Chapter 6

1. Chapter 6 BULK DEFORMATION PROCESSES IN METAL WORKING

2. Rolling Performed as cold, warm, and hot working Forging Bulk Deformation Extrusion Metal Forming Wire and bar drawing Mainly cold working Large group of mfg processes in which plastic deformation is used to change the shape of metal workpieces Bending Sheet Metalworking Shearing Deep and cup drawing Overview of Metal Forming

3. rolling extrusion Wire/bar drawing forging Bulk Deformation(Overview Cont’d)

4. Deep/cup drawing bending shearing Sheet Metalworking(Overview Cont’d)

5. Formability (workability) • Formability of the material depends on: • Process variables • ……………… • ……………… • ……………… temperature • Desirable material properties in metal forming: • Lowyield strength and highductility strain rate stress (2) Metallurgical changes (properties changes such as hardness)during deformation ,formation of voids, inclusions, precipitation, .... etc. Ductility increases and yield strength decreases when work temperature is raised  Any deformation operation can be accomplished with lower forces and power at elevated temperature

6. Process T/Tm Cold working < 0.3 Warm working 0.3 to 0.5 Hot working > 0.6 Metal Forming Processes: Homologous Temp. • What is the parameter that determine working temperature??? • Metal forming process temperature is measured by Homologus temperature • Homologous temperature expresses the temperature of a material as a fraction of its melting point temperature using the Kelvin scale • T:working temperature such Stainless steels have good strength and good resistance to corrosion and oxidation at elevated temperatures • Tm:melting point of metal (based on absolute temperature scale) • e.g. lead • Tm = 327 C • Formed at room temperature (20 C), • …………………………………. T/Tm = (20 +273)/(327 + 273) = 0.5 Warm working • Most metals strain harden at room temperature according to the flow curve (n > 0)---- elastic + strain hardening • But if heated to sufficiently high temperature and deformed, strain hardening does not occur • Instead, new grains are formed that are free of strain • The metal behaves as a perfectly plasticmaterial; that is, n = …. 0

7. Perfectly plastic • When the material is heated to sufficiently high temperature, and tension test is conducted the material will exhibit a perfectly plastic behavior • Perfectly plastic: once the stress reaches yield stress, Y, it continues to undergo deformation at the same level. • When the load is released, the material has undergone permeant deformation; there is no elastic recovery

8. Ao = original cross-sectional area Ad= deformed cross-sectional area Ductility ……...…. with cold work Yield and tensile strength …………… Strain or Work Hardening • Strain hardening (work hardening) is where a material becomes less ductile, harder and stronger with plastic deformation. • Encountered during cold working. • Percentage cold work can be expressed as: decreases increase

9. Strain or Work Hardening • Yield strength (sy) increases. • Tensile strength (UTS) increases. • Ductility (%EL or %AR) decreases. • Dislocation density increases with CW • Motion of dislocations is hindered as their density increases. • Stress required to cause further deformation is increased. • Strain hardening is used commercially to improve the yield and tensile properties. • cold-rolled low-carbon steel sheet • aluminum sheet • Strain hardening exponent n indicates the response to cold work (i.e. larger n means greater strain hardening for a given amount of plastic strain). The influence of cold work on the stress–strain behavior for a low-carbon steel.

10. Copper Cold work  D =15.2mm D =12.2mm o d sy (MPa) UTS (MPa) ductility (%EL) 6 0 8 00 7 00 4 0 6 00 5 00 Cu 2 0 Cu 3 00 4 00 Cu 100 0 2 00 0 2 0 4 0 6 0 0 2 0 4 0 6 0 0 2 0 4 0 6 0 % Cold Work % Cold Work % Cold Work s %EL =7% =300MPa TS=340MPa y Example: Cold Work Analysis • What is the tensile strength & ductility after cold working?

