## 1 Introduction  
Since the advent of the first additive manufacturing (AM) technique, the Stereolithography process by 3D Systems in 1987, additive manufacturing has been under continuous and rapid development to meet growing industrial demands. Formerly, it was mainly used as a prototyping technique for pre-production, testing and analysis. However, different additive manufacturing techniques, which have recently emerged, play a significant role in modern industries [1; 2]. This is principally because additive manufacturing has shown high potential to produce intricate 3D geometries with short lead-times at a relatively low cost. This helps strengthen supply chains and boosts their profitability and flexibly [3]. Presently, AM has been successfully exploited in electronic, aerospace and biomedical industries where highly specialized and customizable components are required [4; 5; 6; 7]. The selective laser melting (SLM) technique makes especially great strides in the field of metal AM. This is especially achievable with the commercial emergence of titanium and nickel based super alloys that have exceptional material properties [7; 8]. The SLM process is a powder bed additive manufacturing technique which uses a laser as a power source. The interaction between the laser and the metal powder causes the powder to selectively melt according to the desired slices. Once one layer is scanned, the platform is moved down by the height of this layer and another layer of powder is applied on top of the formerly built layer. This process of melting and bonding layers together continues successively until the desired part is built. The non-melted powder remains in the build chamber and provides support to the part being built. This non-melted powder can be subsequently removed after building is completed and sieved; it can therefore be reused in successive builds [9]. Though noticeable progress has been made, there have been some challenging issues that still need addressing to allow AM and SLM to be used among mainstream manufacturing processes. Limited precision of the fabricated products and the repeatability of the processes are considered especially high technological barriers to the maturation of additive manufacturing techniques [4]. In particular, AM parts are often built with high surface roughness, which necessitates some post processing steps to refine the resultant roughness and makes it suitable for a wide range of engineering applications. These post-processing steps are rather expensive and time consuming. The SLM process exhibits the so-called staircase effect in both up-facing and down-facing surfaces as depicted in Figure 1.  
This staircase effect contributes to the increased roughness of these surfaces. Down-facing surfaces, especially ones that are at an angle less than 45\({}^{\circ}\), with respect to the build platform, show very high roughness. This is mainly attributed to the formation of cross and spatter due to the high laser absorptivity of powders compared to the solid metal in the bulk of the part as can be seen on Figure 2. The surface topology of parts produced by SLM are highly dependent on their orientation. This is why, in order to produce parts with good surface quality, down-facing surfaces with angles less than 45\({}^{\circ}\) are usually avoided by reorienting the part. Otherwise, there is a need for the building of support  
Figure 1: The staircase effect in up-facing and cross formation in down-facing surfaces in additive manufacturing (AM) parts. structures. However, this in turn results in the increase of process steps, in particular, removal of the support could exhibit defects, such as burr formation, leading to even higher roughness [10]. There have been some research attempts which try to correlate process parameters with quality marks of the parts, [PERSON] et al. concluded that parts built with a 30 \(\upmu\)m layer thickness demonstrated higher strength and lower elongations than parts built with a 50 \(\upmu\)m layer thickness [11]. [PERSON] et al.