Research On AC Flash Butt Welding Based On Quantitative Analysis Of Beam Blasting Characteristics

Flash butt welding is the most widely used welding method in the field of rail welding,but the problem of flat spot defects in flash butt welding has seriously affected the progress of flash butt welding technology for a long time.In this paper,using high-frequency electrical signals and high-speed images as technical means to observe the flash welding process of small samples on the test platform,the feature extraction method of beam blasting for AC flash butt welding is studied.The method is used to extract the characteristics of beams with different parameters during the accelerated flash stage,and the influence of welding parameters on the characteristics of beam blasting was studied.At the same time,the influence of welding parameters on joint performance and flat spot defects was studied.Based on the characteristics of beam blasting of different joints,the reasons for the large-area flat spot defects were analyzed,and the formation mechanism of flat spots was established in combination with the composition and properties of flat spot inclusions.The main research work and achievements of this paper are as follows:（1）The high-frequency electrical signals and high-speed images of the flash butt welding process of the test platform were collected,and combined with the welding curves,the characteristics of the beam blasting at each stage were systematically analyzed and studied.Based on the characteristics of the equivalent contact resistance in the welding process,the formation and blasting mechanism of the beam are perfected:the formation of the beam is divided into two types,the first type is due to the current heating the solid phase contacts and melting them;The second type is due to the flow of liquid metal remaining on the end surface under the action of surface tension,which has no connection with electric heating.Electromagnetic contraction force F_c can not significantly shrink the diameter of the beam and accelerate the blasting of the beam.The retreat action of welding will destroy the stability of the beam and accelerate the blasting of the beam.（2）Analyze the high-frequency electrical signals of AC flash welding,and divide them into four types according to the half cycle:short circuit,open circuit,single beam blasting and multiple beam blasting.Using the power time-domain map generation method based on data adaptive decomposition and the CNN image recognition method,the recognition rate of high-frequency electrical signals reaches 98.2%.For each beam blasting within a half cycle,extract beam blasting times N,beam duration time T₁,beam blasting time T₂,beam blasting current change△I,beam blasting power change△P and beam input energy W as beam blasting characteristics.And the accuracy of characteristics extraction of beam blasting based on high-frequency electrical signal is verified by using high-speed images to extract the time of beam blasting.（3）The stability of beam blasting and process curve in each stage of flash welding are analyzed and studied,and the stability of accelerated flash stage is the best.At this stage,the influence of welding parameters on the characteristics of the beam blasting was studied:the influence of welding voltage on the beam blasting rate was limited,and when the forward speed increased,the beam blasting rate increased from about 30%to about50%.When the welding voltage increases,the beam blasting times N increases;the mean value of beam duration time T₁ increases significantly in the high-voltage region;the beam blasting time T₂ has no significant change;the mean value of beam blasting power change△P,beam blasting current change△I and beam input energy W decreases.When the forward speed increases,the beam blasting times N and the mean value of beam input energy W increase,but the beam blasting times N is limited in the low-voltage region;the beam duration time T₁ and beam blasting time T₂ have no significant changes;the mean value of beam blasting power change△P and beam blasting current change△I increases in the low-voltage region,but has no obvious change in the high-voltage region.When the welding voltage and forward speed increase,the maximum values of beam duration time T₁,beam blasting power change△P,beam blasting current change△I and beam input energy W will be significantly increased,which increased the maximum value of beam blasting strength.（4）The process parameters in the accelerated flash stage have limited influence on the tensile strength and impact absorbed energy of the joint,the area of a single flat spot defect S_max is the key factor affecting the fracture and performance of the joint.In the tensile specimen,there is a linear relationship between the percentage of single maximum flat spot defect area and the percentage of joint tensile strength.The corresponding fitting formula is R_m/R_m’=-2.1568S_max/S_W+101.7597,and the fitting R2 reaches 0.9461.In the impact specimens,large flat spot defects reduce the impact absorbed energy about 30%on average.（5）The increase of welding voltage and forward speed in the accelerated flash stage will increase the tendency of flat spot defects in joints.Under the conditions of 400V and1.2mm/s,the probability of large flat spot defects in joints reaches 62.5%.By analyzing the beam blasting characteristics of different joints,it is found that when strong single beam blasting occurs before upsetting,and the subsequent half cycle is open circuit;this type of beam blasting characteristics will retain the crater state formed by strong single beam blasting to the upsetting stage.A large area of flat spot defects will appear in the joint.The critical beam blasting power change△P of small sample flash butt welding is about 60k W.（6）According to the beam blasting characteristics of large area flat spot defects and the composition and melting point analysis of flat spot inclusions,the formation mechanism of flat spot defects is proposed:During the flash butt welding process,liquid phase inclusions are oxidized to form,which are insoluble in molten steel.Due to the low temperature of the end surface or the unevenness of the end surface,the extrusion of liquid phase inclusions is greatly hindered during the upsetting process.The flat spot defects will be formed if liquid phase inclusions remain in the weld of the joint.