Forming Processes Course Notes PDF

Manufacturing Processes 2024-2025 Course notes Block III. Forming processes EPIG Degrees Block III: Forming processes Contents CONTENTS Contents...............................................

Manufacturing Processes 2024-2025 Course notes Block III. Forming processes EPIG Degrees Block III: Forming processes Contents CONTENTS Contents......................................................................................................................................... i 12 Fundamentals of forming processes................................................................................. 1 12.1 Introduction.................................................................................................................. 1 12.1.1 Historical Introduction.......................................................................................... 1 12.1.2 Concept of plastic deformation............................................................................. 1 12.1.3 Classification of plastic-deformation processes.................................................... 2 12.1.4 Products manufactured by plastic-deformation processes.................................. 2 12.2 Plastic deformation of metals....................................................................................... 3 12.2.1 Structure of polycrystalline metals....................................................................... 3 12.2.2 Strain hardening.................................................................................................... 4 12.2.3 Recrystallization.................................................................................................... 5 12.2.4 Characteristics of plastic deformation of polycrystalline metals.......................... 6 12.3 Characterization of the mechanical properties of the material.................................... 8 12.3.1 Stress-strain curves............................................................................................... 8 12.3.2 Nominal stress vs. nominal strain......................................................................... 8 12.3.3 True stress vs. True strain................................................................................... 10 12.3.4 Plastic behavior equations.................................................................................. 11 12.3.5 Flow (yield) criteria.............................................................................................. 12 12.3.6 Other tests for characterizing the plastic behavior............................................. 13 12.4 Influence of temperature on deformation forming.................................................... 14 12.4.1 Cold work............................................................................................................ 14 12.4.2 Hot work.............................................................................................................. 14 13 Metal rolling.................................................................................................................... 17 13.1 Introduction................................................................................................................ 17 13.1.1 Historical perspective.......................................................................................... 17 13.1.2 Basic concepts..................................................................................................... 17 13.1.3 Types of rolling and starting material................................................................. 18 13.2 Fundamentals of flat rolling........................................................................................ 19 13.2.1 Process description............................................................................................. 19 13.2.2 Reduction of rolling force.................................................................................... 20 13.3 Equipment in flat rolling.............................................................................................. 21 13.3.1 Rolls..................................................................................................................... 21 13.3.2 Rolling stands (rolling mills)................................................................................ 21 13.3.3 Types of rolling stands......................................................................................... 22 13.3.4 Tandem mill (Rolling train).................................................................................. 23 13.3.5 Hot rolling lines................................................................................................... 23 13.3.6 Cold rolling lines.................................................................................................. 24 i Block III: Forming processes Contents 13.4 Flat rolling capabilities and defects............................................................................. 25 13.4.1 Geometrical quality............................................................................................. 25 13.4.2 Defects................................................................................................................. 25 13.4.3 Residual stresses................................................................................................. 26 13.5 Other rolling processes............................................................................................... 26 13.5.1 Shape rolling........................................................................................................ 26 13.5.2 Thread rolling...................................................................................................... 27 13.5.3 Ring rolling........................................................................................................... 27 13.5.4 Rotary tube piercing............................................................................................ 28 13.5.5 Rolling of near-net shape parts........................................................................... 28 14 Metal extrusion and metal drawing................................................................................ 29 14.1 Introduction................................................................................................................ 29 14.1.1 Common aspects and differences....................................................................... 29 14.1.2 Historical perspective.......................................................................................... 29 14.2 Metal extrusion........................................................................................................... 31 14.2.1 Process description............................................................................................. 31 14.2.2 Materials for extrusion........................................................................................ 32 14.2.3 Material flow during extrusion............................................................................ 32 14.2.4 Classification of extrusion processes.................................................................. 33 14.2.5 Equipment for metal extrusion........................................................................... 35 14.2.6 Stages in the extrusion of aluminum.................................................................. 37 14.2.7 Defects in extrusion............................................................................................. 38 14.3 Metal drawing............................................................................................................. 40 14.3.1 Process description............................................................................................. 40 14.3.2 Drawing force and power requirement.............................................................. 40 14.3.3 Equipment used in metal drawing...................................................................... 40 14.3.4 Defects................................................................................................................. 42 15 Metal forging................................................................................................................... 43 15.1 Introduction................................................................................................................ 43 15.1.1 Basic concepts..................................................................................................... 43 15.1.2 Forgeability.......................................................................................................... 43 15.1.3 Historical perspective.......................................................................................... 44 15.2 Open die forging.......................................................................................................... 45 15.2.1 Fundamentals...................................................................................................... 45 15.2.2 Variants of open die forging................................................................................ 45 15.2.3 Open die forgings................................................................................................ 