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A novel method to prepare columnar grains of TiAl alloys by controlling induction heating
International Communications in Heat and Mass Transfer 108 (2019) 104315
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
A novel method to prepare columnar grains of TiAl alloys by controlling induction heating
T
⁎
Yangli Liua, Xiang Xuea, Ruirun Chena,b, , Yingmei Tana, Yanqing Sua, Hongsheng Dinga, Jingjie Guoa a b
National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat treatment Temperature field β transformation Directional morphology
In order to obtain columnar grains of TiAl alloys by heat treatment instead of directional solidification, a new method named as directional induction heat treatment (DHT) has been put forward. Numerical simulation of temperature field and experiments for DHT has been carried out. Temperature fields of TiAl ingots with different diameters under different loaded power have been calculated. The results show that temperature difference between the surface and axis of TiAl ingots increases with the increasing of diameter from 20 mm to 50 mm or power from 10 kW to 30 kW, under skin effect and heat conduction inner. But there is almost no temperature difference in the ingot with diameter of 20 mm, no matter how high the power is. Based on above calculated results, an ingot with diameter of 20 mm was pushed upwards at intervals of 15 mm and was induction heated under different loaded power correspondingly. The temperature of DHT area could be controlled by adjusting loaded power. In the range of 1732 K–1773 K, β phase transformation will occur and β grains grow up continuously with the pulling of the Ti44Al ingot. In this way, columnar grains of Ti44Al alloy have been obtained.
1. Introduction TiAl based alloys have been considered as the best candidate for exchanging the Ni based alloys due to lower density, higher specific strength and their excellent mechanical properties at elevated temperature [1–4]. Generally, the microstructure of TiAl based alloys are composed of α2 (Ti3Al) and γ (TiAl) phase at room temperature. Their structure types are DO19 and L10, respectively. Due to the difference of Al and Ti atom in radius, the crystal symmetry of γ phase decreases. Additional slip systems are less in α2 phase, leading to poor plasticity of TiAl based alloys at room temperature [5,6]. Hence the industrial application of TiAl based alloys are limited significantly [7,8]. The plasticity of TiAl based alloys at room temperature has been greatly improved due to the development of electromagnetic cold crucible directional solidification technology. The Ti-46Al-0.5 W-0.5Si alloy prepared by electromagnetic cold crucible directional solidification (CCDS) technology [9] has great plasticity at room temperature up to 3.6%. Our team have dedicated to study the control mechanism of liquid-solid (L-S) interface shape and solute distribution at the forefront of L-S interface by optimizing the directional solidification process parameters in order to obtain uniform chemical component, excellent
⁎
directional microstructure and comprehensive mechanical properties of TiAl based alloys [10–15]. It is very fine and complicated task to control the shape of L-S interface and keep it flat during CCDS. The main reason is that melt contacts with the water cooled copper crucible wall extremely easily, causing L-S interface curvature on different degree. Furthermore, columnar grains at border would deflect due to radial heat dissipation. In order to solve the deflection of columnar grains at border in the process of CCDS, directional induction heat treatment (DHT) technology is invented in this study. The core idea of this technology is that without water cooled copper crucible, β transformation occurs in effective heat treatment area and β grains grow up continuously in solid phase transition process. And then directional columnar grains are finally obtained. The technology is a method which combines induction heating with direction motion of ingot to realize dynamic heat treatment of metals or alloys. Namely, in the process of heat treatment, the ingot was moved along with certain direction so that grains situating in effective heat treatment area would transfer β phase and grow up continuously. Numerical simulation is very important for parameter optimization and experiment guidance. Sheikholeslami et al. [16,17] carried out a finite element method (FEM) simulation for heat transfer behavior of
Corresponding author at: National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China E-mail address: [email protected] (R. Chen).
https://doi.org/10.1016/j.icheatmasstransfer.2019.104315
Available online 04 September 2019 0735-1933/ © 2019 Elsevier Ltd. All rights reserved.
International Communications in Heat and Mass Transfer 108 (2019) 104315
Y. Liu, et al.
nanoparticle. They found that introducing nanoparticles result into improve the discharging rate and affect solid front. Then they applied finite volume method (FVM) to simulate heat transfer of nanofluid considering entropy generation and solve the equations for the flow and energy balance. They found that thermal boundary layer thickness is affected by secondary flow and thermal irreversibility is greater than frictional irreversibility [18,19]. Due to the obvious skin effect of induction heating, the temperature field distribution and heat transfer process in the ingot should be studied before DHT. By controlling the loaded power, pulling speed and limiting the size of the ingot, the radial temperature distribution in the ingot becomes uniform and the temperature interface is straight. In this study, a finite element (FE) model was established to calculate temperature fields for different parameters and then simulation results were verified. The experimental results are consistent with the simulation results and provide a strong theoretical basis and practical guidance for the subsequent induction heat treatment experiments.
qs =
The principle diagram of DHT is shown in Fig. 1(a) and the experimental equipment is shown in Fig. 1(b). Induction coils with alternating current generate an electromagnetic field, and the ingot is heated. By controlling the loaded power, the temperature of effective heat treatment area in ingot can reach a set value. The solid phase transition will occur in grains locating at this area. Then the pulling device is switched on and pulling speed is set. Since the ingot is placed in GaeIn alloy pool, grains suffering solid phase transition will grow up along with certain direction under the single heat conduction. So the directional grains morphology will be formed in ingot after DHT.
q b = h (T − TGa − In )
(5)
In actual situation, DHT is a complex process. In the current study a simplified model was applied shown in Fig. 2(a). A five-turn induction coil with the height of 54 mm was used. The power frequency is set as 50 kHz. The charge with a height of 96 mm is in the center of coils and its top is 22 mm far from the first turn coil. The thermal fields of the charge with a diameter of 20 mm under different loaded power and the charges with different diameters under the same loaded power (10 kW) were calculated by commercial ANSYS 14.5 software (distributed by ANSYS HIT). The FE model and grids are presented in Fig. 2(a). And the surface region of the charge is fine-meshed in view of skin effect. The calculation procedure is shown in Fig. 2(b). The thermal radiation occurs on surface of the charge and the boundary condition is expressed as Eq. (4). Meanwhile, Eq. (5) states the heat exchange between the charge and GaeIn alloy. The physical properties including resistivity，specific enthalpy and thermal conductivity applied in calculation are referred in Ref. [24].
