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Aiming at the removal of native oxide film on 2024-T3 Al alloy before plasma electrolytic oxidation, the methods to reduce the fatigue damage caused by grit blasting are explored. Effects of the grit size and pressure on surface roughness, residual compressive stress (RCS), and fatigue life of the Al alloy are studied. Increasing grit size and pressure leads to the increase in the surface roughness and RCS. Based on the theorem of momentum and tool cutting principle, mechanism of the grits on the craters and RCS is revealed. It is found that the angularness of the small-sized grit results in the formation of long scratches. The grit with high kinetic energy induces the large craters and high RCS. The craters are the primary fatigue crack initiation source. Moreover, the craters with long scratch and large height lead to the premature fatigue failure of the grit blasted specimens. However, Fatigue behavior is insignificantly affected by the RCS. The results provide new insights into the fatigue deterioration mechanism and life optimization of the grit blasted samples. Consequently, a parameter optimization scheme for the fatigue life of the grit blasted Al alloy is proposed. more...

The stress analysis of micro-arc oxidation (MAO) coated aero Al alloys is the basis to reveal the mechanism of fatigue deterioration. To clarify effects of the coating defects on fatigue life, a stress calculation model with micropores and residual stress is established based on shear lag model in this study. The effects of porosity, substrate elastic modulus, coating thickness and residual stress on the stress are explored. The results indicate that the coating porosity and thickness are the critical factor affecting coating normal stress and interfacial shear stress, respectively. In addition, the residual stress significantly affects the interfacial normal stress. The increasing stress caused by low porosity and residual tensile stress and stress concentration at the large-sized defects are the key factors for fatigue degradation. more...

To predict the geometric characteristics and improve the forming quality and macro-micro defects of multi-track overlapping cladding layer effectively, the experimental research of laser melting deposition single-track Co-based alloy on 42CrMo substrate was carried out. The influence trends of laser process parameters on the single-track cladding height and cladding width were analyzed, and the regression prediction models were established. A variable-process alternated overlapping strategy was proposed using the geometric functions to fit the single-track profile. The surface forming quality and macro-micro defect of different overlapping cladding layers were compared and evaluated. The results showed that the cladding height and cladding width regression models established by an improved optimization algorithm had an excellent prediction effect. The variable-process alternated overlapping cladding layer prepared by the parabolic function had a flat surface, and the fluctuation distance was 81.56 μm, which was significantly improved compared with other overlapping strategies. The surface flatness of the continuous overlapping cladding layer was the worst and the spheroidization defect was the most unsatisfied. The internal defects of the constant-process alternated overlapping cladding layer were the most terrible, with more non-fusion and cracks caused by oxides. The variable-process alternated overlapping strategy proposed in this study can effectively prepare a multi-track cladding layer with a smooth surface and dense interior, which provides essential theoretical guidance for the near-net forming of component surface deposition and repair. more...

Unavoidable issues of micro-arc oxidation (MAO) coated Al alloys are the micro-defects and residual stress generation. Some scientists have reported the effects of micro-defects and residual stress on the fatigue performance of MAO coated samples. However, the influence of the residual stress sign in the MAO coating and substrate beneath the interface on the fatigue damage behavior of the substrate was controversial. Moreover, the coupling mechanism between micro-defects and residual stress on the fatigue performance of the MAO coated samples was nebulous. These present a challenge for the collaborative design of micro-defects and residual stress to optimize fatigue performance. Aiming at the issues, this paper summarized the influencing factors of MAO coatings on the fatigue performance of the substrate. From two aspects of microstructure characterization and stress analysis, the coupling mechanism between micro-defects and residual stress in fatigue failure was elucidated. And the mechanism of the micro-defects and residual stress on the fatigue damage behavior was developed. Finally, strategies to optimize the fatigue performance of MAO coated Al alloys were proposed. The results of this study have great guiding significance in regulating micropores, cracks, interface defects, and residual stress in MAO coatings in order to extend fatigue life. more...

Preparation of coatings with excellent properties is an effective way to address poor wear and corrosion resistance of aluminum (Al) alloys. Micro-arc oxidation (MAO) coatings exhibited high hardness, good adhesion, excellent corrosion and wear resistance. Unfortunately, defects such as micropores, cracks, overgrowth regions, and rough surface, as well as residual stress in MAO coatings are unavoidable. This entails that surface properties associated with corrosion resistance, wear resistance, and adhesion, as well as fatigue performance (SPAFP) of the MAO coated samples were more difficult to be controlled. In response to this issue, defects and residual stress control of MAO coating aiming at SPAFP optimization were studied. In this paper, the formation mechanism of MAO coating–effects of parameters on defects and residual stress–effects of defects and residual stress on the SPAFP of the MAO coated Al alloys were reviewed. Furthermore, an optimized control strategy for SPAFP was summarized. Coupling effect of micropores, cracks, rough surface, overgrowth regions, and residual stress on the SPAFP of MAO coated Al alloys was focused on. Aiming at the fatigue damage behavior, the anti-fatigue design method for the MAO coated Al alloys was proposed. more...

Ambiguous fatigue failure mechanism of micro-arc oxidation (MAO) coated samples was an urgent issue to be solved. In this paper, the influence mechanism of coating defects on the fatigue property of the samples with thin MAO coating was studied. Based on the stress analysis of the MAO coated 2024-T3 Al alloy, the effect of the MAO coatings produced using 8%, 10%, 15%, and 20% duty cycles on fatigue behavior was discussed. Fatigue fracture and strain under cyclic stress were measured with SEM and extensometer, respectively. The results showed that the maximum cyclic stress affected the initiation of fatigue cracks. With the increase in the external loading, stress variation of the coatings was the critical issue that caused the changes in the fatigue performance under the high and low cyclic stresses. Furthermore, the fatigue failure mechanism of the samples with the thin MAO coatings was revealed. This study provided a method basis for further exploring the fatigue property of the samples with the thick MAO coatings. more...