Real-Time Defects Analyses Using High-Speed Imaging during Aluminum Magnesium Alloy Laser Welding

In this study a continuous wave Ytterbium-doped Yttrium Aluminum Garnet (Yb: YAG) disk laser has been used for welding of AlMg3 casted alloy. A high-speed imaging camera has been employed to record hot vapor plume features during the process. The purpose was to identify a mechanism of pores detection in real-time based on correlations between metallographic analyses and area/intensity of the hot vapor in various locations of the samples. The pores formation and especially the position of these pores had to be kept under control in order to weld thick samples. Based on the characterization of the hot vapor, it has been found that the increase of the vapor area that exceeded a threshold value (18.5 ± 1 mm2) was a sign of pores formation within the weld seam. For identification of the pores’ locations during welding, the monitored element was the hot vapor intensity. The hot vapor core spots having a grayscale level reaching 255 was associated with the formation of a local pore. These findings have been devised based on correlation between pores placement in welds cross-section microscopy images and the hot vapor plume features in those respective positions.

Observation of Vapor Plume Behavior and Process Stability at Single-Track and Multi-Track Levels in Laser Powder Bed Fusion Regime

Laser powder bed fusion (LPBF) is a promising additive manufacturing technology for producing metal parts with complex geometric features. However, the issue concerning process stability and repeatability still hinders its future acceptance by the industry. Gaining a better understanding of the behavior and stability of the evaporation process is an important step towards further insights into the complex interaction between laser and material. In this study, we used off-axis high-speed camera to observe vapor plume evolution in single-track formation on bare Ti-6Al-4V plates; the results showed that evaporation has a strong effect on melting quality even if the keyhole is not developed. We then expanded the experiments to multi-track level and found that the melting mode can change as the result of heat accumulation. The results show the possibility that keyhole regime may be reached even if it starts with a combination of parameters below the threshold for keyhole formation in single-track-level observation.

Analysis of Photodiode Monitoring in Laser Cutting

The paper presents the results of an analysis based on the photodiode monitoring signals obtained during the laser cutting of aluminum and stainless steel plates. The mean level of the monitoring signal was measured and related to the process parameters and the quality achieved. The investigation was conducted in the visible and infrared spectra simultaneously for each experiment and a similar behavior at both spectra was observed, concluding the existence of a relationship between the monitoring signal, the quality of the performed cut, and the characteristics of the cutting scenario. Both visible and infrared monitoring signals were found not to vary as long as the parameter used values ensuring that the cut quality was good. Nevertheless, their mean values tended to increase as the cutting quality became worse. The measured intensity of the visible spectrum signal was associated with the vapor plume formation during the cutting process, whereas the infrared signal was related to the temperatures reached.

The Design of a Vapor-Condensing Plume Abatement System and Devices for Mechanical Draft Cooling Towers

Cooling towers are widely used in many fields, but the generation of visible plumes has a serious impact on the environment. Moreover, the evaporation losses also cause a great waste of water. In this paper, a vapor-condensing plume abatement system was designed for a mechanical-draft cooling tower based on the mechanism of vapor plume generation. An effective method to achieve water-saving and eliminate the water fog generated in the cooling tower was proposed, and its feasibility was verified by using thermodynamic analysis. Next, the vapor-condensing plume abatement device was designed and used for both the confined space cooling tower (CSCT) and the free space cooling tower (FSCT). The surface type heat exchanger was adopted to design the vapor-condensing plume abatement device. Then a basic calculation flow and method were proposed to obtain thermodynamic operating parameters. According to the comparison between the results of theoretical calculation and practical engineering application, it was found that the designed vapor-condensing plume abatement system obviously benefits the water-saving of a mechanical-draft cooling tower and considerable economic benefits can be obtained. The contents presented provide the theoretical basis and technical support for the upgrade of the cooling tower and the design of the new cooling tower.

Vapor plume and melted zone behavior during dissimilar laser welding of titanium to aluminum alloy

The present study deals with continuous Yb:YAG laser welding of pure titanium to aluminum alloy A5754 performed with different beam offsets to the joint line. Spectroscopic and morphological characterization of vapor plume exiting the keyhole was combined with post-mortem observation and energy-dispersive X-ray spectroscopy (EDX) analysis of the welds. The laser beam centered on the joint line resulted in periodic transversal inclination of a vapor jet on the aluminum side associated with a local increase of melt width and an intense spatter formation. Such behavior can be attributed to the instability of the keyhole wall from the aluminum side. The beam offset on the titanium side led to the stabilization of vapor plume, the attenuation of spattering, and a minimal mixing between the two liquid metals. On the contrary, the beam offset on the aluminum side produced a violent formation of spatters and a highly unstable, diffuse vapor jet. In this case, the liquid metals underwent a violent mixing that was progressively reduced along with the decrease in quantity of melted Ti. The observed spectral regions contained the emission peaks of neutral Ti and Mn. Very few emission lines of Al were observed, because they require higher excitation energies compared to Ti and Mn. Boltzmann linearization using Ti emission peaks allowed the estimation of vapor temperature to be between 5000 and 6000 K, except for the welds with the beam offset on Al.