46 15.3 Closed die forging........................................................................................................ 47 15.3.1 Fundamentals...................................................................................................... 47 15.3.2 Closed die forging (with flash) and flashless forging........................................... 47 ii Block III: Forming processes Contents 15.3.3 Stages in closed die forging................................................................................. 48 15.3.4 Defects in closed die forging............................................................................... 49 15.3.5 Closed die forgings.............................................................................................. 50 15.4 Equipment................................................................................................................... 50 15.4.1 Forging dies......................................................................................................... 50 15.5 Process variations........................................................................................................ 53 15.5.1 Coining................................................................................................................. 53 15.5.2 Heading............................................................................................................... 53 15.5.3 Rotary swaging (rotary forging).......................................................................... 54 16 Sheet metal bending processes...................................................................................... 55 16.1 Introduction................................................................................................................ 55 16.2 Sheet metal bending................................................................................................... 56 16.2.1 Sheet metal behavior.......................................................................................... 57 16.2.2 Conditions for sheet metal bending.................................................................... 57 16.2.3 Calculation of the blank length........................................................................... 59 16.2.4 Bend radius.......................................................................................................... 60 16.2.5 Other considerations related to the design........................................................ 61 16.2.6 Springback (elastic recovery).............................................................................. 62 16.2.7 Bending force...................................................................................................... 63 16.2.8 Tools for bending................................................................................................. 64 16.2.9 The press brake................................................................................................... 65 16.2.10 The bending sequence.................................................................................... 66 16.3 Roll forming................................................................................................................. 66 16.3.1 Process description............................................................................................. 66 16.3.2 Profiled products................................................................................................. 67 16.3.3 Equipment and tools........................................................................................... 67 16.3.4 Industrial process................................................................................................ 69 16.3.5 Problems and Defects......................................................................................... 69 16.4 Roll bending................................................................................................................. 70 16.5 Tube bending............................................................................................................... 70 17 Other forming processes................................................................................................. 73 17.1 Introduction................................................................................................................ 73 17.2 Deep drawing.............................................................................................................. 73 17.2.1 Process parameters............................................................................................. 74 17.2.2 Stress state in deep drawing............................................................................... 74 17.2.3 Defects in drawing............................................................................................... 75 17.3 Hydroforming.............................................................................................................. 76 17.3.1 Sheet metal hydroforming.................................................................................. 76 17.3.2 Tube hydroforming.............................................................................................. 77 iii Block III: Forming processes Contents 17.4 Spinning....................................................................................................................... 78 17.5 Shearing and combined processes.............................................................................. 79 17.5.1 Introduction......................................................................................................... 79 17.5.2 Shear cutting....................................................................................................... 79 17.5.3 Die cutting........................................................................................................... 80 17.5.4 Stamping.............................................................................................................. 82 17.5.5 Punching.............................................................................................................. 83 iv Block III: Forming processes 12. Fundamentals of forming processes 12 Fundamentals of forming processes 12.1 Introduction 12.1.1 Historical Introduction Forming processes were the first processes used to shape and improve the properties of metal parts obtained in casting processes. It is estimated that around 9500 BC, the forging of bronze began, by hitting the metal with stones until it became a sheet. Since then, blacksmiths made pieces and weapons with greater mechanical resistance and hardness, taking advantage of plastic deformation processes. Metalworking began with cold working, but soon moved on to hot working, in which the metal was worked with less effort. Today, certain products are still being manufactured using these ancient techniques of craftsmanship (Figure 12.1), although with greater knowledge and control of the processes. Figure 12.1: Forging by a blacksmith. 12.1.2 Concept of plastic deformation From a conceptual point of view, plastic deformation processes can be analyzed with a simple example, such as the behavior of a clip wire. If the force or stress exerted to bend the clip is plotted against the deformation (angle) caused in the clip, two clearly differentiated zones can be observed (Figure 12.2). When the force is low, the wire deforms, but as soon as force ceases, the wire recovers its original form. This is the Elastic zone. If the so-called elastic limit of the wire is exceeded, by applying a greater force, a permanent deformation is achieved in the wire, even when the force ceases. The Plastic zone of the material has been reached. However, even if the Plastic zone has been reached, part of the deformation is always recovered when the force ceases, phenomenon known as springback. Stress Plastic zone Elastic zone Strain Figure 12.2: Concept of elasticity and plasticity. 1 Block III: Forming processes 12. Fundamentals of forming processes Therefore, the objective of a plastic-deformation process is to apply a stress that exceeds the elastic limit, so that the piece enters the plastic field and produces permanent deformations in the piece. 12.1.3 Classification of plastic-deformation processes Plastic-deformation processes can be classified according to the type of stress that is applied: tension, compression, bending, torsion, shearing, or combinations of these basic stresses. Thus, rolling, extrusion and forging are processes in which compression stresses predominate, drawing is dominated by tensile stresses, sheet metal bending is dominated by bending stresses, punching by shear stresses, and roll forming and deep drawing are more complex, since different type of stresses are combined (Figure 12.3). Rolling Extrusion Drawing Forging Bending Roll forming Deep drawing Punching Figure 12.3: Several plastic-deformation processes. 12.1.4 Products manufactured by plastic-deformation processes Most of the metal components present in the products used in our daily lives have been subjected to one or more plastic-deformation processes. However, most of the products will need subsequent processes, so they are considered as semi-finished products (Figure 12.4). Bars Profiles Tubes Plates Thick plate Sheet metal Wire Solid Parts Hollow Parts Figure 12.4: Examples of products manufactured by plastic-deformation processes. 