Abstract:The effect of deep cryogenic treatment on the microstructure and corrosion resistance of an AZ61 magnesium alloy metal inert-gas (MIG) welded joint was investigated. The welded joints were deep-cryogenically treated using different parameters, and were analyzed by metallographic observations, X-ray diffraction, microhardness, and NaCl immersion test. The results show that the treatment changes the microstructure of the entire joint by causing grain refinement and increase in the β-phase. The crystal structure, chemical composition, and electrode potentials are different for the α and the β-phases. Therefore, any change in content, status, and distribution of the β phase has important implications on the corrosion resistance of the samples. With an increase in the holding time, the microhardness value of the whole joint increases, and the difference between the three regions of the joint decreases. The results of the NaCl immersion test show that there is an improvement in the corrosion resistance of the treated joints. Compared with the untreated samples, the corrosion potential of the samples treated by deep cryogenic treatment for 4 h at −180 • C is about 0.017 V higher, and the corrosion current density decreased by one order of magnitude (from about 2.769 × 10 −5 A·cm −2 to 1.578 × l0 −6 A·cm −2 ).

In this paper, a numerical scheme is utilized to solve three-dimensional nonlinear system of Volterra-Hammerstein integrals equations, which is based on the three-dimensional block-pulse functions (3D-BPFs) and their operational matrices. Then the primary nonlinear system is transferred into a linear system of algebraic equations by applying the approximate expression and operational matrices, which can be easily solved through any numerical techniques. According to the convergence of 3D-BPFs, the new convergence analysis and error estimation theorem of the research system is detailed investigated. Lastly illustrative examples are included to demonstrate the validity and applicability of the technique.

No abstract

This work analyzes the Tungsten Inert Gas (TIG)welded joint of AZ31 magnesium alloy and the pitting corrosion stress field at different depths under tensile load via the ANSYS finite element method. The strengths of the stress and strain fields inside the corrosion pit were substantially higher than the average strength level of the matrix. Therefore, under the influence of the applied load, the corrosion pit will fail first. Based on the study of pitting corrosion, a model was developed to explain how pitting corrosion cracks propagate in magnesium alloy welded joints. Finally, the variation of residual strength was revealed to study the corrosion dynamic mechanism of the joints.

The welding test of 7mm thick AZ61 magnesium alloy plate was carried out by MIG welding; the joint was cryogenically treated at -180℃, and the holding time was 4h, 8h and 12h respectively; Weight loss method and electrochemical method were used to test the influence of cryogenic treatment on the corrosion resistance of welded joints before and after cryogenic treatment, and the corrosion morphology of specimens was observed by SEM. The test results show that the corrosion resistance of the joint sample reaches the best when the cryogenic treatment temperature is -180℃ and the holding time is 8h.