FUNDAMENTALS OF METAL FORMING
There are four basic production processes for producing desired shape of a product. These are casting, machining, joining (welding, mechanical fastners, epoxy, etc.), and deformation processes. Casting process exploit the fluidity of a metal in liquid state as it takes shape and solidifies in a mold. Machining processes provide desired shape with good accuracy and precision but tend to waste material in the generation of removed portions. Joining processes permit complex shapes to be constructed from simpler components and have a wide domain of applications.
Deformation processes exploit a remarkable property of metals, which is their ability to flow plastically in the solid state without deterioration of their properties. With the application of suitable pressures, the material is moved to obtain the desired shape with almost no wastage. The required pressures are generally high and the tools and equipment needed are quite expensive. Large production quantities are often necessary to justify the process.
Fig 1.1 State of the stresses metal undergo during deformation.
As a metal is deformed (or formed, as often called) into useful shape, it experiences stresses such as tension, compression, shear, or various combinations there of Fig 1.1 illustrates these states of stresses. Some common metal forming processes are schematically given in Fig 1.2 along with the state of stress(es) experienced by the metal during the process.
| Number | Process | State of Stress in Main Part During Forming |
| 1 | Rolling  | Bi-axial compression |
| 2 | Forging  | Tri-axial compression |
| 3 | Extrusion  | Tri-axial compression |
| 4 | swaging  | Bi-axial compression |
| 5 | Deep drawing  | In flange of blank, bi-axial tension and compression. In wall of cup, simple uni-axial tension. |
| 6 | Wire and tube drawing  | Bi-axial compression, tension. |
| 7 | Straight bending  | At bend, bi-axial compression and bi-axial tension |
Fig 1.2 Common metal forming processes. State of stress experienced by metal is also given
To understand the forming of metal, it is important to know the structure of metals. Metals are crystalline in nature and consist of irregularly shaped grains of various sizes. Each grain is made up of atoms in an orderly arrangement, known as a lattice. The orientation of the atoms in a grain is uniform but differs in adjacent grains. When a force is applied to deform it or change its shape, a lot of changes occur in the grain structure. These include grain fragmentation, movement of atoms, and lattice distortion. Slip planes develop through the lattice structure at points where the atom bonds of attraction are the weakest and whole blocks of atoms are displaced. The orientation of atoms, however, does not change when slip occurs.
To deform the metal permanently, the stress must exceed the elastic limit. At room temperature, the metal is in a more rigid state than when at higher temperature. Thus, to deform the metal greater pressures are needed when it is in cold state than when in hot state.
When metal is formed in cold state, there is no recrystallization of grains and thus recovery from grain distortion or fragmentation does not take place. As grain deformation proceeds, greater resistance to this action results in increased hardness and strength. The metal is said to be strain hardened. There are several theories to explain this occurrence. In general, these refer to resistance build up in the grains by atomic dislocation, fragmentation, or lattice distortion, or a combination of the three phenomena.
The amount of deformation that a metal can undergo at room temperature depends on its ductility. The higher the ductility of a metal, the more the deformation it can undergo. Pure metals can withstand greater amount of deformation than metals having alloying elements, since alloying increases the tendency and rapidity of strain hardening. Metals having large grains are more ductile than those having smaller grains.
When metal is deformed in cold state, severe stresses known as residual stresses are set up in the material. These stresses are often undesirable, and to remove them the metal is heated to some temperature below the recrystalline range temperature. In this temperature range, the stresses are rendered ineffective without appreciable change in physical properties or grain structure.
COLD AND HOT WORKING OF METALS
Cold Working:
Plastic deformation of metals below the recrystallization temperature is known as cold working. It is generally performed at room temperature. In some cases, slightly elevated temperatures may be used to provide increased ductility and reduced strength. Cold working offers a number of distinct advantages, and for this reason various cold-working processes have become extremely important. Significant advances in recent years have extended the use of cold forming, and the trend appears likely to continue.
In comparison with hot working, the advantages of cold working are
1. No heating is required
2. Bettter surface finish is obtained
3. Better dimensional control is achieved; therefore no secondary machining is generally needed.
4. Products possess better reproducibility and interchangeablity.
5. Better strength, fatigue, and wear properties of material.
6. Directional properties can be imparted.
7. Contamination problems are almost negligible.
Some disadvantages associated with cold-working processes are:
1. Higher forces are required for deformation.
2. Heavier and more powerful equipment is required.
3. Less ductility is available.
4. Metal surfaces must be clean and scale-free.
5. Strain hardening occurs ( may require intermediate annealing ).
6. Undesirable residual stresses may be produced
Cold forming processes, in general, are better suited to large-scale production of parts because of the cost of the required equipment and tooling.
Warm Working:
Metal deformation carried out at temperatures intermediate to hot and cold forming is called Warm Forming . Compared to cold forming, warm forming offers several advantages. These include:
• Lesser loads on tooling and equipment
• Greater metal ductility
• Fewer number of annealing operation ( because of less strain hardening )
Compared to hot forming, warm forming offers the following advantages.
• Lesser amount of heat energy requirement
• Better precision of components
• Lesser scaling on parts
• Lesser decarburization of parts
• Better dimensional control
• Better surface finish
• Lesser thermal shock on tooling
• Lesser thermal fatigue to tooling, and so greater life of tooling.
Hot Working:
Plastic deformation of metal carried out at temperature above the recrystallization temperature, is called hot working. Under the action of heat and force, when the atoms of metal reach a certain higher energy level, the new crystals start forming. This is called recrystallization. When this happens, the old grain structure deformed by previously carried out mechanical working no longer exist, instead new crystals which are strain-free are formed.
In hot working, the temperature at which the working is completed is critical since any extra heat left in the material after working will promote grain growth, leading to poor mechanical properties of material.
In comparison with cold working, the advantages of hot working are
1. No strain hardening
2. Lesser forces are required for deformation
3. Greater ductility of material is available, and therefore more deformation is possible.
4. Favorable grain size is obtained leading to better mechanical properties of material
5. Equipment of lesser power is needed
6. No residual stresses in the material.
Some disadvantages associated in the hot-working of metals are:
1. Heat energy is needed
2. Poor surface finish of material due to scaling of surface
3. Poor accuracy and dimensional control of parts
4. Poor reproducibility and interchangeability of parts
5. Handling and maintaining of hot metal is difficult and troublesome
6. Lower life of tooling and equipment.
FORGING
Forging is a process in which material is shaped by the application of localized compressive forces exerted manually or with power hammers, presses or special forging machines. The process may be carried out on materials in either hot or cold state. When forging is done cold, processes are given special names. Therefore, the term forging usually implies hot forging carried out at temperatures which are above the recrystallization temperature of the material. Forging is an effective method of producing many useful shapes. The process is generally used to produce discrete parts. Typical forged parts include rivets, bolts, crane hooks, connecting rods, gears, turbine shafts, hand tools, railroads, and a variety of structural components used to manufacture machinery. The forged parts have good strength and toughness; they can be used reliably for highly stressed and critical applications.
A variety of forging processes have been developed that can be used for either producing a single piece or mass – produce hundreds of identical parts. Some common forging processes are:
1. Open – die hammer forging
2. Impression – die drop forging
3. Press Forging
4. Upset Forging
5. Swaging
6. Rotary Forging
7. Roll forging
Open – Die Hummer Forging. It is the simplest forging process which is quite flexible but not suitable for large scale production. It is a slow process. The resulting size and shape of the forging are dependent on the skill of the operator.
Fig 2.1
Open die forging does not confine the flow of metal, Fig 2.1. The operator obtains the desired shape of forging by manipulating the work material between blows. Use may be made of some specially shaped tools or a simple shaped die between the work piece and the hammer or anvil to assist in shaping the required sections (round, concave, or convex), making holes, or performing cut – off operations. This process is most often used to make near – final shape of the part so that some further operation done on the job produces the final shape. Forging Force. In open die forging operation, the forging force F, to be applied on a solid cylindrical component can be determined from the relation.
Example. Using open-die forging operation, a solid cylindrical piece of 304 stainless steel having 100 mm dia x 72 mm height is reduced in the height to 60 mm at room temperature. Assuming the coefficient of friction as 0.22 and the flow stress for this material at the required true strain as 1000 MPa, calculate the forging force at the end of stroke.
Solution . Initial diameter = 100 mm
Initial height = 72 mm
Final height = 60 mm
If final diameter is d, (100)2 x 72 = d2 x 60
i.e. d =110 mm
The process uses shaped dies to control the flow of metal. The heated metal is positioned in the lower cavity and on it one or more blows are struck by the upper die. This hammering makes the metal to flow and fill the die cavity completely. Excess metal is squeezed out around the periphery of the cavity to form flash. On completion of forging, the flash is trimmed off with the help of a trimming die.
Most impression – die sets contain several cavities. The work material is given final desired shape in stages as it is deformed in successive cavities in the die set. The shape of the cavities cause the metal to flow in desired direction, thereby imparting desired fibre structure to the component.
Auto – Forging:
This is a modified form of impression – die forging, used mainly for non – ferrous metals.
In this a cast preform, as removed from the mold while hot, is finish – forged in a die. The flash formed during die forging is trimmed later in the usual manner. As the four steps of the process – casting, transfer from mold to the forging die, forging, and trimming are in most applications completely mechanized, the process has acquired the name Auto – forging.
