Neste trabalho foi realizado um estudo comparativo entre juntas dissimilares produzidas pelo processo FSpW sem e com a utilização de Zn como metal intermediário tendo a finalidade de investigar a eficácia da adição deste metal no comportamento mecânico das juntas, bem como avaliar sua influência nas características microestruturais das juntas soldadas. As juntas foram produzidas com ligas de Al AA5754-H24 e Mg AZ31-O, de particular interesse do setor automotivo. Nas condições de soldagem com metal intermediário, chapas de Al revestido com Zn por galvanização foram usadas. As propriedades mecânicas das juntas foram avaliadas através de ensaios com carregamento quasi-estáticos e dinâmicos, sendo então correlacionados com as características microestruturais. Através das análises constatou-se que a aplicação de Zn como metal intermediário mostrou-se efetiva na redução do volume de compostos intermetálicos formados na interface da solda e consequentemente, obteve-se um aumento do desempenho mecânico de cerca de 48 % no ensaio de cisalhamento por tração, 230% no ensaio de tração cruzada e de 35,31 % no ensaio de fadiga quando em comparação a juntas soldadas diretamente entre as chapas de AA5754-H24 e AZ31-O. / In this research was carried out a comparative study between dissimilar joints produced by means of the process FSpW without and with applying Zn as an intermediary metal with the purpose of investigating the effectiveness of the addition of this metal in the mechanical behavior of joints, as well as to assess its influence on the microstructural characteristics of the welded joints. The joints were produced with the AA5754-H24 Al alloy and the AZ31-O Mg alloy, materials of particular interest to the automotive industry. For the welding conditions with the Zn interlayer Al sheets coated by hot deep galvanizing were used. The mechanical properties of the welded joints were evaluated in terms of quasi-static and dynamic load and then correlated with the microstructural features. Through the analyzes it has been verified that the addition of Zn as an intermediary metal proved to be effective in reducing the amount of the intermetallic compound formed at the weld interface and consequently the mechanical performance increased about 48 % in the lap shear strength test, 230 % in the cross tensile strength test and 35,31 % in the fatigue test when compared to the welded joints produced by the direct contact among the AA5754-H24 and the AZ31-O sheets.

Neste trabalho foi realizado um estudo comparativo entre juntas dissimilares produzidas pelo processo FSpW sem e com a utilização de Zn como metal intermediário tendo a finalidade de investigar a eficácia da adição deste metal no comportamento mecânico das juntas, bem como avaliar sua influência nas características microestruturais das juntas soldadas. As juntas foram produzidas com ligas de Al AA5754-H24 e Mg AZ31-O, de particular interesse do setor automotivo. Nas condições de soldagem com metal intermediário, chapas de Al revestido com Zn por galvanização foram usadas. As propriedades mecânicas das juntas foram avaliadas através de ensaios com carregamento quasi-estáticos e dinâmicos, sendo então correlacionados com as características microestruturais. Através das análises constatou-se que a aplicação de Zn como metal intermediário mostrou-se efetiva na redução do volume de compostos intermetálicos formados na interface da solda e consequentemente, obteve-se um aumento do desempenho mecânico de cerca de 48 % no ensaio de cisalhamento por tração, 230% no ensaio de tração cruzada e de 35,31 % no ensaio de fadiga quando em comparação a juntas soldadas diretamente entre as chapas de AA5754-H24 e AZ31-O. / In this research was carried out a comparative study between dissimilar joints produced by means of the process FSpW without and with applying Zn as an intermediary metal with the purpose of investigating the effectiveness of the addition of this metal in the mechanical behavior of joints, as well as to assess its influence on the microstructural characteristics of the welded joints. The joints were produced with the AA5754-H24 Al alloy and the AZ31-O Mg alloy, materials of particular interest to the automotive industry. For the welding conditions with the Zn interlayer Al sheets coated by hot deep galvanizing were used. The mechanical properties of the welded joints were evaluated in terms of quasi-static and dynamic load and then correlated with the microstructural features. Through the analyzes it has been verified that the addition of Zn as an intermediary metal proved to be effective in reducing the amount of the intermetallic compound formed at the weld interface and consequently the mechanical performance increased about 48 % in the lap shear strength test, 230 % in the cross tensile strength test and 35,31 % in the fatigue test when compared to the welded joints produced by the direct contact among the AA5754-H24 and the AZ31-O sheets.

