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Bulk Deformation Processes in Metalworking

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Bulk Deformation Processes in Metalworking Metal-Forming Processes Overview of Metal Forming Rolling Performed as cold, warm, and hot working Forging Bulk Deformation Extrusion Wire and bar drawing Metal Forming Mainly cold working Bending Sheet Shearing Metalworking Deep and cup drawing Bulk Deformation (Overview Cont’d) rolling extrusion Wire/bar drawing forging Sheet Metalworking (Overview Cont’d) bending Deep/cup drawing shearing Formability (workability) Formability of the material depends on: (1) Process variables - temperature……………… Desirable material properties in metal - strain……………… rate forming: – Low yield strength and high ductility - ………………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 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 Process T/T oxidation at elevated temperatures m Cold working < 0.3 • Tm: melting point of metal (based on absolute temperature scale) Warm working 0.3 to 0.5 e.g. lead Hot working > 0.6 – 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 But if heated to sufficiently high temperature and deformed, strain hardening does not occur 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: Ao = original cross-sectional area Ad = deformed cross-sectional area Ductility ……decreases...…. with cold work Yield and tensile strength ……………increase 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 The influence of cold work on the greater strain hardening for a given stress–strain behavior for a low- amount of plastic strain). carbon steel. 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) Cold Working Disadvantages of Cold Forming: • Equipment of higher forces and power required to shape material. • Surfaces of starting work-piece 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.. Annealing involves three steps Annealing • Material in this condition (cold worked) is annealed, changes will begin to take place. These changes may be classified under three headings: 1. Stress relief 2. Recrystallization 3. Grain growth 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 Annealing 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: o Lower forces and power than in cold working. o More intricate work geometries possible. o Need for annealing may be reduced or eliminated. 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] Hot Working 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. Advantages of Hot Working vs. Cold Working • Work-part 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 work-piece. Work surface oxidation (scale), poor………… surface finish. Shorter…..……… tool life Hot Work vs. Cold Work Hot Work Cold Work • Recrystallization takes place • NO Recrystallization • Less than <0.3 Tm • > 0.5 * Tm • Requires less force • Requires more force • Less residual stresses • Residual Stresses • Greater deformation • Strain Hardened possible • Better Surface Finish • Dimensional Variation • No oxides on the surface [Lower dimensional accuracy] after operation • Poor Surface Finish • lower costs for process • Oxidation of Surfaces and equipment • Expensive costs for process and equipment 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, tool wear – Better surface finish – Removes heat from the tooling Material Behavior in Metal Forming . Plastic region of stress-strain curve is primary interest because material is plastically deformed . In plastic region, metal's behavior is expressed by the flow curve: s K n where K = strength coefficient; and n = strain hardening exponent . Flow curve based on true stress and true strain ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Flow Stress . For most metals at room temperature, strength increases when deformed due to strain hardening . Flow stress = instantaneous value of stress required to continue deforming the material n Yf K where Yf = flow stress, that is, the yield strength as a function of strain ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Average Flow Stress . Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest _ K n Y f 1 n _ where Y f = average flow stress; and = maximum strain during deformation process ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e BULK DEFORMATION PROCESSES IN METALWORKING 1. Rolling 2. Other Deformation Processes Related to Rolling 3. Forging 4. Other Deformation Processes Related to Forging 5. Extrusion 6. Wire and Bar Drawing ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Bulk Deformation Metal forming operations which cause significant shape change by deforming metal parts whose initial form is bulk rather than sheet . Starting forms: . Cylindrical bars and billets . Rectangular billets and slabs and similar shapes . These processes stress metal sufficiently to cause plastic flow into the desired shape . Performed as cold, warm, and hot working operations ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Importance of Bulk Deformation . In hot working, significant shape change can be accomplished . In cold working, strength is increased during shape change . Little or no waste - some operations are near net shape or net shape processes . The parts require little or no subsequent machining ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Four Basic Bulk Deformation Processes 1. Rolling – slab or plate is squeezed between opposing rolls 2. Forging – work is squeezed and shaped between opposing dies 3. Extrusion – work is squeezed through a die opening, thereby taking the shape of the opening 4. Wire and bar drawing – diameter of wire or bar is reduced by pulling it through a die opening ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e METAL FORMING PROCESSES Rolling Deformation process in which work thickness is reduced by compressive forces exerted by two opposing
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