Coining:
It is a closed – die forging process used mainly for minting coins and making of jewelry. In order to produce fine details on the work material the pressures required are as large as five or six times the strength of the material. Lubricants are not employed in this process because they can get entrapped in the die cavities and, being incompressible, prevent the full reproduction of fine details of the die.
Net - shape Forging (Precession Forging)
Modern trend in forging operation is toward economy and greater precision. The metal is deformed in cavity so that no flash is formed and the final dimensions are very close to the desired component dimensions. There is minimum wastage of material and need for subsequent machining operation is almost eliminated.
The process uses special dies having greater accuracies than those in impression – die gorging, and the equipment used is also of higher capacity. The forces required for forging are high. Aluminum and magnesium alloys are more suitable although steel can also be precision – forged. Typical precision – forged components are gears, turbine blades, fuel injection nozzles, and bearing casings.
Because of very high cost of toolings and machines, precision forging is preferred over conventional forging only where volume of production is extremely large.
Forging Force Requirement:
The forging force, F, required to forge material by impression – die forging operation can be determined by the relation
F = k . s f . A
where k is a constant (whose value can be taken from Table 2.1 s f is the flow stress of material at the forging temperature, and A is the projected area of the forging including the flash.
In hot forging of most non – ferrous metals and alloys, the forging pressure is generally in the range of 500 MPa to 1000 MPa. | Table 2.1 Range of value of k |
| Simple shape of part, no flash produced 3 to 5 Simple shape of part, flash produced 5 to 6 Intricate shape of part, flash produced 8 to 12 |
Press forging, which is mostly used for forging of large sections of metal, uses hydraulic press to obtain slow and squeezing action instead of a series of blows as in drop forging. The continuous action of the hydraulic press helps to obtain uniform deformation throughout the entire depth of the workpiece. Therefore, the impressions obtained in press forging are more clean.
Press forgings generally need smaller draft than drop forgings and have greater dimensional accuracy. Dies are generally heated during press forging to reduce heat loss, promote more uniform metal flow and production of finer details.
Hydraulic presses are available in the capacity range of 5 MN to 500 MN but 10 MN to 100MN capacity presses are more common.
Upset Forging
Upset forging involves increasing the cross – section of a material at the expense of its corresponding length. Upset – forging was initially developed for making bolt heads in a continuous manner, but presently it is the most widely used of all forging processes. Parts can be upset – forged from bars or rods upto 200 mm in diameter in both hot and cold condition. Examples of upset forged parts are fasteners, valves, nails, and couplings.
The process uses split dies with one or several cavities in the die. Upon separation of split die, the heated bar is moved from one cavity to the next. The split dies are then forced together to grip the and a heading tool (or ram) advances axially against the bar, upsetting it to completely fill the die cavity. Upon completion of upsetting process the heading tool comes back and the movable split die releases the stock.
Upsetting machines, called upsetters, are generally horizontal acting.
When designing parts for upset – forging, the following three rules must be followed. 1. The length of unsupported bar that can be upset in one blow of heading tool should not exceed 3 times the diameter of bar. Otherwise bucking will occur.
2. For upsetting length of stock greater than 3 times the diameter the cavity diameter must not exceed 1.5 times the dia of bar.
3. For upsetting length of stock greater than 3 times the diameter and when the diameter of the upset is less than 1.5 times the diameter of the bar, the length of un – supported stock beyond the face of die must not exceed diameter of the stock.
Roll Forging This process is used to reduce the thickness of round or flat bar with the corresponding increase in length. Examples of products produced by this process include leaf springs, axles, and levers.
The process is carried out on a rolling mill that has two semi – cylindrical rolls that are slightly eccentric to the axis of rotation. Each roll has a series of shaped grooves on it. When the rolls are in open position, the heated bar stock is placed between the rolls. With the rotation of rolls through half a revolution, the bar is progressively squeezed and shaped. The bar is then inserted between the next set of smaller grooves and the process is repeated till the desired shape and size are achieved.
SWAGING
In this process, the diameter of a rod or a tube is reduced by forcing it into a confining die. A set of reciprocation dies provides radial blows to cause the metal to flow inward and acquire the form of the die cavity. The die movements may be of in – and – out type or rotary. The latter type is obtained with the help of a set of rollers in a cage, in a similar action as in a roller bearing. The workpiece is held stationary and the dies rotate, the dies strike the workpiece at a rate as high as 10 - 20 strokes per second.
Screwdriver blades and soldering iron tips are typical examples of swaged products. Fig 3.1 shows these and other products made by swaging.
Fig 3.1 Typical parts made by swaging.
In tube swaging, the tube thickness and / or internal dia of tube can be controlled with the use of internal mandrels. For small – diameter tubing, a thin rod can be used as a mandrel; even internally shaped tubes can be swaged by using shaped mandrels. Fig 3.2 shows the process.
Fig 3.2 (a) Swaging of tubes without a mandrel. Wall thickness is more in the die gap.
(b) Swaging with a mandrel. The final wall thickness of the tube depends on the mandrel diameter.
(c) Examples of cross-sections of tubes produced by swaging on shaped mandrels.
(b) Swaging with a mandrel. The final wall thickness of the tube depends on the mandrel diameter.
(c) Examples of cross-sections of tubes produced by swaging on shaped mandrels.
The process is quite versatile. The maximum diameter of work piece that can be swaged is limited to about 150 mm; work pieces as small as 0.5 mm diameter have been swaged. The production rate can be as high as 30 parts per minute depending upon the complexity of the part shape and the part handling means adopted.
The parts produced by swaging have tolerance in the range ± 0.05 mm to ± 0.5 mm and improved mechanical properties. Use of lubricants helps in obtaining better work surface finish and longer die life. Materials, such as tungsten and molybdenum are generally swaged at elevated temperatures as they have low ductility at room temperature. Hot swaging is also used to form long or steep tapers, and for large reductions.
Swaging is a noisy operation. The level of noise can be, however, reduced by proper mounting of the machine or by the use of enclosure.
WIRE DRAWING
Wire drawing is primarily the same as bar drawing except that it involves smaller – diameter material that can be coiled. It is generally performed as a continuous operation on draw bench like the one shown in Fig 3.3
Fig 3.3 Wire drawing on a continuous draw block. The rotating draw block provides a continuous pull on the incoming wire.
Large coil of hot rolled material of nearly 10 mm diameter is taken and subjected to preparation treatment before the actual drawing process. The preparation treatment for steel wire consists of :
· Cleaning. This may be done by acid pickling, rinsing, and drying. Or, it may be done by mechanical flexing.
· Neutralization. Any remaining acid on the raw material is neutralized by immersing it in a lime bath. The corrosion protected material is also given a thin layer of lubricant.
To begin the drawing process, one end of coil is reduced in cross section upto some length and fed through the drawing die, and gripped. A wire drawing die is generally made of tungsten carbide and has the configuration shown in Fig 3.4 for drawing very fine wire, diamond die is preferred.
Fig 3.4 Cross section through a typical carbide wire drawing die.
Small diameter wire is generally drawn on tandom machines which consists of a series of dies, each held in a water – cooled die block. Each die reduces the cross section by a small amount so as to avoid excessive strain in the wire. Intermediate annealing of material between different states of wire may also be done, if required.
Wire drawing terms :
Where Do , Df , Lo and Lf are the original and final diameter and length. Ao and Af are original and final cross sectional area.
For a single cold – drawing pass, the percent area reduction that can be done depends upon many factors. These include the type of material, its size, initial metallurgical condition, the final size and mechanical properties desired, die design and lubrication efficiency. The percent of area reduction per pass can range from near zero to 50%.
Die pull
The force required to pull the stock through the die (under frictionless conditions) can be computed as follows.
Where F = die pull, i.e. the force required to pull the stock through the die
Yavg = average true stress of the material in the die gap
Ao , Af = original and final areas of cross section of material.
Alternatively, the following expression can be used
F = c st (Ao - Af )
where c is a constant whose value is in the range 1.5 to 3.0 depending upon the % area reduction, (lower value for higher % reduction), and st is tensile strength of material before drawing.
The pull force determines the machine capacity needed.
TUBE DRAWING
The diameter and wall thickness of tubes that have been produced by extrusion or other processes can be reduced by tube drawing process. The process of tube drawing (Fig 3.5) is similar to wire or rod drawing except that it usually requires a mandrel of the requisite diameter to form the internal hole.
Tubes as large as 0.3 m in diameter can be drawn.
Fig 3.5
Drawing Equipment
Drawing equipment can be of several designs. These designs can be classified into two basic types; Draw bench, and Bull block. A draw bench (Fig 3.5) uses a single die and the pulling force is supplied by a chain drive or by hydraulic means. Draw bench is used for single length drawing of rod or tube with diameter greater than 20mm. Length can be as much as 30 m. The drawing speed attainable on a draw bench ranges from 5 m/min to 50 m/min. Draw benches are available having capacities to provide pull force of upto 1 MN.
Bull block or rotating drum (Fig 3.3) is used for drawing rods or wires of very long length.
FORMABILITY OF SHEET METAL
Formability may be defined as the ease with which material may be forced into a permanent change of shape.
