Drop forging

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Closed die forging with a burr
Open-die forging. (Free forms)
Procedure

Die forging or die forming is a manufacturing process from the main group of forming . There it is assigned to the group of pressure forming together with rolling , free- forming , indenting and pressing . According to DIN 8583, it is pressure forming with forming tools, the dies, moving against one another . The shape to be produced is at least partially contained in the die as a negative. As drop forgingall steps are referred to that are necessary for the production of closed-die forgings. This also includes separating the blanks from semi-finished products , heating and descaling as well as heat and surface treatment . The actual forming process is known as die forming . It is divided into die forms with a partially enclosed die and die forms with a completely enclosed die. Drop forging is mostly used at workpiece temperatures above the recrystallization temperature , as hot forming . Sometimes it is also used at room temperature or at a temperature just below the recrystallization temperature ( warm forming ). Steels are often used as materials, especially structural steels, hot-work steels and stainless steels. In addition, aluminum and magnesium alloys are used, especially special wrought alloys . Titanium , nickel - cobalt and molybdenum alloys require a great deal of force and in some cases only have poor formability . Special variants of closed-die forging are exact and precision forging as well as thixo-forging .

history

Bronze and copper were forged in ancient times around 2500 BC . Around 1500 BC One-sided hollow forms were used for drop forging. To do this, the blank was placed in the die and hammered into the mold. From 600 BC Two-piece bronze tools were used for minting coins . In the Middle Ages, disc brooches were made in this way . In 1848 knives were drop forged for the first time. Modern drop forging tools existed in England towards the end of the 19th century. This was facilitated by the development of the steam hammer , which also allowed very large parts to be machined. In the course of the 20th century, the levels of accuracy that can be achieved with the machines improved. At the turn of the 21st century, precision forging and thixo-forging emerged, both of which no longer require machining. In addition to increasing accuracy, modern developments aim at a higher degree of automation .

Economical meaning

Drop forging plays an important role in metal processing . Drop forged parts are, for example, screws , nuts and bolts or components for automobiles and aircraft . Typical drop forged parts are e.g. B. connecting rods , chassis parts and crankshafts .

Drop forgings meet high requirements in terms of strength and safety and also withstand high dynamic loads . The mass of the individual parts ranges from a few grams to well over a ton.

Many drop forged parts require post- machining . However, it can be omitted in precision forging. In Germany, of the total production volume of forged parts of 1.4 million tons (2007), about 2/3 is forging. About half of EU production was made in Germany. This is followed by Italy with 20% and France with 8%. In relation to the proportions by mass, half of all closed-die forgings are supplied to the automotive industry and a further 39% to mechanical engineering . About 37% of German production is exported.

Achievable accuracies

The achievable accuracies, measured as ISO tolerance , are around IT14 for normal closed-die forging, IT13 to IT10 for precision forging and IT9 to IT7 for precision forging.

Range of materials and application examples

Most drop forgings are made from steels. The most important are unalloyed structural steels , tempered steels , hardened steels , nitrided steels , steels for flame and induction hardening (DIN EN 10083-1 / -2 / -3), bearing steels (ISO 683-17), heat resistant steels , cold-resistant steels , stainless steels and AFP steels .

Structural steels are used for bolts , rings, flanges , levers, hubs and pistons . Quenched and tempered steels for bearing caps, wheel hubs, camshafts , crankshafts , axles, clutch wheels , steering knuckles , hubs, running wheels or planetary gear carriers ( differential housings for differential gears ). Case-hardening steels are used for pins , levers, rollers, gears and gear parts.

Classification of die forming according to DIN 8583

Shape stretching

Die forming is divided in DIN 8583 into:

Drop forging with a burr

The entire process can be divided into: Separation of the raw parts from larger semi-finished products, raw part heating, preforming / mass distribution, pre-forging / finish forging, deburring and punching , cooling and heat treatment . A slightly different classification results from cutting, heating, forming, heat treatment and surface treatment.

