Laser Welding Of AZ31B Magnesium Alloy With Beam Oscillation

Magnesium and its alloys have high specific strength,good electromagnetic shielding effect and excellent damping performance.They are applied to the automotive,electronics,aerospace and other fields.However,there are some problems such as air hole and coarse grain due to the special physical and chemical properties of magnesium.Forced stirring of the molten pool by the beam oscillation can improve the metallurgical characteristics of the magnesium alloy pool and the quality of the weld.However,relevant research has not been carried out yet.Therefore,this article carried out laser welding of AZ31 B magnesium alloy with beam oscillation including butt and lap weld,systematically studied the law of the weld forming and the evolution of microstructure and properties,and discussed the relevant mechanisms.The main results are as follows.The beautiful shape can be observed in the butt weld,when the scanning amplitude is between 0.5mm-2mm and the frequency is between 25Hz-75 Hz.The observation of highspeed photography reveals that the opening area of the key hole is significantly increased compared to the non-scanning weld.When the frequency is 50 Hz and the amplitude is 1mm,the area peak is 6 times that of the non-scanning weld.It is considered that the behavior of the scanning laser beam periodically passing through the molten pool drives the melt flow along the scanning direction,which promotes a more uniform temperature gradient in the pool.The increase in the opening of the key hole is beneficial to the eruption of the metal vapor and the stability of the pool.The interaction between the two improves weld formation and inhibits the occurrence of defects such as pits and humps.It is found that the weld mainly consists of ?-Mg phase and a small amount of ?-Mg17Al12 phase.Among them,the center of the fusion zone presents a snowflake-like equiaxed dendrite.When the scanning amplitude is 0.5 mm and the frequency is 50 Hz,the proportion of the equiaxed crystal region is increased from 72% of the non-scanned weld to 92%.The above changes depend on three points.Firstly,the scanning behavior reduces the temperature gradient and increases the components supercooling of the pool.Secondly,the stirring effect of the pool caused by the oscillating scan promotes the secondary dendrite broken.Furthermore,the strong convection inside the weld pool and the strong eddy current effect can quickly take the broken dendrite from the edge of the weld to the center of the weld,and uniform temperature gradient in this area avoids the melting of some nucleation sites,thereby promoting the formation of equiaxed dendrites.When the scanning amplitude was increased from 0.5 mm to 2 mm,the tensile strength and elongation of the weld decreased gradually from 241 MPa and 9.6% to 209 MPa and 7.5%,respectively.Scan frequency has no significant effect on tensile strength,but it has a significant effect on elongation.When the frequency is increased from 25 Hz to 75 Hz,the tensile strength is basically around 230 MPa,and the elongation rate is increased from 5.3% to 9.2%.The weld tensile strength is in good agreement with the grain size,but the change of the elongation is not only related to the grain size,but also closely related to the highangle grain boundary and twin deformation.In this interval,although the equiaxed grains are coarsened,the degree of twinning deformation increases and the elongation increases.The microstructure of the lap weld is basically the same as that of the butt weld,and the tensile shear force of the weld depends on the melting width of the overlap interface.Increasing the interface melting width can weaken the edge stress concentration and increase the joint tensile shear force.When the scanning frequency is 50 Hz and the amplitude is 2.5 mm,the tensile shear force is 5.87 kN,which is about twice that of the nonscanning weld.