Top 9 Methods of Forging | Metals | Industries | Metallurgy

The following points highlight the top nine methods adopted for forging. The methods are: 1. Drop Forging 2. Press Forging 3. Hammer and Press Forgings 4. Hot Bar Forging 5. Upset Forging 6. Electrical Upset Forging 7. Swing Forging 8. Cored Forging 9. Rotary Forging.

Method # 1. Drop Forging:

The difference between drop forging and smith forging is that in drop forging closed- impression dies are used and there is drastic flow of metal in the dies due to repeated blows the impact of which compels the plastic metal to conform to the shape of the dies. Whereas in the smith forging open face dies are used and the hammering of the heated metal is done by hand tools to get the desired shape by judgement.

Two types of hammers are used in drop forging:

(a) Gravity drop hammer,

(b) Direct powered drop hammer.

In drop forging, the final shape of the product from the raw material is achieved in a number of steps and this is done in order to ensure proper flow of metal. The dies generally contain impressions for several operations. The number of steps required varies according to the size and shape of the part, properties of metal in respect of forging and the tolerances required.

The various steps in drop forging may be as follows:

(i) Shear off bar stock to proper length and bring it upto forging temperature in a furnace adjacent to the forg­ing machine.

(ii) Perform preliminary hot working operation in or­der to achieve some properties of metal.

(iii) Reduce the cross-sectional area, if any, at some places in the final shape of article and make it flow to some projected portions.

(iv) Blocking or semi-finishing operation in which the desired definite form is obtained. In this operation the die is subjected to continuous intermittent blows till the definite shape (i.e. approximate size and shape of the finished forg­ing) is obtained.

(v) Finishing die operation:

In the die also, the metal is subjected to several blows and the final shape is obtained which is having thin projections are excess metal extending around the parting line. This excess metal is purposely provided so that complete filling of the die is ensured. The finishing dies are of the exact size and shape of the forging with true corner radii and with allowance made for shrinkage.

(vi) Removing of the flash around the edges of the finished forging in a separate press by trimmer dies immediately after the finishing operation is completed. Small forgings may however, be trimmed in cold condition also, the care being taken not to distort the part. In trimming operation the forging is held uniformly by the die in the ram and pushed through the trimming edges. Other supplementary operations include cleaning by blasting, pickling, or tumbling.

(vii) Heat treatment of forging so as to relieve the various stresses induced into it and to attain certain other desired properties.

The drop forging machines are generally of two types viz. Steam Hammer type and Gravity Drop or Board Hammer type. In the steam hammer type drop forging machine, the ram and hammer are lifted up by the steam and the force of blow is controlled by throttling the steam. It can work upto 300 blows per minute and the machines are available in various capacities depending upon the steam pressure used.

They are usually of double-housing design, with an overhead steam cylinder assembly providing the power for actuating the ram. The ratio of the energy developed at die to the weight of the machine is more than that for board or gravity type drop hammer.

In the gravity-type drop forging machine, the impact pressure on the die is developed by the force of the falling ram and the die as these strike upon the lower fixed die. Suitable arrangements are provided in the press to lift the ram up after the forging stroke is completed.

After completion of forging operation, the forgings are given a cleaning operation by pickling in acid, shot peening or tumbling, as the forgings are covered with scale. Sometimes a straightening operation is also required if some distortion has occurred in the forging.

Forging operation can be carried out on the following materials:

Carbon and alloy steels, wrought iron, copper base alloys, aluminium alloys and magnesium alloys.

In order to ensure that the material fills the cavities, a small amount of excess material is provided in the slug or performs. This material is squeezed out between the faces of the dies and is known as flash. It is usually removed in a separate dressing operation.

Limitations of drop forging are on account of:

(i) Poor forge ability of material and

(ii) Design of the component.

a. The material must be capable for being forged. Some materials such as most steels, specially those with low carbon content forge readily. A similar process occurs with a 60/40 brass, but the blow is not so severe and is more likely a ‘squeezing’ thin a ‘hitting’ operation. However, a component of 70/30 brass should not be drop forged. This material is ‘hot short’, i.e. it does not flow satisfactorily when hot.

b. The geometrical shape of the component may prevent it from being drop-forged. A conventional drop forging rarely has holes in it and certainly not deep holes of small diameter, while components with re-entrant forms cannot be drop-forged. Small radii (especially with fillets), sharp corners and abrupt changes of section must be avoided.

