Manipulating molten pool dynamics during metal 3D printing by ultrasound

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Abstract

Ultrasound-assisted direct energy deposition (UADED) attracts increasing attention due to its capability to tailor the grain structure. However, the involved molten pool dynamics, particularly the complex interaction of ultrasound-flow-solidification, remain unclear to date, which hinders quantitative prediction and regulation of the microstructures and mechanical properties of UADED components. Here, in situ high-speed imaging and high-fidelity multi-physics modeling are leveraged to investigate flow characteristics and liquid-to-solid transformation in UADED for Inconel 718. The inertial force activated by ultrasound is revealed to drive the molten pool to flow forward and backward along the vibration direction, resulting in poor surface quality. A hybrid deposition strategy is developed to minimize ultrasound-induced defects and produce superior microstructure with alternating coarse- and fine- grains. Such a layered microstructure results in 28% and 15% improvement in the yield strength and ultimate tensile strength compared to the counterpart by additive manufacturing without ultrasound. This work provides unprecedented understanding into the molten pool dynamics in the UADED process as well as valuable guidance to manipulate molten pool flow.

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3D printing of high-strength aluminium alloys

Brennan Yahata, Jacob Hundley, Justin A. Mayer … (2017)

Metal-based additive manufacturing, or three-dimensional (3D) printing, is a potentially disruptive technology across multiple industries, including the aerospace, biomedical and automotive industries. Building up metal components layer by layer increases design freedom and manufacturing flexibility, thereby enabling complex geometries, increased product customization and shorter time to market, while eliminating traditional economy-of-scale constraints. However, currently only a few alloys, the most relevant being AlSi10Mg, TiAl6V4, CoCr and Inconel 718, can be reliably printed; the vast majority of the more than 5,500 alloys in use today cannot be additively manufactured because the melting and solidification dynamics during the printing process lead to intolerable microstructures with large columnar grains and periodic cracks. Here we demonstrate that these issues can be resolved by introducing nanoparticles of nucleants that control solidification during additive manufacturing. We selected the nucleants on the basis of crystallographic information and assembled them onto 7075 and 6061 series aluminium alloy powders. After functionalization with the nucleants, we found that these high-strength aluminium alloys, which were previously incompatible with additive manufacturing, could be processed successfully using selective laser melting. Crack-free, equiaxed (that is, with grains roughly equal in length, width and height), fine-grained microstructures were achieved, resulting in material strengths comparable to that of wrought material. Our approach to metal-based additive manufacturing is applicable to a wide range of alloys and can be implemented using a range of additive machines. It thus provides a foundation for broad industrial applicability, including where electron-beam melting or directed-energy-deposition techniques are used instead of selective laser melting, and will enable additive manufacturing of other alloy systems, such as non-weldable nickel superalloys and intermetallics. Furthermore, this technology could be used in conventional processing such as in joining, casting and injection moulding, in which solidification cracking and hot tearing are also common issues.