Experimental and numerical study of submerged arc welding induced thermal history, residual stresses, distortion and weldment characterization

The addition of surface active elements increases the penetration of the weld, the effect of the same was studied. 4.2 (a) Devices designed to hold samples, (b) Top view of the device (c) Front view of the device. 111 5.18 Numbered fillet weld connection diagram 113 5.19 Temperature distribution for the first side of the double-sided thread weld 113 5.20 Temperature distribution for the second side of the double-sided thread weld 114 5.21 Comparison of angular distortion of the warped plate near the plate and parallel.

115 5.22 Comparison of numerical and experimental thermal profile (Table Plate dimensions and control points for measuring distortion 116 5.24 Experimental and numerical comparison of angular distortion for welding. 118 5.26 Welded sample with flux and thermocouples attached 2c0mer welded 5 fillet joints. ) double sided (b) single sided 120 5.28 Comparison of longitudinal residual stress with published literature 121 5.29 Comparison of transverse residual stress with published literature 121 5.30 (a) Longitudinal, (b) transverse and (c) von-Misses residual stress. 123 5.32 Comparison of longitudinal residual stress distribution 124 5.33 Comparison of transverse residual stress distribution 124 5.34 Comparison of von-Misses residual stress distribution 125 5.35 Comparison of longitudinal residual stress distribution along the weld line 125 5.36 Comparison of von-Misses residual stress distribution 125 5.35 Comparison of longitudinal residual stress distribution along the weld line 125 5.36 Comparison of residual stress distribution i longitudinal direction along the welding line 15.36 Comparison of residual stress distribution in the longitudinal direction along the welding line 5.36 Comparison of residual stress distribution in the longitudinal direction along the welding line 15.36 Comparison of residual stress distribution along the welding line 15. plot of angular deformation of double-sided fillet welding 126 5.38 Contour plot of angular deformation of single-sided fillet welding 127 5.39 Comparison of angular deformation between single and double sided fillet.

148 5.59 Comparison of distortions along the line AB (parallel to the Y axis and near the edge of the plate) in butt-welded plates considering different cases (Table 5.16). 151 5.62 Comparison of distortions along the line AB (parallel to the Y axis and near the edge of the plate) in fillet-welded plates considering different cases (Table 5.16).

NOMENCLATURE

Introduction

General background
Submerged arc welding (SAW) process
Welding as a multi-physics problem
Residual stress and distortion in SAW process
Research Objectives
Layout of thesis

Both residual stress and distortion of the weld can significantly degrade the performance and reliability of the welded structures [5]. Residual stress and angular deformation is one of the areas where some researchers concentrate. Again, due to the non-uniform heating and cooling, volumetric change occurs in the fusion zone and thermal stress develops, leading to the development of residual stress and deformation of the weld structure after cooling.

Depending on the application (location and orientation), residual stress can have positive and negative effects on the stability of the welded structure. Depending on the shrinkage pattern and the shape of the welded structure, different deformations occur. Prediction of the temperature distribution and weld pool dimensions using the newly developed avocado configuration heat source model.

Introduction to the subject, main objective and layout of the report is included in Chapter 1. The results and discussion of the experimental investigation and the numerical analysis are presented in Chapter 5.

Figure 1.1 Schematic illustration of submerged arc welding process and equipment

Literature Review

General Introduction
Modelling of heat sources in welding
Thermo-mechanical analysis in fusion welding
Characterization of residual stress
Measurement of angular deformation
Mechanical and metallurgical characterization of weld joint
Effect of surface active flux
Optimization of welding process parameters
Summary
Scope of the thesis

The main objective of welding heat source modeling is to represent the real heat source distribution mathematically. 52] but the prediction results of the half-ellipsoidal 3-D heat source were less accurate than the double-ellipsoidal one. The dimensions of the heat source change when moving at different speeds according to the requirements of the welding process.

58] developed a combined heat source model to study the tandem submerged arc welding (T-SAW) process. Three ellipsoids were combined, where the parameters of the heat source of the double ellipsoid model were determined by regression analysis. Knowing the comparison of weld joint strength between yield strength (YS) and ultimate tensile strength (UTS) with the base metal is very important, researchers [135].

The results have also been tried to be analyzed on the basis of the heat input. Here an attempt has been made to model the heat source for fusion welding by considering the asymmetry of the moving heat source.

