Y.Q. Su

Lamellar orientation control of directionally solidified Ti–46Al–0·5W–0·5Si alloy by self-seeding technology

Abstract Lamellar structures of Ti–46Al–0·5W–0·5Si (at-%) alloy solidifying through the primary α phase were successfully aligned parallel to the growth direction using a self-seeding technology in a Bridgman type directional solidification furnace. At the growth rate of 20 μm s−1 and temperature gradient of 12·1 K mm−1, the α phase grows along the direction. Therefore, the parallel lamellar structures, whose lamellar orientation is direction, can grow continuously until the end of solidification under these solidification parameters. The original lamellar structures of the unmelted region parallel to the growth direction and suitable solidification parameters are necessary for TiAl alloys solidifying through the primary α phase to control the lamellar orientation. The self-seeding technology gets rid of the cutting and fixing of seeds and simplifies the processing of controlling the lamellar orientation of TiAl alloys. It can promote the engineering applications of the lamellar orientation control of TiAl alloys.

Castability of thin walled titanium alloy castings in vertical centrifugal field

Abstract Different casting technologies were investigated on the thin walled titanium alloy castings in this paper, and the experimental results show that it is hard for the titanium alloy melts to fill the thin walled cavities in the gravity field. However, with the action of centrifugal force and Coriolis force on the melts in the centrifugal field, the cavities can be fully filled with the molten metal. The castability of thin walled titanium alloy castings was influenced by some casting parameters, such as rotational velocity, rotational radius and solidifying rate. Experimental results indicate that the forming ability of thin walled titanium alloy increases with increasing rotational velocity. However, the filling ability of molten metal changes a little with increasing rotational radius. In addition, under the experimental condition used in this paper, the number of internal defects of the thin walled castings decreases with increasing rotational velocity, but increases with increasing rotational radius. Filling sequence and solidifying rate play important roles in the defect formation of thin walled titanium alloy castings.

Effect of boron doping on microstructure of directionally solidified Ti-46Al-2Cr-2Nb alloy

Abstract In this paper, Ti–46Al–2Cr–2Nb and Ti–46Al–2Cr–2Nb–0·2B alloys were directionally solidified under different cooling rates. Based on the comparison, the effects of minor B doping on the primary dendrite arm and lamellar spacing were investigated. TiB was formed during solidification before the formation of the lamellar structure. The primary dendrite arm spacing was slightly reduced by TiB when the α phase was solidified as the primary phase. However, the lamellar spacing significantly increased in the Ti–46Al–2Cr–2Nb–0·2B alloy because TiB reduced the undercooling temperature required for the formation of the γ phase.

Phase and microstructure evolution in Cu-In-Cr alloys

Abstract Cu-In-Cr ternary alloy specimens were prepared with a metal mould and analysed by OP, EPMA, SEM, etc. The results show that the phase composition of Cu-11In-10Cr (wt-%) ternary alloy in casting microstructure includes Cu solid solution, Cr solid solution and Cu9In4 (δ) phase. Cu solid solution has dendritic morphology while Cr solid solution is star or petal shaped and is dispersed. In solidification, the solid/liquid interfaces of Cu solid solution and Cr solid solution are nonfaceted and they form by continuous growth, Cu solid solution forms dendrites under the influence of constitutional supercooling, while Cr solid solution forms equiaxed dendrites owing to the restricted chromium content. Chromium content has an influence on the morphology of Cu solid solution dendrites and Cr solid solution.

Microstructure and mechanical properties of ZrNbMoHfV high entropy alloy

Abstract A novel high entropy alloy, ZrNbMoHfV with equiatomic concentrations, was prepared by vacuum arc melting. This alloy exhibits a typical dendrite and interdendrite structure and contains three phases: two body centred cubic phases and a Laves phase. Owing to the formation of Laves phase, this alloy possesses high hardness and compressive strength while it is fragile. At 500°C, ZrNbMoHfV alloy remains the high compressive strength and fragile. At 1000°C, the ductility of this alloy increases significantly, while the compressive strength decreases dramatically.

Numerical prediction for columnar to equiaxed transition in solidified Ti-Al intermetallic alloy ingots by stochastic model

Abstract A stochastic model of macro- and microtype for predicting the columnar–equiaxed transition (CET) during solidification processes of Ti–Al intermetallic alloy ingots is developed in the present paper. The macroscopic part is based on a finite difference method (FDM) for modelling of heat transfer. The microscopic part consists of a cellular automaton method (CA) for modelling of nucleation and growth. The formation of a shrinkage cavity at the top of ingot is taken into account. The effects of casting variables on CET are presented and discussed. A quantitative relation between CET position and casting variables is obtained. The columnar zone is found to expand with decreasing alloy composition, mould preheated temperature and convection, and increasing thermal conductivity of mould, superheat and heat transfer coefficient at mould/metal interface. And the highly influential variables are superheat, heat transfer coefficient and convection.

Microstructure evolution and interfacial reaction of TiAl–Si alloy solidified in alumina crucible

The microstructure evolution of Ti–43Al–3Si (at-%) alloy solidified in alumina crucible was investigated by directional solidification technology. After directional solidification, the microstructure of the alloy is consisted of γ-TiAl, α2-Ti3Al, ξ-Ti5Si3 phases and Al2O3 particles. There are three morphologies of ξ phases formed in the alloy, namely, long rod-like, cluster-like with eutectic morphology, and needle-like shape. The volume fraction of ξ phases decreases with increasing growth rates. Al2O3 particles broke from the crucible and enter into the melt by the thermal physical erosion. Al2O3 particles enrich in the liquid phase with the moving of solid-liquid interface, and are captured or entrapped by dendrites during solidification. The Al2O3 particles mainly distributed in the interdendritic region, and some particles exist in dendrites.

Modelling generation mechanism of defects during permanent mould centrifugal casting process of TiAl alloy exhaust valve

Abstract In this paper, the TiAl liquid filling process during vertical centrifugal casting into a permanent mould has been described analytically. A model has been established to simulate the forward filling and backward filling process, and two parameters were used to provide a quantitative description of the defect generation that is associated with the forward filling of the mould cavity. One of the parameters used was the forward filling cross-sectional area, and the other was the inclined angle of the free surface of forward filling flow. The cross-sectional area decreases, and the inclined angle increases when the rotational speed increases, the tendency of which becomes more obvious near the mould cavity entrance. The residual volume of the mould cavity after the forward filling is related to the volume of trapped pores. The filling process has also been investigated using numerical simulation. Based on the filling and solidification characteristic of a TiAl valve, the off-centre porosity distribution is also discussed.