2 Block III: Forming processes 12. Fundamentals of forming processes For example, bars of various sections (circular, square, hexagonal, etc.) and sizes can be manufactured by processes like rolling, extrusion or drawing. These bars are then transformed into shafts for all kinds of mechanisms and applications, normally by machining processes. Other typical products are metal profiles and tubes, which are manufactured by extrusion, rolling or roll forming. Sheet metal, produced by rolling, is another very versatile product, both in its thick version, used in boiler making, and in its thin version, used for household appliances, automobiles, fences, etc. Wire is also a product obtained by combining plastic deformation processes, first rolling to produce wire rod, and then drawing to produce wire of different gages (sizes). Finally, it is worth highlighting various types of discrete parts, either solid or hollow, to which plastic-deformation processes provide not only their basic shape but also mechanical properties that are far superior to those of casting processes. 12.2 Plastic deformation of metals 12.2.1 Structure of polycrystalline metals During the solidification of liquid metal, the atoms are arranged orderly in structures called crystals. At the atomic level, there are different possible packing configurations, depending on how the atoms are arranged (body-centered cubic, face-centered cubic, hexagonal close- packed, etc.). In a polycrystalline metal, the crystals are generated independently of each other, so they have random orientations that are not related to each other. The solidification process begins simultaneously at several points, each with a different orientation. The growth of the crystals gives rise to crystalline structures called grains, which may consist of a single crystal, in the case of pure metals, or of a polycrystalline aggregation in metal alloys. When these structures are studied under a microscope, typical images of microstructures with grain boundaries and random structures appear (Figure 12.5). Figure 12.5: Internal structure of metals. To understand plastic deformation, it will be first studied on an ordered single crystal structure. The stresses that produce deformation in these structures are shear stresses; tension, compression and bending stresses only contribute to deformation in terms of the shear stress they generate. 3 Block III: Forming processes 12. Fundamentals of forming processes When a crystal is subjected to an external force, it undergoes elastic deformation, which means that once the external force is removed, the crystal will return to its original shape. If the force is increased above a certain value, the deformation becomes plastic or permanent, and when the force is removed only the elastic deformation (springback) is recovered. Plastic deformation occurs through different mechanisms, but the most important one involves the sliding of one atomic plane over another under a certain shear stress. This shear stress causes the gliding of atomic blocks along slip planes, as can be seen in Figure 12.6. Slip plane Plano Cizallado o Deslizamiento (a) Single crystal structure (b) Elastic deformation (c) Plastic deformation Planos de Slip Slip planes Deslizamiento planes Slip steps (d) Tension (e) Compression (f) Bending Figure 12.6: Mechanisms of deformation in single crystal metals. However, the actual shear stress required for causing a plastic deformation is much lower than expected. This is due to the existence of defects in the crystalline structure, which makes the sliding of atomic blocks easier. Among the defects that aid in the plastic deformation, the role of dislocations stands out. 12.2.2 Strain hardening Dislocations are defects in the arrangement of the atomic structure of the metal. Figure 12.7a shows a type of dislocation, edge dislocation, where there is an additional column of atoms in the lattice. The movement of the dislocation across the crystal lattice under a shear stress can be seen in Figure 12.7b. (a) Edge dislocation (b) Movement of the edge dislocation across the crystal lattice Figure 12.7: Dislocations and movements. However, as the material deforms, dislocations can become entangled and interfere with each other (Figure 12.8d), and they can also be blocked by other mechanisms (Figure 12.8a, b, and c). Thus, as the material deforms and dislocation slippage becomes progressively impeded, a constantly increasing stress is required to overcome the aforementioned barriers and for the deformation of the part to continue. 4 Block III: Forming processes 12. Fundamentals of forming processes (a) Solid solution strengthening (b) Precipitation hardening (c) Grain boundary hardening (d) Work (strain) hardening Figure 12.8: Strengthening mechanisms during plastic deformation. The number of dislocations also increases with plastic deformation. Thus, the greater the deformation of the material, the greater the impediments to the movement of dislocations and the greater the strength of the material. This phenomenon is known as strain hardening or work hardening, and it has two opposing effects. On the one hand, greater strength implies an increase in the stresses to be applied and greater manufacturing difficulty. On the other hand, the material resulting from the deformation has improved strength with respect to the original material, which is a positive aspect that is used to produce much stronger parts. In a polycrystalline metal, there are numerous grains with different crystallographic orientations. Therefore, when stress is applied, only some grains will have an orientation that is favorable to the slip planes. The deformation will begin with these grains, but to maintain the continuity of the metal, it also causes the deformation of the adjacent grains, which will require the activation of other slip mechanisms. This implies that the stress to deform a polycrystalline structure will be much higher than that expected for a correctly oriented single crystal. 12.2.3 Recrystallization The energy applied during plastic deformation is largely converted into heat. This energy is stored in the form of heat until the so-called activation energy is reached, at which point the defects in the crystalline structure are repaired and a rearrangement occurs. For example, dislocations begin to cancel each other out as they move due to internal stresses, and new grain boundaries begin to form within the grains themselves, becoming organized and growing until a new crystalline structure is generated. These changes at the microscopic level lead to changes in the macroscopic behavior of the material. Thus, the internal residual stresses of the material are reduced, while the mechanical properties hardly vary. If the temperature continues to increase, new grains nucleate producing a complete reorganization of the microstructure, with the original microstructure disappearing. These changes do reduce the mechanical strength and hardness acquired during the plastic deformation process, while the reorganization increases the ductility of the material. This recrystallization process can be forced a heat treatment called annealing (Figure 12.9), in which the part temperature is raised to 30%-50% of the melting point of the material. 5 Block III: Forming processes 12. Fundamentals of forming processes Annealing Residual stresses Strength Ductility Strength, hardness, ductility Hardness Cold-worked (and recovered) New grains Grain size Recrystal- Grain Recovery lization growth ( 0.3 ~ 0.5Tm )​ Temperature Figure 12.9: Recovery and Recrystallization To fix a crystalline microstructure with small grains, recovery and recrystallization must be avoided by rapidly cooling the part, for example, by tempering. A note on residual stresses, which have been commented previously. Residual stresses are stresses that remain in the material when the external loads that produced them cease. They have to do with the manufacturing process of each material. The part is in internal equilibrium and maintains its shape despite the existence of these residual stresses, as it can be seen in the rolled part of Figure 12.10. If in subsequent processes, for example machining, material is removed in some regions, the equilibrium of internal stresses will be broken and the part will deform and warp until it reaches a new equilibrium point. Figure 12.10: Internal residual stresses. 12.2.4 Characteristics of plastic deformation of polycrystalline metals When a polycrystalline metal with ideally equiaxed grains is subjected to a deformation stress, these grains deform by elongating, resulting in a deformation of the whole part. Plastic deformation does not alter the grain boundaries, and the mass is preserved, but the mechanical strength of the piece increases due to strain hardening. Analyzing the structure of a material under a microscope, as observed in Figure 12.11 (forged bolt), the deformed grains appear aligned in the direction of application of the stress. 