In coaxial powder feeding laser cladding, the interaction between powder and laser will directly affect the precision and quality of cladding forming. Infrared camera cannot directly obtain the melting behavior of powder in laser. Therefore, by analyzing the absorption of heat by powder, a high-speed camera system is used to collect the dynamic behavior of powder melting, and a dynamic analytical model of powder melting process is established. The influence of laser power on different melting stages and the temperature characteristics of powder entering the molten pool are analyzed by simulation. The results show that there are three typical melting characteristic stages of “solid state → solid-liquid two-phase state → liquid state” in the dynamic melting behavior of powder collected by high-speed camera system in laser. The dynamic behavior of powder melting can be analyzed by mathematical analytical model, and the thermophysical behavior of different stages has a dynamic analytical model of thermal interaction between powder and laser. The influence of laser power, defocusing amount and powder-carrying gas flow rate on powder melting behavior is analyzed. At the same time, the influence of different laser powers on the duration of each characteristic stage is simulated and analyzed to predict the temperature distribution of powder particles reaching the substrate. It is found that when the laser power increases from 100 W to 1500 W, the temperature of powder entering the molten pool changes nonlinearly, and the temperature increases from 750 ℃ to 3250℃.
The laser cladding process has the advantages of strong energy focusing, small cladding heat affected zone, good forming, etc. The cladding process is easy to control, the production cost is low, and it has good effects on the repair and surface strengthening of metal parts. Because the coaxial powder feeding laser cladding mode has good light-powder coupling, high forming accuracy, high spatial freedom, and strong isotropy, it has become an important choice for laser additive manufacturing technology. Forming accuracy and quality control are the key to laser additive manufacturing. At present, in production practice, most of them rely on a large number of process tests and manual experience for regulation. Theoretical models are established from the perspective of forming mechanism and organizational evolution to achieve the prediction of accuracy and quality. Among them, the thermal interaction between laser and material, heat distribution mechanism, etc. have an important influence on the precise forming and high-performance forming of laser cladding. Therefore, to carry out research on the complex mode of thermal interaction between laser and powder, it is necessary to combine experiments with modeling, analyze the thermal interaction between coaxial powder feeding powder and laser, establish a dynamic analytical model, and clarify the mechanism of the influence of laser heat source characteristics on the thermal physical state of powder about to enter the molten pool.
At present, the research on the thermal physical behavior of coaxial powder feeding laser cladding mainly focuses on the absorption and scattering mode of powder on laser, the form of thermal interaction between light and powder, and the thermal physical state of the molten pool. Among them, the thermal interaction between light and powder has an important influence on other physical processes. Many scholars at home and abroad have carried out a lot of research on this, such as Shrey et al. For the pre-set laser cladding process, a parameter integrated analysis model considering the energy transfer and loss mechanism and the surface tension of the molten material is proposed to predict the molten pool temperature, cladding geometry and substrate dilution; Yang Yicheng et al. used the “back image enhancement” transient image capture method and image information processing technology to study the changing characteristics of powder beam and particles under laser irradiation, extracted the number of particles in the highlight state, the total area of the bright area and the average area of the bright area of a single particle as characteristic parameters, combined with the characterization of the influence of process parameters on the laser coaxial powder feeding additive manufacturing process, and proposed that the light-powder interaction process can be regulated by reasonably matching the main process parameters; Zhu Ming et al. modeled and simulated the interaction behavior between the pre-set powder and the laser. In summary, the relevant research mainly focuses on the energy transfer during the light-powder action process and the spatial temperature field distribution of the light-powder action process, while there is less research on the dynamic process of the light-powder action, the evolution of the thermal physical state of the powder during the light-powder action, and the state of the powder particles entering the molten pool.