The Fourier thermal conduction differential equation for the process of induction heating of the charge without convective heat transfer is as follows [20,21]. ⎟
λ ρcp
(4)
2.3. Modeling of thermal field
2.2. Heat transfer equations
α=
ξ ⋅σb⋅(T 4 − T∞4 ) ∂T + =0 λ ∂n
where ξ is the emissivity, σb is the Stefan-Boltzmann constant as 5.67 × 10−8 W/m−2 k−4 and T∞ is the ambient temperature(K) as 298 K. qb is the heat flux density. h is the convection heat transfer coefficient and TGa-In is the liquid alloy temperature as 298 K. Eqs. (4) and (5) are the boundary conditions for Eq. (1). DHT is a complex and continuous process, which is accompanied by phase transition and heat transfer. The mathematical relationship between temperature of the charge before and after pulling (Tc) and (T'c) under a certain pulling rate (u) is that the increase of u will reduce T'c, which is consistent with the relationship between temperature and pulling rate during CCDS [23]. T'c → Tβ (1733 K for Ti44Al alloy) when u → +∞. By contrast, T'c → Tc (1813 K for Ti44Al alloy) when u → 0.
2.1. Principle of directional induction heat treatment
⎜
(3)
where T is the temperature, α is the thermal diffusion coefficient and qs is the heat flux density induced by skin effect. ρ, cp and λ are material A is the density, heat capacity and heat conductivity, respectively. amplitude of magnetic vector potential. The heat flux density is determined by Joule heat, as stated in Ref. [22]. The thermal radiation from surface of the charge and the heat conduction at bottom of the charge are respectively expressed as:
2. Experimental procedure and numerical model
q ∂T λ ⎛ ∂ 2T ∂ 2T ∂ 2T ⎞ = + + + s 2 2 ∂t ρcp ⎝ ∂x ∂y ∂z 2 ⎠ ρcp
∧ J2 = σ (jωΑe jωt )2 σ
(1)
(2)
Fig. 1. Directional induction heat treatment. (a) 3-D diagram; (b) experimental equipment. 2
International Communications in Heat and Mass Transfer 108 (2019) 104315
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(a)
Radiation
(b)
FE modeling EM field set and save
22mm
Convert to temperature field and set Coil NO
h1
t+Δt EM field solution
YES Physical properties update
Temperature field solution Convergence adjustment Charge Reach the final time Calculation end
Heat transfer with Ga-In liquid Fig. 2. FE model for temperature calculation of DHT. (a) FE model; (b) calculation procedure.
ingot locating at effective heat treatment area is measured under different loaded power. The loaded power is 16 kW, 21.6 kW and 28 kW, respectively. When the loaded power is 16 kW and 21.6 kW, the ingot should be thermal stabilization for 10 min before temperature measurement.
2.4. Temperature measurement for DHT under different loaded power The temperature of effective heat treatment area should be measured before DHT. Referring to distribution results of magnetic field and temperature field during CCDS [14,23,24]and combining with previous related experiments, effective heat treatment area of the square coils with 5 turns (cross section of 60 × 60 mm2) is located at the center lower parts. The thickness of this area is about 15 mm and the top boundary of effective heat treatment area is about 23 mm far from the top plane of induction coils (marked as h), shown in Fig. 3. Using WRe5-WRe26 thermocouple, the top plane temperature of Ti44Al alloy
2.5. The microstructure of Ti44Al alloy under different loaded power Square coils with 5 turns and Ti44Al alloy ingot with a diameter of 20 mm are chosen to verify the simulation results. The installation diagram is similar to Fig. 3 without the thermocouple. During experiment, there is cooling water in the induction coils. A molten pool was formed at the top of ingot by continuously increasing of operating power to 28 kW. Then, the loaded power was reduced to 21.6 kW and 16 kW at intervals, and at the same time the ingot was pushed upwards with a speed of 0.5 mm/min. The moving distance of ingot is 15 mm when the load power is 21.6 kW and 16 kW, respectively. 3. Results and discussion 3.1. Simulation results for thermal field The diameter of charges is 20 mm, 30 mm, 40 mm and 50 mm, respectively. Using square coils with 5 turns to heat, the temperature distribution of charges at the same loaded power (10 kW) is shown in Fig. 4. And Fig. 5 shows the temperature distribution of the charge with a diameter of 20 mm under loaded power being 10 kW, 18 kW, 22 kW and 30 kW, respectively. It can be seen clearly when the same power is loaded in induction coils, the area of the high temperature (effective heat treatment area) decreases gradually with the diameter of the charge increasing, and skin effect is gradually obvious, seen in Fig. 4. When charges with the same diameter (Φ 20) are heated under different loaded power, all of the effective heat treatment areas penetrate the charge along radial direction and the temperature interfaces of effective heat treatment area are flat, seen in Fig. 5. This is mainly due to a complicated heat transfer process during DHT. According to the characteristics of induction heating, the surface layer of the charge will be first to induction heat. Subsequently, high temperature area at surface layer of the charge will spontaneously transfer heat to the core of the charge under temperature gradient. The effective heat treatment area within the charge
Fig. 3. Temperature measurement system for DHT. 3
International Communications in Heat and Mass Transfer 108 (2019) 104315
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Fig. 4. The temperature distribution of the charge with different diameters. (a) 20 mm; (b) 30 mm; (c) 40 mm; (d) 50 mm.
