N. R. Mandal

Numerical and experimental study on prediction of thermal history and residual deformation of double-sided fillet welding

Abstract Distortions occur in almost every arc welded joint. The nature of the created distortion depends on several parameters including the welding speed, plate thickness, welding current, voltage, and restraints applied to the job. The distortions and thermal history of a joint can be measured experimentally but the measurement procedure may be costly and time-consuming. In the present work a numerical elasto-plastic thermomechanical model has been developed for predicting the thermal history and resulting angular distortions of submerged arc welded double-sided fillet joints. A moving distributed heat source was used in the finite element modelling of the double-sided fillet welding to create a realistic simulation of the process. The effect of filler metal deposition was taken into account by implementing a birth-and-death process for the elements. The transient temperature distributions were predicted using temperature-dependent material properties. The angular distortion profiles were predicted based on the transient temperature distributions of the fillet welds. The model yielded results that match the experimental values (with a variation of 5–10 per cent for the maximum values of the distortions and a variation of 8 per cent for peak temperatures).

Friction stir welding of aluminum alloy with varying tool geometry and process parameters

The present investigation had two objectives: first, to study the effect of the geometry of the tool pin, and second, the effect of tool rotation and welding speeds on the mechanical properties of friction stir welded joints made for samples of commercial grade aluminium alloys. The results obtained from the performed experiments showed that tools with tapered pins created superior mechanical properties for the friction stir welded joints. It was also found that overall mechanical response depended on the ratio of the tool rotation speed to the tool traverse speed. Suitable values for this ratio were isolated that lead to high quality mechanical properties for the created joints.

Thermomechanical finite element analysis and experimental investigation of single-pass single-sided submerged arc welding of C—Mn steel plates

Abstract Determination of distortions are important while designing the arc-welded joints. These distortions occur in a varied way in almost every type of welded joint, depending on several parameters, i.e. welding speed, plate thickness, welding current and voltage, restraints applied to the job while welding, thermal history, etc. In the present work a numerical model based on the finite element package ANSYS was developed for single-pass single-sided submerged arc welding of square butt joints. Suitable macros were developed to simulate the situation of a moving distributed heat source, metal deposition to account for top and bottom reinforcements, a time step, and a meshing scheme. In this model the effect of bead geometry was incorporated. The effect of filler metal deposition was taken into account by implementing the element birth and death technique. Submerged arc welding (SAW) can be conveniently used to weld a range of plate thicknesses in a single run using a suitable backing strip achieving adequate top and bottom reinforcements. The welding methodology established in the present work resulted in minimizing angular distortions in butt welds. The numerical model yielded results comparing well with those of the experimental ones.

Three-dimensional finite element prediction of transient thermal history and residual deformation due to line heating

The parameters of the line heating process have significant effect on thermal history and the resulting residual deformation of the heated plate. The thermal transients are dependent on torch speed, torch height, gas pressure, and nozzle size, which in turn controls the residual deformation of the plate. In the present work, three-dimensional transient finite element thermomechanical analysis of line heating using oxyacetylene gas flame is presented. The analysis is carried out using temperature-dependent material properties, Newtons convection, and Gaussian distribution of heat. To reduce the computation time, a fine mesh was used along and near the heating line that was coarsened at the edges and away from the heating line. Numerical thermal transients and residual deformation were presented for various torch speeds, heat inputs, and different thicknesses of plate (6 mm, 8 mm, 10 mm, and 12 mm thick plates). The results compared fairly well with experimental and previously published results. The reduction in peak temperature, for similar heating conditions, and the increase in structural stiffness of thicker plates resulted in a reduction in out-of-plane deformation. In this work, the computational model for correlation of deformation pattern with heat input for a single heating line has been achieved. This model will further be extended to predict deformation patterns due to multiple parallel and oblique heating lines.

Modelling the effects of constraints and single axis welding process parameters on angular distortions in one-sided fillet welds

Abstract Welding process parameters and joint fit-ups significantly affect angular distortion patterns. By using constraints at proper positions the angular distortion can be minimized. In the present work the effect of constraints in the form of tack welds to minimize angular distortion in one-sided fillet welds has been analysed. It has been observed that proper positioning of tacks plays an important role in controlling angular distortion. The process was modelled using the three-dimensional finite element technique. Three-dimensional transient thermal analyses were done for predicting temperature distributions by considering a moving heat source. The element birth and death technique was used for simulating filler material deposition. The fillet welds were sectioned and microstructure zones were measured. The thermal model was verified by comparing the temperatures obtained from the thermal analysis with experimental results. Transient thermal and non-linear structural analyses were carried out in order to predict angular distortions.

