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Overview of processes

Overview of processes. Metal forming. Principle of the process Structure and configurtion Process modeling Defects Design For Manufacturing (DFM) Process variation. Principle of Metal Forming. Metal Forming.

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Overview of processes

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  1. Overview of processes Module 8

  2. Metal forming Principle of the process Structure and configurtion Process modeling Defects Design For Manufacturing (DFM) Process variation Module 8

  3. Principle of Metal Forming Module 8

  4. 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 Module 8

  5. Metal Forming • Metal with low yield strength and high ductility is in favor of metal forming • One difference between plastic forming and metal forming is 1. Plastic: solids are heated up to be polymer melt 2. Metal: solid state remains the solid state in the process - (temperature can be either cold, warm or hot) Module 8

  6. Metal Forming Metal forming is divided into: (1) bulk and (2) sheet • Bulk: • significant deformation • massive shape change • surface area to volume of the work is small Sheet: Surface area to volume of the work is large Module 8

  7. Bulk deformation processes Forging Rolling Traditionally Hot Extrusion Drawing Module 8

  8. Sheet deformation processes (Press working/ Stamping) Drawing Bending Shearing Actually Cutting Module 8

  9. We discuss: • General mechanics principle • Individual processes: • mechanics principles • design for manufacturing (DFM) rules • equipment Module 8

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

  11. L0: the initial length of a specimen L: the length of the specimen at time t the true strain at time t • True Stress: Applied load divided by instantaneous value of cross-section area • True strain: Instantaneous elongation per unit length of the material Module 8

  12. More interested in the plastic deformation region Plastic deformation region Module 8

  13. The stress-strain relation in the plastic deformation region where K= the strength coefficient, (MPa),  = the true strain, σ=the true stress, n= the strain hardening exponent. Remark: Flow stress (Yf) is used for the above stress (which is the stress beyond yield). The equation is called flow curve. Module 8

  14. FLOW STRESS • As deformation occurs, increasing STRESS is required to continue deformation • Flow Stress: Instantaneous value of stress required to continue deforming the material (to keep metal “flowing”) Module 8

  15. AVERAGE FLOW STRESS • Average stress: total stress in one complete operation (e.g., extrusion) • Integrating the flow stress along the trajectory of straining, from zero to the final strain value defining the range of interest, divided by the final strain Strength Coefficient Max. strain during deformation Average flow stress Strain hardening exponent Module 8

  16. 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 Module 8

  17. The above analysis 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 particular Temperature up Yield strength down and ductility up Module 8

  18. Instantaneous height of work-piece being deformed Speed of deformation (could be equal to velocity of ram) Strain Rate h Flow stress • Strain rate (related to elevated temperatures) • Rate of the straining • Strain affecting flow stress h Module 8

  19. Strength coefficient but not the same as K where C  strength constant m  strain-rate sensitivity exponent C and m are determined by the following figure which is generated from the experiment Module 8

  20. Lg Lg Module 8

  21. C and m are affected by temperature Lg Temperature Up C Down m Up Lg Module 8

  22. 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, C • All these coefficients, A, n, m, are functions of temperature Module 8

  23. 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 m Solution: 23000=C(10)^m 45000=C(300)^m From these two equations, one can find m=0.1973 Module 8

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