The formability of a material depends on several factors. The important one concerns the properties of material like yield strength, strain hardening rate, and ductility. These are greatly temperature - dependent. As the temperature of material is increased, the yield strength and rate of strain hardening progressively reduce and ductility increases. The hot working of metal, therefore, permits relatively very large amount of deformation before cracking.
There are several methods of predicting formability. A brief description of some important methods follows.
Cup or Radial Drawing:
Cup drawing test uses a circular blank from the metal to be tested. It is inserted in a die, and the severity of the draw it is able to withstand without tearing called the drawing ratio, is noted. The drawing ratio is the ratio of the cup diameter to the blank diameter.
Where Rd = drawing ratio
D = blank diameter
d = punch diameter
A drawing ratio of 50 % is considered excellent. As shown in Fig 4.1(a), either a flat bottom punch with lubricated blank may be used to draw the cup, or as shown in Fig 4.1(b) a blank may be drawn by a lubricated hemi – spherical punch. In the first case, the action is principally that of drawing in which cylindrical stretching of material takes place. In the second case, there will be bi – axial stretching of the material. For drawing, the clamping force is just sufficient to prevent buckling of the material at the draw radius as it enters the die. The deformation takes place in the flange and over the draw radius.
Fukui Conical – Cup Test:
It utilizes a hemispherical, smoothly polished punch. No blank holder is required. In each test, a drawing ratio which will result in a broken cup is determined. Formation of wrinkles is avoided by using a fixed ratio between the thickness of the sheet, the size of the blank, and the punch and die diameters. Under these conditions, the test produces a known amount of stretching, drawing, and bending under tension.
Normal Anisotropy Coefficient:
The material is subjected to uni-axial tensile test. The anisotropy coefficient is derived from the ratio of the plastic width strain eW to the thickness strain et . A material with a high plastic anisotropy also has a greater “thinning resistance.” In general, the higher the anisotropy coefficient the better the material deforms in drawing operations.
Strain-Hardening Coefficient:
Strain hardening refers to the fact that as a metal deforms in some area, dislocations occur in the microstructure. As these dislocations pile up, they tend to strengthen the metal against further deformation in that area. Thus the strain is spread throughout the sheet. However, at some point in the deformations, the strain suddenly localizes and necking, or localized thinning, develops. When this occurs, little further overall deformation of the sheet can be obtained without it fracturing in the necked region.
The strain – hardening coefficient therefore reflects how well the metal distributes the strain throughout the sheet, avoiding or delaying localized necking. The higher the strain – hardening coefficient, the move the material will harden as it is being stretched and the greater will be the resistance to localized necking. Necks in the metal harm surface appearance and affect structural integrity.
For many stamping operations, stretching of the metal is the critical factor and is dependent on the strain – hardening coefficient. Therefore, stampings that need much drawing should be made from metal having high average strain – hardening coefficients. Yield strength should be low to avoid wrinkles or buckling.
Forming Limit Curve:
The forming – limit curve is a good index of determining the formability of sheet metal. Essentially, it requires to draw a curve that shows a boundary line between acceptable strain levels in forming and those that may cause failure, Fig 4.2.
Fig 4.2 The relationship of major, e1 , and minor, e2 , strains is established by measurement after forming.
The curve indicates the relation between major and minor strains that are perpendicular to the plane of the sheet. To determine these strains, a grid of circles is marked on the sheet metal, say by an electrolytic stencil – etching process. After the metal is deformed, the circles are measured to obtain the major strain e1 and the minor strain e2 , as shown in Fig 4.2 Typically, ten to fifteen data points are obtained from a test specimen in the region of fracture. Ellipses lying both in the failed region and just outside of it are measured. The forming – limit curve is then drawn to fall below the strains in the necked and fractured zones, and above the strains found just outside these zones (Fig 4.3)
With controlled variation in specimen size it is possible to plot an entire forming – limit curve from one test setup. A reasonably accurate forming limit curve may be obtained with four specimens while a precision curve may be obtained with eight specimens.
In may be noted that “local” ductility varies for different metals, so no universal forming – limit curve can be developed. For example, two metals may have peak local ductilities of 20% and 50% at a given minor strain. The metal with the 20 % local ductility (high strain – hardening coefficient) may turn out to be the best choice because the strain will then have a better distribution throughout, allowing the entire sheet to be stretched 20%. If the other sheet showed little strain hardening, it might stretch by 50% in local area, but leave the rest of the sheet relatively unstrained.
Through the use of formability – prediction techniques. Designers and fabricators are able to make a wiser choice of metals and obtain date quickly on newer metals. The essential data can be obtained before the die is designed. Also metal suppliers will be able to establish whether a material possesses required formability before it is shipped from the plant.
Fig. 4.3
SHEARING
Shearing is a cutting operation used to remove a blank of required dimensions from a large sheet. To understand the shearing mechanism, consider a metal being sheared between a punch and a die, Fig 5.1 Typical features of the sheet and the slug are also shown in this figure. As can be seen that cut edges are neither smooth nor perpendicular to the plane of the sheet.
Fig 5.1 (a) Shearing with a punch and die (b) features of a punched hole and (c) features of the slug.
Shearing starts as the punch presses against the sheet metal. At first, cracks form in the sheet on both the top and bottom edges (marked T and T', in the figure). As the punch descends further, these cracks grow and eventually meet each other and the slug separates from the sheet. A close look at the fractured surfaces will revel that these are quite rough and shiny; rough because of the cracks formed earlier, and shiny because of the contact and rubbing of the sheared edge against the walls of the die.
The clearance between the punch and the die plays an important role in the determination of the shape and quality of the sheared ege. There is an optimum range for the clearance, which is 2 to 10% of the sheet thickness, for the best results. If the clearance increases beyond this, the material tends to be pulled into the die and the edges of the sheared zone become rougher. The ratio of the shining (burnished) area to the rough area on the sheared edge decreases with increasing clearance and sheet thickness. The quality of sheared edge is also affected by punch speed; greater the punch speed better the edge quality.
Shearing Operations
For general purpose shearing work, straight line shears are used. as shown in Fig 5.2, small pieces (A, B, C, D……….) may be cut from a large sheet.
Fig 5.2
Shearing may also be done between a punch and die, as shown in Fig 5.1. The shearing operations make which use of a die, include punching, blanking, piercing, notching, trimming, and nibbling.
Punching/Blanking
Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal. If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking, see Fig 5.3.
Fig 5.3 Comparison of basic stamping operations.
In punching, the metal inside the part is removed; in blanking, the metal around the part is removed.
In punching, the metal inside the part is removed; in blanking, the metal around the part is removed.
A typical setup used for blanking is shown in Fig 5.4.
Fig 5.4 Blanking punch and die.
The clearance between the die and punch can be determined as c = 0.003 t. t where t is the sheet thickness and t is the shear strength of sheet material. For blanking operation, die size = blank size, and the punch is made smaller, by considering the clearance.
The maximum force, P required to be exerted by the punch to shear out a blank from the sheet can be estimated as
P = t. L. t
where t is the sheet thickness, L is the total length sheared (such as the perimeter of hole), and t is the shear strength of the sheet material.
Stripping force. Two actions take place in the punching process – punching and stripping. Stripping means extracting the punch. A stripping force develops due to the spring back (or resiliency) of the punched material that grips the punch. This force is generally expressed as a percentage of the force required to punch the hole, although it varies with the type of material being punched and the amount of clearance between the cutting edges. The following simple empirical relation can be used to find this force
SF = 0.02 L.t
where SF = stripping force, kN
L = length of cut, mm
t = thickness of material, mm
Example: A circular blank of 30 mm diameter is to be cut from 2 mm thick 0.1 C steel sheet. Determine the die and punch sizes. Also estimate the punch force and the stripping force needed. You may assume the following for the steel : Tensile strength: 410 MPa ; shear strength : 310 MPa
Solution:- For cutting a blank, die size = blank size
\ Die size = 30mm
Clearance = 0.003 x t x t = 0.003 x 2 x 310
= 1.86 mm
Punch size = blank size – 2 clearance
= 30 – 2 x 1.86 = 26.28 mm
Punch force needed = L. t. p = p x 30 x 2 x 310
= 58.5 kN
Stripping force needed = 0.02 L.t
= 0.02 x p x 30 x 2
= 3.77 kN
Piercing:
It is a process by which a hole is cut (or torn) in metal. It is different from punching in that piercing does not generate a slug. Instead, the metal is pushed back to form a jagged flange on the back side of the hole.
A pierced hole looks somewhat like a bullet hole in a sheet of metal.
Trimming:
When parts are produced by die casting or drop forging, a small amount of extra metal gets spread out at the parting plane. This extra metal, called flash, is cut – off before the part is used, by an operation called trimming. The operation is very similar to blanking and the dies used are also similar to blanking dies. The presses used for trimming have, however, relatively larger table.
Notching:
It is an operation in which a specified small amount of metal is cut from a blank. It is different from punching in the sense that in notching cutting line of the slug formed must touch one edge of the blank or strip. A notch can be made in any shape. The purpose of notching is generally to release metal for fitting up.
Nibbling:
Nibbling is variation of notching, with overlapping notches being cut into the metal. The operation may be resorted to produce any desired shape, for example flanges, collars, etc.
