Fundamentals How does the forming of sheet metal work?
Forming and forming technology is used to provide components with specific shapes. Cold forming and hot forming comprise different processes.
Forming is divided into six different manufacturing processes as defined by DIN 8580. In addition to rolling, these forming processes include bending, deep drawing, extrusion and bar extrusion as well as drop forging and open die forging. During the manufacturing processes of forming, raw parts consisting of materials such as metals or thermoplastics are converted into another shape without the removal of materials from the raw parts.
In other words: Forming is defined as the targeted modification of the shape as well as the surface and properties of a (metallic) body, whereby the material cohesion and mass of the body are maintained. As defined by DIN 8050, not only the shape of metallic bodies is specifically modified during the forming process, but also their surface and properties. For this reason, forming technology is primarily concerned with the best possible predeterminability of the final body properties. Accordingly, the respective material retains its cohesion and mass during forming. This is not the case, for example, in joining and separating processes, where cohesion and mass are either increased or reduced. In contrast to deformation, forming results in a targeted shape.
Historical background: History and significance of forming technology
The beginning of forming goes back to prehistoric times - this is verified by archaeological finds, both written records and the contents of burial chambers. On the basis of these finds, a reconstruction of the technological development during these ages is possible.
According to current knowledge, the beginnings of metal processing at the end of the 5th millennium BC were located around the Mediterranean Sea. The main metals processed were copper, silver and gold. Scientists also assume that from Scandinavia to the Alps, it was not forming but foundry technology that had priority at the time. Forming technology was only used in these regions to produce smaller parts which could only be cast with difficulty. These were, for example, simple jewelry profiles or needles.
In the Copper Age, around 4000 B.C., forming consisted of manual expanding with the help of stone tools. In the Bronze Age (2800 BC) copper alloys were produced in the Mediterranean region and pressed sheet dies for silver and gold sheets were invented. In Northern Europe, on the other hand, decorations and smaller products were produced with the help of punch stamps. At the beginning of the Iron Age, around 900 BC, the helve hammer was invented, and bronze replaced stone and wood as tools. The first contoured punches for sheet metal forming are proven. Although iron had poor mechanical properties, it was also introduced. During the period from the Roman Empire to the 13th century, forming was not significantly improved, but it was widespread. Between the 14th and 18th centuries, iron processing became more and more important: Hydropower-driven hammers were developed, sheet metal forming (free forming) was refined, the screw press is introduced, and the so-called pile driver was used as a forming machine. In the 18th and 19th centuries James Watt built the first steam hammer and - also in England - only a few years later the first hydraulic press went into operation. In addition, drop forging was introduced for the production of bulk goods and progress has also been made in forming and sheet metal forming, as the rolling of sheet metal for the manufacture of fittings was optimized.
Finally, in the 20th century, forming and forming technology became a research area at universities. Moreover, specific forming processes allow to manufacture safety-relevant and highly stable components for the aerospace and automotive industries. Moreover, lightweight construction uses forming technology, and materials such as titanium, magnesium and aluminum are also processed. With regard to the importance of forming and forming technology, it should be noted that, especially in times of scarcity of raw materials and energy, forming processes, in contrast to metal-cutting processes, have significant advantages in terms of material utilization. Although the different forming processes - i.e. cold, warm and hot solid forming - require a higher amount of energy in relation to the production of the workpiece, it is important to put this in relation to the energy required to produce the raw parts. Accordingly, due to the high material utilization during forming, these costs are negligible in comparison to the costs for the production of a raw part when calculating the costs for a finished part, unlike in the case of machining processes.
Forming: The specific procedures
In the following, the individual procedures that are relevant in the context of forming are briefly explained.
Cold forming as defined by DIN 8582 refers to the plastic forming of metals. In this process, the forming takes place below the recrystallization temperature. During cold forming, the strength of the material increases continuously as a result of the strain hardening process. If no material hardening is desired, this is prevented by heat treatment afterwards. This ensures that the hardening can be degraded again. As a rule, this forming process is used when good surface properties and narrow dimensional tolerances are to be achieved or when the material strength has to be increased. In general, complex geometries cannot be achieved with this forming process.
So-called cold massive forming is not "forging" in the narrower sense, because in forging the material is heated beforehand, rather the forming is carried out at room temperature. This means that no energy is required to heat the workpieces. However, in addition to high forces, a lot of energy is also essential in forming. This makes it possible for the workpiece to heat up to 150 °C as a result of the energy generated during forming. This is not the case with cold forming, because forming does not take place at room temperature. Vice versa, however, not all forming processes carried out at room temperature are also referred to as cold forming. A relevant aspect is whether the so-called recrystallisation temperature is exceeded or not. For instance, steel forming at a temperature of 200 °C is still considered cold forming because the recrystallisation temperature is not exceeded. The situation is different when forming lead, since the recrystallisation temperature of lead is already exceeded at a temperature of more than 3 °C. This is because the temperature at which lead is recrystallized is higher than the temperature at which lead is formed. Due to the fact that a specific fiber structure is created in the workpiece during cold forming, the end products withstand a high mechanical load. This type of forming is therefore frequently used to manufacture safety-relevant parts.
