Microstructure And Mechanical Property And Deep Cryogenic Treatment Regulation Of Laser Melting Deposited CoCrFeMnNi High Entropy Alloy

High-entropy alloy(HEA)has attracted extensive attention due to its stable solid solution structure and excellent mechanical properties.Controlled by the conventional preparation methods,the fabrication of HEA components often exhibits some disadvantages such as limited shape and sample size,casting defects like inclusion,porosity and elemental segregation,which limit the industrial application of HEA components.Laser melting deposition(LMD)can manufacture metallic parts with the fine grains and excellent mechanical performance,and it processes tremendous potential in preparing the HEA components with complex shape.However,the temperature gradient and residual stress generated during the LMD process will affect the mechanical properties of the HEA components.At present,the following key problems have not been solved in the manufacturing of HEA by LMD.Firstly,it has not been explicitly established that the relations among the thermal history,microstructure and mechanical performances in an LMD process of preparing the CoCrFeMnNi HEA.Secondly,the magnitude and distribution of residual stress generated during the LMD process have not been reconstructed.Thirdly,a post-processing method that can effectively control residual stress without destroying the shape of metal components has not been found.Therefore,this work will put more emphasis on solving the above three scientific issues,using LMD technique as the manufacturing method and selecting CoCrFeMnNi HEA as the research object.The purpose of this study is to establish the relations among the thermal history of LMD,microstructure and mechanical performances,and propose a post-processing method which can effectively tailor the residual stress and microstructure to improve the mechanical properties of the LMD-fabricated HEA components.Firstly,the evolution of temperature field and stress field during the LMD process was analyzed by numerical simulation.The solidification map of the process of manufacturing CoCrFeMnNi HEA components was successfully established,and the relationship between thermal history,microstructure and mechanical properties during the LMD process was revealed.The results indicated that,the temperature gradient gradually decreased,the solidification rate and the constitutional undercooling increased as the deposition height increased.It can significantly accelerate the CET behavior,which is abbreviation of columnar to equiaxed transition.Besides,the constraint caused by substrate is reduced and the residual tensile stress decreased with the increasing deposition height,thus the dislocation density the microhardness gradually decreased in the columnar region.Contrarily,the dislocation density and the microhardness both gradually increased in the equiaxed region.The influences of the laser energy density(ED)on the microstructural characteristics and mechanical performances of the single-track CoCrFeMnNi HEA samples via LMD were studied,and the microstructural evolution and strengthening mechanism were revealed.The results indicated that under the condition of different energy densities,all of the single-track deposited HEA samples processed a single face-centered cubic(FCC)solid solution structure,and the columnar to equiaxed transition occurred during the LMD process.In the columnar region,the columnar grain size gradually increased,the dislocation density and the microhardness decreased with the increasing deposition height.Contrarily,in the equiaxed region,the equiaxed grain size decreased,the dislocation density and the microhardness significantly increased with the increasing deposition height.In addition,the strengthening mechanisms of the single-track deposited HEA samples mainly consisted of refinement strengthening and dislocation strengthening.Obviously,dislocation strengthening played a dominant role in the above strengthening mechanisms,accounting for 44%-53% of the total yield strength of the single-track deposited HEA samples.The optimized processing parameters were selected to fabricate the bulk CoCrFeMnNi HEA components.Then,the residual stress distribution,microstructural characteristics and mechanical performances along the deposition height in the bulk CoCrFeMnNi HEA parts were studied in detail,in order to understand the microstructural evolution and the strengthening mechanisms.The results indicated that the complex temperature field evolution during the LMD process led to the heterogeneous residual stress distribution,which was experimentally revealed as the residual compressive stress in the core region and residual tensile stress near the deposited surface.As the deposition height increased during the process of manufacturing the bulk CoCrFeMnNi HEA samples,the columnar grain size gradually increased,and the dislocation density,the microhardness and the tensile strength decreased.The strengthening mechanisms of the bulk CoCrFeMnNi HEA samples are composed of refinement strengthening and dislocation strengthening.The influences of deep cryogenic treatment(DCT)on the magnitude and distribution of residual stress,microstructural morphology and mechanical performances of the bulk LMD-fabricated CoCrFeMnNi HEA parts were investigated under the condition of single and cyclic DCT process,respectively.In addition,the microstructural evolution during the DCT process and their strengthening mechanisms were revealed.The results showed that single DCT would effectively introduce the residual compressive stress and massive crystalline defects into the samples.In addition,with the prolonging DCT duration,the residual compressive stress in the core region of the bulk HEA samples gradually increased.Additionally,the density of the crystalline defects also increased,and the tensile properties of the LMD-fabricated CoCrFeMnNi HEA at room temperature were significantly improved,which broke the strength-ductility tradeoff.The strengthening mechanisms mainly consisted of dislocation strengthening,grain boundary strengthening and twin strengthening,of which twin strengthening was the most crucial strengthening mechanism with strength contribution of 51.0-246.8 MPa to the total yield strength of the DCT-processed CoCrFeMnNi HEA samples.Cyclic DCT would not only effectively introduce the residual compressive stress and massive crystalline defects into the samples,but also homogenize the residual stress distribution,relieve the stress gradient and induce the phase transformation,thus improving the overall mechanical properties of the LMD-fabricated CoCrFeMnNi HEA samples.The strengthening effects came from dislocation strengthening,grain boundary strengthening and twin strengthening,of which twin strengthening was the most significant strengthening mechanism with strength contribution of 82.4-323.2MPa to the total yield strength of the DCT-processed CoCrFeMnNi HEA samples.