During the coaxial powder feeding laser cladding process, due to the presence of metal vapor and plasma on the surface of the molten pool, it is difficult for conventional thermal imaging methods to accurately reflect the temperature and state of the powder about to enter the molten pool. It is also very difficult to quantitatively analyze the mechanism of the laser heat source on the final stage of powder melting. In order to accurately study the influence of process parameters on the temperature and state of the powder at the final stage, a design The coaxial powder feeding laser cladding test platform, infrared thermal imaging acquisition system and high-speed camera acquisition system were developed. According to the different melting stages of the powder, a dynamic thermophysical analytical model that can describe the melting behavior of the powder was established. The duration of the characteristic stage of powder melting under different laser powers was simulated and calculated. The model was corrected and optimized according to high-speed camera. Finally, the temperature and state of the powder when it reached the molten pool under different laser powers were quantitatively obtained, which provided a theoretical basis for further studying the heat transfer behavior of the powder to the molten pool, the thermodynamic state of the molten pool, etc., and provided a theoretical basis for realizing the control of the powder melting behavior.
1 Test method
Test selection No. 45 carbon structural steel was used as the substrate with a size of 120 mm × 80 mm × 6 mm. High-hardness Ni60A alloy powder was selected as the powder material with a powder particle size of 80 ~ 160μm. The chemical composition is shown in Table 1. Before the test, the powder was placed in a 120 ℃ resistance furnace for drying for 1 h to remove the moisture in the powder. At the same time, the No. 45 steel was polished with sandpaper to remove the surface rust and oxide film, and then wiped with acetone alcohol to remove the surface oil.
The FL-Dlight-1500 laser heat source used in this paper is mainly composed of a direct output rectangular spot semiconductor laser. The minimum spot size is 1 mm × 3 mm, the wavelength is 976 nm ± 10nm, and the maximum output power is 1 500 W. The powder feeding equipment is an ECPF 2-2 LC plasma powder feeder, and is equipped with a high-precision coaxial annular powder feeding nozzle, using the DIAS manufactured by Aidis. The short-wave high-temperature infrared thermal imager was used to observe the thermal process of the light-powder thermal interaction. The measured temperature was 900 ~ 2 500 ℃, the error was 1%, and the measurement frequency was 60 Hz. The laser additive and remanufacturing process acquisition system used a VEO 410L high-speed camera, the shooting frame rate was 10 000 fps, the exposure time was 1 μs, the lens was a Nikon AF60 mm f/2.8D fixed-focus macro lens, and the auxiliary light source used a HSX-F300 xenon lamp to improve the contrast of the acquisition process. The test and acquisition system are shown in Figure 1.
2 Detection, modeling and simulation of powder melting behavior
2.1 Infrared thermal imaging acquisition and analysis of powder melting process
In order to explore the temperature distribution of powder in the light-powder thermal interaction space and the thermal physical state of the powder entering the molten pool, the powder feeding rate was 0.25r/min, the carrier gas flow rate was 7 L/min, the powder feeding height was 20 mm, the laser defocus was 0 mm, and the scanning speed was Under the experimental condition of 4 mm/s, the thermal interaction between laser and powder under different laser powers was collected using an infrared thermal imager, as shown in Figure 2, and the positions of the nozzle and substrate are shown in Figure 3.
As shown in Figure 2, as the laser power gradually increases, the temperature in the light-powder thermal interaction field gradually increases, the high-temperature area of the powder temperature gradually increases and gradually approaches the powder feeding nozzle, and the powder temperature gradually distributes evenly along the longitudinal axis. Through analysis, it can be seen that as the laser power gradually increases, the laser energy density in the light-powder thermal interaction field also gradually increases. The increase or decrease of laser power does not affect the movement trajectory of the powder. However, if the laser power is increased, after the light-powder action for the same time, the powder absorbs higher laser energy, and the powder temperature rises immediately. Therefore, the high-temperature area of the powder stretches longitudinally and gradually approaches the powder feeding nozzle. The proportion of powder melted before reaching the molten pool increases. When the laser power is 700 W or above, the powder temperature rises rapidly, and metal vapor appears near the molten pool and the area near the molten pool, and gradually increases with the laser power. Metal vapor has a great influence on the infrared thermal imaging measurement of powder temperature. The maximum range of infrared thermal imaging is 2 500 ℃, while the temperature of the area covered by metal vapor exceeds this range, and this area increases with the increase of laser power. Therefore, only when the powder under low laser power enters the molten pool, the temperature can be measured by infrared imaging.