Perforating:
Perforating is an operation is which a number of uniformly spaced holes are punched in a sheet of metal. The holes may be of any size or shape. They usually cover the entire sheet of metal. SHEET METAL PROCESSES
BENDING
Bending is one very common sheet metal forming operation used not only to form shapes like seams, corrugations, and flanges but also to provide stiffness to the part (by increasing its moment of inertia).
As a sheet metal is bent (Fig 6.1), its fibres experience a distortion such that those nearer its outside, convex surface are forced to stretch and come in tension, while the inner fibres come in compression. Somewhere, in the cross section, there is a plane which separates the tension and compression zones. This plane is parallel to the surface around which the sheet is bending, and is called neutral axis. The position of neutral axis depends on the radius and angle of bend. Further, because of the Poisson's ratio, the width of the part L in the outer region is smaller, and in the inner region it is larger, than the initial original width.
Fig 6.1 Sheet metal bending. It may be noted that the bend radius is measured to the inner surface of the bent part.
BEND ALLOWANCE
It is the length of the neutral axis in the bend, Fig 6.1. This determines the blank length needed for a bent part. It can be approximately estimated from the relation
Lb = a ( R + kt )
where, Lb = bend allowance (mm)
a = bend angle (radian)
R = bend radius (mm)
t = thickness of sheet (mm), and
k = constant, whose value may be taken as 1/3 when R < 2t, and as 1/2 when R ³ 2t.
Example
A 20 mm wide and 4 mm thick C 20 steel sheet is required to be bent at 600 at bend radius 10 mm. Determine the bend allowance.
Solution.
Here, bend radius R = 10 mm
Sheet thickness t = 4 mm
Since R > 2t, k = 0.5
Bend allowance
MINIMUM BEND RADIUS
As the ratio of the bend radius to the thickness of sheet (R / t) decreases, the tensile strain on the outer fibres of sheet increases. If R / t decreases beyond a certain limit, cracks start appearing on the surface of material. This limit is called Minimum Bend Radius for the material.
Minimum bend radius is generally expressed in terms of the thickness of material, such as 2t, 3t, 4t, etc. Table 6.1 gives the minimum bend radius allowed for different materials.
Table 6.1 Minimum Bend radius for Various Materials at Room Temperature
| Material | Condition | |
| Soft | Hard | |
| Aluminum alloys Beryllium copper Brass,low-leaded Magnesium Steels Austenitic stainless Low-carbon,low-alloy Titanium Titanium alloys | 0 0 0 5t 0.5t 0.5t 0.7t 2.5t | 6t 4t 2t 13t 6t 4t 3t 4t |
Bending Force :
There are two general types of die bending : V – die bending and wiping die bending. V – die bending is used expensively in brake die operations and stamping die operations. The bending force can be estimated from the following simple relation.
P = k.Y.L.t2 / D
where P is bending force, g is the yield stress of the material, L is the bend length ( bend allowance ), t is the sheet thickness, D is the die opening and k is a constant whose value can be taken as 1.3 for a V-die and 0.3 for a wiping die. Fig 6.2 shows various types of bending dies.
Fig 6.2 Die-bending operations.
Bending force varies as the punch progresses through the bending operation. The force is zero in the beginning. It rises and reaches the maximum value as the punch progresses and reaches the bottom of the stroke.
Example:
A 400 mm long and 2.5 mm thick piece of carbon steel sheet is required to be bent at 900 using a V – die. You may assume the yield stress of the material as 500 MPa and the die opening as 10 times the material thickness. Estimate the force required for the operation.
Solution : Here, Y = 500 MPa
L = 400 mm
t = 2.5 mm
k = 1.3 (for V – die)
D = 25 mm
Bending force P = k.Y.L.t2 / D
= 1.3 x 500 x 400 x (2.5)2 / 25
= 65 KN
Example :
If the material as mentioned in the above example is to be bent at 900 using wiping die with radius = 3.75 mm, what is the force requirement?
Solution : Here,Y = 500 MPa
L = 400 mm
t = 2.5 mm
k = 0.3
D = 2.5 + 3.75 + 3.75 = 10mm (see Fig 6.3)
Fig 6.3
Bending force P = k.Y.L.t2 / D
= 0.3 x 500 x 400 x (2.5)2 / 10
= 37.5 KN
DRAWING
It is a process of cold forming a flat blank of sheet metal into a hollow vessel without much wrinkling, trimming, or fracturing. The process involves forcing the sheet metal blank into a die cavity with a punch. The punch exerts sufficient force and the metal is drawn over the edge of the die opening and into the die, Fig 6.4. In forming a cup, however, the metal goes completely into the die, Fig 6.5.
Fig 6.4 Drawing operation.
Fig 6.5 Drawing operation.
The metal being drawn must possess a combination of ductility and strength so that it does not rupture in the critical area (where the metal blends from the punch face to the vertical portion of the punch). The metal in this area is subjected to stress that occurs when the metal is pulled from the flat blank into the die.
OPERATION . A setup similar to that used for blanking is used for drawing with the difference that the punch and die are given necessary rounding at the corners to permit smooth flow of metal during drawing. The blank of appropriate dimensions is place within the guides on the die plate. The punch descends slowly on the blank and metal is drawn into the die and the blank is formed into the shape of cup as punch reaches the bottom of the die. When the cup reaches the counter – bored portion of the die, the top edge of the cup formed around the punch expands a bit due to the spring back . On the return stroke of the punch, the cup is stripped off the punch by this counter – bored portion.
The term shallow drawing is used when the height of cup formed is less than half its diameter. When drawing deeper cup (height greater that ½ diameter) the chances of excessive wrinkle formation at the edges of blank increases. To prevent this, a blank holder is normally provided, see Fig 6.4. As the drawing process proceeds the blank holder stops the blank from increasing in thickness beyond a limit and allows the metal to flow radially. The limiting thickness is controlled by the gap between the die and the blank holder, or by the spring pressure in the case of a spring loaded blank holder.
Some lubricant is generally used over the face of the blank to reduce friction and hence drawing load.
Blank Size
It is generally difficult to find the exact size of the blank needed for drawing a given cup, because of thinning and thickening of the metal sheet during the drawing operation. The following simple relations can be used for determine the blank diameter D:
where d = outside diameter of cup
h = height of cup
r = corner radius on punch.
Drawing Force.
For drawing cylindrical shells having circular cross section, the maximum drawing force P can be determined from the relation
P = k.t.d.t.Y
where d = outside diameter of cup
t = thickness of material
Y = yield strength of material
k = factor whose value is approx. equal to [D/d – 0.6]
D = blank diameter
EMBOSSING
Embossing is an operation in which sheet metal is drawn to shallow depths with male and female matching dies, Fig 6.6. The operation is carried out mostly for the purpose of stiffening flat panels.The operation is also sometimes used for making decoration items like number plates or name plates, jewelry, etc.
Fig 6.6 Embossing operation with two dies. Letters, numbers and designs on sheet-metal parts can be produced by this operation.
COINING
Coining is a severe metal squeezing operation in which the flow of metal occurs only at the top layers of the material and not throughout the values. The operation is carried out in closed dies mainly for the purpose of producing fine details such as needed in minting coins, and medal or jewelry making. The blank is kept in the die cavity and pressures as high as five to six times the strength of material are applied. Depending upon the details required to be coined on the part, more than one coining operations may be used.
The difference between coining and embossing is that the same design is created on both sides of the work piece in embossing (one side depressed and the other raised ), whereas in coining operation, a different design is created on each side of work piece.
PRESSES FOR SHEET METAL WORKING
Classification of presses.
Types of presses for sheet metal working can be classified by one or a combination of characteristics, such as source of power, number of slides, type of frame and construction, type of drive, and intended applications.
Classification on the basis of source of power.
· Manual Presses. These are either hand or foot operated through levers, screws or gears. A common press of this type is the arbor press used for assembly operations.
· Mechanical presses.These presses utilize flywheel energy which is transferred to the work piece by gears, cranks, eccentrics, or levers.
· Hydraulic Presses. These presses provide working force through the application of fluid pressure on a piston by means of pumps, valves, intensifiers, and accumulators. These presses have better performance and reliability than mechanical presses.
· Pneumatic Presses. These presses utilize air cylinders to exert the required force. These are generally smaller in size and capacity than hydraulic or mechanical presses, and therefore find use for light duty operations only.
Classification on the basis of number of slides.
· Single Action Presses. A single action press has one reciprocation slide that carries the tool for the metal forming operation. The press has a fixed bed. It is the most widely used press for operations like blanking, coining, embossing, and drawing.
- Double Action Presses. A double action press has two slides moving in the same direction against a fixed bed. It is more suitable for drawing operations, especially deep drawing, than single action press. For this reason, its two slides are generally referred to as outer blank holder slide and the inner draw slide. The blank holder slide is a hollow rectangle, while the inner slide is a solid rectangle that reciprocates within the blank holder. The blank holder slide has a shorter stroke and dwells at the bottom end of its stroke, before the punch mounted on the inner slide touches the workpiece. In this way, practically the complete capacity of the press is available for drawing operation.
Another advantage of double action press is that the four corners of the blank holder are individually adjustable. This permits the application of non uniform forces on the work if needed.
A double action press is widely used for deep drawing operations and irregular shaped stampings.
