Estudou-se o efeito dos tratamentos de modificação com estrôncio, com sódio e com antimônio sobre a formação de microporosidades em liga de alumínio A356. Através da fundição de barras em moldes de areia, verificou-se que os tratamentos de modificação com sódio e com estrôncio aumentam a propensão a formação de microporosidades, enquanto o tratamento com antimônio diminui levemente esta tendência. Ao mesmo tempo, verificou-se que os tratamentos de modificação alteram a morfologia das microporosidades. Com o objetivo de determinar os mecanismos envolvidos na formação das microporosidades em função da técnica de modificação, efetuaram-se experimentos críticos que procuram evidenciar a influência destas técnicas sobre a gênese das microporosidades: formação de micro-rechupes decorrentes de dificuldades de alimentação das contrações de solidificação e formação de micro-bolhas decorrente da segregação de hidrogênio durante a solidificação. Dessa forma, verificou-se que o super-resfriamento da reação eutética, observado em ligas modificadas com estrôncio e com sódio, não é o responsável pela maior propensão a formação de microporosidades nestes casos.

The effects of Na, Sr and Sb modification in the propensity to microporosity formation in A356 aluminum alloy were studied. The propensity to microporosity formation in Sr-, Na- and Sb-modified alloys with fixed hydrigen content was compared. Microporosity contents increased by up to three times when Na or Sr modification was used, while a slight decrease was observed in the Sb-modified melts. The results also showed that modification treatment has a marked effect on microporosity morphology. In order to determine the mechanisms involved in the microporosity formation in each case, some critical experiments were carried out. As a starting point, it was considered that the formation of microporosity is caused by micro-shrinkage formation and hydrogen segregation. The hypothesis that the modification-induced widening of the freezing range was the main reason for the increasing propensity to microporosity formation observed in Na- and Sr-modified melts was investigated. The results showed that increasing eutectic undercooling by modification exerts only a very small effects on microporosity formation. Using a quenching-during-solidification technique, it was possible to observe the fraction of solida t which the pore nucleation started. The results showed that, in Na- and Sr-modified melts, the more nucleates earlier than in the unmodified ones, while in Sb-modified melts, the pore nucleation is delayed. At the same time, these experiments clarified the mechanisms involved in the evolution of the pore morphology during the solidification. The interdendritic feeding capacities of unmodified and modified A356 alloys were evaluated. Na- and Sr-modified melts presented lower interdendritic feeding capacity than unmodified ones, while Sb-modified melts presented larger feeding capacity. Also, a fluidity test was performed and showed that Na modification reduces the aluminum flowing capacity. Finally, observations of deep etched samples in scanning electron microscope evidenced that modification treatment promotes some changes in the -dendrite morphology. These results could explain the differences in the interdendritic feeding capacity. The most important changes were: (i) tendency to rounded morphology; (ii) grain size reduction; (iii) dendritic arm spacing reduction. As a result of these changes in the dendrite morphology, the permeability of dendrite network was reduced, hindering the interdendritic feeding mechanism and promoting micro-shrinkage formation. Specifically in Na-modified melts this feeding limitation was emphasized by the higher liquid viscosity.In Sb-modified melts, the changes in the dendrite morphology were more intense than in Sr- and Na-modified ones. As a consequence, the dendrite network rigidity was reduced allowing the activation of another feeding mechanism, the channel feeding, that ensured an efficient interdendritic feeding, delaying the microporosity formation.