A Study Of Powder Pool Coupled Activating TIG(PPCA-TIG) Welding For Stainless Steel

The Powder Pool Coupled Activating TIG Welding（PPCA-TIG）is a kind of activating TIG welding method based on GPCA-TIG welding and GTFA-TIG welding,which is a novel type of double layer gas protection and transfer activating powder TIG method.Compared with the previous activating TIG welding,the most difference is that tungsten is protected by inner layer argon or other inert gas,meanwhile the activating flux powder mixing protective gas is transported into the outer layer of arc-molten pool area by the automatic powder feeding device to accrue the aim of increasing the depth of penetration,improving the welding efficiency and welding mechanization and automatization.The method can be used to realize the welding of various metal alloys,such as stainless steel,aluminum,magnesium,etc.,and can a void the pollution of the activating powder to the tungsten electrode.First of all,the SUS304 stainless steel used in industry is chosen as the test plate for welding,and 7 kinds of reagents,such as oxide,halide,fluoride and eleme ntal,are selected as the activating powder.By contrast with the traditional TIG welding,the effects of different activating powder on 304 austenitic stainless steel PPCA-TIG weld forming,arc shape,and tungsten electrode were investigated,the results showed that the oxide increased the weld penetration to the conventional TIG welding is more than 3 times,tellurium can make the weld penetration increased to about 2.4 times.Silicon,halide and fluoride were not obviously increase the poor penetration.Silicon oxide,tellurium,and fluoride halide,can make the arc voltage increases and the arc contraction.Oxide leads to change the marangoni convection of the molten pool from the outside to the inside.The microstructure of the weld is mainly consist of austenite and ferrite,as well as the ferrite morphology is mainly the skeleton.At th e same time,the PPCA-TIG method can effectively avoid the pollution on the tungsten electrode from the activating flux powder.Choosing the single component SiO₂ as the welding process performance experiment of activating flux,analysis of welding speed,arc length effect on weld surface formation,and using orthogonal design method,study the current,inner and outer layers of gas flow,the motor speed effect on weld depth and weld depth to width ratio.Optimizing the results and get the optimum parameters of PPCA-TIG welding with SiO₂ activating flux.Finally the organization and the performance are analyzed.The results show that Welding current has a very important influence on the weld penetration,but has no effect on the depth to width ratio;Outer gas flow has the most influence on the depth to width ratio,while the impact on the weld penetration is lesser,within the scope of the test,weld depth and width is synchronous change;The inner gas flow has more influence on weld penetration,but relatively small effect on the depth to width ratio;And motor speed influence on the weld penetration and the depth to width ratio are all relatively small,which belongs to the secondary factors;Under the optimum technological parameters,PPCA-TIG welding can continuously penetrate 8 mm thick stainless steel plate without opening groove,in the premise of guarantee weld surface forming good issued to the effect of one-side welding of forming;PPCA-TIG weld shows good capability of plastic deformation and low temperature impact toughness.In the process of SiO₂ activating powder particles from arc to molten metal,using electrostatic probe powder capture technology combined with SEM/EDS analysis of the morphology and composition changes of activating powder of arc,and then use XRD,SEM,EDS,AES and hydrogen nitrogen oxygen measuring instrument analysis the weld surface welding slag,surface oxidation in the weld metal layer and the composition distribution and phase distribution,finally using the thermodynamic analysis of the reaction process of activating flux with arc,molten metal.At the last,establishing the transition model of SiO₂ activating powder from the arc to molten metal.Results showed that SiO₂ particles dissociated into O and Si atoms in the arc;Dissociation of Si,O elements into the molten pool,such as Cr,Mn,Si and the effect of alloy elements,through direct oxidation,the oxidation reaction of indirect oxidation and final cooling,a total of three stages to form the weld surface oxide layer;Weld surface oxide layer is mainly composed of macromolecular structure of SiO₂,close the six-party structure of Cr2O3 and with spinel structure of Fe3O4,Mn3O4 and Fe Cr2O4 oxide,this diversity with the crystal structure of the weld metal,make the weld and the bonding force betwe en welding slag,weld slag off good;Using SiO₂ flux of PPCA-TIG welding does not enable a large increase oxygen and silicon for the weld.PPCA-TIG welding can solve the problem that GPCA-TIG welding can not weld aluminum,magnesium and other active metals,and can effectively protect the tungsten electrode compared with GTFA-TIG welding.The welding method is simple and reliable,does not require precise powder conveying device and gas mixing device,can be used for different welding plate by flexible selection of the corresponding activating flux.Trace amounts of activating flux of the outer gas can immensely increase the penetration depth of weld,and the weld has good mechanical properties,also can realize the automatic welding.Therefore,PPCA-TIG welding has a broad application prospect in the future.The work of this paper is of great significance to promote the application and research of the welding method.