1 Chapter 2 Bulk Deformation Forming - Forging. 2 Forging Process Application of compressive force...
-
Upload
adele-walsh -
Category
Documents
-
view
228 -
download
1
Transcript of 1 Chapter 2 Bulk Deformation Forming - Forging. 2 Forging Process Application of compressive force...
2
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
3
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
4
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
5
Forging Process Capabilities Tolerances of 0.5% to 1% can be
achieved Material properties can be tailored by
appropriate die design
– Directed material flow
6
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
8
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
10
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
11
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
16
Impression and Closed Die
Forging Advantages – Produces near net shape – Material properties tailored
to application Disadvantages – High die costs – Highly skilled labor required
17
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
18
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
19
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
20
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.
22
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
23
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
24
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
25
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
28
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.
29
Defects
Effect of fillet radius on defect formation in forging. Small fillets (right side of drawings) cause the defects.
30
Forging Machines Mechanical Presses – Hydraulic – Mechanical – Screw – Hammers – Gravity Drop – Power Drop – Counterblow – High Energy Rate
31
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
32
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
33
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
34
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
35
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