The microstructure of TiAI based alloys is sensitive to growth rates. In this paper, Bridgman directional solidification of Ti-46Al-2Cr-2Nb-0.2B （at.%） alloy was carried out at a constant temperature gradient （G） to investigate the effects of various growth rates （v） on characteristic lengths （primary dendritic arm spacing, secondary dendritic arm spacing and lamellar spacing） of the microstructure. Results show that under the experimental conditions of G = 18 K-mm-1 and v = 15 IJm.s-1 to 70 iJm.s-1, the primary phase of directionally solidified Ti-46AI-2Cr-2Nb-0.2B alloy is a phase, the values of primary dendritic arm spacing （2,）, secondary dendritic arm spacing （λ2） and lamellar spacing （λta） decrease with the increase in growth rate. The results were compared with theoretical models and similar experimental results of TiAI based alloys. The Bouchard-Kirkaldy model agrees well with the relationship between primary dendritic arm spacing and growth rate obtained in the experiment; the relationship between them can be expressed by ,; = 758.6v-039. The relationship between the secondary dendritic arm spacing and the growth rate can be expressed by 2 = 113.9v-045, while the relationship between the lamellar spacing and growth rate can be expressed by ,; = 22.88v-v94.
A theoretical investigation of fluid flow,heat transfer and solidification(solidification transfer phenomena,STP)was presented which coupled with direct-current(DC)magnetic fields in a high-speed strip-casting metal delivery system.The bidirectional interaction between the STP and DC magnetic fields was simplified as a unilateral one,and the fully coupled solidification transport equations were numerically solved by the finite volume method(FVM).While the magnetic field contours for a localized DC magnetic field were calculated by software ANSYS and then incorporated into a three-dimensional(3-D)steady model of the liquid cavity in the mold by means of indirect coupling.A new FVM-based direct-SIMPLE algorithm was adopted to solve the iterations of pressure-velocity(P-V).The braking effects of DC magnetic fields with various configurations were evaluated and compared with those without static magnetic field(SMF).The results show that 0.6 T magnetic field with combination configuration contributes to forming an isokinetic feeding of melt,the re-circulation zone is shifted towards the back wall of reservoir,and the velocity difference on the direction of height decreases from 0.1 m/s to 0.Furthermore,the thickness of solidified skull increases uniformly from 0.45 mm to 1.36 mm on the chilled substrate(belt)near the exit.
The present work numerically investigates two-dimensional （2-D） solidification transport phenomena （EM-STP） during continuous casting （CC） process in the absence and the presence of static magnetic fields （SMFs）, based on a unified numerical model. For the purpose of controlling vortexes, the electromagnetic brake （EMBR） effects of various SMFs under the given depth of submerged entry nozzle （SEN） and the same casting velocity V0 are investigated. ANSYS software is used to analyze the SMFs that applied to the EMCC process, and then a data-conversion program based on the principle of linear interpolation proposed previously is used to deal with the issue of data-format-matching between FEM and FVM. The simulation results indicate that, an appropriate SMF can effectively suppress the bulk liquid flow in CC-process of steel plate, and with in- crease of the intensity of applied magnetic fields, the vortexes become weaker and the oscillating amplitude of impinging jet decreases. Based on the knowledge gained from the EMCC-STP analysis and by comparing the re- sults with the applied magnetic fields with those without magnetic fields, it is found that a SMF with 丨 Bmax 丨 = 5.5 × 10 -3 T can meet the need of braking, and consequently improve the quality of casting by reducing the penetration of non-metallic inclusions, as well as avoiding breakout, macro-segregation and crack ultimately.
Bridgman-type directional solidification experiments were conducted for Ti-46Al-8Nb （mole fraction, %） alloy. The effects of the growth rate and the diameter on the microstructure, phase transition and hardness of the alloy were investigated. The results show that with the increase of the growth rate and the decrease of the diameter, the fullyβphase solidification changes to the peritectic solidification, and the final microstructure is composed of theα2/γlamellar structure and a multiphase microstructure （B2 phase,α2/γlamellar structure） respectively, which can be attributed to the solute enrichment resulting from the decreasing diffusion and convection ability. The occurrence of peritectic reaction at high growth rate promotes the solute segregation heavily and the coarse lamellar spacing in Al-and Nb-rich region, which greatly decreases the hardness values and leads to the discontinuity of the hardness curves with the increase of the growth rate. Comparatively, the Ti-46Al-8Nb alloy has lower hardness values than the other applied TiAl-based alloys in previous studies.
