Hidekazu Murakawa

Numerical analysis of the competitive influence of Marangoni flow and evaporation on heat surface temperature and molten pool shape in laser surface remelting

Abstract A one-domain mixture continuum model is introduced to simulate numerically solid/liquid phase transformation with a mushy region in laser surface remelting process of a type 304 stainless steel. Emphasis is given to the competitive influence of laser-induced alloying element vaporization and Marangoni flow on the heating surface maximum temperature and its distribution as well as the molten pool shapes. The molten pool shapes and fluid flow, the temperature distribution and its peak values on the heating surface have been computed for six calculation cases corresponding to six different pool-surface heat flux balances. The results show that the Langmuir-type vaporization heat loss due to Fe, Mn, Cr, Ni elements can significantly reduce the heating surface peak temperature and the pool profile, while |∂σ/∂T| is small or the buoyancy force is considered as one and only driving force. However, when vaporization-type heat loss and Marangoni flow are coexisting, the free surface temperature distribution and its peak value are markedly affected by the magnitude order of surface tension gradient and the nature of the relationship between surface tension temperature coefficient and temperature. When |∂σ/∂T|>3.0×10−4 N/m K, the influence of Langmuir-type vaporization heat loss is very small and can be ignored.

Actual application of FEM to analysis of large scale mechanical problems in welding

Abstract The finite element method is a powerful tool for predicting welding distortion. However, the mechanical phenomena are strongly non-linear and transient and thus, thermo–elastic–plastic finite element analyses of the phenomena require very long computational time. To overcome this problem, an interactive substructure method was developed as an approach to reduce the computational time in three-dimensional analyses. In this paper, in order to confirm calculation efficiency for a large scale problem, a pipe model with 538 200 degrees of freedom was computed. Furthermore, the residual distortion in an engine component was computed and the applicability of the calculation for problems in welding practices was demonstrated.

Effect of low transformation temperature weld filler metal on welding residual stress

Abstract The effect of weld filler metal austenite to acicular ferrite transformation temperature on the residual stresses that arise during the gas metal arc welding of a low carbon steel has been examined using a finite element model. It was found that the stress levels in the weld can be tailored by the appropriate selection of the filler metal and compressive, near zero or tensile residual stresses produced. Reasonable agreement was obtained between the model and the stresses measured using neutron diffraction both in welds using conventional and low transformation temperature filler metal.

Prediction of Welding Distortion by Elastic Finite Element Analysis using Inherent Deformation Estimated Through Inverse Analysis

During welding, deformation is produced as an unavoidable consequence. The inherent deformation method, in which the inherent deformation is introduced into the elastic finite element method (FEM) as the initial strain, is one of the effective methods to predict the welding distortion of large structures. However, the values of the inherent deformations for all weld joints included in the structure must be known beforehand. Generally, the inherent deformations are influenced by various factors such as materials, welding method, welding condition, joint geometry, plate thickness, and weld length. Thus, it is meaningful to develop a simple method to obtain the inherent deformation. In this report, a simple and efficient method to estimate inherent deformation of typical weld joints is proposed. Further, using the estimated inherent deformation, the welding deformation of joint specimens and large plate structures are predicted.

Predicting welding deformation in thin plate panel structure by means of inherent strain and interface element

Abstract In this study, welding distortion in a large thin plate panel structure was predicted by means of elastic finite element method based on inherent strain theory and interface element formulation. The welding distortions in the thin plate model computed by large deformation theory and small deformation theory were compared. The comparison suggests that the geometrical non-linearity should be carefully considered when welding distortion in a thin plate structure is predicted. In addition, the influences of welding procedure and assembly sequence on the final distortion were examined numerically. Simulation results indicate that both welding procedure and assembly sequence significantly affect the final deformation.

Numerical simulation for the residual stresses of Stellite hard-facing on carbon steel

Abstract Hard-facing is an effective method for modifying the surface properties of metal parts, such as valves. The residual stresses in the hard-facing layer are important factors affecting the service properties and service life of the parts. This research has studied the decrease in the tensile residual stress in the hard-facing layer as well as the residual stress distributions and their affecting factors using a numerical simulation method with the aid of the commercial software abaqus . The computed results indicated that the residual stresses in the hard-facing layer are dependent on several factors, which include the heat-transfer coefficient, the thermal expansions of the materials, the thickness of the base metal and the pre-heating temperature, etc. In addition, local post-heating on the base metal surface can effectively reduce the tensile residual stress and this tensile stress can also be cut down if there is an austenitic stainless steel interlayer between the carbon base metal and the Stellite hard-facing layer. These results indicate that the numerical simulation method is a useful and convenient means by which to investigate the residual stresses of the hard-facing process.

