UNIT-3 Metal Forming (1)

UNIT III - METAL DEFORMATION: HOT AND COLD WORKING OF METALS

Definitions

  • Bulk Deformation/Machine Forming Processes: Classified into two categories: Hot Working and Cold Working.

    • Hot Working: Processes carried out above the recrystallization temperature.

    • Cold Working: Processes that occur below the recrystallization temperature.

Recrystallization

  • A process where atoms reach a high energy level due to heat and force, leading to the formation of new crystals.

  • Examples:

    • Lead and tin have a recrystallization temperature below room temperature, thus considered hot working at room temperature.

    • Steel has a recrystallization temperature in the range of 600-700 °C, making room temperature work cold working.

Hot Working

Advantages:

  • No strain hardening occurs since work is above recrystallization temperature.

  • High ductility of materials at elevated temperatures allows for easier manipulation, even for brittle materials.

  • Shear stress decreases with temperature increase, requiring less force for deformation compared to cold working.

  • Improved mechanical properties achievable through controlled heating.

Limitations:

  • Scaling occurs at high temperatures, leading to material loss and poor surface finish.

  • Difficulties in maintaining dimensional accuracy due to thermal expansion.

  • Handling hot metals can be challenging.

Cold Working

Advantages:

  • High dimensional accuracy achieved.

  • Surface defects are removed through processes.

  • Capable of producing thin gauge sheets.

  • No oxidation or scaling occurs as processes are done at room or low temperatures.

COMPARISON OF HOT WORKING AND COLD WORKING

  • Comparison summarized in a tabular format (details not provided in the notes).

FORGING

  • Forging: A bulk deformation process involving the application of compressive force to shape metals.

    • Typically a hot working operation, though cold forging can also be employed.

  • Forging enhances metal strength and maintains grain structure, producing stronger components than casting or machining.

Common Applications:

  • Used in structures where reliability and safety are critical (e.g., airplanes, automobiles, oil rigs).

Cold-working Processes:

  • Squeezing, Bending, Shearing, Drawing, Special Forming Processes.

Classifications of Processes:

  • Squeezing:

    • Rolling, Swaging, Staking, Sizing, Coining.

  • Drawing:

    • Blank Drawing, Tube Drawing, Wire Drawing.

  • Shearing:

    • Punching, Blanking, Cutting Off.

  • Bending:

    • Angle Bending, Roll Forming, Plate Bending.

FORGING OPERATIONS

Common Forging Procedures:

  • Upsetting: Increases cross-sectional area at the expense of height.

  • Punching: Produces cylindrical holes in the workpiece.

  • Fullering: Reduces cross-section area for further operations like drawing.

  • Drawing Down: Increases length; thickness and width may decrease.

DRAWING DOWN

  • Ironing: Produces drawn parts with varying thickness.

  • Welding: Joins two metal pieces by applying pressure after heating (forge welding).

  • Cutting Welding: Cuts large pieces for further operations.

  • Trimming: Removes flash material from forged components.

Forging Classifications:

  1. Smith Forging (Open-Die Forging):

  • Manual and Power Forging techniques.

  1. Impression Forging (Closed Die Forging):

  • Drop Forging and Machine Forging.

Open Die Forging:

  • Solid workpiece placed between two flat dies for deformation, with no restriction on material flow.

HAND FORGING

  • Done with repeated blows on an anvil; traditionally used by blacksmiths.

Applications:

  • Making simple shapes like chains, hooks, and agricultural tools.

IMPRESSION DIE FORGING

  • When a workpiece is sandwiched between shaped dies, undergoes plastic deformation, and excess material flows out as flashing.

MACHINE FORGING

  • Utilizes powered machines for faster and larger productions, improving quality through uniform deformation.

Drop Forging:

  • Most common forging method defined by the upper die dropping onto the lower die.

Press Forging:

  • Applies continuous pressure with minimal blows, best suited for larger components.

COLD FORGING

  • Variation of impression die forging performed at normal temperature.

ROLLING

  • A deformation process reducing thickness through compressive forces of opposing rolls.

Types of Rolling:

  1. Hot Rolling: Metal heated just below melting point.

  2. Cold Rolling: Retains shape acquired during rolling.

Rolling Operations:

  • Used extensively in steel-making plants to produce various structural shapes.

Rolling Mill Classifications:

  • Two-high, Three-high, Four-high, Cluster Rolling Mills.

SHEET METAL OPERATIONS

  • Involves working with metal sheets (16 to 30 gauge) through cutting, forming, and joining.

Cutting and Shearing Operations:

  • Involves mechanical cutting without forming chips.

  • Includes punching, slitting, trimming, etc.

BENDING

  • Transforming lengths into curves; outer surface under tension, inner surface under compression.

Types of Bending Operations:

  1. Supported Bending

  2. Unsupported Bending

  3. V-die Bending

  4. U-die Bending

DRAWING

  • Converts flat pieces into hollow shapes using a punch.

Types of Drawing Processes:

  1. Pure Drawing

  2. Ironing

METAL SPINNING

  • Involves forming circular cross-section parts over a rotating form block.

Applications:

  • Used to create items like funnels, bells, and radar dishes.

Advantages and Drawbacks:

  • Advantages: Low equipment and tooling costs.

  • Drawbacks: Operator skill-dependent and lack of uniformity.

COMPARISON OF SPINNING AND DRAWING

  • Spinning is ideal for low volume production due to low tooling and equipment cost, while drawing is preferred for mass production.