Porosity Characteristic Of Laser-arc Hybrid Welding Of Aluminium Alloy And Hollow Profiles Weldability

It is of great significance for the rail train lightweight-manufacturing for improving the transport capacity and reducing energy consumption.Thanks to the low density,high strength and fine weldability characteristics,aluminium alloy was the preferred material for high-speed trains manufacturing and has been used in the train body welding manufacturing.Laser-arc hybrid welding method not only keeps the high energy density of laser beam but also reduces the requirements of workpiece assembling tolerance by the arc bridging,and it has gradually become the main welding method for aluminium alloy high-speed trains manufacturing.However,hydrogen porosities due to the sudden changes of hydrogen solubility in aluminum alloy welding pool and process porosities due to the instability of keyhole in laser beam welding are easily generated in the weld seam and will weaken the effective bearing area of the weld cross section and cause serious damage to the joint properties.Therefore,the existence of porosities in aluminium alloy welding is an key issue which cannot be ignored.In this paper,the main research material is the A7204 aluminium alloy for high-speed trains.Firstly,taking laser-cold metal transfer(laser-CMT)and laservariable polarity TIG(laser-VPTIG)hybrid welding methods to make a systematically study of the influence of welding parameters on porosities in the seam.Secondly,aiming at reducing porosity rate during welding for sheet metal,experiments of adding Nb foil were carried out with laser-CMT and oscillating laser-CMT hybrid welding respectively.The porosities distribution characteristics of weld cross section under different adding amount of Nb foil were studied and the influence mechanism of Nb element and oscillating laser beam on weld porosities was revealed as well.Finally,taking 6A01-T5 aluminum alloy profiles with lock bottom for high-speed trains as the research object,the process applicability of the oscillating laser-CMT hybrid welding method on the lock bottom structure was studied by combining with the weld bead porosities distribution characteristics,microstructure and mechanical properties of the joint.About the effects of process parameters on welding porosity for laser-CMT hybrid welding,experimental results show that the porosity rate remains within a low level range that less than 2.00% and does not change significantly with the welding parameters when the plate thickness is 4 mm and process porosity became the main reason for porosity when the plate thickness increased to 6mm.Under the influence of large-size process porosities,the porosity rate of the weld is significantly higher than that when plate thickness is 4mm.The porosity rate decreased with the increase of power and welding speed,while the wire feeding speed changes had no significant effect on the porosity rate.By reserving a certain gap between the butt joints,the occurrence rate of process porosities in the weld can be effectively reduced.For laser-VPTIG hybrid welding of plate with 6mm thickness,the sensitivity of process porosity in the weld is obviously lower than that of laser-CMT hybrid welding.With the increase of laser power and arc current,the porosity rate in the weld decreases.Under the high power and high current,the incidence of process porosities is significantly reduced.The experimental results of Nb foil adding in laser-CMT hybrid welding show that the number and size of hydrogen porosities in the cross-section of the weld bead decrease with the increase of the amount of Nb foil added.Nb added reduces the occurrence of hydrogen porosities in welds by its grain refinement function,hydrogen absorption and fixation function.For the oscillating laser-CMT hybrid welding,the porosity rate in welds is still at a relative low level with the addition of Nb foil,and the obvious suppression of the occurrence rate of process porosities is the most significant feature of the oscillating laser hybrid welding.Thanks to the oscillating of laser beam,the stability of keyhole and the changes of flow behavior of the welding pool play a key role in restraining process porosities during welding.Finally,taking the oscillating laser-CMT hybrid welding method with better porosities inhibition effect applied to the 6A01-T5 lock bottom hollow profile welding and the process applicability of the technology was studied as well.The results show that besides well appearance of the lock bottom join,the internal porosity rate is less than 1.00%,and the fatigue limit of the hybrid joints is 105.0MPa and the average tensile strength of joints with reinforcement is 223.19 MPa,while the tensile strength is up to 84.22% of the base metal.The oscillating laser-CMT hybrid welding method shows the fine weldability for the aluminium lock bottom hollow profile.