11. Cold Working • Performed at room temperature or slightly above. • Many cold forming processes are important mass production operations. • Minimum or no machining usually required (no oxidation). • These operations are near net shape or net shape processes. • Advantages of Cold Forming vs. Hot Working: • Better accuracy, closer tolerances. • Better surface finish. • Strain hardening increases strength and hardness. • Grain flow during deformation can cause desirable directional properties in product. • No heating of work required (less total energy)

12. Annealing involves three steps Cold Working Disadvantages of Cold Forming: • Equipment of higher forces and power required to shape material. • Surfaces of starting workpiece must be free of scale and dirt (to avoid surface defect during cold working). • Less ductility and high strain hardening limit the amount of forming that can be done. • In some operations, metal must be annealed to allow further deformation. ANNEALING-A heat treatment to eliminate the effects of cold working. Purposes of annealing: - ………….. - …………………… - ………………………….. relieve stress [residual stress] increase ductility produce a specific structure

13. Annealing • Material in this condition (cold worked) is annealed, changes will begin to take place. These changes may be classified under three headings: • Stress relief • Recrystallization • Grain growth

14. Effect of cold working on properties The grain boundaries here is the disorder structure of high density dislocation which replace by the original fragmented grain boundaries

15. Annealing

16. Annealing

17. Annealing • Stress relief: • As the temperature of the material is raised so the vibrational energies of the individual atoms are increased and atomic movements can occur. Comparatively minor atomic movements result in the removal of the residual stresses associated with the locked-in elastic strains . • This change, which occur at comparatively low temperature, has a negligible effect on the strength and hardness of the material, and the microstructure of the metal is unchanged in its appearance.

18. Annealing • Recrystallization • When the temperature is raised further, the process of recrystallization begins. New unstressed crystals begin to form and grow from nuclei until the whole of a material has a structure of unstressed polygonal crystals. • This change in structure is accompanied by a reduction in hardness, strength and brittleness to the original values prior to plastic deformation.

19. Annealing • Recrystallization • The driving force for the recrystallization process is the release of strain energy stored in the zones of high dislocation density.[ grain bondaries] • The temperature at which recrystallization occurs is, for pure metal, within the range from one-third to one-half of melting temperature (k). • Recrystallization temperature is not constant for all material. Why????

20. Annealing • Recrystallization • Recrystallization temperature is not constant for all material as its value is affected by: • The a mount of plastic deformation prior to heating (its lower for very heavily cold worked metals than for samples of the same material which have received small amounts of plastic deformation). • The composition (the presence of impurities or alloying elements will increase the recrystallization temperature of the material

21. Annealing • Grain growth • If the temperature is raised further, grain growth may occur following the completion of recrystallization, with some crystal grains growing in the size at the expense of others by a process of grain boundary migration or merge between small grain and large grain • Small grains have larger GB areathan largegrains, and Since the dislocations are concentrated in these large GB area, these large GR becomes a high energy area. • Consequently, these small grains with (large GB area), will have high energy GB areas. The High energy GB area wants to go to lower energy GB region (large grains). • The driving force for grain growth is the release of grain boundary surface energy as the amount of total grain boundary surface is reduced, this will lead to the reduction of grain boundary area

22. Grain Growth • Growth of new grains will continue at high temperature. • Grain growth occurs in both metals and ceramics at elevated temperature. • Involves the migration of grain boundaries. • Large grains grow at expense of small ones (small grains merge). • Reduction of grain boundary area (driving force) for grains to grow. is the release of strain energy stored in the zones (grain boundaries) of high dislocation density. Reducing size Schematic representation of grain growth via atomic diffusion. Why do small grains merge with large grain? Small grains have larger GB area than large grains. Since dislocations are concentrated in the GB area, becomes a high energy area. So, small grains (large GB area), have high energy GB areas. High energy GB area wants to go to lower energy GB region (large grains).

23. Warm Working • Performed at temperatures above room temperature but below recrystallization temperature. Warm working: T/Tm from 0.3 to 0.5 • Advantages of Warm Working: • Lower forces and power than in cold working. • More intricate work geometries possible. • Need for annealing may be reduced or eliminated.

24. Hot Working • Deformation at temperatures above recrystallization temperature: • In practice, hot working usually performed somewhat above 0.5Tm • Metal continues to soften as temperature increases above 0.5Tm, enhancing the advantage of hot working above this level [produce a specific structure] Why Hot Working? • Capability for substantial plastic deformation of the metal ‑ far more than possible with cold working or warm working. • Why? • Strength coefficient (K) is substantially less than at room temp. • Strain hardening exponent (n) is zero (theoretically). • Ductility is significantly increased.

