Swing Arc Characteristics And Weld Formation Mechanism Of Narrow GAP GMAW

In view of the existing problems in narrow gap gas metal arc welding(GMAW),such as serious wear of contact tip,difficult formation in spatial position welding,and poor environmental adaptability,this study took the swing arc narrow gap welding process as the research object,developed swing arc narrow gap welding torch and its control system,studied the electrical characteristics,mechanical characteristics and thermal characteristics of swing arc and weld formation mechanism,and finally established regulation approaches of swing arc narrow gap weld formation by selecting swing parameters and controlling weld deviation.This study laid a theoretical foundation for improving the welding joint quality of thick plate narrow gap GMAW and promoting the application of swing arc narrow gap GMAW process.To improve the adaptability of swing arc narrow gap welding,this paper designed the swing arc welding torch system by double single-chip microcomputers which worked in time-sharing control pattern,and proposed a telescopic adjustable nozzle to improve the gas protection of narrow gap welding torch.On this basis,this paper realized the digital setting and real-time display of swing frequency,at-sidewall staying time and swing angle,the automatic positioning control of arc swing trajectory midpoint and real-time detection of arc swing frequency,which provided a reliable platform for the subsequent swing arc narrow gap welding experiments.By establishing a mathematical model of swing arc length,this paper studied the effects of swing frequency and at-sidewall staying time on arc length and welding current,and verified the effectiveness of the model by welding experiments.In I-type welding groove,arc length in the middle of groove is longer than it at groove sidewalls,while the corresponding instantaneous arc current in the middle of groove is smaller than it at the sidewalls.The raising of swing frequency and at-sidewall staying time can increase the average arc current,thus promoting the groove sidewalls penetration.Some swing arc narrow gap flat welding experiments with flux-cored wire were carried out,and the effects of swing parameters on flux-cored wire arc narrow gap weld formation,slag detachability and arc stability were then discussed.By increasing arc swing frequency and at-sidewall staying time,the groove sidewall penetration in flux-cored wire arc welding grew,while the slag detachability became worse.The droplet transfer process of flux-cored wire and solid wire were observed contrastively by using arc high-speed camera and electrical signal acquisition system.The core melt-retardation of flux-cored wire guides droplets to transfer along with the axis of wire,while the higher swing frequency can also help droplet transfer,thus improving the flux-cored arc stability.Besides,an approach of swing arc narrow gap one-side welding with back formation was proposed,and steady weld formation was obtained by selecting small swing frequency with long at-sidewall staying time.After single pass multilayer welding,a joint of 50 mm thick high strength steel was successfully obtained by swing arc narrow gap one-side welding with back formation.Then,the approach of swing arc narrow gap vertical-up welding with free weld formation was proposed.After arc force impulse was defined,by the mathematical model of average arc force,the effects of conductive rod bending angle and groove gap on vertical-up weld formation were investigated.With an increase of bending angle and a decrease of groove gap,the upward component of average arc force is increased,which can restrain the downward flow of welding pool.On the other hand,the decrease of average arc force component which points to groove sidewall thus reduces sidewall penetration.Meanwhile,this paper presents the mathematical models of arc force impulse ratio and average arc linear heat input ratio in arc staying stage to in arc moving phase,which will help to reveal the effects of swing frequency and at-sidewall staying time on weld formation.It is shown that,the increase of swing frequency and at-sidewall staying time can raise the average arc linear heat input ratio and decrease relatively the heat input into the groove center,thus restraining the downward flow of welding pool;On the other hand,the increase of arc force impulse ratio pushes the liquid metal of welding pool to groove middle,which easily causes the downward flow of welding pool.The above two factors jointly affect the free formation of vertical-up weld.Compared with solid wire,the slag of low melting point can effectively restrain the downward flow of welding pool in flux-cored wire swing arc narrow gap vertical-up welding,which results in the concave surface of weld.From these,swing arc narrow gap vertical-up welding with free formation is realized successfully by selecting bending conductive rod with proper swing parameters.A three-dimensional analysis model of welding thermal field and pool flow filed was established for swing arc narrow gap welding,and the flow behaviors of welding pool were respectively simulated for flux-cored wire and solid wire in flat welding and vertical-up welding,by which the flow behavior of swing arc narrow gap welding pool and its effects on weld formation were revealed.In flux-cored wire narrow gap welding,the slag of low melting point easily gathers in groove middle,which restrains the downward flow of welding pool in vertical-up welding and thus yields a concave surface of weld.Finally,the practical welding experiments verified the validity of simulation model.Furthermore,the effect of weld deviation on narrow gap weld formation was discussed,and the passive visual sensing detection approach of weld deviation was proposed.When the weld deviation reaches over 0.5mm,the penetration of groove sidewall that is far away from the swing arc center sharply decreased,meanwhile,the undercut defect appeared on the opposite sidewall,which significantly affected the cross sectional formation of weld.Accordingly,a passive visual sensing detection approach was proposed according to the welding images that were captured in synchronous triggering actions of arc swing position and pulsed arc current base period,and a detection algorithm of weld deviation was then presented based on local pattern recognitions of sidewall edge and wire positon.Also,the effects of welding process interferences on welding image feature extraction were revealed.Under the strong interferences of welding spatter,arc light and welding fume,the detection precision of weld deviation fell within ±0.1mm,and the detection time of one weld deviation cost less than 30 ms,so that the real-time monitoring on the weld deviation was realized while establishing a technological foundation for the future weld track.