# Investigation of Laser Polishing of Four Selective Laser Melting Alloy Samples
## 3 Results and Discussion  
### Characterisation of Surface Morphology  
The three-dimensional contours of the nine regions were obtained using a self-programming code. Figures 4-7 show the surface morphologies of the Ti6 Al4V, AlSi10 Mg, 316L and IN718 samples, respectively.  
Figure 4: Three-dimensional contours of the Ti6 Al4V sample after laser polishing.  
Figure 5: Three-dimensional contours of the AlSi10 Mg sample after laser polishing.  
Figure 6: Three-dimensional contours of the 316L sample after laser polishing.  
Figures 4-7 show the morphologies of the four sample surfaces after laser polishing, with the polishing strategies listed in Table 2. For the Ti6 Al4V alloy, the roughness values obtained with the eight laser-polishing strategies clearly differ considerably, ranging from 3.32 \(\upmu\)m to 8.14 \(\upmu\)m. It is obvious that the surfaces of the sample after laser polishing are smoother than the initial surface. For the AlSi10 Mg alloy, the roughness values for each laser-polishing strategy range from 3.33 \(\upmu\)m to 13.42 \(\upmu\)m. For the 316L alloy, the roughness values for each laser-polishing strategy range from 3.91 \(\upmu\)m to 13.95 \(\upmu\)m. For the IN718 alloy, the roughness values for each laser-polishing strategy range from 2.76 \(\upmu\)m to 14.12 \(\upmu\)m. By comparing the three-dimensional contours of the four kinds of alloy materials, it was found that, for the given laser-polishing parameters (laser power and scanning speed), the rules of polishing effect are the same regardless of the material investigated in this work.  
The initial surface roughness of the four alloys differs greatly, this being related to their powder characteristics, such as the sphericity and flowability. From Figure 1, it can be seen that the Ti6 Al4V alloy powder has a higher level of particle sphericity than the other three alloys, which is linked to the flowability of the power. Thus, the uniformity of the layer is affected, as well as the surface roughness of the final parts. Although the initial surface roughness values of each alloy are different, after laser polishing, all of the material surfaces tend to be smooth, and the final surface roughness is about 3 \(\upmu\)m.  
The line profiles of the processed surfaces are shown in Figure 8. They were acquired along the centreline of the polished surfaces shown in Figures 4-7. Figure 8 shows the best polishing parameters for the four typical alloy samples and the morphology curves obtained with those parameters. The best polishing parameters are a laser power of 400 W and a scanning speed of 0.5 m/s, which gives the largest energy density. It is obvious that the surface profile after polishing is much smoother than that for the initial surface. The larger peaks and valleys on the initial surface have basically disappeared, and the surface profile is smooth. Although the roughness of the initial surfaces of the four samples are not uniform, the surface roughness of the surfaces of all four materials has been significantly reduced after laser polishing. For the Ti6 Al4V alloy, the initial peak-valley value is more than 70 \(\upmu\)m, and it is reduced to about 10 \(\upmu\)m after laser polishing. For the AlSi10 Mg, 316L and IN718 alloys, their largest peak-valley values decrease from 45, 48 and 77 \(\upmu\)m, respectively, to about 10 \(\upmu\)m.  
Figure 7: Three-dimensional contours of the IN718 sample after laser polishing.