Design,Preparation And Properties Research Of Printable Dedicated Nickel-based Superalloy

Compared to the traditional forming methods such as casting and forging processes,additive manufacturing(AM)technology exhibits the advantages of reducing costs and increasing the freedom of part design.These superiorities receive attention from many industries in terms of the forming of high value and complex nickel-based superalloy parts.Additionally,the solidification process of high cooling rate and high temperature gradient brought by high energy density endows formed parts more outstanding microstructure controllability and excellent mechanical properties than equal and subtractive manufacturing.However,the unique non-equilibrium metallurgical kinetic characteristics of AM can easily lead to crack defects inside the components,which affect the service reliability of the components.The substantial cause is the mismatch relationship between the composition and process of nickel-based superalloys.For this reason,there is an urgent need to establish a printable model of the alloy components to properly guide the design of crack-free,high-strength nickel-based superalloy components.In view of this,this study used thermodynamic software based on calculation of phase diagrams(CALPHAD)theory to study the potential relationship between the composition and printability of typical and specially designed nickel-based superalloys for AM,and established a printable model about nickel-based superalloys based on thermodynamic interpretation.Based on this model,a printable nickel-based superalloy composition is calculated and designed in combination with the traditional nickel-based superalloys design process.By studying the influence of different process parameters on its defect distribution,the rationality of the printable model was verified,and the microstructure and mechanical properties of the deposition and heat treated alloys were explored.The following conclusions were obtained:Based on thermodynamic calculation software,the hot crack susceptibility(HCS),Strain aging crack(SAC)and thermophysical parameters were calculated to isolate the printable nickel-based superalloys and a printable model were established.The results indicate that the printable nickel-based superalloys are isolated by coupling HCS and SAC of nickel-based superalloys is only suitable for some low-SAC nickel-based superalloys,and the printability of high-SAC nickel-based superalloys cannot be explained.By introducing thermophysical parameters,the reason of printable nickel-based superalloys with high SAC can be attributed to high thermal conductivity and low coefficient of thermal expansion.Thus,the coefficient of thermal expansion and thermal conductivity were calculated when the solid-phase fractions are 1 wt.%,0.99 wt.%,0.95 wt.% and 0.90 wt.% respectively.When the solid-phase fraction is 0.95 wt.%,the thermal conductivity and coefficient of thermal expansion coupling evaluation model has a higher tolerance.The result can provide practicable thermodynamic indicators for the design of printable nickel-based superalloys.Based on the design indicator proposed by the printable model,the effects of different Al/Ti ratios,W and Mo content,Ta and Nb content on the printability and mechanical properties of the alloy system were calculated.The results show that with the increase of Al/Ti ratio,HCS and creep life increase first and then decrease,while the coefficient of thermal expansion and thermal conductivity gradually increase.With the increase of W and Mo content,the coefficient of thermal expansion and thermal conductivity decrease,and the creep life increase,while the variation of HCS from increasing sequentially to increase first and then decrease.With the increase of Ta and Nb content,the thermal conductivity decreases,the creep life increases,the changes of HCS from increasing first and then decreasing to decreasing sequentially,and the coefficient of thermal expansion changes from decreasing sequentially to gradually increasing.Considering the above changes and the final composition of the target alloy was determined to be: Ni-19.5Cr-13.5Co-2Al-2.5Ti-1Mo-5W-2Nb-1Ta-0.01C-0.005 B.In the meantime,the new alloy has good printability and thermal stability.It is predicted that the creep life is 662.59 h at 900 ℃/200 MPa in new alloy and the yield and tensile strength reach 1000.14 MPa and 1350.66 MPa at 900 ℃,respectively.Based on the examples processed by selective laser melting(SLM),the influence of process parameters on defect distribution,molten pool micromorphology,grain characteristics and mechanical properties was studied.The results show that when the laser power,scanning rate,layer thickness and scanning spacing are 240 W,850 mm/s,0.04 mm and 0.07 mm respectively,the #2 example obtains the lowest defects density,and fine columnar crystals and cellular crystals exist in the fish-scale molten pool.The #1,#2 and #9 examples exhibit obvious {100} <001> texture characteristics and#2 obtains the highest texture intensity.The anisotropy of grain size and average geometry necessary dislocations(GNDs)density distribution of XOZ and XOY plane of the example is consistent with the mechanical properties at room temperature.In addition,the #2 example paralleling to build direction exhibits excellent strength-ductility synergy effect,its yield strength,tensile strength and elongation is743.6 MPa,1049.73 MPa and 67.98% respectively.When it was tested at 650 ℃,the yield strength was basically unchanged,the tensile strength decreased,and the elongation was 10.43%;at 750 ℃,the yield strength increased significantly to 928.01 MPa,while the tensile strength decreased,showing brittle fracture characteristics;At850 ℃,brittle fracture characteristics occurs,and the tensile strength and elongation are only 435.87 MPa and 0.40%.Based on the samples produced by optimal process parameters,the phase precipitation,grain boundary characteristics and mechanical properties of treatment examples by solution treatment(ST)+ double aging(DA)were studied.The results show that with the increase of solution temperature,recrystallization occurs at1150 ℃ST+DA,and the columnar crystals with <001> orientation are transformed into equiaxed crystals,and the average grain size increase.In addition,the primary γ′phase and the δ phase dissolved,and the secondary γ′ phase size gradually increased.Compared with 1050 ℃ST+DA samples with yield strength,tensile strength and elongation of 1178.07 MPa,1496.86 MPa and 11.2% respectively,the yield strength,tensile strength and elongation of 1200 ℃ST+DA samples decrease to 1126.54 MPa,1414.91 MPa and 8.7% respectively,despite forming high proportion of annealed twins.During the high temperature test,the weakening of grain boundaries led to obvious brittle fractures at 650 ℃,750 ℃ and 850 ℃ in 1050 ℃ST+DA samples.