Guo Jingjie

Composition control of a TiAl melt during the induction skull melting (ISM) process

Abstract The properties of intermetallic compounds are sensitive to alloying composition determined by the melting process. This article describes research that aims to control the composition of a TiAl melt during the ISM process. Firstly, the vapor pressures of Al and Ti in the TiAl melt were established and the results show that P Al / P Ti is over 300. The ratio points out that Al will evaporate more predominantly than Ti in the TiAl melt during the vacuum melting process. Then, the mass transport coefficient K and the mass evaporation rate N m were calculated. The vacuum pressure has the same influence on K and N m . There are two contraflexure points of the vacuum pressure, namely the critical point and the impeding point. When the vacuum pressure surpasses the impeding point, the evaporation of Al can be neglected. When the vacuum pressure is below the critical point, the evaporation of Al will take on a free evaporation and the Al content will decreases to 40at% from 48at%. The calculated results were compared with the experimental data, which verified the calculation results.

Effects of ultrasonic vibration on the microstructure and mechanical properties of high alloying TiAl

To modify the microstructure and enhance performances, the ultrasonic vibration is applied in the mould casting of TiAl alloy. The effects and mechanism of ultrasonic vibration on the solidifying microstructure and mechanical properties are investigated and the model for predicting lamellar colony size is established. After ultrasonic vibration, the coarse microstructure is well modified and lamellar colony is refined from 534 μm to 56 μm. Most of precipitated phases are dissolved into the lamellar colony leading to a homogenous element distribution. The phase ratio of α2-Ti3Al and γ-TiAl is increased, and the chemical composition is promoted to more close to equilibrium level by weakening the influence of β-alloying elements. The microhardness and yield strength are gradually improved by 23.72% and 181.88% due to the fine grain strengthening, while the compressive strength is enhanced by 24.47% through solution strengthening. The critical ultrasonic intensity (Ib) for TiAl alloy is estimated at 220 W cm−2 and the model for average lamellar colony size is established as . The ultrasonic refinement efficiency exponentially increases as the ultrasonic vibration time with a theoretic limit maximum value of Elim = 88% and the dominating refinement mechanism by ultrasonic vibration is the cavitation-enhanced nucleation rather than cavitation-induced dendrite fragmentation.

Coarsening mode and microstructure evolution of Al-In hypermonotectic alloy during rapidly cooling process

Abstract A numerical model reflecting the real physical processes well is developed to predict the coarsening mode of the second phase droplets and the microstructural evolution under the common action of nucleation, diffusional growth, collision coagulation during rapid cooling Al–In hypermonotectic alloys.

Study of Interfacial Reaction between TiAl Alloys and Four Ceramic Molds

Abstract With the aid of SEM and DTA, the microstructures of metal side of the interface between Ti48Al2Cr2Nb alloy and Y 2 O 3 , ZrO 2 (Y 2 O 3 stabilized), ZrO 2 (MgO stabilized), and Zircon sand were investigated, respectively. The initial reaction temperatures were measured for Ti48Al2Cr2Nb alloy and Y 2 O 3 , ZrO 2 (Y 2 O 3 stabilized), ZrO 2 (MgO stabilized) and Zircon sand, respectively. The results show that the microstructure is the coarsest with rosettes-like shape after Ti48Al2Cr2Nb alloy reacts with Zircon sand, and even the smallest with granular-like shape after Ti48Al2Cr2Nb alloy reacts with Y 2 O 3 . The order of the initial reaction temperatures between Ti48Al2Cr2Nb alloy and four oxide ceramic materials is non-reaction at 1500, 1400, 1380, and 820 °C, respectively.

