Simulation Of Heat And Mass Transfer Of Molten Pooland Solidification Microstructure Evolutionin L-DED Of Multilayer

As a high-performance additive manufacturing technology for free solid formed metal parts,Laser cladding forming technology has broad application prospects in the fields of aviation,shipbuilding,chemical industry and machinery.The microstructure of the cladding layer directly affects the mechanical properties of the workpiece.In order to establish the microstructure and performance control mechanism of the cladding layer,it is necessary to deeply study the heat and mass transfer behavior of the molten pool during laser cladding forming and the evolution mechanism of the non-equilibrium solidification structure.Starting from the heat and mass transfer in a moving molten pool,the effect of substrate cooling on the microstructure of cladding layer and the evolution of solidification structure of multilayer laser cladding were studied.First,according to the laser cladding forming technology,a three-dimensional heat and mass transfer model and solidification microstructure evolution model of multi-component alloy laser cladding molten pool was established.The model takes into account the influence of process parameters on the geometry of the cladding layer,and uses a more accurate composite heat source.The self-developed macro-micro coupling interface program realizes the coupling of the three-dimensional molten pool model and the solidification microstructure evolution model of multiple alloys,which can simulate the heat and mass transfer and microstructure evolution process of the mobile molten pool under different process parameters.Secondly,the influence mechanism of substrate water cooling on solidification conditions was studied for the substrate water cooling method commonly used in laser cladding.Based on the macro-micro coupling numerical model,the multilayer laser cladding thin-wall model is solved to obtain the solidification conditions such as temperature gradient,solidification speed and cooling speed at the tail of the molten pool under different process parameters,and the substrate thickness and cladding layer are analyzed.The effects of process parameters such as height and substrate cooling coefficient on solidification conditions are verified by experiments.The research results show that: 1)Applying continuous cooling water circulation to the bottom surface of the substrate can increase the cooling rate of the molten pool in the cladding layer,and the larger the cooling coefficient,the more significant the cooling rate improvement effect.2)The height of the cladding layer as a heat conduction channel increases,the heat transfer path lengthens,and the heat transfer effect decreases.Therefore,the cooling rate decreases non-linearly as the cladding layer height increases.3)Under the same conditions of the substrate cooling coefficient,as the thickness of the substrate increases,most of the heat released during the solidification of the liquid metal is absorbed by the substrate,and the ability of the continuous cooling water cycle to absorb energy decreases,resulting in a significant decrease in the water cooling effect of the substrate.Finally,the macroscopic heat transfer and mass transfer model of the molten pool was coupled with the calculation of solidification structure evolution to further simulate the grain growth behavior during the solidification process of the molten pool under non-equilibrium solidification conditions.The model was used to analyze the initial grain size and heterogeneous check.The effects of microstructures were studied under different process parameters,the influence of the shape of the molten pool,the instantaneous solidification conditions and the multilayer cladding scan path on the solidification morphology of the cladding layer,and the simulation results were verified experimentally.The results show that: 1)As the substrate grain size increases,the number of columnar grains of epitaxial nucleation in the cladding layer decreases,and the average size increases.2)As the rate of heterogeneous nucleation increases,the heterogeneous nucleation appears closer to the fusion line,and the new nucleated equiaxed crystals can gain an advantage earlier in the competition with the columnar crystals,and thus get closer.The CET transition is completed at the fusion line.3)When there is no heterogeneous nucleation inside the molten pool,at a lower scanning speed,the competitiveness of the grains grown epitaxially from the tail of the molten pool is greater than that of the grains grown epitaxially from the bottom of the molten pool to form an axial crystal structure;with the scanning speed And the laser power increases,the component undercooling area increases,the axial crystal area becomes smaller,and eventually disappears in the cladding layer.When heterogeneous nucleation exists in the molten pool,the number of newly nucleated grains increases,and the newly nucleated grains will prevent the columnar crystals from continuing to grow,and the columnar crystal to equiaxed crystal transition behavior(CET)occurs on the top of the cladding layer;With the increase of scanning speed and laser power,new nucleation grains become more competitive,CET transformation occurs near the fusion line,and the number of equiaxed grains increases and the size decreases.4)For the simulation results of the solidification structure of multilayer thin-walled parts,columnar crystals and equiaxed crystals alternately appear to form a multilayer cladding structure.Because the scanning direction of each layer is the same,the macroscopic morphology of each layer is curved in the same direction.The experimentally verified results show that the morphology of the solidified structure is consistent with the simulation results.