We have given a brief description and analysis of each of the various basic forming operations. It is hoped that the reader has by now acquired some idea about these processes, especially about the mechanics involved. However, there are many minor and major variations of such processes. In this article, we shall discuss some of these along with the associated technological aspects.
Forming Process: Forming Operations of Materials
Operation # 1. Rolling:
It is seldom possible to achieve the final cross-section in one step. Generally, rolling is performed with a number of passes, using different polling equipment, in a continuous manner. The whole shop is usually called a rolling mill. When rolling flat strips, it is possible to perform the successive stages, using the same pair of rolls.
The upper roll is normally adjusted to control the gap after each pass. To avoid the problem of extensive material handling, it is desirable to have the provision of reversing the direction of roll rotations. As a result, the work piece moves back and forth in successive passes. Sometimes, the space can be optimized by using a three-high rolling mill.
During hot rolling, the lapse of time should be minimized as the job continuously cools down. This should be one of the major considerations in the layout of a rolling mill. Normally, the job movement is facilitated by providing support rolls. If a job is sufficiently long and flexible, a three-high rolling mill can be provided with some arrangement for feeding the second pass even before the first pass is completed. This is achieved by what is commonly known as a looping mill.
The looping can be done mechanically by using a bent tube or trough, known as repeater. A continuous multi pass rolling can also be performed for a flexible, long job by suitably arranging the rolling equipment with one roll pass near the other.
For a given reduction in area, commonly known as the draft, the roll separating force, which tends to bend the rolls, increases linearly with roll radius R given by equation (3.20).
Hence, the bending deflection of the rolls cannot be very effectively and economically controlled by using large drive rolls. A better and more economical way to reduce the roll deflection is to use backing rolls.
In this figure, two different methods of using the backing rolls are shown. Since the roll separating force depends on the radius of the drive rolls, these are always kept small in size, whereas the backing rolls are provided with a larger radius to increase the rigidity.
However, a certain amount of roll bending is unavoidable, but this can be taken care of by having non cylindrical rolls (Fig. 3.36a) which, under the roll separating force, bend, thus providing a uniform gap between the rolls (Fig. 3.36b). The rolls shown in Fig. 3.36a are called rolls with convex camber. With un-cambered rolls, the thickness of the rolled strip is more at the centre, as explained in Fig. 3.36c. Considering the rolls as thick, short beams simply supported at the ends, the deflection at the centre can be expressed as-
The typical values of λ1 and λ2 are 1.0 and 0.2 for a strip with width l, and 0.5 and 0.1 for a strip with width l/2.
The input stock to a rolling mill is normally of a rectangular cross-section, called a bloom or billet depending on the size. To obtain a different cross-section after rolling, the job has to undergo several passes, using form rolls with a gradually changing geometry. For example- Fig. 3.37 shows how the geometry of the gap between two rolls changes while producing a circular, thin rod from a square billet.
Rolls are normally made of cast or forged steel. Alloyed cast irons are sometimes used to lower down the cost. Superior strength and rigidity characteristics can be obtained by using special alloy steel which, obviously, is costlier. Hot rolls are roughened (even notched sometimes) to provide a good bite on the job, whereas cold rolls are ground to provide a fine surface to impart a good finish to the final product.
The main parameters of rolling include – (i) the temperature range (in hot rolling), (ii) the rolling speed schedule, and (iii) the allotment of reductions to various passes. All these, in turn, influence the dimensional accuracy of the product and also its physical and mechanical properties.
Operation # 2. Forging:
There are many variations of the basic forging operation, and the most commonly practised are:
(i) Smith Forging:
Smith forging is probably the most ancient metal working process. Here, a hot work piece is given the desired shape by using hand-held tools and hammers. Nowadays, power-driven hammers are used to impart the repeated blows. The anvil and the hammer are mostly flat and the desired shape (of course with limited varieties) is obtained by a manipulation of the job between the blows.
(ii) Drop Forging:
In drop forging, the impact loads (blows) are applied to the work piece to cause metal flow for Ailing up the cavity formed by the two halves of the closed die. To ensure complete filling, normally an excess amount of material is provided. This excess material flows out circumferentially to form a flash which is subsequently trimmed. When the product geometry is complicated, a set of dies may be necessary to obtain the final form.
