IOPscience Google Scholar.
Author e-mails. Heat Mass Transf.
D: Appl. A Crossref Google Scholar. Stress 29 Crossref Google Scholar. The inherent strain theory and interface element were employed in the elastic FEM. Heinze et al. Through comparisons with experimental results, it was found that the distortion is sensitive to fractions of martensite, bainite, and ferrite. Wang et al. Inherent bending deformation and initial deflection were found to be influential factors to trigger buckling. To minimize the welding distortion, it is necessary to take advantage of both the numerical method and the experimental measurement.
To simulate the welding-induced buckling distortion, the modelling procedure was presented by Yang et al. Yang and Dong [ 27 ] showed a computational framework for mitigation of buckling distortion. Deo and Michaleris [ 28 ] validated the numerical simulation models and then replaced expensive experimental trials for prevention of buckling distortion.
Because the quantitative numerical simulation of transient distortion in welding needs to be validated by the in-situ measured information, the development of advanced measuring techniques for the full-field welding deformation is especially important. In the present study, the out-of-plane distortion of a Tungsten Inert Gas TIG welded bead-on-plate was investigated by both in-situ digital image correlation DIC measurement and 3D thermo-mechanical analysis. The transient welding distortion of a rectangular plate of Q steel was measured during the welding and cooling processes. The initial geometry measured in the experiment was mapped into the numerical model so that the deformation mode and magnitude in the time history can be accurately simulated.
To clarify possible ways to minimize the out-of-plane distortion in welding, parametric studies on heat input and initial deformation were performed using thermal elastic—plastic FEM solver JWRIAN developed by authors.
X-ray Diffraction Systems & Services
The DIC full view of initial plate geometry and high-fidelity computation offers an accurate way of transient welding distortion prediction and control. A rectangular plate of hot rolled mild steel Q was prepared for the welding experiment as shown in Figure 1 a. It should be noted that, initial residual stresses in steel plates can be large depending on the plate thickness and heat treatment condition [ 8 , 29 ].
The holding time at peak temperature was 2 h.
Through such heat treatment, initial residual stresses could be greatly released so that its influence on welding distortion were limited. Figure 1 b shows the experiment setup. Tungsten Inert Gas TIG welding process was employed to avoid the effect of filler metal and reinforcement.
Therefore, the material and geometrical model could be kept simple for mechanical analysis by the finite element method. The welding piece was placed on the multi-functional welding platform without restraint so it could deform freely. The welding parameters are summarized in Table 1. No additional heat treatment was performed after cooling. Welding and measuring setup. The digital image correlation method was proposed by Chu et al. Now it is widely used in experimental test as the non-contact measuring method [ 31 , 32 ].
Using the DIC technique to measure deformation in welding has two challenges: the intensive arc light and filler metal dust can affect measuring accuracy. Recently, researchers Xiao et al. Chen et al. In the experimental measurement, the speckles were prepared on the back side of the plate before welding. The DIC camera was placed underneath the weld plate with a distance of about mm.
Before speckle painting, the region near the weld was sandblasted to create enough roughness and reduce light reflection. Thus, the adhesive can deform with the plate surface to the same extent during the welding without introducing error by thermal expansion mismatch.
Local Cooling during Welding: Prediction and Control of Residual Stresses and Buckling Distortion
The measured accuracy on x , y and z coordinates was 0. The coordinates of the points were measured at a time interval of 0. The sampled data was output on a grid of 38 columns and 22 rows, which were used for comparison with the analyzed results of FEM. Coordinates of all nodal points were transformed by fixing the out-of-plane displacement Uz at points O, A, and B. The numerical model was also constrained in the same way for a direct comparison of welding distortion. Distortion measurement by the digital image correlation DIC technique. We know that the distortion of a structure under external load is sensitive to initial stress and geometrical imperfection.
Therefore, the initial deformation of the plate before welding was captured by DIC measuring technique. In the numerical model, initially flat geometry was modified by mapping the initial deformation from the measured points to FE nodes. Then thermo-mechanical analysis was performed based on the measured geometry, and the welding-induced distortion was evaluated by comparing the current coordinate with initial coordinates.
The welding process involves thermal—mechanical—metallurgical behavior which makes a coupled simulation very challenging. From the viewpoint of the heat transfer process, the heat converted from the mechanical process is much smaller than the welding heat. Though research by Sun et al. In addition, the microstructural change in SS steel has a minor effect on stress and strain as demonstrated by Nagai et al.
Thus, it is reasonable to perform a thermal—mechanical analysis in the sequentially manner. The equation of thermal conduction [ 8 ] with a volumetric heat generating rate q for TIG welding can be described by Equation 1. These material properties are temperature dependent as shown in Figure 4.
The distribution of volumetric heat in a moving ellipsoidal zone can be given by the following equation in the numerical model.http://www.sanvalentinrun.com/images/347/el-lobo-conocer-gente.php
Measurement of Residual Stresses in Welded Joints by DCP Method
The parameters a , b , c denote the semi-axes of the ellipsoidal zone in welding direction, a transverse direction, and plate thickness direction, respectively. On the free surface of plates, the heat is transferred to the air and surroundings through convection and radiation. Radiation can be taken into account by convection using an equivalent heat transfer coefficient as described by Ueda et al. The equivalent heat flux going out of the surface can be written as:. The energy equation is integrated over the finite element domain and solved in time increment form.
In thin-walled structures, the out-of-plane deformation is greatly affected by the in-plane deformation components.
The temperature gradient through the thickness is also critical to the formation of bending stress and strain. For the thermal mechanical problem, the total strain consists of three basic components if phase transformation induced strain in neglected. To accurately predict the deformation in thin plates, 3D thermal-elastic—plastic FEM together with large deformation theory is necessary. The Green strain expression in the Lagrangian form can be used as described by Zienkiewicz and Taylor [ 40 ]. The detailed mechanical properties including the yield strength and thermal expansion coefficient are plotted in Figure 5.
The temperature dependent material properties of Q are referred to SS [ 41 ] since the two materials have very close chemical compositions. Isotropic hardening at the plastic deformation zone was assumed, and the increment of plastic strain can be expressed as:. Given the displacement increment, the above equations can be used to obtain the updated stress field and nodal force vector. After solving the simultaneous linear equations, a new displacement field can be obtained.
Related Processes and Mechanisms of Welding Residual Stress and Distortion
Copyright 2019 - All Right Reserved