03-CH3

Topic 3Metal Forming &Shaping Processes

(a) An Overview Compiled by:Norliana Mohd Abbas

Sources:Manufacturing Engineering & Technology 5th Edition, Serope Kalpakjian, Steven Schmid, Prentice Hall, 2006Fundamentals of Modern Manufacturing: Materials, Processes and System, Mikell P. Groover, John Wiley & Sons (Asia), 2007MYM Handout 2009

OverviewWhat is Metal Forming?Material Properties in Metal FormingDeformation ProcessesPlastic Deformation in Metal FormingMaterial Behavior in Metal FormingTemperature in Metal FormingFriction in Metal FormingLubrication in Metal FormingSelecting Manufacturing Process

What is Metal Forming?Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces

The 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 die

Material Properties in Metal FormingDesirable 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 friction

Basic Types of Deformation ProcessesBulk deformationRollingForgingExtrusionWire and bar drawingSheet metalworkingBendingDeep drawingCuttingMiscellaneous processes

Bulk Deformation ProcessesCharacterized by significant deformations and massive shape changes

"Bulk" refers to workparts with relatively low surface areatovolume ratios

Starting work shapes include cylindrical billets and rectangular bars

Rolling

Figure 18.2 Basic bulk deformation processes: (a) rolling

ForgingFigure 18.2 Basic bulk deformation processes: (b) forging

Extrusion

Figure 18.2 Basic bulk deformation processes: (c) extrusion

Wire and Bar DrawingFigure 18.2 Basic bulk deformation processes: (d) 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 presses perform these operationsParts are called stampingsUsual tooling: punch and die

Sheet Metal BendingFigure 18.3 Basic sheet metalworking operations: (a) bending

Deep DrawingFigure 18.3 Basic sheet metalworking operations: (b) drawing

Shearing of Sheet MetalFigure 18.3 Basic sheet metalworking operations: (c) shearing

Material Behavior in Metal FormingPlastic region of stress-strain curve is primary interest because material is plastically deformed

Plastic Deformation of Polycrystalline MetalsWhen a polycrystalline metal with uniform equiaxed grains is subjected to plastic deformation at room temperature (cold working), the grains become deformed and elongated.

During plastic deformation, the grain boundaries remain intact and mass contribution is maintained.

Deformed metal has high strength as the entanglement of dislocations with grain boundaries

Plastic Deformation of Polycrystalline Metals

Recovery, Recrystallization and Grain GrowthPlastic deformation can be reversed by heating the metal to a specific temperature range for a period of time called annealing.Three events take place during the heating process:1. RecoveryDuring recovery, which occurs at high temperature range below recrystallization temperature of the metal, the stresses in the highly deformed regions are relieved. Subgrain boundaries will begin to formed, called polygonization.

Recovery, Recrystallization and Grain Growth2. RecrystallizationIt is the process in which new equiaxed and strain-free grains are formed.The recrystallization temperature is defined as the temperature at which complete recrystallization occurs within one hour.Recrystallisation decrease dislocations lower the strength and raise the ductility of the metal. Fig 1.14 shows the effects of recovery, recrystallization and grain growth on mechanical properties and on the shape and size of grains.

Recovery, Recrystallization and Grain GrowthThe effects on recrystallization of temperature, time and plastic deformation by cold working are as follow:- For a constant amount of deformation by cold working, the time required for recrystallization to occur decreases with increasing temperature.- The more the prior cold work, the lower the temperature required for recrystallization.- The greater the degree of deformation, the smaller the grain size becomes during recrystallization.- Anisotropy due to preferred orientation usually persists after recrystallization .

Recovery, Recrystallization and Grain Growth3. Grain GrowthWhen increase temperature of metal continuously, the grains will grow and their size may eventually exceed the original grain size and affects mechanical properties.

Temperature in Metal FormingAny deformation operation can be accomplished with lower forces and power at elevated temperature Three temperature ranges in metal forming: Cold workingWarm workingHot working

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 required

Disadvantages of Cold FormingHigher forces and power required in the deformation operationSurfaces of starting workpiece 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 to allow further deformationIn other cases, metal is simply not ductile enough to be cold worked

Warm WorkingPerformed at temperatures above room temperature but below recrystallization temperature

Advantages of Warm WorkingLower forces and power than in cold workingMore intricate work geometries possibleNeed for annealing may be reduced or eliminated

Hot WorkingDeformation at temperatures above the recrystallization temperature Recrystallization temperature = about onehalf 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

Why Hot Working?Capability for substantial plastic deformation of the metal far more than possible with cold working or warm workingWhy?Strength coefficient (K) is substantially less than at room temperatureStrain hardening exponent (n) is zero (theoretically)Ductility is significantly increased

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 hardening Advantageous in cases when part is to be subsequently processed by cold forming

Disadvantages of Hot WorkingLower dimensional accuracyHigher total energy required (due to the thermal energy to heat the workpiece)Work surface oxidation (scale), poorer surface finishShorter tool life

Friction in Metal FormingIn most metal forming processes, friction is undesirable: Metal flow is retarded Forces and power are increasedTooling wears fasterFriction and tool wear are more severe in hot working

Lubrication in Metal FormingMetalworking 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 tooling

Considerations in Choosing a LubricantType of forming process (rolling, forging, sheet metal drawing, etc.)Hot working or cold workingWork materialChemical reactivity with tool and work metals Ease of applicationCost

Selecting Manufacturing ProcessProper selection on suitable manufacturing process between metal forming, machining & casting process are based on

a) volume of productionb) quality & properties of product expected e.g. surface finish, accuracy, etc.c) technical viability (practicality of the process)d) economy (cost & time)

END OF PART A