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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The birth and death techniques
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 birth and death techniques
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