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DEFORMING. Stress strain behavior of ductile materials. σ. 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.

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slide2
Stress strain behavior of ductile materials

σ

ε

Elastic-plastic with strain hardening

Elastic-perfectly plastic

Rigid perfectly plastic

Rigid plastic with strain hardening

slide3
Topics for today’s lecture

Deformation processes

⊙Deforming process characteristics

⊙Deforming physics

⊙Common deforming processes

⊙DFM

slide4
Process characteristics

Material/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

slide5
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

slide6
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….

slide7
Stress strain behavior of ductile materials

σ

ε

Elastic-plastic with strain hardening

Elastic-perfectly plastic

Rigid perfectly plastic

Rigid plastic with strain hardening

slide8
Example: 12L14 Steel in Duratec

12L14 Steel True Stress-Strain Behavior

Intercept

Tangent modulus

0.41 GPa [0.06 Mpsi]

Stress, kpsi

Stress, MPa

Young’s modulus

190 GPa [27.4 Mpsi]

Strain, mm /mm

slide9
Forging force and friction

Axisymmetric upsetting

Purpose

⊙ Find Fz(µ )

⊙ Sensitivity

Assumptions:

⊙ Tresca flow

⊙ Constant friction coefficient

⊙ Plastic deformation

slide10
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

slide11
Forging force and friction

Affect of friction on contact pressure

Y=75000 psi

h = ½ inch

mu = 0

mu = .1

mu = .2

mu = .5

Contact pressure [psi]

Increasing µ

Distance from center line [inches]

slide12
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 2µR/h

slide13
Forging force and friction

Y=75000 psi

h = ½ inch

Affect of part size and friction on forging force

mu = 0

mu = .1

mu = .2

mu = .5

Forging force [tons]

Increasing µ

Part size [inches]

slide14
Common processes

Sheet metal

Forging

Extrusion

Rolling

slide16
Forging

Blank

Edging

Blocking

Finishing

Trimming

slide18
Affect of grain size

Properties affected by grain size

⊙ Strength

⊙ Hardness

⊙ Ductility

⊙ For example (Ferrite)

We desire smaller grains for better properties

slide19
Recrystallization

Recrystallization and Grain Growth in Brass

Figure 7.21 Callister

Cold- worked(33% CW)

3s at 580 C: initial

recrystallization

4s at 580 C: partial replacement

8s at 580 C: completc

recrystallization

l5 min at 590 C: grain

growth

10 min at 700 C: grain

growth

http://www.mmat.ubc.ca/other/courses/apsc278/lecture11.pdf

slide20
DFM for deforming processes

Parting line and flash

Draft angle

Radii

Lubrication

slide21
Mechanics of spring back

Q: Why do we see spring back?

A: Elastic recover of material

slide22
Example: Elastic spring back

Example: Bending wire

slide23
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

Increasing

Steel