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University of Nevada Las Vegas |
MEG426/626 Manufacturing Processes |
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Department of Mechanical Engineering |
Fall Semester 2000 |
Fundamentals of Forming
Metal Forming
includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces.
Category:
1). Bulk deformation processes and
2). Sheet metalworking processes.
Bulk deformation processes: low area-to-volume ratio.
Sheet metalworking processes: high area-to-volume ratio.
Bulk deformation processes
Rolling
: is a compressive process in which the thickness of a slab or plate is reduced by two opposing tools called rolls.Forging: is a compressive process in which a workpiece is compressed between two opposing dies so that the die shapes are imparted to the work.
Extrusion: is a compressive process in which the work metal is forced to flow through a die opening, thereby taking the shape of the opening as its own cross section.
Drawing: similar as an extrusion process, the difference is in a drawing process, the diameter of a wire or bar is reduced by pulling it through a die opening.
Sheet metalworking processes
Bending
: involves straining of a metal sheet or plate to take an angle along a straight axis.Drawing: refers to the forming of a flat metal sheet into a hollow or concave shape. Usually called cup drawing or deep drawing to distinguish from bar and wire drawing.
Shearing: cuts the work apart using a punch and die (not really a forming process).
Material Behavior in Metal Forming
Stress-strain relationship
In plastic deformation, the metal_s behavior is expressed by the flow curve:
where k = the strength coefficient, lb/in.2; and n is the strain hardening exponent.
Temperature Range in Metal Forming
Cold Working
: is metal forming performed at room temperature or slightly above, also called cold forming.Advantages: Better accuracy, better surface finish, better strength and hardness.
Disadvantages: Higher forces and power requirement, mostly used for small part, can only be used for materials with good ductility.
Warm Working: is metal forming performed above room temperature ( at 0.3 Tm) but below the recrystallization temperature (at 0.5 Tm).
0.5 Tm > T > 0.3 Tm
Where Tm is the melting temperature.
Advantages: Lower forces and power needed, and more intricate work geometries possible.
Hot Working: involves deformation at temperatures above the recrystallization temperature (usually at 0.5 Tm).
0.7 Tm > T > 0.5 Tm
Advantages: More plastic deformation, therefore the shape of the workpart can be significantly altered, lower forces and power, and some materials with less ductility can be processed.
Disadvantages: Total energy required increases, less part accuracy.
Created by Dr. Wang