Numerical Simulation Of Temperature Field And Flow Field In Laser Cladding Amorphous Coatings

Amorphous alloys,which exhibit excellent properties such as high hardness,high strength and high corrosion resistance,have attracted an increasing attention.Laser cladding amorphous coatings on metallic substrate have promising application prospects.However,complex physical phenomena,such as heat transferring,convection,mass transferring,diffusion and solid-liquid,gas-liquid interface reactions occur in the laser molten pool,which will affect the amorphous structure formation during laser cladding processing.The numerical simulation has become an important means of tracking laser cladding temperature field and flow field,because of the time-consuming and labor-intensive of the on-line and real-time monitoring.In this paper,the mathematical model and the physical model of laser cladding are established using FLUENT software.The calculation module of moving heat source and thermal physical parameters of the cladding material are compiled by the user defined function(UDF).An ellipse heat source model is put forward according to the double elliptical coating shape with top thicker and bottom thinner.The simulation results of laser cladding temperature field show that the molten pool reached quasi steady state after 0.5s,when the laser power is 3500W and scanning speed is400mm/min.And then,the temperature field exhibits a typical dragged phenomenon with a wider end and deeper center molten pool.During the solidification processing,the temperature gradient from the bottom to the surface of the molten pool is decreased from3.6×10~5K/m to 1.3×10~4K/m,and the maximum cooling rate is increased from 9.7×10~3K/s to9.4×10~4K/s.The maximum temperature and dilution rate is increased with the increase of laser power.However,the temperature gradient and the cooling rate on the boundary of the molten pool are decreased.With the increase of scanning speed,the maximum temperature and dilution rate is decreased,and the temperature gradient and the cooling rate on the boundary of the molten pool are increased.Based on rapid solidification theory,the solidification morphologies are determined by the shape control factor G/R,the solidification size is determined by the cooling rate?_c.From the bottom to the top of the molten pool,G/R is decreased from 7×10~8K·s/m~2 to 9×10~6 K·s/m~2,?_c is increased from 9.7×10~3K/s to 9.4×10~4K/s.With the decrease of G/R and increase of?_c,the planar crystalline,dendrites,amorphous zone and fine equiaxed crystalline are appeared in the solidified microstructure of laser cladding amorphous coatings.The simulation results of laser cladding flow field show that the convective flow in the molten pool was mainly distributed in longitudinal sections and a convective flow mechanism was formed when the laser power was 3500W and scanning speed was 400mm/min.The vortex in the rear end of the molten pool was obviously larger.The melt flow velocity in bottom of the molten pool had a peak of 0.18m/s at z=0.04m.The maximum velocity is bout0.46m/s and appeared on the surface of the molten pool.The maximum flow velocity in the lower part of the molten pool was increased from 0.13m/s to 0.21m/s with the increase of laser power.However,the maximum flow velocity in the lower part of the molten pool was increased from 0.13m/s to 0.19m/s with the increase of scanning speed.The distance between the maximum flow velocity positions in lower part to the bottom of molten pool was decreases with the increase of scanning speed,which was approximately agreed with the measured distance of the epitaxial growth layer thickness.The cooling rate at the tip of epitaxial growth layer was increased with the thickness increasing.It can be seen that the abrupt interruption of the epitaxial growth layer at the bottom of the molten pool is the result of a combination of melt flow and cooling rate,when the cladding material composition is certain.