Maodong Kang

Tensile properties and microstructures of investment complex shaped casting

Abstract The tensile properties of complex shaped nickel based superalloy casting were investigated at elevated temperature. Specimens were prepared from investment casting with/without hot isostatic pressing (HIP) treatment. Grain size and microstructure of specimen are examined through optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the tensile properties change remarkably, although the specimens were cut from the same casting; the ultimate tensile strength and elongation in semilogarithmic scale of the specimens have obvious linear relationship with grain size. The HIP treatment can not only improve the tensile properties but also increase the sensitivity of elongation in terms of grain size, which roots from the morphology and redistribution of bifilms, δ phase, Laves phase and γ″ precipitates. The present work will provide a better understanding of the local profile–property relationships of complex shaped casting in bottom filling system and suggest that the local profile should be considered seriously in design and manufacture of the complex shaped superalloy casting.

Analysis of Microporosity-Dependent Fatigue Crack Behavior in Alloy 718 by Using Synchronic Radiation X-Ray CT and FEM

Influence of microporosity on the mechanical properties of superalloy have been studied by numerical and experimental methods for long time. In this paper, the real morphology of microporosity in polycrystalline Alloy 718 specimens was inspected by synchronization radiation X-ray micron computerized tomography (μCT) and 3D model was reconstructed for the numerical simulation based on finite element method (FEM). Fatigue testing of the heat treated samples and heat isostatic pressured samples were conducted to verify the numerical analysis. Response surface analysis based on FEM was used to establish the linear function relationship between fatigue life and factors including of porosity, pre-deformation and residual stress. The simulation results showed that the interdendritic region on the surface of microporosity was the place of stress concentration in metal and the crack site of fatigue failure. Experimentally, heat treated sample fractured at microporosity, while the HIPed samples disrupted due to the matrix ductile fracture. The good agreement was obtained between the simulation and experimental results.

Study on the castability and mechanical properties of thin-walled nickel-base superalloy

The castability and mechanical properties of thin-walled nickel-base superalloy castings fabricated by gravity casting and centrifugal casting were investigated. It is shown that, despite its microporosity has slight change, the casting fabricated by centrifugal casting presents fewer misruns and less Laves phase than that of gravity casting. The ultimate tensile strength, yield strength and stress rupture lives of centrifugal casting are improved by about 2.2, 7.4 and 41.1%, respectively. However, the elongation (EL) and reduction in area are sharply decreased by about 10.9 and 25.0%, respectively. Paradoxically, the stress ELs of both castings are similar. The seemingly contradictory results in mechanical properties stem from the opposite effects of centrifugal force on hardening elements macrosegregation and melt turbulence.

Effect of Solidification Behavior on Microstructures and Mechanical Properties of Ni-Cr-Fe Superalloy Investment Casting

The effect of solidification behavior on the microstructures and mechanical properties of Ni-Cr-Fe superalloy investment casting is given. Metallographic and image analysis have been used to quantitatively examine the microstructures’ evolution. For the parts with the thickness of 3 mm and 24 mm, the volume fraction and maximum equivalent radius of the Laves phase increases from 0.3% to 1.2%, from 11.7 μm to 23.4 μm, respectively. Meanwhile, the volume fraction and maximum equivalent radius of carbides increase from 0.3% to 0.5%, from 8.1 μm to 9.9 μm, respectively. In addition, the volume fraction of microporosity increases from 0.3% to 2.7%. As a result, the ultimate tensile strength is reduced from 1125.5 MPa to 820.9 MPa, the elongation from 13.3% to 7.7%, and the quality index from 1294.2 MPa to 954.0 MPa, respectively. A typical brittle fracture is observed on the tensile fracture. As the cooling rate decreases, the microstructures become coarser.