Weld Reinforcement Research Articles

Numerical analysis of double-electrode gas metal arc welding process

With consideration of process characteristics of the newly developed DE-GMAW (double-electrode gas metal arc welding), a finite element analysis (FEA) model is developed to numerically analyze this novel process. To this end, the authors have extended the functions of SYSWELD, a standard software package for numerical analysis of welding processes; simplified the method for processing the weld reinforcement; and proposed an appropriate mode for the welding heat source in this novel process. Based on the FEA results of temperature profile in DE-GMAW, the weld dimension at cross section and temperature distribution are obtained. The predicted and experimentally measured results match satisfactorily. The results show that DE-GMAW has advantages in terms of increasing deposition rate, lowering heat input to base metal, and reducing residual stress and distortion.

Numerical simulation of variable polarity vertical-up plasma arc welding process

Three-dimensional transient governing equations were developed based on conservation laws of energy, momentum and mass. These equations described physical phenomena of convection in weld pool and heat transfer in workpiece during variable polarity vertical-up plasma arc welding process. Boundary conditions for the developed governing equations were given. Welding energy input for variable polarity vertical-up plasma arc welding process was quantitatively expressed. Free surface deformation of the keyhole molten pool was coupled into calculation. Effect of wire filling on the geometry of molten pool and weld reinforcement was considered in the simulation. Correlations of temperature and thermophysical properties for aluminum alloy 2219 were quantitatively established. A control volume based finite difference method was used to solve the discrete governing equations. Moreover, dynamic evolutions of geometrical profile, dimension and fluid flow for the molten pool and keyhole were simulated through the developed computational routines, which achieved transient solution of fluid flow field coupling with thermophysical properties, temperature field and weld pool free surface deformation. Besides, the effect of the workpiece thickness on the moments of keyhole formation and stable keyhole establishment was analyzed, and thermal cycles for the main welding stage were calculated. In addition, experiments via variable polarity vertical-up plasma arc welding technique were conducted, and the established models were experimentally verified through weld cross-section profiles.

Corrosion Fatigue Behaviour of Ferritic Stainless Steel Welded Joints in 3%NaCl Aqueous Solution

Axial fatigue tests have been performed using welded specimens of a ferritic stainless steel, SUS 444, in 3%NaCl aqueous solution, and the effect of corrosive environment on fatigue behaviour was studied. The fatigue strength of the welded specimens in 3%NaCl aqueous solution decreased considerably compared with that in laboratory air. Fracture occurred at the toe of weld due to stress concentration, but fracture surface near crack initiation site was brittle, indicating enhanced small crack growth by 3%NaCl aqueous solution, thus in turn leading to decreased fatigue strength. In order to further understand the corrosion fatigue behaviour of welded joints, additional fatigue tests have been conducted using welded specimens whose reinforcement of weld was removed. The fatigue strength of the welded specimens without reinforcement of weld in 3%NaCl aqueous solution decreased significantly compared with in laboratory air. Fatigue cracks initiated from corrosion pits generated in HAZ near the boundary between the weld metal and HAZ at much earlier stage of fatigue cycling and then grew faster than in laboratory air. It was concluded, therefore, that the decreased fatigue strength in 3% NaC1 aqueous solution was attributed to corrosive enviromment-assisted crack initiation and growth.

Application of the Bead Flush Method to Welded Pipes (Improvement of Accuracy by Sequential Removal)

A new nondestructive method of evaluating welding residual stresses, the Bead Flush Method, has been proposed and its validity have been confirmed for simple butt-welded plates. In the previous work, a basic formulation by assuming axisymmetric condition is attempted for welded pipes and problems to be solved were made clear. In this study, further development of this method was attempted by conducting 3-dimentional formulation. In there, a reinforcement of weld was removed in a sequential manner and the formulation was attempted by incorporating that sequential process. It is shown that by adopting sequential process, accuracy to evaluate residual stresses has significantly improved compared with that of conventional method.

