Design and Qualification of Wrought and Additively Manufactured 4340 Steels for Fatigue-Critical and Damage-Tolerant Applications

Ryder, Matthew

Etd

Design and Qualification of Wrought and Additively Manufactured 4340 Steels for Fatigue-Critical and Damage-Tolerant Applications

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Additively manufactured (AM) parts offer unparalleled design flexibility and are increasingly considered for use in structural applications. The unique processing-structure-performance relationships in AM materials distinguish them from conventional materials and require study of their fatigue lifetimes and damage-tolerant behavior for adoption in such applications. A low alloy steel (4340) has been studied in AM as-fabricated and heat treated conditions and compared with equivalent wrought materials having identical heat treatments. Processing parameters for the bulk AM builds were first established by melt pool optimization studies to identify an ideal melt pool morphology from a large matrix of laser power and scan speed combinations. The optimized depositions were systematically investigated to establish an understanding of the highly refined, anisotropic, and hierarchic microstructures of the as-fabricated AM materials, together with the evolution of bulk fabrication residual stresses. The effect of the heat treatments on residual stress minimization and microstructure homogenization was further assessed and correlated to changes in the materials’ mechanical properties. Fatigue (up to the gigacycle regime) and fatigue crack growth studies have been conducted in different conditions to understand the crack initiation and propagation mechanisms with respect to the materials’ characteristic microstructural features (e.g. melt pool boundaries, prior austenite grain boundaries, and martensite packets) and typical inclusions (e.g. MnS and Al2O3 – wrought materials) or build defects (e.g. gas porosity, unmelted powder particles, and lack of fusion pores – AM materials). Fatigue studies further reveal the impact of porosity and inclusion mitigation in AM materials as compared to the wrought counterparts and highlight the ability for AM materials to achieve near-identical performance when the amount/morphology of these features are well controlled. The knowledge and data developed in this research will be used to support the optimization of the AM-fabrication and post-processing of these materials and develop fatigue design tools, including S-N and Kitagawa-Takahashi diagrams, which will enable the use of these materials in safety-critical applications and establish suitable operating parameter spaces.

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