Dr. Amitava De.pdf

8/13/2019 Dr. Amitava De.pdf

1/25

[email protected], www.me.iitb.ac.in/~amit.html

Mechanical Engineering

Department; Indian

Institute of Technology

Bombay

WRT_2012; IIT Chennai; 23/11/12 24/11/12

Computational Modeling of FusionWelding Process State of the art

Amitava DeMechanical Engineering Department, Indian Institute of Technology Bombay, India


2/25



Department; Indian


Bombay

Outline

Why Process Models are important in Welding ?

How do these models lack reliability ?

What is referred to as Integrated process model ?

How integrated models can serve as real design tool ?

What are the future challenges ?


3/25



Department; Indian


Bombay

Why process models are important in welding?

1 mm

Weld geometry

High peak temperature

Rapid change in thermal cycle

Continuous change in S-L boundary

Addition of filler material (if required)

Experiments provide with thefinal weld dimensions.

Experiments convey very littleabout evolution process of theweld pool.

Melt pool

Heat affectedzone

Solidifiedzone

HeatSource

W

P

Weldingspeed


4/25



Department; Indian


Bombay

Experiments alone can be a Puzzle

5200 W

(a) (b)

20 ppm sulfur 150 ppm sulfur

1900 W

(d)(c)

* Minor changes in composition => major changes in geometry

* Does not always happen!

W. Zhang, et al., Per.Comm. (2003).


5/25



Department; Indian


Bombay

So, what do we expect from process models?

Welding involves interactions of many physical processes which are

affected by several parameters such as,

Physical Processes

Heat Transfer

Fluid Flow

Deformarion

Phase Change

Evaporation

Diffusion

Welding Parameters

Work Piece Dimensions

Weld Joint Geometry

Current, Voltage, Speed or Time

Preheat and Inter-pass Temperature

Material and Shielding Gas

Welders Technique

We need

- fundamental understanding of the welding process and,

- to provide candidate conditions for a target weld attribute.

(e.g. joint dimensions, cooling rate, controlled defects, etc.)


6/25



Department; Indian


Bombay

What are the real task of process models?

Solving a set of Conservation and Constitutive Laws along with realisticboundary conditions to compute temperature, velocity and deformation.

Thermal Energy

Conservation ofMomentum

Conservation of

Mass

j

i

i

j

ijjij

ij

ji

i

jx

u

x

uwhereFxx

Puux

ut

ijij

k

kp

mm

px

TuC

x

Tk

xt

TC

0x

u

m

m

Static forcebalance

)}G3/H(1{

dG3)TT()G23()d()d(G2d

x

dx

x

dx

2

1d;

)2)(1(1

E;

)2(1

EGwhere0F

x

2

kl

d

kl

d

ij

refijijijijij

i

j

j

iiji

j

ij


7/25



Department; Indian


Bombay

Governing Equations need Boundary Conditions

2

eff

2

2

eff

2

2

eff

sryd-

rxd-exp

rdPq

X

Y


8/25



Department; Indian


Bombay

Computed results must be validated !!!

Nd-YAG Laser Spot Weldingof low carbon steel.Beam diameter = 1.3 mmBeam power = 1.6 kW

after 10 ms

after 14 ms

Assumed parameters:

= 0.37d = 3.0

keff = 108.0 W m-1

K-1

= 0.07 kg m-1 s-1

S. Bag and A De, Int. J. of Thermal Science, 2009


9/25



Department; Indian


Bombay

Fitting of uncertain variables is challenging

Modeling results do not always agree with experimental results- lack reliability, mainly due to the uncertain input variables.

Input variables

Current, Voltage, Speed,

Work Piece Dimensions,

and Material properties.

Process Model

Equations of conservation

of mass, momentum and

energy, constitutive eqns

Results

Weld geometry, Cooling

rate, Micro-structure,

Mechanical properties

?

Current models usually need strong domain knowledge for fitting of

uncertan parameters - not helpful in design for welding.


10/25



Department; Indian


Bombay

An Approach to Integrated Modelling

1Process model for welding

considering physical laws

Integrated Model that can learn

+

Optimizer (e.g. GA)

(Finds uncertain parameters)Improve

reliability

2

Experimental data

Compare

Calculate weld

attribute

(geometry,

cool ing rate..)

Test

reliability

5Targetweld

attribute(s)

Compare

Different combinations

of welding variables

(I, V, U) that g ive a

specified weld attribute

Test

ability to

tailor weld

attributes

4Genetic

algorithm+

Develop

ability to

tailor weld

attributes

Reliable Process

Model

3

6Verify withselected

experiment

Compare

Select best set based

on peak temp., cooling

rate, distor tion and

residual stress

Final

Design


11/25



Department; Indian


Bombay

An Approach to Integrated Modelling

M

1m

2

m

M

1m

2

m

M

1m

2

EXP

m

EXP

m

CAL

mM

1m

2

EXP

m

EXP

m

CAL

m

]1W[]1P[

W

WW

P

PPfO

321 fff}f{

For optimizer, we have used

real parameter GA with PCX

operator and G3 Generationmodel*

Uncertain Input Set

Objective Function

(P: penetration; W: weld width;CAL: Calculated thru model;EXP: Corresponding measured)

*K. Deb, A. Anand, and D. Joshi, Evolutionary Computation, 10 (4), 371 395, 2002.


