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porosity formation in EBW.pdf

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  THE CHARACTERISATION AND MODELLINGOF POROSITY FORMATION IN ELECTRONBEAM WELDED TITANIUM ALLOYS ByJIANGLIN HUANG A dissertation submitted toThe University of Birminghamfor the degree of DOCTOR OF PHILOSOPHYSchool Metallurgy and MaterialsThe University of BirminghamSeptember 2011    University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation.  Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.  Abstract This thesis is concerned with the porosity formation mechanism during electronbeam welding of titanium-based alloys. During the welding of titanium alloys forstructural applications, porosity is occasionally found in the solidified welds. Hencethe key factors responsible for porosity formation need to be identified, and guidanceto minimise porosity occurrence needs to be provided.The aim of this project is twofold. First, porosity formed in electron beam weldedtitanium samples is characterised to rationalise the porosity formation mechanism.Second, models based on sound physical principles are built to aid understandingof porosity formation, and to provide predictive capability. Chapter 1 contains theintroduction and background of the project. A literature review is reported in chapter2 which covers the metallurgy of titanium and its alloys, the electron beam weldingprocess, and porosity formation in titanium welds.In chapter 3, electron beam welds of commercially pure titanium (CP-Ti), Ti-6Al-4V,Ti-6246, IMI 834 are characterised to rationalise the porosity formation mechanismby using metallographic sectioning, high resolution X-ray tomography, residual gasanalysis, scanning electron microscopy (SEM) and energy and wavelength dispersivespectroscopy (EDS/WDS) analysis. The results confirm porosity formed in electronbeam welded titanium-based alloys is associated with gas dynamics; hydrogen is verylikely to be responsible for porosity formation.In chapter 4, numerical models are developed to improve the understanding of theelectron beam welding process, including heat cycling, weld pool and keyhole for-mation, which are prerequisites for further investigation of the physical phenomenaoccurring, such as hydrogen behaviour and bubble formation and entrapment.In chapter 5, based on the numerical models for electron beam welding process, acoupled thermodynamic/kinetic model is proposed to study the hydrogen migrationbehaviour. The modelling results confirm hydrogen migrates from the cold regiontowards the hot region and thus causes hydrogen accumulation inside the weld pool.This model enables the prediction of hydrogen content inside the weld pool. Thei  comparison between the predicted hydrogen distribution and previous experimentaldata is shown to be reasonable.In chapter 6, a hydrogen driven bubble growth model is proposed to study thehydrogen effect on porosity formation, in which bubble is assumed to be initiateddue to the effect of asperities at the joint surfaces. This model is used to estimatethe hydrogen effect on stationary bubble growth in the melt, and thus to makepredictions of the hydrogen concentration barrier needed for pore formation. Theeffects of surface tension of liquid metal and the radius of pre-existing micro-bubblesize on the barrier are also investigated.In chapter 7, to study the effect of hydrogen on porosity formation and to confirmwhether hydrogen is the root cause for porosity formation, Ti-6Al-4V samples wereelectrochemically charged to achieve different hydrogen levels before welding. Theresults confirm that bubbles are nucleated at the melting front during the weldingprocess. With optimised electron beam parameters and perfect joint alignment,porosity can be suppressed even at a very high hydrogen levels; on the other hand,porosity is exacerbated when a small beam offset (BOF) is employed. This is becauseany BOF alters the size of the liquid zone at the melting front, where joint edgesare melted. Thus the thickness of the liquid film at the melting front is crucial forbubble nucleation and their survival in the weld pool. It would appear that thenucleation rate in the liquid zone at the melting front determines the likelihood of porosity occurrence. This suggests that BOF is likely to be one factor influencingporosity formation in these circumstances.Finally, in chapter 8, conclusions are drawn and suggestions for further work made.ii
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