Understanding Processing-Microstructure-Fatigue Performance Relationships for Defect-Tolerant Design in Laser Powder Bed Fabricated Al-10Si-0.4Mg Alloys

Piette, Timothy

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Understanding Processing-Microstructure-Fatigue Performance Relationships for Defect-Tolerant Design in Laser Powder Bed Fabricated Al-10Si-0.4Mg Alloys

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Additive manufacturing enables fabrication of lightweight components with high geometric complexity, and its broad adoption in high-integrity structural applications requires development of processing-microstructure-fatigue performance relationships and fatigue design tools. Laser powder bed fabricated Al-10Si-0.4Mg in several heat treated conditions (stress relieved, T6, and HIP+T6) was investigated with respect to microstructure characterization, tensile properties, and fatigue and long fatigue crack growth behavior. Crack initiation in polished specimens occurs from lack-of-fusion pores, which are not fully eliminated by HIP, although their partial closure improves fatigue lives. Crack growth properties of as-fabricated and stress-relieved conditions are lower than conventionally cast materials due to the fibrous eutectic silicon phase network not inhibiting crack propagation, whereas properties of T6 and HIP+T6 materials are improved by thermally modified silicon producing crack closure. Damage mechanism maps and Kitagawa-Takahashi diagrams were developed to relate loading conditions to fatigue behavior at all growth stages. To complement the establishment of crack initiation and growth mechanisms, the fatigue performance of as-printed surfaces was studied using specimens with varied surface contouring parameters. Surface-contoured specimens have longer fatigue lives than those without contouring but shorter than those with polished surfaces, which is attributed to crack initiation from surface intrusions in specimens with as-printed surfaces. Fractographic defect sizes are well-predicted from surface roughness scans, and in conjunction with fracture mechanics-based models, are used to extrapolate a limiting printing roughness for crack initiation from the surface. Finally, conventional and ultrasonic fatigue experiments, with and without the introduction of artificial defects, were used to study the defect tolerance of this alloy. The transition from size-dependent small crack growth to long crack growth was established, and fatigue lifetime predictions using Murakami’s √area parameter are made. These studies collectively establish fundamental insight into damage mechanisms in the novel Al-10Si-0.4Mg microstructure and develop pathways for its efficient design, optimization, and testing.

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