6 Block III: Forming processes 12. Fundamentals of forming processes Forged Bolt Cast Bolt Machined Bolt Aligned grain flow No grain flow Cut grain flow Figure 12.11: Grain flow in a bolt produced by different processes. In the case of a bolt, various manufacturing processes could have been chosen: a) Casting (molding process): the grain structure would not present any directionality, and the mechanical strength and behavior would be isotropic. b) Forging (plastic-deformation process): here the grain structure will be aligned with the direction of application of the stresses, giving rise to characteristic lines or fibers. The behavior will not be isotropic, due to this high directionality. Now the grain boundaries will not be cut, and it will present improved resistance to fatigue and crack nucleation. c) Turning (machining): the bolt would present a grain flow from the previous process (a plastic-deformation process) but cut by the material removal process. The behavior is not isotropic, as it was not isotropic in the starting material. When cutting the grains, strength is lost, as grain boundaries will remain open. Through these open grain boundaries cracks can progress, especially under cyclic stress situations (fatigue). One of the effects of this deformation is the appearance of anisotropies in the material, so that its properties vary depending on the direction of observation (Figure 12.12). The anisotropy of the material depends on the temperature at which the deformation takes place, and on its degree of uniformity. It has effects on the mechanical properties, but also on other properties, such as magnetic properties. Thus, the rolled sheets used in electrical transformers have anisotropic magnetic properties, which are used to reduce magnetic losses and improve their efficiency. Note the alignment of grain boundaries along a horizontal direction in Figure 12.12. This effect is known as preferred orientation. Figure 12.12: Anisotropy in plastic deformation. 7 Block III: Forming processes 12. Fundamentals of forming processes 12.3 Characterization of the mechanical properties of the material 12.3.1 Stress-strain curves Metals can be characterized by various physical properties, quantifiable through various tests. The most relevant mechanical properties, from the manufacturing point of view, would be strength, hardness, toughness and ductility. In the case of plastic deformation processes, the behavior of the material can be determined through stress-strain curves. The degree of deformation corresponding to a certain stress level is relevant in the field of plastic deformation because it will allow calculations of the required force and power, which will help to dimension equipment or select its working conditions. Fixed crosshead Column Test specimen Moving crosshead Table Base and actuator Figure 12.13: Uniaxial tension test. Stress-strain curves represent the relationship between the deformation stress applied to the material (characterized by stress) and the alteration experienced by the geometry of the material (characterized by strain or deformation). These curves can be obtained through various tests. The uniaxial tension test is the most widely used test, and consists of subjecting a test specimen, with a standard geometry, to a tensile force. The most widely used test geometries are cylindrical in shape, with threaded ends to join the specimen to the clamps of a testing machine. In these machines, one end remains fixed and the other moves away from the first during the test. When they move away, the tensile force generates stress inside the material, which undergoes a deformation, first elastic and then plastic. The machine simultaneously records the displacement between two marks on the specimen and the force applied. 12.3.2 Nominal stress vs. nominal strain Thus, the nominal stress can be calculated as the ratio of the applied load, 𝐹, to the original cross-sectional area, 𝐴𝑜 , of the specimen: 𝐹 𝜎𝑛 = 𝐴𝑜 The nominal strain (or elongation) is the ratio of the increase in length to the initial length: 𝐿 − 𝐿𝑜 𝑒= 𝐿𝑜 8 Block III: Forming processes 12. Fundamentals of forming processes Nominal stress, necking starts U.T.S. (Ultimate Tensile Strength) fracture Y (Yield strength, elastic limit) Elastic zone Plastic zone strain, e necking Figure 12.14: Uniaxial tensile test nominal stress–nominal strain curve. From the test, curves like the one represented on Figure 12.14 will be obtained. Initially, the test specimen is elongated and, due to the conservation of volume, the cross- sectional area is uniformly reduced. This deformation will be elastic as long as a threshold value for the nominal stress is not exceeded, which is known as the yield strength or elastic limit (𝑌 for Yield), and which depends on the material. If this limit is exceeded, a plastic deformation of the material occurs; almost all the deformation becomes permanent. During the beginning of the plastic deformation, the cross-section of the test specimen still decreases progressively until a new threshold for the nominal stress is reached, known as the Ultimate Strength. At this point, the deformation is no longer homogeneous, and a local reduction is produced at the middle of the specimen, forming a neck. The nominal stress decreases as the force required for deforming the material also decreases. If the deformation progresses, the neck reduces its section until the material fractures. From this curve, other parameters of interest regarding the mechanical behaviour of the material can be determined. Thus, the ratio between the nominal stress and the nominal deformation in the elastic zone is a constant known as the modulus of elasticity or Young's modulus (𝐸): 𝜎 𝐸= 𝑒 Similarly, the reduction in area at fracture (𝑟𝑓 ) is used as a measure of the ductility of the material, and is calculated as: 𝐴𝑜 − 𝐴𝑓 𝑟= × 100 𝐴𝑜 Finally, the stress value that corresponds to the appearance of necking in the specimen is called Ultimate Tensile Strength (𝑈𝑇𝑆). 9 Block III: Forming processes 12. Fundamentals of forming processes 12.3.3 True stress vs. True strain The use of the initial area in the calculation of the nominal stress is misleading, in the sense that it does not represent the real stress to which the material is subjected, since the elongation of the test piece implies a progressive reduction of the cross section and, therefore, the resistant section changes over time. As a consequence of this, another measure of the stress, the True Stress, denoted as σ is used , which is calculated as the ratio of the force 𝐹 with respect to the cross-sectional area of the specimen at each instant (𝐴): 𝐹 𝜎𝑛 = 𝐴 Similarly, instead of the nominal strain, the True Strain, 𝜀, is used: 𝑙 𝑑𝑙 𝑙 𝜀=∫ = ln 𝑙0 𝑙 𝑙0 True stress Fracture Plastic zone : strength coefficient : strain-hardening exponent Elastic zone Hooke’s law : modulus of elasticity or Young’s modulus True strain Figure 12.15: Uniaxial tensile test true stress – true strain curve. For small deformations, the values of the nominal and true strains are approximately equal. However, the values diverge as the deformation increases, but the true strain represents better the physical phenomena. Thus, a hypothetical compression of a piece between an initial length (𝑙) and a null final length (0) would provide an infinite value for the true strain, regardless of the value of 𝑙; on the contrary, the nominal strain would provide a meaningless value of −1. Another advantage of true strain over nominal strain is additivity. Nominal strain expresses strain as a percentage, whereas true strain is not a percentage. Thanks to the properties of Neper logarithms it is possible to evaluate the final true strain as the addition of the true strain increases of the several stages of deformation: 𝜀𝑓 = 𝜀0 + 𝜀01 + 𝜀12 + 𝜀23 + ⋯ + 𝜀𝑓−1,𝑓 𝑙1 𝑙2 𝑙3 𝑙𝑓 𝑙1 · 𝑙2 · 𝑙3 · … · 𝑙𝑓 𝜀𝑓 = 𝜀0 + ln + ln + ln + ⋯ + ln = 𝜀0 + ln = 𝜀0 + 𝜀0𝑓 𝑙0 𝑙1 𝑙2 𝑙𝑓−1 𝑙0 · 𝑙1 · 𝑙2 · … · 𝑙𝑓−1 10 Block III: Forming processes 12. Fundamentals of forming processes 12.3.4 Plastic behavior equations In the plastic zone, the relationship between the true stress (also called flow stress), 𝜎, and the true strain, 𝜀, can be modelled by the following expression developed by Hollomon: 𝜎 = 𝐾 ∙ 𝜀𝑛 where 𝐾 is the strength coefficient and 𝑛 the strain hardening exponent. These coefficients are determined from tests and are used to determine the required force and power in plastic deformation operations. Table 12.1 collects these coefficients for various materials, and they have also been graphed in Figure 12.16. Table 12.1: Hollomon coefficients of various materials. Source: Manufacturing, Engineering and Technology. 𝑲 𝒏 Pure Annealed Aluminum 175 0,2 Annealed Aluminum Alloy 240 0,15 Heat Treated Aluminum Alloy 400 0,1 Pure Annealed Copper 300 0,5 Bronze (Copper Alloy) 700 0,35 Annealed low carbon steel 500 0,25 High Carbon Steel 850 0,15 Alloy Steel, Annealed 700 0.15 Stainless Steel, Austenitic 1200 0,4 1200 Stainless steel 1000 (austenitic) High-Carbon steel 800 True stress [Mpa] Alloyed steel, annealed 600 Bronze Low-Carbon steel, annealed Aluminium alloy, 400 heat-treated Copper, annealed Aluminium alloy, 200 annealed Aluminium, annealed 0 0 0,2 0,4 0,6 0,8 1 True strain Figure 12.16: Plastic behavior of different materials. The values obtained in the test and the coefficients 𝐾 and 𝑛 derived from them are valid for a given temperature, which must be constant during the test. However, mechanical properties of metals are modified by temperature: the ductility of the material increases with temperature, while its strength and strain hardening decreases. Sometimes the elastic and plastic behavior of the material will be ideally approximated by a straight line. As it can be seen in Figure 12.17, three cases are possible: perfectly elastic, elastic and perfectly plastic, and elastic with strain hardening. 