Fluidized Bed Surface Finishing of Metal Objects Produced By Additive Manufacturing

Fluidized Bed Machining (FBM) of additively manufactured objects is being considered as a viable post-processing method for object finishing and improvement of surface texture. As-built objects are characterized by high surface roughness, thus, post-processing is required. Conventional finishing methods are limited to objects with low geometrical complexity, while in principle, fluidized bed finishing can be suitable for complex geometries, thanks to the mobility of fluidized particles.

The study addresses a comprehensive assessment of FBM technology aimed at the surface finishing of square flat metal specimens produced by Laser-Powder Bed Fusion method. The experimental campaign aimed at characterizing the effectiveness of two operational modes: FBM of stationary specimens immersed into a fluidized bed of abrasive material; FBM of specimens immersed in the fluidized bed under controlled rotational motion. The effect of different abrasive materials, processing time and tilt angle of the specimen was investigated. Surface modifications were analyzed in terms of mean surface roughness, skewness and primary profile root mean square slope, as well as by Scanning Electron Microscopy and monitoring of time-resolved weight loss. The results showed moderate smoothing of the sample surface under stationary sample configuration, while good results were obtained for rotation-assisted FBM tests. A maximum reduction of the surface roughness of 12% and 67% of the initial value was measured for stationary and rotation-assisted FBM, respectively. An appreciable influence of the bed material and of the tilt angle was observed. Steel particles were the most effective bed material for the finishing process, with particle density overtaking hardness as the key particle property. The optimal tilt angle was the one that maximized sliding and shear. The experimental results are consistent with a mechanistic framework according to which surface finishing of the specimens is driven by surface shear forces dominated by inertial stresses in the particulate phase.