Welding distortion investigation in fillet welded joint and structure based on iterative substructure method

Abstract In this paper, prediction and controlling angular distortion in fillet welded joint and structure were investigated. First, two methods to reduce angular distortion in fillet welded joint were investigated by experimental and numerical analysis. One was to apply a constant external force in-process and the other was rigid clamping. In numerical analysis, a new in-house finite element code has been developed based on the idea of iterative substructure method (ISM) to calculate welding distortion in rational time. During experimental analysis, the constant external force was designed and applied in-process to reduce angular distortion of fillet welded joint. The results showed that the distortion can be efficiently predicted by ISM, which were in good agreement with the experimental ones. Applying constant external force in process was a more effective method to reduce distortion than using rigid clamping. In addition, with a constant load distance from weld bead, the locations of the applied constant external force and rigid clamping along the longitudinal direction (welding direction) have little influence on the magnitude of welding angular distortion. Finally, the angular distortion of a large fillet welded structure was predicted with ISM and also controlled with applying a constant external force based on the simulation results of the fillet welded joint.

Finite element analysis on relationships between the J-integral and CTOD for stationary cracks in welded tensile specimens

Abstract In this study the effects of weld strength mismatching and geometry parameters on the relationship between the J-integral and the crack tip opening displacement (CTOD) are investigated. Numerical analysis was carried out by an ABAQUS two-dimensional elastic–plastic analysis mode. The work was performed for center-cracked welded specimens with uniform tensile load. In the specimens the weld strength mismatch, M, was from 0.8 to 1.2, the crack length, a/W, from 0.1 to 0.5, the weld width, h/c, from 0.1 to 0.5. The main results indicate that weld strength mismatching has only a weak influence on the relationship between the J-integral and CTOD at low load levels, but there is a strong effect at high load levels. The yield strength of weld metal may be used for low load levels and the yield strength of base metal may be used for high load levels, when the basic relationship of J-integral versus CTOD is utilized to treat the problem of welded joints. The results also show that the crack size and weld width have an influence on the relationship between the J-integral and CTOD at high load levels. Because the equivalence between the J-integral and CTOD breaks down at high load levels, the relationship between J-integral and CTOD becomes more complex, and the weld strength and geometry mismatching factors must be included.

The effect of mechanical heterogeneity and limit load of a weld joint with longitudinal weld crack on the J-integral and failure assessment curve

Abstract According to the CEGB R6 (Rev.3) approach, the influence of strength mis-matching and the limit load of a weld joint with a longitudinal weld crack on the J -integral and failure assessment curve can be studied by using an elastic–plastic finite element method for Center-Cracked Panel (CCP) specimens. The results indicate that the values of the J -integral and the shapes and positions of the failure assessment curves are greatly affected by the strength mis-matching factor M , a ratio of weld metal yield strength to that of base metal. If the limit load of the base metal is adopted to normalize the applied load, then the greater the value of M , the larger the safe area is in the failure assessment curve (FAC). However, if the limit load of the weld metal is adopted to normalize the applied load, then the greater the value of M , the smaller the safe area is. Therefore, for the undermatched and evenmatched joints, it is safer to choose the limit load of the base metal as the normalized load, and for the overmatched joints, it is safer to choose the limit load of the weld metal as the normalized load. Moreover, when M is less than 0.8 for the former situation, the option 1 curve of the R6 is not a conservative assessment curve. Considering that there is no simple theoretical formula which is suitable for calculating the limit load of a mechanical heterogeneous weld joint under plane stress and a variety of crack geometries, it is recommended that no matter what the strength of the overmatched or undermatched weld joint is, it is safer to use the limit load of that metal which has the higher strength grade of base metal and weld metal as the normalized load.

Parametric studies of welding distortion in fillet welded structure based on FEA using iterative substructure method

Abstract Using an in house finite element (FE) code developed based on the idea of iterative substructure method (ISM), this paper presents a numerical study to evaluate the effects of welding sequence, fixture condition and prestrain on the residual welding distortion in a fillet welded structure. The results show that welding sequence has appreciable influence on residual distortion, appropriate fixture can greatly reduce angular distortion in the workpiece and application of prestrain can significantly reduce the residual distortion. Taking advantage of the high efficiency of ISM, the optimum scheme of prestrain has been numerically predicted within practical time based on a systematic investigation.