Investigation Of The Corrosion Behavior Of Friction Stir Welded Joints In 20mm Thick 7075 Aluminum Alloy

The corrosion behavior of 20 mm thick 7075 aluminum alloy FSW joints was investigated in this paper. The microstructure and the second phase in different zones of the joints were analyzed along the thickness direction. 3.5% NaCl solution immersion test, electrochemical corrosion test and intergranular corrosion test about cross section of joints and surface of the layered sample along the thickness direction have been conducted. The results shown that:Downward along the thickness direction of the weld, the size of second phase and the grains of the center zones of the welds gradually increased while that of thermal mechanical affected zone(TMAZ) increased first and then decreased, and that of heat affected zone(HAZ) gradually decreased. The second phase and the grains size in the center zones of the welds is the smallest. The size of TMAZ is between that of the center zones and HAZ.In the 3.5% NaCl solusion, corrosion characteristics in the center zones of the welds and BM mainly is pitting, but in TMAZ and HAZ firstly pitting and intergranular corrosion occurs and finally the spalling corrosion is presented. The corrosion degree in the center zones of the welds is the lightest while in the arer near the junction of TMAZ and HAZ is most serious. The corrosion degree of the middle-lower part of the junction of the advancing side of TMAZ and HAZ is more serious than that in the top. The corrosion degree of the middle-upper part of the junction of the retreating side of TMAZ and HAZ is more serious than that of the bottom. In the center zones of the welds, the corrosion degree of SAZ is more serious than that of nugget zone(NZ). Downward along the thickness direction of the weld, the corrosion degree of NZ gradually increased. Also it is found that the better the corrosion resistance is, the higher the hardness in the zones is.In the center zones of the welds, the corrosion potential of SAZ is the smallest while its corrosion current density is the largest. Downward along the thickness direction of the weld, the corrosion potential of NZ gradually decreased and its corrosion current density gradually increased. The corrosion potential of TMAZ decreased first and then increased, while its corrosion current density increased first and then decreased. The corrosion potential of HAZ gradually increased and its corrosion current density gradually decreased.The intergranular corrosion behavior of the layered sample is investigated. The results show that, the corrosion degree in the center zones of the welds is the lightest and that of HAZ is the most serious. The periodic aggregation zone of SAZ aggravated its intergranular corrosion degree. Downward along the thickness direction of the weld, the intergranular corrosion degree of NZ gradually increased while that of TMAZ increased first and then decreased, and that of HAZ gradually decreased.Also it is found that the chemical element composition of the second phase have great influence on corrosion rate of aluminum matrix nearby the second phase. The corrosion rate of aluminum matrix nearby S phase(Al2MgCu) is faster than that nearby the Fe-containing phase.