Papers by Author: Guo Qing Zhang

Abstract: In this paper, the contact interface between FGH96 superalloy melts and refractory slurry with corundum powder and silica sol at 1600°C with different soaking time in 10-240 min range was investigated. The morphology and composition of the contact interface were studied by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the reaction layer formed on the contact interface between the melting superalloy and the refractory slurry, and is mainly consist of Alumina and contains a small amount of other metal elements such as Ti and Cr. The reaction layer was not smooth on the micro level, and there was a peninsula-like structure protruding into the internal part of the melting superalloy on the reaction zone, and even started to fall off at some places to form islands. With the increase of soaking time, the reaction between melt of superalloy and refractory slurry increased gradually and the reaction layer began to combine with the refractory slurry substrate and form obvious interaction layered structure, resulting in the corrosion of refractory slurry substrate. With the soaking time over 120 min, the stable contact interface was destroyed. Thermodynamic calculation shows that the substitution reaction between Al in superalloy and SiO₂ in refractory slurry meets the thermodynamic conditions, and the reaction can proceed forward.

Abstract: The effect of particle size and shape on flowability of FGH96 superalloy powder was investigated by field emission scanning electron microscopy (FE-SEM), laser particle size analyzer (LPSA) and X-ray photoelectron spectroscopy (XPS). The results showed that the powder flowability basically presented a decreasing trend as the median diameter decreased. The Hall velocity of the five median diameter powders (𝑑₅₀=203.9 μm, 106.3 μm, 83.2 μm, 73.8 μm, 19.9 μm) was 27.18 s/50g, 23.25 s/50g, 23.86 s/50g, 23.42 s/50g and none, respectively. The surface oxides/ hydroxide/nitride of the five median diameter powders were mostly the same, mainly including Al₂O₃, Cr₂O₃, MoO₃, Nb₂O₅, Ni (OH)₂, TiO₂ and TiN. The median diameter 𝑑₅₀, shape factors (circularity, aspect ratio, roundness, solidity) and fractal dimension were selected to quantitatively characterize particle size and shape. For the same fluctuation value of powder flowability, the roundness and solidity showed lower sensitivity. Compared with the two shape factors, the sensitivity of circularity and aspect ratio was at an intermediate level, while the median diameter and fractal dimension displayed higher sensitivity. The median diameter and fractal dimension can be used to characterize the principal variation of flowability. The circularity and aspect ratio can be utilized to characterize the variation of flowability supplementally.

Abstract: Working conditions, failure forms and property requirements of cold working die steels were presented. This review attempted to introduce the development of cold working die steels, the ways to improve its quality, heat treatment and surface treatment technology at home and abroad. Meanwhile, the characteristic and application prospects of spray-formed cold working die steels were analyzed.

Abstract: The transient temperature field in the dual-property disk of alloy FGH96 was investigated during the solution heating process of the gradient heat treatment by numerical simulation. The temperature curves for the different locations of the disk were attained. Then, the gradient heat treatment experiment was carried out, and the heat profiles were obtained. The numerical and the experimental results were almost consistent. The method of heat transfer for the bore of the disk was changed evidently through effective fixture design during the gradient heat treatment. The gradient of temperature (ΔT) can reach 121 or more, which lead to a supersolvus heat treated rim and subsolvus heat treated bore for the disk.

Abstract: The hot deformation behaviors of spray formed superalloy GH738 were investigated by using of Gleeble-3500 simulator in the temperature range of 950~1200, with a strain rate of 0.13~6.5s^-1 and reduction of 50%. The corresponding flow curves were determined and hot deformed microstructures were observed. The results showed that the flow stress decreased with increasing deformation temperature or decreasing strain rate. A full dynamic recrystallization microstructures with fine-equiaxed grains were obtained at the temperature of 1100~1150 and strain rate of 2.6~6.5s^-1. The hot deformation activation energy Q was 580.81kJ.mol^-1, and the constitutive equation was derived by means of linear regression.

Abstract: The non-metallic inclusions in master alloy, P/M superalloy and HIP powder billet were studied in this paper. The results show that the amount of inclusions in master alloy is higher than that of the superalloy powers. The EB-button analysis shows that the main non-metallic inclusions in both the master alloy and the HIP powder billet is Al₂O₃.The amount of the inclusion in master alloy is about 0.166cm²/kg and the size of most inclusions is in the range of 100μm to 200μm, while the maximum inclusion size reaches 400μm.In the P/M superalloy billet, the content of inclusion is only 0.01cm²/kg and the size of most inclusions is less than 50 μm.

Abstract: Powder metallurgical TiAl alloy was fabricated by gas atomization powders, and the effect of heat treatment temperature on the microstructure evolution and room tensile properties of PM TiAl alloy was investigated. The uniform fine duplex microstructure was formed in PM TiAl based alloy after being heat treated at 1250/2h followed by furnace cooling (FC)+ 900/6h (FC). When the first step heat treatment temperature was improved to 1360/1h, the near lamellar microstructure was achieved. The ductility of the alloy after heat treatment improved markedly to 1.2% and 0.6%, but the tensile strength decreased to 570MPa and 600MPa compared to 655MPa of as-HIP TiAl alloy. Post heat treatment at the higher temperature in the alpha plus gamma field would regenerate thermally induced porosity (TIP).

Abstract: The precipitation behavior of γ' phase after continuous solution cooling tests was examined in superalloy FGH96. The results indicate that, with the increase of cooling rate, the mean diameter of secondary γ' precipitates decreases as well as the shape changes from cuboid to spherical. Either under water cooling or cooling at 10 °C/min and 25 °C/min, the highest strength can not be obtained.

Abstract: Low cycle fatigue behavior of spray formed superalloy GH738 at 650°C has been investigated under fully reversed total strain-controlled mode. When strain amplitude (Δε_t/2) is between 0.32% and 0.4%, cyclic stress response is stable under fully reversed constant total strain amplitude. The stabilized hysteresis loops narrowing sharply to a straight line indicates that the alloy exhibits typical elastic strain. The crack initiates single site from the surface. When strain amplitude is between 0.6% and 1.0%, cyclic hardening is observed until fracture. The tendency for hardening is found to increase with strain amplitude. The hyperesis loops expand gradually, which indicates that plastic deformation happens during cyclic deformation process. The crack initiates multi-sites from the surface. The cyclic strain-stress relationship of spray formed GH738 at 650°C can be illustrated by Δσ/2 =2017(Δε_p/2)0.1489.The total strain-life function can expressed by Δε_t/2=0.0071(2N_f) _-0.0781 +0.0647(2N_f) ) _-0.4914.

Abstract: Ti6Al4V powders were produced by Argon gas atomization, the powder fraction ＜ 250μm was hot isostatically pressed (HIP) at 920°C and 140MPa. The properties of pre-alloyed powders and the compact were investigated in this paper. Powder particles are almost perfectly spherical. The microstructure of powder surface is approximate hexagonal cellular structure, the inner structure exhibits cellular αphase and needle-like martensiteα′ phase, these are resulting from the rapid solidification. After HIP, Ti6Al4V alloy has a Widmanstaten microstructure consisting of continuous grain boundary α（GBα）phase and β transformation structure, the grain size of GBα phase is in the range of 5～15μm . The tensile test at room temperature shows that strength of samples is 880MPa, the fracture surface exhibits obvious brittle cleavage fracture features including cleavage facets with river pattern and a few elongate dimples of different sizes and big voids at localized area.