  • The starting material is usually separated from a semi-finished product, such as a billet, rod, slab or strip. This can be done with shear cutting or sawing .
  • The heating during hot forming takes place in furnaces or by means of induction . Requirements for this process step are a uniform temperature of various raw parts, low scale formation and decarburization , low environmental pollution from heat or noise, and low costs. Steels are heated to temperatures between 900 ° C and 1300 ° C.
  • The actual forming process can be divided into several stages, which can be implemented with a single tool if the degree of forming is small enough or the forming capacity of the material is large enough. Often, however, reshaping takes place in several stages. Initially, an attempt is usually made to roughly match the mass distribution of the later prefabricated part. The final shape is then forged out more precisely. By forming in several stages, the required force can be reduced, a favorable fiber orientation can be achieved, the amount of material that migrates into the ridge can be reduced, which leads to lower tool loads and wear , and sufficient mold filling can be achieved. The higher costs for the additional machines are disadvantageous.
  • The burr is then removed and the component perforated if necessary, for example in the case of connecting rods and wheel hubs. This is usually done using shear cutting. The removal of external burrs is called deburring, and internal burrs (mirrors) are called punching. Since tighter tolerances can usually be adhered to during trimming than in the actual forging process, dimensional changes due to die wear can be compensated for. It can also be used to create flat surfaces that can be used as clamping surfaces during subsequent machining. Basically, deburred surfaces can also be functional surfaces ready for installation. Areas created by shear cutting have a shear surface in the upper area and a fracture surface in the lower area. In principle, the fracture surfaces should be as small as possible and there should be no trimmings. Otherwise a score-free surface and parallelism of the reference surfaces is desired. If deburring takes place in the warm state, very small fracture zones arise, but mostly pronounced grooves; in the cold state, large fracture surfaces arise that can also extend beyond the original burr area.
  • The final step is heat treatment and surface treatment. The heat treatment conventionally consists of normalizing in order to obtain an even, fine-grained structure and, after hardening, tempering for a good combination of hardness and ductility . In order to save costs, attempts are made not to allow the forgings to cool down after forging so that they do not have to be reheated, which is known as forging heat treatment. On the one hand, surface treatment involves cleaning the surface, in particular removing scale. On the other hand, internal pressure stresses should also be introduced into the workpieces in a targeted manner in order to increase their strength. Blasting is used for this , in which particles are shot at the workpiece, and rolling is used for cylindrical parts. When rolling, the surface to be machined rotates around its axis of symmetry and a freely running roller is pressed onto the surface, so that high surface pressures arise, which lead to internal compressive stresses.

Drop forging without a burr

In die forging without a burr, tools without a burr gap are used. Excess material cannot then flow off. Therefore, the dimensions of the blanks and their positioning within the die must be very precise (less than 0.5% deviation). On the other hand, material savings of 10% to 40% are feasible, there is no need for burring (but not for punching) and machining is reduced. In addition, the machining forces are lower, as excess material does not have to be pushed through the burr gap. Process combinations with forging with burrs are possible in order to combine the advantages of both variants.

Precision forging

If workpieces that are almost ready for installation are produced by forging , it is called precision forging . It is irrelevant which forging process is used, only the accuracy achieved defines a forging process as precision forging. A tolerance of IT8 to IT6 is usually assumed here. Precision forging is widely used in industry. Mainly parts in the drive train of motor vehicles , e.g. B. transmission gears are made in this way.

Thixo forging

Here the material is in a special “semi-liquid” state, which is called thixotropic . This results in a very high formability, low machining forces and high accuracy.

Die forging machines

In principle, all forming machines can be used for drop forging by installing the appropriate tools. Particularly with hot forming, the shortest possible contact time between tool and workpiece is sought in order to keep the temperature load on the tool low, since it leads to thermal expansion and promotes wear, both of which have a negative effect on the accuracy that can be achieved. The most common machine types are eccentric presses , forging hammers , screw presses and hydraulic presses .

advantages and disadvantages

Drop forging has the following key advantages and disadvantages:

advantages

  • Higher strengths with the same or lower weight than when using casting processes
  • Adaptation of the fiber orientation in the component to its contour during forming, without the component being broken as is the case during machining
  • The components produced with closed-die forging are extremely resistant to high loads

disadvantage

  • Elaborate die construction
  • Excess material for forming the burr
  • Complicated design of the process
  • Die forging without a burr cannot be used to produce components with complex geometry

Web links

Individual evidence

  1. Hartmut Hoffmann , Reimund Neugebauer , Günter Spur : Handbook Forming. Hanser, 2012, p. 244.
  2. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 253.
  3. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 244.
  4. a b Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 244.
  5. Eckart Doege , Bernd-Arno Behrens : Handbook of forming technology. Springer, 2010, 2nd edition, p. 497.
  6. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 244.
  7. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 273.
  8. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 253 f.
  9. Eckart Doege, Bernd-Arno Behrens: Handbook of forming technology. Springer, 2010, 2nd edition, p. 498.
  10. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 245.
  11. Eckart Doege, Bernd-Arno Behrens: Handbook of forming technology. Springer, 2010, 2nd edition, p. 526.
  12. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 255.
  13. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 257.
  14. Eckart Doege, Bernd-Arno Behrens: Handbook of forming technology. Springer, 2010, 2nd edition, p. 497.
  15. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 261.
  16. Eckart Doege, Bernd-Arno Behrens: Handbook of forming technology. Springer, 2010, 2nd edition, p. 501.
  17. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 265 f.
  18. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 268.
  19. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, pp. 269-271.
  20. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 271 f.
  21. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 272 ​​f.
  22. Hartmut Hoffmann, Reimund Neugebauer, Günter Spur: Handbook Forming. Hanser, 2012, p. 276 f.
  23. Eckart Doege, Bernd-Arno Behrens: Handbook of forming technology. Springer, 2010, 2nd edition, p. 621 f.
  24. Drop forging: process, advantages, process and implementation. Retrieved March 30, 2020 .