Marco Antonio Durlo Tier

Preliminary study on the mechanical behavior of friction spot welds

O processo de Soldagem a Ponto por Friccao (FSpW - Friction Spot Welding) caracteriza-se por possibilitar a uniao no estado solido de duas ou mais chapas de material sobrepostas sem deixar o furo residual caracteristico do processo FSW. O presente trabalho apresenta os primeiros resultados de estudos visando determinar a resistencia mecânica de juntas em ligas de aluminio soldadas por FSpW. Recentemente diversos estudos sobre o modelamento numerico de soldas realizadas por processos de soldagem baseados em friccao tem sido realizados, entretanto a maioria destes estudos esta relacionada a fisica do processo. Alguns trabalhos estabelecem a relacao entre as variaveis de processo e as propriedades mecânicas da junta resultante, mas e muito dificil encontrar na literatura resultados quantitativos que possam ser usados diretamente para projeto destas unioes. O presente trabalho objetiva desenvolver um processo de analise baseado nas caracteristicas do processo que permita avaliar como a geometria e microestrutura das regioes da solda afeta o comportamento mecânico das juntas resultantes. Com este objetivo, os resultados de ensaios mecânicos realizados em juntas de aluminio AA2024-T3 foram usados para validar e calibrar um modelo numerico desenvolvido para prever o modo de falha da uniao. O modelo numerico reproduz as dimensoes e carregamento utilizados nos ensaios de cisalhamento e tracao. As dimensoes e propriedades das regioes principais da junta (ZM - zona de mistura, ZTMA - zona termo-mecânicamente afetada, ZAC - zona afetada pelo calor e MB - metal base) foram definidas com base na medicao da dureza e analises macrograficas de corpos de prova soldados com diferentes parâmetros. Pode-se comprovar uma otima correlacao entre os resultados obtidos nas simulacoes e os dados experimentais. O modelamento numerico das juntas pernite a previsao das propriedades mecânicas da junta e auxilia na compreensao da influencia da geometria e propriedades de cada regiao sobre o seu desempenho em servico. Baseado nos resultados obtidos, o procedimento de analise pode ser facilmente adaptado para o processo de Soldagem a Ponto por Friccao e Mistura Mecânica (FSSW - Friction Stir Spot Welding).

Microstructural and Mechanical Observations of Galvanized TRIP Steel after Friction Stir Spot Welding

Friction stir spot welding was done in transformation-induced plasticity steel sheets coated with zinc. The influence of tool rotational speed and dwell time on the microstructure and mechanical properties of lap-joints were investigated. After processing, different zones were formed in the joints. Microstructures in each zone depended on the welding conditions employed. Higher dwell time coupled with higher rotational speed promoted the deposition of a large amount of allotriomorphic ferrite beside the keyhole left by the pin. Coalesced bainite formation was stimulated by the deformation. Mechanical and chemical stabilization of the austenite occurred in different welding zones. Some zinc from the coating remained in the joint, in the stirring zone, representing a partial bonding between the steel sheets. The strength of the welds depended on a complex interaction between geometrical features, such as bonding ligament length and distance between the zinc and the keyhole left by the pin and the resultant microstructure in the stirring zone. The highest joint strength was observed for the “lowest tool rotational speed–highest dwell time” combination of welding parameters.

Microstructure evaluation and mechanical properties of a friction stir spot welded TRIP 800 steel

A TRIP 800 steel was friction stir spot welded using three different tool rotational speeds, 1600, 2000 and 2400 rpm, and the dwell time was kept constant at 2 s. The resultant microstructures formed in each weld zone were analysed as well as their hardness. Higher hardness values were observed for the lowest rotational speed, 1600 rpm, where the heat input in theory was lower and, therefore, the cooling rate was faster. However, for this rotational speed, allotriomorphic ferrite was also observed in the stir zone. In the lap shear tests, samples welded at 1600 and 2400 rpm did not reach the minimum value recommended by the AWS D8.1M standard, which was attributed to the lower bonding ligament length and also lower distance between the keyhole left by the pin and the end of the zinc line, which is formed in the stir zone. The fracture of the samples occurred along this line. As a result, the influence of the microstructure on the failure process could only be inferred when the zinc line disappeared.

Avaliação Microestrutural e Propriedades Mecânicas de um Aço TRIP Soldado a Ponto por Fricção e Mistura Mecânica (Microstructural Evaluation and Mechanical Properties of a Friction Stir Spot Welded TRIP 800 Steel)

A TRIP 800 steel was friction stir spot welded using three different tool rotational speeds, 1600, 2000 and 2400 rpm, and the dwell time was kept constant in 2s. The resultant microstructures formed in each weld zone were analyzed, as well as their hardness. Higher hardness values were observed for the lowest rotational speed, 1600 rpm, where the heat input in theory was lower and, therefore, the cooling rate was faster. However, for this rotational speed allotriomorphic ferrite was also observed in the stir zone. In the lap-shear tests, samples welded at 1600 rpm and 2400 rpm did not reach the minimum value recommended by the AWS D8.1M standard, which was attributed to the lower bonding ligament length and also lower distance between the keyhole left by the pin and the end of the zinc line, which is formed in the stir zone. The fracture of the samples occurred along this line. As a result the influence of the microstructure on the failure process could only be inferred when the zinc line disappeared.