| The microstructure on the top surface of friction-stir weld(FSW) joints of 7B04-T6 aluminum alloys were characterized by optical microscopy, transmission electron microscope(TEM) and differential scanning calorimetry(DSC). Types, size, distributions, dissolution and precipitation behaviors of the precipitates in different regions of the joints of the weld were investigated. The mechanism of local corrosion on the surface of the joint and the relationship between welding parameters and joint local corrosion sensitivity were evaluated by immersion corrosion and electrochemical experiments, using scanning electron microscopy(SEM) and metallographic microscope analysis.The effect of Al clad and welding parameters on stress corrosion sensitivity of FSW joints was investigated by slow strain rate tensile test.The results of metallographic observation showed that the precipitates of 7B04 aluminum alloy are mainly for η′ phase and η phase. The precipitates of shoulder active zone(SAZ) were almost completely dissolved in the welding process and some η phase precipitated while the joint was cooling. In thermal mechanical affected zone, grains are elongated, the main phases in the zone are coarsened η phase and amount of fine η phase which was reprecipitated.The results of immersion experiment showed that the FSW joints in NaCl + H2O2 solution occured only pitting, indicating 7B04 aluminum alloy FSW joint has no intergranular corrosion sensitivity. The immersion experiment in EXCO solution showed that SAZ occured pulverization. But in the area, grains have no deformation, the second phase particle discontinuous distribution on grain boundaries. The electrochemical analysis showed that SAZ had the owest corrosion potential in EXCO solution, so the shoulder function area as relatively anode accelerated the corrosion of the region. TMAZ cored seriously, this is because the grains of this area was stretched out of shape, and the continuous distribution η phase particles on grain boundary.Slow strain rate tensile test results showed that the FSW joint with Alclad were fractured along the line of ―S‖, while the joint without Alclad were fracture along the junction of heat affect zone and thermal mechanical affected zone. The joint without Alclad had a lower stress corrosion sensitivity than that with Alclad. |
