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

Finite element analysis of localised rolling to reduce residual stress and distortion

Fusion welding processes cause residual stress due to the uneven heat distribution produced by the moving welding torch. These residual stresses are characterised by a large tensile component in the welding direction. Due to the self-equilibrated nature of the residual stress, compressive ones are present in the far field next to the weld seam, which can cause different kind of distortion such as bending or buckling. Welding residual stress can be responsible of premature failure of the components, such as stress crack corrosion, buckling, and reduction of fatigue life. Localised rolling is a stress engineering technique that can be used to reduce the residual stress and distortion caused by welding. It induces plastic strain in the rolling direction, counteracting the plastic strain produced during welding. In this thesis three techniques were investigated, pre-weld rolling, post-weld rolling, and in situ rolling. These techniques have been seldom studied in the past, particularly pre-weld rolling; consequently the mechanisms are poorly understood. Finite element models allow stress and strain development during both welding and rolling processes to be better understood, providing an improved understanding of the mechanisms involved and aiding process development. A literature survey was done to find the state of the art of the computational welding mechanics simulations, stress management, and the residual stress measurement techniques, as well as the knowledge gaps such as, the thermal losses through the backing-bar in the thermal simulation, the frictional interaction in the rolling process, and the material properties of the steel used in the models. In the literature not many models that investigate the management of welding residual stress were found. After this, the general considerations and assumptions for the welding thermal mechanical models presented in this thesis were discussed. The effect of different backing-bar conditions, as well as different material properties where investigated. Both influenced the residual stress profile to varying degrees. In particular, temperature dependent heat loss to the backing-bar was necessary to capture the improved heat loss near the weld. The distortion predicted by the model was investigated to determine whether it was due to bending or buckling phenomena. Lastly, the temperature distribution and residual stress predictions were validated against thermocouple and neutron diffraction measurements conducted by Coules et al. [1–3]. Pre-weld rolling was the first of the three rolling methods considered, in which rolling is applied to the plates before performing GMA butt-welds. The principle behind this technique consisted in inducing tensile residual stress in the weld region before welding; therefore, it is similar to mechanically tensioning the weld, which can significantly reduce the residual stress and distortion. However, there was no significant change in the tensile residual stresses. On the other hand, it was possible to achieve a small reduction in the distortion, when the plates were rolled on the opposite surface to the weld; rolling in this way induced distortion in the opposite direction to the distortion induced by welding, reducing the magnitude of the latter. These results were compared with experiments conducted by Coules et al. [1,4]. A subsequent investigation combined pre-weld rolling with post-weld heating. With this additional process the residual stress and distortion were significantly reduced, and flatter residual stress profile was achieved. The post-weld rolling and in situ rolling techniques were discussed afterwards. In the post-weld rolling models, rolling was applied after the weldment was cooled to room temperature. In in situ rolling the roller was applied on top of the weld bead at some distance behind the torch, while it was still hot. The principle behind these techniques consisted in applying positive plastic strain to the weld bead region by a roller, counteracting the negative plastic strains produced in the welding process. Two roller profiles were investigated, namely, grooved, and double flat rollers. The post-weld rolling on top of the weld bead models, which used the grooved roller, showed good agreement against experimental results, producing a large reduction of the residual stress and distortion. Some discrepancies were present when the weld toes were rolled with the dual flat roller. The former roller was more efficient for reducing residual stress and distortion. The influence of different friction coefficients (between the roller and weldment, and between the backing-bar and the weldment), were investigated. It showed significant dependency on the residual stress distribution when high rolling loads were used. The frictional interaction constrained the contact area inducing more compressive stress in the core of the weld bead; therefore it produced more tensile residual stress in the surface of the weldment. Additionally, the influence of rolling parameters on the through-thickness residual stress variation was investigated. Low loads only influence the residual stress near the surface, while high loads affected the material through the entire thickness. When the dual flat roller was used to roll next to the weld bead, significant compressive residual stress was induce in the weld bead; however, the residual stress reduction was very sensitive to the contact of the roller to the weld toes; therefore, when rolling a weld bead that varies in shape along the weld, the residual stress reduction is not uniform and varies along the length. On the other hand, the in situ rolling did not produced significant residual stress or distortion reduction in all the cases analysed. The rolling occurred when the material was still hot and the residual stress was subsequently formed as the material cooled to room temperature. Numerical modelling was a very useful tool for understanding the development of stress and plastic strain during the welding and rolling processes

Assessment of thermal cycles by combining thermo-fluid dynamics and heat conduction in keyhole mode welding processes

