Metal Forming Processes and Principles

Production Technology: Metal Forming Operations

• Metal forming is a process that changes the shape of a metal through plastic deformation, using external forces.
• Metal forming processes use various types of stresses to deform the metal.
  • Compressive stresses are common in rolling, forging, and extrusion.
  • Tensile stresses are used in some forming processes to stretch the metal.
  • Tensile and compressive stresses are involved in bending operations.
  • Shear stresses are used in shearing.
• Starting materials for metal forming are typically ingots, which are obtained by casting molten metal.
• Continuous casting methods create slabs, blooms, or billets.
• Slabs are rectangular and used for flat products like plates and sheets
• Blooms are used in the manufacture of structural profiles and columns, rails.
• Billets are round or square and are used to make bars, rods, pipes, tubes, and wire coils.
• Metal shapes are further altered using metal forming operations to create standard products.

Types of Deformation Processes

• Metal forming operations are categorized by temperature ranges:
  • Hot working: Performed above the recrystallization temperature.
    • This allows for large deformations and minimal strain hardening.
    • Hot working has advantages like lower forces/power, enabling large deformations with fewer passes on metal that would crack in cold working, and no strain hardening.
    • However, it has disadvantages such as greater energy required (heating the workpiece), work surface oxidation (scale), and shorter tool life.
  • Warm working: Performed at temperatures above room temperature but below recrystallization temperature.
    • Dividing line between cold & warm working often described in terms of a percentage of the metal's melting point (0.3 -0.5).
  • Cold working: Performed at room temperature or slightly above, below the recrystallization temperature.
    • Cold working results in a better accuracy, surface finish, and increased strength and hardness due to strain hardening.
    • Disadvantages include higher forces/power required, limitations on the amount of forming due to ductility and strain hardening, and potentially needing annealing in some cases.

Classification of Metal Forming Processes

• Metal forming processes are categorized into:
  • Bulk deformation: Involves large shape changes and low surface area-to-volume ratios.
    • Processes include rolling, forging, extrusion, wire drawing.
  • Sheet metalworking: Involves forming operations on metal sheets, strips, and coils, with high surface area-to-volume ratios.
    • Processes include bending, deep drawing, shearing.

Bulk Deformation Processes

• Rolling: Reduces the thickness of a slab or plate by compressing it between two rotating rolls.
  • Types of rolling include flat rolling (rectangular cross-section reduction) and shape rolling (complex shapes).
  • Equipment includes two-high, three-high, four-high, cluster, planetary, and tandem roll mills.
  • Thread rolling and ring rolling are also included as specialized rolling processes.
• Forging: Compresses a workpiece between shaped dies to impart the die shapes into the final part.
  • Types of forging include open-die forging, impression-die forging, and flashless forging.
    • Open-die forging: Work is compressed between flat dies allowing lateral flow without constraint.
    • Impression-die forging: Workpiece is formed under pressure in closed cavities; some material that isn't constrained flows outward as a flash.
    • Flashless forging: The entire work piece remains confined within the die and no material flows outward from the die during compression.
• Extrusion: Compresses the workpiece into the die opening, taking on the die's cross-sectional shape.
• Wire drawing: Similar to extrusion, the workpiece is pulled through the die opening to take the die's cross-sectional shape.

Sheet Metalworking Processes

• Bending: Strains sheet material with a punch to create a bend.
• Deep drawing: Stretches flat sheet metal into a hollow shape like a cup using a blank holder on the die that clamps the blank while the punch pushes into the metal.
• Shearing: Cuts sheet metal using a punch and die.