Microstructure Simulation Of Solid State Transformation Of Laser Welding On 5A06 Aluminum Alloy

The welding temperature fields have significant effects on the microstructure of welding joints.Similarly,the microstructure of welding joints also affect the temperature fields because different microstructure changes material thermo physical properties.In most commercial finite element softwares,rarely consider the impact of the phase changing which results in different thermal parameters of materials.In this paper,basing on the 5A06 aluminum alloy laser welding experiment to design the finite element simulation program to calculate phase transformation and temperature field.The phase change model and the thermal-phase transformation coupled model are established.And with those models and the finite element equations,the solid state phase transformation and temperature field distribution in the welding process are solved.And the welding process is optimized by combining the results of the welding test and the coupled simulation.First of all,with the laser power and welding speed as the main welding parameters of the laser welding test,the laser welding experiments were designed for 5A06 aluminum alloy with 3 mm and 4 mm thickness respectively.The weld penetration depth was measured,and it was found that the penetration depth of 5A06 laser welding increased with the increasing of the heat input,until the maximum value of the thickness.The microstructure of laser welding joint is analyzed,and the XRD test is carried out to make sure that the microstructure is α（Al）+β（Al₃Mg₂）at normal temperature.The precipitation of β phase is a large mount after welding in the welding joint.By using the laser welding without filling wire,the elements are mainly Al and Mg which are uniform distribution.In addition,this paper developed the finite element software to simulate the temperature field and the solid state phase transformation.The suitable geometric model is set up for the finite element simulation and the combination model of welding heat source is used in welding process.The linear interpolation method is adopted to establish the material model and the phase transformation model is improved on the basis of JMAK equation.Combined with the phase transformation model,the thermal-phase transformation coupled simulation is accomplishied.Then,the phase transformation and thermal-phase transformation coupled temperature field of 5A06 aluminum alloy laser welding are calculated by the finite element equations.By analyzing simulation results of the temperature field in different section of XYZ three axial directions,it is found that the peak temperature of its thermal cycle is decreased and the cooling process does not have a sudden change step.The reason is that thermal-phase coupled simulation changes the material properties parameters which makes relevant matrix changed.The phase transformation of the 5A06 aluminum alloy in the laser welding process is mainly the α-β phase transformation.A large number of β phases are dispersed,especially in the welding joint area,where the volume fraction is approximately 50%.In the middle part of the heat source,the phase transformation is more accurate and stable because of the smaller boundary influence.At the starte and the end of the heat source,the temperature field is not stable and appears the transformation edge.Finally,based on the analysis of the thermal-phase transformation simulation with different laser power and different welding speed,combined with the macro profile and microstructure of welding joint,the welding procedure can be optimized.With the results of laser welding test,the hardness test and the XRD analysis of the weld joint,the accuracy of simulation is improved.