ME364_forming_overview&BDP (1)

ME364_forming_overview&BDP (1) - CHAPTER CONTENTS...

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Unformatted text preview: CHAPTER CONTENTS 13 METAL FORMING 2 2.1 Overview of Metal Forming De¡nitions Material Considerations Temperature in Metal Forming Friction Effects 2.2 Bulk deformation Processes Classi¡cation of Bulk Deformation Processes Rolling Forging Extrusion 2.3 Sheet Metalworking Classi¡cation of Sheet Metalworking Processes Cutting Operations Bending Operations Deep Drawing Other Sheet Metalworking Operations High-energy-rate Forming (HERF) 2.1 OVERVIEW OF METAL FORMING Defnitions Plastic Deformation Processes Operations that induce shape changes on the workpiece by plastic deformation under forces applied by various tools and dies. Bulk Deformation Processes These processes involve large amount of plastic deformation. The cross-section of workpiece changes without volume change. The ratio cross-section area/volume is small. For most operations, hot or warm working conditions are preferred although some operations are carried out at room temperature. Sheet-Forming Processes In sheet metalworking operations, the cross-section of workpiece does not change—the material is only subjected to shape changes. The ratio cross-section area/volume is very high. Sheet metalworking operations are performed on thin (less than 6 mm) sheets, strips or coils of metal by means of a set of tools called punch and die on machine tools called stamping presses. They are always performed as cold working operations. Material considerations Material Behavior In the plastic region, the metal behavior is expressed by the ¢ow curve: σ = Κε n where K is the strength coef¡cient and n is the strain-hardening (or work-hardening) exponent. K and n are given in the tables of material properties or are calculated from the material testing curves. Flow stress For some metalworking calculations, the ¢ow stress Yf of the work material (the instantaneous value of stress required to continue deforming the metal) must be known: Y f = Κε n Metal Forming 14 Valery Marinov, Manufacturing Technology Average (mean) fow stress In some cases, analysis is based not on the instantaneous fow stress, but on an average value over the strain-stress curve From the beginning oF strain to the ¡nal (maximum) value that occurs during deFormation: ! Y f Y Y f " " f ! =K " n Specific energy u Stress-strain curve indicating location oF average fow stress Y F in relation to yield strength Y and ¡nal fow stress Y F The mean fow stress is de¡ned as here ε F is the maximum strain value during deFormation. n K Y n f f ! " 1 # Work-hardening It is an important material characteristic since it determines both the properties oF the workpiece and process power. It could be removed by annealing. Temperature in metal forming The fow curve is valid For an ambient work temperature. ¢or any material, K and n depend on temperature, and thereFore material properties are changed with the work temperature: log !...
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ME364_forming_overview&BDP (1) - CHAPTER CONTENTS...

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