Establishing Process-Structure Relationships in Laser Powder Blown Directed Energy Deposition Through In-situ Investigation

Webster, Samantha

doi:https://doi.org/10.21985/n2-tdg1-bf09

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Establishing Process-Structure Relationships in Laser Powder Blown Directed Energy Deposition Through In-situ Investigation

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Laser powder-blown directed energy deposition (DED) is an additive manufacturing process that utilizes a co-axial nozzle and laser to melt metal powders onto a substrate in a line-by-line fashion. This coupling gives rise to interactions between the laser, powder, and melt pool. To address fundamental process understanding, physical models were established for high frequency (i.e., particle impact and surface oscillations) and low frequency (i.e., volume) behavior of the melt pool. These behaviors were connected to structures within DED clads such as pores, clad dimension, and dilution. The process-structure relationship for low frequency melt pool behavior was established through an energy density model, and high frequency melt pool behavior was analyzed through air-cushioning models of solid particle impact on a liquid surface. Through the established process-structure relationships, one continuous build on a machine can manufacture several different scales of high quality clads.Fundamental relationships in DED were studied using in-situ observations, with single powder-particle models informing the macro-scale experiments and characterization. Experiments were accelerated with a novel DED system, designed and constructed to conduct experiments within an X-ray hutch at the Advanced Photon Source (APS) in Argonne National Laboratory. From highspeed, in-situ X-ray images collected with this setup, it was determined that particle impact in powder-blown DED causes pore formation. This result stands in stark contrast to currently accepted process maps of porosity, which incorrectly assume that pore formation is independent of particle momentum, size, and wettability. Residual pores were also characterized using X-ray computed tomography (CT), and the percentage of pores formed through bubble entrapment was estimated by classifying pores from their orientation and aspect ratio in CT data. Since pore formation depends not only on the particle’s momentum but also the size of the receiving melt pool, a connection between particle impact area and the melt pool surface area needs to be established. Thus, research concluded with the development of an analytical model of ‘energy density’, a term for how much power (or energy) is being used to melt the metal powders in an area and at a certain speed. This physics-grounded model accounts for laser-particle interactions and predicts clad quality. In all, this research can be used to standardize process conditions across DED machines that utilize different hardware and enable more reliable material processing. Further research investigated the potential of closed-loop control and Hybrid-AM processes to control DED process conditions and expand the technology to include additional energy sources that can further control material structure.

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