Fatigue Propagation of Cracks in Aeronautics Aluminium Alloys, Welded by Laser

Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e TecnologiaEsta Dissertação de mestrado tem como principal objetivo o estudo da influência dos parâmetros de soldadura no comportamento à fadiga e na propagação de fissuras de juntas de T dissimilares de ligas de alumínio, soldadas a laser, com a finalidade de estas poderem vir a ser aplicadas na indústria aeronáutica. Fabricaram-se juntas de T, recorrendo ao processo de soldadura a laser, com as ligas de alumínio AA2024-T4(skin) e AA7075-T6(stringer), tendo cada chapa 2 mm de espessura e a soldadura tendo sida efetuada sequencialmente de ambos os lados do stringer. Foi utilizado como material de adição a liga de alumínio AA4047, liga esta rica em silício (Si), componente este que previne a fissuração a quente. Para o estudo foram efetuadas diferentes séries, com a variação dos parâmetros de soldadura tais como o diâmetro de spot, o ângulo e posição de incidência do feixe de laser e a velocidade de soldadura. Foi efetuada uma análise da influência da variação destes parâmetros quando testados à fadiga, em 4 séries, utilizando uma relação de tensão de R=0,02, realizando inicialmente um estudo sobre o efeito dao número de ciclos total de fadiga, curvas S-N, e por fim um estudo dea propagação de fendas para uma outra série, diferente das anteriores, realizando a curva da/dN-K, e o comportamentotambém aa medição do fechocaracterização de fenda, sendo neste teste utilizado uma relação de tensão de R=0,05.Com a análise realizada da resistência à fadiga, curvas S-N, e recorrendo ao método de elementos finitos, foram obtidas as curvas S-N de cada série com base na tensão efetiva. Durante a realização dos ensaios de propagação de fissura foi observado analisado o comportamento de fecho da fenda, para posteriormente poder ser possível caracterizar a fenda.This Master's Dissertation has as main objective the study of the influence of the welding parameters on the fatigue behavior and the propagation of dissimilar T joints of aluminum alloys, welded to the laser, so that they can be applied in the industry.T-joints were fabricated using the laser welding process AA2024-T4 (skin) and AA7075-T6 (stringer) aluminum alloys, each plate being 2 mm thick and the welding having been performed sequentially from both the sides of the stringer. The AA4047 aluminum alloy was used as the addition material, which is a silicon-rich alloy (Si), which prevents hot cracking. For the study, different series were performed, with variation of the welding parameters such as the spot diameter, the angle and position of incidence of the laser beam and the welding speed.An analysis was made of the influence of the variation of these parameters when tested to fatigue, in 4 series, using a tension ratio of R = 0.02, initially performing a study on the total number of cycles of fatigue, SN curves, and finally a crack propagation study for a different series, different from previous ones, by performing the da/dN-K curve, as well as the measurement of the screwdriver, in this test a voltage ratio of R = 0.05 was used.With the analysis of the fatigue strength, S-N curves, and using the finite element method, the S-N curves of each series were performed based on the effective stress. During the crack propagation tests, the slit closure behavior was analyzed, and the U-K and the Pab / Pmax-K graphs were subsequently performed.This dissertation aims to study the influence of welding parameters on fatigue behavior and crack propagation of dissimilar T joints of laser welded aluminum alloys, in order to be applied in the aeronautical industry.T-joints were fabricated using laser welding with AA2024-T4 (skin) and AA7075-T6 (stringer) aluminum alloys, each plate being 2mm thick and the welding having been performed sequentially on both sides of the stringer. The AA4047 aluminum alloy, rich in silicon (Si), was used as the addition material in order to prevent cracking. For the study, different series were performed, varying the welding parameters such as the spot diameter, the angle and position of incidence of the laser beam and the speed of welding.An analysis was made of the influence of the variation of these parameters when tested to fatigue, initially performing a study on the number of total fatigue cycles, S-N curves, and finally a crack propagation study, curve, and the characterization of it.With the analysis of fatigue strength, S-N curves, and using the finite element method, the S-N curves were performed based on the effective stress. During the crack propagation tests, the crack closure behavior was analyzed to obtain the U-K graph and the Pab / Pmax-K graph