## 1 Introduction  
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 \(\upmu\)m [2], which not only affects the appearance, but also greatly influences several functional properties, including fatigue resistance, friction performance and heat transfer [3; 4; 5]. Accordingly, post-process finishing is required to improve the surface quality of AM parts [6; 7]. 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; 11], 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; 12]. 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; 14]. 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; 16; 17]. 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; 19]. 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].