Neste trabalho foi realizado um estudo comparativo entre juntas dissimilares produzidas pelo processo FSpW sem e com a utilização de Zn como metal intermediário tendo a finalidade de investigar a eficácia da adição deste metal no comportamento mecânico das juntas, bem como avaliar sua influência nas características microestruturais das juntas soldadas. As juntas foram produzidas com ligas de Al AA5754-H24 e Mg AZ31-O, de particular interesse do setor automotivo. Nas condições de soldagem com metal intermediário, chapas de Al revestido com Zn por galvanização foram usadas. As propriedades mecânicas das juntas foram avaliadas através de ensaios com carregamento quasi-estáticos e dinâmicos, sendo então correlacionados com as características microestruturais. Através das análises constatou-se que a aplicação de Zn como metal intermediário mostrou-se efetiva na redução do volume de compostos intermetálicos formados na interface da solda e consequentemente, obteve-se um aumento do desempenho mecânico de cerca de 48 % no ensaio de cisalhamento por tração, 230% no ensaio de tração cruzada e de 35,31 % no ensaio de fadiga quando em comparação a juntas soldadas diretamente entre as chapas de AA5754-H24 e AZ31-O. / In this research was carried out a comparative study between dissimilar joints produced by means of the process FSpW without and with applying Zn as an intermediary metal with the purpose of investigating the effectiveness of the addition of this metal in the mechanical behavior of joints, as well as to assess its influence on the microstructural characteristics of the welded joints. The joints were produced with the AA5754-H24 Al alloy and the AZ31-O Mg alloy, materials of particular interest to the automotive industry. For the welding conditions with the Zn interlayer Al sheets coated by hot deep galvanizing were used. The mechanical properties of the welded joints were evaluated in terms of quasi-static and dynamic load and then correlated with the microstructural features. Through the analyzes it has been verified that the addition of Zn as an intermediary metal proved to be effective in reducing the amount of the intermetallic compound formed at the weld interface and consequently the mechanical performance increased about 48 % in the lap shear strength test, 230 % in the cross tensile strength test and 35,31 % in the fatigue test when compared to the welded joints produced by the direct contact among the AA5754-H24 and the AZ31-O sheets.

Tendo em vista a tendência mundial de se buscar a diminuição de peso de estruturas metálicas de veículos, o desenvolvimento de aços de resistência mecânica superior apresenta-se como um importante fator para a competitividade e sustentabilidade da indústria brasileira, além da segurança do usuário. No setor automobilístico, as reduções no peso e no consumo de combustível requerem não somente a pesquisa de novos aços, mas também a utilização de componentes híbridos, resultantes, entre outros, da união entre os aços austeníticos inovadores de alta liga e baixo custo (TRIP) e aços ferríticos comerciais (ARBL). Na presente dissertação foram investigadas as propriedades mecânicas microestruturais, antes e após o processo de soldagem TIG, dos aços TRIP e ARBL utilizados pela indústria automotiva. O principal objetivo do trabalho foi avaliar possíveis estruturas e modificações nas propriedades das juntas soldadas de acordo com parâmetros de soldagem pré-determinados. Primeiramente, verificaram-se as extensões da zona termicamente afetada dos corpos de prova soldados. Os perfis de microdureza indicaram que não houve uma alteração significativa de dureza na zona termicamente afetada do aço de efeito TRIP, enquanto que para o aço ARBL observa-se um considerável aumento. Os limites de escoamento determinados pelos ensaios de tração realizados permitiram a obtenção dos níveis de resistência mecânica das juntas soldadas. Através dos ensaios de microscopia eletrônica de varredura e microscopia ótica foi possível observar a influência do aporte térmico na zona termicamente afetada do aço ARBL. A análise de EDX nas juntas dissimilares permitiu observar a presença de zonas parcialmente diluídas na zona fundida do lado do metal de base ARBL, a caracterização da composição química nas interfaces entre os metais de base e o metal de solda e prever a microestrutura destas regiões. As fases presentes nos metais de base e na zona fundida foram caracterizadas também pela difração de raios x. Por meio da estereologia quantitativa caracterizou-se as fases presentes na zona fundida e determinou-se suas proporções em função do aporte térmico. / In view of the global trend of searching for the weight reduction of metal frames in vehicles, the development of superior mechanical resistance steels presents itself as an important factor for the competitiveness and sustainability of the Brazilian industry, as well as to user safety. In the automotive industry, the reductions in vehicle weight and fuel consumption not only require research of new types of steel but also the use of hybrid components, which result, amongst other things, between the union of austenitic innovative high alloy and low cost (TRIP) with commercial ferritic steels (HSLA). This dissertation analyses the microstructural mechanical properties before and after the TIG welding process, the TRIP steel, and HSLA used by the automotive industry. The main objective of this study was to evaluate possible phase changes in steel structures and transformation in the properties of the welded joints according to pre-determined welding parameters by analyzing extensions of the heat-affected and melted zones of welded specimens. The micro hardness profiles showed no significant changes in hardness in the heat affected HSLA steel, while the TRIP steel showed considerable increase. Additionally, the yield point determined by the traction tests performed, allowed the obtainment of the mechanical resistance levels of the welded joints. Through the tests of scanning electron and optical microscopy was observed the influence of heat input on heat affected zone of HSLA steel. EDX analysis allowed us to observe the presence of molten zones partially diluted in the side of the base metal HSLA, characterization of the chemical composition at the interface of the base metal and weld metal and predicting the microstructure in these regions. The phases present in the base metal and weld zone were characterized too by x-ray diffraction. By quantitative stereology characterized by the phases present in the weld zone and their proportions were determined as a function of heat input.

