Metal forming is a general term for a large group, that includes a wide variety of manufacturing processes which are based on deformation. These processes have been designed to exploit a remarkable property of some engineering materials (most notably metals) known as plasticity, the ability to flow as solids without deterioration of their properties.

In metal forming process the metal is plastically deformed to shape it into the desired geometry. In order to plastically deform a metal, a force must be applied that will exceed the yield strength of the material.

Elastic and Plastic Deformation

When a small force is applied to a metal it will change its geometry slightly, in correspondence to the force that is exerted. The amount of deformation will be directly proportional to the force applied. The material will return to its original shape once the force is released. Think of stretching a rubber band, then releasing it, and having it go back to its original shape. This is called elastic deformation.

Once the stress on a metal increases past a certain point, it no longer deforms elastically but starts to undergo plastic deformation. In plastic deformation, the geometric change in the material is no longer directly proportional to stress and geometric changes remain after the stress is released, meaning that the material does not recover its shape.

The actual level of stress applied to a metal where elastic deformation turns to plastic deformation is called the proportional limit and is often difficult to determine exactly. The 0.2% offset convention is usually used to determine the yield point, which is taken for practical purposes as the stress level where plastic deformation, (yielding), begins to occur.

It can be seen by the stress-strain graph that once the yield point of a metal is reached and it is deforming plastically, higher levels of stress are needed to continue its deformation. The metal actually gets stronger, the more it is deformed plastically. This is called strain hardening or work hardening. As may be expected, strain hardening is a very important factor in metal forming processes. Strain hardening is often a problem that must be overcome, but many times strain hardening, when used correctly, is a vital part of the manufacturing process in the production of stronger parts.

Material Properties in Metal Forming

• Low yield strength and
• High ductility

These properties are affected by temperature.  Ductility increases and yield strength decreases when work temperature is raised. Other factors which affect these properties are strain rate and friction.

Workability

Workability is defined as the extent to which material can be deformed in the specific metal working process without formation of cracks. Materials differ in their ability to undergo plastic deformation. The extent of plastic deformation in a material is dependent on the materials grain structure, nature of bonding, presence of defects like dislocation and external factors such as temperature.

In the case of ductile materials, the limit of workability is determined by the beginning of necking. Once necking starts, due to localized deformation, further deformation of the workpiece to finished shape becomes impossible. Therefore, in most of the materials, the starting of necking is considered as the limit of working or forming. Workability is dependent on material characteristics and external factors such as tool and die geometry, friction, strain rate etc.

The other criterion for workability may be the formation of cracks on the surface or within the material during the forming process. Cracks on external surface may form due to excessive tensile loads or friction. Internal cracks may form due to the presence of voids, second phase particles etc. Necking during tensile deformation may result in the formation of voids, which may grow in size during loading. Cracks result due to excessive growth of voids and their coalescence. In compressive loading, generally, surface cracks are formed due to excessive tensile stresses induced on the bulged surfaces. Bulging is a non-uniform deformation during compressive loading of billets.

The forming process requires stress above flow stress of the material being deformed. The effect of external work done on the work piece during forming is converted into heat. About 5 to 10% of the work is stored within as internal energy. Friction can also result in heating and increase in internal energy of work piece.

Assuming frictionless deformation, the temperature increase during metal forming operation can be written as:

   \[ \bigtriangleup T = \frac{W_p}{\rho C} \]

Where:
\(W_p\) = Plastic work done per unit volume of work piece
\(\rho\) = Density
\(C\) = Specific heat of material

With friction:

   \[ \bigtriangleup T = \frac{\lambda W_p}{\rho C} \]

Where: \(\lambda\) is a fraction of deformation work converted to heat. Normally, \(\lambda = 0.95\; to\; 0.98\).

Temperature rise is calculated using the stress-strain curve, as the plastic work is calculated as the area under the stress-strain curve for plastic flow.

For slow deformations, the temperature rise of the work piece may be small as the heat generated gets dissipated through the die, surrounding air, etc. However, the adiabatic condition may prevail under large deformation speeds, resulting in a large rise in temperature of the work piece. This may cause incipient melting. Therefore, strain rate also influences the temperature rise during working.

For low carbon steel, the temperature rise for a true strain of 1 has been estimated to be \(553^\circ K\). This is without heat lost from the billet.

