Unit 6c

Metal Manufacturing Process

• Metal forming changes metal workpiece shapes.
• Dies apply stresses exceeding the metal's yield strength.
• Metal's shape aligns with the die's geometry.
• Plastic deformation stresses are typically compressive.
• Examples include rolling, forging, and extrusion.
• Some processes use tensile stresses (stretching) or both tensile and compressive stresses (bending), or shear stresses (shear spinning).

Material Properties in Metal Forming

• Desirable metal properties include low yield strength and high ductility.
• These properties change with temperature; ductility increases and yield strength decreases as temperature rises.
• Other factors influencing forming include strain rate and friction.

Basic Types of Deformation Processes

• Bulk deformation: Significant shape changes in large components.
  • Examples: Rolling, forging, extrusion, and wire/bar drawing.
• Sheet metalworking: Forming operations on thin metal sheets (e.g., strips, coils).
  • Examples: Bending, deep drawing, and cutting.

Bulk Deformation Processes

• Rolling: Thick metals are passed between rollers to reduce thickness. Types include hot (high temperature) and cold (room temperature) rolling.
• Forging: Metal is shaped by hammering or pressing.
• Extrusion: A metal billet is pushed through a die to create desired profiles ("extrudates").
• Wire and Bar Drawing: Drawing metal through dies to reduce cross-sectional area.

Metal Extrusion

• A metal forming process where a billet is forced through a die to produce specific cross-sections (extrudates).
• This is done using either mechanical or hydraulic presses.

Sheet Metalworking (cont.)

• Bending: Shaping metal sheets into desired profiles.
• Deep drawing: Forming a sheet of metal into a cavity, often resulting in hollow shapes.
• Shearing: Cutting metal sheets using specialized dies.

Metal Drawing

• A manufacturing process that reduces metal stock cross-section by forcing it through a die.
• Similar to extrusion, but drawing pulls the metal through the die opening.

Drawing Process

• Performed in cold conditions for better accuracy.
• Metal is hardened beforehand.
• Surfaces are cleaned (e.g., shot blasting).
• Metal is washed and dried.
• Lubricating chemicals are applied.

Temperature in Metal Forming

• Deformation is often more efficient at elevated temperatures.
• Metal forming temperatures are categorized into:
  • Cold working (room temperature or slightly above)
  • Warm working (above room temperature, but below recrystallization)
  • Hot working (above recrystallization temperature).

1. Cold Working

• Performed at or slightly above room temperature.
• Many mass production operations use cold forming.
• Minimal machining is typically needed.
• Advantages include better accuracy, tolerances, and surface finish; no heating is required.
• Disadvantages include higher forces and power, strain hardening limitations.

2. Warm Working

• Performed above room temperature, but below recrystallization temperature.
• Often uses approximately 0.3 times the metal's melting point (absolute temperature).
• Advantages include lower forces and power compared to cold working; more intricate shapes are possible, and annealing may not be needed.
• Disadvantages include scaling on the workpiece's surface.

3. Hot Working

• Performed at temperatures above the recrystallization point.
• Roughly at 0.6 times the metal's melting point.
• Advantages include significantly easier shaping, lower forces and power, and the ability to form metals that would fracture in cold working.
• Disadvantages include lower dimensional accuracy, higher energy consumption, surface oxidation, and shorter tool life.

Impact of Cold Work

• Increased cold work leads to increased yield and tensile strength, and decreased ductility.