| Magnesium alloy as the lightest metal structure material is widely used in practical projects,and is very useful for lightweighting,energy saving and emission reduction in aerospace,transportation and other fields.The welding quality of magnesium alloys is closely related to the safety of vehicles.However,due to the lively chemical nature of magnesium alloys,under the combined influence of external stress and corrosive environment,the risk of stress corrosion damage and corrosion fatigue damage of magnesium alloys and their joints is greatly increased.There are great security risks.Therefore,the analysis of the crack mechanism and laws by studying the corrosion resistance and corrosion crack propagation behavior of magnesium alloys and their joints is of great value for practical engineering applications.In this paper,well-formed rolled AZ31 magnesium alloy joints were obtained by the friction stir butt welding,and the basic mechanical properties of the joints and the corrosion resistance of each area were tested,proving that the retreating side is relatively weak.The slow strain rate tensile tests were performed on each area of the joints,and the stress corrosion sensitivity index(ISSRT=0.776)of the joint is greater than the base material(ISSRT=0.635).Hydrogen embrittlement cracks were found on the side of the specimen fracture.The constant load method was used to conduct stress corrosion crack growth test in each area of the joint in 1 wt.%Na Cl solution.The test shows that the threshold value of the stress corrosion intensity factor KISCCfrom large to small is followed by thermal-mechanically affected zone in advancing side(23.240 MPa·m1/2),welding nugget(20.623 MPa·m1/2),thermal-mechanically affected zone in retreating side(15.605 MPa·m1/2),heat affected zone(13.201 MPa·m1/2)and base material(12.663 MPa·m1/2).The thermal-mechanically affected zone in advancing side has the best resistance to stress corrosion cracking,which is the least prone to stress corrosion cracking,and the worst is base metal.The stress corrosion crack growth rate da/dt from large to small is followed by weld nugget(5.90×10-3mm/s),thermal-mechanically affected zone in advancing side(2.26×10-3mm/s),heat affected zone(2.21×10-3mm/s),thermal-mechanically affected zone in retreating side(2.20×10-3mm/s)and base metal(1.98×10-3mm/s).The cracks in weld nugget propagated fastest during the stable expansion stage,and the base metal propagated the slowest.The stress corrosion cracks in each area of the joint were dominated by transgranular growth,and the crack growth involved anode dissolution and hydrogen embrittlement.The?-Mg is distributed with?-Mg17Al12second phase particles in AZ31 magnesium alloy,which act as cathode to accelerate the anode dissolution of the material.Hydrogen embrittlement occurs when the hydrogen generated on cathode and from 1wt.%Na Cl solution diffuses into the metal.Twin crystals were often found in the coarse grains near the crack tip of the base material and the heat-affected zone,and the crack always propagates toward the soft-oriented structure.Corrosion fatigue crack growth tests in various areas of joints show that the addition of corrosive media significantly deteriorated the fatigue performance of the material,and reduced the threshold of crack growth in each area and increased the crack growth rate.The corrosion fatigue crack growth rate of the thermal-mechanically affected zone in retreating side is the fastest,which has coarse grains.The crack growth of the weld nugget zone is the slowest.The cracks were dominated by transgranular propagation and involved anode dissolution and hydrogen embrittlement.The crack tip opened and closed alternately under the action of cyclic load,which greatly reduced the contact of the corrosive medium.At this time,the corrosion resistance is not the dominant factor,and the microstructure of each area seriously affects the corrosion fatigue crack growth behavior. |
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