In this new article, we continue our conversation with Miguel Queirós, an engineer that works as a sales technician at ETMA, to find out more about the Cold Forging production process. After explaining what this process consists of in the 1st article, as well as how it fits into ETMA’s production context, we will now look at its main technological aspects.
In the previous article, we covered several general aspects of forging. Can we go into more detail about this process from a technical point of view?
Of course. Forging is a manufacturing technology classified as plastic deformation within the forming processes. Unlike processes such as machining and even stamping, which involve cutting and removing material to give the object the intended shape, forging only changes the shape of the volume of material. For this reason, it is a technology that generates little or no material waste, making it a more sustainable process for obtaining components.
Technologically, the forging process can be categorised into two main groups:
- Cold forging
- Hot forging
Hot forging is normally used for medium or large parts, since increasing the temperature of the material reduces its resistance to plastic deformation, in other words, it improves its ductility.
Does ETMA make parts by hot forging?
ETMA only uses cold forging. The parts we produce are small and there is no need to use heat to overcome the resistance to deformation of small volumes of metal. Not using heat to forge the parts also allows us to take advantage of the phenomenon of the hardening of materials by mechanical labour, which means the parts are more resistant than the base material used.
This process has many advantages:
- Allows the manufacture of components with high dimensional accuracy;
- Complex geometries close to the use of the part (near-net-shape);
- Good surface finish of the parts;
- High production rate;
- Preserves and improves the internal structure of the material.
I would like to focus on two of these advantages.
The first advantage is the possibility to obtain complex geometries close to the final shape. Machining processes make it possible to obtain more complex shapes and better dimensional tolerances and geometric quality than forging.
However, they produce a lot of filings, wasting a lot of material. Considering the importance of sustainability in the industry and that reducing waste is an essential goal for that sustainability, it is easy to understand why there is a growing demand for the possibility of obtaining preforms close to the final shape that will then be machined.
ETMA has developed this capacity and is equipped not only with the equipment to obtain forged preforms, but also with machining machinery, in particular turning, adapted to the piece-by-piece work that this option makes mandatory (as opposed to working from bolts, which is typical of turning).
Image 1: Forged preform. Turned final part.
The other advantage I would like to emphasise is the preservation and improvement of the internal structure of materials. In the case of metals, this structure is crystalline, because there is a strongly orientated three-dimensional arrangement of the atoms that make up the metallic material. When a part is machined, this pre-existing structure in the material is cut away, eliminated. The internal alignment of the atoms remains, but is interrupted.
On the other hand, when the part is forged, the structure is simply rearranged. There is a repositioning of the atoms and a reorientation of their alignments, but the bonds of these alignments are preserved.
The strength of the materials in which the internal structure is preserved is higher because they are more resistant to crack initiation and, consequently, to fracture.
Image 2: Comparison of the internal structure of metal parts obtained by forging (left) and machining (right)
Circling back to the energy issue. ETMA manufactures metal components, changing the shape of metal parts cannot be easy…
It certainly is not! Like other metal transformation processes, such as casting or machining, considerable energy is required. In casting, for example, the metal must be heated above its melting point. In machining, the cutting tool must cause a tension in the material greater than its tensile strength in order to cut it. However, any of these material characteristics has an associated energy greater than the elastic limit, which is the internal stress state of the material that must be exceeded for plastic deformation to occur.
From a purely theoretical point of view, forging is supposed to require less energy than the other processes mentioned. In practice, this may not be the case: plastic deformation occurs throughout the entire volume of material and not just on cutting planes, as in machining. Moreover, calculating the energy required must take into account not only the physical process itself, but also the functioning and operation of the machines and their auxiliary systems, such as feeding and transporting materials, which is why it is tremendously complex.
The material characteristics I mentioned earlier, elastic limit (Re) and tensile strength (Rm), are very important in characterising the mechanical behaviour of materials. This behaviour is described graphically in a stress versus strain diagram. Stress (σ) is the state of internal stress in which the material finds itself due to the action of an external force or request. Deformation (ε) evaluates the change in geometry of the object under study as a result of this state of stress.
Metals have a behaviour known as elasto-plastic: up to a certain stress limit they are elastic, which means they deform as a result of an external force applied to them and recover their original shape when this external stress ceases.
