Considering data quantity, reliability, cost and other factors, it is an effective method to monitor the additive process of powder bed fusion-laser (PBF-L) by using photodiode to collect radiation information of molten pools in engineering practice. The on-line radiation monitoring of molten pools based on photodiode can collect a large amount of information in real time reflecting the internal conditions of molten pools in real time, and thus plays a very important role in predicting and controlling the quality of molten pools. The key is to continuously collect the radiation information of molten pools and to extract and analyze the data characteristics in a wide time scale (from several seconds to tens of hours) by the statistical methods, so as to realize the stable state analysis and quality prediction of the forming process. In PBF-L, all process factors (laser parameters, scanning strategy, powder state, and air flow protection) eventually influence the change of radiation signals of the molten pool, among which as for the main effects of laser power, scanning speed, preheating temperature and others on temperature and radiation intensity of a molten pool, a lot of research has been done. However, in addition to radiation intensity, the radiation signal change of the molten pool over time also contains a lot of other technological information and process stability information and it is worth further digging and studying. In the actual production process, PBF-L is a very complex process and the forming quality is still uneven even when the same equipment and process parameters are monitored and controlled. In order to ensure the repeatability and process consistency of additive manufacturing, the influence of process factors such as floor height, substrate position and scan line angle on the radiation signals of the molten pool is analyzed, and it is found that these characteristics can be used as an important indicator of quality control in the future.
In this paper, the forming experiment of K438 superalloy powder is carried out, and the photodiode is used to collect the radiation signal of the molten pool in the process of PBF-L. First it is analyzed and pretreated. Then it is segmented corresponding to the sample one by one. Statistical methods are used to process the segmented data, and the mean and standard deviations are selected as indicators to evaluate the signal characteristics, and the influence of process factors such as floor height, substrate position and scan line angle on the radiation signals of the molten pool is finally analyzed.
The influence of the process factors of PBF-L on the radiation intensity of the molten pool can reflect some classical laws. With the increase of layer height, the mean radiation intensity of the molten pool shows an overall increase trend, and the mean radiation intensity of the printing layer No.190 increases by about 6% (Fig. 4). In the wind field, the intensity waveform of the upwind molten pool shows the feature of "right deviation" , while that of the downwind molten pool shows the feature of "left deviation" (Fig. 6). The influence of relative position in the wind field on the radiation intensity of the molten pool is that the farther away from the air outlet, the greater the molten pool strength, and the closer to the air outlet, the smaller the molten pool strength (Fig. 8). In the scanning strategy, the scan line angle is exactly consistent with the incline angle of the sample (based on the horizontal direction), in which the mean intensity is the maximum (Fig. 11).
Through the processing and analysis of the intensity signal data of the molten pool, the influence law of some process factors on the radiation intensity of the molten pool is obtained, and the relationship between the radiation intensity law of the molten pool and the physical mechanism is established. The analysis shows that the effect of layer height on the radiation intensity of the molten pool is mainly due to heat accumulation among layers. In the wind field, the influence of wind direction and relative position on the radiation intensity of the molten pool is mainly the smoke masking effect. The effect of scan line angle on the radiation intensity of the molten pool is mainly due to the laser duty cycle. Through establishing the correlation between law and physical mechanism, it is clearer that in the practical engineering application and monitoring closed-loop control, it is necessary to consider the influence of layer height, scanning strategy, wind field conditions and other factors on the radiation intensity of molten pools. And its typical rules show that these characteristics can be used as an important indicator of quality control in the future.