11 Block III: Forming processes 12. Fundamentals of forming processes σ σ σ ε ε ε Perfectly elastic Elastic and perfectly plastic Elastic and strain hardening Figure 12.17: Ideal plastic behavior. 12.3.5 Flow (yield) criteria In most of the forming processes tensile stresses are not the predominant type of stresses; in many cases, compressive stresses or combined stresses are applied. On the other hand, the geometry of the workpieces may be rather different than the geometry of the specimen. Therefore, it is necessary to determine if the applied stress will cause a permanent deformation on the workpiece material. For this purpose, there are various flow criteria that can be applied: Tresca and Von Mises. The Tresca criterion is based on the maximum value of the shear stress: if the stress state in a specific point of the workpiece is such that the maximum shear stress exceeds a critical value that is dependent on the material (the Tresca constant for the material, 𝐶𝑇 ), the material will deform permanently at that point. The maximum shear stress, 𝜏𝑚𝑎𝑥 , is determined from the first and third principal stresses, 𝜎1 and 𝜎3 : 𝜎1 − 𝜎3 𝜏𝑚𝑎𝑥 = ≥ 𝐶𝑇 2 Z Y X =0 Figure 12.18: Determination of principal stresses for a generic stress state on a generic workpiece. In the Uniaxial Tensile test, there is only one tensile stress, which will be the applied force divided by the cross-sectional area. Therefore, the tensile stress is the first principal stress at any point of the test specimen. Applying the Tresca criterion: 𝐹 𝜎1 = =𝑌 𝐴 𝑌−0 𝑌 𝜏𝑚𝑎𝑥 = = ≥ 𝐶𝑇 2 2 Where Y is the flow stress determined in the Uniaxial Tensile test. Von Mises establishes a different criterion to Tresca's, based on shear strain energy. When the shear strain energy, due to a certain stress state, exceeds a critical value that depends on the workpiece material (Von Mises constant, 𝐶𝑉𝑀 ), then it is considered that the material is plastically deformed. 12 Block III: Forming processes 12. Fundamentals of forming processes The shear strain energy 𝐸 may be expressed as a function of the principal stresses, by: 1 𝐸= [(𝜎 − 𝜎2 )2 + (𝜎2 − 𝜎3 )2 + (𝜎1 − 𝜎3 )2 ] ≥ 𝐶𝑉𝑀 6⋅𝐺 1 where 𝐺 represents the modulus of rigidity of the material. Therefore, and given that 𝐺 is constant that depends on other properties of the workpiece material, it can be stated that: [(𝜎1 − 𝜎2 )2 + (𝜎2 − 𝜎3 )2 + (𝜎1 − 𝜎3 )2 ] ≥ 𝐶 ′ 𝑉𝑀 where the new constant also represents a critical value dependent on the material to be transformed. As a consequence, it will be sufficient to calculate the principal stresses corresponding to the existing stress state and check whether or not the previous expression is fulfilled. If it is, permanent deformation will occur. 12.3.6 Other tests for characterizing the plastic behavior The disadvantage of the tensile test is that the specimen breaks very quickly, without reaching large deformations. Therefore, the results obtained in the test are applicable exclusively to plastic deformation processes where the deformations are very small. To allow the measurement of larger deformations, other tests were devised, such as compression tests. In the uniaxial compression test, a cylindrical specimen is used, which is compressed by applying a load in the direction of the specimen axis. The main advantage of this test is that the specimen takes much longer to break, which allows greater deformations to be measured. The disadvantage is that, because of friction in the contact area between the tools and the material, the test specimen deforms (widens) to a greater extent in the central area than at the edges, so that it gradually acquires a barrel shape. To avoid the loss of the cylindrical shape, the process is stopped when the barrel shape appears, the test specimen is removed and turned in a lathe before being mounted again in the testing machine. This operation is repeated several times during the test, which complicates its performance. The flow stress measured in this test is the same as that measured in the uniaxial tensile test, but in compression, 𝑌. Uniaxial compression Compression test with plane strain Torsion test test Yield stress Yield stress Yield stress T T w h Figure 12.19: Other tests for material characterization. In the plane compression test (under plane strain conditions), a prismatic specimen is subjected to compressive stresses. The dimensions of the tested piece must be chosen so that in one direction (for example, the one corresponding to dimension 𝑤 in Figure 12.19) it can be assumed that the deformation is negligible compared to the deformations in the other two spatial directions, that is, in the length and thickness dimensions. Under these plane strain conditions, 13 Block III: Forming processes 12. Fundamentals of forming processes the measured flow stress will be designated by 𝑆. The results of this test are of great importance for its practical application to the analysis of plastic forming processes, since the conditions under which many of these processes are carried out can be associated with that of a plane strain state: sheet metal rolling, forging, etc. In the torsion test, a cylindrical specimen is subjected to a torque, the value of which increases until it finally breaks. In this case, the applied torque causes internal shear stresses, and the deformations measured are transverse deformations. In this test, large deformations can be measured before the specimen breaks. For this reason, the results obtained can be successfully applied to plastic forming processes where large deformations are applied, such as in hot plastic deformation processes. The flow stress measured in this test will be designated by 𝐾. The flow stresses determined in each test can be compared with each other by applying the Tresca and Von Mises flow criteria. The following equivalences are obtained: 𝑌 𝑆 𝑇𝑟𝑒𝑠𝑐𝑎: 𝜏𝑚𝑎𝑥 = = = 𝐾 = 𝐶𝑇 → 𝑌 = 𝑆 = 2𝐾 2 2 3 2 √3 𝑉𝑜𝑛 𝑀𝑖𝑠𝑒𝑠: 2 ⋅ 𝑌 2 = ⋅ 𝑆 = 6 ⋅ 𝐾 2 = 𝐶 ′ 𝑉𝑀 → 𝑌= 𝑆 → 𝑆 = 2𝐾 2 2 12.4 Influence of temperature on deformation forming 12.4.1 Cold work In cold working, plastic deformation processes are carried far below the recrystallization range of temperatures, that is: 𝑇𝑤 ≤ 0,3 ∙ 𝑇𝑓 When cold processed, the material has higher strength values, so higher forces are required to achieve the same deformation. Likewise, low ductility and high strain hardening impede to achieve large strains. Sometimes cold working requires the application of heat treatments such as annealing, to increase the material ductility. On the other hand, cold processing provides clear advantages in terms of geometrical quality (better dimensional and geometric tolerances and better surface finish), and in terms of mechanical properties (greater strength and hardness). Additionally, the high directionality of the resulting grain structures gives rise to anisotropy of their properties that, as in the case of electromagnetic ones, can be used for certain applications. Finally, by not requiring heating, the associated energy consumption is reduced. 12.4.2 Hot work In hot working, the material is deformed at temperatures above the recrystallization temperature but below the melting point, that is: 0,5 ∙ 𝑇𝑓 ≤ 𝑇𝑤 ≤ 0,75 ∙ 𝑇𝑓 The great advantage of hot working is that the strength coefficient, 𝐾, is significantly lower than in cold working, and that the strain hardening exponent, 𝑛, is theoretically zero. This means an increase in ductility, which makes it much easier to deform the material, resulting in lower stresses and less power, or in greater deformations for the same applied stress. In addition, metals that would fracture in cold deformation can be processed by hot deformation. Furthermore, since there is no directionality in the grains, the properties of the material will be isotropic. 14 Block III: Forming processes 12. Fundamentals of forming processes The disadvantages of hot working are lower dimensional and geometric accuracy, worse surface finishes, risks of surface oxidation (formation of scale in steels), the demand of more energy, worse mechanical properties (strength and hardness) in the processed material and shorter tool life. There is a third category of work, warm work, in which the material is heated to an intermediate temperature: 0,3 ∙ 𝑇𝑓 ≤ 𝑇𝑤 ≤ 0,5 ∙ 𝑇𝑓 In this working range, the forces required are lower, so that more complex geometries can be achieved and annealing can be avoided. 15 Block III: Forming processes 13. Metal rolling 13 Metal rolling 13.1 Introduction 13.1.1 Historical perspective The origin of the manual rolling process dates back to the Sumerians, in the 7th century BCE. In this case, rolls for the manual rolling of gold and precious metals for jewelry have been found. Through this process, thin sheets or small diameter rods could be created, for example, to make links for chains. The modern practice of rolling began in 1783, with a patent for the production of iron bars using grooved rolls. An engraving of such a machine can be seen in Figure 13.1. This practice experienced tremendous growth with the industrial revolution and the beginning of the manufacture of railway rails and structural I-profiles. The process benefited from the invention of the steam engine, which was replaced in the 20th century by electric motors. Figure 13.1: Manual rolling of profiles on grooved rolls. 