In the selective laser melting (SLM) process, the stability of the melt pool plays a crucial role in determining the surface and internal quality of the formed parts. To address the limitations imposed by unstable factors during the SLM process, introducing in-process monitoring technology is essential. The technology can reveal the relationship between the melt pool radiation signals and the quality of the formed parts. By combining these signal characteristics with process parameters, it is possible to control the quality of the formed parts, which is of great significance to the advancement of SLM technology.

Based on the principle that metal powder absorbs laser energy and generates optical radiation upon melting during the SLM process, this study developed a coaxial in-situ monitoring device for SLM, utilizing photodiodes. This device captured the radiation time-series signals of the high-temperature melt pools during the printing process that was carried out under an unprotected atmosphere. After preprocessing the collected melt pool radiation signals, we examined the mapping relationships among process parameters, forming performance, and optical radiation signals intensity. Subsequently, signal processing techniques, such as fast Fourier transform (FFT) and wavelet transform, were employed to analyze the characteristic changes of the optical signals in the time-frequency domain. This study provides theoretical guidance for applying process monitoring in SLM.

Laser power has a direct effect on the time-domain mean of the optical signal, which in turn influences the mechanical properties and surface roughness of the formed parts. However, the scanning speed shows minimal impact on the mean value of the optical signal intensity. FFT and wavelet transform analyses revealed a strong relationship between the appearance and distribution of characteristic peaks and scanning speed. The study also shows that reducing input energy can improve the stability of the melt pool and significantly suppress the splashing phenomenon caused by the high-energy beam impact. This is particularly significant for improving both the surface and internal quality of SLM-formed parts.

(1) At low laser power, the surface melt of the sample is discontinuous, and an obvious spheroidization phenomenon occurs. As the laser power increases, the surface melt becomes continuous and complete. As scanning speed increases, the amount and size of residual splashes on the sample surface and powder bed gradually decrease, although this may lead to insufficient powder melting. When this occurs, the melt becomes discontinuous, cracks expand, and the density of the samples is significantly reduced. At lower scanning spacing, a severe oxidation reaction occurs during the forming process. Additionally, as scanning spacing increases, gaps form between each melt, and more pores and incomplete fusion defects appear.

(2) The amplitude variation of the time-domain signals is mainly influenced by laser power rather than scanning speed. As laser power increases, the amplitude of the optical signal increases, but its stability decreases. As scanning distance increases, the mean and standard deviation of the optical signal intensity initially increase and then decrease.

(3) When scanning speed is below 1000 mm/s, increasing the laser power leads to a decrease in surface roughness with an increase in the average optical signal intensity. As the optical signal intensity increases, the porosity of the sample decreases significantly. At optimal laser power, the surface roughness and porosity show a trend of initially decreasing and then increasing with the increase in scanning speed, while the average optical signal remains relatively stable. At different scanning spacing, surface roughness and porosity decrease as the amplitude of the optical signal increases. Meanwhile, the average density and signal intensity mean of each process group decrease with decreasing laser power. At the same scanning speed, the coefficient of variation also decreases with decreasing laser power. Increasing scanning spacing causes the density of the sample and the optical signal intensity mean to follow a trend of first increasing and then decreasing. The performance of the tensile samples increases with an increase in the mean radiation signal intensity of the melt pool.

(4) The frequency range of the optical signals is primarily concentrated in the low-frequency band. Additionally, spectral peaks in the low-frequency part distribute at certain intervals. The periodic signals exhibited in the spectrum are closely related to the generation of splashing. Based on wavelet transform, we find that the appearance and distribution of the characteristic peaks in the frequency domain signal of the melt pool are not significantly related to laser power, but are closely tied to scanning speed. Further analysis through three-layer wavelet packet decomposition reveals that the signal is mainly concentrated in the frequency range of 0‒12500 Hz. The kurtosis value of the time-domain signals decreases with increasing scanning speed. The splashing induced by high-energy beam impact is significantly suppressed as scanning speed increases.