Introduction

Forging is the process by which metal is heated and is shaped by plastic deformation by

suitably applying compressive force. Usually the compressive force is in the form of

hammer blows using a power hammer or a press.

Forging refines the grain structure and improves physical properties of the metal. With

proper design, the grain flow can be oriented in the direction of principal stresses

encountered in actual use. Grain flow is the direction of the pattern that the crystals take

during plastic deformation. Physical properties (such as strength, ductility and toughness)

are much better in a forging than in the base metal, which has, crystals randomly

oriented.

Forgings are consistent from piece to piece, without any of the porosity, voids, inclusions

and other defects. Thus, finishing operations such as machining do not expose voids,

because there aren't any. Also coating operations such as plating or painting are

straightforward due to a good surface, which needs very little preparation.

Forgings yield parts that have high strength to weight ratio-thus are often used in the

design of aircraft frame members.

A Forged metal can result in the following

• Increase length, decrease cross-section, called drawing out the metal.

• Decrease length, increase cross-section, called upsetting the metal.

• Change length, change cross-section, by squeezing in closed impression dies. This

results in favorable grain flow for strong parts

Common Forging Processes

The metal can be forged hot (above re-crystallization temperatures) or cold.

Open Die Forgings / Hand Forgings: Open die forgings or hand forgings are made with

repeated blows in an open die, where the operator manipulates the workpiece in the die.

The finished product is a rough approximation of the die. This is what a traditional

blacksmith does, and is an old manufacturing process.

Impression Die Forgings / Precision Forgings: Impression die forgings and precision

forgings are further refinements of the blocker forgings. The finished part more closely

resembles the die impression.

Design Consideration:

• Parting surface should be along a single plane if possible, else follow the contour of

the part. The parting surface should be through the center of the part, not near the

upper or lower edges. If the parting line cannot be on a single plane, then it is good

practice to use symmetry of the design to minimize the side thrust forces. Any point

on the parting surface should be less than 75º from the principal parting plane.

• As in most forming processes, use of undercuts should be avoided, as these will

make the removal of the part difficult, if not impossible.

• Recommended draft angles are described in the following table.

Material Draft Angle (º)

Aluminum 0 - 2

Copper Alloys (Brass) 0 - 3

Steel 5 - 7

Stainless Steel 5 - 8

• Generous fillets and radius should be provided to aid in material flow during the

forging process. Sharp corners are stress-risers in the forgings, as well as make the

dies weak in service. Recommended minimum radiuses are described in the

following table.

Height of Protrusion mm

(in)

Min. Corner Radius mm

(in)

Min. Fillet Radius mm

(in)

12.5

(0.5)

1.5

(0.06)

5

(0.2)

25

(1.0)

3

(0.12)

6.25

(0.25)

50

(2.0)

5

(0.2)

10

(0.4)

100

(4.0)

6.25

(0.25)

10

(0.4)

400

(16)

22

(0.875)

50

(2.0)

• Ribs should be not be high or narrow, this makes it difficult for the material to flow.

Tolerances:

• Dimension tolerances are usually positive and are approximately 0.3 % of the

dimension, rounded off to the next higher 0.5 mm (0.020 in).

• Die wear tolerances are lateral tolerances (parallel to the parting plane) and are

roughly +0.2 % for Copper alloys to +0.5 % for Aluminum and Steel.

• Die closure tolerances are in the direction of opening and closing, and range from 1

mm (0.040 inch) for small forgings, die projection area < 150 cm2 (23 in

2), to 6.25

mm (0.25 inch) for large forgings, die projection area > 6500 cm2(100 in

2).

• Die match tolerances are to allow for shift in the upper die with respect to the lower

die. This is weight based and is shown in the the following table.

Material

Finished Forging Weight Trimmed kg (lb)

< 10

(< 22)

< 50

(< 110)

> 500

(> 1100)

Die Match Tolerance mm (in)

Aluminum,

Copper Alloys,

Steel

0.75

(0.030)

1.75

(0.070)

5

(0.200)

Stainless Steel,

Titanium

1.25

(0.050)

2.5

(0.100)

6.5

(0.260)

• Flash tolerance is the amount of acceptable flash after the trimming operation. This

is weight based and is shown in the following table.

