Experimental Study On High Current MIG/MAG Welding

In China and even the world,the amount of welding work is extremely larg,as the main welding method of arc welding,MIG/MAG welding is the best choice for high efficiency welding due to its advantages of high welding quality and ea sily realize automation,exploring more efficient MIG/MAG welding method s and processes is a hot topic in the international welding industry.Increasing welding current has a significant influence on the weldin g efficiency of MIG/MAG welding,however,when the current exceeds the second critical value,the change of droplet transfer mode causes the welding arc instability and the spatter suddenly increasing,this hinders the raising of welding efficiencyIn this topic,the commercial MIG/MAG welding machine was retrofitted to increase the wire feeding speed from 22m/min to 50m/min,thereby the welding current reaches 600A.Based on this,the droplet transfer behivour and movement frequency of wire end fluid tip during high current MIG/MAG welding were studied,the influence strength and regularity of each factor on the critical current value of the droplet swinging transfer during MAG welding were analyzed,and a simple model was established to explain the formation mechanism of droplet swinging transfer and mixed transfer.In addition,the influence of welding current on spatter rate and metal evaporation rate was analyzed during MAG welding,the above research contents lay a theoretical foundation for the subseque nt realization of high efficient MAG welding.In order to further enhance the welding efficiency of MAG welding,the welding current and wire ex tension length were tested for their contribution to wire melting rate of MAG welding,because wire melting rate represents welding efficiency to some extent.In order to solve the problem of unstable droplet transfer under high current and large extension length,an alternating magnetic field is applied in welding process,alternating magnetic field interacts with the arc to generate an alternating Lorentz force,the force affects the droplet transfer pattern,arc shape,and weld formation during high current MAG welding.Studies shown that when the welding current exceeds the second critical current in MAG welding,the droplet transfer becomes swinging transfer,or the mode of mixed transition,that is simultaneously exist of swinging transfer and rotating spray transfer,but for MIG welding,exceeding the second critical current,the mode of droplet transfer becomes rotating spray transfer.During the rotating spray transfer of MIG welding,most of the fluid tip and the liquid stream are always surrou nded by the arc,which is a kind of stable rotating spray transfer;in the MAG welding,the mode of swinging transfer and rotating spray transfer are extr emely unstable,with lots of welding spatters and weld formation is poor.In addition,the rotational frequency of the rotating spray transfer during MIG welding is always around 500Hz,during MAG welding,the transfer frequency increases with the increase of welding current,but when the wire feeding speed reaches a certain value,due to the influence of short circuit between the fluid tip and the pool will hinder the movement of fluid tip and reduce the frequency.Through orthogonal test,found that wire extension length is the most significant effect on the critical current value of the unstable transfer of droplet,CO₂ proportion and arc voltage is relatively weak,and the critical current value decreases with the increase of the CO₂ proportion,as the voltage increases,the critical current increases.The root cause of the unstable transfer of the MAG welding is constriction disconnected and fluid tip upturned when the droplet projected transfer,the biased fluid tip deviates from the axis of wire to swing or rotate under the co-operating force of the electromagnetic force and the metal evaporation reaction force,the liquid stream and arc are driven along with the fluid tip,and then forming swinging transfer or rotating spray transfer,the longer wire extension length,the easier for fluid tip to deviate from the axis of the wire and the easier to create unstable transfer.Tested the welding spatter rate and evaporation rate,found that the spatter rate is closely related to the droplet transfer mode;however,the evaporation rate is not affected by the droplet tra nsfer mode and is related to arc temperature,as the welding current increases,the evaporation rate continues to increase.In MAG welding,increasing welding current and wire extension length can increase the wire melting rate to more than 20Kg/h,more than twice the upper limit of the traditional MAG welding 10 Kg/h,therefore,increasing welding current and extension length are the most direct route in enhancing the welding efficiency,and statistical regression established test formula of efficient MAG welding wire melting rate for 1.2mm-diameter wire.Applied alternating magnetic field can effectively improve the arc stiffness and stability of the high current MAG welding,thereby shortening the length of liquid stream,reducing the fluid tip deflection degree,making the droplet unstable transfer stable,decreasing the welding spatter,improving the weld formation and the welding efficiency is greatly increased.