Study On Pulsed MIG High Speed Welding Of Stainless Steel Based On Method Of Compensating Gas Jets To The Weld Pool

High speed welding is an important branch of the study on high-efficiency welding,and is of great significance to the improvement of welding technology.However,the problems of humping bead and undercut defects that may occur during high speed welding have gravely hindered the development of high speed welding technology for a long period of time.In this paper,the author put forward the method of "momentum analysis on weld pool flow" to explain the formation mechanism of humping bead and undercut defects,and proposed the method of "compensating gas jets to the weld pool" to control humping bead and undercut defects,and conducted theoretical and experimental studies.The main contents and innovations of this paper are as follows:1.In this paper,the author put forward the method of "momentum analysis on weld pool flow" to explain the formation mechanism of humping bead.This method takes the molten droplet,base metal and weld pool liquid flow during the welding process as three independent objects for study.It compared the difference between conventional weld pools and high speed weld pools from the perspective of movement velocity and momentum,and revealed that the momentum of the weld pool liquid flow was the main factor of humping bead formation during high speed welding.During high speed welding,the forward initial momentum of the molten droplet is large,and the action time of arc force and gas jet force on the weld pool liquid flow is short.Hence,the momentum of the weld pool liquid flow relative to the base metal would be small,the liquid flow's capacity of transporting liquid metal would drop remarkably,and it is unable to form a stable liquid metal accumulation with a constant height at the tail of the weld pool.Under such circumstances,the geometrical morphology of liquid metal in the weld pool is mainly dependent on surface tension,hence it is prone to contract at the periodic stagnation points and solidify to form humping bead defects.2.In this paper,the author put forward the method of "compensating gas jets to the weld pool" to control humping bead and undercut defects and hence achieve a "smoothing" effect.By introducing the method of applying gas jets to impact the unset weld pool,the author realized the aim of "human intervention into postwelding molten pool movement".Moreover the paper studied the feasibility of the method in theory,expounded its main characteristics and functions as well as its working mechanism,revealed the "smoothing" effect of the method of compensating gas jets to the weld pool in high speed welding,and put forward the general idea of type 304 stainless steel MIG high speed welding based on this method.Preliminary tests showed that gas jets had a visible effect on changing weld appearance and improving the thermal efficiency of welding.On the section where gas jets were compensated,weld reinforcement was lowered,weld width was enlarged,and weld penetration depth was increased slightly,hence the appearance and the overall penetration ratio of the welding beads are obviously superior to that obtained from the traditional welding technique.3.In this paper,the author established a smart welding platform for the method of compensating gas jets to the weld pool and a system of high speed welding parameter optimization and welding quality evaluation.The author constructed the gas path system for compensating gas jets,and adopted smart control strategies to control and optimize the arc welding power supply.The author established an expert database self-learning and selfregulation process parameter mechanism with large step length calibration and the concept of local Newton interpolation method.The author conducted multiple high speed welding process tests with and without gas jet compensation through optimizing the process parameters of high speed welding.The author adopted modern image processing technologies to establish a new method to extract and evaluate weld appearance,which uses the binary image method to extract weld profile information and utilizes the image intensity curve to represent the straightness feature of the welding beads,conducts pretreatment on the welding lines with and without gas jet compensation for edge detection and extraction through an edge detection operator and the mathematical morphology method,and uses the global image entropy and the picture similarity technology to conduct comparison and analysis on the pretreated welding images to obtain the quantitative evaluation results of weld appearance that uses gas jet compensation for welding.The test showed that under the same or similar parameters,the ratio of welding speed with gas jet compensation to that without gas jet compensation reached 220%-288.1%;the analysis on the cross-section of the welding line showed that the form factor of the bead and the penetration ratio were notably optimized,the maximum regulation to metal on the convex portion reached 68.16%,and the cross-section thermal efficiency of the metal in the base metal witnessed an increase of up to 22.6%.4.The author conducted a contrast study on the microstructure analysis and mechanics performance testing of welding lines with and without gas jet compensation.The result showed that the method of compensating gas jets to the weld pool could notably control the humping bead and undercut defects in high speed welding,significantly increase welding speed and improve weld appearance and mechanical properties of welds.Gas jet compensation could exert a visible stirring effect on the weld pool,and change the growth environment of coarse columnar crystals and dendrites,thus refining grains in the weld center,weld toe area,penetration zone,heat affected zone,weld surface and other regions.Tensile strength testing showed that the welding lines using the method of gas jet compensation prevailed in all index parameters including maximum load of displacement,maximum displacement of strain,maximum load of stress,yield point strain displacement,fault displacement of strain,fracture stress and breaking load.The ratio of the capacity to bear the breaking load of welding lines with gas jet compensation to those without gas jet compensation reached 110.4%-120.9%.The hardness of the central area of the welding line using gas jet compensation was higher,while the bend test indicated that the weld toughness with gas jet compensation was good,with no ruptures and crack sources.