Investigation On Process,microstructure And Properties Of V-5Cr-5Ti Alloy Fabricated Using Laser Melting Deposition Technique

Vanadium alloys have been considered as promising candidates as structural materials for fusion reactors,due to many unique properties such as superior high temperature strength,low activation characteristics,resistance to severe radiation,chemical inertness in liquid metal systems.The preparation processes,microstructures and properties of V-(4-5)Cr-(4-5)Ti alloys have been widely explored for the applications in nuclear systems.Traditional methods to prepare V-(4-5)Cr-(4-5)Ti alloys mainly include melting/wrought-based processes and powder metallurgy processes.However,these processes are often costly and time-consuming.Recently,a laser melting deposition(LMD)technique has attracted extensive interest for the preparation of near net shaped components with high performance.This technique combines laser cladding and rapid prototyping,offering advantages of time saving,single-step fabrication,and materials waste-free.Thus,LMD is particularly attractive for the fabrication of high valued vanadium components.In this paper,firstly,LMD was used to form single-layer deposits of V-5Cr-5Ti alloy.Based on the morphologies of single-layer deposits,optimized parameters were applied to prepare the V-5Cr-5Ti thin-wall samples.The process parameters,microstructure and mechanical properties of the thin-wall samples were systematically studied.In the first chapter of this paper,the research background and significance of vanadium alloys are summarized first,followed by an overview of V-5Cr-5Ti alloys,about the studies of microstructure,second phase research,and mechanical properties.A brief introduction of LMD technique is also present.In Chapter 2,we briefly introduc the experimental devices and raw materials used in laser melting deposition of V-5Cr-5Ti alloy.The methods of analysis and testing are also described.Chapters 3 and 4 introduce in detail the process parameters,geometric features,microstructures,and mechanical properties of laser-deposited V-5Cr-5Ti single–layer deposits and thin-wall smples.In the third chapter,effects of process parameters such as laser power and scanning strategy on the grain morphologies,microstructures,and mechanical properties of the deposited alloys are investigated.The results show that:(1)A high dense microstructure has been obtained for thin wall samples.Microstructures of these walls contain both columnar grains and equiaxed grains.The proportion of columnar grains and equiaxed grains,their morphologies,and textures can be modified by adjusting scanning strategy,laser power,and deposition duration,due to the change of solidification mode and the heat flux direction.(2)The effects of processing parameters on microstructures can be represented by a metric of volumetric energy density.A decrease of energy density will lead to CET.(3)Columnar dendritic growth takes place for all columnar grains.Although columnar dendrites grow following one [100] axis closest to the heat flux direction,the integral growth directions of columnar grains are opposite to the heat flux direction.Including the small deviation of one [100] axis from heat flux direction,the growth space of columnar dendrites also counts for the success in competitive growth for columnar grains.(4)Both columnar dendrites and equiaxed dendrites are delineated by some white clusters containing lath-like precipitates.Post heat treatment will be applied to the as laser melting deposited samples to modify the morphology and distribution of the precipitates in future work,for the purpose of optimizing the microstructures and properties.The main purpose of the fourth chapter is exploring methods to optimize the microstructures and properties of LMD vanadium alloys.First,the post heat treatment is applied to the fabricated thin-wall samples to study the size,shape and density distribution of precipitated phases.However,in the applied temperature range,no significant changes have been observed in the microstructure,neither the properties.Secondly,TiC nanoparticles are added in the raw powders to work as grain refiner in the LMD process,which in the end does not work well either.The experimental arrangement and design,the process parameters,microstructure and properties of laser-deposited V-5Cr-5Ti alloy studied in this paper are expected to provide practical reference values for the future work to optimize the microstructures and properties of the vanadium alloys prepared by laser melt deposition additive manufacturing technology.