Parts from design shape, size itself is a criterion. There is a limit to the size of a drop forging due to the size of machine. Although machines are getting large, it is rare for a drop- forging to have face exceeding 400 mm square.

Drop hammers are relatively inexpensive and versatile machines but where large quantities of forgings are required, press forging is preferable.

Method # 2. Press Forging:

Generally in the drop hammer press a greater amount of energy is absorbed by the machine and foundations. Therefore, this process is not considered to be an economical one. For forgings symmetrical in shape, press forging which employs a slow-squeezing action in deforming the plastic metal can be employed. Presses provide a faster rate of production because the die in press forging is filled in a single stroke.

Working conditions are improved as noise and vibrations are reduced. Forgings can be pressed with cored holes in both vertical and horizontal axes. The press is generally of vertical type and the squeezing action is carried completely to the centre of the part being pressed.

The modern forging press has the necessary rigidity and power to forge carbon and alloy sheets, or stainless steels, aluminium alloys and magnesium alloys. Forging presses are of two types, mechanical and hydraulic. Both types generally make use of impression dies to produce forgings to commercially exact shape and size.

Mechanical presses may be either of screw type used for brass forging only; for crank type. The screw-type forging press turns a threaded spindle and moves the slide downward. The slide gains momentum as it moves downward but stops when the total energy is expended.

For small press forgings closed impression dies may be used and operation is completed in one stroke only. In bigger forgings, generally all the cavities are put into a single block. For press forging operation, the drive should be capable of giving huge force which is needed at the end of stroke when metal is forced into the desired shape.

For this process, copper alloys are very well suited as these flow easily in the die and are readily extruded. In this process, the area of reduction of metal is faster and the cost of operation is less. It produces quite smooth surface on the forged part. The process if however, limited only to symmetrical shaped parts. Presses can be readily automated.

The normal steps in press work are:

(i) Flattening of the forging stock placed in an end-up position,

(ii) Forging in a blocking impression,

(iii) Forging in the finishing impression.

Edging and fullering operation in press forging are not used to that extent as in drop forging. Press forgings are especially well adapted to forging preshaped blanks to the final size and shape. Coining and forming are two special operations generally carried out on the forging press.

Power presses are used to obtain close dimensional tolerances by coining and sizing on sections, such as bosses, or to impart closer alignment and smoother surfaces than are obtained in the regular forging operation. Scoring and special markings can be imparted by coining when the forging it either low red hot or cold. Sections of a drop forging may be formed or bent into an alignment that is not possible or practical in normal drop forging practice.

Impression dies used for press forging may be similar in character and shape to drop-forging dies in their construction with all the forging steps incorporated in a single set of solid die blocks. The progressive solid die blocks are used for convenience and economy, and with their use the amount of impact vibrations set up in the forging press is minimised. Individual blocks, when used, may be fastened to the press in special holding shoes.

Method # 3. Hammer and Press Forgings:

Hammer forgings may be made from cut pieces or from the end of heated bars and press forgings are invariably made from cut pieces. Fig. 5.30 shows the cross-section of a typical die impression and the terms used.

For complex shapes, it is usual to use perform bar stock. Performing may be achieved by fullering and rolling in dies on a hammer, by free-hand forging. After preforming and forging the component is surrounded by surplus material (flash) which is removed in a trimming press.

Method # 4. Hot Bar Forging:

This process is used to reduce an ingot heated at around 1300°C into a bar. Two flat dies placed opposite to each other squeeze the metal to reduce its thickness as shown in Fig. 5.31. Thus it becomes a very slow process compared to hot rolling but this process is most advantageous when good penetration of the metal is required upto the centre to improve the properties of metal near the centre of the forging.

If the bar has to be of square cross- section then work has to be turned through 90° between passes. The squeeze ratio, i.e. ratio t_xlt₂ should be less than 1.3, otherwise there is possibility of formation of laps (discontinuity or break in metal as it comes out of dies) which introduce scale into the forging, weakening it.

