CAREER: In Operando Investigation of Laser Powder-fed Directed Energy Deposition: Process Physics, Microstructure Evolution, and Mechanical Properties

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The disruptive innovation of additive manufacturing relies on its ability of mass customization of complex-shaped products that are impossible or difficult to make by conventional manufacturing methods. However, poor processing repeatability is one of the major obstacles to its widespread use, especially for critical load-bearing metallic components. The huge variability, for laser powder-fed direct energy deposition in particular, is due to extremely high heating and cooling rates, which occur during stochastic interactions of the powder flow stream with a shape-changing liquid metal pool and cause far-from-equilibrium and highly dynamic phenomena that are extremely challenging to study. This Faculty Early Career Development (CAREER) award supports the study of using high-energy, high-speed x-rays to achieve fundamental understanding of the aforementioned transient phenomena. Experiments at different time and length scales will be performed to establish process and characteristic relationships to predict quality, repeatability and properties of additively manufactured parts. The educational and outreach activities aim to promote a full participation of women and underrepresented minorities in advanced manufacturing and to develop a well-trained, diverse and sustainable manufacturing workforce. Over the project course, the PI will work with middle school girls, undergraduate women and underrepresented minorities through organizing workshop series on additive manufacturing with hands-on activities, attending lunch series and mentoring students in a 10-week summer program, respectively.The main research objective of this project is to transform the powder-fed direct energy deposition by understanding the fundamental physics of the process. A custom-built directed energy deposition apparatus will be interfaced with the facility at Cornell High Energy Synchrotron Source to perform unique experiments. Synchrotron hard x-ray imaging and diffraction techniques with unprecedented spatial and temporal resolutions are combined to study the powder flow stream, melt pool dynamics and non-equilibrium phase transformation of titanium and nickel-based alloys, as well as pore formation. Moreover, experiments with time-resolved x-ray measurements will be conducted to investigate the influence of cyclic heat treatments, intrinsic to additive manufacturing, to the microstructure and residual stress evolution. Correlations between compositions, processing parameters, process signatures and the resultant microstructure, porosity and residual stresses will be established. The findings will be used to overcome the long-lasting limitations of additive manufacturing by i) reducing anisotropy within the accessible process parameter range by promoting equiaxed solidification, ii) minimizing residual stresses and distortions by managing the heat accumulation during the process, iii) maximizing the process efficiency while minimizing melt pool discontinuities and iv) constructing process maps with the best combination of part properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.