- Triple Action Presses. A triple action press has three moving slides. Two slides (the blank holder and the inner slide) move in the same direction as in a double – action press and the third or lower slide moves upward through the fixed bed in a direction opposite to that of the other two slides. This action allows reverse – drawing, forming or bending operations against the inner slide while both upper actions are dwelling.
Cycle time for a triple – action press is longer than for a double – action press because of the time required for the third action.
Classification on the basis of frame and construction.
· Arch – Frame Presses. These presses have their frame in the shape of an arch. These are not common.
· Gap Frame Presses. These presses have a C-shaped frame. These are most versatile and common in use, as they provide un – obstructed access to the dies from three sides and their backs are usually open for the ejection of stampings and / or scrap.
· Straight Side Presses. These presses are stronger since the heavy loads can be taken in a vertical direction by the massive side frame and there is little tendency for the punch and die alignment to be affected by the strain. The capacity of these presses is usually greater than 10 MN.
· Horn Presses. These presses generally have a heavy shaft projecting from the machine frame instead of the usual bed. This press is used mainly on cylindrical parts involving punching, riveting, embossing, and flanging edges.
Fig 7.1 shows typical frame designs.
Fig 7.1 Typical frame designs used for power presses.
Press Selection:
Proper selection of a press is necessary for successful and economical operation. Press is a costly machine, and the return on investment depends upon how well it performs the job. There is no press that can provide maximum productively and economy for all application so, when a press is required to be used for several widely varying jobs, compromise is generally made between economy and productivity.
Important factors affecting the selection of a press are size, force, energy and speed requirements.
Size. Bed and slide areas of the press should be of enough size so as to accommodate the dies to be used and to make available adequate space for die changing and maintenance. Stroke requirements are related to the height of the parts to be produced. Press with short stroke should be preferred because it would permit faster operation, thus increasing productivity. Size and type of press to be selected also depends upon the method and nature of part feeding, the type of operation, and the material being formed.
Force and Energy. Press selected should have the capacity to provide the force and energy necessary for carrying out the operation. The major source of energy in mechanical presses is the flywheel, and the energy available is a function of mass of flywheel and square of its speed.
Press Speed. Fast speeds are generally desirable, but they are limited by the operations performed. High speed may not, however, be most productive or efficient. Size, shape and material of workpiece, die life, maintenance costs, and other factors should be considered while attemping to achicve the highest production rate at the lowest cost per piece.
Mechanical versus Hydraulic Presses:
Mechanical presses are very widely used for blanking, forming and drawing operations required to be done on sheet metal. For certain operations which require very high force, for example, hydraulic presses are more advantageous. Table 7.1 gives a comparison of characteristics and preferred application of the two types of press.
Table 7.1 Comparison of Mechanical and Hydraulic Presses
| Characteristic | Mechanical Presses | Hydraulic Presses |
| Force | Depends upon slide position. | Dose not depend upon slide position. Relatively constant. |
| Stroke length | Short strokes | Long strokes,even as much as 3 m. |
| Slide speed | High. Highest at mid-stroke. Can be variable | Slow. Rapid advance and retraction. Variable speeds uniform throughout stroke. |
| Capacity | About 50 MN (maximum) | About 500 MN, or even more. |
| Control | Full stroke generally required before reversel. | Adjustable, slide reversal possible from any position. |
| Application | Operations requiring maximum pressure near bottom of stroke. Cutting operations(blanking, shearing, piercing, Forming and drawing to depths of about 100 mm. | Operations requiring steady pressure through-out stoke. Deep drawing. Drawing irregular shaped parts. Straightening. Operations requiring variable forces and /or strokes. |
Press Feeding Devices:
Safety is an important consideration in press operation and every precaution must be taken to protect the operator. Material must be tried to be fed to the press that eliminates any chance of the operator having his or her hands near the dies. The use of feeding device allows faster and uniform press feeding in addition to the safety features.
• Blank and Stamping Feeds.
Feeding of blanks or previously formed stampings to presses can be done in several ways. Selection of a specific method depends upon factors like production rate needed, cost, and safety considerations.
Manual feeding . Feeding of blanks or stampings by hand is generally limited to low production rate requirements which do not warrant the cost of automatic or semi- automatic feeding devices. Manual feeding, however, is accomplished with the use of a guard or, if a guard is not possible, hand feeding tools and a point – of – operation safety device. Some commonly used hand feeding tools are special pliers, tongs, tweezes, vacuum lifters and magnetic pick – ups.
Chute feeds . For feeding small blanks or stampings, simple chutes are often used. The blank slides by gravity along rails in the bottom of the chute. Slide chutes are designed for a specific die and blank and are generally attached permanently to the die so as to reduce setup time. Slide angle of 200 - 300 is sufficient in most cases. Chute feeds need barrier guard enclosure for operation protection, with just enough opening in the enclosure for the blanks to slide through to the die.
Push feeds . These feeds are used when blanks need orientation in specific relation to the die. Work piece is manually placed in a nest in a slide, one at a time, and the slide pushed until the piece falls into the die nest. An interlock is provided so that the press cannot be operation until the slide has correctly located the part in the die. To increase production rate, push feeds can be automated by actuating the feed slide through mechanical attachment to the press slide.
Lift and transfer devices . In some automatic installations vacuum or suction cups are used for lifting of blanks one at a time from stacks and then moved to the die by transfer units. Separation of the top blank from a stack is achieved by devices which are operated magnetically, pneumatically or mechanically.
• Dial Feeds.
Dial feeds consist of rotary indexing tables (or turntables) having fixtures for holding workpiecs as they are taken to the press tooling. Parts are placed in the fixtures at the loading station (which are located away from the place of press operation) manually or by other means like chutes, hoppers, vibratory feeders, robots etc. Such feeds are being increasingly used because of higher safety and productivity associated with them.
• Coil Stock Feed.
Two main classifications of automatic press feeds for coil stock are slide (or gripper) and roll feeds. Both of these may be press or independently driven.
Mechanical slide feeds. Press – driven slide feeds have a gripper arrangement which clamps and feeds the stock during its forward movement and releases it on the return stroke. Material is prevented from backing up during the return stroke of the gripper by a drag unit like a frictional brake. Grippers reciprocate on rods or slides between adjustable positive stops to ensure accuracy. Slide feeds are available in a variety of sizes and designs. These are generally best for narrow coil stock and short feed lengths.
Hitch – type feed. This feed differs from press – driven mechanical slide feed in that actuation is by a simple flat cam attached to the ram or punch holder instead of by the press. On the downward stroke of the ram, one or more springs are compressed by the cam action, then on the upstroke, the springs provide the force to feed stock into the die.
These feeds are best suited for coil stock of small to medium thickness and for relatively short feed progression. These are one of the oldest and least expensive feeding devices still used very widely. Due to their low cost, they are generally left permanently attached to the dies, thus reducing setup time.
Pneumatic slide feeds. These feeds are similar to mechanical slide feeds in that they have grippers or clamps that reciprocate on guide rails or slides between adjustable positive stops to push and / or pull stock into a die. However, these differ in that they are powered by an air cylinder, with actuation and timing of valves by cam – operated limit switches.
These feeds are best for short progression, and find wide applications in job shops because of their low cost and versatility.
Roll feeds. In these feeds, coil stock is advanced by pressure exerted between intermittently driven, opposed rolls which allow the stock to dwell during the working part of the press cycle. Intermittent rotation (or indexing) of the feed rolls, with the rolls rotating in only one direction, is accomplished in many ways. In one common design, the rolls are indexed through a one – way clutch by a rack – and – pinion mechanism that is actuated by an adjustable eccentric on the press – crankshaft.
These feeds are available in several types and sizes to suit almost any width and thickness of stock. Though their initial cost is slightly higher, their greater durability and lower maintenance cost account for their extensive use.
DIE AND PUNCH
A typical die and punch set used for blanking operation is shown in Fig 8.1. The sheet metal used is called strip or stock. The punch which is held in the punch holder is bolted to the press ram while die is bolted on the press table. During the working stroke, the punch penetrates the strip, and on the return stroke of the press ram the strip is lifted with the punch, but it is removed from the punch by the stripper plate. The stop pin is a gage and it sets the advance of the strip stock within the punch and die. The strip stock is butted against the back stop acting as a datum location for the centre of the blank.
Fig 8.1.
The die opening is given angular clearance to permit escape of good part (blank). The waste skelton of stock strip, from which blanks have been cut, is recovered as salvaged material.
The clearance angle provided on the die (Fig 8.1) depends on the material of stock, as well as its thickness. For thicker and softer materials generally higher angular clearance is given. In most cases, 2 degree of angular clearance is sufficient. The height of cutting land of about 3 mm is generally sufficient.
Clearance
In blanking operation , the die size is taken as the blank size and the punch is made smaller giving the necessary clearance between the die and the punch. Die size = blank size
Punch size = blank size – 2 x clearance
Clearance = k . t . t
where t is the shear strength of material, t is the thickness of sheet metal stock, and k is a constant whose value may be taken as 0.003.
In a piercing operation , the following equations hold.
Punch size = blank size
Die size = blank size + 2 x clearance
Clearance = k . t . t
TYPES OF DIES
The components generally incorporated in a piercing or blanking die are shown in Fig 8.3. This Figure shown the die in the conventional closed position. The die set is made up of the punch holder which is fastened to the ram of the punch press and the die shoe which is fastened to the bolster plate of the punch press.
