Browsing by Author "Coules, Harry E."
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Item Open Access Application of local mechanical tensioning and laser processing to refine microstructure and modify residual stress state of a multi-pass 304L austenitic steels welds(Elsevier, 2015-03-03) Sule, Jibrin; Ganguly, Supriyo; Coules, Harry E.; Pirling, ThiloAISI Type 304L austenitic stainless steels are extensively used in industries, and welding is an indispensable tool used for joining these materials. In a multi-pass weld, the development of residual stress to a large extent depends on the response of the weld metal, heat affected zone and parent material to complex thermo-mechanical cycles during welding. Earlier researchers on this area used either mechanical tensioning or heat treatment to modify the residual stress distribution in and around the weld. In this research, microstructural refinement with modification of residual stress state was attempted by using high pressure cold rolling followed by laser processing in 12 mm thick 304L austenitic stainless steels which is a novel technique. The hardening of the weld metal was evaluated after welding, post weld cold rolling, and post weld cold rolling followed by laser processing. The residual stress was determined non-destructively by using neutron diffraction. Residual stress analysis show that post weld cold rolling was effective in modifying the longitudinal residual stress distribution throughout the entire thickness. Post weld cold rolling followed by laser processing performed in this research was to induce recrystallization of the cold rolled grains. However, post weld cold rolling followed by laser processing showed minor grain refinement but was not effective as it reinstated the stress state.Item Open Access Characterising the effects of high-pressure rolling on residual stress in structural steel welds(Cranfield University, 2012-09-12) Coules, Harry E.; Colegrove, Paul A.The large residual stresses which occur in welded objects are an unavoidable consequence of the non-uniform cycle of thermal strain inherent in most welding processes. Furthermore, the particular distributions of residual stress which are characteristic of welding can adversely influence several material and structural failure mechanisms, including fatigue fracture, elastic fracture and buckling. This thesis describes an experimental investigation into the use of localised high-pressure rolling of the weld seam for the purpose of residual stress reduction in steel welds. In preliminary experiments, it is demonstrated that the transient stresses which occur in an object while part of it is welded or rolled, can be inferred from strain measurements taken during the process. Furthermore, such measurements can be used to estimate the resulting residual stresses. Good agreement is observed between residual stress distributions found using this method and determined using neutron diffraction. The effect of rolling on residual stress in structural steel welds is then investigated using both of these measurement techniques. Rolling is shown to greatly reduce tensile residual stress at the weld seam, even introducing compressive stress when a greater rolling force is used. However, this is only the case when rolling is applied post-weld: by contrast, methods involving rolling prior to or during welding do not improve the residual stress distribution. It is proposed, on the basis of transient stress measurements, that this is because the deformation which occurs in a weld during cooling greatly exceeds its yield strain, and so any effect of high-temperature deformation on residual stress is subsequently erased. Other effects of rolling on the properties of a weld have also been studied. Using mechanical tests and microstructural analysis it is shown that while post-weld rolling causes work-hardening of structural steel welds, rolling the weld at high temperature results in refinement of the weld microstructure, also hardening it. The effect of roller geometry on residual stress and fatigue life of rolled specimens has been investigated: the induced residual stress distribution is relatively insensitive to the roller’s crosssectional profile, while the fatigue life is shown to be reduced by post-weld rolling. The implications of these findings for the practical implementation of weld rolling, along with many other applied aspects of the process are discussed. While rolling is undoubtedly a useful and highly effective tool for residual stress mitigation in welds, its secondary effects should always be carefully considered.Item Open Access Comparative study of evolution of residual stress state by local mechanical tensioning and laser processing of ferritic and austenitic structural steel welds.(Scientific & Academic Publishing Co., 2015-02-21) Sule, Jibrin; Ganguly, Supriyo; Coules, Harry E.; Pirling, T.Complex thermal stresses generated in welded structures are undesirable but inevitable in fusion welding. The presence of residual stresses can be detrimental to the integrity of a welded joint. In this research, redistribution of residual stress magnitude and profile was studied and compared in two multi-pass welded structural alloys (API X100 and 304L stainless steel) after cold rolling and laser processing. The residual stress field was studied by neutron diffraction using the SALSA strain scanner at their reactor neutron source at ILL, Grenoble. In addition to a complex distribution of residual stress state, multi-pass welds also forms dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. Dendritic grain structure is weaker and segregation of alloying elements may result in formation of corrosion microcells as well as reduction in overall corrosion prevention due to depletion of alloying elements in certain areas. The modification of as-welded residual stress state was done by cold rolling which was followed by laser processing to create a recrystallized microstructure to minimise segregation. The main objective of this study is to understand the suitability of this novel manufacturing technique to create a stress free weldment with recrystallised grain structure. Hardness evolution in the welded structures was scanned following welding, post weld cold rolling and cold rolling followed by laser processing. Hardness distribution in both the structural alloys showed a significant evidence of plastic deformation near the cap pass of the weld metal. Residual stress redistribution was observed up to 4 mm from the capping pass for ferritic steel, while in austenitic steel weld, post weld cold rolling was effective in modifying the residual stress redistribution throughout the entire thickness. Laser processing in both cases reinstated the as-welded residual stress distribution and resulted in softening of the strained area.Item Open Access Effect of crack-like defects on the fracture behaviour of Wire + Arc additively manufactured nickel-base Alloy 718(Elsevier, 2020-09-11) Seow, Cui Er; Zhang, Jie; Coules, Harry E.; Wu, Guiyi; Jones, Christopher A.; Ding, Jialuo; Williams, Stewart W.The fabrication of large components using a high deposition rate, near-net shape process like Wire + Arc Additive Manufacturing (WAAM) is a promising option for many industries, due to the potential for reduction in material wastage and shorter lead times in comparison to conventional methods. Specialist materials like nickel-base superalloys, which are typically used in high temperature and corrosive environments, are particularly attractive options due to their high raw material costs. Although nickel-base Alloy 718 seems well suited to the process due to its good weldability, process-induced defects can arise from unfavourable deposition conditions and elimination of these defects may not always be possible. In WAAM Alloy 718 deposited under such conditions, crack-like defects with planar morphology and hot cracking characteristics were observed. These defects were observable using conventional non-destructive testing techniques and displayed directionality relating to the deposition path. The fracture behaviour of WAAM Alloy 718 containing these defects was “semi-stable” – a mixture of fracture instability and stable crack extension. The apparent fracture toughness of WAAM Alloy 718 containing these defects was found to be anisotropic, which can be attributed to the interaction of the notched crack with pre-existing defects. WAAM Alloy 718 displayed an apparent fracture toughness comparable to that of wrought Alloy 718 when notched perpendicular to the defects; but only half that of wrought when notched parallel to the defects. Therefore, careful consideration of defect orientation and their effects on mechanical properties is important in assessing the fitness-for-service of WAAM Alloy 718Item Open Access The effect of pre-weld rolling on distortion and residual stress in fusion welded steel plate(Transtec Publications; 1999, 2011-03-28T00:00:00Z) Coules, Harry E.; Cozzolino, Luis D.; Colegrove, Paul A.; Wen, S.Local rolling and other mechanical tensioning techniques can be highly effective at reducing residual stress and distortion in thin plate welds prone to buckling. However, the issues of high capital cost and low scalability currently prevent wider adoption of such processes. Pre-weld rolling aims to address these issues and can be applied easily to each component prior to fabrication. The results of an initial trial are presented, and indicate that post-weld distortion can be reduced by an average of 38% when correct rolling parameters are used. Finally, the mechanism by which prerolling acts to modify the state of residual stress around a weld line is discussed.Item Open Access Investigation of post-weld rolling methods to reduce residual stress and distortion(Elsevier, 2017-04-27) Cozzolino, Luis D.; Coules, Harry E.; Colegrove, Paul A.; Wen, ShuwenThe mechanisms of post-weld rolling and how it reduces and eliminates residual stress and distortion are poorly understood. Finite element analysis was applied to two different methods of rolling: rolling the weld bead directly with a single roller and rolling beside the weld bead with a dual flat roller. The models showed that both rolling techniques were able to induce compressive stress into the weld region, which increased with rolling load. The distribution of stress was sensitive to the coefficients of friction between the workpiece and the roller and the backing bar. High friction coefficients concentrated the plastic deformation and compressive stress within the centre of the weld bead. Distortion can be eliminated by rolling; however, the experiments indicated that this was only achieved when applied to the weld bead directly.Item Open Access Measurement and modelling of the transient thermal-mechanical strain field during GMA welding(2011-04-07T00:00:00Z) Coules, Harry E.; Cozzolino, Luis D.; Colegrove, Paul A.; Wen, S. W.; F, VollerstenThe state of residual stress in welded joints is a well-studied topic, and many methodsof residual stress measurement are now available. However, far less is known aboutthe transient strains which occur during the welding process, even though these are thedirect cause of residual stresses. Efforts to model welding processes usually includetransient stresses and strains among their results, but in general these models can onlybe easily compared against experimentally measured residual stresses: there is a lackof published data covering strain in the transient regime.In this study, electrical resistance strain gauges were used to measure transient strainsduring welding, and comparison is made between these measurements and the resultsfrom a sequential thermo-mechanical finite element model of the process. Well-definedmechanical boundary conditions were used for the experiments to ease interpretationof the measured strain data, and to enable close approximation with the boundaryconditions of the computational model. The transient biaxial state of strain wasmeasured during Gas Metal Arc Welding (GMAW) of S355 steel samples in a bead-onplateconfiguration. Measured transient strains were found to be consistent betweensamples, and showed good agreement with the modelling results. It is hoped thatfuture study of welding transient strains will aid in the optimisation of in-processmethods to reduce residual stress, such as rolling and quench