Effects of Dy on cyclic oxidation resistance of NiAl alloy

The NiAl alloys modified by reactive element(RE),dysprosium(Dy),were produced by arc melting.The microstructures of the modified alloys were investigated by field emission-scanning electron microscope(FE-SEM)equipped with energy dispersive spectroscope(EDS)and back scatter detector.Cyclic oxidation tests at 1 200℃were conducted to assess the cyclic oxidation performance of the alloys.The Dy dopant prevents the surface rumpling of the oxide scale and the formation of cavities beneath the oxide scale.The pegs consisting of Dy-rich oxide inclusion core and an outer alumina sheath develop deeply in the alloy and improve the oxide scale adhesion.0.05%-0.1%(molar fraction)Dy dramatically improves the cyclic oxidation resistance of the NiAl alloy. Too high concentration of Dy is deleterious because of the fast oxidation rate caused by severe internal oxidation.

Bifurcation control of nonlinear oscillator in primary and secondary resonance

A weakly nonlinear oscillator was modeled by a sort of differential equation, a saddle-node bifurcation was found in case of primary and secondary resonance. To control the jumping phenomena and the unstable region of the nonlinear oscillator, feedback controllers were designed. Bifurcation control equations were obtained by using the multiple scales method. And through the numerical analysis, good controller could be obtained by changing the feedback control gain. Then a feasible way of further research of saddle-node bifurcation was provided. Finally, an example shows that the feedback control method applied to the hanging bridge system of gas turbine is doable.

Investigation on temperature field of unidirectional carbon fiber/epoxy composites during drilling process

The phenomenon of heat accumulation and transportation in the composite materials is a very typical and critical issue during drilling process.In this study,a three-dimensional temperature field prediction model is proposed using finite difference method,based on the partly homogenization hypothesis of material,to predict temperature field in the process of drilling unidirectional carbon fiber/epoxy(C/E)composites.According to the drilling feed motion,drilling process is divided into four stages to study the temperature distributing characteristics.The results show that the temperature distribution predicted by numerical study has a good agreement with the experimental results.The temperature increases with increasing the drilling depth,and the burn phenomena is observed due to the heat accumulation,especially at the drill exit.Due to the fiber orientation,an elliptical shape of the temperature field along the direction is found for both numerical and experimental studies of C/E composites drilling process.

Effect of temperature on vacuum hot bulge forming of BT20 titanium alloy cylindrical workpiece

Temperature is one of the key parameters for BT20 titanium alloy cylindrical workpiece manufactured by vacuum hot bulge forming. A two-dimensional nonlinear thermo-mechanical coupled FE model was established. Numerical simulation of vacuum hot bulge forming process of titanium alloy cylindrical workpiece was carried out using FE analysis software MSC Marc. The effects of temperature on vacuum hot bulge forming of BT20 titanium alloy cylindrical workpiece were analyzed by numerical simulation. The simulated results show that the Y-direction displacement and the equivalent plastic strain of the workpiece increase with increasing bulge temperature. The residual stress decreases with increasing bulge temperature. The optimal temperature range of BT20 titanium alloy during vacuum hot bulge forming is 750-850 ℃. The corresponding experiments were carried out. The simulated results agreed well with the experimental results.

Numerical Simulation and Technological Parameter Optimization for Quenching Process of a Gas Turbine Compressor Disk Based on Metallo-Thermo-Mechanics

Thermal, mechanical and microstructural phenomena are involved in the process of steel quenching. Based on the coupled metallo-thermo-mechanics theory, a calculation model has been developed in this study to simulate the quenching process of a gas turbine compressor disk by finite element method. The thermal physical and mechanical properties were treated as a functions of temperature. Moreover, a series of subroutines were developed on the MARC software platform. Consequently, simulated results on temperature, internal stress and distortion during the quenching were illustrated. With the aid of the simulated results, an optimum quenching scheme was proposed. The quenching process simulated in this study appears to be a promising tool in design of heat-treatment processing parameters for gas turbine compressor disks.

Surface hardening of titanium alloys by oxygen-diffusion-permeation

The surface oxygen-diffusion-permeation behaviors of Ti based alloys were investigated. MEF4A optical microscopy and HMV-2000 micro-hardness tester were employed to characterize the microstructure and micro-hardness of the oxygen-permeated alloys. The results show that the micro-hardness of Ti based alloys are sharply enhanced by the permeation of oxygen. The microstructure and micro-hardness of oxygen-permeated layer are strongly related to the oxygen-diffusion-permeation techniques. The solid solution of oxygen in α phase can improve the transformation temperature from α phase to β phase and enlarge the region of α phase so as to improve the micro-hardness of surface layer. Therefore, surface oxygen-diffusion-permeation would be a feasible method to reinforce Ti based alloys based on the solid solution of oxygen in α-Ti. At last, a diffusion-solution model was put forward.

Electrochemical Behavior of Oxide Films of Stainless Steel in 40 kHz Sonicated Sulphate Electrolytes

This paper describes effects of 40 kHz ultrasound on the oxide films of stainless steel in sulphate electrolytes so as to determine the transmitted power and to characterize mass transfer and peak current density on the electrode surface. Emphasis was mainly laid on electrochemical oxidations and peeling mechanism of oxide films in sonicated sulphate solutions （0.5 and 1.0 mol/L）. Polarization voltammetry, current response traces and SEM analysis were carded out in order to provide full information as to oxide films surface. Results shows that the rate of electrochemical oxidation, the shape of polarization curves and the surface micrographs in sonicated sulphate electrolytes are different from those obtained without introduction of ultrasound. It is concluded that ultrasound can change the electro-chemical behavior of oxide films by its cavitaion effects, which would produce transient mechanical impulsive force and enhance electrochemical reactions.

Experimental and Numerical Research on Deformation Behavior of Thin-Walled and Large Expansion Ratio Guide Vane Liner in Hydroforming Process

Some tube hydroforming process tests and further research work were conducted to manufacture hollow guide vane liners( made of super alloy GH3030).The relative thickness( t0/ OD) of the tubular blank is approximately 0. 01,and the maximum expansion ratio( Dmax/ OD) of the needed part is more than 40%,and the length to diameter ratio of the expansion regionis more than 3. 0. It is very hard to manufacture this kind of ultra-thin-wall,curved axis and large expansion ratio tubular part without fracture and wrinkles. The success of the process is highly dependent on useful wrinkles with appropriate internal pressure and axial feeding. A simplified finite element model and a theoretical model are used for detecting the deformation behavior and forming laws. Further study results demonstrate that the useful wrinkles do not appear at the same time and middle-wrinkles need bigger axial force than tube-end-wrinkles and feeding-wrinkles. The wrinkles can transfer bigger axial force after its wave peak has come into contact with the die inner surface. The thickness thinning rate of the element at the peak is bigger than that at the trough. With the increase of the axial and hoop stress ratio,the critical buckling stress also increases. Microstructure examination results show that the grain size in the maximum thinning zone has been stretched and refined after the large deformation and annealing treatment.The process is feasible and the finished part is qualified.

Effects of shot peening and laser remelting on oxygen-permeation treatment of titanium alloy

The influence of surface pre-treatments, shot-peening (SP) and laser remelting (LR), on oxygen permeation behaviors of titanium alloy TC11 was investigated. Optical microscope, SEM with EDAX, XRD, and microhardness tester were employed to characterize the microstructure, composition and hardness of this alloy. The results show that the surface roughness is increased by shot-peening, and the microstructure with fine-grain can be obtained by LR pre-treatment. The pre-treated samples were oxygen-permeated at (810± 10) ℃ for 10 h in atmospheric air. The outer layer consists essentially of TiO2, trace Ti2 N, Ti3Al and Ti3AlN for the SP pre-treatment and thin oxygen solution layer is found in the subsurface layer. As for LR pre-treatment, the outer layer consists mainly of TiO2 and small amounts of TiO, and the inner layer consists of alpha crystals, rich in interstitial atoms. Samples by LR pre-treatment has thicker hardened layer with higher hardness values in comparison with SP pre-treated ones.The boost diffusion of oxygen and hardening mechanisms were discussed based on the experimental results.

