Direct fabrication of metal tubes with high-quality inner surfaces via droplet deposition over soluble cores

Droplet-based 3D printing is very promising for the fabrication of complex thin-wall microwave devices such as antenna horns and waveguide tubes since it can print a shell by utilizing only several layers of droplets. However, due to the naturally scalloped shape of metal droplets, conventional droplet-based 3D printing methods cannot produce thin-wall tubes with high-quality inner surfaces that can meet the requirement of electromagnetic transmission. Here, combining the conventional casting procedure and droplet printing, a hybrid printing process is proposed. Uniform aluminum droplets were first rotationally deposited on a soluble core, and then the core was dissolved and left behind a tube with a high-quality inner surface. The deposition parameters (i.e., number of layers and deposition frequency) were adjusted according to the rotation of the core to form a dense shell over its surface. A hexagonal metal tube was fabricated by using the proposed method to prove its effectiveness. The standard Archimedes test shows that the density of the formed part was up to 98.89%. The industrial CT scanning results also prove a porosity-free inner structure. The inner surface roughness (Ra) was measured to be 4.38 mu m by using a laser confocal microscope scanning, and the roughness is only 0.37% of the droplet diameter.