Metal Forming and Metal Working Processes
Metal Forming and Metal Working Processes
Metal working consists of deformation processes in which a metal billet or blank is shaped by tools or dies. In metal working, an initially simple work-piece (e.g., a billet or a blanked sheet) is plastically deformed between tools (or dies) to get the desired final configuration. The design and control of such processes depend on the characteristics of the work-piece material, the conditions at the tool / work-piece interface, the mechanics of plastic deformation (metal flow), the equipment used, and the finished-product requirements. These factors influence the selection of tool geometry and material as well as processing conditions (e.g., work-piece and die temperatures, and lubrication). Because of the complexity of several metal working operations, models of different types, such as analytical, physical, or numerical models are frequently relied upon to design such processes.
The term metal forming refers to a group of manufacturing processes by which the given material, normally shapeless or of a simple geometry, is transformed into a useful part without change in the mass or composition of the material. This part normally has a complex geometry with well-defined (i) shape, (ii) size, (iii) accuracy and tolerances, (iv) appearance, and (v) properties.
Metal forming processes are primary shaping processes in which a mass of metal or alloy is subjected to mechanical forces. Under the action of such forces, the shape and size of metal piece undergo a change. By mechanical working processes, the given shape and size of a machine part can be achieved with high economy in material and time.
Metal forming is a very important manufacturing operation. It enjoys industrial importance among different production operations because of its advantages such as cost effectiveness, improved mechanical properties, flexible operations, higher productivity, and considerable material saving. Metal forming processes are used to produce structural parts and components which have widespread applications in several industries including automobile, aerospace, defense, and other industries. Metal forming processes include a wide range of operations which deform billet, sheet, or pipe / tube metal to form the component with the desired geometry.
Plastic properties of metals are used during the process of metal forming, i.e., the ability to change without damage the shape and dimensions in hot and cold condition under the pressure of forming tools. The knowledge of metal forming principles permits the realizing of the forming at optimum deformation regimes and using the appropriate main and auxiliary equipment. The variety of methods and kinds of metal forming permits producing a wide range of metal products with high productivity, exact dimensions, and needed mechanical properties.
Materials are converted into finished products though different manufacturing processes. Manufacturing processes are classified into shaping, casting, forming, joining, coating, dividing, machining, and modifying material property. Of these manufacturing processes, forming is a widely used process. Wrought forms of materials are produced through bulk or sheet forming operations. Cast products are made through shaping, moulding, and casting. Typical automobile uses of formed parts are wheel rims, car body, valves, rolled shapes for chassis, and stamped oil pan etc. In the daily life, a large number of formed products are used. Examples are cooking utensils, tooth paste tubes, bicycle body, chains, pipe fitting, and fan blades etc. Forming is the process of getting the needed shape and size on the raw material by subjecting the material to plastic deformation through the application of tensile force, compressive force, bending force, shear force, or combinations of these forces. Fig 1 shows different manufacturing operations on materials.
Fig 1 Different manufacturing operations on materials
The term manufacturing processes represents the main shape-generating methods such as casting, moulding, and forming processes, as well as traditional and non-traditional machining processes. Fig 2 gives classification of these processes. This classification provides a guide for the selection of the manufacturing processes which can be suitable contenders for a component. In majority of the cases, there are several processes which can be used for a component and final selection depends on a large number of factors, mainly associated with a range of technical capabilities and process economics, not the least component size, geometry, tolerances, surface finish, capital equipment, and labour costs. Fig 2 gives classification of manufacturing processes.
Fig 2 Classification of manufacturing processes
Some of the main process selection drivers are (i) product quantity, (ii) equipment costs, (iii) tooling costs, (iv) processing time, (v) labour intensity and work patterns, (vi) process supervision, (vii) maintenance, (viii) energy consumption, (ix) overhead costs, (x) material costs and availability, (xi) material to process compatibility, (xii) component form and dimensions, (xiii) tolerance requirements, (xiv) surface finish requirements, (xv) bulk treatment and surface engineering, (xvi) process to component variability. (xvii) process waste, and (xviii) component recycling. These drivers are not necessarily of equal importance or do not occur in a fixed sequence.
Characteristics of manufacturing processes – There are four main characteristics of a manufacturing process namely, geometry, tolerances, production rates, and human and environmental factors.
Geometry – Each manufacturing process is capable of producing a family of geometries. Within this family there are geometries, which can be produced only with extraordinary cost and effort. As an example, the forging process allows production of parts, which can be easily removed from a die set, i.e., upper and lower die. By use of a ‘split die’ design, it is possible to produce forgings with undercuts and with more complex shapes.