12/25



Department; Indian


Bombay

Application of Integrated Modelling in GTAW

M

1m

2

m

2

m

M

1m

2

EXPm

EXP

m

CAL

m

2

EXPm

EXP

m

CAL

m )1W()1P(W

WW

P

PPfO

flS

eff

r

b4321

k

k

e

rkrfffff

Initial candidate solutions

Initial Performance


13/25



Department; Indian


Bombay

Application of Integrated Modelling in GTAW

Optimum Values of

Uncertain Parameters

= 0.53

rb = 1.98 mmkeff= 245.5 W.m

-1.K-1

= 0.07 kg.m-1.s-1

120 A, 11.6 V, 5.0 mm.s-1

After 13 Generations

120 A, 11.6 V, 5.0 mm.s-1 140 A, 11.0 V, 5.0 mm.s-1

200 A, 13.8 V, 7.0 mm.s-1 180 A, 12.3 V, 9.0 mm.s-1

S Bag, A De, T DebRoy, Mat. &Mfg. Proc., 2010.


14/25


15/25



Department; Indian


Bombay

Modelling of Heat Source

Conduction heat transfer based models cannot undertake greater rate of transport

of heat in the growing weld pool. Use of a volumetric heat source term can surpass the shortcoming due to the

neglect of convective transport of heat to a certain extent.

Assigning volumetric heat source dimensions remain as a challenge.

A novel approach can be an adaptively defined volumetric heat source term.

i

i

i

2

2

2

2

2

2volwDE

pc2

wb

la

wherec

z3

a

y3

b

x3exp

abc

Pf36z,y,xQ

Spot welding Linear Welding

Y


16/25



Department; Indian


Bombay

Use of Adaptive Volumetric Heat Source

25 % effective load

100 % effective load

75 % effective load

50 % effective load

Computed weld pool is adapted as heat

source in successive load-steps.

% of effective load

25 50 10075

Desired weld width

Width(mm)


17/25



Department; Indian


Bombay

Use of Adaptive Volumetric Heat Source

0.2 s

0.3 s

0.4 s

0.5 sOn-time (s)

0.2 0.3 0.50.4

Desired weld penetration

Penetration(mm

)

Computed weld pool is adapted as heat

source in successive time-steps.


18/25



Department; Indian


Bombay

Modelling deposition process is always difficult

Kiran et al., Welding in the World, 2012


19/25



Department; Indian


Bombay

Estimate volumetric heat source terms a-priori

The volume of the semi ellipsoidal heat source utilized to heat the base

metal is

Ui is the unfilled volume of the V-groove beneath either the lead or the

trail wire and the same is given by ULE and UTR

[1] [2]

[3] [4]

aLE, bLE, aTR, bTR and hTR refers to the semi major, semi minor axis of the lead

and trail wire heat sources and the reinforcement height respectively

Kiran et al., Welding in the World, 2012


20/25



Department; Indian


Bombay

Apply for a complex welding process

All dimensionsare in mm

300

90 90 90 90

780

8.5

25

180

450

400

Kiran et al., Science & Technology of Welding and Joining, 2012


21/25



Department; Indian


Bombay

Apply for a complex welding process

The dimensions of the

volumetric heat source is

estimated either by

numerical experiments or

with the knowledge of

final weld dimensions tilldate

Simple energy balance

principle is used to

estimate the dimensions

of the volumetric heatsources corresponding to

leading and trailing arcs

Kiran et al., ISIJ International, 2011


22/25



Department; Indian


Bombay

Validate computed results

Kiran et al., ISIJ International, 2011


23/25



Department; Indian


Bombay

Conclusions

Although transport phenomena based weld pool models can

undertake greater physical phenomena, the conduction heat

transfer based models have come a long way to also provide a

sound recourse to weld pool simulation..

In contrast to a typical surface heat flux to represent the

welding arc, the consideration of the same through a volumetric

heat source has emerged as numerically rewarding.

Defining volumetric heat source either analytically as function of

welding conditions and original joint geometry, or in an adaptive

manner is an ideal roadmap for conduction heat transfer based

weld pool models to be predictive in true sense.


24/25



Department; Indian


Bombay

Conclusions

The proposed methods do not need any a-prioriassumption of the heat source dimensions and rather allow

the heat source to get mapped with the actual evolution of

weld pool as it occurs in real-time.

The predicted weld pool shapes and thermal cycles

following the volumetric sources, as proposed in this work,

has been extensively validated for a number of welding

processes and conditions.

Further evaluation of the same approach is needed in

other fusion welding processes.


25/25



Department; Indian


Bombay

Acknowledgement

All the results presented here are from the work my PhD students and in particular,

I wish to thank Dr. Swarup Bag (currently a faculty at IIT Guwahati) and Dr. Kiran

(currently a Post-doc at KAIST, South Korea).

Thank you

Documents

Dr. Amitava De.pdf