Properties of a metal change with changes in temperature. Therefore, the metal will react differently to the same manufacturing operation if it is performed under different temperatures and the manufactured part may possess different properties. For these reasons, it is very important to understand the materials that we use in our manufacturing process. This involves knowing their behaviour at various temperature ranges. There are three basic temperature ranges at which the metal can be formed, cold working, warm working, and hot working.

Cold Working

Cold working, (or cold forming), is a metal forming process that is carried out below the recrystallization temperature of the material. In cold working process, the work hardening is not relieved. The deformation is usually performed at room temperature, but mildly elevated temperatures may be used to provide increased ductility and reduced strength.

Cold working increases the strength or hardness and at the same time, it decreases the ductility of the material. At cold working temperatures, the ductility of a metal is limited, and only a certain amount of shape change may be produced. If the stress exceeds a certain limit, the metal will fracture before reaching to desired shape and size. In order to avoid fracture of metal, cold working operations are generally carried out in several steps with intermediate annealing operation to restore the ductility.

Hot Working

Hot working, (or hot forming), is a metal forming process that is carried out at a temperature range that is higher than the recrystallization temperature of the metal being formed. In this process recovery and recrystallization occur during or immediately after the deformation. It means no strain hardening takes place.

The upper limit for hot working is usually determined by factors such as excess oxidation, grain growth, or undesirable phase transformations. The lower limit is the lowest temperature at which the rate of recrystallization is rapid enough to eliminate strain hardening. Metals with high thermal conductivity will require higher working temperatures or rapid working. To keep the forming forces as low as possible and enable hot deformation to be performed for a reasonable amount of time, the starting temperature of the workpiece is usually set at or near the highest temperature for hot working.

The higher the temperature the more reactive the metal is likely to be. Also if a work piece for a hot working process is too hot then friction, caused during the process, may further increase heat to certain areas causing melting, in localized sections of the work. In an industrial hot metal working operation, the optimum temperature should be determined according to the material and the specific manufacturing process.

Over 90% of the energy imparted to a deforming work piece will be converted into heat. If the deformation process is sufficiently rapid, the temperature of the work piece may actually increase. More common, however, is the cooling of the work piece in its lower-temperature environment. Heat is lost through the work piece surfaces, with the majority of the loss occurring where the work piece is in direct contact with lower temperature tooling.

During most metal forming processes, the die is often cold or slightly heated. For metal forming, in general, the temperature gradient between the die and the work has a large effect on metal flow during the process. The metal nearer to the die surfaces will be cooler than the metal closer to the inside of the part, and cooler metal does not flow as easily. High-temperature gradients, within the work, will cause greater differences in flow characteristics of different sections of the metal, these could be problematic. For example, metal flowing significantly faster at the centre of the work compared to cooler metal near the die surfaces that is flowing slower can cause part defects. Work cooling during the process can also result in more metal flow variations. Thin sections cool faster than thick sections, and this may further complicate the flow behaviour.

Warm Working

Warm working, (or warm forming), is a metal forming process carried out above the temperature range of cold working, but below the recrystallization temperature of the metal. Working temperature depends upon yield or flow strength, ductility, dimensional tolerances, scaling and oxidation losses.

Compared to cold forming, warm forming offers the advantages of reduced loads on the tooling and equipment, increased material ductility, and a possible reduction in the number of anneals due to a reduction in the amount of strain hardening. The use of higher forming temperatures can often expand the range of materials and geometries that can be formed by a given process or piece of equipment. High-carbon steels may be formed without a spheroidization treatment.

Compared to hot forming, the lower temperatures of warm working produce less scaling and decarburization and enable the production of products with better dimensional precision and smoother surfaces. Finish machining is reduced and less material is converted into scrap. Because of the finer structures and the presence of some strain hardening, the as-formed properties may be adequate for many applications, enabling the elimination of final heat-treatment operations. The warm regime generally requires less energy than hot working due to the decreased energy in heating the work piece (lower temperature), energy saved through higher precision (less material being heated), and the possible elimination of post-forming heat treatments. Although the tools must exert 25 to 60% higher forces, they last longer since there are less thermal shock and thermal fatigue.

Friction

Friction arises between two surfaces in contact as a result of interlocking of the asperities. Asperities are microscopic valleys and depressions on surfaces. Metal forming processes are characteristic of high pressures between two contacting surfaces. In hot forming operations, these high pressures are accompanied by extreme temperatures. Friction and die wear are a serious consideration in metal forming.