Generally, the punch is fastened to the punch holder and aligned with the opening in the die block. Fig 8.2 shows one type of stripper plate and push – off pins. The stripper holds the scrap strip so that the punch may pull out of the hole. The push – off pins are needed to free the blank in instances where the material strip clings to the bottom of the punch. This may be necessary for thin material, or where lubricants are used on the material.
Fig 8.2
Sometimes the die and the punch positions may be interchanged. This may become necessary when the opening in the bolster plate is too small to permit the finished product to pass through the bolster opening. Fig 8.3 shows such a die.
Fig 8.3
Inverted die (Fig 8.3) is designed with the die block fastened to the punch holder and the punch fastened to the die shoe. During the downward stroke of ram, the blank is sheared from the strip. The blank and shedder are forced back into the die opening, which loads a compression spring in the die opening . At the same time the punch is forced through the scrap strip and a spring attached to the stripper is compressed and loaded. On the upstroke of the ram, the shedder pushes the blank out of the die opening and the stripper forces the scrap strip off the punch. The finished part (blank) falls, or is blown, out the rear of the press.
Compound die (Fig 8.4) combines the principles of the conventional and inverted dies in one station. This type of die may produce a workpiece which is pierced and blanked at one station and in one operation. The piercing punch is fastened in the conventional position to the punch holder. Its matching die opening for piercing is machined into the blanking punch. The blanking punch and blanking die opening are mounted in an inverted position. The blanking punch is fastened to the die shoe and the blanking die opening is fastened to the punch holder.
Fig 8.4
Progressive dies are made with two or more stations arranged in a sequence. Each station performs an operation on the workpiece, or provides an idler station, so that the workpiece is completed when the last operation has been accomplished. Thereafter each stroke of the ram produces a finished part. Thus after the fourth stroke of a four – station die, each successive stroke will produce a finished part. Operations which may be carried out in a progressive die are piercing, blanking, forming, drawing, cut – off, etc. The list of possible operations is long. The number and types of operations which may be performed in a progressive die depends upon the ingenuity of the designer.
Fig 8.5 shows a four – station progressive die. The die block is made up of four pieces and fastened to the die shoe. This permits easy replacement of broken or worn die blocks. The stock is fed from the right and registers against a finger strop (not shown). The first stroke of the press Fig 8.5(a) produces a square hole and two notches. These notches form the left end of the first piece.
During the upstroke of ram, the stock is moved to the next station against a finger stop (not shown). The stock is positioned for the second stroke. The second station is an idler, Fig 8.5(b). The right end of the first piece, the left end of the second piece, and a second square hole are pierced.
Fig 8.5
The ram retracts and the scrap strip is moved to the third station against an automatic stop, Fig 8.5(c). This stop picks up the notched V and positions the scrap strip. The third stroke of the ram pierces the four holes as shown in Fig 8.5(c). The fourth stroke, Fig 8.5(d), cuts off and forms the radii at the ends of the finished piece. Thereafter every stroke produces a finished part, Fig 8.5(e).
Progressive dies generally have the cut – off or blanking operation as the last operation. It is preferred to have piercing operation as the first operation so that the pierced hole can be advantageously used as a pilot hole. Alternatively, special pilot holes are pierced in the scrapped part of the stock. In certain special cases, blanking is done at the first station, and the blank returned to the die by using spring plates and then moved to the subsequent station by mechanical means or manually.
Progressive dies are used where higher production rates are desired and the material is neither too thick nor too thin. Their use helps in cutting down the material handling costs.
HIGH ENERGY RATE FORMING PROCESSES
In these forming processes large amount of energy is applied for a very short interval of time. Many metals tend to deform more readily under extra – fast application of load which make these processes useful to form large size parts out of most metals including those which are otherwise difficult – to – form.
The parts are formed at a rapid rate, and thus these processes are also called high – velocity forming processes. There are several advantages of using these forming processes, like die costs are low, easy maintenance of tolerances, possibility of forming most metals, and material does not show spring-back effect. The production cost of components by such processes is low. The limitation of these processes is the need for skilled personnel.
There are three main high energy rate forming processes: explosive forming, magnetic forming, and electro hydraulic forming. We shall discuss these processes.
Explosive Forming
Explosive forming, is distinguished from conventional forming in that the punch or diaphragm is replaced by an explosive charge. The explosives used are generally high – explosive chemicals, gaseous mixtures, or propellants. There are two techniques of high – explosive forming: stand – off technique and the contact technique.
Standoff Technique . The sheet metal work piece blank is clamped over a die and the assembly is lowered into a tank filled with water. The air in the die is pumped out. The explosive charge is placed at some predetermined distance from the work piece, see Fig 9.1. On detonation of the explosive, a pressure pulse of very high intensity is produced. A gas bubble is also produced which expands spherically and then collapses. When the pressure pulse impinges against the work piece, the metal is deformed into the die with as high velocity as 120 m/s.
Fig 9.1 Sequeuce of underwater explosive forming operations.(i) explosive charge is set in position (ii) pressure pulse and gas bubble are formed as the detonation of charge occurs, (iii) workpiece is deformed, and (iv) gas bubbles vent at the surface of water.
The use of water as the energy transfer medium ensures a uniform transmission of energy and muffles the sound of the explosive blast. The process is versatile – a large variety of shapes can be formed, there is virtually no limit to the size of the work piece, and it is suitable for low – quantity production as well.
The process has been successfully used to form steel plates 25 mm thick x 4 m diameter and to bulge steel tubes as thick as 25 mm.
Contact Technique. The explosive charge in the form of cartridge is held in direct contact with the work piece while the detonation is initiated. The detonation builds up extremely high pressures (upto 30,000MPa) on the surface of the work piece resulting in metal deformation, and possible fracture. The process is used often for bulging tubes, as shown in Fig 9.2.
Fig 9.2 Schematic illustration of contact technique of explosive forming.
The process is generally used for bulging of tubes.
The process is generally used for bulging of tubes.
Applications. Explosive forming is mainly used in the aerospace industries but has also found successful applications in the production of automotive related components. The process has the greatest potential in limited – production prototype forming and for forming large size components for which conventional tooling costs are prohibitively high.
Electro Magnetic Forming
The process is also called magnetic pulse forming and is mainly used for swaging type operations, such as fastening fittings on the ends of tubes and crimping terminal ends of cables. Other applications are blanking, forming, embossing, and drawing. The work coils needed for different applications vary although the same power source may be used.
To illustrate the principle of electromagnetic forming, consider a tubular work piece. This work piece is placed in or near a coil, Fig 9.3. A high charging voltage is supplied for a short time to a bank of capacitors connected in parallel. (The amount of electrical energy stored in the bank can be increased either by adding capacitors to the bank or by increasing the voltage). When the charging is complete, which takes very little time, a high voltage switch triggers the stored electrical energy through the coil. A high – intensity magnetic field is established which induces eddy currents into the conductive work piece, resulting in the establishment of another magnetic field. The forces produced by the two magnetic fields oppose each other with the consequence that there is a repelling force between the coil and the tubular work piece that causes permanent deformation of the work piece.
Fig 9.3 Various applications of magnetic forming process. (i) Swaging, (ii) Expanding, and (iii) Embossing or blanking.
Either permanent or expandable coils may be used. Since the repelling force acts on the coil as well the work, the coil itself and the insulation on it must be capable of withstanding the force, or else they will be destroyed. The expandable coils are less costly and are also preferred when high energy level is needed.
Magnetic forming can be accomplished in any of the following three ways, depending upon the requirements.
· Coil surrounding work piece. When a tube – like part x is to fit over another part y (shown as insert in Fig 9.3(i)), coil is designed to surround x so that when energized, would force the material of x tightly around y to obtain necessary fit.
· Coil inside work piece. Consider fixing of a collar on a tube – like part, as shown in Fig 9.3(ii). The magnetic coil is placed inside the tube – like part, so that when energized would expand the material of the part into the collar.
· Coil on flat surface. Flat coil having spiral shaped winding can also be designed to be placed either above or below a flat work piece, see Fig 9.3(iii).These coils are used in conjunction with a die to form, emboss, blank, or dimple the work piece.
In electromagnetic forming, the initial gap between the work piece and the die surface, called the fly distance , must be sufficient to permit the material to deform plastically. From energy considerations, the ideal pressure pulse should be of just enough magnitude that accelerates the part material to some maximum velocity and then let the part come to zero velocity by the time it covers the full fly distance. All forming coils fail, expendable coils fail sooner than durable coils, and because extremely high voltages and currents are involved, it is essential that proper safety precautions are observed by the production and maintenance personnel.
Applications
Electromagnetic forming process is capable of a wide variety of forming and assembly operations. It has found extensive applications in the fabrication of hollow, non – circular, or asymmetrical shapes from tubular stock. The compression applications involve swaging to produce compression, tensile, and torque joints or sealed pressure joints, and swaging to apply compression bands or shrink rings for fastening components together. Flat coils have been used on flat sheets to produce stretch (internal) and shrink (external) flanges on ring and disc – shaped work pieces.
Electromagnetic forming has also been used to perform shearing, piercing, and rivettting.
