bulk deformation forming - forging

8/17/2019 Bulk Deformation Forming - Forging

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Bulk Deformation Forming -Forging

2Overview - Shaping and Forming

F

i ni s h i n g

Ingot

casting

Molten

Material

Powders

Casting

Shapes

RollingForging/

Press forming

Stamping

Pressing

Sheet metal

forming

ContinuousCasting/Rolling

Injection

Molding

M a c h i n i n g

R a w M

a t e r i a l

Special

Extruding

Single crystal

pulling

Firing/

Sintering

SLS

Increasing level of detail

Blow

moldingCurrent Lecture


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Forging Process

• Application of compressive force appliedthrough various mechanisms

• The forming of workpieces through a

succession of tools and dies• One of the oldest metalworking operations

• Initially just a hammer on an anvil (jewelry,horse shoes, sword making)

• Used to improved properties as well as forma shape

• Produces discrete parts

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Forging Process History

• Molds of stone helped initial forming efforts

• Now forces are – Mechanical (hammer presses) – Hydraulic

• Dies are tool steel• Near net shape forming


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Forging Practice

• Prepare raw material including cleaning• Heat workpiece (for hot forging)• Descale if necessary• Preheat and lubricate dies (hot forging)• Forge in appropriate dies and in correct

sequence• Remove excess material (flashing)• Clean• Check dimensions• Straighten if necessary• Machine to final dimensions• Heat treat if necessary

• Inspect

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Forging Process Capabilities

• Tolerances of 0.5% to 1% can be achieved

• Material properties can be tailored byappropriate die design

– Directed material flow


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Forging Processes

• Advantages – Metal flow and grain structure can be

controlled

– Results in good strength and toughness – Near net shape (low buy to fly ratio) – Parts of reasonable complexity can be created

• Landing gear

• Connecting rods

• Complex shafts

• Disadvantages – Dies are expensive, particularly for hot forging – Highly skilled labor required

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Forging Process Categories

Hot

Warm

Isothermal

Slug

Preform

Billet

Heading, Coining,

Cogging, Fullering,

Gathering, Hubbing,Piercing, Roll Forging,

Skew Rolling

Open die,Impression Die,

Closed Die,

Precision,

Orbital, Swaging

Trimming,Machining,

Cleaning

Heat

Treating

Preparation ForgingSubsequentOperations

I n s p e c t i on


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Open Die Forging and Cogging

• Simplest and cheapest

• Also called upsetting or flat-die forging

• Advantages – Cheap – Can form a wide variety of simple shapes with

the same dies• Squares, cylindrical

– Useful for preparing material for other forms offorging or machining

– Can handle large items (35 tons)

• Disadvantages

– Barreling of shape due to high friction

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Open Die Forging and Cogging


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Open Die Forging Force

F = Yf r 2 (1 + 2 r/3h)

where Yf is the flow stress of the material

is the coefficient of friction

r is the radius

h is the height of the workpiece

Examples

– Stainless steel workpiece, 150 mm diameter, 100 mmhigh reduced with flat dies to 50% of original height.Coefficient of friction is 0.2

– Force is 5000 tons

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Impression and Closed Die Forging

• Use dies with the approximate end shape

• Usually requires more than one die tocomplete process

• Fullering and Edging dies prepare material to

take up die shape – Fullering moves material away from center – Edging moves material away from edges

• Flashing produced from excess material

• Often used to ensure good die filling


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13Stages in Impression

Die Forging

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Load in impression-die forging


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15Stages in the forging of a crank

shaft

16Terminology of Impression

Forging


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17Impression and Closed Die

Forging

• Advantages – Produces near net shape

– Material properties tailored to application• Disadvantages

– High die costs – Highly skilled labor required

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Precision Forging

• A further development of closed die forging

• Close calculation of material required to fill dieminimizes scrap and flashing

• Dies have more detail minimizing subsequent

shaping operations• Advantages

– Little subsequent shaping – Good to excellent properties

• Disadvantages – Expensive – Difficult to control


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Closed Die Forging ForceF = k Yf A

where Yf is the flow stress

A is the area and

k is a factor given below

Shapes k

Simple, no flashing 3-5

simple, with flashing 5-8

Complex, with flashing 8-12

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Related Processes

• Coining – Similar to precision forging but much older – Die cavity completely closed – Very high pressures involved

– Used in coin making• Heading

– Used mostly for bolts


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Related Processes

• Isothermal forging – Dies at same temperature of workpiece – No workpiece cooling – Low flow stresses – Better material flow – More close tolerances and finer details can be

achieved

• Swaging – Cylindrical parts subjected to radial impact

forces by reciprocating dies – Used to reduce tube diameter and introduce

rifling into gun barrels

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Die Design

• Requires knowledge of – Material strength – Sensitivity of these to deformation rate and

temperature – Friction and its control

– Shape and complexity of workpiece – How the metal will flow to fill the die cavity – Great skill and expertise – Multiple dies to move the material in the right

direction


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Forgeability

• Defined as the capability of a material toundergo deformation without cracking

• Common test is the upset test

– Upset cylindrical specimen to fixed, largedeformation – Examine barrel surfaces for cracks

• Another is the hot torsion test – Twist long cylindrical specimen around its axis – No of twists to failure is forgeability – Also used for rolling and extrusion

deformation capabilities

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Hot forging Temperatures

Material TemperatureRange (oC)

Aluminum alloys 400-550

Magnesium alloys 250-35-

Copper alloys 600-900Carbon and low alloy steels 850-1150

Stainless steels 1100-1250

Titanium alloys 700-950

Iron based superalloys 1050-1180

Tungsten alloys 1200-1300


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Product Quality Issues

• Surface cracks (forgeability limitation)

• Buckling• Laps

• Internal cracks

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Defects


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Defects

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Forging Machines

• Mechanical Presses – Hydraulic – Mechanical – Screw – Hammers

– Gravity Drop – Power Drop – Counterblow – High Energy Rate


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Hydraulic Presses

• Constant speed

• Load limited

• Compared to mechanical – Typically slower – Higher initial cost – Less maintenance

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Mechanical Presses

• Crank or eccentric types

• Stroke limited

• Energy dependent on that stored in flywheel

• Very large forces can be generated at bottomdead center

• Hence must be careful in die design andplacement to avoid die fracture


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Screw Presses

• Derive energy from flywheel like mechanicalpresses

• Flywheel drives a screw, not a ram

• Energy limited• Process stops when flywheel energy

exhausted

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Hammers

• Ram is raised by some mechanism and letfall onto workpiece

• Derives energy from potential energy of thehammer

• They are energy limited• High speeds

• Minimal cooling

• Different types – Gravity drop – Power drop – Counterblow – High energy rate machines


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35Characteristics of Forging

ProcessesProcess Advantages Limitations

Open die Cheap dies limited to simple shapesSmall quantities Poor tolerancesWide range of sizes Low production rates

Good strengths high skill required

Closed Die Good utilization of material High die costsBetter properties Machining necessaryGood tolerances and accuracyHigh ratesGood reproducibility

Precision Close tolerances Machining necessaryMachining unnecessary High forcesExcellent material utilization Intricate diesThin webs and flanges Need way of removing part

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Comparison of Forging Presses


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Forging Economics

• Setup and tooling costs are high initially

• Good for large production quantities

• Material costs as a fraction of total costs varywith material – High percentage for stainless steels (70-85%) – Low percentage for carbon steel (25-45%)

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Summary

• General characteristics of forming processes(metals only)

• General

• Bulk deformation processes – Forging• Open Die

• Closed Die

• Other

• Summary

bulk deformation forming - forging

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