Formation Mechanism Of Weld Appearance Defects In High-speed GTAW And Their Suppression Measures

Gas tungsten arc welding(GTAW)is one of the most crucial and widely used joining processes in manufacturing industry.With the development of modern industry,it is required to achieve higher productivity without sacrificing weld quality.The most direct and simplest strategy is to increase welding current and welding speed proportionately.However,when the welding current or welding speed is above a critical value,undercut defect which is generally defined as groove or depression located at the toe of weld bead is easily formed.As the welding current and welding speed further increase,the weld bead may be undulated along the welding direction,namely humping defect.The sharp weld bead profile may cause stress concentration or even crack,which severely deteriorates weld properties.The formation of undercut and humping is closely related with complicated and multi-coupled weld pool behavior.In this thesis,a three-dimensional transient computational fluid dynamic(CFD)model is developed to calculate the temperature distribution,fluid velocity,deformation of weld pool free surface and solidified weld bead profile in high-speed GTAW using commercial software ANSYS FLUENT.Considering the significant free surface deformation in high-speed GTAW,the arc heat flux,arc pressure,arc shear stress and electromagnetic force are modelled varying with weld pool surface evolution self-adaptively.The detailed morphology of undercut and humping in GTAW,for the first time,can be simulated numerically.The comparison between numerical and experimental results shows good agreement.The numerical data are combined with parametric experiments and visual inspection to investigate the defects' formation mechanism quantitatively.The numerical study of undercut shows that the weld pool free surface is significantly depressed under strong arc forces during undercut formation.A thin liquid layer exists in the gouging region of weld pool.When severely depressed region occupies the maximum width of weld pool,the thin liquid layer at periphery easily solidify prematurely,which initiates undercut formation.The liquid metal flows along trailing periphery of teardrop-shaped weld pool to generate an inward velocity component with magnitude of 0.1 m/s.The undercut formation is promoted by this inward velocity component.The numerical study of humping shows that the backward momentum of liquid metal is increased when welding current and welding speed further increasing.The liquid metal at trailing region of weld pool solidify rapidly due to the high welding speed.The backward flow is much stronger than the backflow and the gouging region is elongated to initiate humping formation.The lateral channel at periphery of gouging region is the main transfer channel for metal flow.When the lateral channel is extended out of the arc heat region,it solidifies instantly to prevent the liquid metal flowing backward.The effect of multi-coupled driving forces on the weld pool behavior and defect formation in high-speed GTAW is investigated quantitatively through combination of numerical model and sensitivity analysis.The arc shear stress is revealed to be the dominant driving force to facilitate free surface deformation and backward flow of liquid metal.The capillary pressure plays a dominant effect on suppressing free surface deformation.Sensitivity analysis also shows that arc shear stress and capillary pressure are the most promoting and suppressive factors for defect formation,respectively.Based on Buckingham ?-theorem,a dimensionless group containing characteristic heat and fluid flow variables of weld pool is derived to evaluate the formation tendency of weld bead defect in high-speed GTAW quantitatively.Then,the relationship between characteristic weld pool variables and welding process variables are studied using scaling analysis,and the obtained scaling laws is verified and calibrated by numerical data.The defect formation tendency at different welding parameters can easily be evaluated or compared using the dimensionless group and scaling equations,only if the welding process variables are known.A high-speed tandem GTAW process is proposed to suppress the undercut and humping formation,in which an assistant arc is introduced to heat the trailing region of the weld pool to prolong the presence of liquid metal and to control metal flow.The influences of assistant electrode parameters and welding current combinations on weld appearance are studied through statistical design-of-experiment.The welding speed of 1.5 mm thick 409L stainless steel plate is increased to 3.0 m/min without sacrificing weld appearance quality.Good mechanical properties and a fine microstructure of welded joint were obtained.