Electro Hydraulic Forming
Electro hydraulic forming (EHF), also known as electro spark forming, is a process in which electrical energy is converted into mechanical energy for the forming of metallic parts. A bank of capacitors is first charged to a high voltage and then discharged across a gap between two electrodes, causing explosions inside the hollow work piece, which is filled with some suitable medium, generally water. These explosions produce shock waves that travel radially in all directions at high velocity until they meet some obstruction. If the discharge energy is sufficiently high, the hollow work piece is deformed. The deformation can be controlled by applying external restraints in the form of die or by varying the amount of energy released, Fig 9.4.
Fig 9.4 Unrestrained and restrained electro-hydraulic forming process.
Advantages
1. EHF can form hollow shapes with much ease and at less cost compared to other forming techniques.
2. EHF is more adaptable to automatic production compared to other high energy rate forming techniques.
3. EHF can produce small – to intermediate sized parts that don't have excessive energy requirements.
Accuracy of parts produced
Accuracy of electro hydraulically formed parts depends on the control of both the magnitude and location of energy discharges and on the dimensional accuracy of the dies used. With the modern equipment, it is now possible to precisely control the energy within specified limits, therefore the primary factor is the dimensional accuracy of the die. External dimensions on tubular parts are possible to achieve within ± 0.05 mm with the current state of technology.
Materials formed
Materials having low ductility or having critical impact velocity less than 30 m/s are generally not considered to be good candidate for EHF. All materials that can be formed by conventional forming processes can be formed by EHF also. These materials are aluminum alloys, nickel alloys, stainless steels, titanium, and Inconel 718.
POWDER METALLURGY
Powder metallurgy (PM) is a metal working process for forming precision metal components from metal powders. The metal powder is first pressed into product shape at room temperature. This is followed by heating (sintering) that causes the powder particles to fuse together without melting.
The parts produced by PM have adequate physical and mechanical properties while completely meeting the functional performance characteristics. The cost of producing a component of given shape and the required dimensional tolerances by PM is generally lower than the cost of casting or making it as a wrought product, because of extremely low scrap and the fewer processing steps. The cost advantage is the main reason for selecting PM as a process of production for high – volume component which needs to be produced exactly to, or close to, final dimensions. Parts can be produced which are impregnated with oil or plastic, or infiltrated with lower melting point metal. They can be electroplated, heat treated, and machined if necessary.
The rate of production of parts is quite high, a few hundreds to several thousands per hour.
Industrial applications of PM parts are several. These include self – lubricating bearings, porous metal filters and a wide range of engineered shapes, such as gears, cams, brackets, sprockets, etc.
Process Details:
In the PM process the following three steps are followed in sequence: mixing (or blending), compacting, and sintering.
Mixing: A homogeneous mixture of elemental metal powders or alloy powders is prepared. Depending upon the need, powders of other alloys or lubricants may be added.
Compacting: A controlled amount of the mixed powder is introduced into a precision die and then it is pressed or compacted at a pressure in the range 100 MPa to 1000 MPa. The compacting pressure required depends on the characteristics and shape of the particles, the method of mixing, and on the lubricant used. This is generally done at room temperature. In doing so, the loose powder is consolidated and densified into a shaped model. The model is generally called “green compact.” As is comes out of the die, the compact has the size and shape of the finished product. The strength of the compact is just sufficient for in – process handling and transportation to the sintering furnace.
Fig 10.1 Typical set of powder metallurgy tools.
To illustrate the process, let us take a straight cylindrical part such as a sleeve bearing. Fig 10.1 shows a typical set of tools used for producing this part. The compacting cycle for this part (Fig 10.2) follows the following steps.
Fig 10.2 Powder metallurgy compacting cycle.
1. With the upper punch in the withdrawn position, the empty die cavity is filled with mixed powder.
2. The metal powder in the die is pressed by simultaneous movement of upper and lower punches.
3. The upper punch is withdrawn, and the green compact is ejected from the die by the lower punch.
4. The green compact is pushed out of the pressing area so that the next operating cycle can start.
This compacting cycle is almost the same for all parts.
Sintering: During this step, the green compact is heated in a protective atmosphere furnace to a suitable temperature, which is below the melting point of the metal. Typical sintering atmospheres are endothermic gas, exothermic gas, dissociated ammonia, hydrogen, and nitrogen. Sintering temperature varies from metal to metal; typically these are within 70 to 90% of the melting point of the metal or alloy. Table 10.1 gives the sintering temperatures used for various metals. Sintering time varies with size and metal of part. Table 10.1 also gives typical range of sintering time needed for various metals.
Table 10.1 Sintering temperature and time for various metal powders
| Material | Temperature ( 0C) | Time |
| Copper, brass, bronze Nickel Stainless steels Ferrites Tungsten carbide Molybdenum Tungsten Tantalum | 760-900 1000-1150 1100-1290 1200-1500 1430-1500 2050 2350 2400 | 10-40 30-40 30-60 10-600 20-30 120 480 480 |
Sintering is a solid state process which is responsible for producing physical and mechanical properties in the PM part by developing metallurgical bond among the powder particles. It also serves to remove the lubricant from the powder, prevents oxidation, and controls carbon content in the part. The structure and porosity obtained in a sintered compact depend on the temperature, time, and processing details. It is not possible to completely eliminate the porosity because voids cannot be completely closed by compaction and because gases evolve during sintering. Porosity is an important characteristic for making PM bearings and filters.
SECONDARY AND FINISHING OPERATIONS
Sometimes additional operations are carried out on sintered PM parts in order to further improve their properties or to impart special characteristics. Some important operations are as under.
1. Coining and sizing. These are high pressure compacting operations. Their main function is to impart (a) greater dimensional accuracy to the sintered part, and (b) greater strength and better surface finish by further densification.
2. Forging. The sintered PM parts may be hot or cold forged to obtain exact shape, good surface finish, good dimensional tolerances, and a uniform and fine grain size. Forged PM parts are being increasingly used for such applications as highly stressed automotive, jet – engine and turbine components.
3. Impregnation. The inherent porosity of PM parts is utilized by impregnating them with a fluid like oil or grease. A typical application of this operation is for sintered bearings and bushings that are internally lubricated with upto 30% oil by volume by simply immersing them in heated oil. Such components have a continuous supply of lubricant by capillary action, during their use. Universal joint is a typical grease – impregnated PM part.
4. Infiltration. The pores of sintered part are filled with some low melting point metal with the result that part's hardness and tensile strength are improved. A slug of metal to be impregnated is kept in close contact with the sintered component and together they are heated to the melting point of the slug. The molten metal infiltrates the pores by capillary action. When the process is complete, the component has greater density, hardness, and strength. Copper is often used for the infiltration of iron – base PM components. Lead has also been used for infiltration of components like bushes for which lower frictional characteristics are needed.
5. Heat Treatment. Sintered PM components may be heat treated for obtaining greater hardness or strength in them.
6. Machining. The sintered component may be machined by turning, milling, drilling, threading, grinding, etc. to obtain various geometric features.
7. Finishing. Almost all the commonly used finishing method are applicable to PM parts. Some of such methods are plating, burnishing, coating, and colouring.
Plating. For improved appearance and resistance to wear and corrosion, the sintered compacts may be plated by electroplating or other plating processes. To avoid penetration and entrapment of plating solution in the pores of the part, an impregnation or infiltration treatment is often necessary before plating. Copper, zinc, nickel, chromium, and cadmium plating can be applied. Burnishing. To work harden the surface or to improve the surface finish and dimensional accuracy, burnishing may be done on PM parts. It is relatively easy to displace metal on PM parts than on wrought parts because of surface porosity in PM parts.
Coating. PM sintered parts are more susceptible to environmental degradation than cast and machined parts. This is because of inter – connected porosity in PM parts. Coatings fill in the pores and seal the entire reactive surface.
Colouring. Ferrous PM parts can be applied colour for protection against corrosion. Several methods are in use for colouring. One common method to blacken ferrous PM parts is to do it chemically, using a salt bath.
8. Joining. PM parts can be welded by several conventional methods. Electric resistance welding is better suited than oxy- acetylene welding and arc welding because of oxidation of the interior porosity. Argon arc welding is suitable for stainless steel PM parts.
METAL POWDERS FOR PM
Metal powders play an extremely important role in powder metallurgy. These are highly engineered materials. The particle size, shape and size distribution of metal powder affect the characteristics and properties of the compacted product. A large number of types and grades of powders available which makes possible the production of a wide variety of components for meeting numerous performance requirements. All metals can be produced in powder form but not all have the desired properties which are necessary for economical production. Some widely used metal powders for manufacturing PM parts are listed in Table 11.1. The characteristic of powders given in this Table are significant from the viewpoint of application feasibility for PM parts
Table 11.1 Widely used Metal Powders
| Pure Metals: Aluminum Antimony Berylium Bismuth Cadmium Chromium Cobalt Copper Iron Lead Manganese Molybdenum Nickel Precious metals(gold, silver, platinum) Rhenium Silicon Tantalum Tin Titanium Tungsten Vanadium Zinc | Alloys: Aluminium-iron Brass Copper-zinc-nickel Nickel-chromium Nickel-chromium-iron Nickel-copper Nickel-iron Silicon-iron Solder Stainless steel Compounds: Borides(chromium, tungsten, etc.) Carbides (molybdenum, tungsten, etc.) Molybdenum disilicide Nitrides (siliconn titanium, etc.) Zirconium hydride |
Powder Production
All metal powders, because of their individual physical and chemical characteristics, cannot be produced in the same way.