(iii) Press Forging:
Instead of the repeated blows, a gradual force is applied in press forging. However, depending on the complexity of the job, a set of dies may be required to obtain the final product. It is obvious that here the alignment of the two halves of the die poses a lesser problem than in drop forging. Since the operation is completed in one stroke, a provision must be made for the air and excess die lubricant to escape.
(iv) Upset Forging:
In many cases, only a portion of the job needs to be forged. A common example is the forging of the bolt head at one end of a rod. Such a localized forging operation is commonly known as upsetting. The upsetting operation may be both closed and open, as shown in Figs. 3.38a and 3.38b, respectively. Clearly, the operation involves a longitudinal compression of the bar stock.
Hence, to prevent buckling, the following rules are observed regarding the unsupported length to be forged:
(a) In an open operation, the length of the unsupported portion (l) should not exceed 3d, d being the diameter of the job.
(b) If l exceeds 3d, a closed operation should be performed with a die diameter D ≤ 1.5 d.
(c) If, during a closed operation, the unsupported length extends beyond the die cavity (Fig. 3.38c) by an amount l1, then l1 ≤ d.
Swaging is a special variation of impact forging where the repeated blows are obtained by a radial movement of shaped dies. This operation is generally used for reducing the diameters and tapering of bars and tubes.
(vi) Roll Forging:
Roll forging is performed with two semicircular, grooved rolls held by two parallel shafts. The process is used for reducing the diameter of rods. The heated work piece is placed between the dies in an open position. After a half revolution of the rolls, the work piece is rolled out. It is then put in the smaller groove and the operation continued until the desired dimension is achieved.
It is obvious that the die is one of the most critical components of the forging operation, and therefore the success of the process depends considerably on the design of the die.
The basic features a forging die should have are as follows (see also Fig. 3.41):
(i) To ease the flow of metal around the comers, a proper fillet radius should always be provided. This also helps in preventing excessive die wear and fracture of metals near the corners.
(ii) As in a moulding pattern, so too here all vertical surfaces should be given a suitable draft for easy removal of the job from the die.
(iii) As already mentioned, a space around the die edges should be provided to accommodate the excess material, known as flash. To receive this flash, it is recommended that a flash gutter be provided.
In hot forging, the die dimensions should include the shrinkage allowance (to compensate for the contraction of the product after cooling) as the forged product is normally not subjected to any subsequent overall finishing operation. The forging die is usually made of a high or medium carbon alloy steel as it is subjected to large workloads. The hardness (Rc) of the die is normally in the range 45-60.
Operation # 3. Drawing:
The drawing operation is mainly used for reducing the diameter of bars and wires. The drawing speed varies from 10 m / min for a large diameter to 1800 m / min for a very thin wire. To begin the operation, the starting end of the stock is swaged to a smaller diameter for easy entry into the die.
Moreover, to prevent any impact action, the operation is started at a slow speed. In large reductions, the operation may be performed in a number of passes. Since sufficient heat is generated due to continuous cold working, it may be necessary to cool the die with water. Sometimes, a tube is also drawn through a draw die, and in this case, the operation is called sinking.
Normally, a large die is made of either high carbon or high speed steel, whereas tungsten carbide is used for a medium-size die. For drawing a fine wire, the die is made of diamond.
Operation # 4. Deep Drawing:
It is evident from our description of the mechanics of the deep drawing process that an attempt should be made to draw the sheet metal into the die as much as possible. This helps in minimizing the thinning of the cup wall. Consequently, the outer circumference of the blank reduces, causing a compressive hoop stress which, when exceeds a limit, may result in a plastic wrinkling of the cup flange. These wrinkles cannot be ironed out afterwards but can be avoided by using a blank holder.
However, an excessive pressure from the blank holder resists an easy drawing of the material into the die. If the drawing ratio (defined as rj / rd) is not more than 1.2, the operation can be conducted even without a blank holder. Higher values of the drawing ratio can be achieved depending on the thickness of the blank and die profile, as shown in Fig. 3.43.
When the ratio of the blank diameter and the final cup diameter is too large, the operation is performed in more than one stage. The successive drawing operations after the first one are known as redrawing. Figures 3.44a and 3.44b show two typical redrawing operations. The operation shown in Fig. 3.44b is termed as reverse redrawing, because, in this, the initially drawn cup is turned inside out. This operation appears to involve a more severe working of the material than the conventional redrawing operation.