Friction between the tool and work piece surfaces plays a considerable role in forming, which are listed below:

• Friction enhances the forming load
• It leads to poor surface finish on the formed part due to improper metal flow
• It leads to considerable increase in temperature-frictional heating
• Die life is reduced due to friction
• Friction also results in non-homogeneous deformation of the material during forming

Therefore, friction has to be understood and ways of reducing it have to be devised if forming loads are to be reduced.

A certain amount of friction will be necessary for some metal forming processes, but excessive friction is always undesirable. For some processes, more than 50% of the input energy is spent in overcoming friction.

In most cases, we want to economically reduce the effects of friction. However, some deformation processes, such as rolling, can only operate when sufficient friction is present. Regardless of the process, friction effects are hard to measure. The specific friction conditions depend on a number of variables, including contact area, contact pressure, surface finish, speed, lubricant, and temperature. Because of the many variables, the effects of friction are extremely difficult to scale down for laboratory testing or extrapolate from laboratory tests to production conditions.

Lubrication

Where friction is involved, lubricants can usually help. For some metal forming processes and materials no lubrication is used, but for many lubrication is applied to contacting surfaces to reduce friction forces. While lubricants are generally selected for their ability to reduce friction and suppress tool wear, secondary considerations may include:

• The ability to act as a thermal barrier, keeping heat in the workpiece and away from the tooling
• The ability to act as a coolant, removing heat from the tools
• The ability to retard corrosion if left on the formed product
• Ease of application and removal
• Lack of toxicity, odour, and flammability
• Lack of reactivity with material surfaces
• Adaptability over a useful range of pressure, temperature, and velocity
• Surface wetting characteristics
• Cost and availability
• The ability to flow or thin and still function as a lubricant

Lubricant selection is further complicated by the fact that lubricant performance may change with any change in the interface conditions. The exact response is often dependent on such factors as the finish of both surfaces, the area of contact, the applied load, the speed, the temperature, and the amount of lubricant. The ability to select an appropriate lubricant can be a critical factor in determining whether a process is successful or unsuccessful, efficient or inefficient. For example, if a lubricant layer can prevent mechanical contact between the tool and the workpiece (full-fluid or solid layer separation), the forces and power required may decrease by as much as 30 to 40%, and tool wear becomes almost non-existent. Considerable effort, therefore, has been directed to the study of friction and lubrication, a subject known as tribology, as it applies to both general metalworking conditions and specific metal forming processes. A substantial information base has been developed that can aid in optimizing the use of lubricants in metalworking.

Lubricants used in industry are different depending upon the type of metal forming process. Lubricants used in the manufacturing industry for metal forming processes include vegetable and mineral oils, soaps, graphite dispensed in grease, water-based solutions, solid polymers, wax, and molten glass.

Stresses to plastically deform the metal are usually compressive, whereas some processes involve other stresses. Depending upon the type of stress the forming processes are listed below.

Compressive Forming

Compressive forming involves those processes where the primary means of plastic deformation is uniaxial or multi axial compressive loading.

• Rolling, where the material is passed through a pair of rollers
• Extrusion, where the material is pushed through an orifice
• Die forming, where the material is stamped by a press around or onto a die
• Forging, where the material is shaped by localized compressive forces
• Indenting, where a tool is pressed into the workpiece

Tensile forming

Tensile forming involves those processes where the primary means of plastic deformation is uniaxial or multi-axial tensile stress.

• Stretching, where a tensile load is applied along the longitudinal axis of the workpiece
• Expanding, where the circumference of a hollow body is increased by tangential loading
• Recessing, where depressions and holes are formed through tensile loading    

Combined tensile and compressive forming

This category of forming processes involves those operations, where the primary means of plastic deformation involves both tensile stresses and compressive loads.

• Pulling through a die
• Deep drawing
• Spinning
• Flange forming
• Upset bulging

Bending

This category of forming processes involves those operations where the primary means of plastic deformation is a bending load.

Shearing

This category of forming processes involves those operations where the primary means of plastic deformation is a shearing load.

Material Behaviour in Metal Forming

While analysing the behaviour of the material in metal forming the plastic region of stress-strain curve is of primary interest because the material is plastically deformed.

