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Lesson 3.1DeformChapter 3 · Bulk forming

Forging (open-die and closed-die)

Squeeze hot metal between flat or shaped dies

Forging shapes a solid piece of metal by squeezing or hammering it, usually while it glows red-hot. Wrenches, car crankshafts and aircraft landing-gear parts all start this way. The metal is never melted, so its grain (the fiber-like pattern of its crystals) follows the part's outline, which makes forgings very tough.

Forging (open-die and closed-die): a real photo
Photo: F. Broer, CC BY-SA 3.0, via Wikimedia Commons
Hot metal is squeezed between dies until it takes their shapeOpen-die: flat dies, the bar is moved and turned between blows. Closed-die: shaped dies copy a cavity.1Heat the billetabout 1,200 °Cfurnacebillet2Open-die forgingramupper dielower dieanvilpress or hammermanipulatorhot barturned 90°between blowsmoved along after each blowsqueezed part:thinner, longerClosed-die forging(also called impression-die): the dies carry the shape of the part3Billet on the lower dieupper dielower die4Dies close, metal fills cavityflashexcess metal squeezes out as flash5Flash trimmed offfinished forgingtrimmed flash, recycledKey:metal being shapeddies and machine partsheat
Forging: heat, then shape in open dies or closed diesCircled numbers match the steps below: step 2 is the open-die route, steps 3 to 5 the closed-die route.

Forging (open-die and closed-die) in a minute, by MecMade

How it works

  1. Heat the billet. A billet (a cut length of bar) goes into a furnace. Steel is typically heated to about 1,100–1,250 °C, until it glows orange-yellow and is soft enough to flow.

  2. Open-die forging: squeeze, turn, repeat. Flat dies (hardened steel blocks) press or hammer the hot bar one section at a time. A manipulator (a large mechanical gripper) turns and moves the bar between blows. Bit by bit, the bar is stretched, flattened or upset (squashed shorter and fatter).

  3. Closed-die forging: place the billet. For mass production, the hot billet sits on a lower die that carries half of the part's shape as a cavity. The upper die carries the other half.

  4. Close the dies. A hammer or press drives the upper die down in one to a few strokes. The metal flows into every corner of the cavity. The excess squeezes out between the dies as flash (a thin fin of surplus metal).

  5. Trim and finish. A trimming press shears off the flash, which is recycled. The part is then usually cleaned of scale (the dark oxide skin), heat-treated, and machined wherever a precise fit is needed.

Key facts

Materials
Carbon and alloy steels, stainless steel, aluminum, titanium, copper alloys, nickel superalloys
Typical parts
Crankshafts, connecting rods, gears, hand tools, flanges, turbine disks, ship and generator shafts
Part size
Closed-die: from a few grams to hundreds of kilograms. Open-die: up to shafts and rings of over 100 tonnes
Precision
Closed-die: about ±0.5 mm. Open-die: several millimeters, then machined
Surface finish
Rough, scaly skin; cleaned by shot blasting, with critical faces machined
Batch size it suits
Open-die: one-offs to a few dozen. Closed-die: thousands to millions
Tooling and machine cost
Closed-die sets: thousands to tens of thousands of dollars. Big presses and hammers: millions
Speed
Closed-die: seconds to about a minute per part. Open-die: minutes to hours per part, with reheating

Strengths and limits

Strengths

  • Very strong and tough: the grain follows the shape and there are no hidden holes
  • Closed-die work is fast and repeatable for big runs
  • Open-die can make huge parts that few other processes can
  • Material-efficient: the flash and offcuts are recycled

Limits

  • Loose tolerances and a rough skin, so most forgings need some machining
  • Closed dies are costly and take weeks to make, so small batches are expensive
  • Heating uses a lot of energy, and the work is hot, loud and heavy
  • Hollow insides, undercuts (overhanging shapes) and very thin walls are hard or impossible

Where it's used

  • AutomotiveCrankshafts, connecting rods, gears, steering knuckles, wheel hubs
  • AerospaceLanding-gear beams, turbine disks, engine mounts, wing fittings
  • EnergyGenerator and turbine shafts, pressure-vessel rings, valve bodies, wind-turbine flanges
  • Tools and hardwareWrenches, hammers, pliers, chisels
  • Oil, gas and miningDrill-bit bodies, pipe flanges, heavy hooks, chain links

How to recognize it

  • A thin band around the part where the flash was trimmed off, often slightly rough or ground (the parting line).
  • Rounded edges and gently sloped sides (draft, a slight taper that lets the part leave the die).
  • A dark or matte, slightly pitted skin on unmachined areas, with bright machined faces only where parts fit together.
  • Raised letters or part numbers, copied from markings cut into the die.

Compare with

Sand casting1.1casting pours liquid metal into a mold; forging squeezes solid metal, so it is tougher but less free in shape.Rolling3.2rolling squeezes metal continuously between turning rolls; forging shapes one piece at a time.Cold heading3.5a fast, cold version of closed-die forging for small parts such as screws and bolts.

Watch it

Scot Forge Company · How a forge heats, presses and turns metal in open dies

Shubham Kola · The main forging types explained side by side

CFS Forge · A crankshaft forged in closed dies, then trimmed of its flash

HBEPRESS · The sheer scale of open-die forging on a glowing ingot

See real photos

Remember

  • Forging squeezes solid, usually red-hot metal into shape; nothing is melted or cut away.
  • Open dies shape big or one-off parts bit by bit; closed dies copy a cavity quickly for mass production.
  • Choose forging when a part must not break: crankshafts, landing gear, hand tools.

More in Bulk forming

All 100 processes
3.2RollingPull metal through the gap between turning rollsOpen the lesson3.3Extrusion (aluminum profiles and plastic pipes)Push metal or plastic through a shaped dieOpen the lesson3.4Wire drawingPull a rod through ever-smaller diesOpen the lesson3.5Cold headingCut wire into blanks and punch a head, coldOpen the lesson3.6Thread rolling and tappingPress a thread onto a blank, or cut or press one in a holeOpen the lesson

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