# Truncated Newton Kernel Ridge Regression for Prediction of Porosity in Additive Manufactured SS316L
## 4 Results and Discussion  
### Analysis of Data Plots  
The properties of the SLM-manufactured parts depend strongly on the input process parameters. Thus, it is crucial to understand the relationships between the main process parameters and the fabricated part characteristics. Although the compiled dataset is a subset of the experimental data available on the relative density of SS316L parts fabricated by SLM, it may provide valuable insight into the relationships between the process parameters and the measured relative density within the investigated range of parameters. Figure 4 shows the relative density versus the laser power, scan speed, hatch distance, and layer thickness of the literature data combined with our experimental results. The variation in the process parameters results in the variation in the relative density. The process parameters in the compiled dataset have the following ranges: laser power between 30 and 400 W, scanning speed between 50 and 2400 mm/s, hatch distance between 0.04 and 0.3 mm, and layer thickness between 0.02 and 0.25 mm. The relative density of the samples ranges from 75% to 100%. Figure 4: Plots of the relative density against (**a**) laser power, (**b**) scan speed, (**c**) hatch distance, and (**d**) layer thickness. Figure 4a shows that the relative density is greater than 90% when the laser power is above 150 W. As the laser power and thermal energy from the laser beam increase, a greater percentage of the material within the volume is heated, possibly above its melting point. As a result, less material is left intact or partially melted, resulting in a higher relative density [22]. In contrast, low laser power may result in insufficient melting of the powder, as well as a reduced depth of the laser penetration into the powder layer, which is insufficient to melt the powder fully and fuse the layers [48]. Thus, increasing the laser power may improve and enhance the relative density of the parts. Furthermore, in Figure 4b, a high relative density is observed when the scan speed exceeds 1500 mm/s. However, below a scan speed of 1500 mm/s, there is no clear correlation between the scan speed and relative density based on this dataset. The lower scan speed may incur irregular molten pools, which may yield larger grains and pores. [PERSON] et al. [49] have reported that a lower scan speed increased laser penetration depth, and inhomogeneous rapid solidification occurred, resulting in lower relative density. A study in [30] has also noted that samples processed at higher scan speeds had better densification and more refined microstructure than samples processed at lower scan speeds. Concerning the hatch distance, as shown in Figure 4c, most of the experiments used small hatch distances in the range of 0.05-0.15 mm. Small hatch distances ensure overlapping melt pools and hence, less pore formation than larger hatch distances that result in weak overlapping and provide good conditions for pore formation [31]. Figure 4d shows the experimental results of different layer thicknesses and the resulting measured relative density of the parts. While there is no clear correlation between the relative density and layer thickness in the range of 0.025-0.05 mm, increasing the layer thickness from 0.05 mm to 0.1 mm results in a higher relative density. The appropriate layer thickness adjustment allows the laser spots to overlap and result in sufficient melting of the powder. However, if the powder layer is too thick, it would be hard to create a melt pool with enough depth to melt and fuse the layers [48]. Based on Figure 4a,b, scan speed greater than 1500 mm/s and laser powers greater than 150 W achieve high relative density. Figure 5 illustrates the combined effects of the laser power and scan speed on the relative density. High laser power appears to compensate for high scanning speed, preventing leftover unmelted powder. When the laser power increases, the same volume of powder must absorb the same amount of thermal energy to melt in a shorter time interval [28].