wedling modeling

7/28/2019 Wedling Modeling

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NARROW GROOVE SAWSIMULATION

Submitted to:

Dr. S. A. Channiwala

Professor, Mech. Engg. Deptt.

S. V. N. I. T, Surat


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Literature of SIMULATION

Welding is complex physical and chemical processes that include

electric arc physics heat transfer metallurgy and mechanics, if we

adopt the experimental method and often cause result lack of

regularity because of many factors influence, and with longer time

and higher costs .

During arc welding, due to uneven heating and cooling cycles,

complex thermal stress and strains are produced resulting inresidual stresses and distortions .


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During welding, the interaction of the heat source with the material results

in melting, vaporization , solidification, subsequent solid-state

transformation stresses, and distortions .

Since most of the fusion welding processes are fast and are completedwithin a few minutes, experimental measurement of all of processes and

their interactions are very difficult. In this regard, computer modelling has

been a very valuable method to evaluate the physics , chemistry, and the

mechanics of the process. In modelling welding processes,phenomenological models are used to seek insight into various aspects

of the processes .


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Welding process parameters like Electrode diameter

Electrode travel speed

Thickness of the work piece, Current and voltage

greatly affect the temperature distribution patternsand hence

residual stresses and distortions .

Like any other fusion welding process the heat supplied during

SAW is responsible for the changes in the microstructures, and

development of residual stresses and distortions.


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With hot structure coupling function of FEM to analysis welding

process cloud overcome the shortcomings of experimental methods,

which is effective method to research the variation of welding

deformation and residual stress that provide the basis for choice

welding manner and technology parameter reasonable .

A large number of models have been reported in the publishedliterature to predict temperature distributions , residual stresses and

distortions in the welded joints .


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Most of them have concentrated on a 2-D approximation of a3-D

problem .

Kamala and Goldak opined that a two-dimensional (2-D)

approximation of a three-dimensional(3-D) problem is not

appropriate to predict temperature distribution, residual stresses

and distortion patterns


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The important process characteristics which are required to be

considered in any simulation are:

(1) Moving heat source,

(2) Arc travel speed,(3) Current and voltage,

(4) Temperature-dependent material properties

(5) Deposition of filler material and(6) geometrical constraints .


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To minimize computational time and cost, an axi-symmetric

modeling approach is adopted widely for welding simulation .

However, recent investigations have advocated 3-D solid modeling

without considering the axi-symmetry for better prediction of

distortions and residual stresses .

To simulate the filler material deposition, the Element Birth and

Death approach has been used in the numerical model .


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In this technique the elements are activated or deactivated as the

welding heat source moves along the weld line.

The model was further verified by comparing the predicted and

experimentally obtained temperature distributions and angular

distortions .


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MODELING METHODOLOGY

THERMAL MODEL

STRUCTURALMODEL


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In the thermal analysis, both the preheating and the inter-pass

temperature were taken into account. A surface and volumetric heat

source with a Gaussian density distribution was used in order to

describe the heat input of the welding arc and the melt droplets .

The arc efficiency factor and the energy distribution between the

surface and volume were adjusted according to the size of the weld

pool from the weld micro-section .

For the boundary conditions during the thermal analysis, convection,radiation and the contact with the clamping tools were considered


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THERMAL MODEL

The heat source was modelled as a distributed heat flux depending on

arc spread.

The rate of arc travel, current and voltage were varied and these

parameters were noted along with the temperature data.

It is not possible to measure the spread of the arc in the SAW

process because it is covered with flux granules.


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THERMAL MODEL

However the radius of arc spread was estimated by considering the

electrode diameter and bead widths of welds formed during

experiments

This arc radius was used for transient thermal analysis with themoving arc and the temperature profiles were verified with the

experimentally measured ones .

After obtaining the temperature profiles for each job coupled thermo-

mechanical analyses were carried out for predicting the angular

distortion .


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THERMAL MODEL

The moving heat load applied in the finite element modelis taken as a distributed heat flux as given by

where r is the region in which 95% of the heat flux is concentrated .

Q is the arc power (W),is the arc efficiency,

V is the arc voltage andI is the arc current


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THERMAL MODEL

The moving heat load was applied on the area bounded by the weld

lines and except for this area other areas of the plate are subjectedto heat loss due to convection.

In this analysis the convection loss is taken as 15W/m2K .

ASUMPTIONSS The following assumptions were made in the present finite element

analysis :

(1) Density is not affected due to thermal expansion.

(2) Linear Newtonian convective cooling was assumed. No forcedconvection was considered.

(3) Convective cooling was assumed on all the surfaces excepting theweld zone.


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THERMAL MODEL

(4) The heat source was assumed to have a Gaussian distribution ofheat flux.

(5) Arc efficiency is 90% was taken to account for other losses .

The governing differential equation for heat conduction in asolid without heat generation is given by


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BOUNDARY CONDITIONS

FIRST BOUNDARY CONDITION :A specified initial temperature covering the entire plate surface is

where T is the ambient temperature.Energy balance at the work surface leads to the secondand third boundary conditions.

Let S1 represents the zone where the arc heat is applied.The rest of the plate represented by S2 is exposed to atmosphere whereheat loss takes place due to convection .


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SECOND BOUNDARY CONDITION

The arc heat acting over the surface S1, is given by


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THIRD BOUNDARY CONDITION

The heat loss due to convection over S2 is given by

To avoid the sharp change in the value of specific heat with melting,enthalpy was used as the material property.

This was done by defining the enthalpy of a material as a function oftemperature .


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STRUCTURALMODEL

The stress strain relationship can be represented as

In the mechanical analysis, the elastic- viscoplastic temperature

dependant material behaviour including the phase dependant

dilatation and the transformation-induced plasticity is considered


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STRUCTURALMODEL

By considering the principle of virtual work it can bewritten as

In the case of nonlinear materials E can be written as

The solution was obtained using ANSYS . Coupled transientthermal and nonlinear structural analysis was done for predictingdistortion.


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STRUCTURALMODEL


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Overview of experimental and modellingtools required

for weld microstructure modelling


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SIMULATION SOFTWARESFOR WELDING

ANSYS

ABAQUS

HYPERWORKS

MARC

WELDSIM

VIRTUAL WELDSHOP (VWS)

SYSWELDFORTRAN 77

PROE


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PROE :For modeling

HYPER WORKS AND MARC :

For meshing

ANSYS ANDABAQUS :

For analysis (solving )


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The birth and death techniques

If material is added to or removed from a system, certain elements

in your model may become "existent" or "nonexistent. In such

cases, you can employ element birth and death options to

deactivate or reactivate selected elements, respectively .

The birth and death feature is available in the ANSYS Multi-physics,

ANSYS Mechanical and ANSYS Structural products


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To achieve the "element death" effect, the ANSYS program does not

actually remove "killed" elements. Instead, it deactivates them by

multiplying their stiffness (or conductivity, or other analogous

quantity) by a severe reduction factor (ESTIF). This factor is set to

1.0E-6 by default, but can be given other values .

Element loads associated with deactivated elements are zeroed out

of the load vector, however, they still appear in element-load lists.

Similarly, mass, damping, specific heat, and other such effects are

set to zero for deactivated elements.


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The mass and energy of deactivated elements are not included in

the summations over the model. An element's strain is also set to

zero as soon as that element is killed .

In like manner, when elements are "born," they are not actually

added to the model; they are simply reactivated. You must create all

elements, including those to be born in later stages of your analysis,

while in PREP7. You cannot create new elements in SOLUTION. To

"add" an element, you first deactivate it, then reactivate it at the

proper load step .


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The birth and death techniques :

When an element is reactivated, its stiffness, mass, element loads,

etc. return to their full original values. Elements are reactivated with

no record of strain history (or heat storage, etc.); however, initial

strain defined as a real constant (for elements such as LINK1) will

not be affected by birth and death operation :

This capability is useful for modelling effects due to phase

changes (as in welding processes, when structurally inactive

molten material solidifies and becomes structurally active).


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ELEMENT SUPPORTING BIRTH ANDDEATH FEATURES


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REFERENCE

Ansys Tutorial

Radaj D. Welding residual stresses and distrotionCalculation and measurement. Dssel dorf: DVS-Verlag;2003 .

Mandal NR. Welding and distortion control, 1st ed. NewDelhi,India: Narosa Publishing House; 2004.

Lindgren LE. Finite element modelling and simulation ofwelding Part1: increased complexity. J Thermal Stresses

2001;24:14192.


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THANK

YOU

wedling modeling

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