# Investigation of Laser Polishing of Four Selective Laser Melting Alloy Samples
## 1 Introduction  
Additive manufacturing (AM) technology is regarded as one of the key technologies that will lead the 'third industrial revolution'. It is highly regarded in countries around the world and has been the subject of a large number of studies. In particular, selective laser melting (SLM) technology has attracted considerable attention. Although the process offers many advantages compared to traditional techniques, the surface roughness produced by the SLM technology limits its developmentand potential high-end applications. [PERSON] et al. concluded that the \(R_{\text{a}}\) value of the parts fabricated by traditional mechanical methods such as milling and grinding is less than 1-2 \(\upmu\)m, and the \(R_{\text{a}}\) value of SLM parts is usually between 10 \(\upmu\)m and 30 \(\upmu\)m [1]. [PERSON] and [PERSON] pointed out that it is critical to obtain the required part top surface roughness in some applications [2]. However, there are many reasons for a rough surface. Many researchers have studied the effect of the SLM process parameters on the surface roughness. [PERSON] et al. concluded that laser process parameters have crucial effects on the surface quality of SLM-fabricated Co-Cr dental alloys [3]. [PERSON] et al. studied the influence of the laser power and laser scanning speed on the surface morphology [4]. Their results showed that, when the scanning speed was increased, the surface morphology initially became flatter, but then, roughness developed again as the scanning speed increased further. As the laser power was increased, the surface morphology gradually became rougher. [PERSON] et al. studied Ti6 Al4V single-track, multi-track and bulk samples formed by SLM using different scanning speeds [5]. They found out that the \(R_{\text{a}}\) of the top of the bulk increased with the scanning speed. [PERSON] et al. analyzed the effect of the beam power, beam velocity and overhang angle in an attempt to further the understanding of the relationship between individual surface characteristics and surface roughness parameters [6]. [PERSON] used a full factorial design to assess the impact of process parameters on surface quality (Q), flatness, overlapping and surface roughness [7]. The best results were obtained with laser powers of 300 W and 400 W and scanning ranges of 450 \(\upmu\)m and 600 \(\upmu\)m. [PERSON] et al. investigated the relationship between the laser energy density and the properties of 316 L stainless steel [8], such as its porosity, surface roughness, microstructure, density and hardness. [PERSON] et al. focused on adjusting the volume energy density with the goal of finding a compromise between an optimum densification state and a minimum amount of contaminating spatter [9], and they finally found an optimum set of parameters that delivered the best surface roughness, combined with low porosity. [PERSON] et al. found that the surface roughness was linearly related to the energy density in the process [10]. [PERSON] et al. studied the effects of the laser energy density (LED) on the densities and surface roughness values of AlSi10 Mg samples processed by SLM [11]. The results showed that the LED has an important influence on the surface morphology of the forming part, with a higher LED possibly producing a balling effect, and a lower LED tending to produce defects, such as porosity and microcracks, subsequently affecting the surface roughness and porosity of the parts. [PERSON] et al. and [PERSON] et al. used a LED formula, as follows [12; 13]:  
\[E_{\text{v}}=\ \frac{P}{v*h*t} \tag{1}\]  
where \(P\) is the laser power (W), \(v\) is the scanning speed (mm/s), \(h\) is the hatch space (mm), \(t\) is the layer thickness (mm) and \(E_{\text{v}}\) is the volumetric LED (J/mm\({}^{3}\)). The LED directly determines the thermal input to the sample's surface and plays an important role in laser polishing. Furthermore, there are other factors that affect surface quality. [PERSON] et al.