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.

A number of recent research studies have investigated the performance of panel zones in seismic-resistant steel Special Moment Resisting Frames (SMF). These recent studies investigated various options for attaching doubler plates to the column at beam-column joints in SMF for purpose of increasing the shear strength of the panel zone. This previous work was primarily focused on doubler plates that extend beyond the top and bottom of the attached beams, and considered cases both with and without continuity plates. As an extension to this previous research, this thesis explores the situation when a doubler plate is fitted between the continuity plates. The objective of this research was to evaluate various options for welding fitted doubler plates to the column and continuity plates through the use of finite element analysis, and to provide recommendations for design. The development and validation of the finite element model are described, along with the results of an extensive series of parametric studies on various panel zone configurations and attachment details for fitted doubler plates. Based on the results of these analyses, recommendations are provided for design of welds used for attaching fitted doubler plates in the panel zone of SMF systems. / text

Les liaisons bimétalliques en acier inoxydable (LBM inox) permettent, au sein des réacteurs nucléaires français actuels, de connecter les gros composants en acier ferritique faiblement allié (cuve, pressuriseur, générateur de vapeur) à la tuyauterie du circuit primaire en acier austénitique inoxydable. De par leurs microstructure et propriétés mécaniques hétérogènes, ces liaisons sont des zones dites "sensibles" pour l'intégrité des structures et il est donc indispensable de caractériser leur tenue mécanique dans les situations de fonctionnement nominal et accidentelles. Ce travail de thèse a pour objectif d'évaluer le risque d'amorçage fragile de la LBM inox dans le haut de la transition fragile-ductile à l'aide d'un critère adapté. Les microstructures au voisinage de l'interface entre l'acier ferritique et le beurrage austénitique ont tout d’abord été caractérisées, et un liseré martensitique d’épaisseur variable ainsi qu’une couche entièrement austénitique ont été observés. Ces deux couches, qui sont le siège d’une intense précipitation de carbures pendant le traitement thermique de détensionnement, forment ensemble une couche dure de martensite et d’austénite carburées potentiellement fragile. Le comportement mécanique de l’ensemble de la LBM inox a ensuite été étudié à 20°C et à -175°C, et des lois de comportement élasto-plastiques isotropes ont été identifiées pour les différentes couches macroscopiques à partir d’essais de traction sur des éprouvettes multi-matériaux travers-joint à diamètre variable. Le comportement mécanique de la couche dure a, quant à lui, été caractérisé à partir d’essais in-situ sur des micro-éprouvettes usinées au FIB et testées à l’aide d’une micro-machine de traction développée dans cette thèse. Une étude des mécanismes de rupture de la LBM inox dans le domaine de la transition fragile-ductile a par ailleurs été réalisée à partir d’essais sur éprouvettes CT et a mis en évidence une fragilité de l’interface MA (entre martensite et austénite) liée à un mécanisme de rupture intergranulaire amorcée sur les carbures et systématiquement activé pour des fronts de préfissure traversant la couche dure. Une modélisation par éléments finis des essais a permis d’analyser les champs de contrainte sur l’interface MA et d’identifier un modèle de Weibull linéique à 3 paramètres basé sur une contrainte seuil et une distance seuil pour les éprouvettes CT. Finalement, l’effet du vieillissement thermique sur les LBM inox a été étudié à partir d’un traitement thermique de 10 000h à 400°C et un durcissement des couches austénitiques résultant d’un mécanisme de décomposition spinodale de la ferrite résiduelle a été mis en évidence à partir d’essais de traction. L’analyse des mécanismes de rupture à l’état vieilli a également montré que ce durcissement provoque une augmentation d’environ 30°C de la température de transition associée à la rupture intergranulaire de l’interface MA. / Stainless steel dissimilar metal welds (SS DMW) are widely used within the French nuclear power plants where they connect the main components (pressure vessel, pressurisor, steam generator) made of low-alloy ferritic steel to the primary circuit pipes made of austenitic stainless steel. Because of their heterogeneous microstructure and mechanical properties, these junctions are critical components for the structure integrity and their fracture resistance has to be demonstrated for all the nominal or accidental operating conditions. This PhD work aims at building a model to evaluate the risk of brittle fracture of the SS DMW in the upper shelf of the brittle-to-ductile transition range. The observation of the microstructures around the fusion line revealed a martensitic layer and a fully austenitic zone, which undergo an important carbides precipitation during the post-weld heat treatment and form a narrow hard layer of carburized martensite and austenite. The mechanical behavior of the SS DMW was then characterized at 20°C and -175°C and isotropic elastoplastic constitutive laws were determined for each macro/mesoscopic layer of the weld from tensile tests on crossweld specimens with variable diameters. The mechanical behavior of the narrow hard layer was also studied with micro tensile tests on specimens extracted by FIB micro processing and tested using an in-situ tensile testing device developed during the PhD. Furthermore, fracture toughness tests were carried out on CT specimens in the brittle-to-ductile temperature range and helped identify the MA interface (between martensite and austenite) as the weakest region in the SS DMW because of an intergranular fracture mechanism initiated at the carbides-rich interface. This mechanism was consistently observed for specimens with fatigue precrack fronts in the hard layer. The stress distributions on the MA interface calculated from the FE numerical simulation of these tests were then analysed and a 1D 3 parameters Weibull model based on a threshold stress and a threshold length was identified for the CT specimens. Finally, the effect of thermal ageing on the SS DMW was explored with a thermal ageing treatment of 10000h at 400°C and a hardening of the austenitic layers was measured by tensile tests and was associated to a spinodal decomposition mechanism of the residual ferrite. The fracture mechanisms of the SS DMW were also analysed in the aged state and showed that this hardening caused an increase of the transition temperature associated with the intergranular fracture of the MA interface by about 30°C.

