Drop forging is a hot forging process which DIN 8583 classifies as a pressure forming process. For this process, forming tools are moved towards each other and push the material in a specific direction.
In this way, the material takes on the shape of the die impressions. The blank is heated to about 1,200 °C during drop forging. Forming takes place in several intermediate dies until the workpiece is finished. The work process is divided into mass distribution, cross-sectional pre-forming, which is frequently carried out by open die forging, and pressure forming. This in turn consists of the basic processes of upsetting, drawing out and rising.
In fact, drop forging is a highly sophisticated branch of forging adapted to the needs of industrial production. Drop forging is characterized by the use of forming tools, i.e. dies, as well as machines that exert high forces and at the same time enable precise guidance of the tools. All metals that can be formed after heating, such as steel, magnesium, aluminum, stainless steel as well as brass, copper and their alloys, are suitable for drop forging.
In principle, drop forging is the most widely used forging method. It is used, for example, to manufacture planes, ships or car parts as well as in the design of various industrial products. In addition to cold, hot and flat forging presses, drop forging hammers are also used for this purpose.
Historical background: Drop forging in history
The tradition of drop forging goes back several millennia. In fact, forging of iron can be traced back to the beginnings of the extraction of iron in bloomeries. At that time, the blanks were forged into rods, which in turn were processed into weapons, tools and jewelry. Due to the fact that forging required not only a lot of strength but also a lot of skill, it was a highly respected job. This is also reflected, for example, in Greek mythology, where the blacksmith was represented by the god Hephaistos. Wieland, who possesses superhuman abilities, holds an equally important position in the Germanic mythological world.
The forerunners of today's forging dies were used, for example, in the production of jewelery - amongst others, on the island of Crete or in the town of Mycenae in the Peloponnese around 1,600 BC. Another interesting point of reference for drop forging is minting, because the one-sided ancient stone hollow forms, which were used to mint gold and silver sheets, are dated back to around 800 BC. In addition, the first bronze tools have been used around 600 BC.
A few centuries later, from 200 A.D. onwards, closed minting dies were used to produced double sided coins in Rome. These dies were equipped with a square guide for the upper punch. At the beginning of the Late Middle Ages, flat, one-sided dies were mainly used to forge iron parts with ornaments. A little later, in the late Middle Ages, drop forging was also used to produce arquebus balls, cannon balls and welded barrels.
Forging dies in their present form have only been in existence since the late 18th century. The basics of steel forging occurred primarily during the 19th century and was the origin of drop forging as we know it today.
In the middle of the 19th century, for example, the first drop forges for knives in Germany were established in the city of Solingen. While drop forging at that time was based to high degree on traditional craftsmanship, it became an industrial process around 1870. At that time, drop forging was practiced both in forging departments of larger companies, but also in the drop forging industry as a separate branch. In this respect, a direct link can be drawn between interchangeable manufacturing - especially in the motor vehicle industry - and drop forging.
In the course of further developments, drop forgings became more and more precise, with the enhancement being driven by the development of tolerance standards and the establishment of delivery conditions in various countries at around 1937. Due to the growing world population and the resulting increase in mass demand, drop forging has been growing in importance. This is due to the nature of drop forging as a part of mass production processes. As one type of forming, drop forging saves material demands and time, but it also meets the high requirements placed on the technical properties of the finished workpieces.
Drop forging: Technical and economic significance
Drop forging is generally considered to be a mass production process for individual workpieces. The forged items can weigh from only a few grams to several tons. The same applies to the dimensions of the workpieces, which range from a few centimeters to several meters, and to the series sizes, which range from just a few pieces to several million items.
Drop forging is used primarily to produce construction parts for machines such as planes or vehicles. Drop forging is also used to produce tools, e.g. wrenches, pliers and hammers.
The main buyers in Germany for workpieces produced by drop forging are the automotive industry, the motorcycle and bike industry, mechanical engineering, the railway industry, mining, shipbuilding and the aircraft industry.
The importance of drop forging for modern vehicle and machine construction can be summarized in the following aspects:
- No matter which metal group, drop forgings are important structural parts. The material of the parts is ideally used within the framework of the forming capabilities of drop forging. The favorable weight/strength ratio of the forgings enables the production of relatively light but highly robust components.
- The combination of drop forging with other manufacturing processes such as welding, extrusion or cold forming results in economical and design related advantages.
- Due to an enormous range of non-ferrous metals and steels and respective heat treatment processes, the properties of the forgings can be adapted to meet their machining and usage properties as well as their intended use.
- Thanks to the high degree of accuracy with regard to shape and dimensions as well as the specified tolerances, die forged parts can be further processed either to finished or unfinished products.
- Drop forgings have neither pores nor other cavities. They feature a homogeneous, dense structure, which can be tested by modern methods. For this reason, drop-forged components are primarily used in areas that demand a high degree of safety.
- Because drop forgings not only require few but very narrowly defined and uniform material additions, unnecessary waste is eliminated and the processing times are optimized.
Drop forging form bar stock
Drop-forging from bar stock uses approximately 2 m long blanks. These bars are heated at one end and then forged in a die. After completion of the forging process, the individual workpieces are separated from the bars - usually by means of a hammer blow.