Material

Finished Forging Weight Trimmed kg (lb)

< 10

(< 22)

< 50

(< 110)

> 500

(> 1100)

Flash Tolerance

mm (in)

Aluminum,

Copper Alloys,

Steel

0.8

(0.032)

3.25

(0.125)

10

(0.4)

Stainless Steel,

Titanium

1.6

(0.064)

5

(0.2)

12.5

(0.5)

A proper lubricant is necessary for making good forgings. The lubricant is useful in

preventing sticking of the workpiece to the die, and also acts as a thermal insulator to

help reduce die wear.

Press Forgings: Press forging use a slow squeezing action of a press, to transfer a great

amount of compressive force to the workpiece. Unlike an open-die forging where

multiple blows transfer the compressive energy to the outside of the product, press

forging transfers the force uniformly to the bulk of the material. This results in uniform

material properties and is necessary for large weight forgings. Parts made with this

process can be quite large as much as 125 kg (260 lb) and 3m (10 feet) long.

Upset Forgings: Upset forging increases cross-section by compressing the length, this is

used in making heads on bolts and fasteners, valves and other similar parts.

Roll Forgings: In roll forging, a bar stock, round or flat is placed between die rollers

which reduces the cross-section and increases the length to form parts such as axles, leaf

springs etc. This is essentially a form of draw forging.

Swaging: Swaging - a tube or rod is forced inside a die and the diameter is reduced as the

cylindrical object is fed. The die hammers the diameter and causes the metal to flow

inward causing the outer diameter of the tube or the rod to take the shape of the die.

Net Shape / Near-Net Shape Forging: In net shape or near-net shape forging, forging

results in wastage of material in the form of material flash and subsequent machining

operations. This wastage can be as high as 70 % for gear blanks, and even 90+ % in the

case of aircraft structural parts. Net-shape and near-net-shape processes minimize the

waste by making precision dies, producing parts with very little draft angle (less than 1º).

These types of processes often eliminate or reduce machining. The processes are

quite expensive in terms of tooling and the capital expenditure required. Thus, these

processes can be only justified for current processes that are very wasteful where the

material savings will pay for the significant increase in tooling costs.

Forging - Glossary

Aircraft quality — A high quality forging involved in controlled manufacturing

processes that must pass a series of required tests before approval.

Alloy steel forging — A forging made from steel containing alloying elements other than

carbon (e.g., Ni, Cr, Mo). These additional elements are used to enhance physical and

mechanical properties and/or heat-treat response.

AMS — Aeronautical Materials Specification

As forged — Refers to the state of a forging as it comes out of the forging die.

ASTM — The American Society for Testing and Materials.

Axisymmetric forging — A type of forging that causes metal flow to move in a

direction away from a common axis in a radial direction.

Batch-type furnace — A single door heat treating furnace used to heat treat materials.

Billet — A term used interchangeably with bloom. A billet is a semi finished metal

product usually of rectangular shape and uniform section. Billets can be cogged, hot-

rolled, or continuous-cast.

Bloom — A term used interchangeably with billet. A bloom is a semi finished metal

product usually of square or rectangular shape. Blooms can be hot rolled or forged.

Brinell hardness — A rating of hardness for a metal part. The Brinell Hardness

Number (BHN) is determined using a standard table. Critical information includes the

diameter of the impression left by a metal ball that is pushed into the surface metal and

the load applied on the ball during the impression.

Carbon steel — Steel that derives its physical properties from the presence of carbon.

Chamfer — Using a straight angle or a grinding wheel to break or remove the sharp

edges or corners resulting from forging.

Charpy impact test — An impact test in which a special V-notched specimen is broken

by the impact of a falling pendulum. The energy absorbed in fracture is a measure of the

impact strength or notch toughness of the sample.

Closed die forging — A forging process that shapes hot metal as two dies exert pressure

from both sides as they come together. Closed die forgings are supplied from ounces to

almost 100 pounds each. Most forgings supplied by Ferralloy are machined complete to

our customer’s requirements. Machined forgings are offered to the marketplace through

our inventory stocking program to assist our customers in better inventory management

as well as providing more timely deliveries.

