| The formability of resistance spot welding of magnesium alloy is not ideal.In order to explore the application of wrought magnesium alloy in the production of automobile body,it is urgent to develop a spot welding technology suitable for automated production.Nd:YAG pulsed laser spot welding is an efficient and precise welding method.It has the advantages of short welding time and high welding accuracy.Moreover,it requires less space for welding structure and fixture.Therefore,it is an ideal spot welding technology for the production of automobile body.At present,the research on Nd:YAG pulsed laser spot welding mainly focuses on stainless steel,low carbon steel and aluminium alloy,while the research on magnesium alloys is rarely reported.In view of the widely used AZ31 wrought magnesium alloy,this paper for the first time carries out the research of pulsed laser spot welding of magnesium alloy.The main research contents and conclusions are as follows:?1?The formation mechanism of pulsed laser spot welding,the microstructures of welding metal and the formation reasons of welding spot defects are analyzed under the default pulse of the Rectangular.The results show that conduction mode occurs at the welds with peak power densities between 2.5 GW/m24.5 GW/m2,and when the peak power density exceeds 4.5 GW/m2,the welds of keyhole mode are generated.The fluids in the molten pool are mainly driven by the Marangoni convection,while the Buoyancy convection has little effect on it.The precipitations in the spot welding metal is Mg17Al12,which is in the form of particles or lamellar.The microstructures near the fusion line of the spot welding surface are columnar crystles,while the microstructures at the center of the spot welding surface are equiaxial crystles.The size of columnar crystal and equiaxial crystal increases with the increase of energy density of pulsed laser beam.In addition,due to the small dimensions of the spot welding pool?0.05 mm36.8 mm3?and the large heat conduction coefficient of AZ31 magnesium alloy(84 W?m-1?K-1),the cooling rate of the welding pool is extremely fast?1×105?/s?3.59×105?/s?,and the defects such as spot welding porosity,spot welding overfill,spot welding undercut and spot welding solidification crack are easily generated in the welding metal.With the increase of the peak power density of pulsed laser beam,the area percentage of porosity and overfill,the undercut depth and solidification crack sensitivity are all increased.?2?The effects of pulse shapes on the spot welding coupling,the surface morphology,metallurgical defects and the microstructures are studied.For the coupling effect of laser spot welding,the results show that the coupling effect of the welds with a Ramp-up pulse?the peak power density is increased from small to big?is the best,followed by the welds with a Rectangle pulse?the peak power density is constant?,and the welds with a Leading-spike pulse?the peak power density is decreased from big to small?is the worst,and the initial pulse shape has little effect on the absorptivity of laser spot welding.In the aspect of morphology and defect suppression of laser spot welding,research shows that the formability of spot welds?Ramp-Down pulse?with slow-down closing characteristics is better than that of spot welds?Rectangle pulse?with instantaneous closure characteristics.The time?t=2 ms?10 ms?of the slow-cooling period of Step-Down-t pulse?a slow-cooling period with a peak power of 0.2 P is appended to the main band of Rectangle pulse?is prolonged,and the percentage of the porosity and overfill,and the undercut depth of the laser spot welding are reduced.When t=10 ms,the defects such as the porosity,the overfill,and the undercut in the laser spot welding disappear.The inhibition of slow-cooling period on the porosity of the laser spot welding is reflected in three aspects:?1?Reducing the total amount of gases involved in the molten pool;?2?Reducing the turbulence of fluids caused by the keyhole collapse in the molten pool,and promoting the escape of magnesium vapor in the weld pool;?3?Prolonging the retention time of the molten pool above the melting point temperature,and increasing the probability of bubbles escaping from the weld pool surface.Increasing the peak power?n=0.1 P?0.5 P?of slow-cooling period of the Step-Down-10ms-n pulse?a slow-cooling period with a pulse time of 10 ms is appended to the main band of Rectangle pulse?,the sensitivity of solidification cracks of spot welds first decreases and then increases,when n=0.3 P,the sensitivity of solidification crack of spot welds is minimum.The inhibition effect of slow-cooling period on solidification crack is reflected in three aspects:?1?Greatly reducing the cooling rate of the molten pool and reducing the tensile stress in the mushy zone;?2?Reducing the non-equilibrium solidification of the liquid metal in the molten pool;?3?Improving the ability of the liquid metal to feedback and heal.In terms of microstructures of laser spot welding,the solidification parameters at liquid-solid interface of laser welding pool with different pulse are calculated by ANSYS simulation software in this paper,and the threshold relationship between crystal morphology and G/R?ratio of temperature gradient to solidification rate?of AZ31 magnesium alloy laser spot welding is obtained.The results show that when the G/R value is less than 2°C?s/mm2,the solidification front generates equiaxed crystals;when the G/R value is greater than 2°C?s/mm2 and less than 700°C?s/mm2,the grain morphology of the solidification front is columnar crystals;when the G/R value is greater than700°C?s/mm2 and less than 2000°C?s/mm2,the grain morphology of the solidification front is cellular crystals;when the G/R value is greater than 2000°C?s/mm2,the theoretical grain morphology of the solidification front should be planar crystals.In addition,the grain size at the fusion line of the spot welding cross-section is smaller than that at the center of the spot welding surface,and the Step-Down pulse can increase the grain size of the spot welding metal.?3?In order to understand the distribution characteristics of solidification cracks,a model of initial and propagation of solidification cracks related to direction and time is established to predict the initiation direction of solidification crack in the mushy zone and to analyze the propagation behavior of solidification crack in this paper.The results show that in the welds with a Rectangular pulse,when the aspect ratio of the spot weld is less than 0.3,a solidification crack is initiated in the 90°direction of the mushy zone;when the aspect ratio of the spot weld is greater than 0.3 and less than 0.8,a solidification crack is initiated in the 45°direction of the mushy zone on both sides of the surface center of the welding cross-section;when the aspect ratio of the spot weld is greater than 0.8,a solidification crack is initiated in the 90°direction of the mushy zone.In the mushy zone,the capillary repulsive force?order of magnitude 106?at the crack tip is much larger than the fluid driving force?order of magnitude 104?.Under the action of contraction/shrinkage,the solidification crack will propagates along the grain boundary with the migration of the mushy zone until the surface center of the spot welding. |
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