time. Therefore, the regional induction heat treatment can be realized by limiting the cross section size of ingot and loaded power of induction coils. The core micro-area and 3r/4 micro-area in effective heat treatment area of ingots with different diameters are marked as point 1(P1) and point 2(P2), respectively, shown in Fig. 6(a). The relation between temperature and load time of P1 and P2 is shown in Fig. 6 under 10 kW loaded power. It can be seen that the temperature of P1 and P2 almost completely changes at the same time for the ingot with diameter of 20 mm. And after about 200 s, the same stable temperature is almost reached at the same time in the two micro-areas, shown in Fig. 6(a). As the diameter of the ingot increases, the effective heat treatment area gradually shrinks and displays skin effect, as mentioned above. And the temperature change of P1 and P2 in Fig. 6(b), 6(c) and 6(d) obviously indicates that the temperature difference (ΔT) from P1 and P2 is generated gradually with the increase of cross section size. The main reason is that the conduction heat transfer in radial and axial direction and surface radiation heat transfer occur at the same time during DHT. Only a dynamic stability namely an identity on the level of space and time of heat transfer being achieved, the effective heat treatment area should
will expand. When the distance for conduction heat transfer in radial direction is controlled within a certain range, induction heating and conduction heating reaches identity within the time range. The effective heat treatment area penetrates the charge along radial direction and the temperature interface of effective heat treatment area is flat, shown in Fig. 5. On the contrary, with the increase of cross section size of the charge, it will take more time for conduction heating. It can not achieve the identity between induction heating and conduction heating. So the skin effect is obvious in temperature field, seen in Fig. 4. Fig. 5 also shows that the temperature of effective heat treatment area increases significantly with the increase of loaded power, which is consistent with the findings of Jieren Yang [10] et al. They have got the same relation between temperature and loaded power in research of cold crucible electromagnetic melting of TiAl based alloy containing Nb element. When the loaded power is 18 kW, the temperature of effective heat treatment area is 1695 K. The temperature is within the range of (α + β) phase field for Ti44Al alloy. The temperature of effective heat treatment area is 1838 K under 30 kW loaded power, which is over the melting point of Ti44Al alloy. In the two situations, the effective heat treatment area always penetrates the charge along radial direction and the temperature interface of effective heat treatment area is flat all the
Fig. 5. The temperature distribution of the charge with 20 mm diameter under different loaded power. (a) 10 kW; (b) 18 kW; (c) 22 kW; (d) 30 kW. 4
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Fig. 6. The temperature curves of P1 and P2 in ingot with different diameters under 10 kW loaded power. (a) 20 mm; (b) 30 mm; (c) 40 mm; (d) 50 mm.
grains at middle position grow up significantly. The main reason lies to the experimental aims and procedure. In order to study morphology evolution of Ti44Al alloy during DHT under different loaded power, the first task must be to determinate the loaded power for melting. An experiment for pushing ingot upwards contiguously has been conducted under different loaded power. When the top of ingot starts to melt and a meniscus area is formed, we can watch the phenomenon from watching windows locating at the top of furnace. The loaded power is switched down quickly to 21.6 kW and the ingot is pushed upwards contiguously. However, the axial temperature gradient is limited in the experiment of pushing ingot upwards. Grains suffering directional heat treatment would further grow up in all directions during air cooling. So gains at bottom position possess directional morphology and grains at middle position grow up contiguously and coarsen obvious. At the top of ingot, grains nearby meniscus area are fine. The reason is that the effective heat treatment area is a certain thickness, about 15 mm. When loaded power is 28 kW, grains nearby meniscus area would melt and recrystallize. And then these grains would pass through effective heat treatment area quickly as the ingot was pushed upwards and load power decreased. So the temperature of this area would decrease quickly and these grains are fine. Fig. 9 shows the microstructure morphology of Ti44Al alloy after DHT. The microstructure on right side is formed under the loaded power of 28 kW, containing the obvious meniscus melting area. The microstructure at middle position is formed under the loaded power of 21.6 kW. It can be seen that grains producing β transformation grow up along axial and full lamellar microstructure is formed. However, under the loaded power of 16 kW, the microstructure would keep the same as the as-cast Ti44Al alloy. Under different loaded power, the top plane temperature of Ti44Al alloy ingot located at effective heat treatment area is measured by WRe5-WRe26 thermocouple. For this thermocouple, the ability to measuring high temperature is up to 2573 K with high accuracy (≤ ± 0.5% t, t represents the measured temperature /°C). When the loaded power is 28 kW, 1743 K is reading on the screen of temperature device. After 5 s the meniscus molten pool is formed and collapse
