Additive Manufacturing (AM) technology has developed rapidly over the past two decades. Metal-based AM techniques represented by Powder Bed Fusion (PBF) have advantages, including the production of fine features with great geometrical accuracy and high-strength-to-weight ratios. However, its forming mechanism, based on the melting of metal powders, induces the agglomeration of partially melted powders and a balling phenomenon in the process [1
]. Therefore, the average roughness (Ra) of metal-based AM parts is generally higher than 10 μm [2
], which not only affects the appearance, but also greatly influences several functional properties, including fatigue resistance, friction performance and heat transfer [3
]. Accordingly, post-process finishing is required to improve the surface quality of AM parts [6
AM parts always have complex shapes and features, which makes the finishing process difficult. Several surface modification processes for AM parts have been reported in the literature. However, each process has limitations and complexities. Implementing hand polishing as one of the main finishing processes for AM parts would increase the labor cost and time, and the surface quality would be poor. Laser polishing, chemical polishing, electrochemical polishing, and abrasive flow polishing have shown great potential for the treatment of the surfaces of AM parts. Laser polishing is efficient and can directly use the laser source of laser melting deposition equipment, but this technique is prone to causing thermal damage [8
] and is more difficult to irradiate with uniform intensity on complex surfaces, which leads to deviations from the designed geometry [9
]. Chemical polishing and electrochemical polishing can effectively treat all kinds of freeform surfaces, lattices and porous structural parts [10
], but do not solve the environment, health and safety problems. In addition, the electric field is limited inside deep cavities in electrochemical polishing, and therefore the internal finishing may be not uniform [11
]. Abrasive flow machining is an effective process of polishing metal-based AM parts with complex internal cavity and channel structures [7
]. However, this method needs to design and control the flow channel for a specific target part, and problems such as the embedding of abrasives and the edge/corner rounding of the parts must be solved [13
]. Therefore, it is important to exploit new solutions for AM part polishing whilst considering how to maintain the satisfactory geometry and application of the parts.
The mechanical effects of ultrasonic power have been widely used in the removal of materials, such as in UltraSonic Machining (USM) technology. A large number of studies have reported the attractive processing capability of USM for fabricating complex shapes, deep holes, and high-aspect-ratio micro-structures on hard and brittle materials [15
]. In the USM process, an ultrasonically vibrated tool is used in conjunction with a liquid slurry for material removal. A large number of micro-sized abrasive particles suspended in the slurry flow through the working area and hammer the workpiece repeatedly by the propulsion of the vibrated tool. This induces countless, tiny brittle fractures and removes the material. Despite this, it is reported that USM can also work on the surface in a ductile removal mode, and form tiny machining marks through bubble cavitation and the micro-cut/impact of the abrasive particles [18
]. Therefore, USM has great potential for use in polishing processes which require no material removal, or only very little material removal.
In this study, the ultrasonic abrasive polishing process is proposed to improve the surface quality of metal-based AM parts, which is based on the USM principle and has several advantages including its low cost, ease of operation, and absence of thermal or chemical damages. The cavitation effect in this process is reported as being effective for removing partially melted powders on the surfaces of AM parts [4
]. In addition, loose abrasive particles in the slurry can reach any intricate surface, which is more suitable for the polishing of geometrical freedom AM components.
This work aims to explore the machining capability of ultrasonic abrasive polishing in smoothing metal-based AM components. Experiments are conducted to improve the surface quality of SLM-built Inconel 625 specimens using a high-power ultrasonic generator. Silicon carbide mixed with water is used as the slurry. The impact action of the abrasive particles is simulated using the Smoothed Particle Hydrodynamics (SPH) method. The roles of cavitation bubble collapse and the impact of abrasive particles in the polishing process are discussed. The effects of important processing parameters, including ultrasonic output power and concentration of abrasive suspension on machining characteristics, are examined.
Based on the results above, two material removal mechanisms can be considered responsible for the surface smoothing process during the ultrasonic abrasive polishing of additive manufactured components. The first is the material removal due to cavitation collapse pressure, which is effective in dislodging the partially melted powders from the surface. The bonding neck between a partially melted powder and the surface acts like a crevice, which entraps gas and improves the cavitation erosion [4
]. The second material removal mechanism is based on the impact of abrasive particles suspended in the slurry. The abrasive particles are accelerated by the cavitation pressure and the ultrasonic energy of the horn, and tend to penetrate the workpiece in different directions. The SPH simulation in this study only demonstrated the vertical impact action of abrasive particles, which, it is believed, would smooth the surface by plastic deformation. However, it should be noted that the impact from other directions would result in material removal by micro-cutting or sliding [27
]. All of these make abrasive particles capable of smoothing and further removing large structures and discontinuities on the surface of AM components. Figure 13
shows the schematic of material removal mechanisms in the ultrasonic abrasive polishing of additive manufactured components explained above.
The two material removal mechanisms are greatly influenced by the process conditions. Even though the general polishing performance under different conditions has been investigated through the experiments, the nature of the change is not fully understood. The real velocities of the abrasive particles accelerated by the cavitation collapse pressure and the ultrasonic energy of the horn should be further studied, so that the polishing process can be improved. In addition, the cavitation erosion is often associated with damage on smooth specimens based on earlier studies, so it may also have a tendency to worsen the surface smoothness in the polishing process. The control of the two conflicting effects of cavitation on AM surface finish is important to ensure efficient polishing, and should be investigated in the future study. It is believed that the technology could be an ideal solution for the surface modification of AM components. Moreover, as the ultrasonic abrasive polishing process is conducted uniformly on the whole working area by using suitable settings, this polishing method can be used to process surfaces with various complex shapes and internal features.