This is the elastic regime and the stress limit is called the Elastic Limit, commonly represented by Re.
However, if the stress associated with the external load is greater than the Elastic Limit, the deformation will be permanent. If the stress continues to increase, the material cannot withstand any more deformation and reaches rupture. This limit to plastic deformation, where rupture is reached, is the material’s Tensile Strength, also known as Mechanical Resistance and is represented by Rm.
These values, Re and Rm, are characteristic of each material. The Stress-Strain Diagram is a graphical form normally used to represent these relationships.
Image 3: Stress-strain graph
Is it important to understand these stress and strain relationships in order to produce good forgings?
Yes. Seeing as the aim of forging is to plastically deform the material, but without reaching rupture, the forming forces applied must generate stress between the material’s Re and Rm values. Furthermore, the greater the difference between these two values, the less sensitive the material will be to variations in the process (room temperature, machine tuning, tool condition, etc.) that could alter the internal stress state and cause it to leave the desired work area.
Another important technological value is Yield Strength (Z%), which evaluates the ductility of the material and is related precisely to the size of the plastic deformation area of the material. A good forging material will have a high Z% value.
The quality of forged parts depends greatly on the ability to control the process. The raw material is a very important variable and its correct selection must be based on these technological variables.
Regarding forging, are there other issues to take into consideration besides the material?
Absolutely. The type or design of the tool is another important aspect. Greater or lesser restriction created by the tool regarding the flow of material during the plastic deformation that occurs during the process is very important for the quality of the forged part.
Tools can be classified as follows:
- Open die forging;
- Closed die forging.
Image 4: Open versus closed die representation
Can you talk a bit more about each type of tool?
Certainly.
In open die forging, the material is compressed in a mould that does not completely restrict the metal during the process.
Image 5: Open die forging
This process allows large parts and simple shapes to be formed in a press using simple tools.
The mould only has two surfaces and the material is simply compressed between these two surfaces.
These tools can also be used to pre-shape parts that will later be subjected to secondary operations, sometimes in a closed die, where they will take the intended, more complex final shape.
Closed die forging is carried out on a more elaborate tool, the inner shape of which is the “negative” of the shape that is to be formed on the outer surface of the material. These shapes can be more complex than in the closed die, with successive diameter reductions. There is no possibility of counter-exits, which means that when you move away from the tool’s parting plane, the section of the part parallel to that plane will always be equal to or smaller than the previous sections.
The part is obtained in its final form in one or more stages, sometimes including the removal of excess material.
Image 6: Closed die forging
The volume of the material that will enter the tool and the volume inside the tool cavity must be equal, otherwise the tool will not withstand the compression pressures and will rupture.
In the current equipment, this volume corresponds to the initial section of wire that is cut in the first operation. Variations in the length of this section will have a major impact on its total volume. It is not possible to prevent this variation with the necessary rigour, which means the tool must have exhaust volumes, or be designed to work with a certain “opening” that allows it to fulfil this function.
This opening can be used, for example, to form the head of a screw, when the shape of the head is simple. Otherwise, this excess volume of material will form a burr that must be cut off in an additional operation, which can be carried out on the machine, if it has this capacity, or externalised.
On the other hand, obtaining a complex shape is usually not possible with one single operation, requiring one or more pre-forging stages. In this situation, it is possible for these stages to combine, alternately or successively, open and closed die operations. This practice is very common in multi-station machine tooling.
Image 8: Screw forging stages (on the right, the last stage with the head contour cut out)
The use of multi-station machines is important to ensure productivity. ETMA has several 4-, 5- and 6-station machines that allow several successive volume reduction and/or burr cutting operations to be carried out.
Image 9: Multi-station forging machine
Finally, closed die forging has more complex (and more expensive) tools. And it is usually used for manufacturing large series, where it is possible to amortise the investment over a large number of pieces.
We end this 2nd article by emphasising, once again, ETMA’s vast experience in the Cold Forging production process, which means that the company is prepared to cooperate with its clients in the development of new projects in this area.
The following topics will be addressed in the next articles:
Design of parts and manufacture of tools
The different applications and examples of parts obtained by cold forging
In the meantime, if you are interested in learning more and, eventually, talking to us about a project, please do not hesitate to contact us.