penetrate the cross section of ingot and temperature interface should be flat. Therefore, a regional dynamic heat treatment can be achieved. When the diameter of ingot is 50 mm, the core micro-area is out of effective heat treatment area, shown in Fig. 4(d). The temperature difference (ΔT) of P1 and P2 is 63 K, seen in Fig. 6(d). For Ti44Al alloy, the temperature range of β phase field is only about 60 K [25]. Therefore, DHT could not be realized in a ingot with a diameter of 50 mm by 5 turns induction coils (cross section of 60 × 60 mm2). Fig. 7 shows that the temperature curves of P1 and P2 in the same ingot with diameter of 20 mm under loaded power being 10 kW, 18 kW, 22 kW and 30 kW, respectively. It can be seen that with the increase of loaded power, the temperature difference (ΔT) of P1 and P2 gradually generates but it is not particularly obvious. When the loaded power is 30 kW, the temperature of P2 is 1838 K which is over the melting point of Ti44Al alloy [25]. And the ΔT achieves peak value about 10 K in this situation. In the process of DHT, the temperature of effective heat treatment area is within the range of β phase field for Ti44Al alloy. The loaded power is about 22 kW. And the temperatures of P1 and P2 are 1749 K and 1757 K, respectively, which are all within the range of β phase field. So DHT can be realized for the ingot with a diameter of 20 mm. 3.2. Experimental verification Fig. 8 shows the macrostructure of Ti44Al alloy after DHT by pushing the ingot upwards. It can be seen that the tip of the ingot is melted when the loaded power is 28 kW, and the meniscus melt area is obviously hump, as marked by the arrows. When the loaded power is 21.6 kW, obvious grain growth occurs in Ti44Al alloy. And as the ingot moves upwards, the bottom grain shows a certain orientation arrangement. When the loaded power is 16 kW, the morphology of Ti44Al alloy hardly changes, seen below the red line in Fig. 8. Fig. 8 also shows that not only directional growth of grains occur in Ti44Al alloy but the temperature interface of effective heat treatment area is relatively flat, as indicated by red line. The directional morphology is shown in the grains at the bottom of ingot. And the equiaxed 5
International Communications in Heat and Mass Transfer 108 (2019) 104315
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Fig. 7. The temperature curves of P1 and P2 in ingot with a diameter of 20 mm under different loaded power. (a) 10 kW; (b) 18 kW; (c) 22 kW; (d) 30 kW. Table 1 The temperatures of ingot under different loaded power/K (300 Pa).
meniscus
28kW
Loaded power
Temperature/K
Phase field
28 kW 21.6 kW 16 kW
1805 ± 7.7 1750 ± 7.4 1634 ± 6.8
L+β β α
Equiaxed grains instantaneously. At that time, 1805 K is reading on the screen of temperature device. Similarly, the temperatures of ingot are estimated under other loaded power, as shown in Table 1. It can be seen that the temperature of ingot is 1750 ± 7.4 K locating in single β phase field when loaded power is 21.6 kW. And the temperature of ingot is 1634 ± 6.8 K locating in single α phase field when load voltage is 16 kW. A.DENQUIN [26] et al. found that the formation of lamellar morphology in TiAl based alloy does not result from eutectic reaction. But γ phase precipitates from disorder α phase or order α2 phase following one of the two reactions: (1) α → α2 → α2 + γ or (2) α → α + γ → α2 + γ [27,28]. Before precipitation, some Shockley incomplete dislocations are formed in hexagonal crystal which is nucleation sites for γ phase. The disorder-order transition (α → α2) is an atomic activity. Therefore, grain size would maintain the same like that before transition. And grain boundary does not move in the formation process of lamellar morphology. Hence, the grain
21.6kW Columnar grains
16kW
Fig. 8. The macrostructure of Ti44Al alloy after DHT.
Fig. 9. Microstructure of Ti44Al alloy under different loaded power after DHT. 6
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treatment area gradually expands and penetrates the charge along radial direction under the dual effect of skin effect and heat conduction. (3) When the diameter is 20 mm for ingot, the effective heat treatment area penetrates along radial direction completely and the temperature interface is flat. There is almost no temperature difference between the surface and the axis of TiAl ingot and the temperature field in the same cross section is homogeneous. (4) When loaded power is 21.6 kW, the temperature of ingot is 1750 ± 7.4 K. Grains in effective heat treatment area are in single β phase field. β solid phase transformation will occur and β grains grow up continuously with the pulling of the ingot. Finally the directional microstructure is obtained. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 51825401, 51741404).
Fig. 10. The schematic diagram of transformation and growth during DHT.
morphology of Ti44Al alloy does not change when the load voltage is 16 kW. For loaded power of 21.6 kW, the temperature of effective heat treatment area is in single β phase field. β transformation will occur among grains in effective heat treatment area. When the grains in effective heat treatment area transfer β phase completely, the ingot is pulled down slowly. Grains in upper of effective heat treatment area will successively enter this area and suffer β transformation shown in Fig. 10. In this way, a directional microstructure of Ti44Al alloy can be obtained. The technology of DHT is effective to avoid deflection of columnar grains, which frequently occurs in the process of cold crucible direction solidification due to the contact between melt and the water cooled copper crucible wall. By reasonably controlling the speed of the pulling device, loaded power of coils and the cross section size of ingot, β transformation will occur completely and β grains grow up continuously. The directional microstructure will be obtained so that the mechanical properties of TiAl based alloys would be improved significantly. In this study, grains in effective heating area participate in β transition and grow up along axial direction, which is controlled by loaded power under a certain withdrawing rate (0.5 mm/min). Columnar grains are obtained finally. Since the β grain growth is affected by both temperature and withdrawing rate. When withdrawing rate is low, thermal radiation on the surface of ingot increases relatively and heat conduction along axial direction decreases relatively. Therefore, it is easier for β grains to grow up along radial direction of ingot and bulky columnar grains are obtained. When withdrawing rate is high, heat conduction along axial direction increases, which will decrease the diffusing rate of atoms on grain boundary. As a result, grain boundary cannot move continuously and columnar grains cannot be obtained.
