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, st rength, 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 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 s ingle plane, then it is good practice to use symmetry of the design to minimiz minimizee 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. Draft Angle (º)
Material
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 Min. Corner Radius Min. Fillet Radius mm mm mm (in) (in) (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
2
2
mm (0.040 inch) for small forgings, die projection area < 150 cm (23 in ), to 6.25 2 2 mm (0.25 inch) for large forgings, die projection area > 6500 cm (100 in ). • 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. Finished Forging Weight Trimmed kg (lb) Material
< 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. Finished Forging Weight Trimmed kg (lb) Material
< 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 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 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 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 elements are used to enhance physical and mechanical properties and/or heat-treat response. AMS — Aeronautical 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, hotrolled, or continuous-cast. Bloom — A term used interchangeably with billet. A bloom is a semi finished metal product usually of square or rectangular 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 Machi ned forgings are offered to the marketplace 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 — tolerances — or or — a — 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 complete rolled rings. The highly engineered ring rolling process utilizing state of the art computer controlled equipment, equipment, NC lathes and the Ferralloy, Inc. distribution model offers the marketplace immediate, measurable economic benefits. SAE — The Society of Automotive Engineers.
— Using either centrifugal force or air pressure to propel metal at a high Shot blasting — Using 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 http://www.efunda.com/processes/metal_pr ocessing/for ging.cfm
