simulation techniques in manufacturing technology lecture 2 · © wzl/fraunhofer ipt forming...

© WZL/Fraunhofer IPT

Forming technology basics

Simulation Techniques in Manufacturing Technology

Lecture 2

Laboratory for Machine Tools and Production Engineering

Chair of Manufacturing Technology

Prof. Dr.-Ing. Dr.-Ing. E.h. Dr. h.c. Dr. h.c. F. Klocke

Seite 2 © WZL/Fraunhofer IPT

Summary 6

Overview of forming processes 5

Tribological interactions 4

Mechanical material behavior 3

Basics of material science 2

Introduciton 1

Table of contents


Introduction

Historical perspective

Source: Industrieverband Massivumformung e. V.

Metal forming is one of the oldest human work techniques.

Already 4.000 BC metals were processed by forging. Around

2.500 BC first copper alloys were found which lead to the name

Bronze Age

Between 700 and 500 BC iron replaced the use of bronze. The

process of melting the iron ore and the following forging process

cohered till the 13. and 14. century

About 1900 the forges produced widespread product lines for the

railway, the automotive industry and agricultural engines with

hammers driven by transmissions

Even today metal forming is an integral part of production industry.

There are hardly possible any technical products without formed

components


Metals account for about two thirds of all

the elements and about 24 % of the mass

of the planet and are still widely available

Wide industrial application of metals

throughout centuries can be explained by

their properties:

– Strength

– Toughness

– High melting point

– Thermal and electrical conductivity

– Ductility

Ductility of metals enables their shaping

by means of forming operations

Introduction

Metals


Summary 6





Introduciton 1

Table of contents


All materials and in particular metals

consist of atoms

The bond between metal atoms is

called metallic bond

At metallic bond the valence electrons

(outer most electrons of an atom) are

free to move in a metal structure

consisting of positively charged ions

Atoms in metals are not free but stay

in an equilibrium state:

– Attraction forces between ions and

electrons

– Repulsive forces between ions

Attraction force

distance

between

atoms X

Resulting Force

Fc - cohesion force

X0 - smallest equilibrium distance between atoms

Repulsive force

X0

Fc

Fo

rce

F

F < Fc F > Fc

+ + + + + + + + +

+ + + + + + + + +

+ + + + + + + + +

Positively charged ions

Negatively charged

electron gas

Basics of material Science

Molecular structure of metals


Yo

un

g‘s

mo

du

lus

[MP

a]

Temperature T

[°C] 0

200

100

50

150

0 200 400 600 800 1000


Spring model by Gottstein

Interatomic forces keep the atoms in a

stable arrangement at a minimum of

potential energy

Elastic deformation of the system due

to an external force leads to the

displacement of atoms and storing of

the potential energy in the system

Load relieve leads to a return of the

atoms to their original position at the

potential energy minimum

Distance between atoms and the

magnitude of forces acting between

them is a material property

Temperature dependency of Young‘s modulus

γ-Fe Cu

Mg

Al

α-Fe

Source: Gottstein, G: Materialwissenschaft und Werkstofftechnik, 4. Auflage, 2014, ISBN: 978-3-642-36602-4

Atomic shell

Interatomic forces

Atomic core

Spring model by Gottstein

x0



Crystal lattice

Atoms of a metal have determined

spatial distribution i.e. crystal lattice

For a description of the crystal lattice

simple geometrical forms unit cells are

used

There are three unit cells of metals:

– fcc – face centered cubic

– bcc – body centered cubic

– hex – hexagonal

The crystal lattice is characterized

through the distance between atoms

(for most of the metals 0.25 – 0.5 nm)

Unit cells usually have anisotropic

material properties along different

directions

Crystal lattice

Unit cell



Lattice types of an unit cell

Face-centred

cubic

(fcc)

Body-centred

cubic

(bcc)

Hexagonal

(hex)

Examples:

Sliding planes:

Sliding directions:

Sliding systems:

Formability:

g-Fe, Al, Cu

4

3

12

Very good

a-Fe, Cr, Mo

6

2

12

Good

Mg, Zn, Be

1

3

3

Poor


fcc

bcc


Iron-Carbon Phase Diagram

Carbon content in weight percent

Cementite content in weight percent

d-Fe

d- + g-Fe

Te

mp

era

ture

in °

C

Liquid + d-Fe

Liquid

Fe3C

(Cementite)

