Research On The Formation Of Porosity In Deep Penetration Laser Welding On Aluminum Alloy

Due to the characteristic such as low density and excellent specific strength,aluminum and its alloy has been widely adopted in automotive in the tendency of auto light-weighting.However,the keyhole is unstable during the welding process,which may lead to the collapse of keyhole and the formation of pores,and that can do great harm to the mechanical property of weld seam and reduce the reliability of the whole structure.In this research,the dynamic behavior of keyhole and flow field in molten pool can affect the formation of keyhole-induced porosity,and adjusting welding parameters can affect the dynamic behavior of keyhole and molten pool effectively,which will then suppress the formation of keyhole-induced porosity in weld,and improve the quality of weld and its structure which will function in vehicles.This research involves both experiment and simulation,to probe into the character of porosity,the effect from welding parameters on keyhole-induced porosity,the variation of metallic plume over the keyhole,the dynamic behavior of keyhole,and fluid flow in molten pool,the formation mechanism and prevention method of bubbles and keyhole-induced porosity during the deep penetration laser welding,then draws some conclusions.First,welding test on 8 mm A356 aluminum alloy plates,1 mm + 2 mm A5182 aluminum alloy,the distribution,outline and microstructure inside the pores were observed and analyzed and found that,the Al-Si dendrites inside the metallurgical porosity are in the same size and distribute uniformly,while it's unevenly distributed and in different size on the inner wall of keyhole-induced porosity.In the light of the results,when increasing the laser output power or reducing the size of laser spot,i.e.increasing the laser energy density,the variation of metallic plume got much more intense,and the pores remained inside the weld rose.When increasing the laser travel velocity from 1.0 m/min to 2.5 m/min,the porosity in weld reduced at first,then rose,reached the minimum value at 2.0 m/min,that's the result of competition between the stability of keyhole and moving velocity of solidification interface.When reducing the ambient atmosphere,the pores got suppressed,meanwhile,because the change of ambient atmosphere can affect the boiling point and recoil pressure,that makes the weld narrower and deeper,and increasing the depth-width rate of weld.When welding on over-lap joints,the gap between two plates can affect the stability of keyhole and the formation of keyhole-induced porosity,and the porosity increases with the size of gap,but when the gap reaches 0.4 mm,the collapse happens more frequent,the depth of keyhole can be kept at a lower value,and leads to the reduction of weld reliability.Then,take the physical process observed and analyzed in the deep penetration laser welding test into consideration,with the help of CFD software Flow3 D,the model of deep penetration laser welding on aluminum alloy overlap joints was set up in the research.The model involves physical models including heat source model,driving force,solidification and phase change model,constant pressure bubble model,and so on,then set initial and boundary conditions in the model,and simulated the process of keyhole laser welding,the dynamic process of keyhole and formation of keyhole-induced porosity successfully.The result reveals that the formation mechanism of keyhole-induce porosity divides into two different ways:(1)bubbles induced due to the collapse inside the keyhole,and then got captured by moving solidification interface to be porosity;(2)the bottom of keyhole got linked with the solid metal,i.e.the depth of keyhole gets larger than molten pool,then collapse happens inside the keyhole and the part contains shielding gas or metallic vapor get isolated in the molten pool from keyhole to be porosity.These two formation mechanism can be greatly affected by the dynamic behavior of keyhole and fluid field.Through the experiment and simulation research,the tight relationship between keyhole-induced porosity and dynamic behavior of keyhole can be found,and the formation mechanism of the porosity is also related to the fluid flow.Through adjusting and optimizing welding parameters can influence the stability of keyhole and fluid field of molten pool,then the generation of keyhole-induced porosity,and improve the quality of weld seam and its structure.