Study On Microstructure And Properties Control And Composition Optimization Design Of Inconel 738 Alloy By Selective Laser Melting

Inconel 738 alloy is a precipitation-reinforced cast nickel-based superalloy reinforced withγ’-Ni₃（Al,Ti）phase.Owing to its high Al+Ti content（>6 wt%）,the alloy forms a high volume fraction of theγ’-reinforced phase,and it has excellent creep resistance,strength,corrosion resistance,and good organisational stability at high temperatures,which is a key material for aero-engine turbine blades.The complex structures of the aero-engine turbine blades with multiple holes and thin walls require conventional casting,forging,and machining methods,having high processing difficulty and long processing cycles.In recent years,selective laser melting（SLM）technology has been particularly suitable for the high-precision and rapid manufacturing of complex thin-walled structures such as medium and small overhangs,complex internal cavities,and profiles,thus providing a new method for the manufacture of aero-engine turbine blades.However,for the numerous conventionally cast Inconel 738 alloy elements,especially those with high Al and Ti contents（high crack sensitivity）,ignoring the extremely high temperature gradient（≥10⁵ K/cm）,extremely fast cooling rate（≥0.01 m/s）,and the process characteristics of cyclic thermal action of the SLM technology can form cracks during SLM forming and heat treatment.The resulting mismatch between the alloy’s printing and mechanical properties（forming performance）severely restricts the application and development of the SLM forming of high Al and Ti content Ni-based high-temperature alloy hot-end parts.Thus,based on the understanding of the crack formation mechanism of the SLM-formed Inconel 738 alloy,this study focuses on the effects of the SLM process,post-treatment,and alloy composition optimisations on the metallurgical defects,microstructure,and mechanical properties of the SLM-formed Inconel 738 alloy to reveal the intrinsic correlation mechanism among alloy composition-process parameters,printing properties,and mechanical properties,and provide a theoretical basis and technical support for improving the printing and mechanical properties of nickel-based superalloys with high Al and Ti contents using laser additive manufacturing.The main conclusions are as follows:（1）The relationship between E and the densities,microstructures,and mechanical properties of SLM-formed Inconel 738 alloy was first investigated by varying the laser power（P）,scanning speed（ν）,and scanning spacing（h）to regulate the laser volumetric energy density（E=P/hvt,i.e.,the ratio between laser power P and scanning speedν,scanning spacing h and powder thickness t）.The formation of various cracks and their mechanisms during the SLM forming of Inconel 738 alloy were further investigated.The results show that the relat ive density of the SLM-formed Inconel 738 alloy increased sharply and then decreased slowly with an increase in E.The density of the alloy reaches a maximum value of 99.4%at E=65.2J/mm³.Simultaneously,the hardness of the SLM-formed Inconel 738 alloy increased with E.For E≥65.2 J/mm³,the hardness of the specimen exceeded the room-temperature hardness value（410 HV）of the cast Inconel 738 alloy.In addition,crack defects still appeared in the densest SLM-formed Inconel 738 alloy,and anisotropy was observed in its microstructure and mechanical properties.Both solidification cracks and liquefaction cracks appeared during the SLM formation of the Inconel 738 alloy.However,liquefaction cracks were dominant.（2）Based on the optimal laser volumetric energy density（E）,the microstructure and properties of the SLM-formed Inconel 738 alloy were regulated by changing the scanning strategy（0°,90°,67°of rotation angle between adjacent layers）.The influence of the scanning strategy on metallurgical defects,residual stress,microstructure,and mechanical properties of the SLM-formed Inconel 738 alloy was investigated,and the mechanism of the effect of grain refinement on the crack inhibition of the SLM-formed Inconel 738 alloy was revealed.The resu lts show that with increasing the complexity of the heat flow direction（0°→90°→67°）,the aspect ratio of elongated columnar grains in the SLM-formed Inconel 738 alloy decreases,whereas the proportion of fine equiaxed grains increases,the former being at tributed to competitive grain growth due to temperature gradient changes and the latter to dynamic recrystallisation.Simultaneously,the crack density in the SLM-formed Inconel 738 alloy decreased with the increased heat flow direction complexity,attributed to the combined effect of grain refinement and residual thermal stress reduction.In addition,the mechanical properties of the SLM-formed Inconel 738alloy with a scanning strategy of 67°had a strong plasticity match in both the horizontal and vertical directions,attributed to the bimodal grain structure of columnar and locally equiaxed grains refined and a few cracks.（3）Based on the optimal SLM process parameters,the effects of metallurgical defects,precipitation phases,and grain structure evolution on the room-and high-temperature mechanical properties of the SLM-formed Inconel 738 alloy under standard heat and hot isostatic pressing（HIP）treatments were investigated.The results showed that HIP treatment reduced the defect density on the XY and YZ surfaces of the SLM-formed Inconel 738 alloy from 0.537%and 0.292%to 0.251%and 0.123%,respectively.At the same time,the HIP treatment changed the microstructure of the SLM-formed Inconel 738 alloy from elongated columnar grains to coarse equiaxed grains and increased the grain size by a factor of 6.In addition,the HIP treatment alleviated the high-temperature brittle fracture of the SLM-formed Inconel 738 alloy.The high-temperature mechanical properties of the HIP+standard heat-treated alloy samples were significantly improved compared with those of the standard heat-treated alloy samples of the low defect density in the alloy and the uniformly distributed M₂₃C₆-type carbides at the grain boundaries.（4）Based on the alloy composition-crack susceptibility-mechanical properties relationship,the design of the conventionally cast Inconel 738 alloy was optimised within the standard composition range by a combination of thermodynamic simulation calculations and experiments,and the influence law of alloy composition on the printability and mechanical properties of the SLM-formed Mod-Inconel 738 alloy was investigated.The results showed that the defect density（0.002%）and the average equivalent volume of defects（54.891μm³）of the SLM-formed Mod-Inconel 738 alloy were reduced by 87.1%and 81.1%,respectively,compared to the defect density（0.015%）and the average equival ent volume of defects（290.542μm³）of the SLM-formed Inconel 738 alloy using alloy-composition optimisation.Alloy-composition optimisation is mainly attributed to the combined effect of the increase in solid solution strengthening elements to improve the matrix strength of the alloy and the decrease in trace elements to reduce theγ+γ′low melting point eutectic phase content.Also,the reduced undercooling of the Mod-Inconel 738 alloy compared to that of the Inconel 738 alloy resulted in a reduced grain aspect ratio of the SLM-formed Mod-Inconel 738 alloy.Moreover,the room-temperature mechanical anisotropy of the SLM-formed Mod-Inconel 738 alloy was reduced compared with that of the SLM-formed Inconel 738 alloy,and the strengths of the horizontal and vertical specimens did not change significantly;however,the elongation of the horizontal specimens increased by 68.8%.