Tolerances – No variable, especially no dimensional variable, can be produced exactly as specified by the designer. Hence, each dimension is associated with a tolerance. Each manufacturing process allows certain dimensional tolerances and surface finishes to be achieved. The quality of these variables can always be improved by use of more sophisticated variations of the process and by means of new developments. As an example, through the use of the lost-wax vacuum casting process, it is possible to get much more complex parts with tighter tolerances than are possible with ordinary sand-casting methods.
Dimensional tolerances serve a dual purpose. First, they allow proper functioning of the produced part, e.g., an automotive brake drum is to be round, within limits, to avoid vibrations and to ensure proper functioning of the brakes. The second role of dimensional tolerances is to provide inter-changeability. Without inter-changeability, the ability to replace a defective part or component (e.g., a bearing) with a new one, produced by a different supplier, modern mass production is unthinkable. Fig 3 shows the dimensional accuracies which are achievable by different processes. The values given in the figure are to be considered as guidance values only.
Fig 3 Approximate values of achievable dimensional accuracies
Forming tolerances represent a compromise between the accuracy desired and the accuracy which can be economically achieved. The accuracy achieved is determined by several factors such as the initial accuracy of the forming dies and tooling, the complexity of the part, the type of material being formed, and the type of forming equipment which is used. Another factor determining the forming accuracy is the type of part being produced.
Production costs are directly proportional to tolerances and surface finish specifications. Under typical conditions, each production process is capable of producing a part to a certain surface finish and tolerance range without extra expenditure. In a production situation, it is better to use the recommendations published by different industry associations or individual organizations.
Surface roughness is normally given in terms of ‘Ra’ (arithmetic average). In several applications the texture (lay) of the surface is also important, and for a given ‘Ra’ value, different processes can result in quite different finishes. It is used to be believed that cost tends to rise exponentially with tighter tolerances and surface finish. This is true only if a process sequence involving processes and machine tools of limited capability is used to achieve these tolerances. There are, however, processes and machine tools of inherently higher accuracy and better surface finish. Hence, higher-quality products can be produced with little extra cost and, if the application justifies it, certainly with higher competitiveness. Still, a fundamental rule, the cost-conscious designer to specifies the loosest possible tolerances and coarsest surfaces which can still accomplish the intended function. The specified tolerances, if possible, are to be within the range achievable by the intended production process so as to avoid additional finishing operations.
Production rate – The rate of production which can be achieved with a given manufacturing process is probably the most significant feature of that process, since it indicates the economics of and the achievable productivity with that manufacturing process. In industrialized countries, manufacturing industries represent 25 % to 30 % of gross national product. However, manufacturing productivity, i.e., production of discrete parts, assemblies, and products per unit time, is the single most important factor which influences the country’s competitive position in international market of trade in manufactured goods.
The rate of production or manufacturing productivity can be increased by improving existing manufacturing processes and by introducing new machines and new processes, all of which need new investments. However, the most important ingredient for improving productivity lies in human and managerial resources, since good decisions regarding investments (when, how much, and in what) are made by people who are well trained and well-motivated. As a result, the present and future manufacturing productivity in a plant, an industry, or a country depends not only on the level of investment in new plants and machinery, but also on the level of training and availability of manufacturing engineers and specialists in that plant.
Environmental factors – Every manufacturing process is to be examined in view of (i) its effects on the environment, i.e., in terms of air, water, and noise pollution, (ii) its interfacing with human resources, i.e., in terms of human safety, physiological effects, and psychological effects, and (iii) its use of energy and material resources, particularly in view of the changing world conditions concerning scarcity of energy and materials. Hence, the introduction and use of a manufacturing process is also to be preceded by a consideration of these environmental factors.
Metal forming is the final stage of metallurgical manufacturing permitting to produce metal ware used in the country’s economy as the finished products or as the billet for further processing. Metal forming is the main method of making metal products and semi-finished products. More than 90 % of smelted metal is processed by different methods of metal forming. Plastic properties of metals are used during the process of metal forming. The knowledge of metal forming rules permits to realize the forming at optimum deformation regimes and to use the appropriate main and auxiliary equipment. The variety of methods and kinds of metal forming permits producing the wide range of metal products with high productivity, exact dimensions, needed mechanical properties.
The development of metallurgical manufacture has resulted in appearance of new kinds of metal forming where the processes of casting and hardening metal reduction are being combined. New technological processes of metal forming give the possibility to shape the product at high strain rate, to get the products with especially high mechanical properties, to reduce the number of process stages and equipment used for it.
Technologically, manufacturing is the application of physical and chemical processes to alter the geometry, properties, and / or appearance of a given starting material to make parts or products. Manufacturing also includes assembly of multiple parts to make products. The processes to accomplish manufacturing involve a combination of machinery, tools, power, and labour, as shown in Fig 4a. Manufacturing is normally carried out always as a sequence of operations. Each operation brings the material closer to the desired final state.
