| Friction stir welding(FSW)can realize the effective welding of Al-Zn-Mg-Cu alloy,but the mechanical and corrosion resistance properties will be affected by the change of microstructure during welding.Generally,the corrosion resistance of weld joint is decreased significantly compared with the base metal,which greatly reduces the service reliability.Al-Zn-Mg-Cu alloys have high local corrosion sensitivity.Therefore,it is necessary to study the effect of microstructure changes on Corrosion Behavior of friction stir welded joints of Al-Zn-Mg-Cu alloys,and to explore the methods to improve the corrosion resistance of welded joints.In this paper,low Cu high strength Al-Zn-Mg-Cu alloy plates were prepared by melt casting,homogenization,multi pass hot rolling and solution artificial aging treatment.FSW and post weld heat treatment(PWHT)were carried out at different welding speeds.The microstructure of welded joint was observed and analyzed by optical microscope,X-ray diffractometer,scanning electron microscope,electron backscatter diffraction and transmission electron microscope.The hardness and corrosion resistance of the joint were tested by microhardness tester,immersion corrosion technology and electrochemical workstation.The effects of welding process and PWHT on microstructure and corrosion resistance of welded joints were systematically studied.The results show that the grains in the nugget zone of the welded joint are fine equiaxed,and the average grain size increases(1.8 ?m to 7.4 ?m)with the increase of welding speed.The main micron particles are broken and uniformly dispersed Al(Fe,Mn,Si,Cr)phase.The hardness of welded joint in nugget zone and heat affected zone is lower than that in HAZ due to the re-dissolution of precipitated phases in grains,and the distribution of hardness is close to "U".With the increase of rotating speed,the amount of precipitates in the heat affected zone increases,the precipitation strengthening effect is weakened,and the hardness value decreases more obviously.The results show that the grain structure and the distribution of micron second phase particles in each region of the joint after PWHT have no obvious change,but the precipitation phase with high density is precipitated in the grain,and the hardness of the joint after solution and re aging treatment is basically restored to the hardness of the base metal.The hardness of PWSA(470/48)sample is the highest.The hardness of the weld nugget zone can be improved obviously by direct aging treatment,but the softening phenomenon has not been improved in the adjacent heat affected zone due to the large precipitates.The hardness of PWAA(120/24)and PWAA(120/48)samples presents a "W" distribution.The corrosion of welded joint experiences pitting corrosion-intergranular corrosion-exfoliation corrosion.The origin of pitting corrosion is Al(Fe,Mn,Si,Cr)phase,which acts as cathode to promote the dissolution of matrix nearby.Continuous precipitation phase at grain boundary has more negative potential than matrix or non precipitation,which promotes the occurrence and development of grain boundary corrosion.With the increase of welding speed,the corrosion sensitivity of nugget zone decreases gradually,and the corrosion sensitive area in heat affected zone becomes wider.After different PWHT,the morphology and distribution characteristics of precipitates in grains and grain boundaries in each region of the joint change.The micro area difference of corrosion potential and current makes corrosion performance regionalized.The results show that PWSA(430/24)and PWAA(120/48)processes can improve the corrosion resistance of the weld nugget zone. |
PDF Full Text Request