Complicated hollow turbine blades and surface grain refinement process

The control of grain size in superalloys is critical in the manufacture of gas turbine blades.The aim of the present research is to provide the technology for producing complicated hollow turbine blades with fine surface grains and better comprehensive mechanical properties.By melt superheating treatment and coating the internal surfaces of shell mould using a cobalt aluminate-bearing coating material,the in-uence of cobalt aluminate as inoculant on the surface grain sizes of turbine blade was studied with addition of cobalt aluminate:0,35%,45%-65% and 100% respectively.At the same time,the effects of cooling circumstances of the blades on surface grain sizes were also experimented under the same addition of cobalt aluminate.The results showed that the melt superheating treatment plays a significant role in the grain size and carbide morphology;and fine surface grains were obtained when the internal surfaces of shell mould were coated using cobalt aluminate coatings.When the addition of cobalt aluminate in coating is between 45%-65%,and the melt is poured into preheated shell moulds with fine silica sand as backing sand,the blades satisfied the surface grain size requirement is over 90%.In addition,comparisons of the surface grain size and the mechanical properties were also conducted between home-made and foreign-made blades.

Bearing loads of dual rotor-bearing system under stationary and transient conditions

Aimed at aeroengine vibration failure, bearing loads of dual rotor-bearing system caused by unbalance are calculated under stationary and transient conditions. The three-dimensional （3-D） finite element method （FEM） model of dual rotor-bearing system was established. Applying the ro- tor dynamics function of Ansys 12.0, bearing loads were calculated under various unbalance force in stationary condition, and the transient vibration characteristics and the effect of acceleration on bearing loads were discussed. On the basis of simulation results, the influence disciplines of unbal- ance on bearing loads and theoretic reference for reducing bearing loads during start-up were ob- tained.

Fractal model for evaluating heat transfer of high temperature porous corundum shell in vacuum investment casting

Under vacuum, heat transfer in porous corundum shell of investment casting depends on the characteristics of the solid materials and the spatial arrangement of solids and pores. In this study, we present a modified fractal approach to model the pore structure of corundum shell and to describe its influence on the thermal conductivity. We assumed that there is no heat convection in the shell. A sectioned view of porous corundum shell was studied and used to describe the geometric structure and to calculate the fractal dimension d. Based on the fractal dimension d, we obtained the relationship between volumetric solid content and pore arrangement in different measure scales. A heat transfer model was thus established using a network of resistors in which we applied an equivalent approach to calculate the effective thermal conductivity of real porous corundum shell that include the effects of heat conduction and heat radiation of solid. From the obtained results we discuss these effects on the effective thermal conductivity including the scale of measurement, the structure of pore and the temperature. At last these results were compared with other empirical model, which computed by assuming even porosity in which effect of pore structure was not being considered. Though the thermal conductivity calculated essentially in agreement with that obtained from empirical model, model used in this study is more close to the real heat transfer process.

Fatigue Life Prediction of Horizontal Press Frame Based on Statistical Probability and Its Redesign

Horizontal press as an important part of hydro-forming machine is used to output the horizontal force to keep the high internal pressure during tube hydro-forming. However,the horizontal press frame is usually mounted on the press bed and not pre-stressed. Meanwhile it will be subjected to the reaction force caused by liquid pressure. Stresses are concentrated severely on the assemble region due to deformation,and total fatigue life will decrease. In order to predict the total fatigue life of the frame,the simulations are used firstly to determine to stress concentration region,and then strain gauge measurements are carried out under different loads. Next,the methods of statistical probability are conducted to calculate the fatigue life based on long-term load history. Finally a structure with the considerable longer fatigue life is redesigned.

Double oxide film defects in Ni-based super-alloy castings

A new class of defects has been recently discovered, which are regarded as double oxide film defects. In this study, a number of Ni-based vacuum-cast test bars were investigated. The fractures in the broken test bars were observed by Scanning Electron Microscopy （SEM）. Energy Dispersive Spectroscopy （EDS）, as well as Wavelength Dispersive Spectroscopy （WDS） were used to characterize the chemical elements of the fracture＇s surface. Observation by SEM revealed the presence of inclusions identified as films that appeared to have initiated the growth of carbides. This study has added to the evidence that oxide film defects do exist in Ni-based super-alloy in certain vacuum casting conditions. It provides a reference for researchers＇ further study on the defects, and provides a possible direction for researchers to improve casting technology so as to remove these defects.

Finite element analysis of welding residual stress of aero engine blisk by controlling heat input

In order to improve aero engine performance, it is necessary to reduce the welding residual stress of aero engine blisk. In this paper, finite element method was employed to simulate electron beam welding process of blisk, in accordance with the deducing formula（ p = kh ） , the heat input is changed with the weld depth to control welding residual stress of blisk. The calculation results show that welding residual stress of blisk can be controlled effectively by reducing the heat input on the conditions of meeting the demand of weld penetration and guaranteeing the welding quality, a new theoretical method and some numerical data are provided for controlling welding residual stress of blisk.

