Study On Mechanical Properties And Stress Corrosion Behavior Of A7N01S-T5 Aluminum Alloy MIG Welded Joints

As a lightweight material, high strength aluminum alloy has been widely used in high-speed trains, subways and light railways. The A7N01S-T5 aluminum alloy, a new kind of material used in high speed train, is essentially applied in the manufacture of the train base buffer, traction force and other key parts of the beam. Due to the variability and complexity of the operational environment, the mechanical behaviors of the welding joint are becoming the key to the stability and effectiveness of welding structures. The micro structure of A7N01S-T5 aluminum alloy welded joints, static mechanical properties, impact toughness or high-frequency fatigue behaviors under low load are investigated in this article, and the stress corrosion behaviors or corrosion mechanism of welded joint are analyzed either, which are of both theoretical and practical significances for the operation safety of high-speed trains and the development of welding materials.MIG welding is employed to joint the A7N01S-T5 aluminum alloy. The acquired joints are processed into standard samples to study microstructure, toughness, failure mechanism and stress corrosion behaviors. The MTS810 servo material test equipment, semi-shock devices and high-frequency fatigue testing equipment are used to carry out mechanical failure tests of welding samples. The slow strain rate tests are performed by using slow strain rate testing machine. Advanced detection analysis methods, such as the metallographic technique (OM), X-ray diffraction (XRD), scanning electron (SEM) and energy spectrum (EDS), are also utilized to study the microstructure, failure mechanism, fracture morphology and physical phase ingredients of the welded joints.Results of experiments show that serious heterogeneity of microstructure exists in the welding joint, and both the fusion zone and heat affected zone (HAZ) have the worst mechanical properties. Microstructures in weld zone are chiefly equiaxed dendrites. Main strengthening phases, the a (Mg2Al) phase and ? (Mg2Al3) phase, are partially dissolved. Coarse columnar grains form in both fusion zone and heat affected zone. The ? phase (MgZn2) agglomerates along grain boundaries but solids in the aluminum substrate with a spherical shape in fusion zone. Some precipitation-free regions are coming into being in HAZ while strengthening phases still distribute uniformly in base metal. Hardness test results show that the hardness of both the weld zone and fusion zone are lower than that of other regions, and there records the lowest value (70HV) near the boundary of the HAZ and the fusion zone. Results of the static tensile test indicate that welding coefficient is only 67% and the tensile strength decreased by 37%. The weld joints fracture in the fusion zone, moreover cracks originate in hydrogen porosities and brittle impurities.Results of the impact tests demonstrate that the weld zone has the best toughness in an instant impact load with an impact fracture energy value of 32.3 J, meanwhile the HAZ has the worst one with a value of only 29.5 J. Analysis indicates that the coarsening effect of grains, transient increases in dislocation motion velocities and thin film of low melting-point eutectic are the most important factors that influence the impact resistance of the weld zone and HAZ. Results of the fatigue tests indicate that the fatigue life of welded joint and the base metal declines along with the increase of frequency and stress amplitude, and the weak positions of welded joint locate at both the weld toe and the fusion zone. Cracks come from the gathering of dislocations at grain boundaries, segregations of impurities and brittle phases at grain boundaries, and the formation of precipitation-free regions by the breaks of fine non-coherent or semi-coherent second phase particles.The polarization experiments and stress-corrosion test suggest that corrosion voltage and the corrosion current density are-1324mV and 0.03mA/cm2, respectively, and the susceptibility of self-corrosion at joint area is higher than that of the base metal. The strengthening phases in materials lose their effect under corrosive environment. When the strain rate increases, the fracture absorbing energy of base metal and joints reduces while the strength and ductility loss increase. The stress corrosion of A7N01S-T5 welded joint accelerates when the strain rate is 5×10-6mm/s. There occurs the lowest mechanical properties of welded joint with a strain rate of 9×10-6mm/s and the sensitivity index of stress corrosion of the joint reaches 0.42, which indicates a severe corrosion. The stress corrosion cracking (SCC) mechanism of A7N01S-T5 welded joints is that pitting in base metal and welded joint is formed from the slip mechanism of oxidized film at surface. Powered by strain and corrosion medium, the generated hydrogen intrude into pitting with the corrosion medium, which accelerates the embrittled propagation of cracks.The coarse strengthening segregation phases along the grain boundary in base metal dissolute as the anode, and the intergranular corrosion tunnels take shape. For the types and content of strengthening phases contained are different, micro batteries in corrosive media are formed between structures in weld zone and HAZ. The weld zone dissolves first as the anode and pitting forms, so corrosion cracking appears in the weld zone close to HAZ. Corrosive dissolution results in a change in content of strengthening phases and alloying elements, and a multi-potential system comes into being in HAZ. The intergranular corrosion tunnels and rings will not stop to be formed by the alternative dissolution of base metal and strengthening phases until the corrosion fracture takes place.