Directional solidification of Ti-45Al-6Nb-xW（x=0,0.4,0.8;at%） alloys was performed by Bridgeman method with the stable growth rate of 5,10,and 20 μm·s^（-1）.The differential scanning calorimeter（DSC） results indicate that both the eutectic temperature and the transition temperature of α（Ti_3Al） to γ（TiAl） increase with W content increasing from 0 at%to 0.8 at%.For the stable growth rate of 10 μm·s^（-1）,the orientations of α_2（Ti_3Al）/γ（TiAl）lamellae change from 45°（0 at%W） to 0° and near 0°（0.4at%and 0.8 at%W） to the crystal growth direction,and the spacing of α_2/γ lamellae decreases with W content increasing from 0 at%to 0.4 at%,while it increases when W content is 0.8 at%.With the increase in growth rate from5 to 10 and 20 μm·s^（-1）,the lamellar spacing of α_2/γbecomes smaller,and the lamellar thickness becomes more uniform.
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.
Cold crucible directional solidification(CCDS)is a newly developed technique,which combines the advantages of the cold crucible and continuous melting.It can be applied to directionally solidify reactive,high purity and refractory materials.This paper describes the principle of CCDS and its characteristics;development of the measurement and numerical calculation of the magnetic field,flow field and temperature field in CCDS;and the CCDS of Ti based alloys.The paper also reviews original data obtained by some scholars,including the present authors,reported in separate publications in recent years.In Ti based alloys,Ti6Al4V,TiAl alloys and high Nb-containing TiAl alloys,have been directionally solidified in different cold crucibles.The crosssections of the cold crucibles include round,near rectangular and square with different sizes.Tensile testing results show that the elongation of directionally solidified Ti6Al4V can be improved to 12.7%from as cast5.4%.The strength and the elongation of the directionally solidified Ti47Al2Cr2Nb and Ti44Al6Nb1.0Cr2.0V are 650 MPa/3%and 602.5MPa/1.20%,respectively.The ingots after CCDS can be used to prepare turbine or engine blades,and are candidates to replace Ni super-alloy at temperatures of 700 to 900°C.
In order to improve the mechanical properties of Ti Al alloys, especially the ductility at room temperature, and to study the effect of boron(B) on Ti Al alloys, different contents(0, 0.1, 0.3, 0.6, 0.9, 1.2, at.%) of B were added into Ti-44Al-6Nb alloys to prepare ingots. The surface quality, macrostructure, microstructure, compressive properties and fracture surface of the ingots were studied. The results show that B has little influence on the surface quality except that there are some dark spots on the surface when the content of B is 0.9%. B can refine the grains. The average grain size decrease from about 0.8 mm to 0.088 mm with increasing B content. Meanwhile, the grain morphology of these ingots changes from big equiaxed grains with lamellars to fine equiaxed grains. When the content of B is 1.2%, the primary Ti B2 phase forms in the liquid phase and increases the nucleation rate, leading to further refinement of the grains. The compressive testing results show that B can increase the strength and the ductility, the compressive strength and compressibility can reach 2,037.8 MPa and 26.7% from 1,156.2 MPa and 10.2% when the boron content is 0.6%, which is resulted from grain refining and grain boundary strengthening. It is found that the compressive strength and the compressibility are relatively stable when the B content is more than 0.3%.
Numerical investigations on the flow field in Ti-Al melt during rectangular cold crucible directional solidification were carried out. Combined with the experimental results, 3-D finite element models for calculating flow field inside melting pool were established, the characteristics of the flow under different power parameters were further studied. Numerical calculation results show that there is a complex circular flow in the melt, a rapid horizontal flow exists on the solid/liquid interface and those flows confluence in the center of the melting pool. The flow velocity v increases with the increase of current intensity, but the flow patterns remain unchanged. When the current is 1000 A, the vmax reaches 4 mm/s and the flow on the interface achieves 3 mm/s. Flow patterns are quite different when the frequency changes from 10 kHz to 100 kHz, the mechanism of the frequency influence on the flow pattern is analyzed, and there is an optimum frequency for cold crucible directional solidification.
YANG Jie-renCHEN Rui-runDING Hong-shengSU Yan-qingHUANG FengGUO Jing-jieFU Heng-zhi
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.
The Ti-46A1-6Nb （mole fraction, %） ingots that were directionally solidified by cold crucible were cyclic heat treated at 1330 ℃ in the a phase region. The microstructures and mechanical properties of the ingots before and after heat treatment were investigated. The results show that the large columnar grains are changed into equiaxed grains after heat treatment. The grain size decreases with increasing the cyclic times, which is caused by the recrystallization and the transition from the large grain of small lamellae to the small grain of large lamellae. Four times of cyclic heat treatment refines the grain size from 1.33 mm to 0.59 turn, nevertheless the lamellar spacing increases from 0.71 ~tm to 1.38 lim. Extending the holding time and increasing the cyclic times of heat treatment eliminate the fl-segregation at the grain boundary and the interlamellar. The compression testing shows that the compressive strength of the directionally solidified ingot in the parallel and perpendicular directions are 1385.09 MPa and 1267.79 MPa, respectively, which are improved to 1449.75 MPa and 1527.76 MPa after two and four times of cyclic heat treatment, respectively, while that is 1180.64 MPa for the as-cast sample. The fracture mode of the sample after cyclic heat treatment is quasi-cleavage fracture.
The nominal Ti44Al6Nb1.0Cr2.0V alloy was newly designed and prepared by vacuum consumable melting technique with the ingot sizes of d225 mm×320 mm. The results show that the average lamella colony size is 780-1830 μm. This as-cast alloy has a modified near lamellar(M-NL) structure that is composed of mainly larger(α2+γ) lamella colonies and smaller(B2+equiaxed γ) blocky morphology. It exhibits the moderate tensile properties at room temperature, in which the Region(5) yields the ultimate tensile strength(UTS) about 499 MPa and the elongation about 0.53%. The obvious brittle fracture characteristics and trans-granular interlamellar fracture are the predominant modes. After room temperature tensile testing, there are some <101] and a few 1/2<112] superdislocations in the γ phase. The as-cast microcrack is the main factor to deteriorate the tensile property, which results in the premature fracture, poor ductility and few dislocations. The addition of Nb, Cr and V can decrease stacking fault energy(SFE) obviously, which is helpful to enhancing the ductility of the alloy.
