A counter-gravity casting system for in situ high-speed synchrotron x-ray imaging characterization.

Counter-gravity casting (CGC) aims to eliminate turbulent melt flow and defect formation during filling and subsequent solidification by pushing high-temperature melt into the mold cavity against gravity with regulated pressure. However, limited by the opaqueness of molten metals and the complexity of the CGC apparatus, it is extremely difficult to directly quantify the high-velocity mold filling and pressurized solidification in real-time. Here, we report the design and characterization of a CGC system capable of in situ monitoring of mold filling and subsequent solidification processes in the synchrotron beamlines by deploying a high-energy, high-speed synchrotron x-ray imaging technique. The high-velocity melt flow and dendrite growth during pressurized solidification have been quantified for systematical process parameter analysis by investigating time-resolved x-ray images of an exemplary Al-Cu alloy. The high-speed imaging results demonstrate that the in situ CGC system provides a useful way to better understand the fundamentals of mold filling, pressurized solidification, and experimental inputs for high-fidelity modeling in scientific and industrial applications.

Publisher Correction: Pore elimination mechanisms during 3D printing of metals.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Pore elimination mechanisms during 3D printing of metals.

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals.

Revealing particle-scale powder spreading dynamics in powder-bed-based additive manufacturing process by high-speed x-ray imaging.

Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior.