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Thesis (M.Comp.Sc.(Hons.))--University of Wollongong, 2003. / Typescript. Includes bibliographical references: leaf 49-55.

Butt fusion welding process is an extensively used method of joining for high density polyethylene (HDPE) pipe. With the increasing number of HDPE resin and pipe manufacturers and the diversity of industries utilising HDPE pipes, a wide range of different standards have evolved to specify the butt fusion welding parameters with inspection and testing methods, to maintain quality and structural integrity of welds. There is a lack of understanding and cohesion in these standards for the selection of welding parameters; effectiveness, accuracy, and selection of the test methods and; correlation of the mechanical properties to the micro and macro joint structure. The common standards (WIS 4-32-08, DVS 2207-1, ASTM F2620, and ISO 21307) for butt fusion welding were used to derive the six welding procedures. A total of 48 welds were produced using 180 mm outer diameter SDR 11 HDPE pipe manufactured from BorSafe™ HE3490-LS black bimodal PE100 resin. Three short term coupon mechanical tests were conducted. The waisted tensile test was able to differentiate the quality of welds using the energy to break parameter. The tensile impact test due to specimen geometry caused the failure to occur in the parent material. The guided side bend specimen geometry proved to be too ductile to be able to cause failures. A statistical t-test was used to analyse the results of the short term mechanical tests. The circumferential positon of the test specimen had no impact on their performance. Finite element analysis (FEA) study was conducted for the long term whole pipe tensile creep rupture (WPTCR) test to find the minimum length of pipe required for testing based on pipe geometry parameters of outer diameter and SDR. Macrographs of the weld beads supplemented with heat treatment were used to derive several weld bead parameters. The FEA modelling of the weld bead parameters identified the length to be a key parameter and provided insight into the relationship between the geometry of the weld beads and the stresses in the weld region. The realistic bead geometry digitised using the macrographs contributed a 30% increase in pipe wall stress due to the stress concentration effect of the notches formed between the weld beads and the pipe wall. The circumferential position of the weld bead had no impact on the pipe wall stresses in a similar manner to the results of the different mechanical tests. IV Nanoindentation (NI) and differential scanning calorimetry (DSC) techniques were used to study the weld microstructure and variation of mechanical properties across the weld at the resolutions of 100 and 50 microns, respectively. NI revealed signature 'twin-peaks and a valley' distribution of hardness and elastic modulus across the weld. The degrees of crystallinity obtained from DSC followed the NI pattern as crystallinity positively correlates with the material properties. Both techniques confirm annealing of the heat affected zone (HAZ) material towards the MZ from the parent material. The transmission light microscopy (TLM) was used to provide dimensions of the melt zone (MZ) which displays an hour glass figure widening to the size of the weld bead root length towards the pipe surfaces. Thermal FEA modelling was validated using both NI and TLM data to predict the HAZ size. The HAZ-parent boundary temperature was calculated to be 105 ⁰C. The 1st contribution of the study is to prove the existence of a positive correlation between the heat input calculated from FEA and the energy to break values obtained from the waisted tensile test. The 2nd contribution providing the minimum length of pipe for WPTCR based on the pipe dimensions. The 3rd contribution is the recommendation for the waisted tensile test with the test using the geometry designed to minimise deformation of the loading pin holes. The 4th contribution related the weld bead parameters to pipe wall stresses and the effect of notches as stress concentrators. The 5th contribution is a new method of visualising a welding procedure that can be used to not only compare the welding procedures but also predict the size of the MZ and the HAZ. The 6th contribution of the study is the proposal of new weld bead geometry that consist of the MZ bounded by the HAZ, for butt fusion welded joints of HDPE pipes.

Le soudage est une méthode d’assemblage couramment utilisée dans l'industrie. Cependant, ce procédé a ses propres inconvénients, notons principalement la présence de porosités ou de contraintes résiduelles, qui peuvent affecter la tenue en service d’une structure. Notre étude se concentre sur un assemblage soudé d'un acier à haute limite d’élasticité. L’objectif de ce travail de thèse est de proposer une démarche mécano-fiabiliste qui permet de prédire la tenue d’une structure soudée en T en présence d'un défaut de type porosité. De ce fait, il est indispensable d’avoir un modèle numérique décrivant finement le comportement de ce type de structure. Une étude expérimentale est donc effectuée permettant d’identifier les caractéristiques des zones de soudure et leurs comportements mécaniques. Des méthodes de mesure (méthode des contours et diffraction de rayon X) permettent de déterminer le champ des contraintes résiduelles dans toute la surface de la structure soudée. Ces résultats expérimentaux ont permis d’enrichir un modèle numérique. Dans le code de simulation par éléments finis, le champ de contraintes résiduelles et la porosité dans le joint de la soudure sont pris en compte. Les simulations numériques réalisées sous ABAQUS avec des modèles élastiques ou élasto-plastiques nous ont permis d'identifier les zones de sécurité et de rupture dans un cordon de soudure ainsi que les configurations critiques. L'approche fiabiliste permet de déclarer un niveau de risque de la soudure avec un niveau de confiance donné compte-tenu des incertitudes des moyens d’inspection du défaut de porosité / Welding is a method of assembly commonly used in the industry. However, this method has several problems, mainly the presence of porosity or residual stress, which can affect the operating performance of a structure. Our study focuses on a welded joint of High-strength low-alloy (HSLA) steel.The objective of this thesis is to propose a mechanical reliability based approach in order to predict the reliability of a T-welded structure containing porosity. Therefore, it is essential to have a numerical model that describes finely the mechanical behavior of this type of structure. An experimental study is conducted to identify the characteristics of the areas of the weld and their mechanical behavior. Two measurement methods (method contours and X-ray diffraction) are used to determine the residual stress field throughout the surface of the welded structure.These experimental results allowed us to enrich a numerical model. In the finite element simulation, the residual stress field and the porosity in the weld are taken into account. Numerical simulations are performed using ABAQUS with elastic or elastic-plastic models to identify the areas of security and fracture in a weld bead and the critical configurations. The reliability approach is used to determine the failure probability of welded structure taking into account the uncertainties of the inspection means of porosity

Das Magnetpulsschweißen ermöglicht das stoffschlüssige Fügen verschiedenartiger Metalle, wobei die intermetallische Phasenbildung im Gegensatz zu herkömmlichen Schmelzschweißverfahren deutlich reduziert werden kann. Im Rahmen dieser Arbeit erfolgt die Entwicklung eines neuartigen optischen Messsystems, welches sich zum Bestimmen der axialen und radialen Kollisionsgeschwindigkeit eignet und die Berechnung des Kollisionswinkels ermöglicht. Es wertet das charakteristische Prozessleuchten aus, das bei der Fügepartnerkollision während dieses Pressschweißverfahrens entsteht. Experimente in Vakuumatmosphäre belegen, dass die Temperatur im Fügespalt bei kleinen Kollisionswinkeln deutlich über den Siedetemperaturen der beteiligten Werkstoffe liegen kann. Die aus dem Fügespalt strömende heiße Partikelwolke schmilzt die Fügepartneroberflächen an, bevor diese aufeinander treffen, sich stoffschlüssig verbinden und schließlich rasch abkühlen. Metallografische Analysen belegen die angeschmolzenen Bereiche in der Fügeverbindung und bilden den Ausgangspunkt für ein numerisches Modell, welches das Aufheiz- und Abkühlverhalten der Oberflächen abschätzt. Das patentierte Messsystem hilft außerdem bei der Prozesseinstellung und -überwachung mit möglichst geringer Impaktgeschwindigkeit, wobei der Einfluss verschiedener anlagenbedingter und geometrischer Faktoren untersucht wird. Der Wärmeeintrag in die Verbindungszone kann außerdem durch exotherm reagierende Zwischenschichten erhöht und dadurch die benötigte Impaktgeschwindigkeit reduziert werden. Die genannten Maßnahmen tragen dazu bei, die thermischen und mechanischen Belastungen auf die Werkzeugspulen zu reduzieren und damit ihre Lebensdauer zu erhöhen.:1 Einleitung 2 Stand der Kenntnisse beim Magnetpulsschweißen 2.1 