Research On Characteristics And Energy Transfer Mechanism Of Laser Welding Under Vacuum For Aluminium Alloys

With the exploration of strategic equipment in aerospace and deep-sea fields,an increasing demand of welding for the thick plate aluminum was advanced.Laser welding displays a strong penetration capbiliyt at preened while porosity defect is still the largest challenge and highest obstacle for the application of thick plate aluminum alloys.Previous researches show that laser welding under vacuum can effectively suppress the plasma plume,obtain a large penetration depth and improve the welding quality.However,the researches focus on the analysis of weld formations and plasma plume characteristics.The mechanisms of laser energy transmission,and the exploration on the stability of keyhole and molten pool behaviors are unclarified.In this study,35-mm-thick 5A06 aluminium alloy plates are used to investigate the effect of ambient pressures on the laser welding characteristics.The experimental results show that the penetration depth increases significantly and the porosity defect and the plasma plume are effectively suppressed with the reduction of ambient pressure.The characteristics of joints with a higher uniform microstructure and a wider heat-affected zone is produced by laser welding under subatmospheric pressures.The fusion zone and bottom zone of weld are mainly composed of equiaxed crystals and columnar crystals,respectively.The mechanical properties of joints improve markedly compared with those produced under atmospheric pressures.The average hardness and tensile strength of the joint increase more than 8.0% and 20.7%,respectively.There are four main reasons: the inhibited porosity defects,the uniform microstructure characteristics,the decreased burning loss of the element Mg,and the larger number of LAGBs and dislocations that exist in the joints.A critical ambient pressure of 101 Pa can be obtained by investigating the effect of ambient pressure on the weld characteristics.The weld characteristics change little when the ambient pressure is lower than this critical value.In addition,the ultra-high power laser welding characteristics of 30 k W under 101 Pa are explored.High qualified 104 mm and 130 mm thick 5A06 aluminum alloy butt joints are produced by a single pass and both sides welding,respectively.The key factors for the improvement of weld quality are the dynamic behaviors of plasma plume,keyhole and molten pool.The experimental results show that the size and brightness of plasma plume are inhibited sharply with the decrease of ambient pressures,while the stability of plasma plume is improved.Besides,the temperature and electron density of the plasma plume are lower,since the severe decrease of mutual collision frequency between particles under subatmospheric pressures.A mass of compounds,such as Al H and Al H2,are dectected in the plasma plume through the collection of mass spectrometry detection.The element H is proved to originate from the dissolution in base materials rather than the surrounding environment.The absorption mechanisms of plasma plume on laser energy transfer are combinations of Inverse-bremsstrahlung absorption,scattering absorption and other forms absorption.Inverse-bremsstrahlung absorption palys a dominant role when welding under atmospheric pressure while the other forms absorption will play a dominant role when welding under lower ambient pressures.The other forms absorption may come from the extinction effect of compounds in the plasma plume during the laser energy transfer.When the ambient pressure is lower than 103 Pa,the plasma plume has little effect on the laser energy transfer.A novel 3D numerical model is developed to describe the keyhole dynamic and melt flow behaviors during laser welding process of 5A06 aluminium alloy under subatmospheric pressures.The effect of ambient pressure on laser welding process is taken into consideration by optimizing the boiling point of aluminium alloy and recoil pressure of evaporated metallic vapor jets based on vapor–liquid equilibria calculation and Wilson equation.A moving hybrid heat source model is employed to describe the laser energy distribution under subatmospheric pressures.Numerical results indicate that a wider and deeper keyhole with less humps is produced under subatmospheric pressure comparing with that of atmospheric pressure.The vortices in the rear keyhole wall became unapparent or even disappeared with the decrease of ambient pressures.The melt flow velocity on the keyhole wall was larger under a lower pressure.A smaller difference between boiling point and melting point was produced and this led to the formation of a thinner keyhole wall and therefore improved the stability of molten pool.Larger recoil pressure produced under subatmospheric pressure was responsible for the weakened vortices and enhanced melt flow velocity.Bigger keyhole opening size,larger melt flow velocity,thinner keyhole and the weakened vortices all resulted into the reduction of porosity defects during laser welding of aluminium alloys.Based on the simulation results,the plasma distribution,weld formation and porosity defects has been demonstrated.The compared results show that the simulation results exhibited good agreements with the experimental ones.In summary,through the above research on laser welding characteristics,plume characteristics,keyhole and molten pool dynamic behaviors,a relatively systematic framework for laser welding under subatmospheric pressures is established.These works can provide some theoretical basis and experimental references for the application of laser welding of thick section structures,and also illuminate the related mechanism of laser welding process under subatmospheric pressures.