Reduction larger than 1.3 can be obtained by chamfering the edges of the dies as shown in Fig. 5.30. In order to avoid the possibility of occurrence of rhombic distortion fault due to poor guide alignment in forging presses, the ratio of width and thickness of billet (unforged) should not be more than 1.5. Further in order that deformation may penetrate sufficiently the unforged thickness t₁ should be less than 3 times the die breadth (b).

Method # 5. Upset Forging:

This is very useful operation for parts having uniform cross-section throughout and some head or other bigger shape at the ends, e.g. bolts, rear-axle drive shafts with flanged ends etc. In upset forging operation, a bar of uniform section is gripped in the fixed half of the die so that the requisite length projects, and pressure is applied at the heated end thus causing it to be upset or formed into some desired shape.

The heated stock is placed between a fixed and a movable die which grips the bar firm when closed. Usually the upsetting is done in a horizontal machine which is rated by the size of bar it will accept. The arrangement is such that a portion of the bar projects beyond the die for the upsetting operation.

A ram having the cavity impression is hammered at the projected portion. The operation of forging is carried out till the metal conforms to the die cavity. For complicated shapes, generally the work is progressively placed into different positions in the die. The impressions can be either in the gripping die or punch or in both. The heating operation might be completed in one position only.

Forging can be made in a single forging step or may require several steps depending on the amount of required movement of metal. This process utilises to the utmost the physical properties inherent in the steel and develops maximum uniform strength and durability throughout the structure of forging.

Fig. 5.31 shows the use of sliding dies in which the bar can be upset at any position (other than the end of the bar) along its length. 

Method # 6. Electrical Upset Forging:

In it the bar is clamped between the electrodes which heat the end. As forging temperature is reached, pressure is applied to cause upsetting. This process is used for forging component with large diameter flanges. Fig. 5.32 shows various stages in fabrication of valve stem.

Method # 7. Swing Forging:

This is a recent develop­ment in the field of automatic and semi-automatic, open die, press forging machines. It employs two pairs of forging tools as shown in Fig. 5.34 whose movement is controlled by rotary mechanisms such that they provide squeezing action at a fast rate. The work is held in rail mounted manipulators and moved axially in synchronism with the action of the forging tools, thus reducing the size of work.

Method # 8. Cored Forging:

The process of cored forging consists of hot-forming parts in dies which include movable cores for internal shaping. The operation requires only one stroke of the press. From the design point of view, this gives a much greater strength-to-weight ratio than a cast structure. The process may be applied to copper, brass and aluminium.

Method # 9. Rotary Forging:

Forging (controlled deformation under pressure using hot or cold working techniques) is an extensive metal working technology for production of a wide range of sizes and configurations of finished products. Conventional forging (using only two dies) is suitable for carbon steel but not for highly alloyed super alloys that maintain high strength levels at elevated temperatures and resist deformation.

Rotary forging is found to be suitable for such cases. The success of forging to a great extent depends on proper heating practice. For high temperature alloys, the forging temperature range is compressed and thus maintenance of desired temperature during forging is very important. Rotary forging helps in such cases by maintaining proper forging temperature due to generation of frictional heat because of its high speed and high reduction rate.

Rotary forging uses four mechanically driven hammers to rapidly exert force simultaneously on four sides of the work piece, resulting in spreading to occur in the desired longitudinal direction. It may be mentioned that with two hammer forging, spreading occurs laterally and there exists possibility of undesired surface tears and centre bursts.

In big rotary forging machines, ingots and billets can be fed to it and rounds, flats, squares, rectangles, hollows etc. can be produced with upto 40% reduction is one pass with possibility of undesired surface tears and centre bursts. In big rotary forging machines, ingots and billets can be fed to it can rounds, flats, squares, rectangles, hollows etc. can be produced with upto 40% reduction in one pass.

The core of the precision rotary forging machine is the robust cast steel forging box which contains four connecting rods arranged at right angles, driven by eccentric shafts and large gears powered by two high motors. Hammers are mounted on the rods. The reacting force are absorbed in the forging box and not transmitted to the foundation.

Forges with as many as 175 strokes (achieved by having short strokes of connecting rod) per minute exerting a force of 1500 tonne have been designed. The positioning of the hammers is accomplished by four adjustable mechanisms mounted in the forging box. The hammers can be adjusted in pairs independently for forging rectangles, or in unison when forging rounds or squares.

