Microstructure And Mechanical Properties Of NiCrFeY And NiCrFeAl Alloy Fabricated By Selective Laser Melting

Oxide dispersion strengthened（ODS）nickel-based superalloys are widely used in aerospace and nuclear reactors because of their excellent creep and radiation resistance.With the trend of customization of aerospace and nuclear reactor components,more and more complex hot-end components are produced by additive manufacturing,by which the microstructure of rapid solidification in additive manufacturing is significantly different from that of traditional casting and forging.It is of great significance to clarify the formation and evolution of oxide particles in the process of additive manufacturing of ODS alloy for realizing controlled forming and improving its mechanical properties.Therefore,NiCrFeY and NiCrFeAl alloys were prepared by selective laser melting（SLM）.The effects of process parameters on density,defect and microstructure of the alloys were studied.The evolution and distribution of Y and Al in additive manufacturing were compared and analyzed.The morphology and distribution of oxide particles in-situ formed during SLM were characterized.The main results are as follows:（1）The density of NiCrFeY alloy produced by SLM increases with energy density in the range of 17.4-77.7 J/mm³ and decreases with energy density in the range of 66.7-222.2 J/mm³.The highest density is 99.6%at66.7 J/mm³.The insufficient energy density was easy to led to lack of fusion.When the energy density is too high,the gas involved in the molten pool does not spill out of the matrix in time,which resulted in the increase of pores.（2）The grains in NiCrFeY alloy produced by SLM are nearly equiaxed and elongated along the direction of deposition.There are cellular structures and high-density dislocations distributed within the grains,and the proportion of low angle grain boundaries is 66%.Y₂O₃ particles are in-situ formed from yttrium in NiCrFeY alloy during SLM.The particles are located at the boundary of cellular structure with an average size of about50 nm and a volume fraction of 0.53%.Due to the interaction of Y₂O₃particles,high density dislocation and low angle grain boundaries,the alloy has excellent mechanical properties,tensile strength of 815 MPa at room temperature,elongation of 16.7%,tensile strength of 563 MPa at 600°C and 256 MPa at 800°C,respectively.（3）Columnar grains distributed along the building direction were formed in NiCrFeAl alloy formed by SLM,and the low angle grain boundaries inside the grains accounted for 37%.Solid and liquid cracks are formed in the alloy,and most of the cracks are distributed at the grain boundary with higher angle.The residual internal stress in SLM led to the formation of solid cracks,and the melt cannot complement the dendrite arms timely to form solidification cracks.The crack density is about 2×10^-9 m^-1 at each process parameter.Due to the cracks,the elongation of NiCrFeAl alloy is less than 6%and the tensile strength is only 535 MPa.The comparison between NiCrFeY and NiCrFeAl alloys showed that Y₂O₃particles in-situ inhibited the columnar grain tendency of NiCrFeY alloy,formed equiaxed grain,refined grain and eliminated cracks.At the same time,Y₂O₃ particles make NiCrFeY alloy exhibit higher tensile strength by pinning dislocation and grain boundary.