1 Chapter 2 Bulk Deformation Forming - Forging. 2 Forging Process Application of compressive force applied through various mechanisms The forming of workpieces.

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Chapter 2 Bulk Deformation

Forming - Forging

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Forging Process Application of compressive force applied

through 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 form

a 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 by

appropriate 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 – 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

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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 of forging 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 r2 (1 + 2r/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 workpieceExamples– Stainless steel workpiece, 150 mm

diameter, 100 mm high 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 to complete

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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Stages in Impression Die Forging

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Load in impression-die forging

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Stages in the forging of a con-rod

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Terminology of Impression

Forging

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Impression 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 die

minimizes 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 belowShapes k

Simple, no flashing 3-5simple, with flashing 5-8Complex, 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 Piercing – Exactly as it sounds – Makes holes – Used in conjunction with closed die forging Hubbing – Like piercing but for making cavities, not complete

penetrations larger areas Roll Forging – Uses rolls to shape parts – Similar to shape rolling but makes discrete parts – (cross-rolling) operation. Tapered leaf springs and

knives can be made by this process with specially designed

rolls.

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Skew rolling

Production of steel balls for

bearings by the skew rolling

process.

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Orbital Forging – Forms the part incrementally – Small forging forces because the die contact is – concentrated on a small part of the workpiece at

anyone time – Applicable to mostly cylindrical shapes Incremental forging – Blank formed in several small steps like orbital – non-rotational parts can be made

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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 to undergo

deformation without cracking Common test is the upset test

– Upset cylindrical specimen to fixed, large deformation

– 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

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Product Quality Issues

Surface cracks (forgeability limitation) Buckling Laps Internal cracks

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Defects

Laps formed by buckling of the web during forging.

Internal defects produced in a forging because of an oversized billet. The die cavities are filled prematurely, and the material at the center of the

part flows past the filled regions as deformation continues.

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Defects

Effect of fillet radius on defect formation in forging. Small fillets (right side of drawings) cause the 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 Large amount of energy can be

transmitted to the workpiece suited for extrusion-type forging Used for both open-die and closed-die

forging The largest H-press in the world is

75000 tons, The largest of our country is 25000tons

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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 bottom

dead center Hence must be careful in die design and

placement to avoid die fracture

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Screw Presses Derive energy from flywheel

like mechanical presses Flywheel drives a screw, not a ram Energy limited Process stops when flywheel energy exhausted

Suitable for producing small quantities, for parts requiring

precision (such as turbine blades), and for control of ram

speed

The largest screw press has a capacity of 16000 tons

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Hammers Ram is raised by some mechanism and let fall onto

workpiece Derives energy from potential energy of the hammer They are energy limited High speeds Minimal cooling Different types – Gravity drop – Power drop – Counterblow – High energy rate machines

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Equipment Selection

The selection of forging equipment depend on:

The size and complexity of the forging

The strength of the material and its sensitivity to strain rate

The degree of deformation

Guideline

Presses are generally preferred for aluminium, magnesium,

beryllium, bronze and brass

Hammer are preferred for copper, steel, titanium and refractory

alloys

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Characteristics of Forging Processes

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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 vary

with material

– High percentage for stainless steels (70-85%)

– Low percentage for carbon steel (25-45%)