COUPLED FIELD ANALYSIS ON
In the present scenario, products should be ensured for good quality. In order to achieve the required quality and reduced product development time, concept of Concurrent Engineering can be applied. Finite Element Analysis is one of the tools in concurrent engineering.
In this paper, the quality of a butt welded joint is analyzed using ANSYS 7.1- Finite Element Analysis Software. It is modeled as a three dimensional transient heat conduction problem. The process of welding is simulated as a moving heat source in the welding zone. The temperature distribution for different heat flux and weld speed are obtained from the Thermal analysis. The result of the Thermal analysis is given as input to the structural analysis and the coupled field analysis is carried out. The effect of input parameters such as heat flux and weld speed is analyzed. The simulation of results for various heat inputs and weld speed are obtained with aid of c++ coding.
It has been found from the investigations that the weld speed and heat flux greatly influence the temperature distribution and residual stress developed in the welded area which in turn improve the quality of weld. Thus, the optimum parameters for quality welding has been found as minimum heat flux and maximum weld speed.
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Welding is a process of joining similar or dissimilar metals by the application of heat with or without the application of pressure and addition of filler material. The result is a continuity of homogenous material of the composition and characteristics of two parts which are being joined together. The application of welding are so varied and extensive that it would be no exaggeration to say that there is no metal industry and there is no branch of engineering that does not make use of welding in one form or another. In fact, the future of any new metal may depend on how far it would lend itself to fabrication by welding.
THERMAL AND STRUCTURAL ANALYSIS OF WELDING
GEOMETRY UNDER INVESTIGATION
The type of welding under investigation is Metal Arc Welding butt joint of Mild Steel. The specimen, which is to be welded, is having a thickness of 10mm for studies. Two weld plates each of 5cm length and 5cm breadth is to be joined by butt joint with the help of Metal Arc Welding. The diameter of the electrode considered is 10mm. Fig.1 shows the geometry of the plate.
The major assumptions employed while carrying out the thermal analysis are as follows:
Cartesian co-ordinate system is employed.
Three dimensional, transient heat conduction is assumed within the weld plates.
The welding electrode is of 10mm diameter, while welding it is expected to spread the heat over an area of 10mm X 10mm.
Though heat flux distribution is of Gaussian type, the heat flux is assumed to be of uniform over the entire area.
The material is isotropic, but the material properties, viz., thermal conductivity and specific heat are assumed to the functions of temperature. The material density is assumed to be constant.
Heat flux is applied normal to the surface and over the surface flux.
Slag formation & addition of filler metals is not considered.
Heat loss due to radiation is not considered.
The governing differential equation for the above conditions will have the following Heat Transfer mechanisms.
3-D transient heat conduction within the solid
Convection heat loss from the surfaces.
The transient heat conduction equation incorporating the heat flux, surface convection to be solved is given by
Where, is the density, Cp is the specific heat, k is the thermal conductivity, q is the surface heat flux (power source) intensity, T is the temperature, is the time, h is the convective heat transfer coefficient, and T is the surrounding temperature.
BOUNDARY CONDITIONS FOR THERMAL ANALYSIS
The following are the Boundary conditions employed during the thermal analysis.
At time =0, the entire solid is assumed to be at an uniform temperature of 30C.
The moving heat source is modeled by applying and deleting heat flux over the corresponding areas with respect to the welding speed.
At time >0, over the entire surface of the solid, convection heat loss is applied by specifying the constant convective heat transfer co-efficient (h) and surrounding temperature (T).
BOUNDARY CONDITIONS FOR STRUCTURAL ANALYSIS
The following are the Boundary conditions employed during the structural analysis.
At time =0, displacement along the directions of plates (Ux, Uy and Uz) are assumed to be zero.
At time >0, displacements of left and right edges of the plates and bottom faces are assumed to be zero.
METHOD OF SOLUTION
Thermal and Structural analysis of the welding structure is carried out using commercial Finite Element Analysis (FEA) software ANSYS 7.1. The procedures for the thermal and structural analysis are discussed below.
MODELING AND FINITE ELEMENT MESHING
o Length -50mm
o Breath -50mm
o Thickness -10mm
o Weld area -50x10mm
The areas in welded region are meshed with quadrilateral elements by free meshing
THERMAL ANALYSIS PROPERTIES
The following properties are given for element while doing thermal analysis.
Thermal conductivity of the material is assumed to be constant
and the value is 50 W/m K.
Density of the material as 8000 kg/mÃ‚Â³ (Mild steel).
