| Gradient composite material is an advanced engineering material whose composition and microstructure change gradually along single or multiple directions in a predetermined manner.It has good comprehensive properties and can meet the working requirements in harsh environments.At present,gradient composite materials have been widely used in aerospace,biological,nuclear and optoelectronic engineering and other fields.Laser Engineered Net Shaping(LENS)technology is a laser-based additive manufacturing technology that directly deposits powder in the molten pool generated by the laser during the manufacturing process.Since it does not rely on powder beds,it has an advantage over other additive manufacturing processes in terms of geometric freedom and construction dimensions.Stainless steel/Ni-based superalloy system gradient composites have attracted much attention due to their excellent corrosion resistance and mechanical strength.Therefore,it is of great significance to systematically study the relationship between the microstructure,microstructure,composition and mechanical properties of stainless steel/nickel-based superalloy system gradient composites.However,due to the large performance mismatch between the two materials,the microstructure often exhibits inhomogeneity and segregation.Therefore,in terms of material connection and technical implementation,it will be more challenging than the joining of same materials.Due to the above problems,The LENS technology was used in this paper,aiming to prepare stainless steel/nickel-based superalloy gradient composites with uniform microstructure and good comprehensive properties.First,SS316 L and IN625 powders were used as raw materials,three processing parameters during additive manufacturing(laser power,powder feeding rate and scanning speed)were explored through orthogonal experimental design,100% SS316 L and 100% IN625 alloy were prepared using different combination of process parameters.Then the results of density and microhardness were statistically analyzed using visual analysis and range method,and the results show that the weight of laser power > powder feeding rate > scanning speed affects the density and microhardness of the specimen,and the optimal combination of processing parameters for SS316 L and IN625 alloys was obtained,further the range of process parameters for preparing SS316L/IN625 composites was determined.Five SS316L/IN625 composite metal materials with different composition ratios were prepared based on the optimal process parameters.Under the metallographic observation,the typical molten pool morphology of samples fabricated by LENS was observed.SEM secondary electron images reveal the relationship between the preferred growth direction of grains and the maximum undercooling at high undercooling rates.The composite metal material with 75% SS316 L + 25% IN625 composition showed optimized mechanical properties,elongation at break was 41.73%,tensile strength was640.9 MPa,and yield strength was 269.33 MPa.Compared with the elongation of 100%SS316L of 40.19%,the 75% SS316 L + 25% IN625 component material show more excellent mechanical properties.The KAM images derived from EBSD show that 100%IN625 has a high dislocation density and excellent resistance to deformation,so that it reaches the highest tensile strength of 845.37 MPa.With the increase of IN625 content,the corrosion weight loss of SS316L/IN625 composite metal material decreased from9.1% to 1.6%.Finally,SS316L/IN625 gradient composites with continuously changing compositions from 100% SS316 L to 100% IN625 were prepared by LENS technology,the gradient step between different components was 25 wt.%.Five composite metal materials with fixed composition ratio were prepared for reference.The study found that SS316L/IN625 gradient composite has dense microstructure and no second phase was found.In addition,with the increase of IN625 content,the proportions of main elements such as Fe,Ni and Cr also changed,and the solid solution strengthening phenomenon caused by the diffusion of elements into the vicinity of the interface was observed,which further enhanced the connection strength of the interface.The microhardness values of SS316L/IN625 gradient composite show a continuous upward trend with the increase of IN625 content,the highest and lowest microhardness value was 265.1HV and 197.7 HV respectively.The elastic modulus,yield strength,tensile strength and elongation of SS316L/IN625 gradient composites were 152.6 MPa,486.4MPa,672.4MPa and 19.96%,respectively,which indicated interfaces have a good metallurgical bond strength.The macroscopic and microscopic analysis of the fracture morphology of the tensile specimens was conducted to reveal the mechanisms of material fracture,as well as the state of stress changes during the tensile process. |
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