Prediction of welding deformations of large stiffened panels using average plastic strain method

Abstract Curved or flat stiffened steel panels used in ships and offshore structures are fabricated mainly by fusion welding. It leads to heat induced plastic strains, distortions and residual stresses. These distortions may adversely affect the subsequent fit-up and alignment of the adjacent panels. A solution methodology named average plastic strain method was developed and adopted based on evaluation of average plastic strains of butt and fillet joints of small scale specimens. These were obtained using the conventional thermomechanical analysis. These plastic strains were then used to determine the overall distortions of large stiffened plate panels. The results obtained were compared with those of established inherent strain method for prediction of distortion of large stiffened plate panels. Computational efficiency of the average plastic strain method was found to be comparable with the inherent strain method. However, the accuracy in the plastic strain method was found to be better than that in the inherent strain method.

Fusion Zone and HAZ Prediction Through 3-D Simulation of Welding Thermal Cycle

An implicit centre difference scheme with a simplified heat source model has been implemented to simulate the transient 3-D thermal cycle. The parameters defining the heat source model, presented in this work, can easily be estimated unlike in the case of ellipsoidal or Gaussian heat source models. The transient temperature distribution was obtained with convection boundary conditions on all the three plate boundaries and top and bottom surfaces. The governing equations were represented in a finite difference form following centre difference implicit scheme. Jacobi iteration scheme with underrelaxation has been implemented to solve the system of difference equations. The temperature dependence of the thermo-physical properties have been incorporated by an iterative process subject to a predefined convergence criterion. The Jacobi iteration and iteration to accommodate variation of thermo-physical properties with temperature run simultaneously. The entire code has been written in VC++ environment. The numerical results for the fusion zone dimensions compared fairly well with those of experiments. Increase in plate thickness indicated increase in cooling rate. For thicker plate, cooling rate stabilised faster as compared to thinner plate.

Study on the effect of tool profiles on temperature distribution and material flow characteristics in friction stir welding

The present investigation describes a computational fluid dynamics approach to study the effects of friction stir welding tool profiles on the thermal conditions and material flow in welds. The aim is to observe the temperatures of the weld and material flow with respect to different probe profiles. For the modeling purpose, temperature dependent material properties and stick–slip conditions between the tool and work material were incorporated. The results from the model exhibited most of the process characteristics of friction stir welding, such as the difference in temperatures of advancing and retreating sides of the weld, temperature contours ahead and behind the tools along the weld, and flow fields. The thermal condition exhibited temperatures that were close to experimental values and below the melting point of the aluminum alloy.

Buckling of Stiffened Panels and Its Mitigation

The increased use of thin plates, like in ships for high-speed operation, results in significantly increased distortion. In-process control of welding distortion is more desirable than postwelding rectification from the point of manufacturing efficiency. This can be achieved by suitably designing and also implementing one or more of the distortion control measures as is suitable for a particular manufacturing situation. In this study, an investigation on buckling phenomenon in fabrication of stiffened panels was carried out. Analytical formulae were used to calculate the weld-induced compressive stresses. Critical buckling stresses considering appropriate buckling coefficient for typical stiffened panels as used in shipbuilding practice were calculated. The predictions from these analytical tools were compared with experimental observations. The experimental results compared fairly well with those of the calculated ones. In the present study the panel design aspect as well as thermomechanical tensioning scheme were studied with regard to buckling mitigation. It was observed that when choosing suitable stiffener spacing in conjunction with plate thickness, buckling distortion can be significantly reduced. Distortion mitigation through thermomechanical tensioning was also found to an effective yet simple-to-implement method of buckling distortion control. However, further investigation is necessary toward fully establishing the method of thermomechanical tensioning. Once this method is established, this pretensioning technique can be applied as an active in-process control method to avoid buckling distortion in stiffened panels.

Modelling of angular distortion of double-pass butt-welded plate

Adequate top and bottom side weld reinforcements are important for single-side submerged arc welded butt joints as the shrinkage forces generated due to the solidification of top side weld reinforcements and bottom side weld reinforcements can cancel out each other to minimize angular distortions. In the present investigation it was experimentally established during submerged arc welding (SAW) that by using proper process parameters adequate bottom and required top weld reinforcements could be obtained by using two welding passes for 12 mm thick square butt joints. During the experimental investigation the first pass of welding was used for adequate bottom reinforcement and for filling up the root gap. The second welding pass was used to achieve the required top reinforcement. The first pass weld acted like a restraint to angular distortion induced due to the weld of the second pass. The process was modelled using three-dimensional finite element analysis considering a distributed moving heat source, reinforcements, filler material deposition in each pass of welding, and temperature- dependent material properties.