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A single-layer pulsed gas metal arc weld of structural steel S355J2+N with a thickness of 5 mm is experimentally and numerically investigated. Two tack welds are considered in the numerical simulation into two different ways. First, the tack welds are represented by elements belonging to the initial material. This implies that the 'tack weld material' was not exposed to any thermal load or phase transformation before actual welding was performed. The weld seam is shortened and there is an influence on the stiffness of the whole structure affecting the calculation result. Secondly, the tack welds were simulated as conducted in the experimental welding procedure. The cases considering tack welding are compared to a simulation neglecting tack welding and to the experimental results.
The influence of tack welds on the calculated welding-induced distortion is clarified and a contribution to an improved simulation-based prediction of welding-induced distortion is possible by modeling tack welding according to the realistic fabrication procedure.
Methods to obtain weld discontinuities in spot-welded joints made of advanced high-strength steels
(2011)
Resistance spot welding is the major joining technique in mass car production. This applies in particular to high-strength steel and advanced high-strength steel (AHSS) joining of thin sheet steel components for lightweight body shell structures. Joining of AHSS in mass production might lead to weld discontinuities under certain circumstances. Those discontinuities in form of cracks might be an initial start of cracking in the spot-welded joints regarding fatigue loads. It is of great interest to figure out, if, in comparison to specimens without weld discontinuities, the crack initiating point changes and if the fatigue resistance might be reduced by the discontinuities. In this contribution, an overview of potential discontinuities is given. Their possible causes are discussed and means for their detection are highlighted. Among the possible causes of weld discontinuities, two major groups are distinguished: the welding parameters as primary influences in the welding process, and the production-specific influences as secondary ones. With emphasis on major cracks penetrating the weld nugget, these influences are analysed. Finally, a combination of extreme welding parameters with production-specific influences is chosen in order to establish a method which enables the preparation of fatigue test specimens with reproducible major cracks in different locations of the spot-welded joints. This method is than applied in order to prepare spot weld specimens for fatigue tests.
Welding is a key technology in the area of industrial production due to its flexibility and efficiency. However, new materials and welding techniques necessitate permanent research activities in order to keep up with the demands. A detailed knowledge about the process itself and the heat effects of welding, e.g., temperatures, distortions, and stresses, is the basis for a target-oriented optimization instead of a trial-and-error approach. Numerical welding simulation is a powerful tool to meet these demands. Complementary to an experimental investigation, it enables the analysis of the specimen during the welding process, commonly known as computational welding mechanics (CWM). Whereas simulation is nowadays a common tool in different development processes, the modeling of welding still remains difficult because of the multiple physical effects taking place. One of the most important problems for the user is the lack of knowledge about the material properties as input data for the simulation. Furthermore, any scattering of the data causes uncertainties that can have major effects on the calculations. The objective of this paper is to give an overview about the experimental determination and analysis of the material properties needed as input data for a welding simulation. The measurement techniques and the occurring deviations of the results are discussed. Additionally, the collected data for three representative alloys (dual-phase steel, austenitic steel, precipitation-hardenable aluminum alloy) are analyzed. Finally, the temperature-dependent thermophysical and thermomechanical material properties for these three alloys are given in a ready-to-use format for a numerical welding simulation.
Welding is one of the most widely used joining processes in structural applications, like in car body production in the automotive industry. It is well-known that distortions and residual stresses occur during and after the welding process. Many procedures exist to decrease these negative heat effects of welding, but are often coupled with highly cost intensive experiments. For several decades, simulation models have been developed to understand and predict the heat effects of welding and to reduce experimental effort. In the production planning of various Original Equipment Manufacturers (OEM), some simulation tools are already well established, e.g. for crash test, forming or casting simulations. For welding, the demand is high but the implementation of welding simulation software is still not established yet. Welding is a complex process and the development of a flexible simulation tool, which produces good simulation results without expert knowledge in simulation, is not an easy task. In this paper, a welded assembly from the automotive industry has been simulated and compared to experimental data. Temperature fields and transient distortion distributions have been measured with thermocouples and with an optical 3D deformations analysis tool, respectively. The simulation has been run with a commercially available welding simulation software. The simulated temperature fields match the numerical ones perfectly. The simulated distortions are also qualitatively in best agreement with the experimental ones. Quantitatively, a difference of approximately 20 % between the simulated and the measured distortions is visible; this is acceptable considering the simplifications and assumptions of the simulation model. The global time to solution to get these results without expert knowledge in welding simulation was between 4 and 6 weeks, which is a reasonable time frame for an industrial application of welding simulation.
