Study On The Process And Mechanism Of Flux Bands Constricting Arc Welding For High-strength Steel Sandwich Panel

Under the requirement of made in China 2025 strategy,how to make high-speed vehicle lightweight through material innovation has become an important issue.High-strength steel sandwich panel has the advantages of high specific strength,high specific height and good energy absorption,which provides a potential solution for lightweight of high-speed vehicles.How to effectively connect the core-plate and the face-plate is always the most difficult problem in the manufacturing of metal sandwich panel.In order to make up for the insufficient weld width of T-joint of laser welded high-strength steel sandwich panel,a new welding method,flux bands constricting arc welding?FBCA?,is proposed in this paper.Aiming at the problems of flux constricting arc mechanism,arc behavior,droplet transfer characteristics and arc stability in the process of FBCA welding T-joint,a complete FBCA welding T-joint equipment system,a high-speed camera system for collecting arc behavior and a real-time current-voltage acquisition system for collecting electric signals in the welding process are built in this paper.The method and equipment of FBCA welding T-joint,the constricting mechanism of flux bands on the arc in ultra-narrow gap weld groove,the weld forming mechanism under different processes,the influence of different processes on the arc behavior,the droplet transfer characteristics and arc stability of FBCA welding T-joint under different processes are systematically studied.The microhardness,microstructure and mechanical properties of the F BCA welded T-joint are systematically studied.Finally,the equipment and method of FBCA welding high-strength steel sandwich panel are proposed,and the advantages and feasibility of using FBCA welding method in the manufacturing of high-strength steel sandwich panel are demonstrated.The completion of this work will put forward a new potential FBCA welding method,provide a systematic theoretical support for further understanding of the arc behavior mechanism under the action of flux bands in the ultra-narrow gap weld groove,enrich the arc physical content,provide a technological basis for the FBCA welding method,and at the same time,provide a process basis for the application of FBCA welding method in the production of high-strength steel sandwich panel.The research results are as follows:The FBCA welded T-joint was successfully realized.Through a lot of welding experiments,it is found that welding deviation,welding leakage and slag inclusion are the main defects in the process of FBCA welding T-joint.Based on the model of one side welding leakage of T-joint,it is found that the deviation of welding wire??x?,welding current?I?and the length of welding root surface?L_r?are the main factors affecting the side leakage of weld.When the error of centering movement of welding wire is less than 1 mm,the welding deviation can be effectively prevented,and the welding leakage can be effectively prevented by adding copper welding gasket s.The slag inclusion and porosity are mainly caused by the collapse of flux bands.The external placement method can effectively ensure the stability of flux bands and avoid slag inclusion and porosity.The mechanism of flux bands constricting arc is obtained.It is mainly reflected in three aspects:1.Solid wall constricting effect?SWCE?:the solid flux bands is nonconductive,and the insulating flux bands can effectively prevent the arc conducting with the side-wall,thus preventing the arc burning position climbing-up phenomenon and improving the arc stability.At the same time,the solid flux bands exert an extrusion effect on the arc,change the arc shape,and accordingly change the droplet transfer characteristics;2.Thermal compression effect?TCE?:the melting of the flux bands will take away a lot of arc heat,so as to regulate the heat distribution of the arc in the groove,change the penetration of the face-plate and the core-plate;at the same time,the melting of the flux bands will compress the arc spots,further change the arc shape and droplet transfer forms;3.Self gas-slag making function:CO₂ gas and CaO-CaF₂ slag system are produced by melting of flux bands,which has special metallurgical protection effect.Moreover,flux droplets are produced by melting of flux bands,and the interaction process of flux droplets and metal droplet forms the special droplet transfer characteristics of FBCA welding process.Through further research on the weld forming under different welding parameters,it is found that the best process range of FBCA welding T-joint is 220-300 A,23-29 V,groove width is 5-8 mm.The morphology of FBCA welding T-joint is different from that of the traditional weld,which is similar to the square shape,and the fan shape appears on the surface of the weld.The fan shape trend of the weld increases with the increase of the arc voltage.The formation mechanism of the weld is revealed by the study of the arc behavior under different welding parameters:the shape of the arc is consistent with the final shape of the weld.With the increase of arc voltage,the arc shape changes from bell to square;with the increase of welding current,the arc shape changes from rectangle to square and then to cone;with the increase of groove gap,the arc shape changes from square to bell.The main reason for the fan-shaped trend of the weld surface is that the arc is squeezed in the ultra-narrow gap groove.The arc is elongated in the welding length direction,and the arc shape becomes dovetail due to the failure of the flux bands,which results in the increase of the weld surface melting width.Based on the study of droplet transfer characteristics in different process of FBCA welding,it is found that there are four modes:short-circuiting transfer,arc-bridging transfer,fine droplets transfer and repelled transfer.The arc voltage affects arc constricting degree.In the case of weak constricting,the arc shape changes little,the metal droplet has enough space to grow up,and the gravity becomes the main force of droplet transfer,forming a typical short-circuiting transfer.Under strong constricting,the arc is extruded,the arc shape changes into a square,the arc anode spots jump to the end of the welding wire,so that the metal droplet overall stress hinders its downward transfer,forming a unique arc-bridging transfer.The"bridge"condition is that the metal droplet force hinders the droplet downward trans fer,and the droplet diameter of the flux droplet is larger than the metal droplet diameter.Through the analysis of the stability of the welding process under different processes,it is found that the arc stability is the highest under the condition of the arc-bridging transfer and fine droplets transfer because of its less arc extinguishing time and short-circuit time.Through the study of the structure and mechanical properties of the F BCA welded T-joint,it is found that there is no obvious softening of the HAZ in the FBCA welded T-joint,and the maximum tensile,bending and shear loads of the T-joints are greater than those of the laser welded T-joint,mainly because it effectively avoids the stress concentration at the root of the T-joint.Finally,on the basis of the above research,the FBCA welding high-strength steel sandwich plate is carried out,the equipment system is built,and the good forming of the FBCA welding long weld is obtained.The X-ray testing is carried out,and no obvious welding defects are found,which provides the process basis of production of FBCA welding high-strength steel sandwich panel.