Drop forging of bars has both advantages and disadvantages. The advantages include good handling, because the workpiece does not have to be held with forging pliers which would require a great deal of effort, and the saving of time because the piece must only be inserted into the die. The disadvantage of this process is the high amount of materials required, because the volume cannot be precisely adjusted.
Drop forging of mandrels is generally used when workpieces are to be forged in an elongated form weighing up to 3 kg and with rod diameters of up to 50 mm.
In addition to stretch forming, this process also comprises gathering by drawing out, rotary swaging, closing in a die and upsetting.
Drop forging from a piece of stock
In drop forging from a piece of stock, either a sheared off or cut off section of the bar stock is used as a blank. The blank can have both a square and a round cross-section. With regard to drop forging of a piece of stock, a distinction is again made between lateral forging and longitudinal forging. While forming in longitudinal forging takes place in the rolling direction of the stock, forming in lateral forging takes place perpendicular to the rolling direction of the stock. The advantage of drop forging of stock pieces is that the fiber orientation follows the outer shape optimally. A disadvantage can be seen in the larger amount of material used.
Drop forging of bar stock pieces is used in various areas: While lateral forging is used to produce workpieces with a pronounced longitudinal axis, longitudinal forging is used to produce disc-shaped, compact workpieces. In general, longitudinal forging of bar stock is used when difficult to form and large parts are to be produced or where a high degree of precision is required.
Drop forging of bar stock can be carried out either with or without burr. In industrial practice, drop forging with burr is the forming process that is most widely used. In this process, the tool forces the material to flow transversely to the direction of the tool movement. The workpiece is almost completely enclosed by the tool. The excess material can escape through the so-called flash gap.
With regard to the filling of die impressions, a distinction is made between three basic types of filling processes. These are upsetting, drawing our and rising. During upsetting, the main material flow occurs parallel to the movement of the tool, the initial height is reduced.
Drawing out displaces the material laterally. This takes place from the inside to the outside. The main material flow in drawing our occurs perpendicular to the movement of the tool. In rising, deep die impressions are filled, with the essential material flow taking place parallel to the movement of the tool.
In contrast to forging with burr, forging without burr completely encloses the workpiece in the tool. In this way no material escapes. The further course of the forming process is analogous to that with burr.
Drop forging from cropped pieces
Drop forging from cropped pieces is based on an initial shape which is cut out of a strip of sheet metal. This process is carried out by surface closure and the sheet metal strips are cut wit almost no material being lost.
This is followed by the creation of an intermediate die, either by bending or upsetting. These intermediate dies are produced from bar stock or from rods in the same way as in drop forging. The workpiece receives its final shape in the die. Drop forging from cropped pieces is primarily used for flat, small workpieces, although this forging process also has advantages and disadvantages.
The advantage of this process is that it is associated with low material consumption, the blank volume is accurate and, for this reason, only a small burr formation occurs. In addition, the forging times are short, which is due to the low material mass distribution. A disadvantage, however, is that the grain orientation cannot be ideally adapted to the shape of the workpiece. In addition to this, the bending fatigue strength may be impaired by the disturbance of the grain flow of the workpiece.
In general, die bending from cropped pieces is used when mass-produced parts such as spanners, pliers, scissors or knives are to be produced which do not have to meet high strength requirements.
The forging workflow
In principle, the workflow can be subdivided into different work steps, whereby the work steps vary according to the respective requirements of the workpiece. In other words: Individual work steps can either be added or omitted. The following overview presents the most common process steps.
Due to the fact that the forming process can only be carried out in a heated state, the essential equipment is combined in forging lines in most plants.
- Heating: The blank is brought to the required forming temperature in a furnace. This is above the austenitization temperature of the respective material, with the following systems being used for this purpose: Inductive heating systems, conductive heating systems, chamber furnaces powered by natural gas.
- Preforming: If necessary, the glowing or heated blank must be preformed with an air-lift hammer, a small press or cross wedge or forging rolls.
- Forging: The actual forming takes place in the course of this work step. The workpiece is located in the die and the main forging unit, i.e. a forging press or a forging hammer, is used. Several press strokes are generally required for the forming process, whereby the die integrated in the forging machine can also contain several impressions with different forming stages. These forming stages include, for example, bending blows, rolling blows, initial and final impressions.
- Deburring and punching: During this step, the protruding burr is removed. Specific trimming tools are used for this purpose. If necessary, this work step can be complemented by punching.
- Postforming: After forging has been completed, it is possible to finish it in a die.
- Heat treatment or quenching and tempering: The forging is then quenched and tempered in a furnace.
- Descaling: In the course of this work step, the scale layer is mechanically removed from the workpiece.
In principle, drop forging results in precise dimensions, especially in comparison to open die forging. In addition, the process is associated with a more appropriate distribution of grains and requires less machining. These advantages extend the service life of the forged pieces.
Forging in a die is also characterized by its simple handling, higher productivity and easy automation and mechanization. However, it also requires investment in equipment. Long production preparation is also a characteristic of drop forging.
This article was first published by MaschinenMarkt.
Original: Stéphane Itasse
Translation: Alexander Stark