Hongsheng Ding

Effects and mechanism of ultrasonic irradiation on solidification microstructure and mechanical properties of binary TiAl alloys

In spite of their high temperature and reactivity, the binary TiAl alloys are successfully imposed by the ultrasonic irradiation and the microstructure evolution, solidification behaviors and mechanical properties are elaborately investigated. After ultrasonic irradiation, a high quality ingot without shrinkage defects and element segregation is obtained and the coarse dendrite structure is well modified into fine non-dendrite globular grains. The coarse lamellar colony and lamellar space of Ti44Al alloy is refined from 685μm to 52μm and 1185nm to 312nm, respectively (similarly, 819μm to 102μm and 2085nm to 565nm for Ti48Al alloy). For Ti48Al alloy, the α peritectic phase is simultaneously precipitated from the melt as well as the β primary phase before the peritectic reaction and the solidification is transformed into the mixed α-solidifying and β-solidifying. Ultrasonic irradiation promotes the peritectic reaction and phase transformation completely and the phase constituent becomes more close to the equilibrium level. The compressive strength of Ti44Al and Ti48Al alloys are increased from 623MPa to 1250MPa and 980MPa to 1295MPa, respectively. The grain refinement and dendrite transformation enhance the grain boundary sliding improving the plastic deformation ability. Ultrasonic irradiation significantly accelerates the melt flow and solute redistribution and the main grain refinement mechanism is the cavitation-enhanced nucleation by inclusion activation and heightened supercooling.

Temperature field calculation on cold crucible continuous melting and directional solidifying Ti50Al alloys

Abstract In order to optimize technological parameters and realize directional solidification, temperature fields of cold crucible continuous melting and directional solidifying Ti50Al (mole fraction, %) at different parameters were calculated. Continuous casting of the model is achieved by distinguishing the moving unit at different positions. The calculation results show that the feeding rod is entirely melted at 200 s, the melt of feeding rod has some superheat degree at 300 s under the conditions of 52 kW and 3.0 mm/min. Both the superheat degree and the molten zone of the feeding rod reduce, the solid-liquid interface becomes concave with increasing velocity from 1.2 mm/min to 6.0 mm/min when the power is 52 kW, and the outside layer of the rod cannot be melted at the velocity of 6.0 mm/min. Both superheat degree and the molten zone of the feeding rod increase, the solid-liquid interface descends and becomes concave with increasing power from 48 to 58 kW at velocity of 3.0 mm/min, and the rod cannot be melted entirely when the power is 48 kW. Cold crucible continuous melting and directional solidification of TiAl alloys will be achieved successfully when the pulling velocity and the power are matched appropriately.

Effect of cyclic heat treatment on microstructures and mechanical properties of directionally solidified Ti–46Al–6Nb alloy

The Ti–46Al–6Nb (mole fraction, %) ingots that were directionally solidified by cold crucible were cyclic heat treated at 1330 °C in the α 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 mm, nevertheless the lamellar spacing increases from 0.71 μm to 1.38 μm. Extending the holding time and increasing the cyclic times of heat treatment eliminate the β-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.

Effect of configuration on magnetic field in cold crucible using for continuous melting and directional solidification

To improve the power efficiency and optimize the configuration of cold crucible using for continuous melting and directional solidification (DS), based on experimental verification, 3D finite element (FE) models with various configuration-elements were developed to investigate the magnetic field in cold crucible. Magnetic flux density (B) was measured and calculated under different configuration parameters. These parameters include the inner diameter (D2), the slit width (d), the thickness of crucible wall, the section shape of the slit and the shield ring. The results show that the magnetic flux density in z direction (Bz) both at the slit and at the midpoint of segment will increase with the decrease of D2 or with the increase of the width of the slit and the section area of wedge slit or removing the shield ring. In addition, there is a worst wall thickness that can induce the minimum Bz for a cold crucible with a certain outer diameter.

Numerical calculation of flow field inside TiAl melt during rectangular cold crucible directional solidification

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.

Uniformity analysis of magnetic field in an electromagnetic cold crucible used for directional solidification

Purpose – The purpose of this paper is to introduce a numerical calculation method to study the uniformity of the magnetic field in a cold crucible used for directional solidification (DS) and provide information for designing a cold crucible that can induce a uniform magnetic field.Design/methodology/approach – To obtain the characteristics of the magnetic field in a cold crucible and its influence on the directional solidification processing, based on experimental verification, 3‐D finite element (FE) models with different crucible configuration‐elements and power parameters were established to study the uniformity of the magnetic field in a cold crucible. In addition, different TiAl ingots were directionally solidified with different cold crucibles, and the solid/liquid (S/L) interfaced were examined to investigate the effect of the magnetic field on the macrostructure of those ingots.Findings – The uniformity of the magnetic field in a given domain can be quantitatively analyzed by statistical methods...

Microstructure and room temperature tensile property of as-cast Ti44Al6Nb1.0Cr2.0V alloy

Abstract The nominal Ti44Al6Nb1.0Cr2.0V alloy was newly designed and prepared by vacuum consumable melting technique with the ingot sizes of d 225 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 ( α 2 + γ ) lamella colonies and smaller ( B 2 +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 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.

Efficient Melt Stirring Induced by the Coupled Effects of Alternating Magnetic Field and Configuration of Cold Crucible

In the present study, the fluid flow in cold crucible and its effects on solute transport were numerically and experimentally investigated. It was found that the coupled effects of alternating magnetic fields and the configuration of cold crucible induce a vigorous three-dimensional melt flow. Further, solute transport results indicate that the three-dimensional flow contributes to more efficient melt stirring, and the stirring efficiency for obtaining uniform melt is improved by 77.5 pct compared with only meridional flow.

Microstructure, Mechanical Properties, and Crack Propagation Behavior in High-Nb TiAl Alloys by Directional Solidification

Titanium aluminide (Ti-47Al-6Nb-0.1C) alloys were prepared using a cold crucible directional solidification technique with an input power range of 35 to 55 kW under a withdrawing velocity of 0.4 mm/min. The macro/microstructure was characterized, and the mechanical properties were evaluated. The results show that the directional solidification (DS) ingots exhibit β-solidification characteristics at an input power range of 35 to 55 kW, and a well-developed DS microstructure was acquired at an input power range of 40 and 45 kW. With the increasing input power, the lamellar spacing decreases, resulting in the increasing tendency in the average room-temperature yield strength. The steady-state creep rate strongly depends on the lamellar spacing, and the creep life depends on the DS microstructure. The well-developed DS alloy can significantly improve the creep properties but has little influence on the room-temperature tensile properties. Moreover, after testing the fracture toughness, the crack propagation showed interlamellar cracks, translamellar cracks, and intercolony boundary cracks that primarily propagated along the colony boundary after creep testing.

Improving the Performance of Nb-Silicide Based Refractory Alloys Through a Novel Cold Crucible Directional Solidification

Nb-silicide alloy with composition of Nb-22Ti-16Si-3Cr-3Al-2Hf (at.%) was directionally solidified into square ingots at different processing parameters through a novel cold crucible directional solidification technology. The effect of directionally solidified processing parameters, such as heat power and withdrawal rate, on the microstructure and mechanical properties was analyzed. Results show that the directional structure composed of coupled growth of (Nb,Ti)ss/(Nb,Ti)5Si3 composite can exhibit good tension performance at 1250°C.