Thermal management for in-situ heat treatment and defects mitigation in wire arc additive manufacturing of maraging 250-grade steel

Xu, Yao

Etd

Thermal management for in-situ heat treatment and defects mitigation in wire arc additive manufacturing of maraging 250-grade steel

Public

Wire arc additive manufacturing (WAAM) is suitable for near-net-shape manufacturing of large-scale components due to high deposition rates and low feedstock costs. However, the accumulated heat during fabrication increases printing instability and results in mechanical anisotropy due to microstructure evolution in the as-fabricated components. Therefore, understanding the impact of thermal history, including thermal cycles and heat accumulation, on the microstructure evolution is critical for process design. The present thesis demonstrates the feasibility of utilizing the accumulated heat to engineer the thermal history based on the process-thermal-microstructure-property relations to realize in-situ heat treatment. The first task focuses on the comprehensive characterization of an as-fabricated 250-graded maraging steel thin wall to investigate the mechanisms contributing to the overall microstructure evolution as a function of wall height. Strengthening mechanisms, including prior austenite grain and martensitic block refinement and the increase in the volumetric fractions of precipitates, are characterized. On the other hand, the softening mechanisms, including martensite tempering and an increase in retained austenite, are observed. The competition between strengthening and softening effects gives rise to the three-stage hardening in the as-fabricated thin walls. The anisotropy of tensile properties, including yield strength, tensile strength, and ductility, are quantified. The formation mechanisms of overflow defects for thin-structure deposition are summarized. The overflow is mitigated by reducing the torch raise-up distance to avoid excessive melting, which expands the process window for preheating and short interlayer dwell time. Lastly, oxide inclusions with AlO@TiO core-shell structures are formed during fabrication, lowering the ductility. The defect of micron-scale oxide inclusions is mitigated by reducing the CO2 content in the shielding gas. The second task focuses on developing in-house capabilities to estimate the thermal history of the WAAM process. A finite element model is established using the COMSOL Multiphysics® platform. A detailed calibration of arc efficiency and arc size is performed using a gradient descent algorithm. The convection coefficient of maraging 250 is also calibrated by fitting the cooling behavior predicted by the model with the temperature profile measured experimentally. The calibrated and validated model can estimate the transient thermal histories, the cooling rates, and the weld pool geometry for different processing parameters. The third task explores the possibility of engineering the thermal history by changing the interlayer dwell time and the preheating temperature for in-situ heat treatment. Simulation results show that the heat accumulation increases with shortening the dwell time. The heat accumulation prohibits austenite to martensite transformation when the workpiece temperature is higher than Ms. The thin walls with substrate preheating temperatures higher than Ms do not show the grain refinement effect minimizing the hardness increase in the as-fabricated components. Together with the negligible age-hardening resulting from short interlayer dwell time, the as-fabricated thin wall demonstrates uniform hardness. This result proves the feasibility of tailoring the mechanical properties by controlling the microstructure evolution via designed thermal history.

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