2.2 Collection and analysis of thermal interaction behavior between coaxial powder feeding and laser
During the manufacturing process of semiconductor laser coaxial powder feeding, the powder will undergo a transition from “solid state → solid-liquid two-phase state → liquid state → volume expansion → gasification → plasma” after entering the laser field. The literature shows that the physical state of the powder obtained under the light-powder thermal interaction is different with different laser irradiation levels, and the difference in brightness between powders can directly reflect the difference in the degree of light-powder thermal interaction. Through “back image enhancement”, the solid powder is coaxially fed into the laser field, and the powder is irradiated by laser energy and heated up. When it reaches the melting point, it will continue to absorb heat. The latent heat of melting is released outward, and solid-liquid transformation occurs. At this time, the powder color can be observed to gradually change from black to bright white through high-speed photography. When all the powder changes from black to bright white, it indicates that the phase change is completed. If the melted powder continues to absorb heat, the volume will expand. When the temperature reaches the vaporization temperature, metal vapor will be generated around the melted powder, and even plasma will appear. The melting behavior of a single powder is very similar to the melting behavior of the overall powder beam. Although there are more powders during coaxial powder feeding laser cladding, the typical melting behavior of a single powder can be studied.
Using Figure 1 The coaxial powder feeding semiconductor laser cladding test system was built, and the typical powder cladding process was selected to analyze and collect the melting behavior of a single powder. When the laser power is set reasonably, the powder entering the laser will not be completely converted into metal vapor or plasma due to the laser action, and this light-powder action thermal process is also regular. Therefore, the melting process of the powder entering the laser action field can be simplified.
Since the probability of all powders being converted into gas or plasma is small and has little effect on the entire thermal process, the powder melting process is simplified to obtain three characteristic stages, as shown in Figure 4. Under the parameters of powder feeding rate 0.25 r/min, powder carrier gas flow rate 7 L/min, powder feeding height 20 mm, laser defocus amount 0 mm, and scanning speed of 4 mm/s, different laser powers are used. The solid powder melting process is collected by high-speed video and processed using Matlab software. The brightness and pixel area of the powder melting in the image are used as characteristic signals. Analysis shows that there are three typical stages from the moment the powder enters the laser action field to the moment it falls into the molten pool, namely 3 Characteristic stage 1: the beginning of the melting stage, the powder is ejected from the nozzle, the movement time is 0 ~ 9.8 ms, and after entering the laser irradiation area, it absorbs heat and begins to change from a black solid to a white liquid. The characteristics of the powder in this stage are grayscale values 0 ~ 160 and pixel values 0 ~ 2 pixels; Characteristic stage 2: the full highlight stage, the powder movement time is 9.9 ~ 12 ms, the powder continues to absorb heat under the laser thermal interaction, melts from top to bottom, and finally becomes a fully bright liquid particle. The characteristics of the powder in this stage are grayscale values 160 ~ 255 and pixel values 2 ~ 5 pixels; Characteristic stage 3: the molten drop enters the molten pool stage, the powder movement time is 12.1 ~ 18 ms, the liquid powder continues to absorb heat through the laser thermal interaction and the volume continues to increase. It is also possible that feathering occurs at this time. Finally, the powder enters the molten pool as a high-temperature liquid. The characteristics of the powder in this stage are grayscale values. 255, the pixel value is greater than 5 pixels. In summary, the powder undergoes a transformation from “solid state → solid-liquid two-phase state → liquid state” in the laser.
