Increasing regulatory pressure to reduce fuel consumption of new vehicles has prompted the automotive industry to seek ways to reduce the weight of their automobiles. The use of steel tailor welded blanks has been successful in reducing vehicle weight while simultaneously reducing manufacturing costs; however, further weight reductions are possible if steel alloys are substituted with aluminum alloys. This has created a need to identify and develop welding techniques that would enable the production of high-quality welds between aluminum sheets of different thicknesses at rates compatible with the demands of the automotive industry. A relatively new welding technique that has been shown to have potential for joining aluminum sheet for tailor welded blank applications is the double-sided arc welding (DSAW) process. In DSAW, an arc is struck between two welding torches situated on either side of the sheets to be welded allowing the aluminum surface oxide to be electrically cleaned simultaneously from both sides of the joint. The demonstrated potential for welding aluminum sheet and the low capital cost compared to conventional laser welding systems typically used for fabricating TWBs makes DSAW an excellent candidate for welding aluminum TWBs. The objective of the research described in this thesis was to assess the feasibility and merits of using a DSAW system to manufacture aluminum TWBs. In this study, a DSAW system comprised of a plasma arc welding (PAW) torch above the work piece and a gas tungsten arc welding (GTAW) torch below the work piece was applied to the high speed welding of 1.0 to 1.5 mm thick AA5182-O aluminum alloy sheets in the butt-joint configuration. A series of conduction-mode DSAW welds were made in the horizontal position to identify the welding conditions that produced good quality welds using visual acceptance criteria and with minimal geometric discontinuity across the weld. Further studies were conducted to determine the influence of the welding process parameters on the hardness, strength, ductility, formability and internal flaws of DSAW welds. DSAW welds were made using a series of welding torch-to-work piece distances, between 1.5 and 6.0 mm, to investigate the influence of varying the relative arc forces acting on the top and bottom of the weld pool on the resulting weld bead dimensions including weld metal drop through. It was found that increasing the torch-to-work piece distance decreased the process efficiency when a constant welding power was used resulting in narrower welds being produced. Weld metal sag or drop through was not observed to be affected by varying the welding torch-to-work piece distance; however, decreasing the PAW torch-to-work piece distance to 1.5 mm was found to produce a pronounced surface ripple pattern on the top surface of the weld. A series of DSAW welds were made to investigate the range of welding speeds and powers that produced visually acceptable welds on 1.0 to 1.5 mm thick AA5182 aluminum sheets. Welding powers ranging from 1.4 to 4.6 kW were found to produce acceptable welds at travel speeds between 10 and 70 mm/s when the net heat input per unit distance was between 60 and 110 J/mm. Above these speeds, unacceptable weld bead quality and lack of fusion defects were observed due to incomplete cathodic etching of the oxide from the surfaces and inconsistent coupling between the welding arcs the sheets. It was found that the visual appearance of the weld was improved and travel speeds could be increased for a given welding power when welding specimens were stainless steel wire brushed prior to welding to break-up and remove most of the pre-existing hydrated aluminum surface oxide. Significant reductions in hydrogen gas porosity were also observed when stainless steel wire brushing was used. The strength, ductility and formability of DSAW welds were found to vary significantly depending on the welding parameters used and the occurrence of porosity defects in the welds. Welds made using welding speeds greater than 30 mm/s were found to exhibit solidification shrinkage micro-porosity and a corresponding degradation in mechanical properties, especially ductility and formability. As the welding speed was further increased, degradation of the material properties continued to increase due to an increase in the quantity of micro-porosity defects in the weld. These defects caused significant strain localization resulting in a marked decrease in ductility and formability. The severity of solidification shrinkage micro-porosity present in the weld metal was found to correspond to the relative length-to-width ratio of the weld pool for all the welding conditions examined. Welds produced at high welding speeds resulted in large length-to-width ratios, a relatively large distance between the liquidus and non-equilibrium solidus and low thermal gradients in the mushy zone at the tail of the weld. These conditions are known to promote micro-porosity in alloys with a wide freezing range. Visually acceptable DSAW welds produced using welding speeds below 25 mm/s were found to have excellent material properties that were nearly indistinguishable from the base metal with excellent ductility and formability. These welds had relatively small length-to-width ratios and little or no evidence of solidification micro-porosity, because the length of the mushy zone at the tail of the weld was much smaller and the thermal gradients were much higher. These conditions are known to prevent solidification micro-porosity during solidification of alloys with a wide freezing range. They also provide more time and opportunity for any hydrogen bubbles that may form during solidification to float up and escape through the top surface of the weld pool thereby further reducing the propensity for hydrogen porosity. The DSAW process has been shown to be capable of successfully producing tailor-welded blanks in 5182 aluminum alloy sheets with excellent ductility and formability provided that all sources of porosity are eliminated. This includes careful cleaning and removal of preexisting hydrated oxides using stainless steel wire brushing prior to welding to minimize hydrogen porosity and welding at slow enough speeds to prevent the formation of solidification micro-porosity at the tail of the weld pool.

Increasing regulatory pressure to reduce fuel consumption of new vehicles has prompted the automotive industry to seek ways to reduce the weight of their automobiles. The use of steel tailor welded blanks has been successful in reducing vehicle weight while simultaneously reducing manufacturing costs; however, further weight reductions are possible if steel alloys are substituted with aluminum alloys. This has created a need to identify and develop welding techniques that would enable the production of high-quality welds between aluminum sheets of different thicknesses at rates compatible with the demands of the automotive industry. A relatively new welding technique that has been shown to have potential for joining aluminum sheet for tailor welded blank applications is the double-sided arc welding (DSAW) process. In DSAW, an arc is struck between two welding torches situated on either side of the sheets to be welded allowing the aluminum surface oxide to be electrically cleaned simultaneously from both sides of the joint. The demonstrated potential for welding aluminum sheet and the low capital cost compared to conventional laser welding systems typically used for fabricating TWBs makes DSAW an excellent candidate for welding aluminum TWBs. The objective of the research described in this thesis was to assess the feasibility and merits of using a DSAW system to manufacture aluminum TWBs. In this study, a DSAW system comprised of a plasma arc welding (PAW) torch above the work piece and a gas tungsten arc welding (GTAW) torch below the work piece was applied to the high speed welding of 1.0 to 1.5 mm thick AA5182-O aluminum alloy sheets in the butt-joint configuration. A series of conduction-mode DSAW welds were made in the horizontal position to identify the welding conditions that produced good quality welds using visual acceptance criteria and with minimal geometric discontinuity across the weld. Further studies were conducted to determine the influence of the welding process parameters on the hardness, strength, ductility, formability and internal flaws of DSAW welds. DSAW welds were made using a series of welding torch-to-work piece distances, between 1.5 and 6.0 mm, to investigate the influence of varying the relative arc forces acting on the top and bottom of the weld pool on the resulting weld bead dimensions including weld metal drop through. It was found that increasing the torch-to-work piece distance decreased the process efficiency when a constant welding power was used resulting in narrower welds being produced. Weld metal sag or drop through was not observed to be affected by varying the welding torch-to-work piece distance; however, decreasing the PAW torch-to-work piece distance to 1.5 mm was found to produce a pronounced surface ripple pattern on the top surface of the weld. A series of DSAW welds were made to investigate the range of welding speeds and powers that produced visually acceptable welds on 1.0 to 1.5 mm thick AA5182 aluminum sheets. Welding powers ranging from 1.4 to 4.6 kW were found to produce acceptable welds at travel speeds between 10 and 70 mm/s when the net heat input per unit distance was between 60 and 110 J/mm. Above these speeds, unacceptable weld bead quality and lack of fusion defects were observed due to incomplete cathodic etching of the oxide from the surfaces and inconsistent coupling between the welding arcs the sheets. It was found that the visual appearance of the weld was improved and travel speeds could be increased for a given welding power when welding specimens were stainless steel wire brushed prior to welding to break-up and remove most of the pre-existing hydrated aluminum surface oxide. Significant reductions in hydrogen gas porosity were also observed when stainless steel wire brushing was used. The strength, ductility and formability of DSAW welds were found to vary significantly depending on the welding parameters used and the occurrence of porosity defects in the welds. Welds made using welding speeds greater than 30 mm/s were found to exhibit solidification shrinkage micro-porosity and a corresponding degradation in mechanical properties, especially ductility and formability. As the welding speed was further increased, degradation of the material properties continued to increase due to an increase in the quantity of micro-porosity defects in the weld. These defects caused significant strain localization resulting in a marked decrease in ductility and formability. The severity of solidification shrinkage micro-porosity present in the weld metal was found to correspond to the relative length-to-width ratio of the weld pool for all the welding conditions examined. Welds produced at high welding speeds resulted in large length-to-width ratios, a relatively large distance between the liquidus and non-equilibrium solidus and low thermal gradients in the mushy zone at the tail of the weld. These conditions are known to promote micro-porosity in alloys with a wide freezing range. Visually acceptable DSAW welds produced using welding speeds below 25 mm/s were found to have excellent material properties that were nearly indistinguishable from the base metal with excellent ductility and formability. These welds had relatively small length-to-width ratios and little or no evidence of solidification micro-porosity, because the length of the mushy zone at the tail of the weld was much smaller and the thermal gradients were much higher. These conditions are known to prevent solidification micro-porosity during solidification of alloys with a wide freezing range. They also provide more time and opportunity for any hydrogen bubbles that may form during solidification to float up and escape through the top surface of the weld pool thereby further reducing the propensity for hydrogen porosity. The DSAW process has been shown to be capable of successfully producing tailor-welded blanks in 5182 aluminum alloy sheets with excellent ductility and formability provided that all sources of porosity are eliminated. This includes careful cleaning and removal of preexisting hydrated oxides using stainless steel wire brushing prior to welding to minimize hydrogen porosity and welding at slow enough speeds to prevent the formation of solidification micro-porosity at the tail of the weld pool.

