Mechanical Properties And Hot Corrosion Behaviors Of CoCrFeMnNi High-Entropy Alloy Fabricated By Laser Additive Manufacturing

High entropy alloys（HEAs）are new multi-components materials.The unique composition design contributes to their superior mechanical performances,such as high hardness,corrosion resistance and outstanding strength,etc.HEAs have wide applications in aviation,aerospace,marine engineering,rail transit and so on.However,the HEAs produced by casting and arc melting usually contain coarse grains and shrinkage cavity due to its slower cooling rate.Laser additive manufacturing（LAM）technology is an advanced manufacturing technology with high flexibility,high efficiency,and good mechanial performance.In this dissertation,a bulk CoCrFeMnNi HEA is successfully prepared using LAM,and the effect of process parameters on the surface morphology,microstructure and mechanical properties of the alloy are studied.Moreover,the hot corrosion behavior and mechanism of LAM-fabricated CoCrFeMnNi HEA are explored through hot corrosion experiment.The main conclusions are summarized as follows:（1）The microstructure evolution of CoCrFeMnNi high-entropy alloy manufactured by laser additive manufacturing is studied.The results show that the LAM-fabricated HEA has a single face-centered cubic（FCC）solid solution phase due to the high ertropy.In LAM process,some spherical particles and un-melted particles appears on the surface of the specimen due to the instability of the molten pool,which contribute to higher surface roughness.The rapid cooling of the molten pool and the high temperature gradient lead to different microstructures at different positions in the cross section.The microstructure consists of columnar grains in the top area,and the mixed columnar and equiaxed grains in the middle and bottom areas.As the laser power increases,the grains become coarser.In addition,the laser power has a great influence on the formation of metallurgical defects.The relative density of specimens increases with the increase of laser power.（2）The tensile properties of CoCrFeMnNi high-entropy alloy manufactured by LAM were studied.By analyzing the microstructure and residual stress,the strengthening mechanism of the mechanical properties was summarized.The results show that the nonuniform microstructure leads to the fluctuation of microhardness on the cross-section.The non-equilibrium solidification behavior inside the molten pool leads to uneven residual stress inside the specimen,and the residual stress parallel to the laser scanning direction is much larger than that perpendicular to the scanning direction.Due to the higher cooling rate,the mechanical properties of the LAM-fabricated CoCrFeMnNi HEA are better than those fabricated by casting and melting.Moreover,the tensile test results indicate that the tensile properties parallel and perpendicular to the scanning direction are different,when the load direction is perpendicular to the laser scanning direction,the sample has excellent tensile properties due to the low residual tensile stress in this direction.（3）The hot corrosion experiment results show that the corrosion rate of the specimens covered by mixed molten salt of mixed salt（75%Na₂SO₄+25%NaCl）and NaCl significantly increases with the increased temperature.In the mixed molten salt,the oxidation reaction mainly occurs during the incubation of hot corrosion.A dense oxide film is formed on the surface to inhibit the progress of corrosion.In NaCl molten salt,the corrosion of the specimens is severe with no incubation period for total corrosion process,and the corrosion rate rapidly increases.The specimen has lower corrosion level in mixed molten salt due to the combined action of oxidation,vulcanization and chlorination.In the NaCl molten salt,NaCl reacts with metal oxides to produce volatile chlorides,thereby forming voids and cracks,which accelerates oxidation peeling,and aggravates corrosion damage of components.