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4. Conclusion Based on numerical simulation and experiment verification for heat transfer behavior and temperature distribution of Ti44Al alloy during DHT, the following conclusions can be obtained: (1) The consistency of simulation and experimental results shows that the FE model for thermal field is reliable. The model can be used to calculate temperature field of TiAl ingot during DHT and the calculation results is accurate. (2) For the square coils with 5 turns (cross section of 60 × 60 mm2), with the decrease of cross section size of ingot, the effective heat 7
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
A novel method to prepare columnar grains of TiAl alloys by controlling induction heating
T
⁎
Yangli Liua, Xiang Xuea, Ruirun Chena,b, , Yingmei Tana, Yanqing Sua, Hongsheng Dinga, Jingjie Guoa a b
National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat treatment Temperature field β transformation Directional morphology
In order to obtain columnar grains of TiAl alloys by heat treatment instead of directional solidification, a new method named as directional induction heat treatment (DHT) has been put forward. Numerical simulation of temperature field and experiments for DHT has been carried out. Temperature fields of TiAl ingots with different diameters under different loaded power have been calculated. The results show that temperature difference between the surface and axis of TiAl ingots increases with the increasing of diameter from 20 mm to 50 mm or power from 10 kW to 30 kW, under skin effect and heat conduction inner. But there is almost no temperature difference in the ingot with diameter of 20 mm, no matter how high the power is. Based on above calculated results, an ingot with diameter of 20 mm was pushed upwards at intervals of 15 mm and was induction heated under different loaded power correspondingly. The temperature of DHT area could be controlled by adjusting loaded power. In the range of 1732 K–1773 K, β phase transformation will occur and β grains grow up continuously with the pulling of the Ti44Al ingot. In this way, columnar grains of Ti44Al alloy have been obtained.
1. Introduction TiAl based alloys have been considered as the best candidate for exchanging the Ni based alloys due to lower density, higher specific strength and their excellent mechanical properties at elevated temperature [1–4]. Generally, the microstructure of TiAl based alloys are composed of α2 (Ti3Al) and γ (TiAl) phase at room temperature. Their structure types are DO19 and L10, respectively. Due to the difference of Al and Ti atom in radius, the crystal symmetry of γ phase decreases. Additional slip systems are less in α2 phase, leading to poor plasticity of TiAl based alloys at room temperature [5,6]. Hence the industrial application of TiAl based alloys are limited significantly [7,8]. The plasticity of TiAl based alloys at room temperature has been greatly improved due to the development of electromagnetic cold crucible directional solidification technology. The Ti-46Al-0.5 W-0.5Si alloy prepared by electromagnetic cold crucible directional solidification (CCDS) technology [9] has great plasticity at room temperature up to 3.6%. Our team have dedicated to study the control mechanism of liquid-solid (L-S) interface shape and solute distribution at the forefront of L-S interface by optimizing the directional solidification process parameters in order to obtain uniform chemical component, excellent
⁎
directional microstructure and comprehensive mechanical properties of TiAl based alloys [10–15]. It is very fine and complicated task to control the shape of L-S interface and keep it flat during CCDS. The main reason is that melt contacts with the water cooled copper crucible wall extremely easily, causing L-S interface curvature on different degree. Furthermore, columnar grains at border would deflect due to radial heat dissipation. In order to solve the deflection of columnar grains at border in the process of CCDS, directional induction heat treatment (DHT) technology is invented in this study. The core idea of this technology is that without water cooled copper crucible, β transformation occurs in effective heat treatment area and β grains grow up continuously in solid phase transition process. And then directional columnar grains are finally obtained. The technology is a method which combines induction heating with direction motion of ingot to realize dynamic heat treatment of metals or alloys. Namely, in the process of heat treatment, the ingot was moved along with certain direction so that grains situating in effective heat treatment area would transfer β phase and grow up continuously. Numerical simulation is very important for parameter optimization and experiment guidance. Sheikholeslami et al. [16,17] carried out a finite element method (FEM) simulation for heat transfer behavior of
Corresponding author at: National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China E-mail address: [email protected] (R. Chen).
https://doi.org/10.1016/j.icheatmasstransfer.2019.104315
Available online 04 September 2019 0735-1933/ © 2019 Elsevier Ltd. All rights reserved.
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nanoparticle. They found that introducing nanoparticles result into improve the discharging rate and affect solid front. Then they applied finite volume method (FVM) to simulate heat transfer of nanofluid considering entropy generation and solve the equations for the flow and energy balance. They found that thermal boundary layer thickness is affected by secondary flow and thermal irreversibility is greater than frictional irreversibility [18,19]. Due to the obvious skin effect of induction heating, the temperature field distribution and heat transfer process in the ingot should be studied before DHT. By controlling the loaded power, pulling speed and limiting the size of the ingot, the radial temperature distribution in the ingot becomes uniform and the temperature interface is straight. In this study, a finite element (FE) model was established to calculate temperature fields for different parameters and then simulation results were verified. The experimental results are consistent with the simulation results and provide a strong theoretical basis and practical guidance for the subsequent induction heat treatment experiments.
qs =
The principle diagram of DHT is shown in Fig. 1(a) and the experimental equipment is shown in Fig. 1(b). Induction coils with alternating current generate an electromagnetic field, and the ingot is heated. By controlling the loaded power, the temperature of effective heat treatment area in ingot can reach a set value. The solid phase transition will occur in grains locating at this area. Then the pulling device is switched on and pulling speed is set. Since the ingot is placed in GaeIn alloy pool, grains suffering solid phase transition will grow up along with certain direction under the single heat conduction. So the directional grains morphology will be formed in ingot after DHT.
q b = h (T − TGa − In )
(5)
In actual situation, DHT is a complex process. In the current study a simplified model was applied shown in Fig. 2(a). A five-turn induction coil with the height of 54 mm was used. The power frequency is set as 50 kHz. The charge with a height of 96 mm is in the center of coils and its top is 22 mm far from the first turn coil. The thermal fields of the charge with a diameter of 20 mm under different loaded power and the charges with different diameters under the same loaded power (10 kW) were calculated by commercial ANSYS 14.5 software (distributed by ANSYS HIT). The FE model and grids are presented in Fig. 2(a). And the surface region of the charge is fine-meshed in view of skin effect. The calculation procedure is shown in Fig. 2(b). The thermal radiation occurs on surface of the charge and the boundary condition is expressed as Eq. (4). Meanwhile, Eq. (5) states the heat exchange between the charge and GaeIn alloy. The physical properties including resistivity，specific enthalpy and thermal conductivity applied in calculation are referred in Ref. [24].
