Microstructure evolution of NiAl alloy seamless tubes formed by an integrated forming process using laminated Ni/Al foil continuous winding

In order to overcome the difficulties in forming NiAl alloy seamless tubes by conventional hot forming, a novel integrated process of internal high-pressure connection and reaction synthesis was proposed using laminated Ni/Al foil continuous winding. The microstructure evolution route and voids inheritance mechanism between element foils were investigated by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The forming defects of the integrated process were analyzed using SEM and energy-dispersive X-ray spectrometer (EDS) microstructure analysis methods. Uniaxial tensile tests and microstructure characterization were conducted to observe the impacts of lay-up patterns and initial foils on the thin-walled components in order to determine the performance control strategy. It was shown that the NiAl alloy seamless tubes were successfully fabricated by the integrated process, and achieved the desired cylindrical shape with a diameter of 39 mm, wall thickness of 1.3 mm, and length of 155 mm. The microstructure of the NiAl alloy tube mainly consisted of Ni and Ni₂Al₃ layers after the first stage of reaction synthesis, with a thickness of 16.07 μm and 126.07 μm, respectively. Voids were observed in the middle of the Ni₂Al₃ layers located parallel to the rolling plane, with a thickness of approximately 8.15 μm. At the end of the first and second stages of reaction synthesis, the detached Al or Ni independent bodies were completely wrapped by the NiAl₃ or Ni₃Al layers, respectively. The reduced pressure led to an insufficient reduction of Kirkendall voids at the NiAl₃/Al and Ni/Ni₃Al interfaces. Consequently, voids were formed in the original position of the Al and Ni layers, and then inherited into the final NiAl alloy component. Furthermore, the Ni/Ni₂Al₃/Ni multilayered structure fabricated under the first stage of reaction synthesis performed superplastic behavior at 900 °C. Ni phase in the deformed Ni/Ni₂Al₃/Ni multilayered structures was completely dissolved in the Ni₂Al₃ matrix after reaction diffusion treatment at 1200 °C. This provided potential opportunities of forming complex shapes for NiAl alloy tubular part. The simultaneous reduction of the initial thicknesses of Ni and Al foils decreased the average grain size in the coarse-grained layers (CGLs) and fine-grained layers (FGLs). The adoption of an Al foil with a double-layer structure of half the initial thickness improved the ultimate tensile strength (UTS).