13.1.2 Basic concepts In the rolling process, rotating rolls are used to apply a compressive force to the workpiece and cause its deformation. In the case of flat workpieces, a reduction in thickness is the main objective, while in other workpieces a modification of their cross section occurs. Flat rolling is the most widespread variant of this process, encompassing 90% of rolling production. Figure 13.2 shows a very simple equipment used for cold rolling jewelry parts. It is operated manually, using a crank to rotate the two rolls, rolls with areas that allow for rolling different cross sections. There is an area for rolling narrow sheet metal, reducing its thickness by adjusting the gap between the rolls after each pass. To do this, there are cranks on the top, which raise and lower the top roll by means of a screw. There are two other areas for profiles, in this case by means of grooves in the rolls, with a progressive succession of decreasing size. Again, it will be a process involving several stages, in which the workpiece will be rolled several times until it reaches the desired shape and dimensions. 17 Block III: Forming processes 13. Metal rolling Figure 13.2: Manual rolling mill for jewelry. The industrial process is similar, although much more expensive, requiring large capital investments and much larger equipment, which implies that its profitability depends on processing large quantities of material in the form of standardized products, such as sheet metal, bars or structural profiles. 13.1.3 Types of rolling and starting material In rolling, the starting material is the output of a continuous casting process, especially in the case of steel. Figure 13.3 shows three different products of the continuous casting of steel, and the processes (mainly rolling) to transform them into other types of products. Slab Hot strip Pickling and oiling Cold strip Skelp Welded pipe Continuous casting or ingots Sheet metal plates Plate Hot- rolled bars Cold-drawn bars Structural shapes Bloom Rods , Wire rod wire and wire products Billet Rails Tube rounds Seamless pipe Figure 13.3: Types of Rolled Products and Starting Material Currently, steel plants can be of two types: electric steelworks, that use electric furnaces for the transformation and smelting of steel; or blast furnace steelworks, such as the one that Arcelor Mittal has here in Asturias. The blast furnace is located in Gijón, where pig iron is obtained in the form of pig iron, and later in Avilés it is converted into steel and the continuous casting process is carried out. The three starting materials for rolling are slabs, blooms and billets. Slabs have a rectangular section up to 350 mm thick, can be up to 2000 mm wide and up to 12 meters long. Blooms have a square or rectangular section between 350 and 450 mm on each side. The billet is the smallest 18 Block III: Forming processes 13. Metal rolling product, usually with a square section up to 250 mm on each side. Continuous casting can also produce circular blooms for extrusion processing. By flat rolling, slabs are transformed into thick sheet metal for the naval or boiler making sectors, or into thin sheet metal for the automotive and household appliance sectors. Some of the sheet metal is also transformed by roll forming into welded tubes, or seam tubes. Blooms are mainly used to obtain structural shapes or rails for trains. The size of the billet is determined by the dimensions of the profile to be obtained. Finally, billets are rolled into steel bars, such as corrugated steel used in concrete slabs, or calibrated bars for later manufacturing shafts or other elements. Another part of the production is dedicated to the manufacture of seamless tubes, by the Mannesman method. Although the most special product could be the wire rod, which will later be used as the raw material for the production of finer wire. The range of products of the steel plants located in Asturias includes the production wire rod and rail in Gijón, thick sheet metal plates in Gijón for the wind and naval industries, and the hot strip train in Avilés, which produces galvanized sheet metal and tinplate. 13.2 Fundamentals of flat rolling 13.2.1 Process description In the flat rolling process, a rectangular cross section strip whose thickness is much lower than its width and length is introduced into the gap between two parallel rolls. The rotation of the rolls and the pressure exerted on the strip, together with the friction forces, cause a movement of the strip in the longitudinal direction, while the cross section is reduced in thickness (the width is supposed to remain practically constant). Since there is no deformation in this transverse direction, rolling is a plane strain process. The volume of material must remain constant, so the deformations are in the other two directions: decrease in thickness and increase in length. Figure 13.4: Flat rolling parameters, speeds and friction. The strip reaches the roll gap at an initial linear speed, 𝑣0 , and emerges from the roll gap at a final speed, 𝑣𝑓. At the roll gap entry, the surface speed of the strip is lower than the peripheral speed of the roll, contrary to what occurs at the exit. Consequently, there must be a point inside the roll gap in which the speeds a of the strip and the roll are equal, without relative sliding between them. This point is called the neutral point or non-slip point. The friction forces also 19 Block III: Forming processes 13. Metal rolling change direction on both sides of the neutral point, so that they favor the entry of the material into the roll gap and hinder its exit. The neutral point should be located close to the exit, so that the effect of the friction forces that cause the flow of the material along the rolling direction (before the neutral point) is greater than that of those that hinder it (after the neutral point). In any case, the exact position of the neutral point depends on each operation. Friction is a loss of energy, which means that more power must be applied to the rolling process. In addition, excessive friction can damage the surface of the product, so lubricants are used in practice to reduce it. Lubricants are a mixture of oils and water, to provide a lubricating action with the oil and a cooling action with the water, which protects the components of the machinery. The rolls are mounted on a system that allows the upper cylinders to be moved by means of a hydraulic mechanism, in order to adapt the roll gap to the required thickness. This mechanism provides the vertical load necessary to prevent the rolls from separating during rolling, so that the final thickness matches the required thickness. 13.2.2 Reduction of rolling force The force exerted by the rolls on the material must be kept to a minimum. The effects of high forces affect the geometric quality of the part, which will be affected by a bending of the roll in the transverse direction or by a flattening in the longitudinal direction. Likewise, the transmission of forces to the support columns can cause a deflection of the assembly that affects the output dimension (thickness) of the rolled product. The required force for rolling, 𝑃, can be estimated by the following expression: (𝑆0 − 𝜎𝑏 ) + (𝑆𝑓 − 𝜎𝑓 ) 𝑃 = 𝑞̅ · 𝐴 = · 𝑤 · √𝑅 · (ℎ0 − ℎ𝑓 ) 2 where 𝑞̅ is the average normal stress, 𝐴 is the surface area of the contact zone, 𝑆0 and 𝑆𝑓 are flow stress values, 𝜎𝑏 and 𝜎𝑓 are external tension stresses, 𝑤 is the width, 𝑅 is the roll radius, and ℎ0 and ℎ𝑓 are the initial and final thickness of the material. To reduce the force applied in rolling, there are several possibilities: Reduce the diameter of the rolls, which affects the surface area of the contact zone between the rolls and material. Perform rolling passes with a small difference between the input and output thickness. Increase the temperature of the material to reduce the flow stress of the material. Apply tensile stresses in the longitudinal direction, to reduce the contribution of compressive stresses to the stress required to achieve plastic deformation. Reduce friction by using lubricants. Tension roll Figure 13.5: Tensile stress caused by a tension roll. 20 Block III: Forming processes 13. Metal rolling 13.3 Equipment in flat rolling 13.3.1 Rolls Rolling rolls are the most expensive part of the equipment and the one that contributes the most to the production cost. Rolls should be designed to minimize problems of wear, surface damage and breakage. A roll consists of three parts: the body (the section that carries out the rolling and is in contact with the strip), the neck (supports the body and transmits the rolling pressure) and the journal (to join to the coupling and transmit the rotation). Neck Body Journal Figure 13.6: Roll for flat rolling. The use of rolls is conditioned by several aspects. For example, the design of the fillet radius between the neck and body must avoid stress concentration, and the design of the rolling passes must be carried out carefully to avoid overloading the rolls. The wear of the rolls is affected by their surface condition, by their hardness and rigidity, and by possible chemical interactions between the roll material and the metal to be rolled. To minimize these problems, it is recommended that the temperature of the rolls does not exceed 80 °C, for which high-pressure water jet cooling systems are used directed at the roll. 13.3.2 Rolling stands (rolling mills) Rolls are arranged in rolling stands. In the simplest configuration, two rolls are placed one on top of the other, and the rolling stand is called two-high stand. The roll necks are mounted on chocks, inside which bearings and bushings allow roll rotation. The chocks are assembled onto the columns of the housing. Two hydraulic cylinders, at the top of the housing, provide the rolling force, transmitted through the housing, chocks, bearings or bushings, to the roll necks. Crosshead Hydraulic cylinder Electric motor Gearbox Coupling Column Chock Base Foundation Figure 13.7: Parts of a rolling stand. 