Lecture 7 Metal Forming Fundamentals.pdf

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Mechanical Engineering
Alokesh Pramanik

Lecture 7: Metal Forming Fundamentals Saturday, 2 November 2013 9:07 PM • Introduction: Definition of metal forming; stresses in forming processes • Variables: Independent and dependent • ProcessModelling • General parametersin Metal Forming: Material characteristics, friction, lubrication • Temperatureclassification of forming Metal Forming • Utilizes a remarkableproperty of metals (and alloys), their ability to flow plastically in the solid state without deteriorationof properties • Plastic deformation:permanent deformation,non-recoverableafter unloading, involves permanent atomic displacements • In metal forming, the tool, usually called a die, applies stresses that exceed the yield strength of the metal. The metal takes a shape determined by the geometryof the die • Advantages: ○ Good mechanical properties of product ○ Favourable grain flow ○ Little waste ○ Cast structure improved,cast defects eliminated ○ No handling of molten • Disadvantages ○ High forces required to deform materials ○ Machinery and tooling often expensive ○ Most economicalfor large quantities Stresses in Metal Forming • Stresses used in metal forming to plastically deform the metal are usually compressive;typical examples are rolling, forging and extrusion • Howeversome forming processes apply other stresses: ○ Stretching: Tensile stresses ○ Bending: Tensile and compressing ○ Shearing: Shear stresses Rolling: Biaxial compression Forging: Triaxial compression Extrusion: Triaxial compression Wire & Tube Drawing: Biaxial compressionand Tension Straight bending: Biaxial tension Deep Drawing: Biaxial tension & compressionand simple and Biaxial compression uniaxial tension Material properties in forming • The mechanical properties of materials desirable in metal forming processes are: • The mechanical properties of materials desirable in metal forming processes are: ○ High ductility: allow for large plastic deformation ○ Low yield strength: Allow for lower deformationforces ○ Values of strain hardening exponent n appropriate: high n is better in shearing, low n better in cold rolling • Materials with these desirable properties are called malleable materials or are said to have good formabilityor workability Independent Variables • Selected or controlled by engineer • Choice of material ○ Initial properties and characteristics ○ Choice dictated by: ease of fabrication, desirable properties (eg. Corrosion resistance), suitability for application • Starting geometryof workpiece ○ Dictated by previous processing ○ Selected from available shapes ○ Economicconsiderations • Tool or die geometry ○ Area of major significance: induces and controls metal flow, affects process success or failure ○ Examples:Diameter/profileof rolling mill roll, die angle extrusion, bend radius and bend angle and die cavity details • Lubrication ○ Up to 50% of power can be used to overcomefriction ○ Other functions: coolants, thermal barriers, corrosion inhibiters, parting compounds ○ Lubricant specifications: Lubrication type, applied amount, methodof application • Starting temperature ○ Temperatureselection and control ○ Temperaturesof workpiece and tooling ○ Affected material properties ○ As temperatureincreases: material strength decreases, ductility increases, rate of strain hardening decreases • Speed of operation ○ Affects productivity ○ Range of speeds available ○ Direct influence also on required force for deformation,lubricant effectiveness,time for heat transfer, speed of deformation(strain rate) • Amount of deformation ○ Depends on material:ductility at forming temperature ○ Controlled by tool design: eg. Impression-die forging ○ Adjusted by operator:eg. Flat rolling • Independent variables affect dependent variables Dependant Variables • Consequences of independent variables • Limited control by engineer • Force or power requirements ○ Depends on material, tool geometry,lubricant, speed of operation, starting temperature ○ Needed to select: appropriate equipment, tool or die materials ○ Estimationnecessary to optimize process • Material properties of product ○ Important to customers:function and lift of product ○ Final properties depend on combined effects of deformationand temperature variation ○ where A io the original cross sectional area and Adis the area after deformation • Final Temperature • Final Temperature ○ Temperaturecontrol important: average temperatureand localised temperature ○ Effect of temperaturechange on: deformationprocess and product properties • Surface finish and dimensional precision ○ Depend on details of forming processes ○ Influence product function, assembly, quality, reliability • Nature of material flow ○ Depends on process and tool geometry ○ Affects product properties ○ Customersatisfaction requires not only the production of a desired geometricshape, but also that the shape possesses the right set of companion properties, without any surface or internal de
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