Study On The Formation Mechanism And Suppression Measures Of Weld Undercut In High-speed GMAW

As an important part of advanced manufacturing technology,welding process has a significant influence on the final quality and efficiency of products.It has become the key factor to enhance the core competition of the enterprise.Recently,gas metal arc welding(GMAW)is predominantly employed in manufacturing because of its advantages,such as relatively low cost,easier operation and better adaptability compared to other welding processes.Increase welding speed is an effective approach to improve productivity.However,if the welding speed is beyond a certain limit,weld formation will be deteriorated and undercut defect will occur.It creates a mechanical notch at the weld interface that lowers the dynamic load capacity of the welded assembly by inducing stress concentration and corrosion cracking.More severely,it can also induce humping defect when the welding speed increases further,which restricts the application of GMAW in manufacturing industry.Therefore,it is of great theoretical and practical significance to make effective improvement for the conventional GMAW to realize low-cost and high-speed welding,under the precondition of understanding the formation mechanism of undercut defect.This paper develops the visual inspection system for arc shape,droplet transfer,temperature filed and molten metal flow on weld pool surface.The heat and force distribution characteristics of arc and droplet are investigated under different welding parameters.The influences of molten metal temperature filed,stress state and fluid flow on weld formation are analyzed quantitatively.The feature parameters are extracted to establish the three-dimensional transient numerical model to study the molten pool fluid dynamics and heat transfer for GMAW process.Then the force characteristics in weld pool on transverse/longitudinal direction,(which including arc force,electromagnetic force,hydrodynamic pressure,surface tension and droplet impinging),are analyzed quantitatively.Their effects on weld pool deformation,temperature distribution,and molten metal flow are further studied.The relationships between "welding parameters-thermal and stress state of molten metal-weld pool behavior-tendency of undercut defect" are established.The undercut formation mechanism and main factors are revealed from the view of weld pool three-dimensional morphology and thermal-stress state.Based on the above research,the assistant gas tungsten arc with low current is used to suppress undercut defect by adjusting the arc and droplet heat distribution,and improving the thermal-stress state and molten metal flow in the weld pool.The GMAW process is systematic analyzed by adopting a method that combines experimental measurement,numerical analysis and experimental verification.The results show that the tendency of undercut defect is mainly affected by the transverse/longitudinal molten metal flow in the front of weld pool.When the ratio of longitudinal flow velocity to transverse flow velocity is greater than the critical value(2.0),the undercut defect will occur.The numerical simulation indicates that the arc pressure and shear stress are so big that there is a very thin liquid metal layer under the arc when using high current in high-speed welding.On one hand,this thin liquid metal layer facilitates the droplet impinging to transform into backward flow,and then leads to accumulation of high temperature filler metal at the tail of weld pool.On the other hand,this thin liquid metal layer solidified rapidly behind the arc center,and then obstructs the transverse spreading of molten metal to fill the weld toe.Thus,undercut defect occurs.In TIG-MIG hybrid welding,the TIG arc markedly reduces the coupled arc pressure and droplet impinging.It slows down the backward flow velocity of molten metal and facilitates its transverse spreading.The undercut defect is suppressed effectively and the welding speed is improved greatly.