A unified numerical model for simulating solidification transport phenomena (STP) of steel slab in electromagnetic continuous casting (EMCC) process was developed. In order to solve the multi-physics fields coupled problem conveniently, the complicated bidirectional coupled process between EM and STP was simplified as a unidirectional one, and a FEM/FVM-combined numerical simulation technique was adopted. The traveling magnetic fields (TMFs) applied to the EMCC process were calculated using the ANSYS11.0 software, and then the EM-data output by ANSYS were converted to FVM-format using a data-format conversion program developed previously. Thereafter, the governing equations were solved using a pressure-based Direct-SIMPLE algorithm. The simulation results of the STP in CC-process show that, due to the influences of Lorentz force and Joule heat, the two strong circulating flows and the temperature field can be obviously damped and changed once TMF with one pair of poles (1-POPs) or 2-POPs is applied, which would accordingly improve the quality of casting. It was found in the present research that the integrated actions of 2-POPs TMF are superior to 1-POPs. All the computations indicate that the present numerical model of EM-STP as well as the FEM/FVM-combined technique is successful.
Gong HaijunLi XinzhongFan XueyiQie JuhongXu DamingGuo Jingjie
Effect of thermal stabilization on the microstructure and mechanical property of directionally solidified Ti-46Al-0.5W-0.5Si （mole fraction, %） alloy was investigated. The specimens were thermal stabilized for different time （t） and directionally solidified at a constant growth rate of 30 μm/s and temperature gradient of 20 K/mm. Dependencies of the primary dendritic spacing （λ1）, secondary dendritic spacing （λ2）, interlamellar spacing （λL） and microhardness （HV） on holding time were determined. The values of the λ1, λ2 and λL increase with the increase of t, and the value of HV decreases with the increase of t. The increase of t is helpful to obtain a good directional solidification structure. However, it reduces the mechanical property of the directionally solidified TiAl alloy. The optimized value of t is about 30 min.
Sn-36%Ni peritectie alloys were directionally solidified at different growth rates under a constant temperature gradient （20 K/mm）, the dependences of microstructural characteristic length scales on the growth rate were investigated. Experimental results are presented, including primary and higher order dendrite arm spacings 21, 22, 23 and dendrite tip radius R of primary NisSn2 phase. Comparisons between the theoretical predictions and the experimental results show that, for the primary dendrites, 21=335.882v-0.21, which is in agreement with the Kurz-Fisher model; for the secondary dendrites, λ2=44.957v-0.277, which is consistent with the Bouchard-Kirkaldy model; for the tertiary dendrites, λ3=40.512v-0.274; for the dendrite tip radius, R=22.7v-0.36. The experimental results also show that the 21/22 changes greatly with increasing growth rate while the 21/23 has no significant change, indicating that tertiary dendrite arms have a more similar growth characteristics to primary dendrites compared with secondary dendrites. The λ1/R ranges from 2 to 2.3 with the increase of growth rate. Key words： Sn-Ni alloy; directional solidification; dendrite arm spacing; dendrite tip radius
Peng PENG Xin-zhong LI Yan-qing SU Dong-mei LIU Jing-jie GUO Heng-zhi FU
Directional solidification experiments were conducted for Ti-46Al-8Nb alloy at the growth rates ranging from 3 to 70 pards. The microstructure evolution and microsegregation pattern were investigated. In the range of growth rate, a regular dendritic structure appears and the primary dendrite spacing decreases with increasing growth rate. The peritectic reaction is observed during the solidification and the final microstructure is composed of α2/γ lamellar structure and retained β（B2） after directional solidification The lamellar orientation is found to be parallel and 45° to the primary growth direction ofβ dendrite. Peritectic reaction leads to significant chemical inhomogeneity, in which aluminum is rich in interdendritic liquid and niobium is rich in the core ofβ dendrite during the solidification. With the nucleation and growth of a phase, the segregation amplitude of niobium increases, which promotes the formation of B2 phase, while aluminum rich in the interdendritic becomes homogeneous gradually.
