Selective Laser Melting Preparation And Cracking Inhibition Of René104 Superalloy

To solve the problem that René104 nickel-based superalloy is easy to crack during selective laser melting（SLM）,the effects of SLM parameters,heat treatment,nano-TiC addition and Sc microalloying on the microstructure,cracking behavior and mechanical properties of the alloy were systematically investigated.The cracking mechanism model of René104 alloy prepared by SLM was established,and the influence mechanism of process parameters and component addition on the cracking behavior of the alloy was discussed.Finally,with the introduction of nano-TiC to control the non-equilibrium solidification behavior of SLM,and by Sc microalloying to improve the weldability of René104 alloy and reduce the cracking sensitivity,René104 alloy parts with no cracks and excellent mechanical properties were successfully prepared.This study provides a new perspective for the research and application of"non-weldable"Ni-based superalloy prepared by SLM.The main research contents and results are as follows:（1）The optimum SLM forming parameters（laser power is 250 W,hatch distance is 90μm,exposure time is 80 s,the optimized energy density is 49.38 J/mm³）were determined by parameter optimization.With the increase of energy density,the relative density of the as-built René104sample first increased and then decreased,and the defects changed from spheroidized and unfused holes to cracks.The microstructure of the as-built René104 alloy is composed of submicron cellular structure in XY plane and columnar grains parallel to the building direction in XZ plane,with a strong<001>fiber texture.The yield strength（YS）,ultimate tensile strength（UTS）and elongation（EL）of longitudinal samples are 11.2%,5.3%and 30.6%higher than that of transverse samples,respectively.The directional growth of columnar grains in the<001>direction and microcracks parallel to the building direction make the microstructure and mechanical properties of as-built René104 alloy obviously anisotropic.（2）There are solidification cracks and liquation cracks in the as-built René104 alloy,which are distributed along the high angle grain boundaries（HAGBs）and propagated along the long and straight columnar grain boundary.In the final stage of solidification,the liquid phase backfill between dendrites is blocked,resulting in continuous and semi-open shrinkage cavity cracks between dendrites.Liquation cracking is attributed to the low melting point phase formed by low melting point boride andγ-γ′eutectic,which melted again in the heat affected zone（HAZ）to form a continuous liquid film,which greatly reduces the grain boundary strength,and leads to hot tearing under the action of tensile stress.（3）The average grain size of the direct aging（DA）sample is 61.1μm,and a large number of dispersed stripedγ′phases with a size of 38.4 nm are precipitated.After solution treatment at 1160℃（S1160）,some columnar grains were recrystallized to form fine equiaxed grains,with an average grain size of 47.5μm and a bimodal distribution ofγ′phase sizes of 10±3 nm and 230±15 nm,respectively.Recrystallization and grain growth occurred in the sample treated by 160℃solution aging heat treatment（SA1160）,with the average grain size of 56.2μm and theγ′phase size of 138.17 nm.The cracking density of DA,S1160 and SA1160samples are increased by 85.1%,30.2%and 35.4%,respectively,and the microhardness and strength are greatly improved,but the plasticity decreased to varying degrees.SA1160 sample exhibits the best comprehensive mechanical properties,with the YS,UTS and EL of 1230MPa,1298 MPa and 2.62%,respectively,and the microhardness of 532HV_0.3.（4）The nano-TiC reinforced René104 superalloy was prepared by ball milling and SLM with micron TiC particles,which solved the problem that nanoparticles were easy to agglomerate during SLM forming.The columnar to equiaxed transformation occurred,the average grain size decreased by 67.4%,the cracking density reduced by 83.3%,and YS,UTS and EL improved by 32.2%,27.1%and 113.9%,respectively.Uniformly dispersed nano-TiC provided heterogeneous nucleation,effectively promoted grain refinement and equiaxed grain formation of René104superalloy,and reduced HAZ area,thus inhibiting crack formation and propagation.（5）With the microalloying of Sc,the reduction of O and S content in powder,which improved the weldability of the alloy and reduced the cracking sensitivity.A crack-free René104-Sc sample was successfully prepared by SLM,and the YS,UTS and EL are 918 MPa,1289 MPa and13.9%,respectively,which are 29.7%,42.4%and 256.4%higher than the original René104 alloy.The in-situ formation of Al₃Sc nanoparticles promoted heterogeneous nucleation of nickel,resulting in a 73.9%decrease in grain size.The greatly refined grains provided more grain boundary area,which retarded the enrichment of elements forming low melting point phase and the accumulation of thermal stress.