2.3 Analysis of the thermal physical process of the melting behavior of powder when it is fed into the laser
As shown in Figure 4, the melting form of powder in the laser is dynamic, and the process of absorbing energy is also dynamic. Therefore, the description of the melting behavior of powder after entering the laser should also be dynamic. However, most of the existing
thermal physical models use static and single heat absorption equations. Therefore, it is necessary to establish a dynamic thermal physical model according to different characteristic stages to describe the melting behavior of solid powder in the laser, and calculate and analyze the state and temperature of the powder entering the molten pool.
2.3.1 Laser heat source model
In order to analyze the influence of heat source characteristics on the melting behavior of powder, a laser heat source model is first established. Since the energy transmission form and carrier in the light-powder thermal interaction are relatively complex, the following assumptions need to be made for the light-powder thermal interaction process: ① The attenuation of laser energy is through powder absorption and scattering, ignoring the influence of plasma; ② The analysis of laser energy density acting on powder is in accordance with trapezoidal distribution; ③ The powder studied finally enters the molten pool after the interaction between light and powder; ④ The metal vapor acts on the powder in the form of heat conduction, but the effect is relatively small, so the influence of metal vapor on the powder temperature is ignored in the study. The rectangular semiconductor laser heat source is Gaussian distributed in the x direction and trapezoidal distributed in the y direction, as shown in Figure 5.
The semiconductor laser heat source is the trapezoidal distribution that best reflects the uniformity of energy distribution in the length direction. At the same time, the laser cladding direction is perpendicular to the length direction of the laser spot. Therefore, in order to simplify the laser heat source model, it is necessary to assume that the powder moves to the molten pool along the plane with the maximum energy perpendicular to the laser width direction. After being affected by laser heat, the thermal physical behavior is analyzed according to the trapezoidal distribution of laser energy density. The simplified laser energy distribution formula is shown in formula (1) in the figure, where: qlaser is the laser energy density at any position in the light-powder action space; P is the laser power; W is the laser spot width; L is the spot length; y is the distance along the length of the laser beam.
When laser cladding is performed in the form of coaxial powder feeding, the powder is affected by the powder-carrying airflow, and the movement form is relatively complex. For annular powder feeding, the powder in the light-powder thermal interaction space is affected by the airflow resistance and its own gravity, so the force form is more complex, and it is more difficult to analyze their force and movement forms. However, due to the high symmetry of the annular powder feeding cladding head and the powder beam, when the powder feeding parameters are constant, the powders with the same cross section have the same force and movement form. Therefore, this paper analyzes the movement model of the powder in the two-dimensional cross section in the center direction of the laser spot width, and the force
mode is shown in Figure 6.
The movement of a single powder from the powder feeding nozzle to the molten pool can be decomposed into horizontal and vertical directions. The movement time t1 and t2 in the two directions can be calculated by kinematics. The maximum movement time in the laser action field can be calculated as t = min[t1, t2], that is, see formula (2) in the figure, where: v0 is the speed of the powder at the powder feeding nozzle; az is the acceleration of the powder in the vertical direction; ay is the acceleration of the powder in the horizontal direction; θ is the powder incident angle.
The laser energy distribution (1) is coupled with the powder motion model (2), and the energy density qlaser at any time t in the laser beam is obtained as (3), which is shown in the figure. Where: t is the time required for the powder to move to any position in the laser.
2.3.2 Modeling of thermal physical processes in characteristic stage 1
In the early stage of entering the laser field, the powder is not melted by the laser interaction, but changes from a low-temperature solid state to a high-temperature solid state. The energy transfer balance equation at this time is shown in the figure. Formula (4) (5). In the formula: the powder is solid; Qp-solid is the heat absorbed by the powder; Qp-solidabs is the laser heat absorbed by the powder in the stage t1; Qp-solidcon is the heat lost by the powder by thermal convection; Qp-solidrad is the heat lost by the powder by thermal radiation; αsolid is the ratio of the powder absorbing the laser; hp-solid is the powder thermal convection heat transfer coefficient; Tp-solid(t) is the final real-time temperature of the powder in the first characteristic stage; ρp-solid is the density of the powder; Cp-solid is the specific heat capacity of the powder; is the radius of a single powder particle; T0 is the ambient temperature; is the emissivity of the powder to the laser; is the Boltzmann constant.
