Integrated Process And Microstructure Control For Forming-in-Situ Reaction Synthesis Of Ni Al Alloy Curved Thin-Walled Shells

The development of new generation hypersonic aircraft urgently needs high performance,light weight and temperature resistant thin-walled components.NiAl alloy has the advantages of low density,high service temperature,high specific strength/rigidity and excellent oxidation resistance.It is a potential material to replace high temperature alloy at 800°C～1000°C.In order to solve a series of problems existing in manufacturing NiAl alloy curved thin-walled components,an integrated process of reaction synthesis-forming was proposed in this paper,to achieve precise forming and microstructure control of the components.In this paper,NiAl alloy cylindrical and conical thin-walled shells are taken to reveal the integrated process of reaction synthesis and forming,to study reaction synthesis process and shape accuracy of components,to clarify microstructure and mechanical properties control mechanism.The study aims to provide a new way for the forming of NiAl alloy curved thin-walled components.Microstructure evolution of Ni/Al laminated foils under different reaction synthesis stages was studied by XRD,SEM and EBSD.Composition and phase change were investigated during the process of reaction synthesis.Formation mechanism and microstructure evolution of the bimodal laminated grain structure was studied.The influence of process on the micro-layer structure and reaction synthesis mechanism was clarified.The process parameters of reaction synthesis were obtained.The reaction synthesis process includes two stages:low temperature and high temperature reaction synthesis.At low temperature stage,Al phase with low melting point is completely transformed into NiAl₃ and Ni₂Al₃phases through diffusion reaction,forming Ni/Ni₂Al₃/Ni layered structure.The NiAl₃ and Ni₂Al₃ layers formed at low temperature stage and the Ni₃Al layer formed at high temperature stage are fine-grained structures.Grain boundary diffusion is an important way for element diffusion,and the concentration gradient is conducive to diffusion at each stage effectively.The composition of NiAl alloy sheet prepared by reaction synthesis process was uniform,the chemical composition was 50.8%Ni-49.2%Al（at.%）,and the chemical composition deviation of Ni and Al in microzone of fine-grained layers and coarse-grained layers were 0.92%and 1.36%,respectively.The mechanical properties of the NiAl alloy sheet prepared by reaction synthesis process were studied by tensile test at room temperature and high temperature.The variation of flow stress during tensile deformaton at high temperatures was studed.Microstructure evolution was study by SEM,EBSD and3D-CT.The NiAl alloy sheet shows brittleness and mixed fracture mode of both intergranular fracture and transgranular fracture at room temperature.During the early stage of hot tensile deformation,high density dislocations first accumulate in the fine-grained layers,providing driving force for dynamic recrystallization.With the increase of strain,the dislocation density in the fine-grained layers decreases,and the dislocation density is harmonized and extends to the coarse-grained layers.Subsequently,dynamic recrystallization occurs in this area.The tensile fracture at high temperature changes dramatically along the thickness direction and presents a jagged pattern,local necking occurs in the coarse-grained layers,and the density and size of voids and microcracks show gradient distribution in 3D space.The feasibility of the new process was verified,and NiAl alloy cylindrical and conical thin-walled shells were successfully formed by the process of reaction synthesis and forming.The setting methods of process parameters were discussed,and the thickness distribution of curved thin-walled shells was analyzed,and the main forming defects were studed.The shells show high forming precision and shape accuracy.The average yield strength and tensile strength at 1000°C are73MPa and 81MPa,respectively.Thinning occurs at whole region of the cylindrical and conical shells.The wall thickness at the top of the cross section is smaller than that at other positions,and the wall thickness gradually increases with the increase of the angle,and the maximum value is obtained at the bottom edge.The thickness of the cylindrical thin-walled shells is distributed uniformly along the axial section direction,while the thickness of the conical shell is gradient distributed,and the thickness of the cylindrical shell increases gradually along the minor diameter to the major diameter.The component show good dimensional accuracy and high mold profile consistency.The maximum deviation of the whole cylindrical shells is within±0.5 mm,and the maximum deviation of the conical shells is within±0.1 mm.Microstructure of the curived thin-walled shells manufactured by the integrated process was invesigatd by XRD,SEM,EBSD.Phase structure,grain morphology and void distribution on each region of the curved thin-walled shells were analyzed,and void evolution and formation mechanism were studed,the correlation between process parameters and microstructure and mechanical properties was investigated,and a new method to control microstructure and mechanical properties was put forward.The microstructure of the shells is uniform in all regions,and all regions are NiAl phase.The grain morphology is similar and presents bimodal laminated structure.With the increase of angle,the average size of coarse grains decreases slightly,while the average size of fine grains changes unobvious.The voids in the curved thin-walled shell show uneven distribution,and increasing the pressing force will reduce the void size in the component.The grain size of the shells is related to the initial thickness of Ni/Al lamination foils.By reducing the initial Ni/Al foils thickness,the grain refinement can be obtained.By adjusting the thickness ratio of Ni and Al foil,the layered structure with Ni/Ni₃Al/Ni can be designed,which will helpful to improve the mechanical properties,in order to achieving the control of microstructure and performance of the components.