21 Block III: Forming processes 13. Metal rolling Each roll is driven by an independent electrical motor, and the speeds are balanced by means of a gearbox. Cardan joints are normally used for transmitting the rotation to the roll journals, as they permit regulating the roll gap. If one roll were to become blocked, it would be abraded, generating a flat area that would damage the roll and the sheet metal strip. 13.3.3 Types of rolling stands The simplest configuration is the duo or two-high stand, which uses only two work rolls and can be reversible, allowing rolling in two opposing directions, which reduces the space needed in the steel plant. The roll gap should be adjusted after each rolling pass. Another common configuration is the trio or three-high stand, which has three non-reversible rolls. The strip is rolled in one direction with the top and center rolls and then, another pass is carried out in the opposite direction with the center and bottom rolls. Both the two-high stand and the three-high stand are employed in the first stages of rolling (roughing) where greater deformations are required. The four-high has four rollers in a vertical arrangement; the two work rolls are of small diameter to reduce the rolling force and are supported by two back-up rolls of larger diameter. The function of the back-up rolls is to absorb the loads of the work rolls, minimizing the deflection or bending of the latter. Four-high stands are employed both in roughing or in finishing rolling passes. There are other configurations, such as the cluster stand (or Sendzimir), which uses several back- up rolls of progressively larger sizes, to achieve less bending and a greater precision. The usual number of rollers is 20 in this type of stand (1-2-3-4 roll arrangement on both sides of the strip). Cluster stands are employed in cold rolling of very thin sheets, in high strength materials. (a) Two-high stand (b) Three-high stand Screw or Housing hydraulic Backing bearing Driven roll mechanism Housing Bearing shaft Back-up Driven roll First intermediate roll roll Second intermediate roll Chocks Work rolls Driven roll Strip Work roll Back-up roll driven roller (c) Four-high stand (d) Cluster stand (Sendzimir) Figure 13.8: Types of rolling stands. 22 Block III: Forming processes 13. Metal rolling 13.3.4 Tandem mill (Rolling train) Rolling mill stands are arranged in series (one after the other) in a tandem mill. In tandem rolling, the metal plate is rolled continuously, so that several rolling stands act simultaneously on the strip, achieving a progressive reduction in its thickness. As the continuity equation is valid in steady-state processes, the mass flow must be kept constant, which requires perfect coordination of the speeds and the thickness achieved at each rolling stand. Coil storage Take-up reel Rolling stands Operator Controls Speeds Take-up reel 5 4 3 2 1 Thicknesses Figure 13.9: Tandem mill. 13.3.5 Hot rolling lines The rolling process does not only use rolling stands, but also requires other stages and processes to achieve a good quality of the final product. Heating furnace Slab Transfer Table Tail/head shearing Finishing Roughing mill mills (4-Hi stand) (tandem) Laminar Cooling Coiler Figure 13.10: Stages of the hot rolling process. 23 Block III: Forming processes 13. Metal rolling For example, in hot rolling (Figure 13.10), the slabs are heated in a furnace to enhance the metal ductility, thus reducing the flow stress. Then, slabs pass through a roughing stand, a two- or four- high stand that is usually reversible and reduces the thickness of the slab in successive passes from the initial 200 mm to about 30 mm. The strip is then transferred to the finishing line, where the head and tail ends of the strip, deformed, are first sheared, and then they pass through the finishing tandem mill to achieve the final thickness. The strip is then cooled in a controlled manner to room temperature, and then it is wound into coils. 13.3.6 Cold rolling lines The cold rolling process also has several stages (Figure 13.11). It starts with previously hot-rolled coils, which first pass through a vertical scale (layer with oxides) breaker. When the sheet is bent, the surface oxide layer is removed in the form of flakes. Next, in the pickling stage, the surface layer is removed chemically. Any remaining oxide particle that could damage the rolls in the rolling stand is removed by immersing the sheet in a hydrochloric or sulfuric acid bath. The sheet is then cold rolled to the final thickness, normally in a Sendzimir type stand. The next phase is electrolytic cleaning, which removes any lubricant residues that may have remained on the sheet. Then, annealing is used to improve the ductility properties, as the sheet may be subsequently formed by drawing. Annealing can be a batch process, in which complete coils are introduced into furnaces, or it can be a continuous process, in which the sheet is heated locally and cooled in a controlled manner. The last phase is called skin pass rolling, in which the sheet thickness of the sheet is reduced by at most 1%, and the objective is to obtain a smooth finish and remove possible surface marks. These processes are applied consecutively, and to avoid wasting time, coils are usually welded together, cleaning first the heads and tails of the sheets to ensure a good joint. Hot Rolled coil Pickling Scale Breaker Cold Rolling mill Electrolytic Washing Annealing Skin Pass Cold rolled coil Figure 13.11: Stages of the cold rolling process. After the finishing process, the sheet metal is ready to be delivered to customers. In some cases, however, galvanization or painting are performed to protect the sheet, depending on the customer's requirements. At Arcelor Mittal, both galvanization and painting options are available, as well as the possibility of coating with tin to obtain tinplate, used in tin cans. 24 Block III: Forming processes 13. Metal rolling 13.4 Flat rolling capabilities and defects 13.4.1 Geometrical quality Different specifications are achieved in hot and in cold rolling. The main geometrical specifications are the thickness dimension and the flatness of the strip. As was explained in the previous lesson, cold working allows achieving greater accuracy in dimensional and geometric tolerances. In hot working, the variability is wider as the material undergoes thermal changes, heating previous to forming, and then cooling to room temperature, which makes it more difficult to control the final specifications. Surface finish is also better in cold working, as the surface in hot rolling products develops an oxide layer called scale. Table 13.1. Comparison of specifications achieved in cold and hot rolling. COLD ROLLING HOT ROLLING Thickness Between ± 0,01 · ℎ and ± 0,025 · ℎ Superior Flatness ±15 mm/m ±55 mm/m Roughness (𝑅𝑎) 6,3 − 0,2 m 50 − 6,3 m 13.4.2 Defects Rolled products can present a multitude of defects, both internal and superficial (Figure 13.12). The presence of defects affects the quality of the material, reducing its mechanical properties. Surface defects include scale, oxidation, scratches, cracks, pores and inclusions. In addition, geometric defects can occur, such as the flatness defects already mentioned, or the waviness of the edges, which is related to the problems of roll bending: when the roller bends, the material is compressed more at the edges than in the central area, because the longitudinal expansion is limited by the central region of the material. In this way, the edges, being longer, end up buckling and adopting a wavy shape, like the ruffles on a dress. Roll bending can also have an impact on the appearance of cracks in the central area of the sheet, called zipper cracks, when this area is subjected to tension. Finally, the appearance of some types of cracks is related to ductility problems of the material at the temperature being rolled. Wavy edges Zipper cracks Alligatoring Edge cracks Figure 13.12: Defects in rolling. Rolls tend to bend elastically during rolling, leading to a non-uniform thickness along the strip width, being thicker at the center than at the sides (Figure 13.13a). To solve this problem, a common method is to grind them so that the diameter at the center is slightly larger than the diameter at the ends, compensating for possible deformation (Figure 13.13e). This is called camber, or rolls with camber. 