Liquid +

Fe3C

Liquid + g-Fe

g-Fe + Fe3C

g-Fe

(Austenite)

a-Fe (Ferrite)

g- + a-Fe

a-Fe + Fe3C

mixed a -Fe + Fe3C



Atomic and macroscopic view of metal structures

Ideal

crystal

structure

Special agglomeration of crystals

Section plane

a

Crystal lattice Unit cell

2D–Cut

of the microstructure

Microstructure

Scheme Photograph

Real

crystal

structure



Macroscopic understanding of metals

Real metals are polycrystals i.e.

consist of many single crystals

The reason for a formation of the

multiple crystalites or grains is that the

crystalization starts at many points of

the molten material simultaneously

Every single grain of a metal has a

different crystallographic orientation

If two grains with different orientations

meet a grain boundary is formed

Different orientation of grains

compensate for the anisotropy of their

mechanical properties so that metals

behave quasiisotropic on the macro

scale (if not textured, i.e. rolled sheet

metal)



Equilibrium state of a crystal lattice

Crystal structure of real metals has a

lot of deviations from the ideal regular

lattice

Each deviation leads to a distortion of

the lattice and results in a misbalance

of the electrostatic forces between

atoms

Additional electrostatic forces increase

the potential energy of the atomic

lattice

Due to lattice defects there is

potentially another equilibrium state of

the crystal which it can take if

provided some external energy (e.g.

mechanical work A)

A

x1 x0

Fre

e e

nerg

y



1D lattice defects

Vacancy Intermediate-lattice atom FRENKEL-matching

Substituting atom Emplacement atom

The foreign atoms induce

stress to the crystal lattice.

This stress affects crystal

strengthening of the

material.



2D lattice defects

Screw dislocation Edge dislocation

Dislocations are linear defects in the lattice



3D lattice defects

Low angle grain boundary High angle grain boundary


Summary 6



Creep 3.3

Plasticity 3.2

Elasticity 3.1



Introduciton 1

Table of contents


Unloaded Tensile-loaded

s - Nominal stress

e - Natural strain

E - Young‘s Modulus

l0 l1

s

s

Mechanical material behavior

Atomic representation of pure elastic-tensile deformation

00

01

l

Δl

l

ll ε

E

ele

s


g

g - Shear angle

- Shear stress

G - Shear modulus

n - Poisson‘s ratio

E - Young‘s modulus


Atomic representation of pure elastic-shear deformation

Gelg

1-

2G

E n

Unloaded Shear-loaded


For elastic behavior:


Stress-strain curve for elastic behavior

00

01

l

l 00 l

Δl

l

ll

l

dl ε

l

dl d

1

0

e

A

F

0

s

tanel

el

e

sa E

ele

s

Str

ess

Strain

Re

sel

eel

Natural strain:

Natural stress:

α

for σ ≤ Re

with: tan α = E

F

F

A

A0 l 0

l

∆l


Summary 6



Creep 3.3

Plasticity 3.2

Elasticity 3.1



Introduciton 1

Table of contents



Types of plastic deformation

Dislocation movement

Low force requirements

Sliding

High force requirements

Before

After



Sliding and dislocation movement

Dislocation movement Sliding



Constraints on dislocation movement

Constraints on dislocation

movement are induced by:

Edge dislocation

Substituting atom

Emplacement atom

Unit cell of

ferrite

0,286 nm Screw dislocation

Grain boundary scarf dispersion Grain diameter

Incoherent dispersions

Coherent, lattice oriented dispersions

High melting point

phase

Slip line

Grain boundary

dispersions

Further dislocations within

the crystal

Grain boundaries and grain

boundary dispersions

Knots (networks of locked

dislocations)

Foreign atoms in the lattice

Dispersions (Orowan

mechanism)

Other phases


Grain boundary hardening – Hall-Petch-Relation

With increasing change in shape close adjacent dislocations impede each other because of surrounding stress fields, which results in a strain hardening of the material


Strain hardening

D

kσRes 0

Dislocation movement

(scheme)