The Fourier thermal conduction differential equation for the process of induction heating of the charge without convective heat transfer is as follows [20,21]. ⎟
λ ρcp
(4)
2.3. Modeling of thermal field
2.2. Heat transfer equations
α=
ξ ⋅σb⋅(T 4 − T∞4 ) ∂T + =0 λ ∂n
where ξ is the emissivity, σb is the Stefan-Boltzmann constant as 5.67 × 10−8 W/m−2 k−4 and T∞ is the ambient temperature(K) as 298 K. qb is the heat flux density. h is the convection heat transfer coefficient and TGa-In is the liquid alloy temperature as 298 K. Eqs. (4) and (5) are the boundary conditions for Eq. (1). DHT is a complex and continuous process, which is accompanied by phase transition and heat transfer. The mathematical relationship between temperature of the charge before and after pulling (Tc) and (T'c) under a certain pulling rate (u) is that the increase of u will reduce T'c, which is consistent with the relationship between temperature and pulling rate during CCDS [23]. T'c → Tβ (1733 K for Ti44Al alloy) when u → +∞. By contrast, T'c → Tc (1813 K for Ti44Al alloy) when u → 0.
2.1. Principle of directional induction heat treatment
⎜
(3)
where T is the temperature, α is the thermal diffusion coefficient and qs is the heat flux density induced by skin effect. ρ, cp and λ are material A is the density, heat capacity and heat conductivity, respectively. amplitude of magnetic vector potential. The heat flux density is determined by Joule heat, as stated in Ref. [22]. The thermal radiation from surface of the charge and the heat conduction at bottom of the charge are respectively expressed as:
2. Experimental procedure and numerical model
q ∂T λ ⎛ ∂ 2T ∂ 2T ∂ 2T ⎞ = + + + s 2 2 ∂t ρcp ⎝ ∂x ∂y ∂z 2 ⎠ ρcp
∧ J2 = σ (jωΑe jωt )2 σ
(1)
(2)
Fig. 1. Directional induction heat treatment. (a) 3-D diagram; (b) experimental equipment. 2
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(a)
Radiation
(b)
FE modeling EM field set and save
22mm
Convert to temperature field and set Coil NO
h1
t+Δt EM field solution
YES Physical properties update
Temperature field solution Convergence adjustment Charge Reach the final time Calculation end
Heat transfer with Ga-In liquid Fig. 2. FE model for temperature calculation of DHT. (a) FE model; (b) calculation procedure.
ingot locating at effective heat treatment area is measured under different loaded power. The loaded power is 16 kW, 21.6 kW and 28 kW, respectively. When the loaded power is 16 kW and 21.6 kW, the ingot should be thermal stabilization for 10 min before temperature measurement.
2.4. Temperature measurement for DHT under different loaded power The temperature of effective heat treatment area should be measured before DHT. Referring to distribution results of magnetic field and temperature field during CCDS [14,23,24]and combining with previous related experiments, effective heat treatment area of the square coils with 5 turns (cross section of 60 × 60 mm2) is located at the center lower parts. The thickness of this area is about 15 mm and the top boundary of effective heat treatment area is about 23 mm far from the top plane of induction coils (marked as h), shown in Fig. 3. Using WRe5-WRe26 thermocouple, the top plane temperature of Ti44Al alloy
2.5. The microstructure of Ti44Al alloy under different loaded power Square coils with 5 turns and Ti44Al alloy ingot with a diameter of 20 mm are chosen to verify the simulation results. The installation diagram is similar to Fig. 3 without the thermocouple. During experiment, there is cooling water in the induction coils. A molten pool was formed at the top of ingot by continuously increasing of operating power to 28 kW. Then, the loaded power was reduced to 21.6 kW and 16 kW at intervals, and at the same time the ingot was pushed upwards with a speed of 0.5 mm/min. The moving distance of ingot is 15 mm when the load power is 21.6 kW and 16 kW, respectively. 3. Results and discussion 3.1. Simulation results for thermal field The diameter of charges is 20 mm, 30 mm, 40 mm and 50 mm, respectively. Using square coils with 5 turns to heat, the temperature distribution of charges at the same loaded power (10 kW) is shown in Fig. 4. And Fig. 5 shows the temperature distribution of the charge with a diameter of 20 mm under loaded power being 10 kW, 18 kW, 22 kW and 30 kW, respectively. It can be seen clearly when the same power is loaded in induction coils, the area of the high temperature (effective heat treatment area) decreases gradually with the diameter of the charge increasing, and skin effect is gradually obvious, seen in Fig. 4. When charges with the same diameter (Φ 20) are heated under different loaded power, all of the effective heat treatment areas penetrate the charge along radial direction and the temperature interfaces of effective heat treatment area are flat, seen in Fig. 5. This is mainly due to a complicated heat transfer process during DHT. According to the characteristics of induction heating, the surface layer of the charge will be first to induction heat. Subsequently, high temperature area at surface layer of the charge will spontaneously transfer heat to the core of the charge under temperature gradient. The effective heat treatment area within the charge
Fig. 3. Temperature measurement system for DHT. 3
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Fig. 4. The temperature distribution of the charge with different diameters. (a) 20 mm; (b) 30 mm; (c) 40 mm; (d) 50 mm.
