduced from one hole to another.

• The most common drawplate have round holes and are used

to reduce the size of round wire.

Drawing wire with the draw tongs drawbench

How such a drawplate hole is made