Fig 4 Two ways to define manufacturing process
Economically, manufacturing is the transformation of materials into items of higher value by means of one or more processing and / or assembly operations, as shown in Fig 4b. The key point is that manufacturing adds value to the material by changing its shape or properties, or by combining it with other materials which have been similarly altered. The material has been made more valuable through the manufacturing operations performed on it. When iron ore is converted into steel, value is added. When sand is transformed into glass, value is added. When petroleum is refined into plastic, value is added, and when plastic is moulded into the complex geometry of a courtyard chair, it is made even more valuable.
Forming processes are frequently used together with other manufacturing processes, such as machining, grinding, and heat treating, in order to complete the transformation from the raw material to the finished and assembly-ready part. Desirable material properties for forming include low yield strength and high ductility. These properties are affected by temperature and rate of deformation (strain rate). When the work temperature is raised, ductility is increased and yield strength is decreased. The effect of temperature gives rise to distinctions among cold forming (work-piece initially at room temperature), warm forming (work-piece heated above room temperature, but below the recrystallization temperature of the work-piece material), and hot forming (workpiece heated above the recrystallization temperature). For example, the yield stress of a metal increases with increasing strain (deformation) during cold forming. In hot forming, however, the yield stress, in general, increases with strain (deformation) rate.
The development in forming technology has increased the range of shapes, sizes, and properties of the formed products enabling them to have different design and performance needs. Formed parts are needed specifically when strength, reliability, economy, and resistance to shock and fatigue are necessary. The products can be determined from materials with the needed temperature performance, ductility, hardness, and machinability.
Metal forming processes in manufacturing – Metal forming consists of a large number of manufacturing processes which are capable of producing industrial products as well as military components and consumer goods. Metal forming processes include (i) bulk forming operations such as forging, rolling, and drawing, and (ii) sheet forming processes, such as brake forming, deep drawing, and stretch forming. Unlike machining, metal forming processes do not involve extensive metal removal to achieve the desired shape of the work-piece. Among the group of manufacturing processes, metal forming represents a highly significant group of processes for producing industrial and military components and consumer goods.
The outlines of some of the important areas of application of work-pieces produced by metal forming underlining their technical significance are (i) components for automobiles and machine tools as well as for industrial plants and equipment, (ii) hand tools, such as hammers, pliers, screw-drivers, and surgical instruments, (iii) fasteners, such as screws, nuts, bolts, and rivets, (iv) containers, such as metal boxes, cans, and canisters, (v) construction elements used in tunneling, mining, and quarrying (roofing and walling elements, pit props, etc.), and (vi) fittings used in the building industry, such as for doors and windows etc.
Metal forming normally needs relatively expensive tooling. Hence, the process is economically attractive only when a large number of parts are to be produced and / or when the mechanical properties needed in the finished product can be achieved only by a forming process. Forming is especially attractive in cases where (i) the part geometry is of moderate complexity and the production volumes are large, so that tooling costs per unit product can be kept low, e.g., in automotive applications, and (ii) the part properties and metallurgical integrity are extremely important, such as load-carrying aircraft and jet engine and turbine components.
The design, analysis, and optimization of forming processes need (i) analytical knowledge regarding metal flow, stresses, and heat transfer, (ii) technological information related to lubrication, heating, and cooling techniques, and (iii) technological information related to material handling, die design and manufacture, and forming equipment.
Classification of manufacturing processes – The manufacture of metal parts and assemblies can be classified, in a simplified manner, into five general categories as given below.
The first category is the primary shaping processes, such as casting, melt extrusion, die casting, and pressing of metal powder. In all these processes, the material initially has no shape but gets a well-defined geometry through the process. The second category is the metal forming processes such as rolling, extrusion, cold and hot forging, bending, and deep drawing, where metal is formed by plastic deformation. The third category is metal cutting processes, such as sawing, turning, milling and broaching where removing metal generates a new shape. The fourth category is the metal treatment processes, such as heat treating, anodizing and surface hardening, where the part remains essentially unchanged in shape but undergoes change in properties or appearance. The fifth category is joining processes, which include (i) metallurgical joining, such as welding and diffusion bonding, and (ii) mechanical joining, such as riveting, shrink fitting, and mechanical assembly. Metallurgical joining processes, such as welding, brazing, and soldering, form a permanent and robust joint between components. Mechanical joining processes, such as riveting and mechanical assembly, bring two or more parts together to build a sub-assembly which can be disassembled conveniently.
Among all manufacturing processes, metal forming technology has a special place since it helps to produce parts of superior mechanical properties with minimum waste of material. In metal forming, the starting material has a relatively simple geometry. The material is plastically deformed in one or more operations into a product of relatively complex configuration. Forming to near-net or to net-shape dimensions drastically reduces metal removal requirements, resulting in considerable material and energy savings.