Experimental investigation of laser peening on TiAl alloy microstructure and properties

In order to study the effect of laser peening on microstructures and properties of TiAl alloy, TiAI alloy samples were treated by Nd：YAG laser system with the wavelength of 1064 nm, pulse-width of 18 ns, and pulse-energy of 0-10J. Surface micro-hardness, roughness, and microstructural characteristics were tested with micro-hardness tester, roughness tester and scanning electron microscope. Residual stress and pole figures were tested with X-ray diffraction and its high-temperature stability was analyzed. The experimental results show that surface micro-hardness increases by up to 30%, roughness increases to 0.37 lain, compressive residual stress increases to 337 MPa, and local texture and typical lamellar microstructure are generated. Residual stress, micro-hardness, and （002） pole figures tests are conducted, compressive residual stress value drops from 337 MPa to 260 MPa, hardness value drops from 377 HV0.2 to 343 HV0.2, and the （002） poles shift back to the center slightly. Laser peening improves microstructure and properties of TiAl alloy significantly.

Evolution of Grain Selection in Spiral Selector during Directional Solidification of Nickel-base Superalloys

The process of grain selection in the spiral selector was investigated by both a ProCAST simulation based on a cellular automaton finite element（CAFE） model and experimental confirmation.The results show that the height of starter block,the spiral diameter and initial angle play an important role in grain selection.The dimension of selector should be maintained in a stable range to optimize the grain orientation and select a single crystal efficiently.A selector which can efficiently select a single crystal had been successfully designed.Grain orientation fluctuation in the spiral part was also studied by means of the variation of thermal condition.

Isothermal Oxidation Behavior of Dysprosium/S-Doped β-NiAl Alloys at 1200°C

A β--NiAI alloy with normal purity, a S-doped and a Dy and S co-doped （Y-NiAI alloys were prepared by arc-melting and their corresponding S contents were less than 20 ×10-6, 33 ×10-6 and 22 × 10-6, respectively. The isothermal oxidation behavior of the alloys at 1200 ℃ was investigated and the extent of S segregation at the scale-alloy interface was determined by scanning Auger microscopy. S-doping had no significant effect on the phase transformation rate from e- to β-Al2O3, while the addition of Dy retarded this process. For the S- doped alloy, scale rumpling occurred only after 2 h thermal exposure and numerous large voids were observed at the scale-alloy interface where S segregated. In contrast to this, the oxide scale formed on the Dy and S co-doped alloy still remained flat even after 50 h isothermal oxidation and only small voids existed at the interface where S segregation was not detected.

Modes of Grain Selection in Spiral Selector during Directional Solidification of Nickel-base Superalloys

The modes of grain selection in spiral selector were investigated by both a ProCAST simulation and experimental confirmation.The results show that the efficiency of grain selection in starter block is associated with the geometry shape.At the early stage of grain selection,the optimization of grain orientation is dominated by competitive grain growth,but the optimization of grain orientation in starter block is gradually dominated by geometry shape at the later stage of grain selection.Besides,the spiral part could also optimize the orientation of the selected single crystal when the initial angle is large enough,and the single crystal selection in spiral parts with different pitch lengths and initial angles is dominated by different modes.The simulation results agree well with experimental ones.

Local deformation and processing maps of Ti-24Al-17Nb-0.5Mo alloy

The processing maps were used to identify the optimal forging parameters of Ti-24A1- 17Nb-0.5Mo alloy by evaluating the flow data according to the DMM model. The actual local strain rate and strain distribution in the samples were obtained by finite element calculations. The local microstructures of the deformed samples were related to the local deformation parameters and correlated with the processing maps at 0.3, 0.4, 0.5 and 0.6 of logarithmic strain. Flow regimes predicted by DMM analysis were then correlated with the local microstructural observations. Five domains of efficient coefficient could be distinguished. Unstable regions were microstructurally related to shear band formation within the （~2~B2 phase deformation field, and to flow localiza- tion at grain boundaries of B2 phase in the near B2 phase deformation field. Stable flow regimes were shown to be associated with dynamic globularization of the plate- like a2 in the a2＋B2 phase deformation zone, and with dynamic recrystallization of B2 in the near B2 phase deformation zone.