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• Anodic oxidation coating's cracking causes a severe substrate stress concentration. • Sealing weakens anodic oxidation coating's cracking resistance during fatigue tests. • Prior shot peening enhances the fatigue property of anodic oxidation coated specimen. In this study, the effects of sealing and prior shot peening treatments on the fatigue behavior of the sulphuric acid anodic oxidation coated 6082-T6 aluminium alloy were investigated. Experimental results indicate that the preferential fracture of anodic oxidation coatings due to its higher brittleness and lower fracture toughness as well as the stress concentration of near-interface substrate caused by the interface cyclic dislocation accumulation could cause the fatigue failure of anodic oxidation coated specimens. Consequently, the fatigue property of anodic oxidation coated specimens notably decreases compared to as-machined specimens, with a decrease of 26.7 % in the fatigue limit corresponding to 2 × 107 fatigue cycles. Additionally, the sealing treatment introduces numerous cracks that penetrate the coating entirely, diminishing the coating's resistance to cracking under fatigue loading; this alteration changes the fatigue failure mechanism of anodic oxidation coated specimens, ultimately resulting in a decrease of 15.5 % in the fatigue limit of anodic oxidation coated specimens. Despite exacerbating the surface integrity and increasing the surface roughness of anodic oxidation coated specimens, the prior shot peening treatment increases the fatigue limit of anodic oxidation coated specimens by 82.8 % since the affecting layer beneath the interface plays a crucial role in inhibiting the further propagation of coating cracks into the substrate as well as the crack propagation in the substrate beneath the interface. [ABSTRACT FROM AUTHOR]

• Prior shot peening improves the CF limit of AO coated aluminum alloy by 50 %. • AO specimens exhibit different CF failure modes under low and high applied stress. • Substrate residual stress and microstructure affect the CF crack propagating path. • Substrate residual stress relaxes in distinct mode under low and high applied stress. Plain (anodic oxidation) and composite (prior shot peening + anodic oxidation) coatings were prepared to investigate their effects on the corrosion-fatigue (CF) property of 6082-T6 aluminium alloy in a 3.5 wt% NaCl aqueous solution. The results indicate that plain coated specimens exhibited similar CF properties to as-machined specimens at low applied stresses. However, under high applied stresses, the plain coating led to a preferential fracture and was unable to effectively prevent corrosion medium. Hence, the CF property of plain coated specimens was found to be inferior to as-machined specimens at high applied stresses. The synergetic effect of substrate residual stress and microstructure beneath the coating-substrate interface reduced the crack propagating rate in the initial stage by deflecting the crack at the grain boundary. Therefore, the CF property of composite coated specimens improved as a whole when compared with as-machined and plain coated specimens. Besides, under low applied stresses, the near-interface substrate residual stress of composite coated specimens occurred the relaxation due to the substrate crack propagation, while under high applied stresses, this was due to the synergetic effect of materials' local cyclic yield and plastic flow, as well as the substrate crack propagation. [ABSTRACT FROM AUTHOR]

Low, medium and high-intensity (0.19, 0.26 and 0.34 mmA) shot peening treatments and 0.34 mmA high-intensity shot peening combined with slight and severe polishing treatments were used as the pre-treatments to improve the fatigue property of micro-arc oxidation (MAO) coated 6082-T6 aluminum alloy, respectively. In consideration of the significance of substrate residual stress beneath the coating-substrate interface on MAO coated specimens' fatigue property, the substrate residual stress of various coated specimens was analyzed by the X-ray's powerful penetrability. Results indicate that the substrate residual stress state of MAO coated specimens is changed from tensile (+93 MPa) to compressive (−65 to −93 MPa) by the above pre-treatments, which is beneficial for inhibiting the crack initiation at the substrate or resisting the crack's further propagation from the coating into the substrate. Whereas, similar to the compressive residual stress induced by shot peening, the compressive substrate residual stress for prior shot peening + MAO composite coated specimens will also relax during the fatigue tests. In addition, the gradient compressive residual stress beneath the interface induced by prior shot peening reduce the crack propagating rate, thus improving the fatigue property of coated specimens; after the 0.26 mmA shot peening pre-treatment, the fatigue strength at 2 × 107 cycles of MAO coated specimens increases by 59.1 %. Finally, the effects of surface and interface roughness, substrate residual stress and in-depth compressive residual stress on the fatigue life of MAO coated specimens are further discussed. • Pre-treatments improve MAO coated Al alloys' fatigue life by 2.41 to 4.35 times. • Grain refinement of substrate increases MAO coatings' pore size and thickness. • Compressive substrate residual stress inhibits crack propagation beneath interface. • Compressive substrate residual stress relaxes by 8 % to 41 % at 104 fatigue cycles. [ABSTRACT FROM AUTHOR]

• CSP and MSP are used to improve the fatigue property of bare and MAO coated Al. • CSP and MSP introduce a grain-refined and CRS affected layer on surface. • CSP and MSP is more appropriate for MAO coated and bare Al, respectively. • Crack initiation stage dominates the fatigue property of bare Al. • Crack propagation stage dominates the fatigue property of MAO coated Al. Conventional shot peening (CSP) and micro-shot peening (MSP) are used to enhance the fatigue property of bare and micro-arc oxidation (MAO) coated 6082-T6 aluminum. The difference in the anti-fatigue strengthening mechanism of CSP and MSP on bare and MAO coated 6082-T6 aluminum alloy is revealed. Results show that MSP provides a more significant improvement in fatigue properties of bare specimens than CSP since the intensely inhibiting effect on crack initiation. Nevertheless, MAO coating impairs the surface integrity of MSP specimens, thus reducing the crack initiating life. In comparison, the effect of deeper affected layer induced by CSP on the improvement of crack propagating life is mostly retained after the MAO process. Hence, for the MAO coated specimen, the CSP prior-treatment is more effective in improving its fatigue property. [ABSTRACT FROM AUTHOR]

The effects of micro-shot peening (MSP) and artificial surface defects were investigated in relation to the defect tolerance of Al-Si-0.6Mg alloy in the framework of fracture mechanics. The results indicated that coral-shaped casting defects inside the alloy could be effectively quantified using the X-ray synchrotron radiation technique. An affected layer with a surface roughness (Ra) of 1.25 and residual stress of − 125 MPa was introduced on the MSP specimen compared to the un-peened (UP) specimen. The fatigue strengths of the UP and MSP specimens containing the artificial defects decreased as the defect size increased under rotating bending fatigue tests, except in the case of defects with the equivalent size of 395 and 490 µm, respectively. MSP could shift the crack initiation site toward the subsurface, while all the cracks of the specimens with the artificial defects originated from the surface and maintained an aspect ratio of 0.66 during the crack growth process. The relationship between the defect sizes and the fatigue strengths could be reasonably assessed by the modified Kitagawa-Takahashi (K-T) diagrams. The critical defect size of the MSP specimens was increased from 450 to 542 µm compared with that of the UP specimens, owing to the beneficial effect of the compressive residual stress. The defect tolerance of the Al-Si-0.6Mg alloy was improved by the MSP process. • Effects of micro-shot peening (MSP) on the fatigue properties of Al-Si-0.6Mg alloy are explored. • Casting defect inside the alloy was identified using synchrotron radiation X-ray computed microtomography (SR-μCT) technique. • The influence of MSP on the fatigue tolerance was characterized by Kitagawa-Takahashi (K-T) diagrams. • The critical defect size harmless to fatigue strength was evaluated using fracture mechanics. [ABSTRACT FROM AUTHOR]