Classification of forming – Metal forming processes are normally classified as per two broad categories namely (i) bulk, or massive, forming operations, and (ii) sheet-forming operations. In the broadest and most accepted sense, however, the term forming is used to describe bulk forming as well as sheet-forming processes. In both types of processes, the surfaces of the deforming metal and the tools are in contact, and friction between them can have a major influence on material flow. In bulk forming, the input material is in billet, rod, or slab form, and the surface-to-volume ratio in the formed part increases considerably under the action of largely compressive loading. In sheet forming, on the other hand, a piece of sheet metal is plastically deformed by tensile loads into a three-dimensional shape, frequently without considerably changes in sheet thickness or surface characteristic.
Rolling, forging, extrusion, and drawing are bulk forming processes. In bulk deformation processing methods, the nature of force applied can be compressive, compressive and tensile, shear, or a combination of these forces. Bulk forming is accomplished in forming presses with the help of a set of tools and dies. Examples for products produced by bulk forming are gears, bushes, valves, and engine parts such as valves, connecting rods, and hydraulic valves etc.
A new class of forming process called powder forming is gaining importance because of its unique capabilities. One of the important merits of powder forming is its ability to produce parts very near to final dimensions with minimum material wastage. It is called near-net-shape forming. Material compositions can be adjusted to suit the desirable mechanical properties. Formability of sintered metals is higher than conventional wrought materials. However, the challenge in powder forming continues to be the complete elimination or near-complete elimination of porosity. Porosity reduces the strength, ductility and corrosion resistance and enhances the risk of premature failure of components. Fig 5 gives classification of metal forming processes.
Fig 5 Classification of metal forming processes
Processes which fall under the category of bulk forming have the two distinguishing features namely (i) the deforming material, or work-piece, undergoes large plastic (permanent) deformation, resulting in an appreciable change in shape or cross section, (ii) the portion of the work-piece undergoing plastic deformation is normally much larger than the portion undergoing elastic deformation, hence, elastic recovery after deformation is negligible.
Sheet metal forming involves application of tensile or shear forces predominantly. Working upon sheets, plates, and strips mainly constitutes sheet forming. Sheet metal operations are mostly carried out in presses either hydraulic or pneumatic. A set of tools called dies and punches are used for the sheet working operations. Bending, drawing, shearing, blanking, and punching are some of the sheet metal operations.
Based on the nature of deformation force applied on the material during forming, metal forming processes are also classified into several types as given shown in Fig 6.
Fig 6 Classification of forming processes based on the nature of deformation force applied
Forming is also classified as cold forming, hot forming, or warm forming. Hot forming is the deformation carried out at temperatures above recrystallization temperatures. Typically, recrystallization temperatures for materials ranges from 0.5 Tm to 0.8 Tm, where ‘Tm’ is melting temperature of the material.
Classification of metal forming processes as per type of stress employed – Primary metal working processes are those in which the bulk material in the form of ingots, blooms, and billets is broken down to the needed shapes and sizes by processes such as forging, rolling, and extrusion etc. These processes can be categorized on the basis of the kind of stress employed on the material. As per this classification metal forming processes are (i) mainly compression type, such as forging, rolling, and extrusion etc., (ii) mainly tension type, such as drawing, (iii) combined compression and tension type, such as deep drawing, and embossing etc. Some of these processes are shown schematically in Fig 7.
Fig 7 Metal working processes
The different forming processes are associated with a large variety of forming machines or equipment, including (i) rolling mills for plate, strip, and shapes, (ii) machines for profile rolling from strip, (iii) ring rolling machines, (iv) thread-rolling and surface-rolling machines, (v) magnetic and explosive forming machines, (v) draw benches for pipe, tube and rod such as wire drawing and rod drawing machines, and (vi) machines for pressing-type operations (presses).
Among the listed, pressing-type machines are the most widely used and are applied to both bulk forming and sheet forming processes. These machines can be classified into three types (i) load-restricted machines (hydraulic presses), (ii) stroke-restricted machines (crank and eccentric, or mechanical, presses), and (iii) energy-restricted machines (hammers and screw presses).
The significant characteristics of pressing-type machines comprise all machine design and performance data which are pertinent to their economical use. These characteristics include (i) characteristics for load and energy, e.g., available load, available energy, and efficiency factor (which equals the energy available for work-piece deformation / energy supplied to the machine), (ii) time-related characteristics, e.g., number of strokes per minute, contact time under pressure, and velocity under pressure, and (iii) characteristics for accuracy, e.g., deflection of the ram and frame, particularly under off-centre loading, and press stiffness.
Metal forming is possible in case of such metals or alloys which are sufficiently malleable and ductile. Mechanical working needs that the material can undergo ‘plastic deformation’ during its processing. Frequently, work piece material is not sufficiently malleable or ductile at ordinary room temperature, but can become so when heated. Hence, there are both hot and cold metal forming operations.