There are several methods for producing metal powders each giving different size and structure of the particles. Table 11.2 gives important characteristics of powders produced by some commercial methods. Also given in this Table are advantages and disadvantages of these methods. A brief description of some of these methods follows.
Table 11.2 Metal Powder Characteristics
| Apparent Density The apparent density or specific gravity of a powder is expressed in kg/m 3 . It should be kept constant. This means that the same amount of powder should be fed into the die each time. Chemical Properties These are the properties like the purity of the powder, amount of oxides permitted, and the percentage of other elements allowed. Compressibility Compressibility is the ratio of the volume of intial powder to the volume of the compressed piece. It varies considerably and is affected by the particle-size distribution and shape. Compressibility affects the green strength of a compact. Fineness Fineness refers to the particle size and is determined by passing the powder through a standard sieve or by microscopic measurement. Flowablity Flowablity is the characteristic of a powder that permits it to flow readily and conform to the mold cavity. It can be described as the rate of flow through a fixed orifice. Particle-Size Distribution Particle-Size Distribution refers to the amount of each standard particle size in the powder. It influences the flowablity and apparent density as well as porosity of the product. Sintering Ability Sintering ability is the suitability of a powder for bonding of particles by the application of heat. |
Atomization: It is as excellent and very widely used method of producing metal powders. In case of low melting point metals, the molten metal is kept in a tank. It is raised by the suction produced by hot air, through a pipe to the atomizing nozzle. A fine stream of molten metal is broken into small droplets, which solidify into metal powder particles. The size of particles can be controlled but the shape of particles remains irregular. However, the technique used for high melting point metals is slightly different. A stream of molten metal coming from an orifice at the bottom of a reservoir is broken up by a jet of atomizing fluid (which may be inert gas, air , water or steam) into metal powder particles. It is possible to control the powder characteristics (average particle size, particle shapes, particle size distribution, particle chemistry, and particle structure) by changing the process variables (such as temperature, stream velocity, etc.) in the atomizing process.
Electrolysis: Electrolytic deposition or electrolysis is a widely used method of producing powders of iron, coppers, silver, and several other metals. For producing iron, for example, a tank containing a suitable electrolyte is taken. In it steel plates are placed as anode and stainless steel plates are placed as cathode. The two electrodes are connected to a powerful de source. In about 50 hours, a 2 mm thick deposit of is obtained on the cathode plates. This deposit of electrolytic iron is stripped, washed, screened, and sized. The iron powder may be annealed if its brittleness is to be reduced.
Reduction: In this process, metal oxide is reduced to metal powder through contact with a reducing gas at temperature below the melting point. For example, in case of iron the iron oxide is crushed and passed through a furnace. The hydrogen atmosphere in the furnace reacts with the oxygen of iron oxide at a temperature of nearly 1050 0 C and pure iron with sponge – like structure is obtained. In addition to iron, other commonly produced commercially by this method include nickel, cobalt, molybdenum, and tungsten.
Machining and Grinding. Machining has been used to produce coarse magnesium powder. Milling and grinding processes utilize various types of rotary mills, stamping mills, crushers, and grinders, break down brittle metals into powders of almost any fineness but of irregular shaped particles.
There are several other methods involving precipitation, condensation and other chemical processes, that are employed for producing metal powders.
Powder Mixing
Mixing of powders precedes compacting.
The process of mixing includes mixing of various metal powders with lubricants as a result of which the powders are thoroughly intermingled. This is carried out in batch mixers. The surface friction properties of the powders to be mixed significantly affect the properties of the mixtures. If the powders differ to much in density, segregation of the heavier powder may occur because gravitational forces may be stronger than the frictional forces.
The temperature during mixing affects the friction between powder particles. With increasing temperature, the friction coefficient between most materials increases and the flow of powders is impaired. It is therefore desirable to maintain lower mixing temperature.
When parts are pressed in rigid dies, the use of lubricant becomes essential in order to reduce friction between powder particles and between the compact being pressed and the die wall and core rod. The lubricant also reduces the pressure required to eject compacts from the die. The lubricant, which is generally ½ to 1% by weight, is introduced as a fine powder mixing time and the intensity of mixing powder and lubricant affect flow and apparent density of the powder mixture.
POWDER METALLURGY
Advantages: Metal in powder form is costlier than in solid form. Further, expensive dies and equipment needed to adapt this process implies that the process is justified by the unusual properties obtained in the products. Powder metallurgy offers the following specific advantages.
i. Parts can be produced from high melting point refractory metals with respectively less difficulty and at less cost.
2. Production rates are high even for complex parts. This is primarily because of the use of automated equipment in the process.
iii. Near net shape components are produced. The dimensional tolerances on components are mostly such that no further machining is needed. Scrap is almost negligible.
iv. Parts can be made from a great variety of compositions. It is therefore much easy to have parts of desired mechanical and physical properties like density, hardness toughness, stiffness, damping, and specific electrical or magnetic properties.
v. Parts can be produced with impregnation and infiltration of other materials to obtain special characteristics needed for specific applications.
vi. Skilled machinists are not needed, so labour cost is low
vii. Parts with controlled porosity can be produced
viii. Bi-metallic products, sintered carbides and porous bearings can be produced only by this process.
Limitations: Powder metallurgy has the following limitations.
i. High cost of metal powders compared to the cost of raw material used for casting or forging a component. A few powders are even difficult to store without some deterioration.
ii. High cost of tooling and equipment. This is particularly a limitation when production volumes are small.
iii. Large or complex shaped parts are difficult to produce by PM process.
iv. Parts have lower ductility and strength than those produced by forging.
v. Uniformly high – density products are difficult to produce.
vi. Some powders (such as aluminum, magnesium, titanium and zirconium) in a finally divided state present fire hazard and risk of explosion.
vii. Low melting point metal powders (such as of zinc, tin, cadmium) give thermal difficulties during sintering operation, as most oxides of these metals cannot be reduced at temperatures below the melting point.
Applications of Powder Metallurgy:
There is a great variety of machine components that are produced from metal powders, many of these are put to use without any machining operation carried out on them. Following are some of the prominent PM Products.
Filters: Permanent metal powder filters have greater strength and shock resistance than ceramic filters. Fiber metal filters, having porosity upto 95% and more, are used for filtering air and fluids. Such filters find use in dehydration for filtering air and fluids. Such filters find use in dehydrators for diffusing moisture – laden air around some drying agent such as silica gel, Fig 12.1.
Fig 12.1 Applications of powder metallurgy parts. Filiers can be used for diffusing or for separating.
These filters find wide usage also in petrol / diesel engines for separating dirt and moisture from fuel system. Metal powder filters are also used for arresting flame and attenuating sound.
Cutting Tools and Dies. Cemented carbide cutting tool inserts find extensive applications in machine shops. These are produced by PM from tungsten carbide powder mixed with cobalt binder.
Machinery Parts. Several machinery parts including gears, bushes and bearings, sprockets, rotors are made from metal powders mixed with sufficient graphite to give to product the desired carbon content. The parts have nearly 20 percent porosity. The pores of the parts which are to rub against another surface in their use, are impregnated with oil to promote quiet operation.
Bearing and Bushes. Bearing and bushes to be used with rotating parts are made from copper powder mixed with graphite. In small quantities, lead or tin may also be added for obtaining better wear resistance. After sintering, the bearings are sized and then impregnated with oil by vacuum treatment. Porosity in the bearings may be as high as 40 percent of the volume. Other machinery parts made by PM include clutch plates, brake drums, ball retainers and welding rods.
Magnets. Small magnets produced from different compositions of powders of iron, aluminum, nickel and cobalt have shown excellent performance, far superior to those cast.
Electrical Parts. The possibility of combining several metal powders and maintaining some characteristics of each has promoted PM for production of electric contact parts. These parts are required to have excellent electrical conductively, be wear resistant, and somewhat refractory. Several combinations such as copper – tungsten, cobalt – tungsten, silver – tungsten, copper-nickel, and silver – molybdenum have been used for production of these parts.
Economics of Powder Metallurgy:
Since it is possible to produce near net shape parts by PM, there is usually very little scrap and also no need for secondary manufacturing and assembly operations. PM is therefore becoming increasingly competitive with conventional manufacturing processes like forging, casting, and machining. The high initial cost of dies, punches, and equipment for PM processing, however, requires sufficiently high production volume to make this process cost – effective.
Design Considerations for PM Parts
The following recommendations should be kept in mind while designing parts to be made by PM
1. The shape of the part must permit ejection from the die.
2. The shape of the part must not require the powder to flow into thin walls, narrow passages, or sharp corners.
3. The shape of the part should permit construction of strong and rigid tooling.
4. The shape of the part should make allowance for the length to which thin – walled portion of the part can be compacted.
5. The shape of the part should have the fewest possible change in section.
6. The special capabilities afforded by PM to produce certain part forms, should be utilized.
Nice Blog, Thanx For Sharing. We Rachna Fasteners is an ISO 9001:2008 certified & CE Approved Fasteners manufacturer As Per EN-15048 Part-1 and 2 from TUV Nord leading manufacturer of high tensile hot and cold forged fasteners in India. EN 14399 manufacturer Supplier in India, Automotive fasteners, superior fasteners.
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