Study On The Stability Of Weld Pool In Variable Polarity Plasma Arc Keyhole Welding Of Thickness Plate High-Strength Aluminum Alloy

Variable polarity plasma arc welding(VPPAW)has become the most ideal welding method in aluminum alloy structure welding due to its more concentrated energy,small deformation,single side welding,double side forming and high weld quality.In the process of variable polarity plasma arc perforation welding of aluminum alloy,the keyhole molten pool steady stable becomes worse with the increased thickness of welding structure.Therefore,the stable mechanism of the keyhole molten pool in VPPAW of the thick plate2A14 high-strength aluminum alloy were studied in-depth which can improve the welding quality of thick plate 2A14 high-strength aluminum alloy and provide a theoretical reference for its extensive application in the engineering field.In the process of VPPA keyhole welding,according to the dynamic characteristics of the small hole,a binary segmentation threshold is designed based on Otsu algorithm,and use edge algorithm to extract the edge of the small hole image and carry out the least square fitting calculation.The research shows that the edge extraction algorithm can extract and calculate the characteristic parameters of keyhole accurately,and adapt to variable polarity plasma arc perforation welding under different welding parameters conditions.On this basis,the evolution of the characteristic size of the back of the keyhole is studied by using the single factor method with different welding current and plasma gas flow rate.The results show that the characteristic size of the keyhole increases with the increased welding current and plasma gas flow rate under certain welding parameters condition.When the change of the keyhole area is large,the keyhole cannot be stable and the weld seam collapses;When the change of the area of the keyhole is small,the keyhole is stable and the weld is formed well.The results show that the optimum welding parameters are: welding current in the electrode negative phase is 320 A,welding current in the electrode positive phase is 360A(average welding current is 326A),the plasma gas flow rate is 3.4l/min,the welding speed is 120mm/min.The influence of the welding parameters on the keyhole molten pool stable was studied by measuring the temperature distribution of the test plate in the welding process.The results show that when the thickness of the test plate is 12 mm,with the increase of welding current,the temperature gradient perpendicular to the weld direction increases gradually,and the temperature gradient in the thickness direction decreases gradually;When the plasma gas is increased,the effect of plasma gas flow rate on the temperature gradient is small,but on the perforation rate is large.When the thickness of the plate is 8mm,compared with the temperature distribution of 12 mm thick plate,the range of temperature gradient in the direction perpendicular to the weld is larger,and the temperature variation in the thickness direction is small.It can be seen that the reason for the poor stable of keyhole molten pool of 12 mm thick plate is that there is a temperature gradient in the thickness direction of thick plate welding compared with thin plate welding.The mechanism of keyhole molten pool stable is analyzed from the point of force analysis.The results show that when the additional pressure caused by the surface tension generated by the temperature gradient is equal to the arc radial force,the expansion rate of the upper part of the keyhole is equal to the contraction rate of the lower part of the keyhole,and the keyhole remains stable.However,in the process of thick plate welding,there is a temperature gradient in the thickness direction resulting in the variation of the surface tension,and the unbalanced force in the molten pool,and it is difficult to establish a stable keyhole pool,which makes it more difficult to weld thick plate.Finally,the mechanical properties and microstructure of 12 mm thick 2A14 highstrength aluminum alloy welded joint were analyzed.The results show that under the optimal parameters,the tensile strength of the joint is 297 MPa,the coefficient of welded joint is 61%,the elongation after fracture is 3.8%,and the microhardness at the center of the weld is 90 HV.