Verfahrenseigenschaften und Anwendungsgebiete 2.2 Wirkprinzip und Einflussgrößen beim elektromagnetischen Umformen 2.3 Theorien zum Fügemechanismus beim Kollisionsschweißen 2.4 Erscheinungsbild und Eigenschaften der Verbindungszone 2.5 Messtechnisches Erfassen von Prozessparametern 2.6 Strategien für eine höhere Prozesseffizienz 2.7 Zwischenfazit zu Kapitel 2 3 Zielsetzung 4 Versuchsaufbau und Bewerten des Schweißvorgangs 4.1 Versuchsaufbau 4.2 Bewerten des Energieeinsatzes 4.3 Bewerten des Schweißergebnisses 4.4 Zwischenfazit zu Kapitel 4 5 Erfassen der kinetischen Kollisionsparameter 5.1 Entwickeln eines Messsystems zum Erfassen des Impaktblitzes 5.2 Numerisches Modell zum Bestimmen der Kollisionsparameter 5.3 Experimentelles Bestimmen der Impaktgeschwindigkeit 5.4 Experimentelles Bestimmen der Kollisionspunktgeschwindigkeit 5.5 Weitere Anwendungsmöglichkeiten der Blitzauswertung 5.6 Zwischenfazit zu Kapitel 5 6 Experimentelle Analyse der Partikelwolkeneigenschaften 6.1 Einfluss der Kollisionsbedingungen auf die Temperatur der Partikelwolke 6.2 Charakterisieren der Partikelwolke 6.3 Zwischenfazit zu Kapitel 6 7 Schweißmodell 7.1 Unterscheidung von Schweißmechanismen 7.2 Zwischenfazit zu den experimentellen Ergebnissen 7.3 Metallurgische Effekte 7.4 Aufbau des temperaturbasierten Schweißmodells 7.5 Einfluss der thermischen und kinetischen Prozessbedingungen 7.6 Zwischenfazit zu den numerischen Ergebnissen 7.7 Wellenbildung 7.8 Zwischenfazit zu Kapitel 7 8 Einstellen der kinetischen Kollisionsparameter 8.1 Frequenzeinfluss 8.2 Wandstärkeeinfluss 8.3 Fügespalt- und Wirklängeneinfluss 8.4 Prozessrobustheit bei geometrischen Abweichungen 8.5 Experimentelle Hinweise zum Ermitteln des Schweißfensters 8.6 Zwischenfazit zu Kapitel 8 9 Exotherm reagierende Zwischenschichten 10 Zusammenfassung / Magnetic Pulse Welding is a pressure welding process that enables material joints between dissimilar metals. Compared to conventional fusion welding processes, the intermetallic phase formation can be minimized to an uncritical minimum due to the reduced and localized heat input. Although the process is already applied in industrial production for hybrid parts, the underlying principle of the bond formation is not yet completely explored. One of the main reasons for this is the difficulty in process monitoring, which also hinders process adjustment or the targeted support of the joining mechanism. Both aspects are of great importance for an efficient welding process and increased life-times of the tool coils. In the present thesis, a new optical measurement system has been developed to get insights into the kinetic conditions during collision welding processes. It evaluates the characteristic flash that occurs during the high-speed collision of the joining partners above a certain impact velocity. Furthermore, the second velocity component of the collision front in axial direction can be measured, which enables the calculation of the collision angle. Experiments in vacuum atmosphere reveal for small collision angles, that the temperatures in the joining gap can exceed the vaporization temperatures of the involved materials. Since the ejected cloud of particles is very hot, the surfaces of the parts are melted before they are pressed together. Afterwards, the bond is formed and the joining zone is cooled down rapidly. Metallographic analysis evidenced melted regions in the joining zone, which serve as an input variable for the numerical model. This model predicts the heating and cooling behavior of the surfaces and shows for large collision angles, that the surfaces are already solidified before they come into contact. This fact inhibits the identified welding mechanism based on fusion. The patented measurement device helps studying the influence of certain machine-related and geometrical parameters during the process adjustment with low impact velocities and serves as a quality assurance system. Furthermore, exothermic reactive interlayers can increase the heat input in the joining zone and thus, decrease the minimum impact velocities. These strategies may contribute to a significant reduction of thermal and mechanical shock loading of the tool coils to increase their life-time.:1 Einleitung 2 Stand der Kenntnisse beim Magnetpulsschweißen 2.1 Verfahrenseigenschaften und Anwendungsgebiete 2.2 Wirkprinzip und Einflussgrößen beim elektromagnetischen Umformen 2.3 Theorien zum Fügemechanismus beim Kollisionsschweißen 2.4 Erscheinungsbild und Eigenschaften der Verbindungszone 2.5 Messtechnisches Erfassen von Prozessparametern 2.6 Strategien für eine höhere Prozesseffizienz 2.7 Zwischenfazit zu Kapitel 2 3 Zielsetzung 4 Versuchsaufbau und Bewerten des Schweißvorgangs 4.1 Versuchsaufbau 4.2 Bewerten des Energieeinsatzes 