deforming. stress strain behavior of ductile materials σ ε elastic-plastic with strain hardening...
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DEFORMING
Stress strain behavior of ductile materialsσ
ε
Elastic-plastic with strain hardening
Elastic-perfectly plastic
Rigid perfectly plastic
Rigid plastic with strain hardening
Topics for today’s lecture
Deformation processes
⊙ Deforming process characteristics
⊙ Deforming physics
⊙ Common deforming processes
⊙ DFM
Process characteristicsMaterial/continuum changes during processing
⊙ Machining = Local, concentrated
⊙ Deforming = Over large volume
Force
⊙ Machining: ~10s - 100s of lbs
⊙ Stamping: ~10s - 100s of tons
Materials - Virtually all ductile materials
Shapes - Limited by strain/flow
Size - Limited by force/equipment
Advantages and disadvantages
Advantages
⊙ Parallel process: rapid bulk formation
⊙ Overall material properties improved
Disadvantages
⊙ Cost of equipment and dies
⊙ Limited flexibility in shapes and sizes (i.e. compared to machining
⊙ Accuracy
⊙ Repeatability
Important physics
Stress/strain
⊙ Affect CQFR
⊙ Affect deforming force -> equipment & energy requirements
Friction
⊙ Affect on deforming force -> equipment & energy requirements
⊙ Why lubrication is needed and can help
⊙ Friction is not repeatable… quality….
Stress strain behavior of ductile materialsσ
ε
Elastic-plastic with strain hardening
Elastic-perfectly plastic
Rigid perfectly plastic
Rigid plastic with strain hardening
Example: 12L14 Steel in Duratec
12L14 Steel True Stress-Strain Behavior
Intercept
Tangent modulus 0.41 GPa [0.06 Mpsi]
Young’s modulus190 GPa [27.4 Mpsi]
Strain, mm /mm
Str
es
s,
MP
aS
tres
s, k
ps
i
Forging force and frictionAxisymmetric upsetting
Purpose
⊙ Find Fz(µ )
⊙ Sensitivity
Assumptions:
⊙ Tresca flow
⊙ Constant friction coefficient
⊙ Plastic deformation
Forging force and friction
Eqxn: A – Equilibrium in r direction
dFinner arc dF friction top & bottom dF hoop dFouter arc
Eqxn: B -Tresca Yield Criterion
Forging force and friction
Y=75000 psi h = ½ inch
Affect of friction on contact pressure
Distance from center line [inches]
Co
nta
ct p
ress
ure
[p
si]
Increasing µ
mu = 0
mu = .1
mu = .2
mu = .5
Forging force and friction cont.
Now use Taylor’s series expansion (3 terms) to approximate the exponential function
Expand about 0, makes this approximation valid for small values of 2µ R/h
Forging force and friction
Affect of part size and friction on forging force
Fo
rgin
g f
orc
e [t
on
s]
Part size [inches]
mu = 0 mu = .1 mu = .2 mu = .5
Increasing µ
Y=75000 psi h = ½ inch
Common processesSheet metal
Forging
Extrusion
Rolling
Bending and shearing
Forging
Blank
Edging
Blocking
Finishing
Trimming
Rolling and extrusion
Affect of grain size
Properties affected by grain size
⊙ Strength
⊙ Hardness
⊙ Ductility
⊙ For example (Ferrite)
We desire smaller grains for better properties
RecrystallizationRecrystallization and Grain Growth in Brass
Figure 7.21 Callister
http://www.mmat.ubc.ca/other/courses/apsc278/lecture11.pdf
8s at 580 C: completcrecrystallization
3s at 580 C: initialrecrystallization
4s at 580 C: partial replacementCold- worked(33% CW)
l5 min at 590 C: graingrowth
10 min at 700 C: grain growth
DFM for deforming processes
Parting line and flash
Draft angle
Radii
Lubrication
Mechanics of spring backQ: Why do we see spring back?
A: Elastic recover of material
Example: Elastic spring backExample: Bending wire
Quantifying elastic spring back
Elastic recovery of material leads to spring back
⊙ Y = Yield stress E = Young’s Modulus t = thickness
⊙
⊙ With increase in Ri / t or Y OR decrease in E spring back increases
Titanium
SteelIncreasing