Several metal forming processes are suitable for processing large quantities (i.e., bulk) of material, and their suitability depends not only upon the shape and size control of the product but also uponthe surface finish produced. There are several different metal forming processes and some processes yield a better geometry (i.e., shape and size) and surface-finish than some others. However, these are not comparable to what can be achieved by machining processes. Also, cold working metal forming processes result in better shape, size, and surface finish as compared to hot working processes. Hot working results in oxidation and decarburization of the surface, formation of scales, and lack of size control because of contraction of the work-piece while it cools to room temperature.
Apart from higher productivity, mechanical working processes have certain other advantages over other manufacturing processes. These are (i) It improves the mechanical properties of material like ultimate tensile strength, wear resistance, hardness, and yield point while it lowers ductility (this phenomenon is called ‘strain hardening’), and (ii) it results in grain flow lines being developed in the part being mechanically worked. The grain flow improves the strength against fracture when the part is in actual use. This is better explained by taking example of a crank-shaft. If the crank-shaft is manufactured by machining from a bar of large cross-section, the grain flow lines get cut at bends whereas in a crankshaft which is shaped by forging (a mechanical working process), the grain flow lines follow the full contour of the crankshaft making it stronger. This is shown in Fig 8.
Fig 8 Comparison of grain flow
During mechanical working, the grains of the metal get deformed and lengthen in the direction of metal flow. Hence, the grains offer more resistance to fracture across them. Because of this, mechanically worked components have better mechanical strength in a certain orientation, i.e., across the grain flow.
A common way of classifying metal forming processes is to consider cold (room temperature) and hot (above recrystallization temperature) forming. Majority of the materials behave differently under different temperature conditions. Normally, the yield stress of a metal increases with increasing strain (or deformation) during cold forming and with increasing strain rate (or deformation rate) during hot forming. However, the general principles governing the forming of metals at different temperatures are basically the same, hence, classification of forming processes based on initial material temperature does not contribute a big deal to the understanding and improvement of these processes. In fact, tool design, machinery, automation, part handling, and lubrication concepts can be best considered by means of a classification based not on temperature, but rather on specific input and output geometries and material and production rate conditions.
Complex geometries, in both bulk and sheet forming processes, can be achieved equally well by hot or cold forming. Of course, because of the lower yield strength of the deforming material at high temperatures, tool stresses and machine loads are, in a relative sense, lower in hot forming than in cold forming.
Difference between hot working and cold working – Cold working (or cold forming, as it is sometimes called) can be defined as plastic deformation of metals and alloys at a temperature below the recrystallization temperature for that metal or alloy. When this happens, then the strain hardening which occurs as a result of mechanical working, does not get relieved. In-fact, as the metal or alloys gets progressively strain hardened, more and more force are needed to cause further plastic deformation. After sometime, if the effect of strain hardening is not removed, the forces applied to cause plastic deformation can in-fact cause cracking and failure of material.
Hot working can be explained as plastic deformation of metals and alloys at such a temperature at which recovery and recrystallization take place simultaneously with the strain hardening. Such a temperature is above recrystallisation temperature. Typical hot working temperatures are (i) 650 deg C to 1,050 deg C for steel, (ii) 600 deg C to 950 deg C for copper and alloys, and (iii) 350 deg C to 485 deg C for aluminum and alloys. Properly done hot working leaves the metal or alloy in a fine-grained recrystallized structure. Here, the recrystallization temperature is not a fixed temperature but is actually a temperature range. Its value depends upon several factors. Some of the important factors are given below.
The first is nature of metal or alloy. Recrystallization temperature is normally lower for pure metals and higher for alloys. For pure metals, recrystallisation temperature is roughly one third of its melting point and for alloys it is around of the melting temperature. The second is the quantity of cold work already done. The recrystallization temperature is lowered as the quantity of strain-hardening done on the work piece increases. The third is strain-rate. Higher the rate of strain hardening, lower is the recrystallization temperature. For mild steel, recrystallization temperature range can be taken as 550 deg C to 650 deg C. Recrystallization temperature of low melting point metals like lead, zinc, and tin can be taken as room temperature. The effects of strain hardening can be removed by annealing above the recrystallization temperature.
Advantages and disadvantages of cold working and hot working processes – Since cold working is practically done at room temperature, no oxidation or tarnishing of surface takes place. No scale formation is there, hence, there is no material loss. In hot working opposite is true. Besides, in case of steel, hot working also results in partial decarburization of the work-piece surface as carbon gets oxidized as CO2 (carbon di-oxidize). Cold working results in better dimensional accuracy and a bright surface. Cold rolled steel bars are hence called bright bars, while those produced by hot rolling process are called black bars since they appear greyish black because of the oxidation of surface.
