FUNDAMENTALS OF METAL FORMING

Rolling

Forging

Extrusion Deep drawingWire drawing

FUNDAMENTALS OF METAL FORMING

• Overview of Metal Forming• Material Behavior in Metal Forming• Temperature in Metal Forming• Strain Rate Sensitivity• Friction and Lubrication in Metal Forming

Metal FormingLarge group of manufacturing processes in which plastic

deformation is used to change the shape of metal work pieces

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

• The metal takes a shape determined by the geometry of the die

Bulk Deformation Processes

• Characterized by significant deformations and massive shape changes

• "Bulk" refers to work parts with relatively low surface area to volume ratios

• Starting work shapes include cylindrical billets and rectangular bars

• Direct-compression-type process• Indirect-compression process• Tension-type process• Bending process• Shearing process

Metal Working Classification

Basis: type of force applied to the work piece

Basic bulk deformation processes: Rolling

Basic bulk deformation processes: Forging

Basic bulk deformation processes: Extrusion

Basic bulk deformation processes: Drawing

Sheet Metalworking

• Forming and related operations performed on metal sheets, strips, and coils

• High surface area to volume ratio of starting metal, which distinguishes these from bulk deformation

• Often called press-working because presses perform these operations− - Parts are called stampings− - Usual tooling: punch and die

Basic sheet metalworking operation: Bending

Basic sheet metalworking operation: Deep Drawing

Basic sheet metalworking operation: Shearing

Stresses in Metal Forming

• Stresses to plastically deform the metal are usually compressive− - Examples: rolling, forging, extrusion

• However, some forming processes − - Stretch the metal (tensile stresses)− - Others bend the metal (tensile and

compressive)− - Still others apply shear stresses

Material Properties in Metal Forming

• Desirable material properties: − - Low yield strength and high ductility

• These properties are affected by temperature:

- Ductility increases and yield strength decreases when work temperature is raised

• Other factors: − - Strain rate and friction

Material Behavior in Metal Forming

• Plastic region of stress-strain curve is primary interest because material is plastically deformed

• In plastic region, metal's behavior is expressed by the flow curve:

where K = strength coefficient; and n = strain hardening exponent

• Stress and strain in flow curve are true stress and true strain

nt Kεσ =

Typical values of K and n (σt = K·εn)MATERIAL K (MPa) n Aluminum, 1100-O 2024-T4 5052-O 6061-O 6061-T6 7075-O Brass, 70-30, annealed 85-15, cold-rolled Bronze (phosphor), annealed Cobalt-base alloy, heat treated Copper, annealed Molybdenum, annealed Steel, low-carbon, annealed 1045 hot-rolled 1112 annealed 1112 cold-rolled 4135 annealed 4135 cold-rolled 4340 annealed 17-4 P-H annealed 52100 annealed 304 stainless, annealed 410 stainless, annealed

180 690 210 205 410 400 895 580 720

2070 315 725 530 965 760 760

1015 1100 640

1200 1450 1275 960

0.20 0.16 0.13 0.20 0.05 0.17 0.49 0.34 0.46 0.50 0.54 0.13 0.26 0.14 0.19 0.08 0.17 0.14 0.15 0.05 0.07 0.45 0.10

Engineering stress – strain curve

True Stress (σt) & Strain (ε)

Flow curve

Flow Stress

• For most metals at room temperature, strength increases when deformed due to strain hardening

• Flow stress = instantaneous value of stress required to continue deforming the material

where Yf = flow stress (true stress), that is, the yield strength as a function of strain

nf KY ε=

Average Flow StressDetermined by integrating the flow curve equation between zero

and the final strain value defining the range of interest Where ε = maximum strain during deformation process

nKY

n

f +=

1

_ ε

True strain

Temperature in Metal Forming

• For any metal, K and n in the flow curve depend on temperature

Both strength and strain hardening are reduced at higher temperaturesIn addition, ductility is increased at higher temperatures

Temperature in Metal Forming

• Any deformation operation can be accomplished with lower forces and power at elevated temperature

• Three temperature ranges in metal forming: − - Cold working− - Warm working− - Hot working

Influence of Annealing Temperature on the Tensile Strength and Ductility of a Brass Alloy

Influence of Annealing Temperature on the Tensile Strength and Ductility of a Brass Alloy

Hot Working

• Deformation at temperatures above recrystallization temperature

• Recrystallization temperature = about one-half of melting point on absolute scale 1. In practice, hot working usually performed

somewhat above 0.5Tm

2. Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level