No description available.

Detta arbete har genomförts vid Volvo Construction Equipment i Braås. I nuläget används konventionell spraybågssvetsning med metallpulverfylld rörtråd i produktionen. Ett alternativ som övervägs är att byta till solidtråd. Huvudsyftet med detta arbete har varit att utreda huruvida några utvalda processer med 1.2 mm solidtråd är kapabla att producera ensträngs kälsvetsar som uppfyller av Volvo CE ställda krav: - Minst 3 mm i-mått. - Teoretiskt a-mått mellan 4 och 5.5 mm. - Svetsklass VD enligt Volvo standard STD 181-0004. - Kompatibel med fogföljning via ljusbåge. - Ingen spalt mellan plåtarna tillåten. Arbetet inleddes med en litteraturstudie för att framförallt beskriva svetsparametrars inverkan på svetsens inträngningsprofil och geometri.  Ett stort antal svetsförsök utfördes med konventionell spraybåge samt de vågformsstyrda processerna Fronius PMC och Lincoln Electrics RapidArc. Försöken genomfördes i PA och i PB läge. I PB läget orienterades förbandet dels med gränsytan mellan plåtarna horisontellt och dels vertikalt. Flertalet av försöken genomfördes med svetstraktor. Kompletterande försök genomfördes med svetsrobot. Inträngning och kvalité bedömdes framförallt genom makroprov och syning, men också genom lasermätning. Vid inledande faktorförsök med RapidArc, PMC och konventionell spraybågsprocess användes trådmatningshastighet 13 och 15 m/min i kombination med svetshastighet 55 och 65 cm/min. Försöken visade att i PB läge med horisontell gränsyta kan 3 mm i-mått uppnås med trådmatningshastigheten 13 m/min och svetshastigheten 55 cm/min. Under avslutande svetsförsök med PMC i PB läge med gränsytan vertikalt vid 60 cm/min svetshastighet, pendling och 14.5-15.5 m/min trådmatningshastighet erhölls i-mått avsevärt överstigande 3 mm. Vid svetsförsök i PB läge med solidtråd uppnåddes kraven på 3 mm i-mått och mjuka fattningskanter med ökad svetshastighet jämfört med ett typiskt produktionsfall. I PA läge erhölls mindre skillnad gentemot ett produktionsfall. Genom ytterligare optimering av svetsprocesserna är det dock inte orimligt att svetshastigheterna kan ökas utan försämrade i-mått och fattningskanter. Möjlig kostnadsreducering för tillsatsmaterialet vid övergång till solidtråd är 30-50% beroende på att solidtråden är billigare än metallpulverfylld rörtråd. Ytterligare kostnadsreducering kan erhållas genom förbrukning av mindre mängd tillsatsmaterial. Vid genomförde svetsförsök har mindre mängd tillsatsmaterial tillförts jämfört med typiska produktionsfall.  Både den konventionella spraybågsprocessen och de vågformsstyrda processerna är kapabla att producera de eftersträvade svetsarna med solidtråd. Här måste dock ett ”varnade finger” höjas; vid svetsningen krävs god kontroll av elektrodvinklarna och elektrodens position relativt fogens rot, annars erhålls betydligt sämre resultat. / This thesis has been performed at Volvo Construction Equipment in Braås. Currently, conventional MAG spray transfer with metal cored wire is used in the production. The company is now considering switching from metal cored wire to solid wire. The main purpose of this thesis has been to investigate whether a few selected processes used with 1.2 mm solid wire are capable of producing single pass fillet welds that conform to the following requirements set by Volvo CE: - Minimum i-dimension 3 mm. - Theoretical throat thickness between 4 and 5.5 mm. - Weld class VD according to Volvo standard STD 181-0004. - Compatible with Through Arc Seam Tracking. - No gap between the plates is allowed. A literature study was performed primarily in order to describe the influence of weld parameters on the weld and penetration profile. A large number of weld trials were performed with conventional spray transfer and the two waveform controlled processes Fronius PMC and Lincoln Electrics RapidArc. The weld trials were performed in the weld positions PA and PB. For PB the joint root was orientated both horizontally and vertically. For the majority of trials a weld tractor was used. Complementary trials were performed by a robot. Weld testing consisted primarily of macro etch testing and visual inspection but also laser scanning. The initial trials with RapidArc, PMC and conventional spray transfer were performed with the wire feed speeds 13 and 15 m/min in combination with the travel speeds 55 and 65 cm/min. The trials showed that when using weld position PB with a horizontal joint root, 3 mm i-dimension can be achieved by using wire feed speed 13 m/min and travel speed 55 cm/min. Final trials with PMC were performed in weld position PB with a vertical joint root using wire feed speeds 14.5-15.5 m/min, a travel speed of 60 cm/min and weaving resulting in i-dimensions much greater than 3 mm being achieved. Using solid wire in weld position PB, welds with 3 mm i-dimension and good wetting of the toes were produced with increased travel speed compared to a typical production case. However, in weld position PA less difference was obtained compared to a typical production case. By further optimizing the weld processes it is not unlikely that the travel speeds can be increased while maintaining the i-dimension and good wetting of the toes. A possible cost reduction for the filler material is 30-50% because solid wire is cheaper than metal cored wire. Using less filler material will result in additional cost reductions. During performed weld trials less filler material was used compared to current production scenarios. Both the conventional spray transfer process and the waveform controlled processes are capable of producing the desired welds with solid wire. However, a cautionary tale must be told: when welding, good control of the electrode angles and position relative to the joint root is needed otherwise much worse results will be obtained.