Tendo em vista a tendência mundial de se buscar a diminuição de peso de estruturas metálicas de veículos, o desenvolvimento de aços de resistência mecânica superior apresenta-se como um importante fator para a competitividade e sustentabilidade da indústria brasileira, além da segurança do usuário. No setor automobilístico, as reduções no peso e no consumo de combustível requerem não somente a pesquisa de novos aços, mas também a utilização de componentes híbridos, resultantes, entre outros, da união entre os aços austeníticos inovadores de alta liga e baixo custo (TRIP) e aços ferríticos comerciais (ARBL). Na presente dissertação foram investigadas as propriedades mecânicas microestruturais, antes e após o processo de soldagem TIG, dos aços TRIP e ARBL utilizados pela indústria automotiva. O principal objetivo do trabalho foi avaliar possíveis estruturas e modificações nas propriedades das juntas soldadas de acordo com parâmetros de soldagem pré-determinados. Primeiramente, verificaram-se as extensões da zona termicamente afetada dos corpos de prova soldados. Os perfis de microdureza indicaram que não houve uma alteração significativa de dureza na zona termicamente afetada do aço de efeito TRIP, enquanto que para o aço ARBL observa-se um considerável aumento. Os limites de escoamento determinados pelos ensaios de tração realizados permitiram a obtenção dos níveis de resistência mecânica das juntas soldadas. Através dos ensaios de microscopia eletrônica de varredura e microscopia ótica foi possível observar a influência do aporte térmico na zona termicamente afetada do aço ARBL. A análise de EDX nas juntas dissimilares permitiu observar a presença de zonas parcialmente diluídas na zona fundida do lado do metal de base ARBL, a caracterização da composição química nas interfaces entre os metais de base e o metal de solda e prever a microestrutura destas regiões. As fases presentes nos metais de base e na zona fundida foram caracterizadas também pela difração de raios x. Por meio da estereologia quantitativa caracterizou-se as fases presentes na zona fundida e determinou-se suas proporções em função do aporte térmico. / In view of the global trend of searching for the weight reduction of metal frames in vehicles, the development of superior mechanical resistance steels presents itself as an important factor for the competitiveness and sustainability of the Brazilian industry, as well as to user safety. In the automotive industry, the reductions in vehicle weight and fuel consumption not only require research of new types of steel but also the use of hybrid components, which result, amongst other things, between the union of austenitic innovative high alloy and low cost (TRIP) with commercial ferritic steels (HSLA). This dissertation analyses the microstructural mechanical properties before and after the TIG welding process, the TRIP steel, and HSLA used by the automotive industry. The main objective of this study was to evaluate possible phase changes in steel structures and transformation in the properties of the welded joints according to pre-determined welding parameters by analyzing extensions of the heat-affected and melted zones of welded specimens. The micro hardness profiles showed no significant changes in hardness in the heat affected HSLA steel, while the TRIP steel showed considerable increase. Additionally, the yield point determined by the traction tests performed, allowed the obtainment of the mechanical resistance levels of the welded joints. Through the tests of scanning electron and optical microscopy was observed the influence of heat input on heat affected zone of HSLA steel. EDX analysis allowed us to observe the presence of molten zones partially diluted in the side of the base metal HSLA, characterization of the chemical composition at the interface of the base metal and weld metal and predicting the microstructure in these regions. The phases present in the base metal and weld zone were characterized too by x-ray diffraction. By quantitative stereology characterized by the phases present in the weld zone and their proportions were determined as a function of heat input.

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.

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