ta 201 l11
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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