Rail is a very expensive component of the railway track. Therefore, research methods extending rail life have great economic importance. During the past thirty years and, particularly during the past ten years there has been an increasing awareness throughout most rail networks in the world of the need to introduce improved design criteria, better construction techniques and higher standard track generally. This implies that quality control at all levels is mandatory if these objectives are to be achieved. With the improved understanding of degradation of track, a more complete comprehension of the costs associated with different operating and infrastructure conditions should also be developed, aiding in the determination of efficient maintenance costs and their contribution to access charges. Track and structures together account for 60% of maintenance costs, with 50% of the total being track. The UIC has done a lot of work on comparative performance indicators, and these show what potential savings much be out there for the taking, just by adopting current best practice. The old wisdom is that it's not enough o do things rights; we have to make sure that we do the right things. These developments have largely resulted from the demand for higher speeds particularly in passenger services and the demand to accept heavier axle loads of freight traffic. Whilst the conventional railway track structure is not likely to change significantly over the next ten years there will be a requirement over that period for better quality track infrastructure. This means less rail surface defects, less internal defects and less wheels irregularities. The presence of rail surface defects generally increases the roughness of the track leading to a poor passenger ride and increased safety risk with freight traffic. In addition, rail surface defects will generally increase the degradation rate of other track components; however, not all defects will produce visible track deterioration. Dynamic impacts produced by the rollingstock running over rail surface defects, such as poor welds, will, over time, create continuous rail defects, loosening of fastenings, abrasion and skewing of sleepers, crushing of ballast and loss of formation geometry. It is only in the recent years that the importance of poor welds in track has been identified. Dips and peaks must be recognised as a severe track irregularity that needs to be addressed and removed. Current maintenance activities have little effect on removing misaligned welds in track and the improvement obtained after the maintenance works is generally short lived. On the other hand, straightening operations have proven to solve the problem and maintain the results following 7 months of traffic. As part of this project, a six kilometre test section was selected on the Mt Isa Line and all welds located in this region were monitored for over 9 months to increase the understanding of the effect of individual maintenance activities on the track roughness. Three 2km Divisions were established; each Division had different maintenance activities and levels of intervention completed over the duration of the project. Over 15,000 readings were recorded and analysed. The following conclusions were drawn. The effect of cycle tamping was clearly identified when comparing the means of weld located in Division 1, 2 to the mean of welds in Division 3. Cycle tamping showed to have a significant positive effect on the dipped welds geometry and an increase in severity of peaked welds prior to their correction. Straightening operations completed in Division 1 and 2 reduced the overall mean of weld misalignments. These Divisions were subjected to different levels of straightening intervention however they produced similar results. Division 1 all dips were straightened and Division 2 only dips &gt0.3mm were straightened. This means that no additional benefit, in terms of overall misalignment of welds, can be gained when straightening operations target dips with a misalignment smaller than 0.3mm. Cycle grinding proved to have little effect on the removal of both dips and peaks. In fact, due to the configuration of the grinding machine, grinding operation produced a slight worsening of the dips misalignments and only a minor improvement of peaks. Although long term monitoring of the site may show minor variations in weld geometry performance, after approximately 3.9 Mgt of traffic the mean of dipped welds in Division 1 and 2 appeared to remain unaltered, as Division 3 showed a minor worsening. Furthermore, the mean of peaked welds in Division 1 and 2 appeared to remain unaltered, as Division 3 showed a minor worsening.