Quality Assessment of Laser Welding Dual Phase Steels

Since non-conforming parts create waste for industry, generating undesirable costs, it is necessary to set up quality plans that not only guarantee product conformity but also cut the root causes of welding defects by developing the concept of quality at origin. Due to their increasing use in automotive industry, dual phase (DP) steels have been the chosen material for this study. A quality plan for welding DP steel components by laser was developed. This plan is divided into three parts: pre-welding, during and post-welding. A quality assessment regarding mechanical properties, such as hardness, microstructure and tensile strength, was also performed. It was revealed that DP steel does not present considerable weldability problems, except for the usual softening of the heat affected zone (HAZ) and the growth of martensite in the fusion zone (FZ), and the best analysis techniques to avoid failures in these steels are finite element method (FEM), visual techniques during welding procedure and digital image correlation (DIC) for post-weld analysis.The present work was done and funded under the scope of projects UIDB/00481/2020 and UIDP/00481/2020—FCT—Fundação para a Ciencia e a Tecnologia; and CENTRO-01-0145-FEDER- 022083—Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund. LAETA/INEGI/CETRIB is acknowledge due to the support provided in all research activities.info:eu-repo/semantics/publishedVersio

An Automated Inverse Method to Calibrate Thermal Finite Element Models for Numerical Welding Applications

Numerical modelling of welding processes is often completed using a sequentially coupled FE thermo-mechanical analysis to predict both the thermal and mechanical effects induced by the process. The accuracy of the predicted residual stresses and distortions are highly dependent upon an accurate representation of the thermal field. Utilising this approach, the physics of the melt pool are replaced with a heat source model which represents the heat flux distribution of the process. Many heat source models exist; however, the parameters which define the geometrical distribution have to be calibrated using experimental data. Currently the most common method involves trial and error, until the predicted thermal history and melt pool geometry accurately represent the experimental data. Although this is a simple approach, it is often time dependant and inherently inaccurate. Therefore, this study presents an automated calibration process, which determines the optimum element size for the FE mesh and then refines the parameters of the heat source model using an inverse approach. The proposed procedure was implemented for laser beam welding, operating in both the conductive and keyhole regimes. To ensure that both the thermal history data and melt pool geometry were predicted with accuracy, a multi-objective optimisation was required. The proposed methodology was experimentally validated through welding nine IN718 samples using a Nd:YAG laser heat source. A good correlation between the experimental and numerical data sets were apparent. With regards to the predicted melt pool geometry, the maximum error for the width, depth and area of the melt pool was 8.4%, 4.0% and 11.0% respectively. The minimum error was 1.5%, 0.3% and 0.3% respectively. For the temperature profiles, the maximum and minimum error for the peak temperature was 8.6% and 1.2%. Overall, the proposed calibration procedure allows automation of a

Research reports: 1991 NASA/ASEE Summer Faculty Fellowship Program

The basic objectives of the programs, which are in the 28th year of operation nationally, are: (1) to further the professional knowledge of qualified engineering and science faculty members; (2) to stimulate an exchange of ideas between participants and NASA; (3) to enrich and refresh the research and teaching activities of the participants' institutions; and (4) to contribute to the research objectives of the NASA Centers. The faculty fellows spent 10 weeks at MSFC engaged in a research project compatible with their interests and background and worked in collaboration with a NASA/MSFC colleague. This is a compilation of their research reports for summer 1991

Index to 1984 NASA Tech Briefs, volume 9, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1984 Tech B Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

NASA Tech Briefs Index, 1977, volume 2, numbers 1-4

Announcements of new technology derived from the research and development activities of NASA are presented. Abstracts, and indexes for subject, personal author, originating center, and Tech Brief number are presented for 1977

