Research On Metal Transfer And Arc Stablity Of MAG Welding In High-strength Steel For Vertical And Horizontal Position

With the development requirements of large-scale and light-weight in ships and offshore platforms as well as ship-to-ship equipment,the application of highstrength steel in ship-building is becoming more and more widespread.In this study,the welding base metal is a 20-50 mm thick 10Cr5 Ni Mo V high-strength steel,which belongs to the welding of thick plate.In order to improve the welding quality and efficiency,a new technique of single-sided MAG for backing welding and double-sided MAG welding for filling welding is selected.This paper focuses on the study of back welding by MAG welding of high-strength steel for vertical and horizontal positions.The droplet transfer images and welding voltage and current signals in the welding process were recorded synchronously by means of a highspeed camera and an electrical signal acquisition system,which helps us observed the arc morphology and metal transfer stability during welding.A comprehensive evaluation and analysis of the arc stability,droplet transfer stability and weld bead formation in the welding process were conducted.The welding specifications were optimized so that backing weld with stable welding process and good weld formation was obtained.The effects of the contents of CO2 and He on the arc stability,metal transfer stability and weld formation were studied in the flat welding position.Studies have found that the addition of CO2 or He to the shielding gas causes the arc to shrink and the arc length to become shorter.When CO2 was added by 2.5% only,there was a serious cathode speckle drifting phenomenon.The instability of the arc led to an unstable one drop for one-pulse and unstable several drops for one-pulse transfer.When the CO2 content increases to 5% and 7.5%,the stability of the arc was enhanced,and the transfer form of the droplets changes from a stable one drop for one-pulse to stable several drops for one-pulse.With the increase of He content,the overall transition frequency showed a downward trend.The shrink of arc will affect the shape of the weld,the more CO2 and He content were,a larger degree of arc shrinkage was,leading to a deeper bowl-like penetration.For upright MAG backing welding,in order to avoid the molten metal squat and the arc heat concentration in the middle of the groove,swing arc was used.Orthogonal test was used to study the effects of welding parameters,including the swing length L,the swing width D,the ratio of the welding gun's one-side dwell time k,and the electrical parameter welding current I on the arc stability,metal transfer stability and weld beading.The results showed that with the increase of welding speed and the ratio of one-side dwell time,the sensitivity of the selfadjustment of the arc increased,but the stability of the arc didn't increase continuously with the increase of the welding speed.When the welding speed was increased to 0.08m/min,the arc stability declined.As the welding current increased,the stability of the arc decreased as a whole.The influence of the swing length on the arc stability was not obvious within the range set in this experiment.With the increase of welding current,the weld thickness increased;the welding speed and ratio of one-side dwell time increases,and the weld thickness decreased.As the welding speed increased,the weld height decreased first and then increased.When the welding speed was small,increasing the speed appropriately can avoid excessive accumulation of molten metal and reduce the height.However,when the welding speed was too fast,the solidification rate of the liquid metal was accelerated accordingly,which was not conducive to the spreading of the liquid metal in the groove,leading to an increase of weld seam height.When the ratio of one-side dwell time increased,the horizontal speed of the arc sweeping over the groove increases,the molten metal does not easily accumulate in the middl e,and the overall height of the weld decreases.Having arc stability,metal transfer stability and weld formation were analyzed and evaluated comprehensively,the specification of vertical welding bottom welding with stable welding process and good weld formation were obtained.The welding torch didn't oscillate in the horizontal position welding,and an asymmetrical groove was adopted.The torch was tilted down and aligned with the position of the upper and lower grooves which can helps the arc affect the upper and lower grooves symmetrically and prevent the melting metal in the upper groove from sagging.The effects of welding speed and welding current on weld formation was investigated.It was found that when the welding speed increased,the penetration depth and weld thickness of the upper groove decreased gradually.As the welding current increased,the depth of penetration of the upper groove and the thickness of the weld increased gradually.Within the set level,the effect of the welding speed on the depth of penetration of the upper groove and the thickness of the weld is significantly greater than that of the welding current.The metal transfer stability and weld formation were evaluated comprehensively.As a result,the best welding specifications were obtained.