02mech JTaylor Copy

7/26/2019 02mech JTaylor Copy

1/36

1Qualmet Services

Metal forming processesEF420 lecture 2

John Taylor


2/36

Metal forming

Using forces to shape solid metalplastically

Avoids problems with solidificationwhich can occur during casting

Minimises the high scrap loss ofmachining processes

Removes segregation and defectspresent in cast ingots

Promotes a desirable fibrestructure

Rolled thread

Machined thread


3/36

3Qualmet Services

Classification of methods

Primary metal working - Processing Forming ingots or other cast forms into simple

shapes (plate, sheet, bar) Often performed hot

Rolling, extrusion

Secondary metal working - Fabricating Producing components from simple shapes.

Often performed cold

Deep drawing, bending, shearing, machining


4/36

4Qualmet Services

Typical stress states


5/36

5Qualmet Services

Process analysis

Used to determine required forces, forselecting equipment

May predict failure Study of yielding behaviour (flow stress)

Total work put into working operation = work

involved in shape change + work to overcomefriction + redundant work.

Mathematical & computer modelling


6/36

6Qualmet Services

Cold working (forming)

Strength is increased and ductility reduced Strength improvements can be dramatic

Work hardening rate depends on material Grain structure is distorted

Dislocation population is increased

From ~104

lines/mm2

in fully annealed metal To ~1010lines/mm2in fully cold worked metal

Dislocation locking


7/367Qualmet Services

Cold work advantages

Strength, fatigue & wearproperties improved by cold

working Good surface finish &dimensional control No oxidation

Finishing processes may not beneeded



Cold work limitations

Higher forces and therefore more powerful equipmentis required

Only a limited amount of cold work can beundertaken before the material fails Brittle materials cannot be cold worked at all (tungsten,silicon carbide, glass)

Intermediate anneals may be required

Undesirable residual stress may be created



Cold worked material

Many materials are available in the coldworked condition

The temper designation is the amount of coldwork Annealed O no cold work

1/4 hard = 25% of the maximum cold work

possible 1/2 hard and 3/4 hard designations

Fully work hardened = no ductility left


10/36

10Qualmet Services

Annealing heat treatment

Removes the effect of cold work,

Increasing ductility

Reducing strength Improves other physical properties

Eg conductivity of copper

At a temperature of about 0.5 of the meltingtemperature in degrees Kelvin

Applied ONLY to cold worked metals


11/36

11Qualmet Services

Effect of annealing

New grains are nucleated where deformationis highest

The more heavy the cold work, the more grainsare nucleated & the finer the grain size

The new grains take over the cold workedmetal by diffusion annihilating the distorted

structure (recrystallisation) Dislocation density is reduced


12/36

12Qualmet Services

Grain growth

Reduction of grain boundary energy

Occurs at higher temperatures or insufficient amountsof deformation

Grain boundaries straighten

Large grains tend to consume small grains

Yield strength and ductility are both reduced

Can be inhibited when second phase particles pingrain boundaries


13/36

13Qualmet Services

Recrystallisation temperature

Depends on amount of initial cold work

Less than critical strain, no recrystallisation

Critical strain for iron - 10%, for aluminium - 1% High amount of cold work, lowerrecrystallisation temperature Also depends on the time at temperature.

Long times reduce the recrystallisation

temperature.


14/36

14Qualmet Services

Typical recrystallisation temperatures

Iron 450CMolybdenum 900C

Nickel 600CZinc 200C

Copper 200CLead 15C

Aluminium 150CTin 15C

Magnesium 150CCadmium 20C


15/36

15Qualmet Services

Hot working

Carried out at a temperature and strain rate atwhich recrystallisation is simultaneous with

deformation. Above about 60% of the absolute melting point New grains are continually formed

Material properties (yield strength, ductility) largely

independent of the amount of hot work, and are thesame as if the material was cold worked andannealed

The amount of deformation is limitless


16/36

16Qualmet Services

Effects of hot working

Crystal structure is refined

Original cast structure eliminated

Facilitates homogenisation Defects can be welded closed

Improves strength, ductility, toughness by

refining grain size.