Simulação numérica de deformações e tensões em soldadura

Welding is one of the best known methods in the industry for joining a wide variety of materials. This process inevitably creates stresses and strains in the components due to the high energy intensity released by the heat source. Nowadays it is almost mandatory to quantify these changes in the parts that go through the welding process. This is the only way to comply with strict quality parameters ensuring that the part fulfils the assigned function. It is very common to use experimental methods to do this analysis. However, the use of computational methods in welding process simulation was being increasing significantly. Numerical simulation, based on the Finite Element Method, appears to make it easier for engineers to predict and analyse complex phenomena. In this work two numerical simulation models of the welding process by laser were developed on Dual-Phase 600 steel plates. Two types of joints were tested: butt and in T. The deformations and stresses caused were quantified using the Simufact software.A soldadura é dos métodos mais conhecidos na indústria para unir uma grande variedade de materiais. Este processo cria inevitavelmente tensões e deformações nos componentes devido à alta intensidade de energia libertada pela fonte de calor. Nos dias que correm torna-se quase obrigatório quantificar estas alterações nas peças que passam pelo processo de soldadura. Só assim é possível cumprir rigorosos parâmetros de qualidade, garantindo que a peça cumpre a função atribuída. É muito comum recorrer a métodos experimentais para fazer esta análise. No entanto, o uso de métodos computacionais em simulação de processos de soldadura tem crescido significativamente. A simulação numérica, baseada no Método de Elementos Finitos, surge para facilitar aos engenheiros a prevenção e análise de fenómenos complexos. No presente trabalho foram desenvolvidos dois modelos de simulação numérica do processo de soldadura através de laser em chapas de Dual-Phase 600. Foram testados 2 tipos de juntas: topo a topo e em T. As deformações e tensões causadas pelo processo foram quantificadas com recurso ao software Simufact.Mestrado em Engenharia Mecânic

Investigation of thermal techniques to mitigate buckling distortion in welding panels

This thesis describes the advancements of the application of thermal tensioning techniques to different weld geometries in order to eliminate buckling distortion. The main goal of this work is to better understand these techniques through experimental and numerical investigation and increase their technological maturity to aid industrial implementation. The thermal tensioning techniques investigated in this work are Thermal Tensioning by Cooling and Thermal Tensioning by Heating. The investigation for both techniques encompasses thermal source characterisation, application to different weld geometries and residual stress measurements and analysis of both butt and fillet welded samples. A detailed technology transfer study of Thermal Tensioning by Cooling was carried out in which different aspects of the application of TTC to arc welding (Gas Metal Arc Welding and Gas Tungsten Arc Welding) was examined. This study focused on the influence of both the liquid CO2 delivery system installation and welding tooling and jigging on the effectiveness of Thermal Tensioning by Cooling in reducing buckling distortion. Experimental and numerical cooling source characterisation was also carried out in the Thermal Tensioning by Cooling work to investigate the characteristics of the cooling source under different cooling conditions. The Thermal Tensioning by Cooling work was then concluded with welding trials and residual stress measurement and analysis. The results of the Thermal Tensioning by Cooling study show that the installation of the liquid CO2 delivery system as well as the welding tooling and jigging has a major influence on the effectiveness of Thermal Tensioning by Cooling in reducing buckling distortion. The cooling source characterisation work reveals that the most important parameter of the cryogenic nozzle delivery system used in this work is the Air Entrainment Gap. A description of a control system of Thermal Tensioning by Cooling is suggested based on controlling the Air Entrainment Gap. The residual stress analysis shows a reduction in the Applied Weld Load and minor changes in the tensile peak of the residual stress distribution of both butt and fillet welded panels. The Thermal Tensioning by Heating investigation includes heat source characterisation, application of Thermal Tensioning by Heating on butt and fillet welds, utilisation of alternative heat sources and residual stress analysis. The results of these investigation show that Thermal Tensioning by Heating is also highly effective in eliminating buckling distortion in butt, fillet and overlapped panels. The applied heating temperature in this work is typically in the range of 160-250 °C but not greater than 330 °C. The residual stress measurements reveal that the additional heating of Thermal Tensioning by Heating generates a positive stress gradient at the location of heating