A numerical framework for simulation of the steady-state thermal behaviour in keyhole mode welding has been developed. It is based on the equivalent heat source concept and consists of two parts: computational thermo-fluid dynamics and heat conduction. The solution of the thermo-fluid dynamics problem by the finite element method for a bounded domain results in a weld pool interface geometry being the input data for a subsequent heat conduction problem solved for a workpiece by a proposed boundary element method. The main physical phenomena, such as keyhole shape, thermo-capillary and natural convection and temperature-dependent material properties are taken into consideration. The developed technique is applied to complete-penetration keyhole laser beam welding of a 15 mm thick low-alloyed steel plate at a welding speed of 33 mm s-1 and a laser power of 18 kW. The fluid flow of the molten metal has a strong influence on the weld pool geometry. The thermo-capillary convection is responsible for an increase of the weld pool size near the plate surfaces and a bulge formation near the plate middle plane. The numerical and experimental molten pools, cross-sectional weld dimensions and thermal cycles of the heat affected zone are in close agreement.DFG, 411393804, Experimentelle und numerische Untersuchung der Entstehungsmechanismen des Bulgings und dessen Einfluss auf die Bildung von Mittelrippendefekten beim Hochleistungslaserstrahlschweißen niedriglegierter Stähle hoher Blechdick

Study of fundamental parameters in hybrid laser welding

This thesis undertakes a study of laser welding in terms of basic laser material interaction parameters. This includes power density, interaction time and specific point energy. A detailed study of the correlation between the laser material interaction parameters and the observed weld bead profiles is carried out. The results show that the power density and the specific point energy control the depth of penetration, whilst the interaction time controls the weld width. These parameters uniquely characterise the response of the material to the imposed laser energy profile, which is independent of the laser system. It is demonstrated that by studying the laser welding with respect to the basic laser material interaction parameters also helps explain some phenomenological phenomena in laser welding, such as the effect of beam diameter on the weld profile. In addition a new approach for parameter selection in laser and hybrid laser welding is investigated. A phenomenological model allowing achievement of a particular laser weld on different laser systems is developed. In the proposed method the user specifies the required weld profile, according to the quality requirements and then the model provides combination of laser parameters, which lead to this particular weld on a given laser system. This approach can be potentially used to transfer laser data between different laser systems with different beam diameters. An extensive study of residual stains in laser and hybrid laser welding is carried out. Both processes are compared either at a constant total heat input or at conditions required to achieve the same depth of penetration. The results demonstrate that there is a trade-off between the fit-up tolerance and the residual stress-induced distortion. Hybrid laser welding provides better ability to bridge gaps than the laser welding, but for the price of increased residual stress and distortion. Additionally, industrial study of the sensitivity to fit-up of hybrid laser welding with high deposition rate MIG sources is carried out. This thesis is a part of NEGLAP (Next Generation Laser Processing) project sponsored by EPSRC (Engineering and Physical Sciences Research Council) and Tata Steel. The main objective is to understand the process fundamentals and exploit the usefulness of laser technology in pipe industry.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The influence of surface residual stress on fatigue crack growth.

Residual stress exists in most structures and although it has been recognised for a long time, its complex mechanism and characteristics are still being intensively studied. Residual stress can be stratified into damaging residual stress and beneficial residual stress. Surface tensile residual stresses are generally known to reduce the mechanical properties of materials while compressive residual stresses improve the fatigue performance of components. This Ph.D. thesis reports the analytical and experimental work conducted to investigate both the damaging and beneficial effects of residual stress on fatigue crack growth in structural components. The detrimental effect of tensile residual stresses is studied through large scale fatigue testing of T-butt welded plates fabricated from High Strength Steels (HSS). Despite the growing use of HSSs in the offshore industry, the fatigue performance and corrosion resistance of welded joints made from such steels are still not clear. Due to their complex metallurgy and relatively poor weldability, there is still a lack of understanding of the residual stresses that arise from the welding process. This study involved modification and development conducted on a variable amplitude (VA) load-time sequence generator for the investigation of long life fatigue performance of HSS. Emphasis was given to the generation of a stationary load-time history and numerous analyses were presented to demonstrate the importance of the long term statistical nature of the load-time sequence on fatigue testing. Fatigue test results obtained were extensively compared with previous HSS corrosion fatigue studies. The effect of tensile residual stress on fatigue crack growth was investigated using a SIF weight function-based fatigue crack growth model. Two newly developed preferential cold working techniques termed stitch cold rolling and stitch shot peening were explored to investigate the beneficial effect of surface compressive residual stress on fatigue crack growth in mild steel plates. One of the main objectives of this study was to control the fatigue crack shape by the manipulation of surface residual stress fields. The stitch cold rolling technique was implemented using a custom-built cold rolling jig. The feasibility of preferential cold working techniques was further investigated by the fatigue testing of stitch shot peened specimens. Both experimental programmes yielded unprecedented fatigue crack growth results. A residual stress monitoring programme was conducted to study the residual stress relaxation behaviour under cyclic loading. The experimental test results enabled the investigation of SIF solutions in non-uniform stress fields. A novel fatigue crack growth evolution model, which takes into account residual stress relaxation effects, was developed using the powerful SIF weight function methods

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

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens