# Abrasive Surface Finishing on SLM 316L Parts Fabricated with Recycled Powder
## 1 Introduction  
3D printing technology has enabled designers to fabricate parts that are functional, lighter, and remarkably more complex from various materials [1, 2, 3]. Remarkably, for metallic parts fabricated with the selective laser melting (SLM) method, the mechanical properties are higher in comparison with traditionally produced ones [4, 5]. Specifically, for SLM technology, it has been advanced so that it can operate with several common alloys utilized in the aerospace, biomedical, and automotive industries [6, 7, 8, 9]. Nevertheless, an obvious drawback of 3D printed parts, in general, and SLM parts, in particular, is the large surface roughness caused by the nature of the fabricating process as investigated in [10, 11, 12, 13]. Specifically, the surface roughness of SLM parts is mainly influenced by different factors, such as the laser power, laser speed, scanning strategies, position and direction of the parts in the building chamber, direction and flow rate of the inert gas flow, and powder characteristics [8, 14, 15, 16]. The arithmetic mean high of a line (Ra) of SLM parts could range from 5 up to 50 \(\upmu\)m, and is primarily found below 20 \(\upmu\)m as in [11, 17, 18]. Subsequently, without undergoing any post surface finishing, the SLM parts cannot satisfy the requirements of Ra being below 0.8 \(\upmu\)m as for machine components and below 1 \(\upmu\)m as for dental implants [19]. To achieve the desired surface roughness, as previously mentioned, SLM parts must then undergo the traditional computer numerical control (CNC) machining process [20]. The two fabricating processes indeed have been combined into a so-called hybrid additive-subtractive manufacturing (HASM) process, where adding and subtracting material from a fabricated component are realized simultaneously in one machine [21, 22]. However, due to the restriction of the tool path when finish machining a 3D printed part, several alternativepost-processes have been investigated, such as blasting, grinding, laser/electromechanical polishing, shot/ultrasonic peening, etc., as summarized in [23]. Among the most effective techniques is the surface mechanical attrition treatment (SMAT), investigated in [23], where the surfaces of the SLM part were bombarded with steel balls vibrating at 40 Hz in 10 min to reduce the Ra from 15 to 1.8 \(\upmu\)m, and longer SMAT time can deliver surface roughness below 0.5 \(\upmu\)m. Besides, in [24], ultrasonic peening reduces the Ra from 10.6 to 1.3 \(\upmu\)m. Utilizing the dry mechanical-electrochemical polishing (DMECP) technique, [25] led to reducing the Ra to the lowest 0.75 \(\upmu\)m. Last but not least, in [19], laser polishing was reported to deliver Sa, the 3D extension of Ra, below 0.51 \(\upmu\)m.  
There have been several studies on different surface treating methods on SLM parts printed from various materials such as steel, titanium, Inconel, and their alloys, etc. Nevertheless, these aforementioned techniques and material usage require heavy investment in the facilities, which may not be applicable for a majority of entities. Thus, in this case study, SLM stainless steel 316L specimens fabricated with recycled powder were examined. They were treated solely with the mechanical abrasive finishing methods aiming at finding a more affordable solution as an alternative for the aforementioned surface treating methods. Specifically, a combination of blasting and tumbling was investigated in this manuscript. In blasting, surface imperfections are removed by streamlines of media being shot directly to the surface of the components [26]. Additionally, in tumbling, components are tumbled in different solid media added for deburring, surface smoothing, degreasing, increasing corrosion resistance with the help of chemical (tumbling compound), etc. [27]. Specifically, in this manuscript, the as-built parts underwent sand or steel-ball blasting, then tumbled in three different media being ceramic, plastic, and porcelain, for different pre-specified durations.