Formation of Metals: 4 Processes | Industries | Metallurgy

The following points highlight the four main processes involved in the formation of metals. The processes are: 1. Explosive Fabrication 2. Hydroforming 3. Electro-Hydraulic Forming 4. Magnetic-Pulse Forming. 

Process # 1. Explosive Fabrication:

It is also known as high-energy-rate forming. It develops high rate of energy transfer by increasing the velocity instead of using high values of mass.

Explosive forming of metals is a new process in which the complicated and difficult shaped articles are produced in no time using suitable propellant. Large forces of plastic deformation in this process are supplied by inertia rather than by very large frames of machines. Further the die-inertia is such that relatively weak, soft dies can be used to form hard metals.

In the explosive working of metals, large differences in the energy requirements and in the resulting behaviour of the workpiece occur. On explosion, a high pressure gas bubble is formed instantaneously whose pressure is around 70000 kg/cm² and it causes a pressure wave to move out spherically through the water.

Energy from the explosion is transferred to the workpiece by the pressure wave as well as by cavitation, water hammer and diffraction.

Depending upon the type of process, it is mainly classified as:

(i) Open type (stand-off operation)

(ii) Closed type (contact operation)

(iii) Multiple type.

In a stand-off operation, the energy is released some distance from the workpiece and is propagated mainly in the form of a pressure pulse through some intervening medium; whereas in the contact operation, the energy is released while the energy source, usually an explosive charge, is in intimate contact with the work piece.

(i) Open Type Process (Stand-Off Operation):

In this process, a high explosive is detonated and its energy is trans­mitted through a fluid medium, commonly water, to workpiece, which is held on a female die and clamped prop­erly in a draw ring placed on the die. The space in the die is usually connected to vacuum forming machine in order to avoid adiabatic compression and heating of the entrapped
area in the die.

Also the presence of air puts more resistance and the air pockets do not allow the required shape to be obtained. For energy transfer either water or air is commonly used though some other media are also possible e.g., molten salts of oils etc., in case of hot forming. Water is very cheap, convenient to use and superior due to its reduced compressibility and shock-impedance matching characteristics.

Since the explosive in this process is kept in open, only a part of the energy is utilised. Vacuum maintained in this process is of the order of 29 mm of mercury. Weight of explosive varies from 0.1 to 2 kg depending upon the shape and size and form to be given on the workpiece.

The shock wave is normally the major energy source in this process. However the secondary bubble phenomenon resulting from detonation becomes important under certain conditions. In this process workpiece is deformed in about one to two milliseconds and, therefore, the strain rate is of the order of 10 or 100 per second.

The advantage of the process is that no punch is required and also different dies for redrawing are not needed. Further it involves less capital cost and can be used for simple bending or any other type of operation.

The disadvantages of the process being that lot of sound and noise is produced and lots of precautions have to be taken as explosion hazards are there and, therefore, the process should be conducted only at safe and remote places.

For this process the following requirements should be provided:

i. Operation area near river,

ii. Power supply connection,

iii. Vacuum pump,

iv. Explosive handling facilities,

v. Material handling facilities,

vi. Inspection area and inert storage area,

vi. Die fabrication and maintenance area etc.

The process is finding great utility and resulting in lot of savings as it is capable of making the following complicated shaped articles in a single operation:

(a) Integral bosses on plates and sheets,

(b) Possibility of maintaining uniform thickness distribution in the dome of pressure vessel,

(c) Explosive forging,

(d) Multi-perforation by explosive charge,

(e) Hot explosive forming.

Energy Utilisation:

Say, W is the energy released by the explosive and E_m is the energy incident on the work and E_ab is the energy absorbed by the work.

Thus from above expression, ƞ increases, if

(a) Radius is increased

(b) For small values of s

(c) Larger value of K₁ (i.e. putting reflectors, better media).

(d) Larger value of K₂ (i.e. absorption of materials is improved).

(ii) Closed Type (Contact Operation):

Since in the closed type process whole of the energy is reflected on the work, therefore, efficiency of this process is much higher than that of open type of process. There is no problem of water splash­ing etc., as the process is conducted under vacuum. How­ever, as the process is performed in an enclosed container, it requires more extensive tooling and greater safety consid­erations in tool design.

An extension of this process is gas mixture process in which the energy released by the chemical reaction of a fuel with an oxidiser it utilized for deforming the workpiece through the transfer medium.

This process is very suitable for many operations because of the following characteristics of gas mixture:

(a) A gas mixture can be placed in position ready to fire, in a short time with no requirements for rigging of charge.

(b) As the gas mixture assumes the shape of its container, therefore, approaches the optimum shape regardless of the quantity used.

(c) The gas mixture can be easily adjusted within inflammability limits by varying the amount of fuel and oxidising agent.

(d) The gas mixture can be detonated like a high explosive, but it is usually used in the adiabatic range.

(iii) Multiple Type Forming:

In this process, energy is utilised for forming more than one blank at a time. Blanks are arranged to form a closed chamber and most of the energy is utilised.

Advantages of Explosive Fabrication:

Explosive forming methods have the following advantages:

(a) Extremely high pressures can be applied. The entire forming process occurs in a small interval of time; the large forces associated with forming being contributed by the inertia, thus large machine frames are not required.

(b) The area of the object being pressed at a given pres­sure is not limited by the maximum total force as it is in the case of a hydraulic piston press.

(c) Very high compact densities can be obtained.

(d) Shrinkage on sintering can be markedly reduced.

(e) Process eliminates final expensive finishing opera­tions.

(f) Mixtures of metals, ceramics etc. can be easily com­pacted and the layers of dissimilar materials can be bonded.

(g) At extreme high velocity die interface melts as high temperatures are reached and allows the interface to act as lubricant.

Process # 2. Hydroforming:

Hydroforming is a metal forming process in which structural/frame members having closed cross-sections are formed from tubes. A prevent tubular blank is loaded in an open die and the tube is filled with fluid. Some pressure is maintained inside the tube before die closure.

Internal hydraulic pressure is applied forcing the tube to conform to the internal die contours. The tube ends are usually forced fed, thus feeding the tube inside the die. A counterbalance holder may also be used to control material flow.

In addition, holes, cutouts, flat proportions, etc. can be formed when the die is closed. A hydraulic press equipped with a die and capable of producing the required die closing force, two horizontal axial feed cylinders, a high pressure hydroforming unit, tube end sealing units and a counterbalance cylinder are used for this process.

Types of Hydroforming:

Hydroforming is categorised as:

(1) Low Pressure Forming

(2) High Pressure Forming

(3) Sequenced Pressure Forming.

(1) Low Pressure Forming:

In Low Pressure Forming, the tube freely expands in the die space without contacting the die at pressures of less than 1,000 bar. Usually, this method is used for tubes with flat areas, large corner radii and simple cross-sections. The internal Hoop’s stress at the minimum corner radius is less than the material yield stress, and the original wall thickness is maintained at the corners.

(2) High Pressure Forming:

High Pressure Forming utilises pressures upto, 7,000 bar to force the tube against the die profile. The Hoop’s stress at the minimum corner radius exceeds the material yield strength and there is a reduction in wall thickness at the radii. However, the tube material must have high enough elongation to withstand forming without bursting.

(3) Sequenced Pressure Forming:

In Sequenced Pressure Forming, initially a low forming pressure is applied to minimise die friction and cause the metal to flow into the corner radii. Then, the pressure is increased so that the metal accurately attains the final form while maintaining a uniform wall thickness. The required forming pressure is less than that required in High Pressure Forming.

The limits for hydroforming are defined by the different failure modes of the tube during pressure forming: buckling, wrinkling and bursting.

In this process, Forming pressure = (Wall thickness x Material stress) / Inside corner radius and Die closing force = Component projected area x Forming pressure

A metal having high ductility, large elongation, high ultimate yield strength and good corrosion resistance is best suited for this process. The tube thickness plays an important role in deciding the internal pressure required. Single or double walled tubes, extruded profiles and tailor-made blanks may be used.

The component shape (i.e., its length, width and depth) and corner radii are also important. They determine the press capacity size. Producing components that are very large or long, and have severe internal radii may be uneconomical, because of the large press size required. Sharp corners should be avoided since they restrict metal flow and lead to part thinning.

Process # 3. Electro-Hydraulic Forming:

In Electro-hydraulic forming process the shock waves and pressure produced due to conversion of electrical energy to mechanical energy in a liquid medium are utilised for metal forming. The release of energy can be more closely controlled and it can be used within the premises of the factory.

The two most common methods of converting electrical energy to mechanical energy are:

(i) Capacitor discharge through a gap and

(ii) Capacitor discharge through a wire.

In the first method, the voltage of the order of 10,000 to 30,000 Volts is generally used so that spark jumps from 1 electrode to another.

