Micro-arc oxidation (MAO) is performed on Ti-6Al-4V electron-beam-welded joints, and the microstructure, phase composition, and corrosion resistance of the joint and surface coating are investigated systematically by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), as well as electrochemical and stress corrosion analyses. SEM analyses revealed that the joint undergoes a phase transformation. The coating morphology of the joint and base materials is similar, but the joint coating is denser and thicker. XRD analyses recognize the rutile and anatase phases in the coating. Polarization and electrochemical impedance spectroscopy (EIS) corrosion tests reveal that the MAO treatment results in a decrease of two orders of magnitude in the corrosion current density of the welded joint and an increment of corrosion resistance. Stress corrosion evaluation reveals that a dense layer is exposed to protect the joint after long-term exposure to a high-stress corrosion environment. No stress corrosion-induced cracking or defects are observed in the joints, indicating the corrosion resistance of the joint has significantly improved.

The fatigue crack growth (FCG) behavior of 34CrMo4 steel, a typical material for gas cylinders, has been investigated. Specimens were taken from the base material (BM) as well as the hot-drawn (HD) cylinder and cold-flow (CF) formed cylinder along the longitudinal and transverse directions. The FCG tests were conducted under different stress ratios for different materials and directions. The main purpose of this research was to explore the influences of the mechanical and thermal processes, sampling direction and stress ratio on the FCG behavior of 34CrMo4 steel. To further reveal the mechanism of crack propagation at different stages, the microstructures and fracture modes of FCG specimens were analyzed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), respectively. The results showed that HD and CF materials exhibited better resistance to fatigue crack propagation than BM. The FCG rates of investigated materials can be accelerated by the increase in stress ratio. However, the sampling direction had little effect on the FCG rate. Finally, a driving force parameter (DFP) model was used to fit the experimental FCG data of three materials with different mechanical and thermal processes. A unified transition stage between the stable and unstable FCG stages of three materials under various experimental conditions was revealed by DFP model, playing an important role on the early warning of fatigue fracture for different types of 34CrMo4 steel.