4.3 Bewerten des Schweißergebnisses 4.4 Zwischenfazit zu Kapitel 4 5 Erfassen der kinetischen Kollisionsparameter 5.1 Entwickeln eines Messsystems zum Erfassen des Impaktblitzes 5.2 Numerisches Modell zum Bestimmen der Kollisionsparameter 5.3 Experimentelles Bestimmen der Impaktgeschwindigkeit 5.4 Experimentelles Bestimmen der Kollisionspunktgeschwindigkeit 5.5 Weitere Anwendungsmöglichkeiten der Blitzauswertung 5.6 Zwischenfazit zu Kapitel 5 6 Experimentelle Analyse der Partikelwolkeneigenschaften 6.1 Einfluss der Kollisionsbedingungen auf die Temperatur der Partikelwolke 6.2 Charakterisieren der Partikelwolke 6.3 Zwischenfazit zu Kapitel 6 7 Schweißmodell 7.1 Unterscheidung von Schweißmechanismen 7.2 Zwischenfazit zu den experimentellen Ergebnissen 7.3 Metallurgische Effekte 7.4 Aufbau des temperaturbasierten Schweißmodells 7.5 Einfluss der thermischen und kinetischen Prozessbedingungen 7.6 Zwischenfazit zu den numerischen Ergebnissen 7.7 Wellenbildung 7.8 Zwischenfazit zu Kapitel 7 8 Einstellen der kinetischen Kollisionsparameter 8.1 Frequenzeinfluss 8.2 Wandstärkeeinfluss 8.3 Fügespalt- und Wirklängeneinfluss 8.4 Prozessrobustheit bei geometrischen Abweichungen 8.5 Experimentelle Hinweise zum Ermitteln des Schweißfensters 8.6 Zwischenfazit zu Kapitel 8 9 Exotherm reagierende Zwischenschichten 10 Zusammenfassung

Fusion welding of dissimilar metals constitutes a crucial processing stage in a variety of applications, and the use of high energy beams (HEB) like lasers and electron beams for such welding applications has several advantages, such as, precision, narrow heat affected zone, and consequently, low distortion. An understanding of microstructural evolution in the weld is a prerequisite for producing sound joints with desired properties. HEB welding of similar metals have been studied extensively. In contrast, fewer studies have been directed toward understanding the fundamental aspects of solidification of dissimilar welds. This thesis presents an effort in that direction by exploring microstructural evolution in Ti/Ni dissimilar welds. Welding of Ti/Ni serves to illustrate the fundamental differences that distinguish dissimilar welding from the welding of similar metals. These are: (i) Thermophysical properties of the base metals are, in general, different, and this can have important consequences in the heat transfer conditions. (ii) Composition can vary over an wide range, the extreme being for the case of a pure binary couple, and the solid–liquid interface cannot be defined by a single liquidus isotherm. (iii) In addition to the surface energy driven Marangoni convection, a strong solutal convection can arise due to a large difference in the density of the base metals. (iv) Nucleation of phases assumes greater importance, especially in systems with intermediate phases. We have carried out laser and electron beam welding (LW and EBW) experiments in a butt welding geometry to join Ti/Ni dissimilar couples. Weld microstructures were characterised using scanning and transmission electron microscopy (SEM and TEM); composition information was obtained from energy dispersive spectroscopy (EDS) of Xrays in the SEM. In addition to the pure binary couple, we have also studied electron beam welding of Ti/Ni with a thin Ta interlayer. We summarise our findings in each set of experiments in the following sections. Laser welding of Ti/Ni We have studied partial penetration welds obtained within the range of experimental parameters used in our study. These welds show the following interesting features: 1. The welds are asymmetric with respect to the initial joint. Despite its higher melting point, Ti melts more than Ni due to its lower thermal diffusivity, making the average composition of the weld richer in Ti (Ti–40at.%Ni). 2. Composition changes very steeply near the fusion interfaces in both Ti and Ni with associated microstructural changes. The variation is of much lesser magnitude in the rest of the weld, reflecting a well mixed melt pool on a macroscopic scale. 3. Growth of base metal grains into the weld pool at the fusion interfaces is severely restricted at both Ti and Ni ends. 4. The Ti fusion interface is marked by a band consisting of Ti2Ni dendrites which grow toward the Ti base metal. 5. Layered structures form at the Ni fusion interface. The sequence of the layers is: solid solution (Ni)→ Ni3Ti→ Ni3Ti+NiTi eutectic → NiTi. We note the absence of the (Ni)+Ni3Ti eutectic in this sequence. 6. NiTi and Ti2Ni are the major phases that appear in the bulk of the weld. Volume fraction and morphology of NiTi vary almost periodically to form microstructural bands. 