Sliding plane

Dislocation source

Movement direction

Grain boundary

S

D

k, σ0

Res

= average grain size

= Hall-Petch-Constant

= Yield stress

S1 S2

Grain 1 Grain 2

Grain boundary

2

D

Piled up dislocations

at a grain boundary



Recording of dislocation movement by an infrared camera

F

Tensile specimen of tempered aluminum

with a reflective surface

F



Stress-strain curve up to the uniform elongation

A

)l(F

0

s

Str

ess

Strain

Nominal stress: (related to starting cross section)

Rm

Re ,se

eel epl

Load

relieving Reload

)l(A

)l(F s

True stress: (related to real cross section)

σ‘

σ

Ag – Uniform elongation

F

F

A

A0 l 0

l

∆l



Flow stress determination using the example of a tensile test

)l(A

)l(F´s

Strain

True stress:

Rm

Re ,se

eel j or epl Ag

Fracture

s0

s‘

kf

l0

l

l

A0

A

elf εA

F(l)

l

l

A

F(l)

A(l)

F(l)k 1

000

Flow stress:

Usable region to

determinate the flow stress

Str

ess

F

F

A

A0 l 0

l

∆l

lΔ



Flow curve

Flo

w s

tress k

f

True strain j

Required stress to overcome

strain hardening

Minimal required stress for

initial plastic deformation



Movie: Methods for determination of the yield stress

Tension test Compression test Torsion test

Low deformation

Uniaxial stress state to the uniform elongation Au , then multiaxial elongation

To true strain φ = 1

Multiaxial stress state in case of insufficient lubrication

Use of Teflon foil for uniform friction

High deformation

Multiaxial stress state

Especially suitable for determining of flow curves at high temperatures

The stress state during determination of the flow curve and the forming should be equal.



Limits for cold forming without annealing

Overload machine

Overload tool

Critical deformation /

fracture

jVB – Fracture strain

Flo

w s

tress k

f

True strain j

With increasing strain hardening the flow stress increases.



Static Recrystallisation

requirements:

- jv > 0

- T > T Recrystallisation

- impact time

Schematic course of recrystallisation of cold formed structure

du

cti

le y

ield

A10,

ten

sil

e s

tre

ng

th R

m

cry

sta

l

reg

en

era

tio

n

temperature, °C

small decrease of Rm

large increase of A10



Effective Strain and Temperature Influence the Grain Size g

rain

siz

e

effective strain

range of

recrystallisation



Static and dynamic recrystallization

Static recrystallization

– Static recrystallization occurs at a

tempering above TR ≈ 0,4TS after a cold

forming process and before reaching the

fracture strain φVB

– Annealing processes for recrystallization

purpose reset the grain structure leading

to a reduction of strain hardening and thus

increase the formability

Dynamic recrystallization

– Dynamic recrystallization only occurs at

hot forming processes (T >> TR) and is a

continuously neutralization of the

dislocation density during the process

φVB1 φVB2 Effective strain j

Annealing process

for recrystallization

(Static recrystallization)

Cold forming T << T Rekri

Hot forming

T >> T Rekri Dynamic recrystallization

Flo

w s

tress k

f



Forming Temperature and Velocity Influence the Flow Stress

forming temperature below

recrystallisation temperature

high forming velocity

low forming velocity

forming temperature above

recrystallisation temperature

effective strain

flo

w s

tre

ss



Influence of temperature on dynamic recrystallization

Hot-compression tests at different temperatures

– Compression on the same compression level = same deformation

– Recrystallization was immediately stopped by quenching after testing

Different grain structure depending on process-temperature

– 700°C: Hardly any recrystallization, heavily deformed structure

– 800°C: Nucleation = beginning recrystallization

– 1000°C: Nearly complete recrystallization, uniform structure

Source: IBF, Exzellenzcluster „Integrative Produktionstechnik für Hochlohnländer“, Teilprojekt B21; WZL

Werkstoff Stauchprobe: 25MoCrS4

T = 700°C

50 µm

T = 800°C

50 µm

T = 1000°C

50 µm


Summary 6



Creep 3.3

Plasticity 3.2

Elasticity 3.1



Introduciton 1

Table of contents



Creep definition

Creep material behavior should be

taken into account for the

constructions operating at

temperature 𝑇 above 0.3𝑇𝑚 (𝑇𝑚 is the

melting temperature)

The applied stress is usually less than

the yield stress of the material 𝑅eS

Uniaxial creep curve is obtained from

tensile creep test at constant load and

constant temperature

Three creep stages are distinguished

at creep strain vs. time diagram: I –

primary creep, II – secondary creep

and III – tertiary creep

Creep is the progressive time-dependent inelastic deformation under constant load and

temperature.