Coining — A process used after initial forging to improve the surface of the forging or to

attain closer tolerances — or— a closed die process that stamps the surface of a metal

part with an imprint of the die.

Cold forging — Various forging processes conducted at or near ambient temperatures to

produce metal components to close tolerances and net shape. These include bending, cold

drawing, cold heading, coining, extrusion (forward or backward), punching, thread

rolling and others.

Cold heading — Plastically deforming metal at ambient temperatures to increase the

cross-sectional area of the stock (either solid bar or tubing) at one or more points along

the longitudinal axis.

Cold lap — An error that is caused by the metal not completely filling the die during the

forging process. Subsequent forging allows metal to fill in around the gap, however a

seam forms between the layers of metal.

Cold shut — An error that occurs as metal folds over itself during the forging process. A

cold shut commonly occurs where the vertical and horizontal surfaces meet.

Cold working — Forging at a temperature below the metal’s recrystallization point.

Concentricity — Adherence of part features to a common axis.

Decarburization — Using heat to remove carbon from the surface of steel.

Dies(forging) — Tools or devices for creating a desired shape, form, or finish with a

source material. Dies range from simple to complex, requiring anywhere from one to a

number of impressions to form the desired shape. In forging, dies are usually paired up to

exert pressure on both sides of a metal part.

Draft — The taper on the side of a forging to allow removal from the dies; also applies to

the die impression. It is Commonly expressed in degrees as the draft angle. As applied to

open die forging, draft is the amount of relative movement of the dies toward each other

through the metal in one application of power.

Draft angle — The angle of taper, expressed in degrees (usually 5° to 7°), given to the

sides of the forging and the side walls of the die impression.

Extrusion — Shaping metal by a process that forces it through the die. Extrusion can

take place using either forward or backwards force depending on product specifications.

Flash — The metal that extends out from the forging because it is in excess of the metal

required to fill the die. Flash is typically removed by trimming.

Heat — The commonly used name for the product of a single melting operation.

Heat-resistant steel — Alloy steel designed for application at elevated temperatures.

Heat treatment — Creating metal parts with desired properties/specific ations by using a

sequence of controlled heating and cooling operations.

Induction heating — Heating metals by means of an alternating magnetic field.

Ingot — A casting that will undergo subsequent rolling, forging, or extrusion.

Lap — An error that occurs as metal folds over itself and causes surface irregularities

that appear as fissures or openings.

Mandrel — A blunt-ended tool or rod used to retain or enlarge the cavity in a hollow

metal product during forging.

Nonferrous — Metals like aluminum, copper and magnesium that contain no appreciable

quantity of iron.

Open die forging — Forging when the metal being shaped is not completely confined

during the process. The open die forging process uses hammers and presses to shape

metal parts, usually using repeated strokes and continuous manipulation.

Pickling — Using a heated acid bath to remove oxide scale from forgings.

Rolled ring forgings — Produced in diameters up to 196” from carbon, alloy and

stainless steel grades. Rolled ring forgings can be supplied in as forged condition or we

can provide rough machining as well as machined complete rolled rings. The highly

engineered ring rolling process utilizing state of the art computer controlled equipment,

NC lathes and the Ferralloy, Inc. distribution model offers the marketplace immediate,

measurable economic benefits.

SAE— The Society of Automotive Engineers.

Shot blasting — Using either centrifugal force or air pressure to propel metal at a high

velocity at forgings. This process is designed to blast clean the forgings.

Shrinkage — The contraction of metal during cooling after hot forging. Die impressions

are made oversize according to precise shrinkage scales to allow the forgings to shrink to

design dimensions and tolerances.

Swaging — (1) Reducing the diameter of or rounding out a section of a forging by a

series of blows, tapering the forging lengthwise until the entire section attains the smaller

dimension of the taper. (2) Tapering forging stock by forging, hammering, or squeezing.

Source

http://www.ferralloy.com/forging_glossary.htm

http://www.efunda.com/processes/metal_processing/for

ging.cfm