Study On Precipitates Distribution And Local Mechanical Properties Of IN718 Superalloy Prepared By Cladding Additive Manufacturing

Based on traditional micro-fusion and cast welding,the metal cladding additive manufacturing process uses a focused thermal energy to melt the metal material and build it layer by layer according to the three-dimensional model of the component,including “volume additive manufacturing” and “surface additive manufacturing”.At present,controlling the internal microstructure quality and ensuring the mechanical properties of the cladding parts is the key to the development and application of this technology.The different thermal history of each region in the cladding volume produces a different microstructure and has the essential characteristics of heterogeneous microstructure and uneven mechanical properties.In order to analyze the microstructure and mechanical properties of the samples prepared by cladding additive manufacturing,the basic processes of Wire and Arc Additive Manufacturing(WAAM),Powder and Laser Additive Manufacturing(PLAM)were studied.An open source low-cost WAAM experimental system was designed and built,and its application potential was evaluated from the aspects of formability,microstructure,and mechanical properties through basic process experiments.Based on the orthogonal test,the experimental parameters of WAAM and PLAM process were determined by volumetric energy approximation,and multi-layer multi-pass IN718 samples were prepared.The results show that by using compulsory cooling in the cladding process can produce high-quality metal thin-walled parts with complex shapes and no macro defects,and the reasonable distribution of total input energy is the key to control whether the cladding bead collapses.The high material utilization ratio is the main reason why the WAAM process is more efficient than the PLAM process.Using the Python language programming development in Abaqus software,the representative volume element(RVE)modeling by using different shapes of hexahedron is realized to describe the overlapping features between the cladding passes,and the effect of the thermal boundary change of the cladding layer on the overall temperature field is considered by simulating the change of the heat transfer surface by dynamically activating the thermal boundary conditions.The general parametric finite element model of the cladding additive manufacturing process was established and the accuracy of the thermal model was verified by in-situ temperature measurement.Subsequently,the temperature field of the multi-layer multi-pass IN718 sample cladding process was simulated,and the thermal history data of the relevant nodes were extracted for the analysis and prediction of precipitation phase evolution in microstructure.The inhomogeneous microstructure and macroscopic tensile mechanical properties of multi-layered IN718 specimens were characterized and tested.The results show that the dendrite size at the layer band region of the microstructure varies alternately,the brittle Laves and MC carbides are inhomogeneously distributed in the interdendritic region.The difference in the effective volumetric energy density causes the difference in the microstructure of WAAM and PLAM.The mechanical properties of each region and each orthogonal direction of the cladding specimen are related to the microstructure inhomogeneity.After high-temperature metallographic microscopy,the microstructure changes of the cladding microstructure at high temperature were simulated and studied.The results reflect that the cladding microstructure is heat-resistant and stable under 650℃.The inhomogeneity of grain boundary liquation at high temperature is affected by the segregation uniformity of the elements in the grain boundary.In order to study the heat-related microstructure and the precipitates distribution characteristics,the thermal history of the related nodes in the finite element thermal simulation results is extracted.Based on the model of isothermal phase transformation kinetics,the distribution and homogeneity of the main precipitated phases in multi-layer multi-pass IN718 samples were predicted by the input of thermal history.The distribution results of the calculated strengthening phase were indirectly verified by microhardness test,and the multi-scale SEM image was used to quantify the statistics and estimate the average content of the nanoscale γ’/γ’’ precipitates in the larger micron scale range.The results show that the degree of phase precipitation in the process of cladding can be quantitatively predicted based on the thermal history curve of the subdivision and the calculation model based on isothermal phase transformation kinetics.The microscopic precipitates in the IN718 cladding samples are spatially inhomogeneously distributed,and there are differences in the content of each local region.Most of the γ’/γ’’ phases are inhomogenously distributed along the dendritic boundaries due to the segregation of Nb.The longer the minimum sustained temperature of the thermal history is in the aging temperature range,the more favorable the precipitation of the strengthened phase γ’/γ’’ is.In order to characterize the elastoplastic mechanical properties of IN718 cladding microstructure on the micrometer scale,nanoindentation tests were carried out on the local regions of the cladding samples and a finite element model of the indentation problem was established.By writing a Python program that combines the particle swarm optimization(PSO)algorithm,the idea of automatically performing inverse optimization analysis based on the indenter load-displacement curve to obtain the plasticity performance parameters of the indented micro-region is realized.The above method was verified by DaoMing’s analytical algorithm,which can be an alternative method for material performance characterization.Afterward,the average plastic mechanical properties of the micron scale local regions on the IN718 cladding samples were determined.The results show that the local plastic mechanical properties of the WAAM and PLAM samples are inhomogeneous,and the local plastic mechanical properties on the Intermediate cross-section of the samples exist fluctuation.The fluctuation of yield strength is more obvious along the height direction,but less along the horizontal direction within the same layer.