A Smart Bi-Directional Model of Heat Transfer and Free Surface Flow in Gas-Metal-Arc Fillet Welding for Practising Engineers

In recent years, numerical heat transfer and fluid flow models have provided significant insight about fusion welding processes and welded materials. A major problem in their practical use is that several input parameters cannot be easily prescribed from fundamental principles. Available inverse models of fusion welding for the determination of these unknown parameters have ignored important physical processes such as convection in the weld pool to make computational tasks tractable. As a result, these models are not very different from the neural network type models that are not required to obey any physical law. A smart, bi-directional, numerical model has been developed to determine three-dimensional temperature and velocity profiles, weld geometry and the shape of the solidified weld reinforcement surface during gas metal arc (GMA) welding of fillet joints. Apart from the transport of heat from the welding arc, additional heat from the metal droplets was also considered in the model. The model is capable of estimating unknown parameters such as the arc efficiency, effective thermal conductivity and effective viscosity from a limited number of data on weld geometry based on multivariable optimisation. Alternative strategies for the optimisation are examined. The calculated shape and size of the fusion zone, finger penetration characteristic of the GMA welds and the solidified free surface profile were in fair agreement with the experimental results for various welding conditions. In particular, the computed values of the leg length, the penetration depth and the actual throat agreed well with those measured experimentally for various heat inputs. The weld thermal cycles and the cooling rates were also in good agreement with the independent experimental data. The research presented here shows that advances in computational hardware and software have now made construction of smart, bi-directional, large transport phenomena based phenomenological models a useful undertaking.

Analysis and application of partial least square regression in arc welding process

Because of the relativity among the parameters, partial least square regression (PLSR) was applied to build the model and get the regression equation. The improved algorithm simplified the calculating process greatly because of the reduction of calculation. The orthogonal design was adopted in this experiment. Every sample had strong representation, which could reduce the experimental time and obtain the overall test data. Combined with the formation problem of gas metal arc weld with big current, the auxiliary analysis technique of PLSR was discussed and the regression equation of form factors (i. e. surface width, weld penetration and weld reinforcement) to process parameters(i. e. wire feed rate, wire extension, welding speed, gas flow, welding voltage and welding current) was given. The correlativity structure among variables was analyzed and there was certain correlation between independent variables matrix X and dependent variables matrix Y. The regression analysis shows that the welding speed mainly influences the weld formation while the variation of gas flow in certain range has little influence on formation of weld. The fitting plot of regression accuracy is given. The fitting quality of regression equation is basically satisfactory.

Fatigue Behavior of Welded Joints in Ferritic Stainless Steel SUS 444

Fatigue behavior of welded joints was studied on a ferritic stainless steel, SUS 444. Plate specimens with weld zone located at the center of gauge section were used for fatigue strength evaluation and two types of CT specimens were employed for crack propagation tests. The fatigue strength of the welded specimens decreased remarkably compared with that of the base metal. Multiple cracks initiated at the toe of weld, then grew and coalesced, leading to final failure. In order to discuss causes of the reduction of fatigue strength in the welded specimens, additional fatigue tests were performed using the annealed specimens and the specimens whose reinforcement of weld was completely removed. The former specimens showed only a slight increase of fatigue strength compared with the welded specimens, while the latter specimens exhibited nearly the same fatigue strength as the base metal. Based on these results, it was concluded that the effect of residual stress was relatively small and thus the significant decrease of fatigue strength in the welded specimens was attributed to the stress concentration resulting from the geometry of the reinforcement of weld. Furthermore, it was found that the crack propagation resistance of HAZ (Heat Affected Zone) and weld metal was higher than that of the base metal due to higher crack closure level.

Improving reliability of modelling heat and fluid flow in complex gas metal arc fillet welds—part I: an engineering physics model

Although numerical heat transfer and fluid flow models have provided significant insight about fusion welding processes and welded materials in recent years, several model input parameters cannot be easily prescribed from fundamental principles. As a result, the model predictions do not always agree with the experimental results. In order to address this problem, the approach adopted here is to develop and test a model that embodies a heat transfer and fluid flow sub-model and an algorithm for optimizing and learning the values of uncertain process variables from a limited volume of experimental data. The heat transfer and fluid flow sub-model numerically calculates three-dimensional temperature and velocity fields, the weld geometry and the shape of the solidified weld reinforcement surface during gas metal arc (GMA) welding of fillet joints. Apart from the transport of heat from the welding arc, additional heat from the metal droplets is also considered in the model. Alternative algorithms for optimization of uncertain welding variables are examined. The overall model is capable of estimating uncertain parameters such as the arc efficiency, effective thermal conductivity and effective viscosity from a limited number of data on weld geometry. Part I of this paper is focused on the details of the numerical model, optimization technique used and an examination of the important features of the model. In an accompanying article (part II), the application of the model to GMA fillet welding of mild steel is described.