Study on Vacuum Suction Casting for TiAl-Based Alloys

Abstract A novel type of foundry method for TiAl-based alloy is presented in the present paper. The filling processes of Ti-47Al alloy sheet and blade have been studied via numerical simulation. The investigation results indicate that the filling fraction is enhanced by the increase of filling velocity and graphite throat diameter. The graphite throat diameter is the main parameter for the backward filling location in the process of vacuum suction casting. With the increase of mould temperature, the solid fraction is reduced. The alloy solidification begins from bottom to top. The TiAl sheet and the blade are produced by this method. The grain size of the sheet is in the range from 10 μm to 40 μm. There is no obvious shrinkage cavity.

Hydraulic modelling of mould filling behaviour during vertical centrifugal casting processing

Abstract In this paper, the effects of different gating systems and varied rotational velocity on mould filling ability were systematically investigated using a water analogue. Using an adjustable velocity centrifugal machine, a high speed camera and water, four kinds of gating systems have been studied, namely top-, bottom-, side- and stepped-gating. Results showed that bottom gating and stepped gating were better than other gating methods and the filling ability was enhanced by increasing the rotational velocity of the mould. However, overly high rotational velocity led to a decrease in the filling velocity. High velocities were also detrimental to the filling ability.

Numerical simulation of off-centred porosity formation of TiAl-based alloy exhaust valve during vertical centrifugal casting

The vertical centrifugal casting method has been used to cast a TiAl-based alloy exhaust valve. Although mould filling and solidification of TiAl-based alloy melt proceed under pressure generated by the centrifugal force, an unfavourable solidification sequence is formed because of the special mould filling mode and finally results in the formation of off-centred porosity. Numerical simulation is applied to study the vertical centrifugal casting process of the TiAl-based alloy exhaust valve in this paper. On the basis of simulations of the flow field and the temperature field, the law of porosity formation during the vertical centrifugal casting of the TiAl-based alloy exhaust valve is discussed by analysing the mould filling and solidification process. The results of the numerical simulation show that off-centred porosity forms easily near the entrance of the cast product and the tendency for off-centred porosity formation decreases with increasing distance from the entrance to the far end. The experimental results agree well with those of the numerical simulation.

Directional Solidification of TiAl-Based Alloys: Determination of the Primary Phase in Ti-50Al-5Nb Alloy

Abstract In order to obtain excellent comprehensive mechanical properties, it is important to control the lamellar orientation of TiAl-based alloys by directional solidification technology. The control of the lamellar orientation is related closely with the primary phase during the directional solidification of TiAl-based alloys. The identification of the primary phase in directionally solidified TiAl-based alloys is insufficient only according to an isolated two-dimensional micrograph of the lamellar orientation. In this paper, we identified accurately the primary phase according to the quasi three-dimensional micrograph of the lamellar microstructure. The results indicate that the primary phase with various lamellar orientations is β phase during the directional solidification of Ti-50Al-5Nb alloy.

Influences of Fe and B on the Columnar Structure of Ti-46Al Alloys

Abstract The microstructures of Ti-46Al, Ti-46Al-0.3B, Ti-46Al-0.5B, Ti-46Al-2Fe, Ti-46Al-2Fe-0.3B, and Ti-46Al-2Fe-0.5B alloys solidified on water-cooled copper crucible were investigated. It is found that the as-cast alloys possess a typical columnar structure, and the mean diameters of the columnar grains decrease significantly as the addition of Fe or B. The mean columnar diameter in Ti-46Al-2Fe-0.5B is found to be the smallest among these alloys. B addition can increase the constitutional supercooling in front of the solid-liquid interface, which refines the columnar structure and the dendritic structure. Moreover, Fe addition can enhance the refining effect of boron on the columnar structure of TiAl-based alloys.

Numerical Simulation of Thermal and Flow Fields in Induction Skull Melting Process

Abstract The electromagnetic, thermal and fluid fields during ISM (Induction Skull Melting)of TiAl alloy are simulated. The effects of ampere-turn, frequency and relative position between crucible and coil on the distribution of thermal and flow fields are numerically studied. The simulation model is verified by experiments and the simulation results are discussed in detail.