Warm forming is also a specific forging technique which is generally used when steel alloys are forged. As a rule, this forming process is carried out at temperatures between 600 °C and 950 °C. The temperature of the metal is between 600 °C and 950 °C. Within this temperature range, forging of numerous metal alloys is possible which cannot be processed by cold forming. In contrast to hot forming, this type of forming does not result in scaling of the respective forming product, or only to a small extent. Furthermore, the dimensional tolerances are low, and the forged part surface is homogeneous. Nevertheless, during this forming process, energy must be applied to heat the raw part. However, the forming of the raw part is much simpler afterwards, resulting in reduced energy consumption. By using this forming process, steel grades with a higher alloy can also be machined and geometrically more demanding blanks can be produced. This is because it is easier to form the steel than it is with cold forming.
The hot forming process, also known as hot forging, is suitable for forming all metallic materials. However, this forming process is a very energy-intensive one, as all raw parts must first be heated to forging temperature. In general, hot forming of steel alloys and iron takes place at temperatures between 1100 °C and 1300° C. If forming takes place in this temperature range, however, the steel is scaled, and the surface becomes rough. In addition, recrystallisation and recovery processes occur in the forged part. This is due to the high temperatures - both during and after the forming process, which have an influence on the properties of the product after forming. Since the workpiece can change its geometry or shrink after forming during the cooling process, both the dimension and the shape of the tool should be adjusted accordingly. In general, all those forming steps that take place above the recrystallization temperature of a blank are subsumed under the term "hot forming". Accordingly, the forming of a blank by hot forming does not necessarily require heating of the blank. For numerous metals, recrystallization already begins at room temperature.
Regardless of whether forming is done by cold, warm or hot forming, it is characteristic for all forming processes that the parts are not produced in a single operation but in several steps. The sequence of the individual operations is referred to as the stage sequence. Because several operations are required for forming, production is usually only economical when large quantities are involved.
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Forming technology: Sheet metal forming
In general, sheet metal forming is carried out by non-cutting processes. The forming of sheet metal has numerous areas of application, for example in the production of parts from different metals or for the production of car bodies. Sheet metal forming can be carried out by means of the following measures: Cutting, stamping, trimming, separating, blanking, punching, deep drawing with presses, embossing, hydraulic die cushions and pneumatic die cushions. Apart from the processes mentioned above, there are other special processes for sheet metal forming. The forming of sheet metal is not only used in the manufacture of series products, but also of individual special parts. This is primarily the case for companies operating in the following sectors: Metal processing, aerospace engineering, electrical engineering, apparatus engineering, medical technology, plant construction, pharmaceutical technology, control technology, ventilation and air-conditioning technology, arts and crafts, measurement technology and heating technology as well as communication technology.
The sheet metal forming process can be found in numerous areas of industrial production: in the automotive industry for the production of mudguards, doors, hoods, household appliance industry, for the production of freezers, extractor hoods or sinks in the aircraft industry, for the production of wings and fuselage in the food industry, for the production of canned food or cooking pots. During sheet metal forming, a flat sheet metal blank is cut into the respective shape so that the desired component is produced. The forming of the material takes place by plastic deformation, there is no machining involved. In addition, elastic deformation may occur in some cases. This is a process that becomes noticeable in the form of springback after forming.
The following sheet metal forming processes are distinguished by DIN 8582:
- Pressure forming, for example by die forming, free forming or rolling
- Tensile compression forming, for example by pressing or deep drawing
- Tensile forming, for example by expanding, elongating, stretch indenting
- Bending forms, e.g. by folding, free bending or die bending
- Shear forming, e.g. with straight tool motion or twisting
In principle, forming can take place explosively or magnetically using air, tools or liquids. There are also special sheet metal forming processes. These are, for example, press hardening or superelastic forming.
In principle, machines used in solid forming must guarantee precise guidance of the tools as well as the transmission of high forces. The investment costs are therefore correspondingly high. In order to keep unit costs as low as possible, forming machines are usually operated automatically. This also ensures that the forming machines, also known as forming robots, are highly efficient. Depending on the area of application, hammers, rollers or presses as well as combined forging machines are used.
In general, it can be claimed that automation has benefits when processing sheet metal.
Forming machines: Cold forming
Mechanical and hydraulic presses are primarily used in cold forming. Hydraulic presses are easier to change over but achieve shorter cycle times than mechanical presses. If small or medium series are to be produced, hydraulic presses are therefore the better choice. This is also because workpieces whose production requires a large working stroke are generally not suitable for mechanical presses. Hydraulic presses are used to manufacture gear parts, axle shafts, gears, hollow or cup parts and drive bevel gears. In contrast, mechanical eccentric presses, joint presses, toggle presses and crank presses are used when large series with shorter cycle times are to be produced automatically.
Forming machines: Hot forming
In hot forming, rollers as well as presses and hammers are used for forming. Automatic hammers have the advantage that they are inexpensive and can be set up quickly. Additionally, they have a short pressure contact time and a high impact frequency. Automatic spindle presses also have a short set-up time and large working strokes. Hydraulic presses offer a large working stroke and have a long pressure contact time. Crank and eccentric presses are characterized by good automation properties and short cycle times during forming. On the other hand, ring rollers score with a high degree of flexibility, longer cycle times and low tool expenditure. This is not the case when forming is done with stretching and cross wedge rollers, as these require a great deal of tooling.
This article was first published by belchnet.
Original by Frauke Finus
Translation by Alexander Stark