The upper most stroke position of hammers is controlled by hammer set designs. Rotary forging is actually a pressing process in which the forging is subjected to a high rate of deformation by the simultaneous, high speed action of the four forging hammers.

For gripping and feeding the forging to the hammers, two manipulators or chuck heads, one on each side of the forge box are used. For forging rounds, the work piece is rotated by the check-heads. When forging squares and rectangles, the chuck-head spindle is locked in position.

The forging of hollows is accomplished with a mandrel. The mandrel is held by a support tube through the chuck-head and is positioned between the hammers. The chuck-head feeds the hollow perform over the mandrel and through the hammers. To avoid sagging of long bars, work-piece supports are used. Additional support is provided in the vicinity of the hammers by centring guides.

Rotary forging allows much higher productivity, and a more precise, metallurgical round product with a uniformity and consistency often not achievable with conventional forging. No reheats are required and as such production is continuous. Surface defect are less likely to occur and there is not risk of tearing and bursting even with materials having limited hot ductility and a very narrow forging temperature range.

Machining allowances are reduced due to greatly improved surface quality with close and reproducible dimensional accuracy. This process consumes about 50% less energy compared to conventional forging. The complete plant comprises of rotary hearth and walking beam furnaces, automatic charge and discharge equipment, a hot abrasive cutting machine, etc.

Wobble forging (Rotary forging): 

In closed die forging, a huge force is required to forge a component because whole of the product is forged at a time. In wobble forging, also known as rotary forging, only a small proportion of the part is deformed at any particular time, thus forging forces are reduced considerably and capital cost is also low.

In this process, the upper die is rotated about an inclined axis which itself is rotating and the lower die is gradually moved up. This process can be applied only to circular products. It is possible to forge very thin flanges. Since preheat temperatures are not as high as for conventional forging process, the scale formation is avoided. Very close thickness tolerances can be achieved by this process.

Rotary metal working processes are becoming popular because of the advantages (of high material utilisation, low tooling costs, excellent product quality) offered by them. Rotary metal working processes include roll forming, ring rolling, wedge rolling, shear forging, rotary swaging, rotary forging.

Rotary forging (also sometimes called rotary swaging, or orbital forging) uses to dies to deform only a small portion of the work piece at a time and the process continues till whole of the work piece is deformed one portion after other. The axis of one die is tilted at a small angle with reference to the other die. (Refer Fig. 5.36).

The inclined upper die rotates around the work piece in such a way that only a small area of the die is in contact with the workpiece at any one time. The lower die is horizontal and rotates at same speed as upper one.

Due to the tilt angle and the shape of the upper die, there is only a small area of contact between the workpiece and the upper die at any one time. Due to lesser area of contact (around one-fifth of workpiece surface area), the force required for forging is considerably less than conventional forging. Because of smaller force, the deformation of die and machine and friction are also less, and thus the process can be high-precision one. It can also be used for cold forging of the parts.

The dies of rotary forging machines, can have three types of motions, viz., rotational (die spinning about its own axis) orbital (axis of one die processing about the axis of the other, causing the die to appear to rock or wobble), and translational (vertical motion or feed motion). Depending on combination of these motions, three types of rotary forges are available (Refer Fig. 5.37).

In case (a), one die may be driver and other a follower, or both dies may be power driven and thus rotate independently. In the former type, there is no slippage at any point along the die-workpiece contact line and thus each point on the upper die has the same tangential velocity at its corresponding contact point on the workpiece.

However a positional discrepancy is created after each revolution due to the difference in radius between the die and the workpiece. In the latter type, the position discrepancy can be eliminated if the rotational velocity of the upper die is equal to that of the workpiece. However the difference in radii causes a difference in the linear velocities on the corresponding contact points and thus slipping occurs.

It is found that first arrangement in case (a) discussed above yields greater precision with fewer repetitive errors. The press frame design is simple in this case because forging always acts in one direction. Bearing design is also simpler because the power drive for axial feed is decoupled from that for the rotating of the lower die. It may be noted that the upper bearing design in cases (b) and (c) is quite complex due to rocking motion.

Since the upper die does not have a rotary drive, the mechanism for varying the tilt angle is simpler. Higher accuracy is achievable because an error in the part is uniformly distributed along the circumference of the part.