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The construction ofmonitoring systems for a process of laser welding with deep penetration requires a choice from all possible signals, bearing information about the process [1], those, which is the most complete satisfy to the technological requirements, as essence ofthe control process. There are: 1 . Presence ofthe clear and physical -adequate connection between parameters ofthe signal measured and behavior ofthe welding bath; 2. Possibility to "recalculate" the signal measured in technologically significant parameters: a depth and width of the welding seam, and perhaps its mechanical properties, during a time, acceptable for the technologists; 3. Possibility to monitor the appearance ofdefects ofthe weld seam, such as pores, root peaks and etc. by the measured signal; 4. Convenience ofmeasurement and processing and protection from external interference. It has to note, that the acoustic signal so far have not physically clear connection with behavior of the welding bath, in spite ofthe fact that a lot of works are devoted to study ofthem in a number of years. Experimental researches ofprocess ofacoustic emission at laser welding [2]togetherwith results of frequent analysis have shown dependence ofthe signal level on intensity ofthe laser radiation and processing regime. Correlation ofthe acoustic emission signal with radiation power, welding speed and level ofradiation focusing are established by the authors ofthe [3]. The similar results are described in a number ofworks [4,5].Itis established specifically, that the acoustic signal bears in selfthe information about presence ofthe through or partial penetration [5,6],however to understand the acoustic emission nature, there are used as a rule either phenomenological reasons [7] or the models which far from reality, for example [8]. The authors of [9] basing on [10] connect the acoustic signal with time derivative of the evaporation mass flux but thus they completely neglect a real physical picture of generation ofthe sound in the active zone. The purpose of given investigation is to establish the connection and decide a question about usefulness ofthe acoustic emission to monitor the process of laser welding. Let's begin from description ofthe phenomenon and formulation of the investigation problems. The cavity is a source of sound oscillations at laser welding with deep penetration. By evaporation from the cavity surface a vapor flow running into surrounding space is formed. Self-oscillations oftemperature of the cavity walls, described in [1 1], result in oscillation ofthe evaporation mass ratio, and oscillating walls of the cavity modulate a vapor jet in addition. By virtue of connection described by gas-dynamic equations the velocity and temperature oscillations ofthe vapor jet lead to those ofpressure, i.e. to appearance of the sound oscillations. When acoustic frequencies observed in experiments on laser welding (f-i 0 -I0000 lIz) are typical, length of the acoustic wave "A" is much more than the cavity depth and especially than its radius. Since for the surrounding space the outlet section ofthe cavity is a point source ofthe acoustic oscillations. Power ofthis source and its frequent spectrum are defined by nature ofthe radiation process of the sound fluctuations from the cavity into the surrounding space. It is necessary to take into account, that the gas temperature and hence the sound velocity inside the cavity and in the surrounding space are essentially distinguished. The size of the transitive area above the cavity, where the sound velocity changes from value which is typical for the cavity up to the value corresponding to the room temperature, is a lot of less than length of the acoustic wave. Therefore we can neglect its sizes and require on some border (coinciding with the metal surface), in order that the solution ofthe problem about internal acoustics of the cavity coincides with the solution of the external problem about spreading of the oscillations in the surrounding space
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