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Manufacturing Processes > Forming Processes > Forging

 

Forging

 

Process description

Hot metal is formed into the required shape by the application of pressure or impact forces causing plastic deformation using a press or hammer in a single or a series of dies.

Materials

  • Mainly carbon, low alloy and stainless steels, aluminum, copper and magnesium alloys. Titanium alloys, nickel alloys, high alloy steels and refractory metals can also be forged.
  • Forgeability of materials important; must be ductile at forging temperature. Relative forgeability is as follows, with the easiest to forge first: aluminum alloys, magnesium alloys, copper alloys, carbon steels, low alloy steels, stainless steels, titanium alloys, high alloy steels, refractory metals and nickel alloys.
Forging

Process variations

  • Presses can be mechanical, hydraulic or drop hammer type.
  • Closed die forging: series of die impressions used to generate shape.
  • Open die forging: hot material deformed between a flat or shaped punch and die. Sections can be flat, square, round or polygon. Shape and dimensions largely controlled by operator.
  • Roll forging: reduction of section thickness of a doughnut-shaped preform to increase its diameter. Similar to ring rolling , but uses impact forces from hammers.
  • Upset forging: heated metal stock gripped by dies and end pressed into desired shape, i.e. increasing the diameter by reducing height.
  • Hand forging: hot material reduced, upset and shaped using hand tools and an anvil. Commonly associated with the blacksmith’s trade, used for decorative and architectural work.
  • Precision forging: near-net shape generation through the use of precision dies. Reduces waste and minimizes or eliminates machining.

Economic considerations

  • Production rates from 1 to 300/h, depending on size.
  • Production most economic in the production of symmetrical rough forged blanks using flat dies. Increased machining is justified by increased die life.
  • Lead times typically weeks.
  • Material utilization moderate (20–25 per cent scrap generated in flash typically).
  • Economically viable quantities greater than 10 000, but can be as low as 100 for large parts.
  • In the case of open die forging: lower material utilization, machining of the final shape necessary, slow production rate, low lead times, commonly used for one-offs and high usage of skilled labor.
  • Tooling costs high.
  • Equipment costs generally high.
  • Direct labor costs moderate. Some skilled operations may be required.
  • Finishing costs moderate. Removal of flash, cleaning and fettling important for subsequent operations.

Typical applications

  • Engine components (connecting rods, crankshafts, camshafts)
  • Transmission components (gears, shafts, hubs, axles)
  • Aircraft components (landing gear, airframe parts)
  • Tool bodies
  • Levers
  • Upset forging: for bolt heads, valve stems
  • Open die forging: for die blocks, large shafts, pressure vessels

Design aspects

  • Complexity is limited by material flow through dies.
  • Deep holes with small diameters are better drilled.
  • Drill spots caused by die impressions can be used to aid drill centralization for subsequent machining operations.
  • Locating points for machining should be away from parting line due to die wear.
  • Markings are possible at little expense on adequate areas that are not to be subsequently machined.
  • Care should be taken with design of die geometry, since cracking, mismatch, internal rupture and irregular grain flow can occur.
  • It is good practice to have approximately equal volumes of material both above and below the parting line.
  • Inserts and undercuts are not possible.
  • Placing of parting line is important, i.e. avoid placement across critical dimensions, keep along simple plane, line of symmetry or follow the part profile.
  • Corner radii and fillets should be as large as possible to aid hot metal flow.
  • Maximum length to diameter ratio that can be upset is 3:1.
  • Avoid abrupt changes in section thickness. Causes stress concentrations on cooling.
  • Minimum corner radii =1.5 mm.
  • Machining allowances range from 0.8 to 6 mm, depending on size.
  • Drafts must be added to all surfaces perpendicular to the parting line.
  • Draft angles ranging 0–8°, depending on internal or external features, and section depth, but typically 4°. Reduced by mechanical ejectors in dies.
  • Minimum section =3 mm.
  • Sizes ranging 10 g–250 kg in weight, but better for parts less than 20 kg.

Quality issues

  • Good strength, fatigue resistance and toughness in forged parts due to grain structure alignment with die impression and principal stresses expected in service.
  • Low porosity, defects and voids encountered.
  • Forgeability of material important and maintenance of optimum forging temperature during processing.
  • Hot material in contact with the die too long will cause excessive wear, softening and breakage.
  • Variation in blank mass causes thickness variation. Reduced by allowing for flash generation, but increases waste.
  • Residual stresses can be significant. Can be improved with heat treatment.
  • Die wear and mismatch may be significant.
  • Surface roughness and detail may be adequate, but secondary processing usually employed to improve the surface properties.
  • Surface roughness ranging 1.6–25 µm Ra.
  • Process capability charts showing the achievable dimensional tolerances for closed die forging using various materials are provided. Note, the total tolerance on Charts 1–4 is allocated +2/3, -1/3. Allowances of +0.3–+2.8mm should be added for dimensions across the parting line and mismatch tolerances ranging 0.3–2.4mm, depending on part size .
  • Tolerances for open die forging ranging ±2–±50 mm, depending on size of work and skill of the operator.

costing