Prediction of weld quality in pulsed current GMAW process using artificial neural network

Weld joint dimensions and weld metal mechanical properties are important quality characteristics of any welded joint. The success of building these characteristics in any welding situation depends on proper selection-cum-optimisation of welding process parameters. Such optimisation is critical in the pulsed current gas metal arc welding process (GMAW-P), as the heat input here is closely dictated by a host of additional pulse parameters in comparison to the conventional gas metal arc welding process. Neural network based models are excellent alternatives in such situations where a large number of input conditions govern certain outputs in a manner that is often difficult to adjudge a priori. Six individual prediction models developed using neural network methodology are presented here to estimate ultimate tensile strength, elongation, impact toughness, weld bead width, weld reinforcement height and penetration of the final weld joint as a function of four pulse parameters, e.g. peak current, base current, pulse on time and pulse frequency. The experimental data employed here are for GMAW-P welding of extruded sections of high strength Al–Zn–Mg alloy (7005). In each case, a committee of different possible network architectures is used, including the final optimum network, to assess the uncertainty in estimation. The neural network models developed here could estimate all the outputs except penetration fairly accurately.

Utilization of thermo-elasto-plastic analysis of welding eigenstrain for improvement of the bead flush method

Measurement of welding residual stresses is one of important tasks to assess the structural integrity of welded structures. By now, authors have proposed a new nondestructive evaluation method of welding residual stresses called the bead flush method, and confirmed its utility for simple welded plates by numerical simulations and experiments. In this method, eigenstrains are determined by the inverse analysis using released strains measured during removing reinforcement of weld. And then residual stresses are calculated by simple FEM analyses using eigenstrains determined above. For wider applicability and higher accuracy of this method, eigenstrain field should be determined accurately beforehand in the inverse analysis. Therefore, in this paper, a concept how solution sets of eigenstrains are determined in the inverse analysis is proposed. Based on this concept, influencing factors of welding eigenstrains are investigated and relations between welding eigenstrains and residual stresses are made clear by the thermo-elasto-plastic analysis.

New Methodology for Crevice-Corrosion Testing of Welds with Reinforcements

Abstract Weld zones in fabricated structures may exhibit preferential corrosion, especially under crevice-corrosion conditions. However, it is difficult to produce crevice conditions in the laboratory because of the nonflat surfaces associated with weld reinforcements. A new methodology has been developed for testing welds with this common geometry. Its application has proven successful in evaluating the effects of weld material, surface condition, and temperature on the crevice-corrosion characteristics of weld zones with reinforcements.

Fatigue crack initiation and growth in AlMg4.5Mn butt weldments

The fatigue life of full‐penetration and partial‐penetration 5 and 25 mm thickness AlMg4.5Mn (AA5083) aluminium alloy butt weldments was investigated under (R = 0 and R = − 1) constant amplitude loading. The fatigue lives of the tested specimens were predicted using an analytical model which estimated both the crack initiation and crack growth portions of the total fatigue life. The fatigue life of partial‐penetration weldments was found to be substantially less than that of full‐penetration weldments because of the greater stress concentrations of the incomplete joint penetration and the consequent absence of a substantial crack‐initiation life period. Tensile mean stresses (R = 0 versus R = − 1‐test conditions) markedly reduced the fatigue life of the weldments studied and greatly diminished the duration of the fatigue crack growth period. The extra material provided by the weld reinforcement noticeably increased the fatigue life of the partial‐penetration weldments. Weld angular distortion‐induced bending stresses greatly affected the smaller thickness (5 mm) full‐penetration weldments offsetting the fatigue strength bonus anticipated for small‐size weldments. Except for the predictions for R = − 1 full‐penetration weldments at long life, which the analytical model underestimated, the agreement between experiment and analytical prediction was within a factor of 2, that is, as good as can be generally expected.

Computer Aided Nondestructive Evaluation Method of Welding Residual Stresses by Removing Reinforcement of Weld. Introduction of Regularization Method and Optimization of Strain Measuring Location.