cooling.Item Open Access Modelling distortion reduction on pre- and post-weld rolled gas metal arc welded plates(2011-04-07T00:00:00Z) Cozzolino, Luis D.; Coules, Harry E.; Colegrove, Paul A.; Wen, S. W.; F, VollerstenResidual stress generated during welding can cause distortion, especially when appliedon relatively thin plates. There are several standard and well documented techniquesto reduce distortion and residual stress after welding, however these techniques areoften costly and time consuming to implement on an industrial scale. In this paper wedemonstrate the pre- and post-weld rolling techniques for reducing weld distortion. Pre-Weld Rolling (PWR) can be applied before fabrication, potentially by the steelmanufacturer. An experimental investigation indicated that there was an average 38%reduction of distortion with this technique. Finite element analysis (FEA) was used todetermine the stress distribution to understand the phenomenon.Item Open Access Study of residual stress and microstructural evolution in as-deposited and inter-pass rolled wire plus arc additively manufactured Inconel 718 alloy after ageing treatment(Elsevier, 2020-10-14) Hönnige, Jan; Er Seow, Cui; Ganguly, Supriyo; Xua, Xiangfang; Cabeza, Sandra; Coules, Harry E.; Williams, StewartThe manufacture of structural components made from nickel-based super alloys would benefit from the commercial advantages of Wire + Arc Additive Manufacturing (WAAM), as it is commonly expensive to process using other conventional techniques. The two major challenges of WAAM are process residual stress and undesired microstructure. Residual stress causes part distortion and build failures, while the as-deposited microstructure does not allow the common heat-treatment to be effective in achieving the desired mechanical properties. This paper focuses on understanding the microstructural features, phase formation and three-dimensional residual stress state variation in as-deposited and inter-pass rolled conditions and after solutionising, quenching and ageing. The thermal history from successive deposition and cold working were correlated to the phase formation and macro residual stress formation and subsequent evolution. The {311} family of crystallographic planes were used as atomic strain gauge to determine the macrostrain and analysis of three dimensional stress state in different processing conditions. The measured strain were corrected for the compositional variation by measuring EDM machined d0 specimens manufactured under similar processing conditions. While the as-deposited part show significant stress redistribution and distortion after removal from the main fixture, inter-pass rolling was found to reduce part distortion significantly, the residual stress profile after inter-pass rolling showed highest tensile magnitude near the substrate while near the top of the deposit it was compressive as can be expected from the rolling process. The other two beneficial effects of inter-pass rolling on the microstructure are mitigation of the formation of undesired Laves-phase, thereby improving the response to solution treatment and aging together with significantly reduced grain size and texture. The application of inter-pass rolling reduces the potential part complexity, which however does not prevent the manufacture of common candidate parts, which are typically 1-to-1 replacements of forged, cast or machined from solidItem Open Access Three-dimensional mapping of the residual stress field in a locally rolled aluminium alloy specimen(Elsevier, 2017-03-08) Coules, Harry E.; Horne, G. C. M.; Kabra, S.; Colegrove, Paul A.; Smith, D. J.Detrimental residual stresses that occur in welded joints can be removed by rolling the weld seam. In this study we show that rolling could be applied to much thicker-section welds than has previously been attempted. A residual stress field introduced by localised rolling of an aluminium alloy specimen was studied to establish whether the plastic deformation caused by rolling would be sufficient to treat thick-section welds. It was modelled using finite element analysis and characterised using detailed neutron diffraction measurements. During rolling, plastic deformation occurs through the entire thickness of the specimen and strongly compressive residual stresses are created in the rolled area. Some features of the three-dimensional residual stress field, such as a region of tensile stress beyond the end of the rolled area, could be detrimental to structural integrity. It is recommended that these should be taken into account in the design of rolling-based weld treatment and surface treatment processes.Item Open Access Wire + Arc Additively Manufactured Inconel 718: Effect of post-deposition heat treatments on microstructure and tensile properties(Elsevier, 2019-08-24) Seow, Cui Er; Coules, Harry E.; Wu, Guiyi; Khan, Raja H.U.; Xu, Xiangfang; Williams, Stewart W.Wire + Arc Additive Manufacturing (WAAM) can be used to create large free-form components out of specialist materials such as nickel-base superalloys. Inconel (IN) 718 is well suited for the WAAM process due to its excellent weldability. However, during deposition, WAAM IN718 is susceptible to micro-segregation, leading to undesirable Laves phase formation in the interdendritic regions. Further, the WAAM process encourages columnar grain growth and the development of a strong fibre texture, leading to anisotropy in grain structure. This unfavourable microstructure can be addressed through specialised post-deposition homogenisation heat treatments. A new modified heat treatment was found to be effective in dissolving Laves phase, whereas a standard treatment precipitated δ phase. Tensile test results revealed that Laves and δ phases lead to low ductility when present in a precipitation-hardened matrix. The modified heat treatment also reduced the anisotropy in grain structure, leading to almost isotropic elevated temperature tensile properties, which meet minimum specifications for conventional cast but not for wrought material. Specialised post-deposition heat treatments, which address the unique microstructure of WAAM IN718, are crucial to achieving optimal mechanical properties.