Resistência à corrosão das ligas de alumínio AA2050-84 e AA7050-T7451 soldadas por fricção e mistura

Abstract

VIVEIROS, Bárbara Victoria Gonçalves. Corrosion resistance in aluminum alloys AA2050-T84 and AA7050-T7451 welded by friction stir welding. 2020. Dissertação (Mestrado em Tecnologia Nuclear-Materiais) Instituto de Pesquisas Energéticas e Nucleares-IPEN/CNEN. São Paulo. The most commonly used aluminum alloys in the aeronautic industry are those of the 2XXX and 7XXX series, due to their good mechanical properties related to addition of alloying elements that lead to hardening. Some parts of aircrafts involve the use of alloys of both series joined by rivets since aluminum alloys present low weldability. The use of rivets results in a significant increase in aircrafts' weight. An alternative to the use of rivets is to weld the alloys by friction stir welding, FSW. By the use of this technique, the materials do not reach their melting point. However, FSW leads to microstructure changes due to the plastic deformation at high temperatures to which the aluminum alloys are exposed during this welding process. This causes changes in the mechanical resistance and corrosion resistance of these alloys. In this work the corrosion resistance of the AA2050-T84 and AA7050-T7451 alloys welded by FSW was investigated using chloride solutions. The weld produced different microstructure zones in the two alloys. These zones are known as heat affected zone (ZTA), thermomechanically affected zone (ZTMA) and stir zone (ZM). The corrosion resistance, microstructure and microhardness of the welded alloys, besides the alloys as received, were studied. Microhardness obtained at cross section and at the top surface of the weld, these measurements identified significant differences between the zones. The characterization of the corrosion resistance was carried out by electrochemical tests, agar gel visualization test and immersion tests. Localized electrochemical techniques were also used in this work, specifically scanning vibrating electrode technique (SVET) and scanning ion-selective electrode technique (SIET). The results showed that the most electrochemically active zone at the cross-section of the welded alloys was the ZTMA. This zone corresponded to the transition between the two alloys. For the top surface of the welded alloys, the most electrochemically active zone corresponded to the transition between ZTMA and ZTA zones of the AA7050. The high electrochemical activity of this last zone was related to the formation of hardening precipitates at the ZTA of the AA7050, whereas for the cross-section area, the high activity was due to the galvanic coupling between the two alloys. In the galvanic coupling, the AA7050 alloy acted as anode, and the AA2050, as the cathode. When the two alloys, AA2050 and AA7050, were exposed separately to 0.005 mol.L-1 NaCl or 3.5 (wt%) NaCl solutions and tested by localized electrochemical techniques and by immersion tests, the AA2050 alloy showed higher electrochemical activity compared to the AA7050 alloy due to the higher content of micrometric precipitates in the AA2050 alloy. This last alloy was cathodi...