However, the real situation is just the opposite, as now explained. In conventional redrawing (Fig. 3.44a), the material bends in the opposite directions around the blank holder and the die comers. On the other hand, in reverse drawing (Fig. 3.44b); the material bends in only one direction, namely, along the outer and the inner die corners. In an extreme case, the die can be provided with a round edge, as shown in Fig. 3.44c, resulting in a less severe working of the material.
Since some amount of strain hardening takes place during the initial operation, annealing is normally advised (to restore the ductility) before commencing the redrawing operation.
In general, the flow of metal is not uniform throughout the work piece and in most cases the drawn parts have to be trimmed to remove the undesired metal. Such a trimming can be done either by a hand-guided operation or by using a separate trimming die.
The stripping of the job from the punch can be achieved by machining a slight recess into the underside of the draw die. During the return stroke, the punch pressure is removed from the cup; as a result, the drawn cup tends to spring back. Due to this action, the recess prevents the drawn cup from moving along with the punch during its upward stroke.
Operation # 5. Bending:
The analysis of the bending operation we have given is applicable only when corners are required to be produced in a sheet metal. However, more complicated shapes can also be obtained by this operation. In general, such an operation may need more than one stage. For producing a complex shape, the bending operation is performed continuously, using a series of contoured rolls. Idle rollers are used when necessary for pressing the job from the side during the production of such a shape.
Tubes and other hollow sections can also be bent by wrapping the job around a form block through the use of a wiper roll. If the wiper roll has a constant curvature, it may be hinged at the centre of the curvature to be produced. Figure 3.48 explains such an operation for bending a tube. The tube can be prevented from collapsing by filling the inside space with some filling material, e.g., sand. The self-explanatory diagrams tube bending operations.
Operation # 6. Extrusion:
Extrusion is one of the most potential and useful metal working processes and has a large number of variations in the mode of application. It can be performed under both hot and cold conditions. Hot extrusion helps reduce the work load (especially for high strength materials) but it poses more problems such as cooling arrangement and rapid die wear.
From the analysis for a simple forward extrusion process we have already given, it is clear that, in this direct process, the whole billet is required to move forward, resulting in a large frictional loss and high working load. As a consequence of this high work load, the container is subjected to high radial stresses.
The foregoing difficulties can be avoided by using a backward extrusion process where the billet remains stationary. Thus, the frictional force is absent between the billet and the container and acts only at the die-container interface. The magnitude of the latter is much less than that of the frictional force encountered in a forward extrusion process. Hence, the work load is reduced and also it is independent of the billet length.
Tubular sections can also be extruded by using a mandrel along with the ram, as illustrated in Fig. 3.51. Both open (Fig. 3.51a) and closed (Fig. 3.51b) end products can be obtained depending on the initial blank shape. The mandrel may either be fixed to the ram or to a separate body, as indicated in Fig. 3.51c.
Thin-walled cans may be obtained by using impact extrusion. This process is limited to soft and ductile materials and is normally performed under cold conditions.
Instead of applying the load on the billet directly by the ram, a fluid medium can be used, as illustrated in Fig. 3.53a. This process is known as hydrostatic extrusion; here, the frictional loss at the billet-container interface is eliminated.
A slight variation of this process offers a possibility of extruding a relatively brittle material. In this, apart from the large hydrostatic pressure applied to the billet, the product in the receiving chamber is maintained under a lower pressure (about one-half the pressure applied to the billet). As shown in Fig. 3.53b, the material is subjected to lower strain gradients. In this process, it is possible to produce very large objects. However, since the process is inherently slow, its application is limited.
To produce a job having a complex shape with non-uniform cross-section, closed cavity extrusion with a split die can be used. The process is similar to closed die forging and is illustrated in Fig. 3.53c.
All billets are usually covered with an oxide layer. During a normal extrusion process, this oxide layer may be drawn into the core of the product (reducing its strength characteristics) unless a laminar flow during the plastic deformation is ensured. Lubricants should be used between the billet, die, and container not only to reduce the work load but also to keep the flow laminar. As a result, the outer surface of the billet forms the skin of the product. This principle of maintaining the surface layer is also used in a hot extrusion of high strength materials and clad products as now discussed.