time. Therefore, the regional induction heat treatment can be realized by limiting the cross section size of ingot and loaded power of induction coils. The core micro-area and 3r/4 micro-area in effective heat treatment area of ingots with different diameters are marked as point 1(P1) and point 2(P2), respectively, shown in Fig. 6(a). The relation between temperature and load time of P1 and P2 is shown in Fig. 6 under 10 kW loaded power. It can be seen that the temperature of P1 and P2 almost completely changes at the same time for the ingot with diameter of 20 mm. And after about 200 s, the same stable temperature is almost reached at the same time in the two micro-areas, shown in Fig. 6(a). As the diameter of the ingot increases, the effective heat treatment area gradually shrinks and displays skin effect, as mentioned above. And the temperature change of P1 and P2 in Fig. 6(b), 6(c) and 6(d) obviously indicates that the temperature difference (ΔT) from P1 and P2 is generated gradually with the increase of cross section size. The main reason is that the conduction heat transfer in radial and axial direction and surface radiation heat transfer occur at the same time during DHT. Only a dynamic stability namely an identity on the level of space and time of heat transfer being achieved, the effective heat treatment area should
will expand. When the distance for conduction heat transfer in radial direction is controlled within a certain range, induction heating and conduction heating reaches identity within the time range. The effective heat treatment area penetrates the charge along radial direction and the temperature interface of effective heat treatment area is flat, shown in Fig. 5. On the contrary, with the increase of cross section size of the charge, it will take more time for conduction heating. It can not achieve the identity between induction heating and conduction heating. So the skin effect is obvious in temperature field, seen in Fig. 4. Fig. 5 also shows that the temperature of effective heat treatment area increases significantly with the increase of loaded power, which is consistent with the findings of Jieren Yang [10] et al. They have got the same relation between temperature and loaded power in research of cold crucible electromagnetic melting of TiAl based alloy containing Nb element. When the loaded power is 18 kW, the temperature of effective heat treatment area is 1695 K. The temperature is within the range of (α + β) phase field for Ti44Al alloy. The temperature of effective heat treatment area is 1838 K under 30 kW loaded power, which is over the melting point of Ti44Al alloy. In the two situations, the effective heat treatment area always penetrates the charge along radial direction and the temperature interface of effective heat treatment area is flat all the
Fig. 5. The temperature distribution of the charge with 20 mm diameter under different loaded power. (a) 10 kW; (b) 18 kW; (c) 22 kW; (d) 30 kW. 4
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Fig. 6. The temperature curves of P1 and P2 in ingot with different diameters under 10 kW loaded power. (a) 20 mm; (b) 30 mm; (c) 40 mm; (d) 50 mm.
grains at middle position grow up significantly. The main reason lies to the experimental aims and procedure. In order to study morphology evolution of Ti44Al alloy during DHT under different loaded power, the first task must be to determinate the loaded power for melting. An experiment for pushing ingot upwards contiguously has been conducted under different loaded power. When the top of ingot starts to melt and a meniscus area is formed, we can watch the phenomenon from watching windows locating at the top of furnace. The loaded power is switched down quickly to 21.6 kW and the ingot is pushed upwards contiguously. However, the axial temperature gradient is limited in the experiment of pushing ingot upwards. Grains suffering directional heat treatment would further grow up in all directions during air cooling. So gains at bottom position possess directional morphology and grains at middle position grow up contiguously and coarsen obvious. At the top of ingot, grains nearby meniscus area are fine. The reason is that the effective heat treatment area is a certain thickness, about 15 mm. When loaded power is 28 kW, grains nearby meniscus area would melt and recrystallize. And then these grains would pass through effective heat treatment area quickly as the ingot was pushed upwards and load power decreased. So the temperature of this area would decrease quickly and these grains are fine. Fig. 9 shows the microstructure morphology of Ti44Al alloy after DHT. The microstructure on right side is formed under the loaded power of 28 kW, containing the obvious meniscus melting area. The microstructure at middle position is formed under the loaded power of 21.6 kW. It can be seen that grains producing β transformation grow up along axial and full lamellar microstructure is formed. However, under the loaded power of 16 kW, the microstructure would keep the same as the as-cast Ti44Al alloy. Under different loaded power, the top plane temperature of Ti44Al alloy ingot located at effective heat treatment area is measured by WRe5-WRe26 thermocouple. For this thermocouple, the ability to measuring high temperature is up to 2573 K with high accuracy (≤ ± 0.5% t, t represents the measured temperature /°C). When the loaded power is 28 kW, 1743 K is reading on the screen of temperature device. After 5 s the meniscus molten pool is formed and collapse
penetrate the cross section of ingot and temperature interface should be flat. Therefore, a regional dynamic heat treatment can be achieved. When the diameter of ingot is 50 mm, the core micro-area is out of effective heat treatment area, shown in Fig. 4(d). The temperature difference (ΔT) of P1 and P2 is 63 K, seen in Fig. 6(d). For Ti44Al alloy, the temperature range of β phase field is only about 60 K [25]. Therefore, DHT could not be realized in a ingot with a diameter of 50 mm by 5 turns induction coils (cross section of 60 × 60 mm2). Fig. 7 shows that the temperature curves of P1 and P2 in the same ingot with diameter of 20 mm under loaded power being 10 kW, 18 kW, 22 kW and 30 kW, respectively. It can be seen that with the increase of loaded power, the temperature difference (ΔT) of P1 and P2 gradually generates but it is not particularly obvious. When the loaded power is 30 kW, the temperature of P2 is 1838 K which is over the melting point of Ti44Al alloy [25]. And the ΔT achieves peak value about 10 K in this situation. In the process of DHT, the temperature of effective heat treatment area is within the range of β phase field for Ti44Al alloy. The loaded power is about 22 kW. And the temperatures of P1 and P2 are 1749 K and 1757 K, respectively, which are all within the range of β phase field. So DHT can be realized for the ingot with a diameter of 20 mm. 3.2. Experimental verification Fig. 8 shows the macrostructure of Ti44Al alloy after DHT by pushing the ingot upwards. It can be seen that the tip of the ingot is melted when the loaded power is 28 kW, and the meniscus melt area is obviously hump, as marked by the arrows. When the loaded power is 21.6 kW, obvious grain growth occurs in Ti44Al alloy. And as the ingot moves upwards, the bottom grain shows a certain orientation arrangement. When the loaded power is 16 kW, the morphology of Ti44Al alloy hardly changes, seen below the red line in Fig. 8. Fig. 8 also shows that not only directional growth of grains occur in Ti44Al alloy but the temperature interface of effective heat treatment area is relatively flat, as indicated by red line. The directional morphology is shown in the grains at the bottom of ingot. And the equiaxed 5
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Fig. 7. The temperature curves of P1 and P2 in ingot with a diameter of 20 mm under different loaded power. (a) 10 kW; (b) 18 kW; (c) 22 kW; (d) 30 kW. Table 1 The temperatures of ingot under different loaded power/K (300 Pa).