Figure 2.1 The Gaussian distributed heat source [30]

Theoretical Background & Modelling Methodology

General introduction
Thermal modelling

Formulation of Finite element (FE) transient thermal analysis
Heat source model

Development of a new heat source model

New heat source for FE thermal model

Welding thermo-mechanical analysis

Formulation of 3D FE stress-strain relationship
Derivation of structural matrices
Structural analysis with material non-linearities

Materials properties
Overall FE solution strategy
Equivalent loading technique

Inherent strain

Optimization of welding process parameters

Design of experiments: A well planned set of experiments, in which all parameters of interest are varied over a specified range, is a much better approach to obtain systematic data
Analysis of variance (ANOVA)
Grey relational analysis
Confirmatory experiment

Summary

In general, the rate of heat transfer through conduction is given by Fourier's law of heat conduction. A specified starting temperature for the weld covering the entire test piece elements:. where T is the ambient temperature. The quantity qn represents the component of the conduction heat flux vector perpendicular to the working surface.

The element-specific heat matrix [Cet] is assessed based on the material's specific heat. If the welding thermal cycle in the vicinity of weld metal and heat affected zone is not of interest. Here, an attempt has been made to model the shape of the moving heat source during welding.

In the present study, a shape of the ellipsoid is changed to get an asymmetric avocado-like shape which was used as a volumetric heat source for heat generation in the weld pool. The final crater shape of the weld bead from submerged arc welding is shown in Figure 3.3 which is similar to the cross section of an avocado. The value of major axis is taken as the average value of the end crater length.

The point (0, 0) is the old location and the point (p, 0) is the new location of the center of the ellipsoid, as explained in Figure 3.8. Therefore, the heat distribution in the front and back of the avocado shape should not be the same. For the moving heat source problem, the coordinate system of the moving heat source can be modeled as (x, y', z), where y'= y - u×t, u.

The average values of the material properties can be used in analysis if the temperature does not vary too much. The overall performance characteristic of the multiple response process depends on the computed gray relational character. This also consists of the development of 3-D transient heat transfer analysis using a newly developed heat source configuration.

Figure 3.1 Various Regions of welding plate

Experimental investigation

Introduction
Pre-stage of the experiments

Trial and error run
Arc initiation
Design of welding fixture
Backing bar
Materials and job specimen

Welding without edge preparation and effect on mechanical properties

Design of experiment

Comparative study of the effect of V-groove angle on mechanical properties
Experimental study of the effect of surface active elements on mechanical properties

Procedure for applying surface active elements
Measurement of temperature profiles
Measurement of bead geometry and mechanical properties

Summery

The voltage of the welding arc has a direct influence on the geometry of the weld bead and the appearance of the bead. A reusable backing tape filled with flux was used to provide support for the weld metal from the underside of the plate. Unlike manual welding, arc starting can be difficult in SAW, due to the flux covering.

The welding parameters and their levels used in the final experiments are shown in Table 4.1. The finer the particle size of the flux used in the backing strip, the better the formation of bottom reinforcement. The chemical composition of the electrode wire and the fluxes are given in tables 4.3 and 3.4 respectively.

Design of experiments is required to properly establish the strategy of experimental investigation. A series of experiments are performed in which deliberate changes are made to the input variables, so that corresponding changes in the output responses can be observed. For the simplification of the experiment in each set of experiments (i.e. for an actuated element) the current and voltage are taken as variable and keeping the travel speed (5.5 mm/s) and the electrode out (25 mm) constant. In total there are 15 number of experiments with different combinations of activated elements on the surface.

This mixture is hand-rushed onto the top surface along the weld line of the specimen prior to welding. Using the small paint brush, oxide powder is applied to the center of the plate. A residual slag remaining on the surface of the weld pool is removed by grinding or aggressive wire brushing.

The Instron 3382 universal testing machine was used to measure the tensile properties of the welded specimens. In this part, the experimental aspects of the entire study were presented, which consists of the preparation of the fixture, support rods, selection of welding parameters, design of experiments. Briefly, the summary of the experiments included in this study was discussed in this section.

Table 4.1 Welding parameters and their levels

Results and discussions

General introduction

Model geometry
Material properties
Experimental details

Numerical thermo-mechanical analysis

Prediction of thermal history and angular deformation of square butt joints
Prediction of thermal history and angular deformation of double sided fillet joint
Comparative study of residual stress and angular deformation of single and double sided fillet joints
Influence of tacking sequences on residual stresses and distortions
Effect of welding sequences on thermal history, residual stresses and welding distortions
Prediction of weld induced distortion of large structure
Development of an avocado shaped new welding heat source model for the numerical simulation of fusion welding processes

Results of experimental investigation

Effect of process parameters on microstructure and mechanical properties
Effect of welding parameters on weld bead geometry & mechanical properties
Effect of surface active elements on weld bead geometry
Effect of included angle on mechanical properties

A comparison of the transverse residual stress distribution between one-sided and two-sided fillet welds is shown in Figure 5.33.