Parameters and some applications of plasma generated during keyhole welding using a highly concentrated energy beam – an overview
Abstract
Keyhole welding using concentrated energy beams (electron beam and laser beam) is at the forefront of welding technology and finding ways to improve welding quality is a pressing issue. Finding optimal welding modes, monitoring weld quality and/or detecting weld defects in real-time during the welding process using nondestructive, costeffective and reliable methods is one of current challenges. Plasma generated in the keyhole and the plasma plume (space above the welding pool) provides an opportunity to study welding stability and optimal modes as well as formation of weld defects.
Generation and characteristics of plasma in the keyhole and above the welding pool are discussed in the paper. For laser keyhole welding spectral analysis data and video image techniques are widely used for control and inspection of laser induced plasma in real time. Electron beam welding is studied using plasma parameter measurements and by studying the current collected by the positively polarized ring electrode above the welding pool. In case of electron beam welding with beam oscillations the method of coherent accumulation is applicable to analyze of the plasma fluctuations process at the plasma electron current.
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Introduction
Electron beam and laser beam welding are widely used technologies for joining of metals due to numerous advantages in comparison to other welding technologies. However, certain problems arise in the keyhole welding process, related to instability of weld joint formation and difficulties in creating and controlling the optimal welding modes. One of the main concerns of the industry is to assure the weld quality in real-time using a cost-effective and reliable method. It would be significant for the industry to be able to find optimal welding modes and/or detect defects non destructively in real-time during the welding process.
One of the phenomena that occur during interaction of concentrated energy beam with metal sample is generation of plasma in the welding zone. Study of plasma characteristics and their relation to process/product performance/quality could help increase knowledge of control of electron beam welding using concentrated energy beams and create approaches for its optimization. Due to the complex character of keyhole welding using high energy beams, differences in interactions of both beams with materials, and lack of adequate models of physical processes in the crater in the welding bath optimization and quality improvement of electron and laser beam welding technologies are empirical and still need more research.
Conclusion
In this paper recent studies are reviewed as well as some older ones that haven‟t been improved upon since their publication. The paper is focused on understanding the processes of plasma generation and transportation when welding using concentrated energy beams, as well as known attempts of use of these processes for monitoring the stability and quality of laser and electron beam welding.
In the case of vacuum electron beam welding the efforts are included in study of secondary particles behavior and of collected by plasma currents. Probably due to limitations of the vacuum chamber results of optical studies of emitted from plasma light are not published. Unfortunately investigations of plasma behavior for EBW at middle and atmospheric pressures were not found.
A model of plasma formation in the keyhole in the liquid metal and above the EBW zone has been described. Plasma parameters for keyhole and space above the welding pool were simulated. Thermionic electron emission was calculated for the keyhole wall. The calculated data is in good agreement with the experimental data. It is shown that there is a need to take into account the effect of the strong electric field in the keyhole near the keyhole wall on electron emission when calculating the current for non-independent discharge.
From reviewed papers one can conclude that optical sensing has become a mature real time monitoring technology for laser welding in laboratory and industrial practice. Photodiode sensors with advantages of low cost and simple structure provide rich information on high frequency features of the emitted light. This makes this method adaptable to larger scale industrial manufacturing. The visual camera gives a great deal of spatial information and has high accuracy in detecting weld defects. The spectrometer captures data for evaluating plasma characteristics during the laser welding process, which help to analyse welding stability and defects of the weld seam. Besides choosing a suitable filter system in video-imaging is considered the key step for obtaining accurate detecting information. Spectral analysis and optical images of plasma plume and of work piece surface in welding pool area are the work horses that are utilized to study stability and defects in laser keyhole welding process.
Signals obtained through different sensors can be preprocessed by way of signal and image processing technologies to extract signal features. It is based on this that classifier connecting feature characteristics and weld defects are setup to conduct quality inspection. Little research has been conducted on adaptive control during the welding process, and the controlled variables are mainly confined to more widely disperse low power laser system.
Nevertheless, it should be noted that real time detecting and controlling technologies for electron beam and laser welding are far from perfect, and the detecting accuracy for different welding statuses and defects is expected to be improved. Unfortunately, there is not a multitude of sensors available to detect all kinds of welding statuses and defects. Moreover, the hardware characteristics of the sensors, such as sampling frequency and resolution, need updating. The limited computing speed of intelligent signal processing and recognition technology also restricts wide use of real time detecting.