During welding, residual stresses build-up created by the steep thermal gradient that occurs in the weld zone from localized heating and cooling, and phase transformations appearing in low-alloyed structural steel is inevitable. Welding of rather simple test plates do not cover the actual structural effects, which have to be considered during real component welding. However, the resulting welding-induced residual stress state is highly influenced by the structural characteristics, i.e. restraint conditions, of the welded construction. Therefore, a unique large-scale testing facility providing a specific shrinkage restraint while welding and subsequent cooling was used for the present investigations. Hereby, a six bead multi-pass gas metal arc weld of 20 mm thick structural steel S355J2 + N was welded under shrinkage restraint. The residual stresses were experimentally and numerically investigated, and compared to an analysis of plates welded under force-free support and free shrinkage conditions.
The experimentally determined and calculated residual stresses using both 2D and 3D numerical models are in a good agreement. Furthermore, the influence of a shrinkage restraint on the residual stress distribution is both experimentally and numerically shown for the present test set-up.
Study of fatigue behavior for spot welded tensile shear specimens of advanced high strength steels
(2012)
For automotive body-in-white applications the influence of manufacture-related gaps between the steel sheets and also of manufacture-related surface cracks on the fatigue behavior of tensile shear specimens for spot welded TRIP steel was analyzed. It was shown that gaps between the steel sheets reduce the fatigue strength, whereas the fatigue behavior is neither influenced by cracks in the electrode indentation area nor in the heat effected zone.
Austenite grain growth during welding is a critical factor for controlling weld microstructure in addition to nominal composition and thermal cycles. Recently, experimental data suggesting a decrease in martensite start temperature with a decrease in prior austenite grain size has been published. However, the actual sensitivity of this phenomenon on residual stresses evolution in the heat-affected zone has not been investigated, yet. Therefore, a numerical model was modified to consider this phenomenon. Numerical simulations were performed for welding of a low-alloy structural steel with minimum yield strength of 355 MPa (S355J2+N) and a heat-resistant steel P91 or 9Cr–1Mo, respectively. The results clarify the influence of prior austenite grain size on the residual stress development and show the importance martensite transformation temperatures and final martensite fraction. Consequently, the residual stress evolution of P91, which completely transforms to martensite while cooling, based on the enhanced model leads to maximum stress differences of 200 MPa in the heat-affected zone.
In this study, the influence of production-related gaps on the shear tension strength and fatigue performance was investigated for resistance spot welded TRIP steel HCT690. Furthermore, the local strain distribution in shear tension test was calculated by the digital image correlation technique (DIC). The static shear tension strength was found to be almost independent of gaps up to 3 mm. The maximum local strain in the spot weld region however decreases depending on which sample side (deformed or undeformed) is considered. In addition, it has been ascertained that gaps of 3 mm lead to a significant drop in fatigue life compared to gap-free shear tension samples. This fact could be attributed to decreased stiffness, higher transverse vibration and higher rotation (θ) between the sheets as well as increased local stress calculated by 2 dimensional FE simulation.
A multi-physics numerical model was developed to investigate the influence of a steady magnetic field aligned perpendicular to the welding direction during partial penetration high power laser beam welding of aluminium in downhand position. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were successfully solved with the finite element differential equation solver COMSOL Multiphysics 4.2. The implemented material model used temperature-dependent properties up to evaporation temperature. Marangoni convection in the surface region of the weld pool, natural convection due to the gravitational field and latent heat of solidliquid phase transition were taken into account. Solidification was modelled by the CarmanKozeny equation for porous media morphology. The flow pattern in the melt as well as the weld bead geometry were significantly changed by the induced Lorentz force distribution in the liquid metal. It reveals that the application of a steady magnetic field to laser beam welding with corresponding Hartmann numbers Ha2 ≈ 104 allows for a suppression of the characteristic wineglass-shape of the weld cross section caused by thermocapillary flow. The numerical results are in good agreement with experimental results obtained with welding of AlMg3 with a 16 kW disc laser. The steady magnetic field was delivered by permanent magnets mounted on both lateral sides of the weld specimen. The maximum magnetic flux density was around 500 mT. It shows, that the applied magnetic field has a predominant dissipating effect on the weld pool dynamics independently of its polarity.
Resistance spot welds were prepared on 3 mm thick sheets of continuous cast and rolled AZ31 magnesium alloy. The microstructure and composition analysis of weld nugget, heat affected zone (HAZ) and base metal were examined using optical and scanning electron microscopy (HR-SEM and EDS/X). The resistance spot welded magnesium alloy joints consist mainly of weld nugget and HAZ. The nugget contains two different structures, i.e. the cellular-dendritic structure at the edge of the nugget and the equiaxed dendritic structure in the centre of the nugget. The structure transition is attributed to the changes of solidification conditions. In the HAZ, grain boundary melting occurred and grain boundaries became coarse. It has been shown that hardness reduction in the weld nugget and HAZ compared with base metal is evident due to dendritic microstructure and grain growth, respectively. The results showed that spot welded joints have failed in interfacial mode under torsion and tensile–shear loading conditions. Digital image correlation during tensile–shear testing showed that low surface strains occur in the interfacial failure mode, because fracture and deformation happened primarily in the nugget area.