Research On Grain Evolution Of Aluminum Alloy In Laser Additive Manufacturing Process

Laser additive manufacturing technology is one of the most promising advanced manufacturing technologies at present,which is suitable for manufacturing complex workpieces.Therefore,it has a broad application prospect in the fields of aerospace,biological medicine,military equipment,automobile and other high-precision fields.However,There is a big challenge to produce high precision and excellent microstructure.The evolution of microstructure in the molten pool has an inevitable influence on the mechanical properties of the shaped parts.The solidification of molten pool is a nonlinear,unbalanced and high temperature dynamic evolution process.At present,it is difficult to observe the internal microstructure evolution process by traditional experiment or monitoring method,so numerical simulation method becomes the main method to study the microstructure evolution of molten pool.In view of the above problems,this paper selected two series aluminum alloy as the research object,and used the most promising phase field method to simulate the evolution process of grains in the molten pool,and coupled the macroscopic temperature field.The forming process of the cladding layer was simulated by macroscopic simulation,the local points on the solid-liquid interface were extracted,and the evolution process of microstructure at the local position was reproduced by phase field simulation.The influence of the cooling rate,the angle of grain competition orientation,the angle between the orientation and the direction of temperature gradient on the evolution of grains in the molten pool was investigated.At the same time,the phenomena of grain elimination,grain boundary nucleation and grain morphology in the molten pool were explained by the simulation results.The specific research content and results are as follows:(1)The grain competition in the molten pool will lead to the difference in grain height,and the severe segregation at the grain boundary will result in nucleation and special grain morphology.Through the solid-liquid interface of the pulsed laser modulating melting pool and the macroscopic simulation,it is concluded that the cooling rate,the angle between the competition with neighboring grains,and the angle between crystal orientation and the direction of temperature gradient all change with the change of the longitudinal height during the growth of grains.At the same time,by matching the experimental results with the macroscopic simulation results,the geometric parameters of the cladding layer and the maximum temperature of the melting pool,the correctness of the cooling rate of the phase field input in this paper is verified.(2)By simulating the effects of three variables on grain selection,explain the reason of the difference of the grain height in the molten pool.The results show that: for divergent competition,with the increase of competition orientation angle,eliminating angle increases first,and after the orientation angle is greater than 30°,eliminating angle will decrease.With the increase of cooling rate,the elimination angle basically remains unchanged.With the increase of grain deflection,the elimination angle first increases and then decreases.For convergence competition,elimination angle is basically unchanged when the three variables change.In the experiment,the difference in local grain height was due to the different orientation and the different angle of deviation from the temperature gradient,which led to the rapid change from preferred grain to non-preferred grain.The reason why the grain height in the central part of the melting pool is different from the bottom is that the change rate of the direction of the bottom temperature gradient is greater than that of the middle part of the melting pool,so the the grains at the bottom rapidly become to be non-preferred grains.(3)By extracting the maximum constitutional undercooling at the grain boundary in the competition,the result that the segregation is severe at the divergent grain boundary is obtain.As the orientation angle increases,the maximum constitutional undercooling increases,when the orientation angle is greater than 25°,the maximum constitutional undercooling decrease.With the increase of cooling rate,the maximum constitutional undercooling increases.The maximum constitutional undercooling and grain deflection shows bimodal relations,and in a particular negative angle or positive angle,the curve will peak;There is no obvious segregation at the convergent grain boundary.Therefore,the reason for the grain boundary nucleation in the experimental phenomenon is that the constitutional undercooling at the grain boundary is greater than nucleation undercooling.In addition,the principle of columnar to equiaxed transition(CET)is further explained by the interdendritic nucleation.(4)The formation mechanism of grain morphology was explained through simulation.The "down triangle" morphology at the bottom of the grain is due to grain nucleation and preferred competition.The "upper triangle" morphology at the top of the grain is due to the large positive deviation between grain orientation and the direction of temperature gradient,and eliminated by adjacent preferred grain.The specific grain morphology is determined by its competition type and solidification condition.