Microstructure evolution, fatigue crack growth, and ultrasonic fatigue in As-fabricated laser powder bed and conventionally cast Al-10Si-0.4Mg: A mechanistic understanding and integrated flaw-sensitive fatigue design methods

Additive manufacturing (AM) enables fabrication of lightweight components with high geometric complexity, and its broad adoption in high-integrity structural applications relies on a fundamental understanding of the processing-microstructure-fatigue performance relationships and development of fatigue design methods. In this study, microstructures, tensile properties, and long and small fatigue crack growth (FCG) behaviors of laser powder bed (LPB) manufactured Al-10Si-0.4Mg were systematically investigated in both orientations with respect to the build direction, and compared to those of an equivalent conventionally cast material. Tensile properties of the LPB material demonstrated orientation dependency, and exceeded on average those of the cast material, with higher yield (>170% increase) and tensile (>150% increase) strengths and enhanced elongation (88% increase in horizontal orientation). In contrast, long FCG properties at various stress ratios showed minor differences associated with orientation (attributed to residual stress), and were lower compared to the cast alloy, particularly in the near-threshold regime. Mechanisms of FCG were identified at all growth stages using fractography and electron backscatter diffraction, and further integrated with the loading conditions by creating dual-parameter load-microstructure-damage mechanisms design maps. Ultrasonic fatigue testing was complementary conducted on the LBP-fabricated materials to study the very-high cycle fatigue regime (for both orientations and different surface conditions), and the results were correlated with the FCG data using Kitagawa-Takahashi diagrams to unify the two fatigue testing methods, and together with the dual-parameter maps, provide a microstructure-sensitive design toolset for fatigue crack initiation and growth resistance.

Automated summary

This study systematically investigated the microstructures, tensile properties, and fatigue crack growth behaviors of laser powder bed manufactured Al-10Si-0.4Mg material, compared to an equivalent conventionally cast material. The LPB material showed orientation-dependent tensile properties with higher strengths and enhanced elongation, while minor differences in fatigue crack growth properties were observed, particularly in the near-threshold regime. Mechanisms of fatigue crack growth were identified and integrated with dual-parameter load-microstructure-damage mechanisms design maps, providing a microstructure-sensitive toolset for fatigue crack initiation and growth resistance.