Study On GMAW-S Droplet Transfer Instantaneous Process Based On Visual Technology

Short-circuit transfer is an important metal transfer mode in gas metal arc welding. Featured with high-quality, great efficiency, low-cost, energy-saving, and high-productivity, it is widely used in sheets welding. And as it belongs to the complex nonlinear time-varying systems, the possibility of the generation of the welding spatter as well as the forming of the final weld are determined by the GMAW-S instantaneous processes. Therefore, researchers over the world have carried out a lot of studies and exploratory work on the behavior of droplet and weld pool in GMAW-S. Such studies, however, have been for a long time conditioned by limited experimental techniques, researching tools and other restrictions, thus have been mainly focused on modeling and process optimization based on signal process, judgment of transferring state based on arc acoustic signal, width and penetration control based on image signal and partly state analysis based on droplet images. The research platforms are relatively dispersive; the models are simplex; the results are not obvious and hard to verify. Therefore, a thorough study of the mechanism of GMAW-S instantaneous process based on integrated research platforms will further deepen the understanding of GMAW-S process and provide scientific basis for new GMAW-S welding process control ideas.In this paper, a set of macro high speed vision sensor system which is composed of backlight system, filtering system and imaging system, was developed.In the design of backlight system, based on the spatial mode of the semiconductor laser AlGaInP adopted in the research, the optimal optical parameters of spatial filter and collimated expander were determined by the results of the calculations based on Gaussian beam transmission and the theory of fluctuations in optical imaging and the top-hat beam was eventually obtained.In the design of filtering system, according to the principles of polarized light, a polarized optical system was designed to replace the traditional pinhole for the purpose of achieving continuous filtering. Together with the protection lens and interference filter, a coaxial-conditioning system was assembled through threaded connectors.In the design of imaging system, based on the parameters of the high speed camera adopted, detailed droplet images of GMAW-S and GMAW-P of higher imaging ratio were obtained by using C/F adapter and macro zoom.The establishment of this system eliminates many technological bottlenecks in high speed image capturing of welding process, i.e. long time consumed in equipment structuring, complexity in parameter setting, poor applicability of welding parameters and low quality of image. It thus provides possibility for carrying out wider range of welding process studies based on image monitoring.In this paper, a GMAW-S welding experimental platform which is composed of welding control system, shielding gas mixing system, real-time electrical signal acquisition system and high speed vision sensor system were developed. Based on this platform, the influence of different components of shielding gas to the state of GMAW-S droplet transfer was studied based on the GMAW-S welding experiment with the same ratio shielding gas as in simulation.For the complex background noise in GMAW-S instantaneous process images such as mass welding fume, diffraction fringes, etc., a linear image processing algorithm which combines frequency-domain calculation was proposed. Through image spatial-frequency domain transformation, attenuation of low-pass filtering, gray-scale mapping and Gaussian low-pass filtering, as well as Laplacian sharpening, the GMAW instantaneous process images with distinct details and much lower background noise were obtained while the nature of the images remain unchanged. On this basis, the cube B-spline wavelet was constructed according to Mallat tower algorithm and the edge extraction of droplet, liquid bridge and weld pool in GMAW-S instantaneous process images was achieved by the means of multi-scale wavelet edge detection algorithm.In the paper, the droplet forming instantaneous process was studied and the mechanical model of this process was established. Based on the calculation of this model, the results demonstrated that the surface tension was the major driving force in droplet forming process, and the drop can maintain droplet shape in the expansion process since the wire melting speed is the same with the drop expansion speed.Based on the magneto hydrodynamics (MHD) theory and computational fluid dynamics equations, a two-dimensional dynamic mathematical model of instantaneous GMAW-S droplet stimulated deforming process was established, with consideration of electromagnetic force, surface tension, plasma flow and gravity. The evolution of droplet profile in growing in shape and falling of current, the flow mechanism of internal metal liquid and the velocity distribution were analyzed, based on numerical simulation.The instantaneous short circuits (ISC) process was thoroughly studied in this paper and an ISC force equilibrium critical equation was established. Using image analysis technology combined with quantitative calculation of electrical signals, it is proved that the critical equation can reflect the real ISC process. Further, the definition of ISC was discussed and the past definition"the electrode touches the weld pool for a very short period of time, but no metal transport takes places"was considered to be inaccurate. The time concept in definition was recommended to be abandoned, and ISC should be defined as a short-circuit phenomenon where melting electrode makes contact with the weld pool without liquid bridge formed. Moreover, a short-circuit transfer process in which liquid bridge formed but not completely extended was defined as instantaneous short circuit transfer (ISCT) process.