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Metal Forming

Metal Forming

• The tool, usually colled a die, applies stresses that exceed yield strength of metal

Stresses in Metal Forming

• Stresses to plastically deform the metal are usually compressive
• However, some forming processes
  • Stretch the metal(tensile stresses)
  • Other bend the metal(tensile and compressive)
  • Still others apply shear stresses

Material Properties in Metal Forming

• Desirable material properties
  • Low yield strength & high ductility
• These properties are affected by temperature
  • Ductility increases and yield strength decreases when work temperature is raised
• Other factors
  • Straint rate(변형속도) and friction

Bulk Deformation Processes

• Characterized by significant deformations and massive shape changes
• “Bulk” refers to workparts with relatively low surface area-to-volume ratios
• Starting work shapes include cylindrical billets and rectangular bars
• Basic bulk deformation processes: rolling, forging, extrusion, drawing

Sheet Metalworking

• High surface area-to-volume ratio of staring metal
• Basic sheet metalwokring operations: bending, drawing, shearing

Temperature in Metal Foraming

• Any deformation operation can be accomplished with lower forces and power at elevated temperature
• Three Temperature range in metal forming:
  • Cold
  • Warm
  • Hot working

Cold Working

• Performed at room temperature(보통 20도) or slightly above
• Important mass production operations
• Minimum or no machining usually required
  • These operations are near net shape or net shape processes

Advantages of Cold Forming vs. Hot Working

• Better Accuracy, closer toleratnces
• Better surface finish
• Strain hardening(변형 강화) increases strength and hardness
• Grain flow during deformation can cause desirable directional Properties in product
• No heating of work required

Disadvantages of Cold Forming

• Higher forces and power required
• Surfaces of staring workpiece must be free of scale and dirt
• Ductility and strainhardening limit the amount of forming that can be done

Warm Working

• Performed at temperatures above room temperature but below recrystallization temperature
• Dividing line between cold working and warm working often expressed in terms of melting point:
  • 0.3Tm, where Tm = melting point for metal(absolute temperature)

Advantages of Warm Working

• Lower forces and power than in cold working
• More intricate work geometries possible
• Need for annealing may be reduced or eliminated

How Working

• Deformation at temperature above recrystallization temperature
• Recrystallization temperature = about one-half of melting point on absolute scale
  • In practive, hot working usually performed somewhat above 0.5Tm
  • Metal continues to soften as temperature increases above 0.5Tm, engancing advantage of hot working above this level

Why Hot Working?

• Capability for substaintial plastic deformation
• Why?
  • Strength coefficient is substantially less than at room temperature
  • Strain hardening exponent is zero(theoretically) Ductility is significantly increased

Advantages of Hot Working vs. Cold Working

• Workpart shape can be significantly altered
• Lower forces and power required
• Metals that usually fracture in cold working can be hot formed
• Strength properties of product are generally isotropic
• No strengthening of part occurs from work hardening
  • Advantageous in cases when part is to be subsequently processed by cold forming

Disadvantages of Hot Working

• Lower dimensional accuracy
• Higher total energy required(due to the thermal energy to heat the workpiece)
• Work surface oxidation, poorer surface finish
• Shorter tool life

Friction in Metal Forming

• In most metal forming processes, friction is undesirable
  • Metal flow is retarted
  • Forces and power are increased
  • Wears tooling faster
• Friction and tool wear are more severe in hot woriking

Lubrication in Metal Forming

• Metalworking lubricants are applied to tool-work interface in many forming operations to reduce garmful effects of friction
• Benefits
  • Reduces sticking, forces, power, tool wear
  • Better surface finish
  • Removes heat from the tooling

Considerations in Choosing a Lubricant

• Type of forming processes(rolling, forging, sheet metal drawing, etc.)
• Hot working or cold working
• Work material
• Chemical reactivity with tool and work metals
• Ease of application
• Cost