The following thesis is a study on the ability to create acceptable welds in thin-plate, ultra-high-strength steels (UHSS) by way of friction stir welding (FSW). Steels are welded together to create tailor-welded blanks (TWB) for use in the automotive industry. Dual Phase (DP) 590, 780, and 980 steel as well as Transformation-Induced Plasticity (TRIP) 590 steel with thicknesses ranging from 1.2 mm to 1.8 mm were welded using friction stir welding under a variety of processing conditions, including experiments with dissimilar thicknesses. Samples were tested under tensile loads for initial determination if an acceptable weld had been created. Acceptable welds were created in both TRIP 590 and DP 590 at speeds up to 102 centimeters-per-minute. No acceptable welds were created in the DP 780 and DP 980 materials. A series of microhardness measurements were taken across weld samples to gain understanding as to the causes of failure. These data indicate that softening, caused by both excessive heat and insufficient heat can result in weld failure. Not enough heat causes the high concentration of martensite in these materials to temper while too much heat can cause excessive hardening in the weld, through the formation of even more martensite, which tends to promote failure mode during forming operations. Laser welding is one of the leading methods for creating tailor-welded blank. Therefore, laser welded samples of each material were tested and compared to Friction Stir Welded samples. Lower strength and elongation are measured in weld failure while the failure location itself determines the success of a weld. In short, an acceptable weld is one that breaks outside the weld nugget and Heat Affected Zone (HAZ) and where the tensile strength (both yield and ultimate) along with the elongation are comparable to the base material. In unacceptable welds, the sample broke in the weld nugget or HAZ while strength and elongations were well below those of the base material samples.

The shipbuilding sector, as well as all modern transportation industries, is faced with demands for greater productivity while at the same time ensuring the manufacture of consistently high quality products, reducing levels of re-working, saving energy, and minimizing operational costs. Furthermore, it is imperative that new designs and all the stages of production comply with stringent environmental regulation. Within this context, the application of Friction Stir Welding (FSW) as a manufacturing process to weld Tailor Welded Blanks (TWB) for Al structures can contribute to the development of high speed craft and lightweight ships that are more fuel efficient, based on a high energetic efficient and environmental friendly welding process. In this work, the heterogeneous mechanical behavior of TWB joints welded by FSW was evaluated using quasi-static and cyclic loading, and the observed microstructural features were analyzed. The TWB joints were manufactured using dissimilar alloys and thicknesses (6 and 8mm) of particular interest in the shipbuilding sector (AA5083, AA5059 and AA6082). An evaluation of local constitutive properties in different regions through the TWB joint was performed by digital image correlation linked to the tensile test system. From the DIC data processing were generated stress concentrations diagrams and true stress-strain curves for several TWB subzones. The DIC methodology used as well as the accuracy of the proposed method are described in detail. The joints exhibited excellent mechanical properties approximately the same as those of the base metal for the joints manufactured with work hardened alloys (AA5059/AA5083) and 76% mechanical efficiency to those manufactured with the heat-treatable alloy (AA6082). The fatigue strength of the TWB joints were higher than the IIW references for welded structures in aluminum and the fracture mechanisms were characterized using SEM. / O setor de construção naval, bem como a indústria moderna, é continuamente sobrecarregada por demandas de aumento de produtividade e ao mesmo tempo precisa garantir a fabricação de produtos com alta qualidade, reduzindo os níveis de retrabalhos, economizando energia e diminuindo os custos operacionais. Adicionalmente a este paradoxo, é imperativo que os novos designs de produtos e todos os estágios de produção sejam compatibilizados com as rígidas exigências ambientais. Neste contexto, a concepção de projetos de estruturas leves soldadas por SALM em configurações sob medidas (Tailor Welded Blanks - TWB) em Al podem contribuir para produção de embarcações com eficiente consumo de combustível e redução dos níveis de eliminação de CO2 através da redução do peso de suas estruturas. Além de utilizar um processo de soldagem eficiente energeticamente e amigável ao meio ambiente. Neste trabalho as características heterogêneas de juntas em TWB soldadas por SALM foram avaliadas através de ensaios mecânicos com carregamentos quasi-estáticos e dinâmicos e, foram criadas relações entre as propriedades mecânicas das juntas e alterações microestruturas resultantes do processo de soldagem. As juntas em TWB foram produzidas com três diferentes ligas de alumínio de particular interesse da construção naval, (AA5083, AA5059 e AA6082) em configurações similares e dissimilar, com combinações de espessuras de 6 e 8mm. Acoplado ao ensaio de tração um sistema de correlação digital de imagens (DIC) foi instalado e o perfil de deformação local das juntas foram investigados durante o carregamento. A partir do processamento dos dados obtidos por DIC, diagramas de concentração de tensão e curvas de tensão-deformação locais foram computados para diferentes subzonas das juntas. O procedimento utilizado, bem como os dados obtidos e a precisão da metodologia proposta foram descritos detalhadamente. As juntas apresentaram excelentes propriedades mecânicas, equivalentes às do metal base para a junta dissimilar produzida com as ligas endurecidas por trabalho mecânico (AA5059/AA5083) e 76% de eficiência para as juntas similares produzidas com a liga tratável termicamente (AA6082). A resistência a fadiga das juntas foram superiores às referências do IIW para juntas soldadas em alumínio e os mecanismos de fratura foram caracterizados por MEV.

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