In cold working, heavy work hardening occurs which improves the strength and hardness of bars, but it also means that high forces are needed for deformation increasing energy consumption. In hot working this is not so. Because of limited ductility at room temperature, production of complex shapes is not possible by cold working processes.
Severe internal stresses are induced in the metal during cold working. If these stresses are not relieved, the component produced can fail prematurely in service. In hot working, there are no residual internal stresses and the mechanically worked structure is better than that produced by cold working. The strength of materials reduces at high temperature. Its malleability and ductility improve at high temperatures. Hence low-capacity equipment is needed for hot working processes. The forces on the working tools also reduce in case of hot working processes.
Sometimes, blow holes and internal porosities are removed by welding action at high temperatures during hot working. Non-metallic inclusions within the work-piece are broken up. Metallic and non-metallic segregations are also reduced or eliminated in hot working since diffusion is promoted at high temperatures making the composition across the entire cross-section more uniform.
Brief description of forming operations – Some of the forming operations are described briefly below.
Rolling – Rolling process is one of the most important and widely used industrial metal forming operations. It provides high production and close control of the final product. It was developed in late 1500s. It accounts for 90 % of all metal products produced by metal working processes. It is a compressive deformation process. Rolling process is plastically deformation of metal passing it between rolls. Rolling is defined as the reduction of the cross-sectional area of the work-piece being rolled, or the general shaping of the metal products, through the use of the rotating rolls.
In rolling, a squeezing type of deformation is carried out by using two work rolls rotating in opposite directions. The principal advantage of rolling lies in its ability to produce desired shapes from relatively large pieces of metals at very high speeds in a somewhat continuous manner. Since other methods of metal-working, such as forging, are relatively slow, majority of large blooms are rolled into billets, bars, structural shapes, wire rods (for drawing into wire), and rounds (for making seamless tubing). Metal slabs are rolled into plate, strip, and sheet.
Rolling of metal is forming process in which metal is passed through a pair of rotating rolls for plastic deformation of the metal. Plastic deformation is caused by the compressive forces applied through the rotating rolls. High compressive stresses are as a result of the friction between the rolls and the metal stock surface. The metal material gets squeezed between the pair of rolls, as a result of which the thickness gets reduced and the length gets increased. Rolling is classified as per the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling.
The process of rolling is a specialized form of metal forming for shaping large bulk material into thin sheets, plates, or different types of cross-sections such as rounds, flats, squares, angles, channels, T-bars, rails, and beams etc. of large lengths. The rolling operation is to ensure the final shape geometry of the work piece being rolled, the uniformity of the material, and the change in property because of the deformation process. Fig 9 shows the concept of rolling of metals.
Fig 9 Concept of rolling of metals
Majority of the metal rolling operations are similar in that the work-piece is plastically deformed by compressive forces between two constantly spinning rolls. These forces act to reduce the thickness of the metal and affect its grain structure. The reduction in thickness can be measured by the difference in thickness before and after the reduction, this value is called the draft. In addition to reducing the thickness of the work-piece, the rolls also act to feed the metal as they spin in opposite directions to each other. Friction is hence a necessary part of the rolling operation, but too much friction can be detrimental for a variety of reasons. It is necessary that in a metal rolling process the level of friction between the rolls and workpiece is controlled. Use of lubricants can help with this.
During a metal rolling operation, the geometric shape of the work is changed but its volume remains necessarily the same. The roll zone is the area over which the rolls act on the material. It is the place where the plastic deformation of the work-piece occurs. An important factor in metal rolling is that because of the conservation of the volume of the material with the reduction in thickness, the metal leaving the roll zone is moving faster than the metal entering the roll zone. The rolls themselves rotate at a constant speed, hence at some point in the roll zone the surface velocity of the rolls and that of the material are exactly the same. This point is known as the no slip point. Before this point the rolls are moving faster than the work-piece, after this point the work-piece is moving faster than the rolls. Fig 10 shows the metal rolling concept.
Fig 10 Metal rolling concept
Forging – Metal forging is a bulk forming process in which the work-piece or billet is shaped into finished product by the application of compressive and tensile forces with the help of a pair of tools called die and punch. It is a deformation process where metal is pressed, pounded, or squeezed under large pressure into high strength products known as metal forgings. The forging process is entirely different from the casting (or foundry) process, as metal used to make forged parts is neither melted nor poured as in the casting process.
Forging is defined as a metal working process in which the specific shape of metal work-piece is achieved in solid state by compressive forces applied through the use of dies and tools. During the forging process, controlled deformation of metal takes place. Forging process is accomplished by hammering or pressing the metal. In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics, or steam.
All the metals and alloys are forgeable, but the forgeability rating of different metals and alloys can vary from high to low or poor. The factors involved are the composition, crystal structure and mechanical properties all considered within a temperature range. The wider the temperature range, the higher the forgeability rating. Majority of the forging is done on heated work-pieces. Cold forging can also take place at the room temperatures.