The temperature range of the billet during the hot extrusion of steels is 1200-1500°C. The die must be kept at a lower temperature (approximately 200°C) to avoid excessive wear rate. Glass fibres (or powders) are normally used as lubricants since the viscosity of glass is sensitive to temperature. Thus, the viscosity is high at the die surface, providing a good protection to die wear and facilitating the formation of a glass skin (about 0.025 mm thick) .on the product. At the same time, the work load is reduced since the viscosity of glass is much lower at the billet-container interface.
Another useful application of this cladding process is in the production of a radioactive nuclear fuel rod of, For example- uranium and thorium. The rod is canned in copper or brasses both of which are less reactive to the atmospheric gases and protect the fuel rod from oxidation and other types of contamination. The billet is prepared with the cover made of a cladding material.
Operation # 7. Punching and Blanking:
Though punching and blanking are the most common sheet metal operations involving shearing of the metal strips, there are other similar operations such as – (i) notching, (ii) lancing, (iii) slitting, (iv) nibbling, and (v) trimming.
In the notching operation, material is removed from the side of a sheet metal, whereas lancing makes cuts partway through the metal without producing any scrap. Lancing is frequently combined with bending to form tabs. Slitting is an operation to cut a coiled sheet metal lengthwise to produce narrower strips.
In the nibbling operation, complicated shapes are cut out from a sheet metal by producing overlapping notches starting either from the outer boundary or from a punched hole. Without using any special tool, a simple, round or triangular punch of small dimensions is reciprocated at a fixed location. The sheet metal is guided to obtain the desired shape of the cut. Trimming refers to the removal of the excess material in a flange or flash.
In reducing the operation time and cost, the design of the die and punch for blanking plays an extremely important role. A typical simple die-punch combination. An accurate relative location of the punch and the die is maintained with the help of a set of guide posts. The stripper helps in removing the sheet metal work piece from the punch during the return stroke, whereas the spring loaded push-off pins help in removing the blank from the punch face. The stripper also acts as a blank holder to prevent drawing.
To optimize space and time, more than one operation can be performed in a stroke, using more than one set of die and punch in the same assembly (Fig. 3.56). Such an assembly is commonly known as a compound die. It should be noted that the blanking punch and die are in the inverted position in Fig. 3.56. It is obvious that piercing of the inner hole has to be performed before blanking. Sometimes, a combination of drawing (or bending) and blanking is also used for economy.
In the foregoing situation, more than one operation is performed in only one location. However, it is also possible to use a series of die-punch elements at different locations. Here, one operation is performed at each station and the metal stock is advanced to the next station. Thus, a continuous operation is possible. Such an assembly of dies is called a progressive die.
Another important aspect of the blanking operation is to minimize the scrap by an optimum layout design (also known as nesting). This is schematically represented in Fig. 3.58. The restrictions on the layout are shown in Fig. 3.58b. The minimum gap between the edge of the blank and the side of the strip is given as g = t + 0.015h, where t is the thickness of the strip and h is the width of the blank.
The gap between the edges of two successive blanks (b) depends on the strip thickness t. Table 3.1 shows the various values of b. Sometimes, the relative direction of grain flow (when a rolled strip is used as stock) with respect to the blank is specified. In such a case, the freedom of nesting is nearly lost.
In a circular blank, some saving in the scrap may be achieved only through a choice of multiple rows.
Operation # 8. High-Energy-Rate Forming Processes:
In all the metal forming processes we have discussed, the conventional energy sources are used. In addition to these, energy sources such as chemical, magnetic, and electrical discharge can be used. Since, in all such processes, the rate of energy flow is of a much higher order, these are commonly called High-Energy-Rate (HER) processes. As the kinetic energy of a moving body is proportional to the square of its velocity, a large amount of energy can be supplied by a relatively smaller body moving at a high speed.
For example- a press of capacity 500 kN moving over a distance of 0.15 m delivers an energy of 75 kJ. Approximately the same amount of energy can be delivered by a hammer weighing 42 kN if it strikes the work piece with a velocity of 6 m / sec. However, a water front, weighing only 26 N, made to move with a velocity as high as 240 m / sec by an explosive charge, can supply the same amount of energy. This principle can be used in making small machines and equipment.