In plastic region, metal's behaviour is expressed by the flow curve:

  \[  \sigma_f = K \varepsilon^n \]

Where:
\( \sigma_f \) = Flow stress
\( \varepsilon \) = True strain
\(K\) = Strength Coefficient
\(n\) = Strain hardening exponent

Stress and strain in flow curve are true stress and true strain.

Flow Stress

During a metal forming operation, it is important to know the force and power that will be needed to accomplish the necessary deformation. The stress-strain graph shows us that the more a workpiece is deformed plastically, the more stress is needed. Flow stress is defined as the instantaneous value of stress required to continue plastically deforming the material i.e. to continue yielding and flow of the work material at any point during the process. Factors such as chemical composition, purity, crystal structure, phase constitution, exit microstructure, grain size, and heat treatment, affect the flow stress.

Flow stress is the yield strength of the metal as a function of strain, which can be expressed as:

   \[ \sigma_f = K \varepsilon^n \]

Where:
\( \sigma_f \) = Flow stress
\( \varepsilon \) = True strain
\(K\) = Strength Coefficient
\(n\) = Strain hardening exponent

Hence, Flow stress can also be defined as the stress required to sustain plastic deformation at a particular strain. The flow stress is a function of plastic strain.

Higher strain hardening exponent values enhance the flow stress. Similarly, flow stress is enhanced with an increase in strain rate during a plastic deformation process. Effect of strain rate on flow stress becomes more pronounced at higher temperatures.

The flow stress considerably changes during the forming process as the material gets work hardened considerably. In such cases, an average flow stress is determined from the flow curve. The average flow stress is given as:

   \[ \sigma_f = Y_f = \frac{K \varepsilon^n}{1+n} \]


Where \(\varepsilon\) is maximum strain during the deformation process.

The flow stress value can be used to analyse what is going on at any particular point in the metal forming process. The maximum flow stress may be a critical measurement in some metal forming operations since it will specify the force and power requirements for the machinery to perform the process. The force needed at the maximum strain of the material would have to be calculated in order to determine maximum flow stress.

For different types of metal forming processes, the flow stress analysis may be different. For a process like forging, the maximum flow stress value would be very important. However, for a process like extrusion, where the metal is continuously being deformed and the different stages of the metal's deformation are occurring simultaneously, it is of interest to analyse the mean flow stress value.

The metal forming processes can be classified into two different categories based on the amount of deformation to the workpiece, which are:

1. Bulk Forming Processes
2. Sheet Metal Forming Processes

Bulk Forming Processes

Bulk deformation processes are those where the thicknesses or cross sections are reduced or shapes are significantly changed. Since the volume of the material remains constant, changes in one dimension require proportionate changes in others. Thus the enveloping surface area changes significantly, usually increasing as the product lengthens or the shape becomes more complex. The bulk forming operations can be performed in all of the temperature regimes. Common processes include rolling, forging, extrusion, and drawing.

Rolling: Rolling is a metal forming process that deforms the work by the use of rolls. Rolling processes include flat rolling, shape rolling, ring rolling, thread rolling, gear rolling, and the production of seamless tube and pipe by rotary tube piercing or roll piercing.

Forging: Forging is characteristic in the use of dies to compress and shape a work piece. The die may be flat or may contain an impression of a certain geometry.

Extrusion: Extrusion involves forming by forcing metal through a die opening, producing work of variable length and constant cross-section.

Drawing: Drawing is similar to extrusion, in that a length of metal is made to flow through a die opening and forming is done over its cross-section. The difference between drawing and extrusion is the application of force to the workpiece. In extrusion the work is pushed through the die opening, in drawing the work is pulled through the die opening.

Sheet Metal Forming Processes

In contrast, sheet-metal forming operations involve the deformation of a material where the thickness and surface area remain relatively constant. Common processes include shearing or blanking, bending, and deep drawing. Because of the large surface-to-volume ratio, sheet material tends to lose heat rapidly, and most sheet-forming operations are performed cold.

Shearing: Shearing is the cutting of the work piece, this would include punching holes. Technically shearing does not involve shaping by plastic deformation, but it is a critical process in sheet metal working operations and should be understood along with metal forming processes.

Bending: Bending involves the deformation of the work by way of bending about a certain axis.

Deep Drawing: Deep drawing is a metal forming process in which a flat piece of plate or sheet is forced into a die cavity to take a shape, such as a cup.