Study On Evolution Of Porosity In Remote Laser Welding Of Aluminum Alloy

Because of its light weight,good corrosion resistance characteristics,Aluminum alloy are widely used in automotive lightweight manufacturing process.Welding shape of laser welding is good,and welding quality is high.Remote laser welding has good application prospects in industrial manufacturing because of its non-contact,high efficiency and good flexibility characteristics.However,due to its fast thermal conductivity,low surface tension and large amount of active elements,aluminum alloy could get many defects in the process of laser welding,especially keyhole-induced porosity,which is one of the main welding defects of aluminum alloy during laser welding.The presence of keyhole-induced porosity could significantly reduce the mechanical properties of the joint,and its irregular shape can induce stress concentration,leading to joint failure.In this study,the process of the keyhole-induced porosity formation in aluminum alloy laser lap welding is studied systematically by taking the A5182 aluminum alloy as the object of study and the numerical simulation as the main research method.The flow behavior of the molten pool and the keyhole dynamics under different conditions are studied,in order to provide a theoretical basis for suppressing the formation of keyhole-induced porosity.Firstly,based on fluid mechanics,the mathematical model of aluminum alloy laser welding was established with Flow-3D computational fluid dynamics software.The simulation of the laser welding process of the aluminum alloy was carried out,in which the laser welding heat source considered multireflection and Fresnel absorption,vapor reaction force,solidification shrinkage and phase transformation were considered.The temperature field,the dynamic change of the keyhole,the flow of the liquid metal in the molten pool and the formation process of the keyhole-induced porosity were calculated.It was found that the formation process of the keyhole-induced porosity consists of three stages: the formation of bubbles,the bubbles are brought into the molten pool and the bubbles are captured by the front of the solidification interface.Keyhole in the welding process is in the constant fluctuations,and the instability of the keyhole fluctuations leading to the collapse of the keyhole and bubbles formation.The mode of molten pool flow shows that the surface of the keyhole liquid metal flow downward under the action of the recoil pressure.There is a clockwise vortex in the lower part of the molten pool,so that the formation of bubbles was brought into the molten pool after forming.Due to the high thermal conductivity of aluminum alloy,the bubble in the molten pool is too late to escape the solidification interface.The bubble was captured and became porosity finally.Secondly,the study of porosity sensitivity under different welding parameters such as welding orientation,laser power,welding speed and laser inclination angle was carried out.It is found that the upright welding direction can restrain the formation of small pores,because the possibility of collapsing of the liquid metal behind the keyhole is weakened by gravity.High laser power,high welding speed can effectively inhibit the formation of porosity.Opened keyhole,gentle liquid metal flow in the molten pool,long lasting time molten pool could effectively inhibit the formation of bubbles and porosity.When the laser is leaning forward,the flow pattern of liquid metal in the molten pool changes.The larger the forward inclination angle is,the weaker the clockwise vortex in the molten pool,and flow direction is away from the keyhole wall.Because the keyhole front wall contains a small amount of liquid metal,it is difficult to collapse so that the forward laser position could also inhibit the formation of porosity.Finally,the effective methods to suppress the porosity are put forward,that is to improve the welding speed and welding power,and to increase the laser forward inclination angle.