Now, let us consider the rate of energy release in the three cases we have mentioned. In the first case, the typical time consumed is about 0.5 sec, indicating a power of 150 kW. The drop hammer takes about 0.06 sec to come to rest and the power involved turns out to be 1.25 MW. The explosive operation is completed in about 0.0007 sec, implying a power of 107 MW. This indicates that the last case results in not only the most compact but also the most powerful machine. High velocity forming operations, viz., explosive and electric discharge forming, are based on the foregoing principle.
We now discuss the three common HER processes:
i. Explosive Forming:
Figure 3.60 shows two schemes of explosive forming. In both, a shock wave in the fluid medium (normally water) is generated by detonating an explosive charge.
For a small part, the entire shock wave front is utilized in a confined space, whereas for a large object, only a part of the wave front is used. Obviously, the unconfined operation is less efficient. However, there is a greater hazard of die failure in the confined operation due to the inevitable lack of control in explosive forming.
The typical explosives include TNT and dynamite for higher energy, and gun powder for lower energy. With high explosives placed directly over the work piece, pressures up to 35 kN / mm2 can be generated. With low explosives, pressures are limited to 350 N / mm2.
With water as the transmitting medium, the peak pressure p obtained is given by –
The distance between the explosive charge and the free surface of water (in
unconfined forming) should be at least twice the stand-off distance. Otherwise much energy is lost, lowering down the efficiency of the operation. Using various types of tooling, we can form a variety of shapes. Generally, the effects of the process on material properties are similar to those in conventional forming.
Electric discharge in the form of sparks, instead of explosives, can also be used to generate a shock wave in a fluid. An operation using this principle of generating a shock wave is called electrohydraulic forming. The characteristics of this process are very similar to those of explosive forming. The capacitor bank is charged through the charging circuit; subsequently, the switch is closed, resulting in a spark within the electrode gap to discharge the capacitors.
The energy level in this process is lower than that in explosive forming. The peak pressure developed over the work piece is a function of the amount of energy discharged (through the spark) and the stand-off distance.
Just as in electrohydraulic forming, so too in electromagnetic forming, the electrical energy is first stored in a capacitor bank. This energy is then discharged through a coil by closing the switch. The coil produces a magnetic field; the intensity of this field depends on the value of the current. Since the metallic work piece is in this magnetic field (varying with time), a current is induced in the job which sets up its own magnetic field.
The directions of these fields are such that the rigidly held coil repels the work piece into the die. The work piece obviously has to be electrically conductive but need not be magnetic. Short life of the coil is the major problem in such an operation.
Operation # 9. Coining:
Coining is a closed die forging operation which imparts the desired variation in the thickness (because of lateral constraints) to thin and fiat work pieces. As the name implies, this process is widely used in producing coins and also other similar objects requiring a well-defined impression of the die face.
Operation # 10. Thread Rolling:
For a mass production of threaded objects, e.g., bolts and screws, two flat, reciprocating dies (or threaded rolls rotating in opposite directions) can be used to obtain the thread in the work piece through plastic deformation. This is basically a rolling operation, and hence the name thread rolling.
Operation # 11. Tube Piercing:
The production of seamless tubes is very important and is commonly achieved by a tube piercing operation. In this operation, a solid bar stock is forced to flow over a mandrel at one end by means of two inclined rollers rotating in opposite directions. The speed and the amount of inclination of the rollers decide the feeding rate. This operation is done in a hot condition.
The simultaneous squeezing and rotating action of the rolls deforms the material to an elliptic shape and develops a crack along the major axis. A further rotation of the deformed material causes the crack to expand and transform into a hole which is finally shaped and sized by the mandrel.
Operation # 12. Spinning:
In the spinning process, an object with surface of revolution is produced from a sheet metal. The blank is held against a form die which is rotated and the sheet metal blank is laid over this die, using a specially-shaped tool or roller. If a simultaneous thinning of the sheet metal takes place during the operation, the process then is called shear spinning.
Operation # 13. Stretch Forming:
In a sheet metal bending operation, compressive stress is always developed and, under certain circumstances, this may be large enough to cause local buckling or wrinkling. Such problems can be avoided by keeping the metal strip under tension during the operation. This process of simultaneous stretching and bending is called stretch forming.