In a previous work, a new method was proposed to estimate 3-D welding residual stresses nondestructively. Firstly, 3-D eigenstrain distributions are estimated by the inverse analysis from surface strain changes due to removal of reinforcement of the weld. Then, residual stresses are easily determined by an FEM analysis using estimated eigenstrains as boundary conditions. Under an existence of possible experimental errors. however, a solution of the eigenstrain distribution becomes unstable due to the ill-posedness of the problem, which is not acceptable as an actual evaluation method. To overcome above ill-posedness, a regularization method based on the singular value decomposition was incorporated. It is confirmed that 3-D residual stress distributions can be obtained in good accuracy. Then, a new optimizing method determining strain measuring locations under reasonably small number, was proposed based on the experimental design theory using the Doptimality criterion and its utility is shown.

Computer Aided Nondestructive Evaluation Method of Welding Residual Stresses by Removing Reinforcement of Weld. A Proposal of New Concept and Its Application to Butt Welded Plate.

Based on previous works, a new concept of a nondestructive stress measuring system for welded structures was proposed. Firstly, reinforcement of weld is removed. From strain changes on the surface due to removal, distributions of eigenstrain are determined by the inverse analysis. Then, residual stresses are estimated by the elastic FEM (Finite Element Method) analysis by using the determined eigenstrain distributions as anihnitial condition. Toward realization of this concept, applicability of this method was examined by simulations for a butt welded plate with anisotropic eigenstrains along and near a welding line. In a preliminary analysis on an effect of each eigenstrain component on welding residual stresses, it was found that an eigenstrain component in bead direction has a significant influence on producing residual stresses perpendicular and parallel to the welding line. Thus, by separating this dominating component from other two, an accuracy in the inverse analysis can be considerably improved. Further attempt to reduce unknown parameters in the inverse analysis was to approximate eigenstrain distributions by using a series of logistic functions, which proved to have a wide applicability as a curve fitting function form to actual eigenstrain distributions under various welding conditions. Finally, it was shown that welding residual stresses along and perpendicular to welding line in any location of the plate can be evaluated by using the proposed method.

Schweißnahtradiographie und -topographie

Correct imaging and estimation of weld reinforcement is a special problem in radiography. Particularly the indications of flaws are overlaid by flaw-like structures of the weld surface while projecting into the film plane, which may yield misinterpretations of the radiographic image. It is shown, that the combination of the radiographic image and the corresponding topometric surface scan improves the reliability of the image interpretation.

Mathematical modeling of three-dimensional heat and fluid flow in a moving gas metal arc weld pool

Mathematical models capable of accurate prediction of the weld bead and weld pool geometry in gas metal arc (GMA) welding processes would be valuable for rapid development of welding procedures and empirical equations for control algorithms in automated welding applications. This article introduces a three-dimensional (3-D) model for heat and fluid flow in a moving GMA weld pool. The model takes the mass, momentum, and heat transfer of filler metal droplets into consideration and quantitatively analyzes their effects on the weld bead shape and weld pool geometry. The algorithm for calculating the weld reinforcement and weld pool surface deformation has been proved to be effective. Difficulties associated with the irregular shape of the weld bead and weld pool surface have been successfully overcome by adopting a boundary-fitted nonorthogonal coordinate system. It is found that the size and profile of the weld pool are strongly influenced by the volume of molten wire, impact of droplets, and heat content of droplets. Good agreement is demonstrated between predicted weld dimensions and experimently measured ones for bead-on-plate GMA welds on mild steel plate.

The effect of residual stresses in weld reinforcement on the propagation of fatigue cracks

The effect of residual stresses in weld reinforcement on the propagation of fatigue cracks

Dural closure with laser tissue welding.

