FUNDAMENTALS OF METAL FORMING

Jurusan Teknik MesinFakultas Teknologi IndustriInstitu Teknologi Sepuluh NopemberManufacturing ProcessFundamental of FormingMetal FormingLarge group of manufacturing processes in which plastic deformation is used to change the shape of metal workpiecesThe tool, usually called a die, applies stresses that exceed the yield strength of the metal The metal takes a shape determined by the geometry of the dieStresses to plastically deform the metal are usually compressiveExamples: rolling, forging, extrusion

Material Properties in Metal Forming2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Desirable material properties: Low yield strength High ductilityThese properties are affected by temperature: Ductility increases and yield strength decreases when work temperature is raisedOther factors: Strain rate and frictionBasic Types of Metal Forming ProcessesBulk deformationRolling processesForging processesExtrusion processesWire and bar drawingSheet metalworkingBending operationsDeep or cup drawingShearing processes

Bulk Deformation Processes(a) Rolling and (b) forging

Bulk Deformation Processes(c) Extrusion and (d) wire and bar drawing

Sheet MetalworkingForming and related operations performed on metal sheets, strips, and coilsHigh surface areatovolume ratio of starting metal, which distinguishes these from bulk deformation Often called pressworking because these operations are performed on presses Parts are called stampingsUsual tooling: punch and dieSheet Metalworking(a) Bending and (b) deep drawing

Sheet Metalworking(c) Shearing: (1) punch first contacting sheet and (2) after cutting

Material Behavior in Metal FormingPlastic 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:

where K = strength coefficient; and n = strain hardening exponent Flow curve based on true stress and true strain

Flow StressFor most metals at room temperature, strength increases when deformed due to strain hardeningFlow stress = instantaneous value of stress required to continue deforming the material

where Yf = flow stress, that is, the yield strength as a function of strain

Average Flow StressDetermined by integrating the flow curve equation between zero and the final strain value defining the range of interest

where = average flow stress; and = maximum strain during deformation process

Stress-Strain RelationshipAverage flow stress in relation to Flow stress Yf Yield strength Y

Source: http://kang-sarmin.blogspot.com/2011/12/material.htmlTemperature in Metal Forming For any metal, K and n in the flow curve depend on temperatureBoth strength (K) and strain hardening (n) are reduced at higher temperaturesIn addition, ductility is increased at higher temperaturesThree temperature ranges in metal forming: Cold workingWarm workingHot workingAny deformation operation can be accomplished with lower forces and power at elevated temperature

Cold WorkingPerformed at room temperature or slightly above Many cold forming processes are important mass production operationsMinimum or no machining usually requiredThese operations are near net shape or net shape processes Advantages of Cold FormingBetter accuracy, closer tolerancesBetter surface finishStrain hardening increases strength and hardnessGrain flow during deformation can cause desirable directional properties in productNo heating of work requiredDisadvantages of Cold FormingHigher forces and power required for deformationSurfaces of starting work must be free of scale and dirtDuctility and strain hardening limit the amount of forming that can be doneIn some cases, metal must be annealed before further deformation can be accomplishedIn other cases, metal is simply not ductile enough to be cold workedWarm WorkingPerformed at temperatures above room temperature but below recrystallization temperatureDividing line between cold working and warm working often expressed in terms of melting point: 0.3Tm, where Tm = melting point (absolute temperature) for metalAdvantages and Disadvantages of Warm WorkingAdvantagesLower forces and power than in cold workingMore intricate work geometries possibleNeed for annealing may be reduced or eliminated DisadvantageWorkpiece must be heatedHot WorkingDeformation at temperatures above the recrystallization temperature Recrystallization temperature = about half of melting point on absolute scale In practice, hot working usually performed somewhat above 0.5TmMetal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level Advantages of Hot WorkingWorkpart shape can be significantly alteredLower forces and power requiredMetals that usually fracture in cold working can be hot formedStrength properties of product are generally isotropicNo strengthening of part occurs from work hardeningAdvantageous in cases when part is to be subsequently processed by cold formingDisadvantages of Hot WorkingLower dimensional accuracyHigher total energy required, which is the sum of The thermal energy needed to heat the workpieceEnergy to deform the metalWork surface oxidation (scale). Thus, poorer surface finishShorter tool life Dies and rolls in bulk deformationFriction in Metal FormingIn most metal forming processes, friction is undesirable: Metal flow is reduced Forces and power are increasedTools wear fasterFriction and tool wear are more severe in hot workingLubrication in Metal Forming2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Metalworking lubricants are applied to toolwork interface in many forming operations to reduce harmful effects of friction Benefits: Reduced sticking, forces, power, tool wearBetter surface finishRemoves heat from the toolingConsiderations in Choosing a Lubricant2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Type of forming process (rolling, forging, sheet metal drawing, etc.)Hot working or cold workingWork materialChemical reactivity with tool and work metals Ease of applicationCost Strain RateStrain rate in forming is directly related to speed of deformation vDeformation speed v = velocity of the ram or other movement of the equipmentStrain rate is defined:where = true strain rate; and h = instantaneous height of workpiece being deformed

In most practical operations, valuation of strain rate is complicated by Workpart geometryVariations in strain rate in different regions of the partStrain rate can reach 1000 s-1 or more for some metal forming operationsStrain Rate (cont.)Effect of Strain Rate on Flow StressFlow stress is a function of temperatureAt hot working temperatures, flow stress also depends on strain rateAs strain rate increases, resistance to deformation increases This is the effect known as strainrate sensitivityStrain Rate SensitivityTheoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n = 0The metal should continue to flow at the same flow stress, once that stress is reachedThe flow stress of a metal is a function of temperature, and at the hot working, flow stress depends on strain rate.Strain rate sensitivity is the effect of strain rate on strength propertiesStrain Rate Sensitivity (cont.)(a) Effect of strain rate on flow stress at an elevated work temperature(b) Same relationship plotted on loglog coordinates

Strain Rate Sensitivity Equationwhere C = strength constant (analogous but not equal to strength coefficient in flow curve equation), and m = strainrate sensitivity exponent

Effect of Temperature on Flow StressThe constant C, indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreasesAnd m (slope of each plot) increases with increasing temperature

Observations about Strain RateSensitivityIncreasing temperature decreases C and increases m At room temperature, effect of strain rate is almost negligibleFlow curve alone is a good representation of material behaviorAs temperature increasesStrain rate becomes increasingly important in determining flow stress