Relaxation is related to creep phenomena, which can be defined as time-dependent

decrease of stress under the condition of constant deformation and temperature.

Time

I II III

creep fracture

minimum creep

rate



Creep modelling

Secondary creep

Models assuming existence of creep

potential:

𝒔 is stress deviator, the function for

equivalent creep rate could be defined as:

Power law 𝜀 𝑒𝑞𝑐𝑟 =

3

2𝑎 𝜎𝑒𝑞

𝑛−1𝒔

Exponential law 𝜀 𝑒𝑞𝑐𝑟 = 𝑏 𝑒𝑥𝑝

𝜎𝑒𝑞

𝜎0− 1

Hyperbolic sine law:𝜀 𝑒𝑞𝑐𝑟 = 𝑎𝑠𝑖𝑛ℎ

𝜎𝑒𝑞

𝜎0

For many materials secondary creep lasts the most life-time of the construction. To predict

long term behavior of the structure only the secondary creep can be described.

a, n, b, 𝜎0, m, k, l are material parameters, which should be determined by fitting the family of creep

curves, obtained from creep tests at a given temperature. Given secondary and primary creep models

are available as standard material behavior in Abaqus Software.

Primary creep

Time hardening 𝜺 𝑐𝑟 =3

2𝑎 𝜎 𝑒𝑞

𝑛−1𝑡𝑚𝒔

Strain hardening 𝜺 𝑐𝑟 =3

2𝑏 𝜎 𝑒𝑞

𝑘−1 𝜀 𝑒𝑞𝑐𝑟 𝑙

𝒔

Tertiary creep and Damage

Continuum damage mechanics approach

𝜺 𝑐𝑟 =3

2𝑎

𝜎𝑒𝑞

1 − 𝜔

𝑛 𝑺

𝜎𝑒𝑞

𝜔 is Rabotnov parameter related to the reduction of

the cross-section area due to accumulated voids,

cavities, etc.

𝜺 𝑐𝑟 =3

2𝜀 𝑒𝑞

𝒔

𝜎𝑣𝑀


Summary 6





Introduciton 1

Table of contents


Workpiece

Lubricant

Tool Substrate

Boundary zone

Interface

Coating

Interaction

Tribological interactions

Tribological system

Friction and wear emerge as a

consequence of relative motion

between tool and workpiece

Interaction between tool and

workpiece is described by

means of the tribological

system

Scientific description of friction

and wear is denoted as

tribology

Friction and wear are minimized

by means of:

– Lubricants and/or

– Coatings

Process forces

Temperature

Relative velocity

…

System input

Friction

Wear

System output

Tribological system

Ambient medium


v


Friction laws and friction models

Model representation Friction laws

Coulomb‘s law

Friction factor law

Reality

Coulomb’s law:

- Sheet metal forming

Friction factor law (shear friction):

- Bulk forming

S

he

ar

str

ess

Normal stress 𝜎𝑁

Body A with mass 𝑀𝐴

Body B with mass 𝑀𝐵

𝐹𝐴 = 𝑀𝐴 ∙ 𝑔

𝑚 –proportionality factor

𝑘 – shear flow stress of the

softer material

𝜏𝑅 = 𝜇 ∙ 𝜎𝑁

𝜏𝑅 = 𝑚 ∙ 𝑘 = 𝑚 ∙𝑘𝑓

3

0 ≤ 𝑚 ≤ 1

𝑚 = 0 – frictionless state

𝑚 = 1 – condition of adhesion

𝜎𝑁

𝜏𝑅



Wear mechanisms (1/2)

Adhesion Adhesion Surface fatigue Surface fatigue

Cleaving or micro cutting by means of an

interlocking contact of abrasive particles or

roughness peaks of an opposite body

Surface material change due to chemical

reaction with the components of

intermediate medium

Wear is a progressive material loss from the surface of a solid body (base body), caused due to

mechanical reasons, i.e. contact- and relative motion of a solid, liquid or gaseous counter body

[GfT Arbeitsblatt 7].