Laser assisted arc welding process for dry hyperbaric deep water application

Hyperbaric Gas Metal Arc Welding (GMAW) is an important technology for repair welding of deep sea pipelines and linking of existing pipeline networks to newer ones through tie-ins and hot-tap welding. With increasing water depth the process becomes susceptible to hydrogen assisted cracking due to the very fast cooling rate of the weld caused by higher habitat gas density and resulting higher thermal diffusivity. Maintaining sufficient heat in the welding zone is vital to avoid a potential cracking tendency especially as moisture pick-up may be difficult to avoid during hyperbaric welding operations. In addition to this, hyperbaric GMAW has a limitation of low heat input because it is operated at a short arc length or dip transfer mode to avoid process instability at high pressure. Also, the short arc length generates weld spatter that may affect weld quality. The research presented in this thesis, investigated the use of an industrial laser in conduction mode for the purpose of providing significant additional heat input to control the weld thermal cycles of GMAW. Advanced GMAW power sources such as the Fronius Cold Metal Transfer (CMT) and EWM ColdArc have also been investigated for reduced weld spatter generation. Studies were conducted to investigate the weld pool thermal cycles and resulting metallurgical phase formation in hyperbaric GMAW at different pressures ranging from 1 bar to 200 bar. This was followed by welding trials at one atmosphere to compare the process characteristics of traditional dip transfer GMAW with some advanced GMAW power sources such as CMT and ColdArc. The main experimental trials to investigate a laser assisted GMAW (CMT) process were performed at one atmosphere condition. A thermal model was developed using Abaqus software to predict the weld metal and heat affected zone thermal cycle in a laser assisted GMAW (CMT) process at one atmosphere and under high ambient pressures. Finally, investigation was carried out to evaluate the benefit of the laser assisted process in lowering diffusible hydrogen content from the weld metal. The hyperbaric GMAW experimental results showed that the weld pool cooling rate increases with pressure due to higher chamber gas density and resulting thermal diffusivity. But this effect is not prominent for thicker plates. Therefore, it was concluded that heat conduction through the steel thickness dominates convective losses to the chamber gas environment. It was also shown that the welding arc shrinks as pressure increases in order to minimise energy loss to the environment. This defined the weld bead profile; although it was found that beyond 100 bar pressure the weld penetration depth remained effectively unchanged. Apart from the hardness of the weld made at 1 bar, there was little difference between those at 18, 100 and 200 bar. However, all of the welds show hardness peaks greater than 350 HV10 recommended for offshore structures. It was observed that CMT produced the lowest weld spatter compared to the traditional GMAW and ColdArc. However, this advantage is constrained to low wire feed speed (3 to 5 m/min) beyond which it becomes relatively unstable. For the laser assisted GMAW (CMT) trials, it was shown that the laser serves as a spatially resolved heat source, reheating the weld bead and reducing the cooling rate. For the laser parameters investigated, over 200% reduction of cooling rate could be achieved when compared with GMAW alone. It was also demonstrated that the additional laser thermal input will extend the weld residence time at high temperature (over 300 °C). This will prolong the weld cooling time such that dissolved hydrogen can diffuse out before it comes to room temperature. The laser was shown to significantly reduce the weld peak hardness from about 420 HV0.5 to values below 350 HV0.5, which will be beneficial for hyperbaric welding. The model prediction of the weld thermal cycles was in good agreement with the experimental results. Therefore, it could be used to predict the weld metal and HAZ cooling rate of a laser assisted GMAW (CMT) process although the model would need to be calibrated for higher pressure data. It was also demonstrated that additional laser heat can reduce the weld hydrogen content to acceptable limits of 5 ml/100 g of weld metal even for high moisture content in the welding environment. In conclusion, the addition of laser heating to GMAW will reduce the weld cooling rate, extend the weld pool cooling time, and expel diffusible weld hydrogen. All of these would be immensely beneficial in terms of improving the quality and reliability of structures fabricated through hyperbaric GMAW

FEM prediction of welding residual stresses in fibre laser welded AA 2024-T3 and comparison with experimental measurement

Welding generates a considerable amount of residual stresses which affect the structural integrity of welded components. It is often assumed that the magnitude of residual stresses around the welded joint is as high as the yield stress of the material. In this investigation, such assumption was found to be overly conservative and failed to accurately represent the distribution of residual stresses in fibre laser-welded aluminium alloy 2024-T3 sheets. Welding simulation based on the finite element method was used to reliably determine the distribution and magnitude of transient residual stress fields and distortions in thin sheets welded using three different sets of welding parameters. The accuracy of the finite element models was checked by calibrating with experimentally measured weld pool geometries and temperature field prior to conducting parametric studies. X-ray and neutron diffraction measurements were performed on the surface and in the bulk of the welded components, respectively, and compared with numerical results. The influence of weld metal softening, welding parameters and restraints on residual stresses and distortion were investigated systematically by numerically simulating ideal conditions which eliminate the practical limitations of conducting experimental studies, for process optimization as well as evaluation of in-service structure integrity and failure modes of the welded sheets