Jingjie Guo

Effect of cooling rate on solidification microstructures in AISI 304 stainless steel

Abstract AISI 304 austenitic stainless steel was cast into a water cooled copper mould with wedge shaped slot to investigate the effect of cooling rates on its microstructural evolution and solidification sequences between δ ferrite and austenite phases. Dendritic δ ferrite forms at the base of the sample where the cooling rate is low. With increasing cooling rate, the solidification mode is transformed from primary ferrite mode to primary austenite mode resulted from the phase selection. The metastable austenite solidifies as the primary phase due to its kinetic advantage when the growth rate is sufficiently high. With the higher cooling rate at the tip of the sample, the morphology of the austenite phase transforms from dendritic to cellular.

Well-aligned in situ composites in directionally solidified Fe–Ni peritectic system

Well-aligned in situ composites obtained in directionally solidified Fe–Ni peritectic system are formed by nonisothermal cellular coupled growth instead of isothermal coupled growth because of morphological instabilities. Peritectic coupled growth, with a plane interface like eutectic coupled growth, is always unstable due to the influence of peritectic reaction around the liquid/δ∕γ trijunctions. However, cellular nonisothermal peritectic coupled growth, in which one of the two solid phases bulges towards the liquid ahead of the other one, can reach a steady state and produce well-aligned in situ composites under proper growth conditions and sample composition.

Producing well aligned in situ composites in peritectic systems by directional solidification

Recently, it was found that cellular peritectic coupled growth (CPCG) can be a candidate method to grow well aligned in situ composites in peritectic alloys. In this letter, we experimentally show that there is a narrow growth region in which CPCG can be stable to avoid the influences of peritectic reaction around the trijunctions and the sidebranching instability to produce well aligned in situ composites. A simplified model was developed to predict the growth region of stable CPCG. Good agreement was obtained between the theoretical predictions and the experimental observations.

Microstructure evolution in AISI 304 stainless steel during near rapid directional solidification

Abstract The microstructure evolution of near rapidly directionally solidified AISI 304 stainless steel was investigated in the present paper. It is found that the microstructure consists of δ ferrite dendrites with developed sidebranches and interdendritic austenite (γ) under the temperature gradient (G) of 20 K mm–1 and growth rate (V) of 1·0 mm s–1. Coupled growth microstructures of thin lamellar ferrite and austenite begin to form at a higher growth rate of 2·0 mm s–1. The formation mechanism of the coupled microstructures is analysed based on the nucleation and constitutional undercooling criterion that the δ ferrite phase and austenite phase form alternately before the steady state growth of each phase is reached due to larger undercooling. With further increase of the growth rate up to 3·0 mm s–1, the morphology of the δ ferrite transforms from lathy to cellular.

Casting defects of Ti-6Al-4V alloy in vertical centrifugal casting processes with graphite molds

Numerical simulation and experimental investigation are utilized to analyze the casting defects of Ti-6Al-4V alloy formed under different vertical centrifugal casting conditions in graphite molds. Mold rotating rates of 0, 110 and 210 rpm are considered in experimental process. Results show that centrifugal forces have significant effects on the quantity of both macropores and microdefects (micropores, microcracks and inclusions). The relative amount of all macro- and micro-scopic casting defects decreases from 62.4 % to 24.8 % with the increasing of the centrifugal force, and the macropore quantity in stepped casting decreases exponentially with the increase of the gravitation coefficient. The relative proportions of both micropores and microcracks decrease with the mold-rotating rate increase, but the relative proportion of inclusions increases significantly. Besides this, the mold-filling sequence is proved to be an important factor in casting quality control.

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.

Evolution of solidification microstructure of centrifugal cast Ti–6Al–4V alloy

Abstract Ti–6Al–4V alloy castings were obtained with different centrifugal holding time in induction skull melting. The effects of centrifugal radius, riser and centrifugal holding time on the solidification microstructure of centrifugal cast Ti–6Al–4V alloy were investigated in detail. Results show that, under a certain centrifugal holding time, the average grain size of Ti–6Al–4V alloy castings decreases with the increasing in centrifugal radius. The microstructure of the Ti–6–Al–4V alloy casting with riser is finer. Meanwhile, the grain size increases greatly as the centrifugal holding time decreasing from 15 to 1·5 min.

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.

EFFECT OF HfO2-CODOPING CONCENTRATION ON THE OPTICAL PROPERTIES OF Er3+-DOPED LiNbO3

A series of Hf, Er co-doped LiNbO3 crystals were grown by Czochralski technique with 1 mol% of Er2O3 and with 2, 4, 6 and 8 mol% of HfO2, respectively. The optical damage resistance of Hf:Er:LiNbO3 crystals was studied by the transmitted beam pattern distortion method. The optical damage resistance of Hf (6 mol%): Er:LiNbO3 crystals is about two orders of magnitude higher than that in Hf:Er:LiNbO3. The X-ray power diffraction, the ultraviolet-visible absorption spectra and the infrared absorption spectrum were measured and discussed in terms of the spectrometric characterization and the defect structure of crystals. The results showed that with mild co-doping with HfO2, Er3+ substitutes Nb5+, whereas with heavy co-doping, a part of Er3+ substitutes Li+. The structure defects were discussed in this paper to explain the improvement of the optical damage resistance in the Hf:Er:LiNbO3.