1. Bulk Deformation Forming - Forging
Chapter 2
Bulk Deformation
Forming - Forging

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

7. Forging Process Categories

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

9. Open Die Forging and Cogging

10. Open Die Forging Force F = Yf p r2 (1 + 2mr/3h)
where Yf is the flow stress of the material
m is the coefficient of friction
r is the radius
h is the height of the workpiece
Examples
– 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

12. Stages in Impression Die Forging

13. Load in impression-die forging

14. Stages in the forging of a con-rod

15. Terminology of Impression Forging

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 Force
F = 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
simple, with flashing
Complex, with flashing

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.

21. Skew rolling
Production of steel balls for
bearings by the skew rolling
process.

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

26. Hot forging Temperatures

27. Product Quality Issues
Surface cracks (forgeability limitation)
Buckling
Laps
Internal cracks

28. Laps formed by buckling of the web during forging.
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 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 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

36. Characteristics of Forging Processes

37. 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%)