Forging can be done in open dies or closed dies. Open die forging is normally used for preliminary shaping of raw materials into a form suitable for subsequent forming or machining. Open die forging is done using a pair of flat faced dies for operations such as drawing out, and thinning, etc. Closed die forging is performed by squeezing the raw material called billet inside the cavity formed between a pair of shaped dies. Forged products achieve the shape of the die cavity. Valve parts, pump parts, small gears, and connecting rods, spanners etc. are produced by closed die forging. Fig 11 shows open die and closed die forging.
Fig 11 Open die and closed die forging
Coining is the process of applying compressive stress on surface of the raw material in order to impart special shapes on to the surface from the embossing punch – e.g. coins, and medallions.
Extrusion process – Extrusion is a compressive deformation process in which a block of metal is squeezed through an orifice or die opening in order to get a reduction in diameter and increase in length of the metal block. The resultant product has the desired cross-section. Extrusion involves forming of axi-symmetric (symmetrical about an axis) products. Dies of circular on non-circular cross-section are used for extrusion. Extrusion normally involves high forming forces. Large hydrostatic stress in extrusion helps in the process by improving the ductility of the material.
Metals like aluminum, which are easily workable, can be extruded at room temperature. Other difficult to work metals are normally hot extruded or warm extruded. Both circular and non-circular products can be achieved by extrusion. Channels, angles, rods, window frames, door frames, pipes tubes, and aluminium fins are some of the extruded products. Difficult to form materials such as steels, nickel alloys are extruded because of inherent advantage of the extrusion process, such as, no surface cracking because of reaction between the billet and the extrusion container takes place. Extrusion results in better grain structure, better accuracy, and surface finish of the components. Less wastage of material in extrusion is another attractive feature of the extrusion process.
Extrusion, in the majority of cases, is a hot working operation but can also be carried out in cold mode. The hot extrusion process is used to produce metal products of constant cross section, such as bars, solid and hollow sections, pipes and tubes, wires, and strips, from materials which cannot be formed by cold extrusion. Three basic categories of hot extrusion are non-lubricated, lubricated, and hydrostatic. These are schematically represented in Fig 12.
Fig 12 Three basic categories of hot extrusion process
There are several different methods of extrusion but a characterization is frequently made with respect to the direction of the extrusion relative to the ram. Extrusion is classified normally into four types. They are (i) direct or forward extrusion, (ii) indirect or backward extrusion, (iii) impact extrusion, and (iv) hydrostatic extrusion. Fig 13 shows direct and indirect extrusion and idealized ram displacement load curves.
Fig 13 Direct and indirect extrusion and idealized ram displacement load curves
Drawing process – Drawing of wire from rod is a metal working process used for the reduction of the cross-section of the rod. Similarly, rods are drawn from metal rounds of larger diameters. During drawing, the volume remains the same and hence there is increased in the length of the drawn wire or rod. It is carried out by pulling the wire / rod through a single or a series of the drawing dies. In the case of series of drawing dies, the subsequent drawing die is to have smaller bore diameter than the previous drawing die. Drawing is normally performed in round sections at room temperature. Hence, it is classified as a cold working process. However, it can be performed at higher temperatures for large diameter rods to reduce forces.
Drawing process (Fig 14) is normally most frequently used for producing round cross sections, but squares and other shapes can also be drawn. In the process of drawing, the cross section of a long rod or wire is reduced or changed by pulling (hence the term drawing) it through a die called a draw die. Pulling of rod through the die is done by means of a tensile force applied to the exit side of the die. The plastic flow is caused by compression force, arising from the reaction of the metal with the die.
Fig 14 Drawing process
The difference between drawing and extrusion is that in extrusion the material is pushed through a die, whereas in drawing it is pulled through it. Although the presence of tensile stresses is obvious in drawing, compression also plays a considerable role since the metal is squeezed down as it passes through the die opening. For this reason, the deformation which occurs in drawing is sometimes stated to as indirect compression.
The major processing variables in drawing are reduction in cross-sectional area, die angle, friction along the die-work piece interface, and drawing speed. The die angle influences the drawing force and the quality of the drawn product.
The basic difference between drawing of rod and wire drawing is the size of the starting material which is processed. Bar drawing is the term used for drawing of rods from the steel rounds, while wire drawing applies to drawing of wires from steel wire rods. Wire sizes down to 0.03 mm are possible in wire drawing.
Sheet forming – sheet forming comprises deformation processes in which a metal blank is shaped by tools or dies, primarily under the action of tensile stresses. The design and control of such processes depend on the characteristics of the work-piece material, the conditions at the tool / work-piece interface, the mechanics of plastic deformation (metal flow), the equipment used, and the finished-product needs. These factors influence the selection of tool geometry and material as well as processing conditions (work-piece and tooling temperatures, and lubrication etc.). Because of the complexity of several sheet-forming operations, models of different types, such as analytic, physical, or numerical models are frequently relied on to design such processes.