25. Advantages of Hot Working vs. Cold Working • Workpart shape can be significantly altered. • Lower forces and power required (equipment). • Metals that usually fracture in cold working can be hot formed. • Strength properties of product are generally isotropic. • No strengthening of part occurs from work hardening. • Disadvantages of Hot Working: • Lower dimensional accuracy. • Higher total energy required. • - Due to the thermal energy to heat the workpiece. • Work surface oxidation (scale), ………… surface finish. poor Shorter • …..……… tool life

26. Cold and Hot Working Review of Materials • Cold working: • Plastic deformation which is carried out in a temperature region and over a time interval such that the strain hardening is not relieved is called cold work. • In the early stages of plastic deformation, slip is essentially on primary glide planes and the dislocations form coplanar arrays. • As deformation proceeds, cross slip takes place. The cold worked structure forms high dislocation density regions that soon develop into networks. • The grain size decreases with strain at low deformation but soon reaches a fixed size. • Cold working will decrease ductility.

27. Cold and Hot Working Review of Materials • Hot working: • Hot working refers to the process where metals are deformed above their recrystallization temperature and strain hardening does not occur. • The resistance of metals to plastic deformation generally falls with temperature. For this reason, larger massive sections are always worked hot by forging, rolling, or extrusion

28. Cold and Hot Working Review of Materials • Hot working: • Metals display distinctly viscous characteristics at sufficiently high temperatures, and their resistance to flow increases at high forming rates. • This occurs not only because it is a characteristic of viscous substances, but because the rate of recrystallization may not be fast enough.

29. Cold and Hot Working Review of Materials • Temperature: • The lower limit of the hot working temperature is determined by its recrystallization temperature. • As a guideline, the lower limit of the hot working temperature of a material is 60% its melting temperature (on an absolute temperature scale ). • The upper limit for hot working is determined by various factors, such as: • Excessive oxidation, • Grain growth, • Or an undesirable phase transformation.

30. Cold and Hot Working Review of Materials • Temperature: • In practice materials are usually heated to the upper limit first to keep forming forces as low as possible and to maximize the amount of time available to hot work the workpiece. • The most important aspect of any hot working process is controlling the temperature of the workpiece. • 90% of the energy imparted into the workpiece is converted into heat. • Therefore, if the deformation process is quick enough the temperature of the workpiece should rise, however, this does not usually happen in practice.

31. Cold and Hot Working Review of Materials • Temperature: • Most of the heat is lost through the surface of the workpiece into the cooler tooling. • This causes temperature gradients in the workpiece, usually due to nonuniformcross sections where the thinner sections are cooler than the thicker sections. • Ultimately, this can lead to cracking in the cooler side. • One way to minimize the problem is to heat the tooling. The hotter the tooling the less heat lost to it, but as the tooling temperature rises, the tool life decreases. Therefore the tooling temperature must be compromised; commonly, hot working tooling is heated to 500–850 °F (325–450 °C).

32. Cold and Hot Working Review of Materials • Temperature: • Usually the initial workpiece that is hot worked was originally cast . The microstructure of cast items does not optimize the engineering properties, from a microstructure standpoint. • Hot working improves the engineering properties of the workpiece because it replaces the microstructure with one that has fine spherical shaped grains . • These grains increase the strength, ductility, and toughness of the material

33. Cold and Hot Working Review of Materials • Temperature: • The engineering properties can also be improved by reorienting the inclusions (impurities). • In the cast state the inclusions are randomly oriented, which, when intersecting the surface, can be a propagation point for cracks. • When the material is hot worked the inclusions tend to flow with the contour of the surface, creating stringers . • As a whole the strings create a flow structure , where the properties are anisotropic (different based on direction). • With the stringers oriented parallel to the surface it strengthens the workpiece, especially with respect to fracturing . The stringers act as "crackarrestors“ because the crack will want to propagate through the stringer and not along it.

34. Friction in Metal Forming • In most metal forming processes, friction is undesirable: • Metal flow is retarded • Forces and power are increased • Wears tooling faster • Metalworking lubricants are applied to tool‑work interface in many forming operations to reduce harmful effects of friction. • Benefits: • Reduced sticking, forces, power, tool wear • Better surface finish • Removes heat from the tooling • Considerations in Choosing a Lubricant: • Type of forming process (rolling, forging, sheet metal drawing, etc.) • Hot working or cold working • Work material • Chemical reactivity with tool and work metals • Ease of application • Cost

35. BULK DEFORMATION PROCESSES IN METAL WORKING • The bulk deformation processes are important commercially and technologically (1) They are capable of significant shape change when hot working is used, (2) They have a positive effect on part strength when cold working is used, and (3) Most of the processes produce little material waste; some are net shape processes

