| Laser additive manufacturing(LAM)has broad application prospects in aerospace,marine,nuclear power and biomedical fields,especially for the fabrication of complex and integral metal parts,attributed to its high flexibility,short cycle,and high processing capacity.However,the development and application of LAM is greatly limited due to the complexly physical process and the numerous influencing factors in LAM.The complexly thermal-physical phenomenon and the non-equilibrium solidification mechanism of the molten pool are still not clear.In this thesis,the solidification behavior of the molten pool and the dendritic growth mechanism in LAM were investigated by comprehensive research methods of experimental study,numerical simulation and theoretical analysis.The detailed research contents and results are as follows:(1)Based on the heat and mass transfer theory and the Ginsberg-Landou theory,a multi-scale model for solidification microstructure evolution during LAM was established using the macroscopic three-dimensional heat and mass transfer model coupled with the phase field method of thin interface analysis.The physical process considered in the macroscopic molten pool mode included the interaction of laser-powder-substrate,convection,radiation,latent heat,evaporation,powder addition,melt flow,etc.The correctness of the mathematical model is verified by experimental results,in which the longitudinal section of the molten pool and the power exponential relationship between primary dendrite arm spacing(PDAS)and cooling rate are well matched with the simulation result.(2)The solidification behavior of the molten pool and dendritic growth behavior in laser additive manufacturing was investigated using a multi-scale mathematical model realized by the parallel FORTRAN program design of mobile grid and MPI.The effects of local thermal behavior of the molten pool on the PDAS,the radius of the dendrite tip and the tip overcooling were studied.The results show that the temperature gradient(G)gradually increases from the top to the bottom of the molten pool along the solid/liquid interface,whereas the solidification rate(R)and the cooling rate shows the opposite trend.The relationship between the PDAS and the cooling rate is yA oc R-1.31 close to experimental result as ??R-1.34.The relationship between the dendrite tip radius and the cooling rate is ??R-1.01.The undercooling degree is gradually reduced.The PDAS at different locations are also compared with both Hunt's and Kurz-Fisher' models.It is found that both models are not able to describe the PDAS along the solid/liquid interface of the molten pool.Near the top of the molten pool with large VS and small G,the PDAS values are close to Hunt's model,while near the bottom of the molten pool with small VS and large G,the PDAS values are close to Kurz-Fisher model.The relationship between ? and G-0.5VS-0.25 is a roundabout in a small range.The map of ??G-0.5VS-0.25 in direct energy deposition process is at the upper right of that in the selective laser melting process.(3)The Nb element distribution during LAM of Inconel 718 was dynamically presented.Effects of solidification parameters,such as temperature gradient,solidification rate,and cooling rate on the spatial distribution of Nb elements were studied.The results show that the concentration of Nb in the dendritic core is positively correlated with the solidification rate,temperature gradient and cooling rate.The concentration of Nb in the dendritic tip is inversely related to the solidification rate and cooling rate,and is positively correlated with the temperature gradient.The Nb element distribution in the liquid phase is not influenced by the solidification conditions.The Nb element concentration in the dendrite gradually decreases,and the Nb element concentration in the dendrite tip gradually decreases,from the top to the bottom of the molten pool along the solid/liquid interface.The relationship between the local cooling rate and the segregation of Nb element was studied by simulation and experimental methods.The experimental results of the Nb element distribution between the dendrites match well with the simulation results.The Nb element is enriched in droplets at interdendritic regions.From bottom to top of the molten pool,the Nb concentration decreases with the increase of cooling rate.It is indicated that both the Nb segregation and the volume fraction of brittle Laves phase are reduced by increasing the cooling rate of the molten pool.(4)The growth behavior of tilted dendritic for LAM fabricated Inconel 718 was studied using the developed multi-scale mathematical model.The effects of temperature gradient,solidification rate,cooling rate and orientation angle on the PDAS,dendritic tip radius,tip overcooling and tilt angle of the tilted dendrites were systematically studied.The formation conditions of seaweed crystals are explored.The experimental verification of double-headed crystals,multi-head crystals and "zigzag"grains was conducted.The results show that the primary dendrite arm spacing is inversely related to the temperature gradient,solidification rate,and shows a positively correlation with G-0.5VS-0.25,orientation angle.The PDAS shows a power exponential relationship with cooling rate,and the exponential and orientation angles are linearly related to b?-0.53(?)0.The dendrite tip radius is inversely related to the solidification rate,and is positively correlated with G-0.5VS-0.25.It is inversely correlated with the orientation angle at low solidification rate,and positively correlated at high solidification rate.The tip radius shows an exponential relation with the cooling rate.The exponential and orientation angles are linearly related to b ? 0.40(?)0.The dendrite tip is positively correlated with the solidification rate,cooling rate and orientation angle,and is inversely related to G-0.5VS-0.25.The inclination angle is inversely related to the solidification rate and cooling rate,and G-0.5VS-0.25 is positively correlated and has a linear relationship with the orientation angle.In addition,the cooling rate is inversely related to the PDAS and tilt angle.The dendrite tip radius is also inversely related to dimensionless undercooling.The experimental and simulation results confirm that the double orientation or even polymorphism is easy to form large orientation angles.The algae crystals are easily formed at a high solidification rate as the orientation angle is 45°.The dendrites grow in the opposite direction at the high G/VS or when the orientation angle exceeds 45°.In summary,the multi-scale model for solidification microstructure simulation in LAM was established.The solidification behavior of the molten pool and the dendritic growth behavior were systematically investigated.This thesis provides a theoretical basis for understanding the solidification microstructure evolution and also provides new ideas for microstructure control in LAM of metals or alloys. |
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