Finite element analysis of arc welded structural steel. Part 1/2 - Thermal Analysis.

Peter ŠKORVAGA

pskorvaga@gmail.com

February 2012

Abstract

This article presents a 3D finite element analysis of a single pass V-groove butt joint with the thermal history during the welding process of a structural steel. The Visual Mesh software is used to mesh the 3D-model, SysWeld developed by ESI Group is chosen as solver and Visual Viewer with Microsoft Excel and Matlab provide the post-processing process for displaying the results. The 3D model is represented by two 6mm thick plates 110 mm bright and 180 mm long which are welded with a single pass V-groove butt joint together. The material of the steel plates is a S355J2G3 structural steel. The heat transfer analysis is carried out to simulate the gas metal arc welding process (GMAW). Goldak’s double ellipsoid model is used to simulate the heat source during the welding procedure.

Keywords: finite element method (FEM), GMAW, V-groove butt joint, thermal analysis, Goldak’s double ellipsoid.

Introduction

Welding induced residual stresses and distortions are the biggest disadvantages of the welded structures. Distortions can cause many failures of the constructions and parts influenced by inaccuracy or the production process can be massively slowed by an unforeseen and unexpected distortion. On the other hand, the residual stresses without further relaxation can lead at the beginning of the use to an overloaded status of the critical area in the whole structure, even if the operational or calculated load conditions are not big enough to cause a failure. It is sometimes necessary to consider these stresses as the initial inner loads in addition to the load conditions. The results of this study are a part of further research which will exactly describe the use of the residual stresses of the welded structures as initial loads in addition to the operating load conditions in finite element calculations of the parts and structures.

Material and welding process

The specimen is represented by two 6mm thick plates welded with a V-groove butt joint. Dimensions of the specimen and V-groove are displayed in Figure 1. The material is represented by S355J2G3 structural steel with chemical composition and mechanical properties shown in Tables 1 and 2 [1, 2]. GMAW (CO2) procedure with welding parameters set in Table 3 [3] is simulated in this example which is a typical welding method for structural steels.

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Chemical composition [%]C Si Mn P S Cr Al Cu Ni

Max. 0.20

Max. 0.55

Max. 1.60

Max. 0.035

Max. 0.035

Max. 0.30

Min. 0.020

Max. 0.30

Max. 0.30

Table 1. Chemical composition of S355J2G3 steel

Yield strength - Re

[MPa]Tensile strength - Rm

[MPa]Elongation - A5min

[%]355 490-630 Max. 22

Table 2. Mechanical properties of S355J2G3 steel

Figure 1. Model geometry with detailed V-groove dimensions

FE model

The 3D finite element model contains 24400 nodes associated with 18840 3D-elements, which are representing the material of the two welded plates and the filler. In addition the meshed model includes 11428 2D-elements, which represent the ambient air that is in contact with the outer surface of the whole 3D-mesh. This kind of 2D-surface mesh was created with the “Extract from 3D Mesh” tool and only the “Open Faces” were selected. Open faces are faces on the 3D-elements which represent the outer surfaces of the model (or faces that are in contact with “ambient air”). The

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Start-node

Reference line

Weld-line

End-node

meshed model contains in addition 120 1D-elements. The 1-D elements are two meshed lines (each one contains 60 elements) to define the weld-line (trajectory of the heat source) and the reference-line which is parallel to the weld-line and must have the same number of elements as the weld-line. This line is used to determine the normal to the trajectory of the heat source [4]. The size of the fusion zone and expected heat affected zone elements was refined and varies from 0.33 to 0.77 mm. The details and explanation of the mesh is shown in Figure 2 (model positioning in the coordinate system) and Figure 3 below.

Figure 2. FE model with mesh refinement in the FZ and HAZ

Figure 3. Ambient air surface mesh - extracted from open faces of 3D elements

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Heat source parameters

For this kind of welding process simulation the double ellipsoid heat source model is used. This kind of volumetric heat source model combines 2 ellipsoidal fields (front and rear) to simulate the arc welding. The power density distribution inside the double ellipsoid transformed in the fixed coordinate system is described by Equations 1 and 2 [5]. All parameters of the heat source 3D-model in this simulation were defined and adjusted using the “Heat Source Fitting Tool” which is a part of the SysWeld software. The welding speed (or the heat source movement speed) is set to 6.25 mm/s. Heat input per unit length of the weld is defined in Equation 5. The exact values are given in Table 4 and the power density distribution on the top surface (z=0) is shown in Figure 5.

Power density distribution qf [W/mm3] in the front half is shown in Equation 1.

q f ( x , y , z , t )=6√3Q f fa f bcπ √π

e−3{[ x+vw ( τ−t )]2

af2 +

y2

b2+z2

c2 } (1)

Power density distribution qr [W/mm3] in the rear half is shown in Equation 2.

qr ( x , y , z , t )=6√3Q f rarbcπ √π

e−3{[ x+ vw (τ−t ) ]2

ar2 +

y2

b2+z2

c2} (2)

Where:

Q=ηUI (3)

f f+ f r=2 (4)

Ql=Qvw

(5)

Q – Energy input rate [W]

Ql – Heat input per unit length [J/mm]

vw – Welding speed [mm/s]

τ – Lag factor needed to define the position of the heat source at time t=0 [s]

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Parameters ValueWelding voltage U [V] 24Welding current I [A] 225Arc efficiency η [-] 0.85Welding speed Vw [mm/s] 6.25

Table 3. Welding parameters

Parameter ValueLength of the front ellipsoidal af [mm] 12.4Length of the rear ellipsoidal ar [mm] 10.2Half width of the heat source b [mm] 5.0Penetration of the heat source c [mm] 6.0Front heat source fraction ff [-] 0.45Rear heat source fraction fr [-] 1.55Energy input rate Q [W] 4590Heat input per unit length Ql [J/mm] 734.4

Table 4. Heat source parameters

Figure 4. Double ellipsoid heat source model

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Figure 5. Power density distribution on the top plane at z=0 with used parameters from Table 4

Analysis set-up

The ambient temperature is set to 20 °C. Welding process begins at the start-node of the weld-line. Heat source starts to move along the trajectory at the speed of 6.25 mm/s (in the x-direction) and reaches the end-node of the weld-line after 28.8 s. In order to receive detailed results during the cooling, the procedure is divided into 7 different time steps: 0 s - 600 s, 600 s - 1000 s, 1000 s - 1500 s, 1500 s - 2000 s, 2000 s - 2500 s, 2500 s - 3000 s, 3000 s - 3600 s. The simulated process lasts up to 3600 s, when the temperatures in the whole model should be balanced (Figure 11).

Results – temperature distribution

Computed thermal simulation results are shown in figures below. Figure 6 shows the temperature history on selected nodes over time from the start to 120 s. The first node is distanced 3.5 mm from the weld center line (y-axis) which is exactly the upper edge of the V-groove. Temperature distribution in lateral direction (y-axis) at 4.15 s after start of the simulation is displayed in Figure 7. At this time the position of the heat source center is in the x-direction x=-165 mm. The temperature rises rapidly as the heat source reaches the selected node in the upper edge of the V-groove (Figure 6 - curve no. 1). Temperature at this node reaches maximum value of 1560 °C at the time of 3.5 s which means that the peak temperature is slightly above the liquidus temperature. In the weld center line are the temperatures much higher and can reach a peak value of 2325 °C on the top surface (Figure 7). For better illustration the weld pool is represented by the isosurfaces with temperatures of 1480 °C and 1530 °C (fusion zone) in Figure 8. The heat affected zone is shown as an isosurface with temperature of 723 °C. Cross section of the weld formation at the time of 3.5 s is displayed in Figure 9.

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Figure 6. Temperature history on selected nodes

Figure 7. Temperature distribution at selected nodes along weld-line (4.15 s after start)

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Figure 8. Weld pool isosurfaces

Figure 9. Weld cross section with fusion zone (FZ) and heat affected zone (HAZ)

Temperature distribution at nodes after 4.15 s of the arc initiation is displayed in Figure 10 and Figure 11 shows temperature contours when the temperatures are balanced at 3600 s (cooling time is 3571.2 s).

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FZ

HAZ

Figure 10. Temperature distribution after 4.15 s

Figure 11. Temperature distribution after 3600 s

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Conclusion

The results of this study will be used in further work, especially in the computation of residual stresses. According to the temperature history and with combination of known material properties (thermophysical parameters, CCT-diagram) the phase transformation process can be exactly described which can lead to detailed knowledge about the influence of welding parameters and cooling speed on the mechanical properties of welded steel structures.

References

[1] http://www.sinthai.co.th, EN 10025 S355J2G3 HIGH TENSILE PLATE.[2] http://www.interfer.de, 1.0570 - S355J2G3 Spezifikation.[3] Grøng, Ø., Metallurgical Modelling of Welding. 1994: The Institute of Materials.[4] ESI Group, WELDING SIMULATION - USER’S GUIDE-SYSWELD® 2008.[5] Goldak, J. and Akhlaghi, M., Computational Welding Mechanics, e-ISBN. 0-387-23288-5, Springer,

2005.

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