In second method, the path of the electrical discharge can be positively predetermined and shaped and more efficient energy conversion takes place and thus the better control can be provided. The advantage of using a wire is that use of lower voltage is possible and the wire will initiate path across a wider gap than a specified voltage will jump without the wire. However it gives a lower production rate as the wire is melted by the discharge and has to be replaced after each firing.

Electrical energy in a charged capacitor is expressed as U = 1/2 CV² where U is energy in watt-second or joules, C is capacitance in farads, and V is the charged voltage.

The physical size of the capacitor chosen is dependent upon the energy storage capability and not on C and V. Therefore, increasing of V alone for high energy will not serve the purpose.

In the process the capacitor bank is charged by means of a power supply which accomplishes both current rectification and voltage set-up. The stored energy is then dumped into the spark gap, or the wire to be exploded, by some extremely fast operating triggering device.

The various applications of electro-hydraulic process include bulging, forming, bending, drawing, blanking and piercing of odd-shaped holes etc. The advantage of using this process for above operations is that the tooling required for conventional method is not needed.

Large amounts of energy can be directed into isolated area of the workpiece by this process. Production rates for this process are very high under favourable conditions.

Various limitations of the process are:

(i) Energy rating of the capacitor bank,

(ii) Amount of energy that can be pumped by the triggering devices,

(iii) Materials should have critical impact velocities above 30 metres per second in order that they may be conveniently worked by this process,

(iv) A liquid as a medium is required for transferring pressure to the workpiece,

(v) Particular attention has to be paid to provisions for air escape from behind the workpiece,

(vi) Dies for drawn shapes should have provision for sealing the liquid away from the cavity of the die.

Power supply requirements for the process depend upon the voltage rating of the capacitor bank, voltage of power source and the desired charging time. Major components of the power supply are a set-up transformer and rectifiers.

The current rating must be sufficient to permit charging the capacitor bank within the desired time limits and commonly used charged times are 6 to 30 seconds. The storage system consisting of suitable bank of capacitors connected together with either heavy conductors or bus bars so as not to create detrimental resistances is important. By this process automation is possible, running cost is less, initial cost is high and smaller and intermediate sizes can be used for fabrication.

Process # 4. Magnetic-Pulse Forming:

In this process the pulsed magnetic fields are used directly for forming of metals. By this process, powerful, uniform impulse of 3300 kg/cm² for a period of many microseconds can be applied to metallic workpiece through the medium of the magnetic field.

A big advantage of the process is that since the magnetic field does not interact appreciably with insulators, the magnetic impulse can be transmitted to the workpiece through the walls of a non-­conducting sheath, which may maintain workpiece in an inert, or other special environment. The process could be used for swaging and expanding tubular forms, for coining, shearing, forming the sheets, forming of conductors etc.

Principle of Magnetic-Pulse Forming:

The basic circuit used for this process is shown in Fig. 10.61 and consists of an energy storage capacitor, a switch, a coil and a power supply that provides energy to charge a capacitor. The current through the coil produces a magnetic field of high intensity between the coil and the workpiece.

During the brief impulse, eddy currents in the workpiece restrict the magnetic field to the surface of the workpiece. This interaction of the magnetic field and the eddy currents creates an inward force on the workpiece as shown in Fig. 10.61.

The coils must be designed in such a way that stray inductance is minimised and current concentrations are avoided. The coils may be made in various shapes like flat or of other configuration to produce magnetic pulse of the configuration required on the workpiece.

It is also possible to concentrate the current and hence the force in certain regions of the workpiece by utilising massive conducting structure as flux-concentrator device and not connecting it directly to the basic coil as shown in Fig. 10.62. In this way the basis coil is relieved from the task of carrying high current and at the same time very high pressures can be produced.

The efficiency of magnetic pulse forming depends upon the resistivity of the metal being formed.

Most suitable materials for this process are those with high electrical conductivity like copper, silver, gold and aluminium. For good practical results the resistivity of the material should be less than 15 micro-ohm-centimetres.

For non-conducting materials, forces must be generated by using a conductive material between the coil and the non-conductive workpiece. For efficient operation the coil used should be good fit in work-piece. Efficiency is maximum when the gap between the coil or field shaper and the workpiece is minimum.

The various typical applications of this process are:

i. Forming the metal to metal seals and elastic seals by using metals of high resistivity for joining tubes,

ii. Forging of structural joints between tubes and fittings,

iii. Assembling of instrument gear assembly,

iv. Bending fabrication of electrical connection,

v. Forming of ceramic assemblies,

vi. Sizing of cups,

vii. Embossing, etc.