What is a mechanism of cold forging
Cold forming of metal
Cold forming is a metalworking process in which the material is forced to assume a certain shape by means of high compressive or tensile forces. The material is not or only slightly heated before processing. This is where cold and hot forming differ. The temperature limit that separates both areas is the recrystallization temperature.
Formation of the material structure when the melt cools
The deformability of the metals can be traced back to their internal structure. At the beginning of metal processing, the materials are in liquid form as a melt. When it cools down, individual atoms first attach to each other and form crystals. A crystal is characterized by the regular geometric arrangement of the atoms. The addition of more atoms leads to crystal growth. The growth continues until the crystals collide.
This is how the material structure is formed. The structure can be seen under the microscope. For this purpose, material samples are ground smooth, polished and etched on a surface. The spatial orientation of the crystals is purely coincidental. They appear in the micrograph as grains that reflect the light differently and are enclosed by the grain boundaries.
Change in the material structure during cold forming
During cold forming, the crystals are compressed or elongated. In order to maintain the new shape, the attacking forces must overcome the forces of attraction between the atoms in the crystal lattice. Dislocations form in the crystal structure. The individual grid levels shift against each other. These dislocations cause internal stresses and strain hardening of the material.
Rising temperatures cause increased heat movements in the particles of a substance. Above a certain limit, these heat movements can compensate for the internal tensions. The atoms organize themselves into crystals, which have their normal structure again. The old structures build up again around the dislocations. The temperature at which these processes start is the recrystallization temperature.
The stress-strain diagrams for the individual materials provide information on the forces required. In the tensile test, a standardized material sample is stressed with increasing force. At the beginning, the sample expands proportionally to the applied stress. Above a material-dependent value, the yield point, the sample begins to constrict. If the tension continues to rise, it will break. The area between the yield point and the elongation at break is used for cold deformation.
Materials suitable for cold forming
In the case of materials for cold deformation, the area between the start of permanent deformation and elongation at break should be as large as possible and the forces to be applied should be as small as possible. This is the case with many aluminum alloys, copper and certain types of steel.
Steel is an alloy of iron and carbon. The carbon content is a maximum of 2%. In order to specifically influence the properties, other metals are mixed in with the molten steel. The exact composition depends on whether the material can be cold-formed or whether a forming process with increased temperature has to be used. According to the 2013 by the Industrieverband Massivumformung e. V. published textbook "Massivumformung briefly and flush" should be for cold forming the carbon content below 0.5% and the content of other alloy elements below 5%.
Cold forming process
Cold forming is divided into massive cold forming and sheet metal forming. Sheet metal is formed, for example, by deep drawing and bending. The material thickness is largely retained. In the case of massive cold forming, significant changes in cross-section are caused. Examples are rolling and extrusion.
When rolling, the metal passes through the gap between rotating tools. The shape and arrangement of the tools determine the geometric shape of the workpiece. In this way profiles, rings or threads are created. The strain hardening of the material areas close to the surface that goes along with the deformation is often a welcome effect. However, if large changes in cross-section are required, the deformation must be carried out in several steps. The increasing strength interferes with this. A targeted heat treatment, the recrystallization annealing between the work steps, can reduce the strength again.
During cold extrusion, the metal is pressed into a die by a punch and takes on its shape. This method is useful for the production of large numbers of parts with a rotationally symmetrical contour. If the individual parts are very voluminous, they can be annealed before forming. This reduces the forces to be applied.
Operating materials for cold forming
During cold forming, heat is generated through friction, which can lead to the oxidation of the workpieces and the cold welding of the tool and workpiece. The friction is reduced by a coating that is applied to the material surface before forming. The layers consist of zinc phosphate, forming oils, waxes, graphite or special soaps and facilitate the sliding movement during massive cold forming.
The forming lubricants must be adapted to both the materials used and the processes. Kluthe offers a wide range of metal-forming lubricants, which are specialized, for example, in the cold heading of fastening elements, the cold drawing of wire or the bending of sheet metal. The portfolio also includes special forming lubricants for cold massive forming.
In order to achieve certain final dimensions, multiple forming processes are often required, which alternate with recrystallization annealing to reduce strength. The coating is then usually removed before the heat treatment and then reapplied. In many cases it is worthwhile to use high-performance lubricants as a substitute for zinc phosphate layers.
In the development and selection of forming lubricants, the options for subsequent cleaning of the parts and the environmental compatibility of the ingredients are of growing importance. Products that are free from chlorine, sulfur, boron and heavy metal compounds can often be used. Forming lubricants without secondary amines and formaldehyde releasers are also available on the market.
Advantages of cold forming
The main advantages of the forming processes compared to metal cutting are the better utilization of the material and the uninterrupted fiber flow. In the case of large quantities, the shorter processing time has the effect of reducing costs.
Cold forming enables the production of parts with very complex geometries and high strengths. This is used successfully in the automotive industry, in the construction of energy systems and in valve construction. Another field of application is apparatus and container construction.
Why cold forming?
Compared to hot forming, cold forming enables a better surface quality and higher dimensional and shape accuracy to be achieved. In addition, there is no need for energy to heat the parts. For this, however, more mechanical force has to be applied.
The work hardening in the formed material area saves the hardening of the finished parts in many cases. Shafts and threaded spindles produced by cold forming processes are more resilient than machined parts with the same cross-section.
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