| Zinc-coated steel sheet is the main material of automobile body-in-white.Considering the existence of multi-layer zinc-coated steel sheet lap joints in the body manufacturing,laser welding with high processing efficiency and small welding deformation becomes increasing common in the automotive industry.However,the evaporation of the low-boiling zinc-coated layer results in violent spatter during the laser welding process,which seriously affects the quality of the weld.Therefore,the spattering in laser lap welding of multi-layer zinc-coated steel sheet is still a key problem that makes the weld formation poor.In this paper,laser lap welding of double-layer hot-dip zinc-coated steel sheets is studied,and a "gas-liquid-solid" multiphase three-dimensional numerical model for laser lap welding considering zinc vapor flow is established.The flow behavior of molten pool and the occurrence of spattering under different process parameters,gap sizes,spiraling and zig-zag paths are studied,the interaction between zinc vapor and keyhole and its influence on molten pool flow are clarified,and the effect of zinc vapor on spattering occurrence is also revealed.Based on these analyses,a spatter control method for laser welding of zinc-coated steel sheets is proposed.Main conclusions are summarized as follows:Firstly,a three-dimensional model of zinc-coated steel laser welding considering the jet pressure force and the drag force is established,and the effect of zinc vapor forces on molten pool flow and spatter size under different laser power and welding velocity is clarified.Numerical results show that when the welding velocity is 50 mm/s and the laser power increases from 2.0 k W to 3.0 k W,the jet pressure force and the drag force increase,the rear wall of the keyhole is seriously dented,and a large amount of fluid behind the keyhole flows upward at high speed,resulting in large spatter.When the laser power is 3.0 k W and the welding speed increases from 50 mm/s to 70 mm/s,the pressure and drag force of the zinc vapor jet decrease,the liquid near the back wall of the keyhole flows upward at high speed to form small spatter,and the size of the spatter is reduced from 0.35 mm to less than 0.1 mm.With the aid of the high-speed photographic observation system,the effect of zinc vapor on the keyhole,molten pool morphology and spatterring during the laser welding process is analyzed.It is found that the zinc vapor between the zinc-coated steel sheets first brakes through the front wall of the keyhole and entered the keyhole,forming a jet of zinc vapor toward the rear wall of the keyhole,which impacts the rear wall to produce an obvious dent,and then the liquid near the keyhole rear wall flows upward to form spatter following the escape of the zinc vapor,which verifies the reliability of the simulation results.Secondly,it is found in the experimental study that zinc vapor enters the keyhole periodically under the condition of zero gap.Based on this,the mass and momentum source models of zinc vapor are established,which are coupled with the threedimensional numerical model to simulate the influence of the flow behavior of zinc vapor on the molten pool flow and spattering occurrence under different gap conditions,and the dynamic mechanism of zinc vapor on the molten pool flow and the basic conditions for spattering occurrence are revealed.The simulation results show that:when there is no gap,the zinc vapor periodically breaks through the front wall of the keyhole and enters the keyhole at an initial velocity of 81 m/s,the zinc vapor firstly impinges on the back wall of the keyhole,causing it to deform significantly,and then escapes upward along the back wall of the keyhole at a speed of up to 20 m/s,transferring momentum to the liquid near the back wall of the keyhole,making the nearby melt flows upward at a velocity upper than 1.97 m/s,forming spatter.When there is a preset gap of 100 μm,the front wall of the keyhole is not kept closed during the welding process,so the zinc vapor escapes from the keyhole and the gap after being generated,resulting in a decrease in the speed of zinc vapor entering the keyhole to 33m/s,and then the zinc vapor escapes upward along the back wall of the keyhole at a velocity of 5 m/s,the liquid near the back wall of the keyhole flows upward at a speed lower than 0.75 m/s without spattering.Finally,a numerical model for laser oscillating welding of zinc-coated steel is established,and the effect of the evaporation behavior of the zinc-coated layer under the spiraling path and the zig-zag path on spattering is studied,and the critical velocity of the liquid in the molten pool to generate spatter is defined.The results show that: in the laser welding with the spiraling path,when the laser oscillates forward,that is,when the component of laser scanning direction is toward the welding direction,the zinc layer evaporates in a large area,and the initial velocity of the zinc vapor entering the keyhole is 93 m/s.A large amount of zinc vapor enters the keyhole,which causes the keyhole to be elongated.At this stage,the liquid in the molten pool is accelerated to 1.7 m/s by the impact of zinc vapor,and the flow velocity of the liquid remaining in the molten pool next to the spatter is 1.36 m/s,so it is proposed that the critical velocity of the spatter generated in the molten pool is 1.36 ～ 1.70 m/s.When the laser oscillates backward,the metal is reheated and almost no zinc vapor is produced,so the welding process is stable at this stage.In the zig-zag oscillating laser welding,since the laser always oscillates towards the welding direction during the welding process,the evaporation of zinc is relatively stable,and the initial velocity of the zinc vapor entering the keyhole is 48 m/s.In this case,the welding process is stable,the flow rate of the liquid in the molten pool is 0.9 m/s,and the laser welding with the zig-zag path could effectively control the spatter during the laser welding of zinc-coated steel,and the result reasonably explains the phenomenon observed in the experiment. |
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