7. Solid state transformation of NiTi results in the formation of the Rphase and martensite, which reflect the composition heterogeneity in the weld. Sometimes, Ni4Ti3 precipitates are observed also, providing indirect evidence of nonequilibrium solidification. 8. Nitrogen pickup from the atmosphere during welding leads to the formation titanium nitride dendrites in the weld. 9. Solutal convection and buoyancy forces manifest themselves through the segregation of the lighter nitride and Ti2Ni phases toward the top surface of the weld; the heavier liquid forms blocky NiTi in the bottom half of the weld. These observations stand in striking contrast with the microstructures of conventional welds. We have proposed a set of composition and temperature profiles in the weld which reflect the diffusive and advective transport processes; when combined with thermodynamic information from the Ti–Ni phase diagram to yield spatial liquidus temperature profiles, these profiles can adequately explain most of the results. Our observations illustrate the importance of (a) nucleation, and (b) the inhomogeneous nature of the melt in which growth takes place. They also highlight the role of convective currents in bringing about local fluctuations in composition and temperature leading to ‘low velocity bands’. Electron beam welding of Ti/Ni We have carried out full penetration EBW of thin plates of Ti and Ni. The major observations are: (i) Average composition of the weld is in the Ni–rich side of the phase diagram (Ni–40at.%Ti). (ii) Fusion interface microstructures are very similar to that in LW exhibiting restricted base metal growth (although little amount of epitaxy can be seen in the Ni side), growth of Ti2Ni dendrites toward the base metal at the Ti fusion interface and the sequence of layers at the Ni interface: (Ni)→ Ni3Ti→ Ni3Ti+NiTi. Unlike LW, however, Ni3Ti, instead of NiTi, reappeared after the third layer on the Ni side. (iii) General microstructure consists of the Ni3Ti+NiTi eutectic, which appears in several anomalous as well as regular morphologies. (iv) Formation of NiTi is restricted mostly to regions near the Ti fusion interface. (v) Segregation of Ni3Ti was observed in a few places. The most prominent change in the microstructure compared to LW is a shift from the Ti2Ni– NiTi phases in the bulk of the weld to a Ni3Ti+NiTi eutectic structure. This is a direct consequence of the shift in the average composition of the weld to the Ni– rich side. The occurrence of different anomalous and regular eutectic structures bear similarity with bulk undercooling experiments conducted on eutectic systems having a strongly faceting phase as one of its constituents. The asymmetric coupled zone, along with composition and temperature fluctuation due to fluid flow, can be attributed to the origin of these structures. Electron beam welding of Ti/Ni with a Ta interlayer Motivated by the report of superior mechanical properties of Ti/Ni welds with an interlayer of Ta, whose melting point is much higher than those Ni and Ti, we performed EBW experiments using a Ni–Ta– Ti configuration. The key observations are: (i) The process is inherently unsteady in nature, and results in partial and irregular melting of the Ta interlayer. This partial melting essentially divides the weld into Ni–rich and Ti–rich halves. (ii) Microstructure near the fusion interface in Ni and Ti show similarities with that of the pure binary Ti/Ni welds; the phases here, however, contain Ta as a ternary addition. (iii) Microstructure in the Ti–rich half consists of dendrites of the Ni(Ti,Ta) phase with a high Ti:Ta ratio, and an eutectic formed between this phase and a (Ti,Ta)2Ni phase having significant amount of Ta. Two Ni(Ti,Ta) type phases dominate the microstructure in the Ni–rich half: the phase having a higher Ti:Ta ratio forms cells and dendrites, whereas the one of a lower Ti:Ta ratio creates an interdendritic network. (iv) Regions near the unmolten Ta layer in the middle show the formation of a sawtoothlike Ta–rich faceted phase of composition (Ta,Ti)3Ni2. Since very scarce thermodynamic data exist for the Ni–Ta–Ti ternary system, we have taken cues from the binary phase diagrams to understand the microstructural evolution. Such extrapolation, although successful to some extent, fails where phases which have no binary equivalents start to appear. In summary, in this thesis, we explore microstructural evolution in the Ti/Ni dissimilar welds under the different settings of laser and electron beam welding processes. This study reveals a variety of phenomena occurring during dissimilar welding which lead to the formation of an extensive range of microstructural features. Although a few questions do remain, most results can be rationalised by drawing from, and extending the knowledge gained from previous studies by introducing physical and thermodynamic arguments.