Sheet forming frequently involves local deformation. During sheet forming, a piece of sheet metal is plastically deformed by tensile loads into a three-dimensional shape, frequently without a considerable change in its thickness or surface characteristics. The characteristics of sheet-metal-forming processes are (i) the work-piece is a sheet or a part fabricated from a sheet, (ii) the deformation normally causes considerable changes in the shape, but not necessarily the cross-sectional area, of the sheet, and (iii) in some cases, the magnitudes of the plastic and the elastic (recoverable) deformations are comparable, hence, elastic recovery or spring-back can be considerable.
Examples of sheet-forming processes include deep drawing, stretching, bending, rubber-pad forming, and other methods (Fig 15). Sheet-forming methods can also be classified as per suitable methods in getting the desired dimensional features, such as surface contours or deep recesses. Some methods of local deformation also extend beyond sheet forming to the bending and forming of solid sections and tubular products.
Fig 15 General classification of sheet forming processes
Deep drawing – Deep drawing is a sheet metal process the process in which a sheet metal is forced into cup of hollow shape without altering its thickness by using tensile and compressive forces. Complex shapes can be produced by deep drawing of blanks in stages such as redrawing, multiple draw deep drawing etc.
Deep drawing is a sheet forming process in which in its simplest form, a cylindrical shape or alike is produced from a thin disc of sheet metal by subjecting it to a compressive force (while it is held between a die and blank holder) through a circular punch which mainly work on the blank thickness. Deep drawing process is used to produce containers from flat circular blanks. The central portion of sheet of blank is subjected to pressure applied by punch into a die opening to get a sheet metal of needed shape without folding the corners. This normally needs the use of presses normally having a double action for blank holding force and punch force. The mechanics of deep drawing of a conical cup are shown in Fig 16, which shows the complexity of the process.
Fig 16 Mechanics of deep drawing of a conical cup
Deep drawing process involves several types of forces and deformation modes, such as tension in the wall and the bottom, compression and friction in the flange, bending at the die radius, and straightening in the die wall. The process is capable of forming beverage cans, sinks, cooking pots, ammunition shell containers, pressure vessels, and auto body panels and parts etc.
Deep drawing can be defined as the combined tensile and compression deformation of a sheet to form a hollow body, without an intentional change in sheet thickness. It is one of the frequently applied methods in sheet metal forming. Deep drawing operation is based on producing engineering parts with specific shapes through major plastic deformation of flat metal sheets. An external force on a metal sheet does this plastic deformation. This external force has to be large enough to place the material in the plastic zone and to ensure that after displacing the external force, the metal part does not spring back or elastic deform again. The final quality of the parts produced through this operation is based on the final wall thickness and being wrinkle-free and fracture-free.
The term deep drawing implies that some drawing-in of the flange metal occurs and that the formed parts are deeper than can be achieved by simply stretching the metal over a die. Clearance between the male punch and the female die is closely controlled to minimize the free span so that there is no wrinkling of the sidewall. This clearance is sufficient to prevent ironing of the metal being drawn into the side-wall of the drawn part. If ironing of the walls is to be part of the process, it is done in operations subsequent to deep drawing. Suitable radii in the punch bottom to side edge, as well as the approach to the die opening, are necessary to allow the sheet metal to be formed without tearing.
In the majority of the deep drawing operations, the part has a solid bottom to form a container and a retained flange which is trimmed later in the processing. In some cases, the cup shape is fully drawn into the female die cavity, and a straight-wall cup shape is ejected through the die opening. For controlling the flange area and to prevent wrinkling, a hold-down force is applied to the blank to keep it in contact with the upper surface of the die. Presses used for deep-drawing operations can be either hydraulic or mechanical, but hydraulic presses are preferred because of better control of the rate of punch travel.
Hydro mechanical deep drawing uses both punch force and hydrostatic force of a pressurized fluid for achieving the shape. Flanges and collars are formed by flanging process. Spinning transforms a sheet metal into a hollow shape by compressive and tensile stresses. Spinning mandrel of given shape is used against a roll head. Embossing imparts an impression on the work-piece by means of an embossing punch. Bending of sheets includes rotary bending, swivel bending, roll bending using rotary die. Die bending using flat die or shaped die is used for bending of sheets, or die coining of sheets.
Deep drawing of sheet metal is used to form parts by a process in which a flat blank is constrained by a blank-holder while the central portion of the sheet is pushed into a die opening with a punch to draw the metal into the desired shape without causing wrinkles or splits in the drawn part. This normally needs the use of presses having a double action for hold-down force and punch force.
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