A Study On The Character And Mechanim Of Penetration Keyhole Instability In CO₂ Laser Welding

The processing stability during laser welding effects seriously on the weld quality, which is difficult to control. Therefore, industry sectors have been making great efforts to resolve this puzzle. Reviewing published study results about process stability, the instability behaviors can be caused by plasma screening under the condition of very high laser power welding, or by the fluctuation of welding parameters, or by laser welding mode transformation between penetration welding and conduction welding. But in practice it has been found that the instability of penetration keyhole welding occurs more universally, and results in the weld imperfection, such as partial penetration and porosity, etc.In this paper, with analysing studies of the instability during laser welding, it is thought that the unstable penetration keyhole process had close relationship with characters of the melten pool, keyhole and plasma/vapor, as well as the reactions among them. Both experimental and theoretically analysizing methods were used in order to investigate the process character, keyhole forming mechanism, and evaluating conditions and monitoring method of instability penetration keyhole in laser welding. The research results can benefit to lay the theoretic foundation for high quality weld structures.Based on the results of spectral analysis in laser penetration welding, two systems were set up. One is a high speed camera system, the other is a monitoring system which can detect visible optical signals, infrared optical signals and acoustic signals. The filter technology are necessory in the experiment. The images and signals monitoring of many transient process were observed, and some special keyhole laser welding phenomena are obtained successfully for the first time, such as plasma/vapor over the surface of melt pool and under the back of pool. According to these results, some new thoughts should be considered for process stability and instability in laser welding.The research results show that plasma/vapor occurs obviously in unstable periodicity. For CO2 laser welding 304 stainless steel( Ar was used as gas shielding), there are two characteristic frequency bands existing, one is 100Hz～500Hz and the other is 1500Hz～3500Hz. The former characteristic frequency band shows the fluctuation of plasma/vapor within the keyhole, and the latter band shows the fluctuation of plasma/vapor above the pool. The research results of keyhole features of the weld back indicate that the period of penetration keyhole mainly ranges from 2ms to 6ms, that is 167Hz～500Hz, which is the same as the plasma/vapor study results. During unstable penetration keyhole process, there is a special phenomenon that plasma/vapor on the weld back occurs and vanishs alternately.The experimental results related the image of the surface and back of the pool show that the time of high temperature is about hundreds ms, and has a close relation with the weld parameters in full penetration laser welding. The fluidity of the melt pool has a close relation with the vapor force of the opening keyhole. The research results also show that length of the back pool is the most suitable characteristic parameter to evaluate penetration keyhole instability process. So it is an available way keeping penetrationg keyhole stability that the welding heat input must ensure the length of the back pool over a threshold, which is affected by the kind of shielding gas and the laser power density. The research results about force effect of the penetrating keyhole process show that the main mechanism of forming an unstable penetrating keyhole is the alternating occurance between a penetrating keyhole and un-penetrating keyhole, with a larger vapor force during the penetration keyhole.Based on the experimental research and the theoretic analysis about penetration keyhole instability process, two methods evaluating penetration keyhole instability were suggested. One method is based on the weld back infrared signal monitoring, and another is based on the weld surface multi-sensor data fusion. For the laser welding 304 stainless steel experiment, both methods result in nearly 100% reliability in a single weld and different welds.In a word, properly choosing welding parameters that can make the length of back pool over a threshold can obtain stable penetrating keyhole process, and reduce porosity in the weld, and also control microstructure of the weld. More research can offer a theoretic foundation and practical method for high quality control of the welding process in the future.