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