Precipitation Behavior And Corrosion Properties Of Marine Al-Mg Alloy AA5083 FSW Joint After Sensitization At Low Temperature

After serving for a long time,the precipitation phase along grain boundaries significantly reduces the corrosion resistance of the AA5083 FSW joint and induces a large number of intergranular corrosion and stress corrosion cracking.In this thesis,the microstructure characterization,corrosion performance and SCC behavior of the AA5083 FSW joint of low-temperature sensitized Al-magnesium alloy were studied.The influences of sensitization,test environment and electrode potential on precipitation behavior,intergranular corrosion,electrochemical behavior and stress corrosion cracking were studied.The second phase particles,Al₆（Mn,Fe,Cr,Si）and Mg₂Si,are dispersed in the base material（BM）,thermal-affected zone（TMAZ）and stirring zone（SZ）of Al-Mg alloy AA5083FSW joint.The diameter of the second phase particles of TMAZ and SZ is smaller due to the mechanical action of stirring pin.The microstructure of the AA5083 FSW joint of Al-Mg alloy leads to the combination of deformed and recrystallized grains,small grain size,the proportion of small Angle grain boundary（LAGB）is large,and large dislocation density.These factors lead to the precipitation of more precipitated phases at grain boundaries after low temperature sensitization.TMAZ has greater electrochemical corrosion sensitivity and SCC sensitivity in both welded and sensitized.With the increase of low temperature sensitization time,the thickness and coverage of grain boundary precipitated phase gradually increased,and the intergranular corrosion sensitivity,electrochemical corrosion sensitivity and SCC sensitivity gradually increased.TMAZ has the highest electrochemical corrosion sensitivity and SCC sensitivity,and the worst corrosion resistance.With the increase of sensitization degree,BM,TMAZ and SZ,the self-corrosion potential gradually becomes negative,the self-corrosion current density gradually increases,and the polarization resistance gradually decreases,indicating that the corrosion sensitivity increases.TMAZ at 70℃/450 days has the most negative self-corrosion potential,the highest self-corrosion current density,the lowest polarization resistance and the worst corrosion resistance.With the increase of sensitization degree,the SCC sensitivity of Al-Mg alloy FSW joint increases gradually,and SCC does not occur in welded specimens.The SCC sensitivity of the FSW joint was the highest at 70℃/450 days,and intergranular brittle fracture occurred.The test environment has an important influence on the electrochemical behavior and SCC behavior of sensitized FSW joints.The electrochemical corrosion sensitivity and SCC sensitivity of FSW joints increase with the increase of Cl^-concentration.The influence of strain rate on the SCC sensitivity of FSW joint is complicated.As the strain rate decreases,the elongation of FSW joint decreases first and then increases,and the most serious SCC occurs at 1×10^-5 s^-1,while no SCC occurs at 1×10^-3 s^-1 and 1×10^-6 s^-1.Sensitized（70℃/180 days）FSW joint has an important effect on the electrode potential.At-0.9 V_SCE and-1.1 V_SCE,the sensitized FSW joint does not have a serious SCC.At-1.5V_SCE and-1.7 V_SCE,the elongation decreases,which is due to the low potential.Overprotection occurred and hydrogen embrittlement occurred in the sample.