Adhesive interactions (secondary forces or

primary valence forces) on the surface can

exceed bonding forces within the material

Cyclic loads lead to the crack initialization,

crack growth and, eventually, to the particle

erosion

Abrasion Abrasion Tribochemical reaction Tribochemical reaction



Wear mechanisms (2/2)

200 μm 200 μm 10 μm

100 μm 100 μm

Smearing on an HSS extrusion punch Fatigued lateral surface of a HSS fine blanking

punch

Abrasion on a ceramic deep drawing ring Tribochemical wear on a borated steel

Tool wear consists of different wear types: adhesion, surface fatigue, abrasion and tribochemical wear

Adhesion Adhesion Surface fatigue Surface fatigue

Abrasion Abrasion Tribochemical wear Tribochemical wear


Summary 6

Sheet metal separation 5.3

Sheet metal forming 5.2

Bulk metal forming 5.1





Introduciton 1

Table of contents


Overview of forming processes

Classification of manufacturing processes (selection)

Forming

Bulk metal

forming

Manufacturing Processes

Casting Cutting

Cold

forging

Semi-hot

forging

Sheet metal

forming

Sheet

cutting

Rolling

Extruding

Compressive

forming

Extruding

Compressive

forming

Semi-hot rolling

Forging

Deep drawing

Stretch forming

Hydroforming

Shearing

Fine Blanking

Tearing

Forging



What is the meaning of forming?

Manufacturing by plastic deformation

Constant workpiece volume

Constant workpiece mass

Cohesion of workpiece is retained

Source: GCFG, Feintool

Manufacturing by cutting

Material cohesion is released locally

No generation of formless substance

(no chips)

Cutting

Forming Bulks and wires

Sheets

Sheet metal forming

Cutting:

Fineblanking, Shearing

Bulk metal forming



Bulk forming – Sheet metal forming – Blanking

Source: Saarstahl AG, Benteler AG, Feintool AG

Geometry

Process temperature

Strain hardening

Tool loads

Cross section changing

Forces

Plane (h << b, t)

Low up to medium

Low

Low

Low

Low up to medium

Sheet metal forming

Plane (h << b, t)

Low

Low

Low

Low

Low up to medium

Blanking

Spatial

Low up to high

Low up to high

Medium up to high

High

High

Bulk forming


Summary 6








Introduciton 1

Table of contents


Bulk metal forming

Component spectrum of bulk forming

Hinge bearing

Cardan shaft

Crank shaft

Gear shaft

Gear wheels

Drive shaft

Axle journal

Wheel carrier

Turbine blade

Source: Infostelle Industrieverband Massivumformung e.V.


1,3 kg

0,4 kg

Bulk metal forming

Advantages of bulk forming

Cutting Forming

Semi-finished part Component Semi-finished part Component


Bulk formed component

Bulk metal forming

Advantages of bulk forming

Source: Infostelle Industrieverband Massivumformung e.V., ThyssenKrupp Presta

Fiber orientation


Bulk metal forming

Process variants of bulk forming

Temperature

Cold forming T ≈ 25 °C Warm forming T ∈ [500, 900] °C Forging T ∈ [900, 1250] °C

Advantages:

Less force and work

requirements

High fracture strain

Disadvantages:

High energy input for heating

High thermal impact on tools

High material costs for tools

Dimension faults by shrinkage

Material loss and increased

finishing caused by scale

formation

Advantages:

Lower tool material costs as for

hot forging

Low influence of forming velocity

No energy costs for heating

No dimensional faults caused by

shrinkage

High surface quality

Increasing strength of the

workpiece due to strain

hardening

Disadvantages:

High force and work

requirements

Limited plastic strain

Often complex and polluting

lubrication necessary

Advantages:

Strengthening of the workpiece

Small range of tolerance caused

by shrinkage

Good surface quality

Disadvantages:

Energy input for heating

High flow stresses


Initial condition

Cold forming

Warm forming

Forging

0,001 – 30 kg

0,001 – 50 kg

0,05 – 1500 kg

< 1,6

< 4

< 6

Less

Low

High

Workpiece

weight

Fracture strain,

φVB

Finishing effort

Bulk metal forming

Efficiency of bulk metal forming


Forming process IT-statement according DIN ISO quality

5 6 7 8 9 10 11 12 13 14 15 16

Cold extrusion

Warm extrusion

Hot extrusion

Mean roughness index Ra / µm

0,5 1 2 3 4 6 8 10 12 15 20 25 30

Achievable with special efforts Achievable without special efforts

Bulk metal forming

Efficiency of bulk metal forming


Summary 6








Introduciton 1

Table of contents


Sheet metal forming

Techniques of metal forming; Bulk forming – sheet metal forming

Bulk half-finished products

High changes in cross sections and dimensions

High plastic deformation

High strain hardening (in cold forming)

High forces

High tool loads

Plane half-finished product: Sheet metal (t << b, l)

No or low unwanted changes of the original wall thickness

Lower plastic deformation

Lower strain hardening

Lower forces

Lower tool loads

Source: G. Siempelkamp GmbH & Co. KG, Saarstahl AG, BMW Group AG

Bulk forming Sheet metal forming


Sheet metal forming

Typical sheet metal products

Half-finished product: Sheet metal

Products: Hollow parts with constant wall thickness

Low deformation compared to bulk forming

Low forces compared to bulk forming

Process example:

– Deep drawing

– Hydroforming

– Spinning

– Stretch forming etc.

Source: Benteler AG

Passenger car structure elements

Roof

reinforcement

Underbody

Door Impact

protection

Base plate

Tunnel

Dashboard

support

Front bumper

Rear

bumper

B-Pillar

reinforcement

Door sill

reinforcement

A-Pillar

reinforcement

Side frame

reinforcement

Window frame

reinforcement


Sheet metal forming

Typical sheet metal processes

Sheet metal forming

Bending Stretch forming process

Quelle: Total Materia

Hydroforming

Deep drawing Ironing


Summary 6








Introduciton 1

Table of contents


Sheet metal separation

Overview

Half-finished product:

Sheet metal

Mechanical cutting of work

pieces

No appearance of formless

material (no chipping)

Process example:

– Blanking

– Fine blanking

Source: Dr. Karl Bausch GmbH Co. KG, Otto Bauckhage



Comparison of blanking and fine blanking (1/2)

Assembly of high-precision work pieces with

smooth cutting surfaces free from cracks

Cutting surfaces often serve as functional

surface without finishing processes

Triple action press required (Punch force, Vee

ring and blank holder force, Counter punch

force)

Sheet metal thickness up to 16 mm

Main field of application: automotive

engineering, medical technology, household

equipment

Shearing is the most used blanking process

Manufacturing of sheet metal components with

very high output

Simple and cheap tool geometry

Manufacturing of sheet thickness up to 20 mm

Main field of application: automotive

engineering, medical technology, household

equipment

Fine blanking Blanking


1 Cutting die

2 Guiding plate

3 Punch

1 Cutting die

2 Vee ring and

blank holder

3 Punch

4 Counter punch

FS – Punch force

FR – Vee ring and blank

holder force

FG – Counter punch

force

Blanking Fine blanking

Fs = Punch force

Fs Fs

FR FR

FG

Die clearance ca. 5,0% of sheet

metal thickness

ca. 0,5% of sheet

metal thickness


Comparison of blanking and fine blanking (2/2)

1

2 3

1

2 3

4

Source: Feintool


In fineblanking, the smooth sheared zone can take a share of 100%


Comparison of sheared edges in blanking and fine blanking

Shearing

Fineblanking

Zone of die roll

Smooth sheared zone

Rupture zone

burr

Smooth sheared zone

burr

Zone of die roll



Comparison of sheared edges in blanking and fine blanking

Method IT - classification Costs Output

high

rough (IT 11) low high

low fine (IT 7)

Sheared

surface

Fineblanking

Shearing

Source: Feintool


Summary 6





Introduciton 1

Table of contents


Summary

Recap of the material science basics concerning

metals structure

Discussion of the elastic and plastic material

behavior

Introduction to the basic tribological aspects

Presentation of the basic information on the creep

behavior of metals

Brief Introduction of the basic forming processes:

– Bulk forming

– Sheet metal forming

– Sheet metal separation

Deep understanding of the boundary conditions of a forming process including material and tribological

behavior are obligatory prerequisites for a set-up of a realistic simulation model

simulation techniques in manufacturing technology lecture 2 · © wzl/fraunhofer ipt forming...

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