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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.

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.

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.

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.

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.

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.

Selective laser melting (SLM), as a revolutionary technology for metal manufacturing, attracts tremendous attention because it can produce complex components to benefit the customised production. Here we report that additively manufactured 304L austenitic stainless steel (SS) with low stacking fault energy (SFE) show superior fatigue resistance than its conventional counterparts due to the unique heterogeneous microstructure despite containing relatively high porosity. A series of detailed microstructural characterisations were applied to systematically disclose the fatigue enhancement mechanism of additively manufactured parts. Direct evidence is offered to show the obvious progressive work hardening and strain rate hardening caused by the heterogeneous microstructure during cyclic deformation, thus enhancing the fatigue crack initiation resistance. The microstructural results reveal that the cellular substructure plays a decisive role in regulating the dislocation motion during cyclic deformation, resulting in the intergranular fatigue cracking along HAGBs rather than twin boundaries.

Due to the stratified nature of the manufacturing process, material extrusion (ME) parts have lower mechanical properties than those fabricated by traditional technology. This is one of the most significant defects hindering the development and application of this rapid prototyping technique. In this paper, vibration was applied to the ME process by using piezoelectric ceramics for the first time to improve the mechanical properties of the built parts. The vibrating ME equipment was established, and the specimens processed in different build directions were individually fabricated without applied vibration and with different applied vibrations. To quantify the effect of applied vibration on their mechanical properties and to summarize the influencing rule, a series of experimental tests were then performed on these specimens. A comparison between the testing results shows that the tensile strength and plasticity of the specimens, especially those processed in the Z direction, can be obviously improved by applied vibration. The orthogonal anisotropy is decreased obviously. The improvement becomes greater with increasing vibration frequency or amplitude. From the microscopic point of view, it can be seen that applied vibration can reduce the part’s defects of porosity and inclusion as well as separation between layers and, thereby, improve the bonding strength.

With the increasingly wide application of fused filament fabrication (FFF) technique, the built products are inevitably exposed to dynamic mechanical loading and vibration. However, there has been no systematic study in the literature on understanding and characterization of dynamic mechanical performance for FFF products. In this paper, the dynamic characteristics of FFF plates are quantified, with the effect of different extrusion width taken into account. A dynamic model of the built plate with cantilever boundary conditions is established, and the inherent characteristics are predicted. Modal tests are then performed on these samples to obtain the measured data. Through the comparison between predictions and measurements, the theoretical model is validated. Different extrusion width makes the material properties of the plates different, resulting in different dynamic characteristics. The scanning electron microscopy (SEM) analysis on the samples confirms that the dynamic characteristic is deteriorated as the extrusion width decreases. This present work provides theoretical basis and technical support for further research in improving the dynamic performance of FFF products and helps extend the applications of this technique.

Medium-carbon steel is commonly used for the rail, wire ropes, tire cord, cold heading, forging steels, cold finished steel bars, machinable steel and so on. Its fatigue behavior analysis and fatigue life estimation play an important role in the machinery industry. In this paper, the estimation of fatigue life of medium-carbon steel with different surface roughness using established S-N and P-S-N curves is presented. To estimate the fatigue life, the effect of the average surface roughness on the fatigue life of medium-carbon steel has been investigated using 75 fatigue tests in three groups with average surface roughness (Ra): 0.4 μm, 0.8 μm, and 1.6 μm, respectively. S-N curves and P-S-N curves have been established based on the fatigue tests. The fatigue life of medium-carbon steel is then estimated based on Tanaka-Mura crack initiation life model, the crack propagation life model using Paris law, and material constants of the S-N curves. Six more fatigue tests have been conducted to validate the presented fatigue life estimation formulation. The experimental results have shown that the presented model could estimate well the mean fatigue life of medium-carbon steel with different surface roughness.