36. BULK DEFORMATION PROCESSES IN METAL WORKING 1. Rolling 2. Forging 3. Extrusion 4. Wire and bar drawing

37. Bulk Deformation • Metal forming operations which cause significant shapechange by deforming metal parts whose initial form isbulk rather than sheet. • Starting forms: • Cylindrical billets • Rectangular billets, slabs and similar shapes • These processes stress metal sufficiently to causeplastic flow into the desired shape • Performed as cold, warm, and hot working operations

38. Importance of Bulk Deformation • In hot working, significant shape change can beaccomplished at high temperature . • In cold working, strength is increased during shapechange. • Little or no waste - some operations are near netshape or net shape processes • The parts require little or no subsequent machining

39. Importance of Bulk Deformation • Hot Working of Metals • Hot working is defined as the process of altering the shape or size of a metal by plastic deformation with the temperature above the recrystallization point. • Being above the recrystallization temperature allows the material to complete grain growth during deformation :and to keep the ductility high and hardness and strength low. • This is important because being above recrystallization keeps the materials from strain hardening, which ultimately keeps the yield strength and hardness low and ductility high.

40. º • Hot Working of Metals TR = recrystallization temperature º

41. Importance of Bulk Deformation • Cold Working • Cold working is the process of altering the shape or size of a metal by plastic deformation with the temperature below the recrystallization point. • Hardness and tensile strength are increased with the degree of cold work ( it becomes brittle depends to cold working percentage) whilst ductility and impact values are lowered. • Processes include rolling, drawing, pressing, and extruding, it is carried out below the recrystallization point usually at room temperature. • The cold rolling and cold drawing of steel significantly improves surface finish. (no oxides on the surface after operation)

42. Hot Work vs. Cold Work • Cold Work • NO Recrystallization • Less than <0.3 Tm • Requires more force • Residual Stresses • Strain Hardened • Better Surface Finish • No oxides on the surface after operation • lower costs for process and equipment • Hot Work • Recrystallizationtakes place • > 0.5 * Tm • Requires less force • Less residualstresses • Greater deformationpossible • DimensionalVariation [Lowerdimensional accuracy] • Poor Surface Finish • Oxidation ofSurfaces • Expensive costs for process and equipment

43. Four Basic Bulk DeformationProcesses 1. Rolling :– slab or plate is squeezed betweenopposing rolls 2. Forging :– work is squeezed and shaped betweenopposing dies 3. Extrusion – work is squeezed through a die opening{has fixed profile},thereby taking the shape of the opening 4. Wire and bar drawing – diameter of wire or bar isreduced by pulling it through a die opening

44. Rolling

45. Rolling • Rolling is the process of reducing the thickness or changing the cross section of a long workpiece by compressive forces applied through a set of rolls, thus the process is similar to rolling dough with a rolling pin to reduce its thickness. • Rolling, which accounts for about 90% of all metals produced by metalworking processes, was first developed in the late of 1500s. • The basic rolling operation is called flat rolling, or simplerolling, where rolled products are flat plate and flatsheet

46. Rolling • Plates: are generally regarded as having a thickness greater than 6mm, and are used for structural applications such as boilers, bridges, machine structure, girders, and ship hulls. • Plates can be as much as 0.3 m thick for large boilers, and 100-125 mm thick for warships and tank armor. • Sheets :are generally less than 6mm thick. They are used for automobile bodies, aircraft fuselages, office furniture and kitchen equipment's.

47. Rolling • Traditionally, the initial form of material for rolling is an ingot; An ingot is a material, usually metal, that is cast into a shape suitable for further processing [materials prepared in bulk form] • Rolling is first carried out at elevated temperature (hot rolling), wherein the coarse-grained, brittle, and porous cast structure of the ingot metal is broken down into a wrought structure, with finer grain size and improve properties

48. Grain Structure During Hot Rolling Figure 13.6 Changes in the grain structure of cast or of large-grain wrought metals during hot rolling. Hot rolling is an effective way to reduce grain size in metals, for improved strength and ductility. Cast structures of ingots or continuous casting are converted to a wrought structure by hot working.

49. Rolling Deformation process in which work piece (slab or plate) thickness is reduced by compressive forces exerted by two opposing rolls. The rotating rolls perform two main functions: Pull the work into the gap between them by friction between workpart and rolls. Simultaneously squeeze the work to reduce cross section. The rolling process (specifically, flat rolling)

50. Rolling • One of the first primary processes to convert raw material into a finished product. • Deformation process in which work thickness isreduced by compressive forces exerted by twoopposing rolls (shown below is flat rolling)