Forging of Metals: 5 Types | Metallurgy

The forging processes may be classified into following types: 1. Open Die Forging 2. Closed Die Forging 3. Impression Die Forging 4. Upsetting and Swaging 5. Rotary Swaging.

1. Open Die Forging:

In open die forging, the work piece is deformed between two flat or simple-curved dies. There is free flow of material in the lateral direction. The only hindrance to free flow is the interfacial friction between dies and the metal. In open die forging the shape of tool or die is not closely related to the final shape of product. Forgings are produced by repetitive blows and manipulation of work piece.

Thus we may try to forge a round bar with flat dies. However, the curved shape made by flat dies can only be an approximate one. Close tolerances on shape as well as on dimensions cannot be obtained in open die forging. The weight of forging may vary from a few grams to 150 tons.

Very large forgings which are required in small numbers are generally made by open die forging. The range of products made by open die forging is really very broad.

Some examples of the products made by this process are described below:

(i) Rotors, spindles, step shafts and other products with large lengths.

(ii) Hollow cylindrical shapes with large and small lengths (rings). The product may have step diameter.

(iii) Shells with contoured surfaces like pressure vessels with protruded (extruded) nozzles.

Metal Flow in Compression between Two Flat Dies:

Compression of Circular Discs and Cylinders:

If we compress a circular disc or cylindrical work piece between two flat dies, the resulting shape of the compressed disc or cylindrical work piece depends on the ratio h/d, (height/diameter) and the interfacial friction between the die and the work material. For long cylindrical billets when the ratio (h/d) is more than 2 the work piece may get a double barrel shape on compression, Fig. 6.5(a).

If the ratio is less than 2 the resulting shape may be a single barrel as shown in Fig. 6.5(b). However, if there is no interfacial friction there will be no barreling. Friction on the interface may be reduced by entrapping the lubricant on top and bottom faces with the help of a small step around the periphery. This technique is used for testing metals in compression wherein barreling is not desired.

In case of frictionless compression the die load is equal to the yield strength of the material multiplied by the area of contact. But with friction, the die load is higher than this value. The increase depends upon the ratio d/h and the co-efficient of friction μ, Fig. 6.6.

Metal Flow in Compression of Non-Circular Discs:

It is interesting to study the resulting shapes if we compress non-circular discs say polygonal discs between two flat dies. Figure 6.7 shows triangular, square and hexagonal discs before and after compression. The upper row is of uncompressed specimen.

For studying the metal flow, lines were inscribed on the surfaces of specimen before compression. In case of triangular specimen these lines emanated from the in-center and ended at vertices and at other points on the sides.

These also include the lines perpendiculars to the sides. During compression, the lines joining the in-center to the vertices and lines drawn perpendicular to the sides remain nearly straight while other lines from the in-center to the sides bend toward the perpendicular line. This gives us an important clue to the flow of material during compression. The material flows in radial as well as in circumferential directions.

During compression of any regular polygonal discs a similar trend is observed. The lines bisecting the angles of the polygonal faces as well as the lines from in-center and perpendicular to the sides remain straight during compression.

All other lines bend towards the perpendicular lines. Juneja has carried out a detailed study of these deformations and has developed analytical solutions which can predict the shape of deformed specimen as well as die load. Sagar and Juneja have extended the solutions to compression of four sided irregular polygonal discs and to closed die forging.

Figure 6.9 shows a similar metal flow study on rectangular discs. The non-uniform flow of material in the plane of disc resulting in curving of sides is called bulging. We have already given the name barreling to non-uniform flow along the height of cylinder. The non-uniformity in flow both in vertical as well as horizontal planes is present in compression of any non-circular disc.

This is illustrated in Fig. 6.10 which shows a compressed rectangular block. In case of rectangular discs (Fig. 6.9), the rectangular face is divided into three regions, i.e. the two end regions each having length equal to half the width, and a central region. If there is no central region the two end regions would make a square.

The metal flow pattern in the end regions is similar to that in a square disc while in the central region it is nearly a plane strain case, i.e. the flow is along the normal to the long central line. The flow is also non-uniform along the thickness of the specimen. The vertical sides become barrel and horizontal sides bulge out. This is illustrated by Fig. 6.10.

Fullering:

It is an open die process which is often carried out before die forging. Its purpose is to draw out and redistribute the material so that die filling is ensured. Two types of fullering tools are illustrated (Fig. 6.11). The curved tool gives higher elongation than the flat one.

2. Closed Die Forging:

In closed die forging the shape of die is closely related to the shape of the product, in fact, it fits the die cavity. In this process the metal is under compressive forces from top and bottom dies and it is contained from all the sides, with the result, the metal flows into grooves or impressions cut in the die. Coining is an example of closed die forging process. Forging of gears and precision components for which the near net size concept is practiced are other examples.

For studying metal flow during closed-die forging of circular discs in a hexagonal shaped die, circular lines were inscribed on the top face of specimen. The deformation of these lines during compression gives an idea of the metal flow, which has been utilized for the analysis of the process by Sagar and Juneja.

3. Impression Die Forging:

In industry most of the forgings are made by impression die forging. The amount of raw material taken in hot impression die forging is usually more than the volume of die cavity. On compression the extra material flows out of the dies through the flat surfaces of dies surrounding the cavity (Fig. 6.14). The thin layer of material thus formed all around the forging is called flash.

The width of flash is in fact an indicator of the pressure developed in the die cavity and it is desirable to have flash because that will ensure complete filling of the die. After the desired flash width the dies have gutter space for pushing the extra material into it. The flash and the gutter material are trimmed off the forging before machining.

For the same area of contact the pressures and consequently the die loads are much higher in impression die forging than in open die forging. Drop hammers, steam and pneumatic hammers, and hydraulic presses are generally used to carry out these processes.

In all forging operations the impurities in material and the grains get elongated in the directions perpendicular to the compressive deforming forces. This makes a fiber like micro- structure (Fig. 6.15). The location of die parting line is an important factor in modifying the metal flow and in reducing forging defects.

4. Upsetting and Swaging:

The upsetting process is used for producing headed components like bolts, rivets, screws, engine valves and other similar components. For small components the process is carried out in cold state. However, for large diameter bars and for high strength materials the bar end may be heated for upsetting. Some of the products made by this process are shown in Fig. 6.16.

The upsetting process is done on machines specially designed for the purpose. The process sequence for producing bolt head is illustrated in Fig. 6.17.

In designing upsetting sequence it is important to take care of the buckling of bar stock during compression. The practice generally followed in industry is given in Table 6.1.

5. Rotary Swaging:

The schematic diagram of rotary swaging is shown in Fig. 6.19. Bar is forged by swages which are pressed on to it when the rotating cylinders ride over them. The process is also used for pointing the wire rod or tube ends for wire/tube drawing purposes.