## 4 Conclusions  
We reported on a study based on the analysis of the influence of the process parameters in laser piercing to create know-how in a field in which commercial devices detect the end of the piercing only improve piercing strategy based on laser power modifications, without considering other process parameters. A monitoring system provided with a sandwich of Si-diode (400-1000 nm) and InGaAs-diode (1000-1800 nm) that simultaneously captured the visible and infrared spectrum was used as a tool to evaluate and relate intensity signal changes to piercing process parameters for mild steel, stainless steel, and aluminum thick sheets. The selection of the monitored spectrum, visible or infrared, was done based on the gas used during the experiments, oxygen or nitrogen, respectively. The visible spectrum was associated with vapor plume detection and infrared spectrum with temperature. Soft piercing was required for mild steel 20 mm thick, observing visible monitored signal dependence with pressure, and duties percentage and ramp time, parameters directly related to the material ejection or to the formation of vapor plume by modifications in laser power, respectively. Based on these results, the piercing time duration was reduced by 25%. For stainless steel and aluminum 10 mm thick, fast piercing was used. In the first case, infrared signal intensity displayed a lower signal intensity in ideal piercing conditions than in non-ideal ones due to having a minimum heating in the piercing zone. Regarding aluminum, the visible signal was saturated when an explosion occurred, as a consequence of material ejection and vapor plume saturation. These results showed that the optimization of the piercing time for soft piercings was feasible thanks to the knowledge obtained from the monitoring inputs. For fast piercings exists also the possibility of establishing a threshold to control the piercing results. This paper shows the potential of the process monitorization, not only to detect the final stage of the piercing, but also to create knowledge that can optimize the actual piercing strategies.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Piercing Parameters** & \multicolumn{3}{c}{**Piercing A**} & \multicolumn{3}{c}{**Piercing B**} \\ \hline Gas type & O\({}_{2}\) & O\({}_{2}\) & O\({}_{2}\) & O\({}_{2}\) & O\({}_{2}\) \\ Power [W] & 5000 & 5000 & 5000 & 5000 & 5000 \\ Piercing time [ms] & 7000 & 7000 & 3000 & 17000 & \\ Gas pressure [bar] & 3 & 3 & 3 & 3 & 3 \\ Focal [mm] & \(-3\) & \(-5\) & \(-7\) & \(-3\) & \(-3\) \\ Initial standoff [mm] & 8 & 8 & 8 & 8 & 8 \\ Final standoff [mm] & 8 & 8 & 8 & 3 & 3 \\ Frequency [Hz] & 500 & 500 & 500 & 500 & 500 \\ Duty minimum [\%] & 8 & 14 & 20 & 14 & 16 \\ Duty maximum [\%] & 14 & 20 & 20 & 16 & 20 \\ Ramp time [ms] & 1000 & 1000 & 1000 & 500 & 500 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Piercing parameters for mild steel 20 mm thick, for two different piercing strategies. Piercing A was programmed as three independent piercings and piercing B as a continuous descendent piercing process.  
Figure 6: Piercing monitored signal for mild steel 20 mm thick, varying piercing strategy.  
**Author Contributions:** Conceptualization, S.M.G. and J.F.; methodology, S.M.G.; investigation, J.R. and S.M.G.; validation, J.R. and S.M.G.; formal analysis, S.M.G, A.L., and J.F.; resources, J.F.; writing--original draft preparation, S.M.G.; writing--review and editing, S.M.G, A.L., and J.F.  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflict of interest.