Traditional Manufacturing Processes

Casting Forming Sheet metal processing Powder- and Ceramics Processing Plastics processing Cutting Joining 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:

 

K



n

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

Y f



K



n

where

Y f

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

Temperature in Metal Forming

• For any metal,

K

and

n

depend on temperature in the flow curve

– 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

operations, hot or warm cross-section area/volume is small. For most

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 ?) V o t t o V f V o t t o V f 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:

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

Surface finish/Dimensional control: Better than casting (typically) Stronger/tougher than cast/machined parts of same material [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, …

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

Similar to extrusion, except:

pulling force

is applied Commonly used to make wires from round bars

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