The area of research dealing with joining of dissimilar metals has been active in recent time. Although fusion and non-fusion techniques of joining have been effectively used for manufacturing components, a comprehensive scientific understanding of the process is lacking. This void exists both in fusion and non-fusion welding methods. The present investigation addresses some of these aspects. The investigation consists of two sections - Part A and Part B. Part A is on Friction welding and Part B deals with Fusion welding using laser. Each section has two chapters each. Following an introductory chapter, basic aspects of friction welding is presented in chapter 2. Chapter 3 deals with the work on friction welding of Fe-Cu couple. Fe-Cu couple is a system with positive heat of mixing. After a brief introduction on various non-equilibrium processes that can occur in this system, experimental details and results are presented. Using the results an attempt is made to understand the flash formation, formation of pores at the interface and the formation of chemically altered zone. It is observed that a chemically altered layer forms predominantly on the Cu side of the interface. It consists of Fe entrapped as fragments/fine crystals and as solid solution in Cu matrix. This zone has higher thickness at the edges than at the center. The mechanism of formation of this interfacial layer which is central to the joining process is related to the fracture and transport of fragments during plastic deformation. Fe forms solid solution in copper under non-equilibrium conditions promoted by shear energy. Using the concept of ballistic mixing, the formation of solid solution is explored. Using nano-indentation experiments mechanical properties of the weldment is estimated and an attempt is made to correlate mechanical properties with the amount of second element present in that location. The chapter 4 in part A deals with the friction welding of Ni-Ti couple. Ni-Ti system has negative heat of mixing and it forms a number of intermetallics. After a brief introduction to the chapter, various experimental techniques and strategies followed to carry out the experiments are explained. Following these, the results are presented. It is observed that TiNi3 formed at initial stage. Theories based on effective heat of formation and surface energy also predict the nucleation of TiNi3. With the continuation of frictional processes, the formation of TiNi and Ti2Ni phases were also observed. Formation of Ti2Ni was shown to greatly accelerate due to shear process. In this system two complementary processes like ballistic mixing and thermal assisted diffusion accelerate Ti2Ni formation. From mechanical tests it is found that Ti2Ni layer in the weldment is weak and hence formation of Ti2Ni in the weldment is detrimental. In chapter 5 an introduction to fusion welding of dissimilar metals is presented as background materials for the subsequent chapters. Chapter 6 deals with nature of segregation of Ag during laser welding of Fe-Ni couple. Ag is used as a tracer to probe fluid flow in the Fe-Ni couple during laser welding. Ag is immiscible both in Fe and Ni whereas Fe and Ni form a complete solution at an elevated temperature and in liquid state. Besides the experimental work, numerical simulation of the weld pool were carried out using homogeneous mixture model using SIMPLER algorithm. Experiments and simulations indicate that fluid flow is asymmetrical and in the deep penetration welding strong convection in the pool drives the tracer to the top of the pool. Overall distribution of the tracer is due to the combined effect of convection and diffusion. In shallow welding there exists a boundary region where tracer does not penetrate. In chapter 7 the results of instrumented indentation experiments on laser welded Fe-Cu weldment has been presented. It was earlier reported that during laser welding of Fe-Cu couple, a variety of microstructures evolves at various locations in the weldment and hardness of the weldment were found to be very high. Here an attempt has been made to explore in details the origin of such a high hardness. The chapter starts with a description of various microstructures that are observed in this weldment followed by the various procedures used for extracting data from instrumented indentation tests. It is followed by the presentation of the experimental results. It is found that rule of mixture along with Hall-Petch strengthening explains the observed increase in hardness of the weldment. The fine scale microstructure consisting of alternate Fe rich and Cu rich layers increases the hardness of the weldment. On copper side of the weldment, composition and scale of microstructure fluctuates and so also the hardness. Finally in chapter 8 overall conclusions of the various chapters in the thesis have been summarised.

This diploma thesis is resolving all the possibilities of enhancing the efficiency of furnace structures welding. Small batch production does not offer many possibilities for implementing mechanization or automation. On the other hand, when using a big batch production, efficiency can be enhanced by means of a robotic workstation. When the production batch is big enough, we will see a costs save after a short time, mainly in labor costs. This means the return of investments will be in short time period. In the LAC company there are three types of products made. At laboratory furnaces the efficiency can be enhance by using fixtures. At other standard and atypical furnaces is very difficult to design fixtures or positioners because of their different sizes. Big complication can be also a company location on second floor. The most suitable product regarding to welding efficiency enhancement is big batch production of heaters. For this type the welding time can be shortened by means of a robotic workstation.