Efeito do Tempo de Atuação da Polaridade Positiva no Comportamento Mecânico e em Corrosão de Juntas Compostas por Ligas Al-Mg com Diferentes Teores de Mg Soldadas pelo Processo TIG CA.

Abstract

The need for repairs and structural modifications in vessels and marine structures built in Al-Mg alloys, often due to the difficulty in obtaining the original alloy, makes it necessary to use alternative alloys within the same series. Alternative alloys have Mg and secondary element contents different from the original alloy. The present work covers a study on the evaluation of the mechanical and corrosion behavior of joints composed of base metals of the 5XXX series (Al-Mg) with different Mg contents welded by the TIG CA process with variation of the positive polarity actuation time (% EP): 30%, 40% and 50% of the cycle. AA 5083 H111 (5.04% Mg) and AA 5052 F (2.81% Mg) alloys were used as base metals, while ER 5183 (5.0%Mg) alloy was chosen as filler metal. Dissimilar joints made with AA 5052 F and AA 5083 H111 alloys as base metals and similar joints made only with AA 5083 H111 alloy were subjected to hardness and tensile tests to evaluate their mechanical behavior. Potentiodynamic polarization tests verified the pitting corrosion resistance of the joints. The characterization by SEM/EDS was carried out before and after the polarization tests, in order to identify the intermetallic phases and their influence on pit formation. The results showed that despite the %EP variation having directly influenced the weld bead width and the cathodic cleaning range, there were no significant differences in the morphology and grain size of the weld metal region. The hardness and tensile tests did not show effect of the %EP variation on the mechanical characteristics of the joints. Through SEM/EDS analysis, the presence of intermetallic phases rich in Fe and Mn, possibly Al6(Mn,Fe), Al(Mn,Fe,Cr,Si) and phases rich in Mg and Si, possibly Mg2Si, were identified in the weld metal regions and ZTA_5083, for the three variations of %EP. In the ZTA_5052 region, however, the presence of phases rich in Fe, possibly Al3F, and phases rich in Mg and Si, possibly Mg2Si, were detected for the three variations of %EP. Electrochemical polarization tests showed that the corrosion potential of welded joints is governed by the weld metal region, regardless of whether the confection is similar or dissimilar, and did not vary with %EP. The initiation and propagation of pits in dissimilar and similar joints occurs through the formation of a galvanic couple between Fe-rich cathode intermetallic phases such as Al6(Mn,Fe), Al3Fe, with the α-Al matrix, which undergoes dissolution in the surrounding areas.