meniscus
28kW
Loaded power
Temperature/K
Phase field
28 kW 21.6 kW 16 kW
1805 ± 7.7 1750 ± 7.4 1634 ± 6.8
L+β β α
Equiaxed grains instantaneously. At that time, 1805 K is reading on the screen of temperature device. Similarly, the temperatures of ingot are estimated under other loaded power, as shown in Table 1. It can be seen that the temperature of ingot is 1750 ± 7.4 K locating in single β phase field when loaded power is 21.6 kW. And the temperature of ingot is 1634 ± 6.8 K locating in single α phase field when load voltage is 16 kW. A.DENQUIN [26] et al. found that the formation of lamellar morphology in TiAl based alloy does not result from eutectic reaction. But γ phase precipitates from disorder α phase or order α2 phase following one of the two reactions: (1) α → α2 → α2 + γ or (2) α → α + γ → α2 + γ [27,28]. Before precipitation, some Shockley incomplete dislocations are formed in hexagonal crystal which is nucleation sites for γ phase. The disorder-order transition (α → α2) is an atomic activity. Therefore, grain size would maintain the same like that before transition. And grain boundary does not move in the formation process of lamellar morphology. Hence, the grain
21.6kW Columnar grains
16kW
Fig. 8. The macrostructure of Ti44Al alloy after DHT.
Fig. 9. Microstructure of Ti44Al alloy under different loaded power after DHT. 6
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treatment area gradually expands and penetrates the charge along radial direction under the dual effect of skin effect and heat conduction. (3) When the diameter is 20 mm for ingot, the effective heat treatment area penetrates along radial direction completely and the temperature interface is flat. There is almost no temperature difference between the surface and the axis of TiAl ingot and the temperature field in the same cross section is homogeneous. (4) When loaded power is 21.6 kW, the temperature of ingot is 1750 ± 7.4 K. Grains in effective heat treatment area are in single β phase field. β solid phase transformation will occur and β grains grow up continuously with the pulling of the ingot. Finally the directional microstructure is obtained. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 51825401, 51741404).
Fig. 10. The schematic diagram of transformation and growth during DHT.
morphology of Ti44Al alloy does not change when the load voltage is 16 kW. For loaded power of 21.6 kW, the temperature of effective heat treatment area is in single β phase field. β transformation will occur among grains in effective heat treatment area. When the grains in effective heat treatment area transfer β phase completely, the ingot is pulled down slowly. Grains in upper of effective heat treatment area will successively enter this area and suffer β transformation shown in Fig. 10. In this way, a directional microstructure of Ti44Al alloy can be obtained. The technology of DHT is effective to avoid deflection of columnar grains, which frequently occurs in the process of cold crucible direction solidification due to the contact between melt and the water cooled copper crucible wall. By reasonably controlling the speed of the pulling device, loaded power of coils and the cross section size of ingot, β transformation will occur completely and β grains grow up continuously. The directional microstructure will be obtained so that the mechanical properties of TiAl based alloys would be improved significantly. In this study, grains in effective heating area participate in β transition and grow up along axial direction, which is controlled by loaded power under a certain withdrawing rate (0.5 mm/min). Columnar grains are obtained finally. Since the β grain growth is affected by both temperature and withdrawing rate. When withdrawing rate is low, thermal radiation on the surface of ingot increases relatively and heat conduction along axial direction decreases relatively. Therefore, it is easier for β grains to grow up along radial direction of ingot and bulky columnar grains are obtained. When withdrawing rate is high, heat conduction along axial direction increases, which will decrease the diffusing rate of atoms on grain boundary. As a result, grain boundary cannot move continuously and columnar grains cannot be obtained.
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4. Conclusion Based on numerical simulation and experiment verification for heat transfer behavior and temperature distribution of Ti44Al alloy during DHT, the following conclusions can be obtained: (1) The consistency of simulation and experimental results shows that the FE model for thermal field is reliable. The model can be used to calculate temperature field of TiAl ingot during DHT and the calculation results is accurate. (2) For the square coils with 5 turns (cross section of 60 × 60 mm2), with the decrease of cross section size of ingot, the effective heat 7
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