25 Block III: Forming processes 13. Metal rolling (a) (b) Figure 13.13: Elastic deformation of rolls. (a) Crown at the strip center, (b) Rolls with camber. Another geometric effect is caused by the thermal gradient of the roll in contact with the material in hot rolling, which can barrel the shape, but this effect can be controlled by acting on the roll cooling, differentially along the contact width. Finally, the effect of roll flattening by the contact pressure has no easy solution, so its effect on the increase in rolling forces must be taken into account in the design of the rolling passes. 13.4.3 Residual stresses The practice of flat rolling generates residual stresses in the material (Figure 13.14). Thus, the use of small diameter rollers tends to generate large compressive stresses on the surface, and smaller tensile stresses in the core of the strip. The behavior in the case of large rollers is the opposite, with large deformations in the core and smaller ones on the surface, especially due to friction restrictions. It is usually preferred that the outer surfaces of the strips be subjected to compressive stresses, so that surface cracks do not occur, since if there are tensile stresses these cracks would grow rapidly. Thickness Tension Compression Tension Compression Small diameters, or small reductions Large diameter, or high reductions Figure 13.14: Residual stresses in rolling. 13.5 Other rolling processes 13.5.1 Shape rolling Shape rolling or profile rolling is used to produce long, straight structural shapes, such as beams, and railway rails. In this process, hot material is passed through pairs of rollers at different stages, in which the shape of the product is progressively brought closer to the final geometry. The design of the sequence of rolling stages is key to minimize the incidence of internal and external defects, achieving the desired dimensional tolerances and minimizing roller wear. As can be seen in Figure 13.15 to obtain a shape close to that of a rail it is usual to rotate the piece during rolling, and in the final stages it is also necessary to use auxiliary rollers that rotate perpendicularly to the main ones, in order to better control the shape and dimensions of the desired piece. 26 Block III: Forming processes 13. Metal rolling Figure 13.15: Rolling of shapes (rails). 13.5.2 Thread rolling Thread rolling is a cold rolling process applied to parts produced in large series from round rod or wire rod, such as bolts and screws. In this case, flat dies or rolls can be used (Figure 13.16). Compared to threading processes by chip removal, it has the advantage of allowing high production rates, without wasting material. In addition, rolled threads have better mechanical properties than machined threads, since the process does not cut the flow lines of the material grain, but deforms the grains. (a) Flat dies (b) Two-roll dies Figure 13.16: Thread rolling. 13.5.3 Ring rolling Ring rolling modifies the dimensions and cross section of a ring-shaped workpiece, previously manufactured by forging. An internal roll is introduced into the ring hole, which compresses the section against an external roll, which is driven by a motor (Figure 13.17). A third roll is used for rounding the ring. As rolling progresses, the thickness of the ring decreases but its size (inner and outer diameters) increases, as the volume should remain constant. The profile geometry may be also modified by using contoured rolls. Sometimes, edging rolls are employed to also change the height of the ring. Figure 13.17: Ring rolling. 27 Block III: Forming processes 13. Metal rolling 13.5.4 Rotary tube piercing Tube rolling provides seamless tubes of a higher quality than tubes obtained from roll formed sheet that is subsequently welded leaving a seam or weld bead. Among the variants used for tube rolling, the Mannesmann process uses skewed rolls to apply compressive stresses onto a solid rod (Figure 13.18). A void is generated in the center of a round bar due to tensile stresses appearing in the transverse direction. An internal mandrel is introduced in the roll gap to enlarge the hole and make the process progress. After producing the tube, variants of the rolling process can be used to reduce the tube diameter and/or wall thickness to the required dimensions. Figure 13.18: Rotary tube piercing (Mannesmann process). 13.5.5 Rolling of near-net shape parts The last two variants of the rolling process again use contoured rolls, which in their rotational movement compress and give shape to the part. In the case of skew rolling (Figure 13.19a), the roll axes form a certain angle, so that the friction makes the rod move into the roll gap. Spiral grooved rolls allow for generating spherical shapes that are separated from the rod during the rolling process. It is one of the ways of obtaining preforms or near-net shapes for spherical parts, which are used in bearings. Cross rolling (Figure 13.19b) employs rolls with profiled grooves matching the desired shape of the final part. The rolls do not turn continuously, but rotate only a portion of one revolution corresponding to the desired deformation to be accomplished on the part. It is typical for axisymmetric shapes, such as bolts and pins, which are safety parts with important requirements for mechanical strength and toughness, which the rolling process provides. (a) Skew rolling (b) Cross rolling Figure 13.19: Rolling of near-net shape parts. 28 Block III: Forming processes 14. Metal extrusion and metal drawing 14 Metal extrusion and metal drawing 14.1 Introduction 14.1.1 Common aspects and differences Metal extrusion is a process in which the application of a compression force on a metal part inside a container causes the material to pass through a die. The material will exit the die through an orifice whose shape will define the cross-section geometry of the extruded part (Figure 14.1a). The extrudate is a long product with constant cross-section, which can be complex and even hollow, as would be the case in tubular pieces or profiles. Like all plastic- deformation processes, the grain structure and mechanical properties are improved. The application of force in this process gives rise to high triaxial compression forces, which allows large deformations without causing fracture. In addition, tight tolerances can be achieved in cold extrusion, with minimal material waste. Typical extruded products are aluminum profiles used in frames and windows, structural and architectural profiles, tubes, pipes, or rails for sliding doors. (a) Extrusion principle (b) Drawing principle Figure 14.1: Extrusion and drawing. Metal drawing is based on applying a tensile force on the final end of the part, after passing through the die (Figure 14.1a). Again, the die orifice will impart the dimensions and shape to the cross-section of the drawn product, which is produced in high lengths in a continuous way. In drawing, two additional problems can be seen: the “threading” of the stock through the die, since the parts needs to be sharpened to pass through the die orifice, and the other problem is the possibility of breaking the material during drawing, due to the applied tensile stresses. To achieve large section reductions, subsequent reduction stages are disposed, especially in the manufacture of wire, in a process called wire drawing. 14.1.2 Historical perspective Drawing was one of the oldest forming processes used throughout history. In the time of the Egyptians (2500 BC), threads of noble materials such as gold were used for ornaments and to weave ceremonial garments for the pharaohs (Figure 14.2a). To produce these threads, forging, rolling and drawing processes were combined until reaching the desired diameter. References to fabrics made with these gold thread appear in hieroglyphics and biblical texts. Tools, similar to those still used nowadays in jewelry, have been found in archaeological excavations of Viking settlements from the 9th-10th century. Examples of these tools are dies in the form of hole-pattern plates, made of stone or metallic materials (Figure 14.2b). During the Renaissance, the technique continued to advance (Biringucio 1540), incorporating traction systems based on winches or even wheels moved by water for pulling the material at 29 Block III: Forming processes 14. Metal extrusion and metal drawing the die exit (Figure 14.2c). The objective was the same, to produce wires of metallic materials, with application in goldsmithing and to produce tools and machinery. During the industrial revolution, Abraham Rees adapted the dies to the steam engine, and dies for wire drawing benches were developed (Figure 14.2e), although part of the development was halted when wire rod rolling techniques were improved. (a) Egyptian gold thread (b) Viking hole plate (c) Renaissance (d) Drawing bench in 1820 (e) Wire drawing bench in 1920 Figure 14.2: Evolution of metal drawing. Extrusion is not a process that has evolved over such a long period of time, as it requires the use of large compression forces, and this was not possible until the development of large hydraulic presses. Previously, the concept of extrusion did exist, but applied to materials such as clay, which