This study investigates the use of tissue-soldering techniques to substitute or reinforce traditional suture closure of dural incisions. Fresh human cadaveric dura was incised and subsequently closed by use of three techniques: (1) conventional interrupted suture with 4-0 silk (n = 25), (2) laser solder reinforced suture closure (n = 25), and (3) laser solder closure alone (n = 25). Anastomosis tensile strength and hydrostatic leak pressures were measured. Dural repair was also performed in 15 live Lewis rats. Dural closure was accomplished with 9-0 Prolene sutures (n = 5), laser-reinforced suture closure (n = 5), and laser solder closure alone (n = 5). Histologic examination of the closure immediately after soldering and 2 weeks later was performed. Suture closure alone had the lowest leak pressure, 9.4 +/- 1.7 mm Hg, and an intermediate break point, 13.3 +/- 2.1 Kgf/cm2. Measurements with laser solder alone revealed a mean leak pressure of 26.2 +/- 3.7 mm Hg and a break point of 4.6 +/- 1.4 Kgf/cm2. Solder-reinforced suture closure leak pressure measured 64.0 +/- 6.7 mm Hg and 21.4 +/- 2.4 Kgf/cm2. There was a statistically significant increase in leak pressure and tensile strength in the closures performed with laser weld reinforcement of traditional suture technique (p = 0.0001). Dural closure with laser tissue welding alone provided an immediate leak-free closure, but with poor tensile strength. Histologic examination of welded dura and underlying brain tissue showed no evidence of thermal injury in four of five animals studied. Laser welding may significantly decrease the incidence of cerebrospinal fluid leak after dural closure. In addition, laser tissue welding also makes dural closure possible where space constraints make traditional suture closure difficult.

Dural closure with laser tissue welding

This study investigates the use of tissue-soldering techniques to substitute or reinforce traditional suture closure of dural incisions. Fresh human cadaveric dura was incised and subsequently closed by use of three techniques: (1) conventional interrupted suture with 4-0 silk ( n = 25), (2) laser solder reinforced suture closure ( n = 25), and (3) laser solder closure alone ( n = 25). Anastomosis tensile strength and hydrostatic leak pressures were measured. Dural repair was also performed in 15 live Lewis rats. Dural closure was accomplished with 9-0 Prolene sutures ( n = 5), laser-reinforced suture closure ( n = 5), and laser solder closure alone ( n = 5). Histologic examination of the closure immediately after soldering and 2 weeks later was performed. Suture closure alone had the lowest leak pressure, 9.4 ± 1.7 mm Hg, and an intermediate break point, 13.3 ± 2.1 Kgf/cm 2. Measurements with laser solder alone revealed a mean leak pressure of 26.2 ± 3.7 mm Hg and a break point of 4.6 ± 1.4 Kgf/cm 2. Solder-reinforced suture closure leak pressure measured 64.0 ± 6.7 mm Hg and 21.4 ± 2.4 Kgf/cm 2. There was a statistically significant increase in leak pressure and tensile strength in the closures performed with laser weld reinforcement of traditional suture technique ( p = 0.0001). Dural closure with laser tissue welding alone provided an immediate leak-free closure, but with poor tensile strength. Histologic examination of welded dura and underlying brain tissue showed no evidence of thermal injury in four of five animals studied. Laser welding may significantly decrease the incidence of cerebrospinal fluid leak after dural closure. In addition, laser tissue welding also makes dural closure possible where space constraints make traditional suture closure difficult. (Otolaryngol Head Neck Surg 1996;115:513-8.)

Comparison between Full Scale Tests and Small Scale Tests in Evaluating the Cracking Susceptibility of Line Pipe in Sour Environment.

Line pipes have possibility to encounter three types of crack morphology: SSC, HIBC/SWC, SOHIC (Stress Oriented HIC) in the sour environment. For modern line pipes, SOHIC is concerned under applied stress, particularly in softened heat-affected zone. In this study, an X65 UOE line pipe was evaluated as a CAPCIS type full ring test and a full pipe test were compared with tensile tests conforming to NACE standard TM0177-90 method A on the susceptibility to SOHIC in weld. A rectangular tensile specimen 6.4 mm in thickness provided the same result as the round bar specimen specified in the NACE standard in terms of threshold stress. Ruptured specimens showed SOHIC preferably induced in the softened HAZ. In contrast, no defect indication was obtained after the completion of the full ring test performed at the applied stress of 90% SMYS above the threshold stress. With regard to the full pipe test, no defect was observed when loaded with 90% SMYS with an exception of bead toe cracking caused by rather high applied stress due to stress concentration. The rectangular tensile test in which a single wide side was exposed to the environment was also conducted. No internal SOHIC was observed at the applied stress of 100% SMYS. And the single side exposure test with weld reinforcement exposed to the sour environment also reproduced the bead toe cracking. This study concludes that the NACE standard is more severe than CAPCIS type full ring test which can be substituted with the one-side exposed rectangular specimen and more severe than the full pipe test from the viewpoint of evaluating the susceptibility to SOHIC. This study proposes the single side exposure rectangular test as a realistic substitute for full scale tests.