Research On The Deposition Behaviors During Uniform Aluminum Droplet Directionally Printing And The Elimination Of Defects On Inner Surfaces

Uniform metal droplet deposition manufacturing is a branch of additive manufacturing(3D printing)technologies.In the manufacturing process,metal billets are first heated and melted in a crucible.A micro metal droplet is then forced out of a nozzle driven by external excitation disturbance.By elaborately regulating of droplet ejection and the three-dimensional(3D)movement of substrate,metal droplets are successively deposited to form a 3D object according to the preset scanning traces.This technique dispenses with special raw materials or expensive equipment,and it has broad application prospects,such as printing of micro and complex structures,functional devices,heterogeneous parts,electric circuit and packaging.However,because this technique uses discrete metal droplets as the basic building cells,the directly printed parts usually possess scalloped surface morphologies that are not smooth.Such uneven surface morphologies can be hardly eliminated via optimizing the processing parameters due to the nature of metal droplet printing technique.The surface qualities of these printed parts should be further improved in virtue of some subtractive post-processing technologies.But the inner cavity surfaces of micro parts with complex structures are typically difficult processing regions,the surface qualities of these inner surfaces are also very difficult to be improved via subtractive post-processing technologies.Therefore,how to improve the inner surface quality has become one technical bottleneck in the fabrication of micro-complex parts with high precision.To this end,for the first time,this thesis combines aluminum droplet-based 3D printing technology with soluble core technique.The method of eliminating the uneven inner surface morphologies of conventionally printed parts via soluble support structures is proposed,i.e.,precise control of aluminum droplets depositing on a soluble core with preset shapes and dimensions.The soluble core is then dissolved and left behind a tubular structure with high-quality inner surfaces.This thesis reveals the formation mechanism of inner surface defects of the parts printed via droplet oriented deposition using both experimental and modeling methods.The corresponding control strategies of the forming qualities are then proposed,and metal tubes with high-quality inner surfaces are directly fabricated,which further verifies the effectiveness of the proposed control strategies of the forming qualities.The research results lay the foundation of engineering application of metal droplet-based 3D printing in the fields of the fabrication of tubes with high-quality inner surfaces.The main research contents of this thesis are as follows:Based on the requirements of orientated printing experiments,a five-axis linkage deposition platform and corresponding control subsystem are designed and developed.The positioning devices for the initial aluminum droplet deposition locations are developed.The dynamic images of droplet deposition manufacturing are captured in real time via ultra-long focal length microphotography subsystem.The real-time maintenance and monitoring of low-oxygen work environment during aluminum droplets oriented printing are achieved via inert atmosphere glove-box,moisture and oxygen content sensors.The formation mechanism of hole-defects on the bottom surfaces of single aluminum droplet after depositing is revealed.The characterized morphologies of toroidal holes formed on the bottom surfaces of the deposited droplets are proposed,and the corresponding characterized dimensions of the holes are quantitatively analyzed.The critical conditions for the instability of gas-liquid two-phase flow are revealed,and the influence of fluid instability on the solidified morphologies of the bottom surfaces of the deposited droplets is investigated.This research lays the foundation for the limiting case predicting of the inner surface qualities of the parts fabricated via aluminum droplet oriented printing.The blocking effects of the surface ripples and the solidified angles larger than 90 degrees on the fusion between neighboring molten aluminum droplets are revealed.The strategy of regulating the solidified morphologies of the droplets via adjusting the heat conductive coefficient of the substrate is proposed.The metallurgical bonding between neighboring droplets is successfully promoted by eliminating the ripples and sharpening the solidified angles.Moreover,the bonding conditions within the deposited layers are further divided from the originally-identified three cases to the detailed four cases.The parametric mapping between the highly-dense inner surfaces and the temperature parameters is built.Surface metallization is employed in the preparation of silver coatings on the soluble core.Rebound behaviors of aluminum droplet from the soluble core surfaces are successfully eliminated by the prepared silver coating.The thermal diffusion,formation and evolution rules of the intermetallic compounds on the aluminum-silver interfaces during the transient impacting processes are revealed.The controllable deposition of aluminum droplets on the soluble support surfaces is achieved.The selection strategies of rotational angles of aluminum droplets rotationally depositing on a soluble core with uniform cross-sections are established.The influence of deposition layer and frequency on the forming qualities of the printed tubes are investigated.The scanning manners of aluminum droplets on inclined and corner surfaces are proposed,and the corresponding optimized scanning traces of aluminum droplets depositing on a soluble core with varying cross-sections are established.A hexagonal tube and a horn both with smooth and dense inner surfaces are directly printed using the optimized processing parameters,which successfully verifies the effectiveness of the quality control strategies of the printed inner surfaces.