Specific heat of the mild steel is specified and the value is 400 J/kg K.
STRUCTURAL ANALYSIS PROPERTIES
The following properties are given for element while doing structural analysis.
Youngâ„¢s modulus of the material is assumed as an isotropic and the value is given as 2.15e11 N/mÃ‚Â²
Thermal expansion co-efficient of the material as 15e-06 /Ã‚Â°C.
APPLYING THE BOUNDARY CONDITIONS
A uniform temperature of 30oC is applied over the entire solid region as the initial condition.
Convective heat transfer is applied to all free surfaces with heat transfer co-efficient of 20 w/m2 k.
For convection, the surrounding temperature of 30Ã‚Â°C is specified with the convective heat transfer.
Heat flux value of 6e9 w/mÃ‚Â² is given to different areas. Details of heat flux applied corresponding to the welding speed of 5 mm/s are listed in Table.1.
Load steps are executed from 1 to 10 to solve the problem.
COUPLING OF THERMAL AND STRUCTURAL ANALYSIS
The temperature distribution obtained from the thermal analysis act as a load for the structural analysis. De-coupling of thermal and structural analysis is done by changing the preference as structural and making switch element type as thermal-struct. Now, the elements used in the thermal analysis are replaced by an equivalent structural element such as plane42, solid45.
In structural analysis, the loads are displacements and temperature from thermal analysis. Because of the plates are fixed at the far most edges, the nodal displacements are set to be zero in all directions. The thermal load i.e., temperature distribution is applied by browsing the result file of the thermal analysis. Then the loads are solved by current load step. Displacement and von mises stresses are obtained as a result. The results are discussed later.
RESULTS FROM THE INVESTIGATION
Thermal and structural analyses are carried out by varying the important parameters that are primarily responsible for determining the quality of weld.
Table 1 Stress and Displacement for different Heat flux and Velocity
No Heat flux, q (W/mÃ‚Â²) Weld speed, v (mm/s) Stress, s max
*10^9 N/mÃ‚Â² Displacement
1. 6e9 4 3.4000 0.234
2. 6e9 5 2.7680 0.190
3. 6e9 6 2.3460 0.161
4. 6e9 7 2.0451 0.141
5. 6e9 8 1.8167 0.125
6. 6.1e9 4 3.4600 0.237
7. 6.1e9 5 2.8300 0.193
8. 6.1e9 6 2.8100 0.164
9. 6.1e9 7 2.0800 0.143
10. 6.1e9 8 1.8430 0.127
11. 6.2e9 4 3.5090 0.241
12. 6.2e9 5 2.8534 0.196
13. 6.2e9 6 2.4160 0.166
14. 6.2e9 7 2.1057 0.145
15. 6.2e9 8 1.8690 0.128
f the plates decreases s
Fig. 4 Stress distributions after 1.25seconds and 1200 seconds
Fig. 5 Displacements after 1.25seconds and 1200seconds on node-19
Fig. 6 Stress variation for different heat flux and for v=4mm/s and 8mm/s
Fig. 7 Stress variation for weld speed with constant heat flux
From the thermal and structural analysis, the following are the optimum values of input parameters for reducing the residual stress and distortion of the plates.
Table.2 Optimum settings of input parameters
S. No FACTORS RESIDUAL STRESS DISTORTION
1. Heat flux in W/m^2 6*109 6*109
2. Weld speed in mm/s 8 8
3. Metal preheat in Ã‚ÂºC 30 30
4. Plate thickness in mm 10 10
The major conclusions drawn from the present investigations are:
Variation in weld speed also affects the temperature distribution in the weld zone as well as in the plate. When the weld speed increases, temperature in the weldment decreases and vice versa.
For the given value of welding speed, when the heat flux increases, residual stress increases and vice versa.
For the given value of heat flux, when the weld speed increases, the residual stress decreases and vice versa.
For the given value of weld speed, when the heat flux increases, nodal displacement value also increases and vice versa.
For the given value of heat flux, when the weld speed increases, the distortion in the plate decreases and vice versa.
In order to obtain better quality welding, the heat flux value is set as minimum and the weld speed as maximum, according to the consideration of residual stress and distortion.
Â¢ Computer Simulation of Flow and Heat Transfer by P.S.Ghoshdastidar.
Â¢ Finite Element Method For Engineers by S.S. Rao
Â¢ Heat Transfer â€œ A Practical Approach Yunus A.Cengel
Â¢ ANSYS 7.1 Userâ„¢s Manual, ANSYS Corporation.