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A utilização de aços na produção de estruturas metálicas e equipamentos, é algo profundamente entranhado na indústria dos dias de hoje. Por consequência, um dos principais processos de união destes materiais, se não mesmo o mais utilizado, é a soldadura. A utilização de aços de alta resistência (muito usados na indústria automóvel) permite uma maior abrangência e capacidade de aplicações. No entanto, com o desenvolvimento de novos materiais, surgem questões relativamente à sua resposta ao processo de soldadura que necessitam de ser respondidas. É extremamente importante entender de que forma cada material responde à soldadura e avaliar as propriedades das juntas soldadas para que estas possam ser utilizados na indústria com segurança e confiança. É sabido que a aplicação de temperaturas elevadas em aços tem um forte efeito nas suas
propriedades mecânicas e consequentemente comportamento. A soldadura é um processo extremamente agressivo para o material, uma vez que os ciclos de aquecimento e arrefecimento são muito rápidos. Deste modo considerou-se importante avaliar a influência de parâmetros de soldadura selecionados, nas propriedades de juntas soldadas de chapas de alta resistência usadas na indústria automóvel. Mais especificamente, estudou-se a variação a intensidade de corrente de soldadura nas propriedades das juntas soldadas. A variação da intensidade de corrente, tem naturalmente uma consequência na quantidade de
calor que é induzida às juntas durante a soldadura, e assim é natural concluir que existe uma relação entre a sua variação e as propriedades do aço. Para levar este estudo a cabo, foram executadas soldaduras com três intensidades de corrente diferentes usando os processos TIG e soldadura por resistência na variante de pontos. De seguida foram executados ensaios mecânicos aos vários provetes realizados de forma a entender as diferentes respostas do material. Adicionalmente foram feitas análises macrográficas e micrográficas e de dureza, relacionando a resposta mecânica dos materiais com as alterações provocadas na microestrutura.
Foi constatado, para os dois processos, a clara influência da variação da intensidade na ductilidade do aço, sendo que quanto mais elevado este parâmetro, mais dúctil se mostrou o material. Essa tendência foi acompanhada por uma diminuição dos valores máximos de dureza. Para o caso dos provetes de soldadura por resistência verificou-se também um aumento da resistência ao corte e arrancamento com o aumento da intensidade de corrente. Foi possível constatar que com intensidades de corrente maiores a quantidade de material fundido aumentava criando uma maior área resistente para a junta. Por fim, as análises de microestrutura, permitiram distinguir as diferentes zonas da soldadura, onde se verificou o aparecimento de martensite na zona termicamente afetada das soldaduras, estruturas dendríticas nas zonas fundidas e um crescimento de grão na aproximação à ZTA.
The use of steel in the manufacture of metallic structures and equipments, is something that is profoundly rooted in today’s industry. By consequence, welding is one of the main processes used to join the different parts of each structure. The use of more complex steels (widely used in the automotive industry), allows us to have a broader spectrum of applications. However, with the development of new materials, it is natural that new questions arrive considering their reactions when submitted to the different welding processes. It is extremely important to understand the way how each material reacts to welding, so that they can be safely used with security and confidence. It is known that the influence of temperature in steels has great and preponderance effect on the mechanical characteristics of the steels and consequent behavior. Welding is an extremely aggressive process to the material, due to the rapid cycles of heating and cooling. With that in mind, it is considered important to evaluate the variation of selected welding parameters that might have influence in the properties of high strength steels used in the automotive industry. More specifically, the intensity of the welding current, which will be the focus of the study. The variation of the welding current, is, naturally, one of the deciding factors that influence the amount of heat induced in welded joints, so it is fair to conclude that there is a relationship between its variation and the properties of the steel. With that in mind, three different current values were changed for the welds produced by TIG and resistance spot welding. Several mechanical tests were conducted to understand the influence of the current variation on the different material responses/ properties. Additionally, several macrographic and micrographic analyses were made, to relate the mechanical responses to the alterations in the material microstructures. It was possible to see, for both processes, the clear influence of the variation of the welding current in the ductility of the steel. The higher this parameter is, the more ductility the material is. This tendency was accompanied by a decrease of the maximum value of hardness. Regarding the specimens produced by resistance spot welding, it was observed that the shear resistance and peeling tests rose when the welding current increase as well. For higher intensities, there was a bigger amount of fused material which in turn formed a bigger area of resistance. Finally, from the microstructural results, it was possible to see a clear distinction between the various areas of the weld, with the appearance of martensite in the heat affected zones, dendritic structures in the fused areas, and grain growth in the steel vicinity of the heat affect zone with the base material.
The use of steel in the manufacture of metallic structures and equipments, is something that is profoundly rooted in today’s industry. By consequence, welding is one of the main processes used to join the different parts of each structure. The use of more complex steels (widely used in the automotive industry), allows us to have a broader spectrum of applications. However, with the development of new materials, it is natural that new questions arrive considering their reactions when submitted to the different welding processes. It is extremely important to understand the way how each material reacts to welding, so that they can be safely used with security and confidence. It is known that the influence of temperature in steels has great and preponderance effect on the mechanical characteristics of the steels and consequent behavior. Welding is an extremely aggressive process to the material, due to the rapid cycles of heating and cooling. With that in mind, it is considered important to evaluate the variation of selected welding parameters that might have influence in the properties of high strength steels used in the automotive industry. More specifically, the intensity of the welding current, which will be the focus of the study. The variation of the welding current, is, naturally, one of the deciding factors that influence the amount of heat induced in welded joints, so it is fair to conclude that there is a relationship between its variation and the properties of the steel. With that in mind, three different current values were changed for the welds produced by TIG and resistance spot welding. Several mechanical tests were conducted to understand the influence of the current variation on the different material responses/ properties. Additionally, several macrographic and micrographic analyses were made, to relate the mechanical responses to the alterations in the material microstructures. It was possible to see, for both processes, the clear influence of the variation of the welding current in the ductility of the steel. The higher this parameter is, the more ductility the material is. This tendency was accompanied by a decrease of the maximum value of hardness. Regarding the specimens produced by resistance spot welding, it was observed that the shear resistance and peeling tests rose when the welding current increase as well. For higher intensities, there was a bigger amount of fused material which in turn formed a bigger area of resistance. Finally, from the microstructural results, it was possible to see a clear distinction between the various areas of the weld, with the appearance of martensite in the heat affected zones, dendritic structures in the fused areas, and grain growth in the steel vicinity of the heat affect zone with the base material.
Description
Keywords
HSS: High Strength Steel Welding TIG Resistance spot welding Aços de alta resistência Soldadura Soldadura por resistência