| The 5083-H321 aluminum alloy plates were jointed by friction stir welding (FSW) based on mini machine. The optical microscope (OM), transmission electron microscope (TEM), electron backscatter diffraction (EBSD) were employed to study the microstructure evolution at different zones of the seams by FSW. The mechanical properties, stress corrosion cracking (SCC), exfoliation corrosion and electrochemical behavior were investigated by tensile machine, micro-hardness tester, scanning electron microscope (SEM) and potentiodynamic scanner. At last, the behaviors of FSW were investigated. The main results are as follows:1.The FSW joint macrostructure was investigated for 5083 aluminum alloy. The well joints are characterized by the nugget zone (NZ), thermo-mechanical zone (TMAZ) and heat affected zone (HAZ). The appropriate heat input is the essential condition for attaining well FSW joints. The insufficient heat input leads to the formation of "kissing bond" defect. On the contrary, the sufficient heat input leads to the formation of flash defect at the advancing side and the "tunnel" defect inside the joint.2.The microstructure evolution was investigated at the different FSW joint zones of NZ, TMAZ and HAZ. The NZ is characterized by fine equiaxed grains, the TMZA is characterized by the grains elongated along the material flow direction, and the HAZ is characterized by growth grains. The average grain diameters at the zones above is approximately 10μm,16μm, respectively and 50μm. The fine structure is attributed to the dynamic recrystallization and severe plastic deformation at the zone of NZ and TMZA. The dynamic recrystallization and severe plastic deformation leads to the increment of high angle grain boundary. In rolling parent metal, the percent of grain boundaries more than 10°is 37%. However, the percent of grain boundaries more than 10°is 66% at the NZ of FS W joint.3.The mechanical properties of FSW joints were investigated for 5083 aluminum alloy plate. The tensile strength is 306MPa, closing to 90% of the parent metal which is uncommonly higher than that of fusion welding. The elongation is 16.3%, equaling to the parent metal. The micro-hardness is 85HV, reaching to 90% of the parent metal. Comparing with other series of aluminum alloy, the hardness distribution is uniform. The refining structure is the substantial reason of the properties above. 4.The joint corrosion resistance of 5083 aluminum alloy plates was investigated, and the mechanisms of stress corrosion cracking and exfoliation corrosion were studied. The FSW joints are not susceptive. In 3.5%NaCl solution, the joint tensile strength is 335 MPa, reaching to 98 percent of the parent metal, and the elongation is 22 percent, far more than that of the parent metal. The joint fracture appears ductile in solution. The uniform exfoliation corrosion occurs on the parent metal surface. For FSW joints, the HAZ has better exfoliation corrosion resistance, and the severe exfoliation corrosion occurs the on the friction ring surface. The increment of specific surface leads to the corrosion easier. The refining structure and the uniform distribution of the second phase are the substantial reason of better corrosion resistance.5.The heat and temperature effect, material flow and mechanism of refining structure were investigated for 5083 aluminum alloy plates. The heat derives from friction and severe plastic deformation. The peak temperature in the weld is more than 500℃. The heat transmission not only affects the material flow, but the heat deformation of dynamic recrystallization. The "onion rings" formation is related to the material flow in the thread flute. The fine structure is attributed to the dynamic recrystallization and severe plastic deformation.
|
PDF Full Text Request