| The ultra-thin sheets(with thickness of 100μm or less)is very sensitive to heat input,so it is easy to burn through and deform during welding.The generation of these defects is closely related to the oscillation process of the molten pool of ultra-thin sheets.The oscillation of molten pool is a phenomenon of periodic fluctuation in the molten pool formed by the base metal heated and melted in the welding process,which includes the natural oscillation of molten pool and the induced oscillation caused by external force.The natural oscillation of the molten pool is directly related to the size of the molten pool,which is related to the material characteristics,while the induced oscillation of the molten pool is related to the action of external forces(mechanical vibration,pulse arc force,etc.)during the welding process.The oscillation process of molten pool contains a lot of characteristic information,such as the shape of molten pool,the dynamic flow process of molten pool,and the state of base metal penetration,which is of great significance to its research.Because of the limitation of the thickness of the base metal,the amplitude of the molten pool is very weak,so it is difficult to observe the fluctuation of the molten pool by naked eyes.It is also difficult to detect the oscillation of the weld pool directly because of the limitation of the position of the welding gun and the strong interference of the electric arc;Therefore,it is of great significance to study and find out the method suitable for the ultra-thin sheets molten pool oscillation with P-MPAW,and comprehensively and deeply study the characteristics of the ultra-thin sheets pulsed micro plasma arc welding pool oscillation based on the detection,so as to reduce the burn through and deformation defects easily occurred in the welding process of ultra-thin sheets(thickness of 100 μm and below).In this study,the P-MPAW process and forming process of 304 stainless steel ultra-thin sheets with 100 μm thickness were studied.Through the different matching of four parameters(peak current,base current,duty cycle,frequency)of pulse current,the heat input of welding 304 stainless steel ultra-thin sheets with thickness of 100 μm was carefully adjusted.The burn through and deformation defects in the welding process of ultra-thin sheets has been successfully avoided,and the single side welding and double side forming of ultra-thin sheets welding were realized.The results show that under the conditions of welding current of 1.8A,welding speed of 4.2 mm/s,torch height of 2 mm,ion gas flow rate and shielding gas flow rate of 0.5 L/min and 3.5 L/min respectively,five kinds of single side welding double side forming welds can be obtained for the ultra thin 304 stainless steel sheets with 100 μm thickness.Among them,the pulse frequency has the greatest influence on the weld formation.With the increase of pulse frequency,the change of weld morphology shows a trend from molten pool discontinuity to continuous unequal width,to molten pool remelting,to fish scale shape,and finally to continuous molten pool.Secondly,based on the arc voltage,the reflected light on the surface of the molten pool and the vision sensor,three methods of detecting the P-MPAW molten pool oscillation were proposed.Based on these three methods,the real-time detection of the P-MPAW molten pool oscillation was carried out.The results show that:(1)Based on the arc voltage,the real-time detection of P-MPAW molten pool can be realized,and the arc voltage signal noise is small and the quality is good;(2)Based on the reflected light of the molten pool surface,the P-MPAW molten pool oscillation of the ultra-thin sheets was detected.The original weak molten pool oscillation was amplified by the optical amplification of the laser,and the real-time detection of the P-MPAW molten pool oscillation of the ultra-thin sheets was realized successfully;(3)The detection of P-MPAW molten pool oscillation based on visual sensor is the most intuitive way to detect the molten pool.After morphological processing of the molten pool image,the area,boundary,diameter and length of the molten pool were obtained.Through the dynamic change of each characteristic information of the molten pool,the oscillation fluctuation and waviness characteristics of P-MPAW molten pool were studied.Finally,through filtering noise reduction and time-frequency analysis information processing technology,the characteristic information of P-MPAW molten pool oscillation was extracted successfully.On this basis,the influence mechanism of pulse parameters on the pool oscillation of P-MPAW ultra-thin sheets was further studied.The results show that:(1)Based on the arc voltage measurement,the oscillation characteristics of the molten pool are as follows: under the same duty cycle and pulse frequency,the natural oscillation frequency of the molten pool decreases with the decrease of the difference between the base current and the peak current;the natural oscillation frequency of the molten pool increases with the increase of the pulse frequency.However,at high frequency,the increasing trend of the natural oscillation frequency with the increase of the pulse frequency becomes slower,and the maximum oscillation frequency of the molten pool will not exceed 300 Hz;(2)The change rule of the natural oscillation characteristics of the molten pool with the pulse parameters based on the reflected light detection of the molten pool surface is basically the same as the change rule of the natural oscillation characteristics of the molten pool with the pulse parameters based on the arc pressure detection.However,due to the existence of interference factors,the characteristic information of P-MPAW molten pool oscillation based on the two methods has errors;(3)The oscillation amplitude of P-MPAW molten pool decreases with the increase of pulse frequency;With the increase of pulse frequency,the distance between the first wave band and the boundary of the molten pool will decrease,the width of the first wave band will decrease,and the gray value of the first wave band will increase. |
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