Module 8Overview of processes*

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Module 8*Metal formingPrinciple of the process

Structure

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

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Module 8Principle of Metal Forming*

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Module 8*Metal Forming Metal forming includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal work pieces Plastic deformation: a permanent change of shape, i.e., the stress in materials is larger than its yield strength Usually a die is needed to force deformed metal into the shape of the die

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Module 8* Metal with low yield strength and high ductility is in favor of metal forming One difference between plastic forming and metal forming isPlastic: solids are heated up to be polymer meltMetal: solid state remains in the whole process - (temperature can be either cold, warm or hot)Metal Forming

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Module 8*Metal forming is divided into: (1) bulk and (2) sheetMetal Forming Bulk: (1) significant deformation (2) massive shape change (3) surface area to volume of the work is smallSheet: Surface area to volume of the work is large

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Module 8*Bulk deformation processesRollingForgingExtrusionDrawingTraditionally Hot

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Module 8*Sheet deformation processes (Press working/ Stamping)BendingDrawingShearingActually Cutting

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Module 8*In the following series of lecture, we discuss:General mechanics principleIndividual processes:- mechanics principles- design for manufacturing (DFM) rules- equipment

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Module 8*1. General mechanics principle The underlying mechanics principle for metal forming is the stress-strain relationship; see Figure 1.Figure 1

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Module 8* True strain: Instantaneous elongation per unit length of the material True Stress: Applied load divided by instantaneous value of cross-section area

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Module 8* In the forming process we are more interested in the plastic deformation region (Figure 1)Plastic deformation region

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Module 8* The stress-strain relationship in the plastic deformation region is described byWhere K= the strength coefficient, (MPa) = the true strain, =the true stress n= the strain hardening exponent, The flow stress (Yf) is used for the above stress (which is the stress beyond yield)

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Module 8* As deformation occurs, increasing STRESS is required to continue deformation (shown in curve) Flow Stress: Instantaneous value of stress required to continue deforming the material (to keep metal flowing) FLOW STRESS

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Module 8* For many bulk deforming processes, rather than instantaneous stress, average stress is used (extrusion) The average flow stress can be obtained by integrating the flow stress along the trajectory of straining, from zero to the final strain value defining the range of interestAVERAGE FLOW STRESSAverage flow stressMax. strain during deformationStrength CoefficientStrain hardening exponent

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Module 8*Example 1:Determine the value of the strain-hardening exponent for a metal that will cause the average flow stress to be three-quarters of the final flow stress after deformation.According to the statement of the problem, we have of

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Module 8* The above analysis is generally applicable to the cold working, where the temperature factor is not considered. The metal forming process has three kinds in terms of temperature: (1) cold, (2) warm, (3) hot In the case of warm and hot forming, the temperature factor needs to be considered, in particularTemperature up The (yield) strength down and ductility up

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Module 8* Strain rate (related to elevated temperatures) Rate at which metal is strained in a forming process In the hot forming or warm forming, the strain rate can affect the flow stressh

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Module 8*whereC strength constantm strain-rate sensitivity exponentC and m are determined by the following figure which is generated from the experiment

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Module 8*C and m are affected by temperatureTemperature UpC Down m Up

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Module 8*Even in the cold work, the strain rate could affect the flow stress. A more general expression of the flow stress with consideration of the strain rate and strain is presented as follows:A is a strength coefficient, a combined effect of K, CAll these coefficients, A, n, m, are functions of temperature

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Module 8*Example 2:A tensile test is carried out to determine the strength constant C and strain-rate sensitivity exponent m for a certain metal at 1000oF. At a strain rate = 10/sec, the stress is measured at 23,000 lb/in2; and at a strain rate = 300/sec, the stress=45,000 lb/in2. Determine C and m23000=C(10)^m45000=C(300)^m

From these two equations, one can find m=0.1973

Solution:

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