Laser welding, which uses a high-energy laser beam as a welding heat source, has been widely used in material processing because of the advantages of energy concentration, high flexibility, and high production efficiency. As an important physical phenomenon in deep penetration laser welding, plasma generation contains considerable information about laser welding processes. The study of plasma behavior is critical for the quality monitoring of laser welding processes. In recent years, plasma electrical signal detection technology has been used because of its advantages of simultaneously acquiring plasma temperature and oscillation characteristics. With the further study of plasma electrical signals, the characteristics of electrical signals under different welds are becoming crucial parameters for monitoring laser welding processes. Some scholars studied the relationship between the oscillation of plasma electrical signals and weld penetration depth. However, the difference in plasma electrical signal characteristics between partial and full penetration welds have received little attention. Therefore, it is essential to analyze the plasma electrical signals under different penetration conditions and improve plasma detection methods for monitoring the laser welding process.

In this study, titanium alloy TC4 is used to investigate the characteristics of the plasma electrical signals under partial and full penetration welds. The titanium alloy plate with a 2-mm thickness is welded using an Nd∶YAG laser. Partial and full penetration welds are realized by adjusting the laser power and changing the welding speed. During the welding process, a synchronous acquisition system for plasma photoelectric signals is used. The short-time autocorrelation analysis method is used to analyze the difference in the oscillation frequency of plasma electrical signals between partial and full penetration welds.

Comparing the collected plasma photoelectric signals, the oscillation characteristics of electrical signals at different periods are different (Fig. 5). The electrical signals are segmented and every section is analyzed using short-time autocorrelation method to investigate the oscillation frequency variation of the plasma electrical signals in the entire welding process. The characteristic oscillation frequency of the plasma electrical signals in the initial stage is 241-669 Hz, which is evidently lower than that in the relatively stable stage, and the characteristic oscillation frequency decreases as the welding heat input increases (Fig. 6). The fluctuation interval length of the plasma characteristic oscillation frequency under the partial penetration weld in the relatively stable stage is only 200 Hz, whereas that under the full penetration weld can reach 500 Hz (Fig. 7), which is an important feature that can be used to distinguish the partial penetration weld from the full penetration weld.

In this study, a photoelectric observation system is constructed to obtain plasma photoelectric signals during laser welding of titanium alloy. The difference in the characteristic oscillation frequencies of plasma electrical signals is analyzed under different conditions of partial and full penetration welds. The main findings of this study are as follows.

In the initial stage, the characteristic oscillation frequency of the plasma electrical signals under the partial penetration weld is much higher than that under the full penetration weld. Simultaneously, the characteristic oscillation frequency of the plasma generated under the partial penetration changes faster than that under the full penetration . The differences in oscillation frequency and variable speed are important features that distinguish the partial penetration weld from the full penetration weld.

In the relatively stable stage, the characteristic oscillation frequency of the plasma electrical signals under the partial penetration weld fluctuats and the deviation is 7%-9% of the average. The characteristic oscillation frequency under the full penetration varies dramatically and the maximum deviation is over 40% of the average. The stability of the characteristic oscillation frequency is another important feature that distinguishes the partial penetration weld from the full penetration weld.