Study On Microstructure And Properties Of 3D Printed Gradient Alloys Based On Continuous Solution Structure

In recent years,heterogeneous metal 3D printing as a new material development technology has been developed rapidly.This technology has a broad application prospect in the field of gradient alloy development and new parts manufacturing,and a large number of related achievements are emerging.This technology has a broad application prospect in the field of gradient alloy development and new parts manufacturing,and a large number of related achievements have been emerging.However,the 3D printing technology of dissimilar metals still faces problems such as surface spheroidization,pore defects,delamination and cracking,and interface defects in the transition zone.Firstly,physical model was used to analyze the evolution of melt pool and the mutual solubility of droplets in 3D printing of heterogeneous metals.The effects of droplet temperature,droplet incidence rate,surface tension on the morphology,material distribution and pore defect formation of melt pool were studied to guide the preparation process of 3D printing gradient alloy.Secondly.based on the numerical analysis results,the single system gradient alloy preparation process was studied using a small 3D printer.The interface properties of steep/smooth gradient alloy were studied by combining the results of preparation process and model analysis of matrix material.Based on the results of the study,the preparation process was optimized,and the new gradient alloy can be used for marine propeller materials.Finally,the results of numerical analysis and single-system gradient alloy interface properties are used as guidance.The preparation process and properties of dual-system steel-nickel composite were studied.The prepared dual-system gradient alloy can be used for reactor manufacturing.A physical model of laser cladding process was established by using the finite element method to analyze the flow of the molten pool.In this paper,the morphology,heat distribution and surface tension distribution of molten pool in 3D printing process have been studied.The law between temperature gradient and surface tension distribution and laser input state during 3D printing is revealed.The influence of initial momentum and temperature gradient of incoming droplets on the fusion process in 3D printing of heterogeneous metals has been analyzed by meshless method.The formation mechanism of pore defects during 3D printing of heterogeneous metals is investigated.When different initial momentum of droplets fall into the pool,the percentage of gas impurities reaches 7%.The temperature gradient between the droplet and the melting pool forms Marangoni convection,with the gas impurity content decreasing to 1%.Based on the results of pool evolution,single-pass cladding test was used to study the preparation process of matrix materials,and the structure and properties of the deposited alloy were analyzed.Scanning electron microscopy(SEM)results show that it is easier to form a stable morphology cladding layer using stainless steel substrates than copper alloy substrates.The morphology of cladding layer at 300～350 W power corresponds well with the simulation results.The metallographic results show that the deposited MnCu alloy has the same structure as the cast alloy.The structure of the alloy is dendritic with Mn-rich phase and can be converted to equiaxed crystals after heat treatment.The mechanical properties of the solution-aging MnCu alloy are higher than that of the deposited state,and the tensile strength reaches 530～550 MPa.The crystal structure of the nickel-aluminium bronze(NAB)alloy produces acicular crystals due to the rapid cooling process,which is different from the bulk crystals of the cast alloy.The tensile strength and elongation of NAB are 746 MPa and 6%,which conform to Hall-Petch theory.Based on the deposition parameters of manganese-copper alloy and nickel-aluminium bronze alloy,steep/smooth gradient alloys were synergistically prepared by using a laser engineering net shaping(LENS)system.The influence of heat treatment and preparation process on the interfacial properties was studied,and the final preparation process of gradient alloy was determined.The mechanical properties of deposited alloys are similar at different power levels,which are above 500 MPa.After solution-aging treatment of gradient alloy,the results of metallographic and EBSD show that the grain size of the transition zone reaches 605 μm,with fine recrystallized structure.The alloy has poor mechanical properties and has no potential for application.The mechanical properties of the gradient alloy are similar to those of the MnCu alloy matrix after optimized preparation by step-by-step shaping.The tensile strength of the gradient alloy reaches 522 MPa.The alloy also takes into account the damping and corrosion resistance.The results show that the continuous solid solution structure can form a good performance interface in gradient alloys.Based on the study of interface properties of single-system gradient alloy,the preparation process of dual-system steel-nickel gradient alloy with stronger application was studied.The steel-nickel gradient alloy boundary has good surface bonding and tensile strength reaches 1041 MPa.The steel-nickel structure reactor is the application direction of gradient alloy.The NiAl-CoCrMo alloy catalysts prepared by the dual-feeders method have good catalytic performance.The CO conversion of the catalyst is maintained above 60%and the CH4 selectivity is maintained above 98%throughout the process.XRD and BET results reveal that Ni and Al element powders are easier to form intermetallic compounds during 3D printing.The steelnickel composite reactor was prepared by repeated cross-printing.No obvious crack defects were found in the SEM images at the steel/nickel interface,and the gradient distribution of elements was obvious.