17/36

17Qualmet Services

Temperature limits

The maximum temperature is determined bythe point at which constituents in the material

melt Low melting point phases may be present

High strain rates cause adiabatic heating

Must be above the recrystallisation

temperature Hot work can be at room temperature (Sn, Pb)


18/36

18Qualmet Services

Formability of a metal

Load required for yielding Reduced by increasing temperature

Material ductility Ability to stand tensile stress without cracking

Stress system imposed by forming Some processes more suitable than others forless ductile materials


19/36

19Qualmet Services

Pure metals

Flow stress decreases with melting point Low formability - tungsten

Good formability - tin, lead, zinc Ductility increases with number of slip planes

fcc - large number of slip planes, Al, Cu

bcc - fewer slip planes, Ferrite

cph - limited slip planes, Mg


20/36

20Qualmet Services

Alloying effect

Increasing alloying leveland complexity Usually lowers melting point

Raises work hardening rate

Generally higher alloylevels and more complexalloys are more difficult to

form

Temperature

Alloy content %

Hotworkingrange

Melting temperature


21/36

Forging

Localised compression is used to form complexshapes

Usually a hot work process, seldom cold

Open die forging Simple shapes, larger forgings, slow process

Closed die forging (stamping) Small items, large numbers, complex shape, expensive dies

Drop hammers (Hammer and anvil)

Press forming - more deeply penetrating


22/36

22Qualmet Services

Features of forging

A batch process, limited productivity

Capable of producing a wide variety of shapes

Components made by forging have betterproperties than those made by casting ormachining from stock

Useful in reducing machining costs Less machining time Less scrap


23/36

Forging types

Edging Fullering

Swaging (Shaft is rotated)

Drawing

Forging

Die

Gutter

Flash

Closed Die


24/36

24Qualmet Services

Forged products

High quality irregular shapes Gears, levers, crankshafts, pipe fittings, gas

cylinders, rings Fasteners (nuts, screws)

Coins (cold forgings)


25/36

25Qualmet Services

Rolling

By compressing between rolls, a material is reducedin thickness and increased in surface area

Can be regarded as a form of continuous open dieforging.

Cylindrical rolls produce flat products (plate & sheet) Thickness variations by controlling roll spacing

Grooved rolls used for long products Angles, channels, tees, beams, columns


26/36

Types of roll stands

2-high(reversing orNon-reversing)

3-high 4-high

Cluster rolls (12-high)

Backing rolls


27/36

27Qualmet Services

Hot rolling

Breakdown of as-cast shapes (ingots &strands) to slabs, blooms or billets

Finished steel sections and plates Simple two- or three-high rolling systems

Product has mill scale and has to be finished

May require pickling Structural steel can be sand blasted and primed


28/36

28Qualmet Services

Cold rolling

Improved finish

Higher forces needed

For better finish and dimensional control,more complex rolls are required


29/36

29Qualmet Services

Extrusion

Material compressed through a hole in a die to makea product of uniform cross section

A mode of deformation that occurs in other workingprocesses, particularly closed die forging

Almost always performed hot

Flow is complex, with a lot of redundant work(bending and unbending)

Friction plays an important part Surface of billet tends to stick to container, extrusion surfaceis new


30/36

30Qualmet Services

Extruded products

Long products, uniform cross section.

Can be complex sections, which cannot be

rolled Reentrant angles

Window frame sections

Small billets used to make containers Beer cans

Ductile materials (aluminium)


31/36

31Qualmet Services

Extrusion processes

Direct extrusion - solid Indirect extrusion - tube

Impact extrusion


32/36

32Qualmet Services

Deep drawing and pressing

The formation of shapes, such as cups and dishesfrom sheet material

Often undertaken cold to allow work hardening andmaintain a high surface quality

Stress systems vary over the surface and includebiaxial tension, bending and unbending,circumferential and ironing compression.

A high work hardening rate is desirable so thatdistortion is shared over the whole surface


33/36

33Qualmet Services

Final forming

Deformation less than primary forming

Bending is the most common process

Includes roll bending of plate, tube and sections Induction bending

Local induction heating of increment

Spinning of dished heads.


34/36

34Qualmet Services

Machining

Passing a tool through the metal, whichshould break off in chips.

Cold working with fracture of the chip from thecomponent

Ease of machining depends on: Design of tool

Lubrication

Material being cut (machinability)


35/36

35Qualmet Services

Machinability

Measured by speed of cutting

Strength of material affects the force

necessary for machining Ductility affects the type of chip formed andthe ease of its removal

Very ductile materials such as copper oraluminium spread under the cutting tool, andcan pressure-weld to it.


36/36

36Q l t S i

Machinability improved by

Low number of slip planes for dislocationglide

Compare fcc aluminium to cph magnesium Presence of brittle or weak second phaseparticles Graphite in cast iron, sulphides in free cutting steel

Cold work hardening mechanisms Solid solution, second phases, etc

02mech JTaylor Copy

Documents

Transcript of 02mech JTaylor Copy