From formula (5), it can be seen that the duration t1 of the characteristic stage 1 increases with qlaser(t) That is, when the laser power P and the powder incident angle θ decrease, the laser defocus D and the powder incident initial velocity v0 increase, the duration t1 of characteristic stage 1 increases, and the growth rate of the powder real-time temperature Tp-solid(t) slows down.
2.3.3 Thermophysical process modeling of characteristic stage 2
The powder begins to undergo solid-liquid phase transition. While the powder absorbs heat, it releases latent heat of melting due to phase transition. At this stage, the thermal physical state of the laser energy carrier changes, and the energy transfer balance equation is shown in formula (6) (7) in the figure. Where: Qp-latent is the energy released when the powder undergoes phase transition.
Where: ∆Hf is the latent heat of melting, and Tm is the melting point of the powder. Since the temperature difference of the powder is small during the solid-liquid transition, this value is approximately equal to Tm, so formula (7) is simplified to formula (8) in the figure.
It can be concluded from equation (8) that the duration of characteristic stage 2 (t2−t1) increases as the laser power P and the incident angle θ of the powder decrease, and also decreases as the average particle size rp of the powder, the initial velocity v0 of the powder, and the laser defocusing amount D decrease.
2.3.4 Thermophysical process modeling of characteristic stage 3
The powder has completed the solid-liquid transition and is still subjected to continuous laser heat before falling into the molten pool. Due to the large difference in the thermophysical parameters of the solid and liquid phases of the powder, the thermophysical process of this stage needs to be adjusted according to the thermophysical parameters of the liquid state transmission. The heat transfer balance equation is shown in equation (9) in the figure. In the formula: the powders are all liquid; Qp-liquid is the heat absorbed by the powder; Qp-liquidabs is the heat of the laser acting on the powder; Qp-liquidcon is the heat lost by the powder due to thermal convection; Qp-liquidrad is the heat lost by the powder due to thermal radiation.
In the formula: the powders are all liquid; αliquid is the absorption rate of the laser; hp-liquid is the convection heat transfer coefficient; Tp-liquid(t) is the real-time temperature; ρp-liquid is the density; Cp-liquid is the specific heat capacity.
From formula (10), it can be seen that the duration of characteristic stage 2 (t3−t2) is related to the real-time temperature of liquid powder Tp-liquid(t), the initial velocity of powder movement v0, the laser power P, the laser positive defocus amount D, the powder incident angle θ and other parameters. If the powder temperature is raised higher in the same time, the laser positive defocus amount D, the initial velocity of powder movement v0 can be reduced. Reduce the laser power P, increase the powder incident angle θ, and at this time, the powder will last longer in the characteristic stage 3.
2.4 Simulation analysis of powder melting behavior
2.4.1 Effect of laser power on powder melting behavior
Although the surface of the molten pool is covered with metal vapor during the semiconductor laser coaxial powder feeding manufacturing process, making it difficult to measure the temperature of the powder when it enters the molten pool, the above model can be used to calculate the temperature of the powder when it enters the molten pool, and predict the physical state of the powder when it enters the molten pool. The model is used to calculate the duration of the powder in the three characteristic stages under different laser powers. Under the same laser power and other parameters, the actual duration of each characteristic stage is recorded by high-speed camera. The accuracy of the model is verified by comparing the duration before and after. On the basis of the obtained model, the temperature and state of the powder when it enters the molten pool are analyzed and judged.
Under the following simulation conditions: powder feeding rate 0.25 r/min, powder carrier gas flow rate 7 L/min, laser defocus 0 mm, powder feeding height 20 mm, powder incident angle 45 °, formula (3), formula (5), formula The real-time powder temperature T(t) corresponding to time t1, t2, and t3 in (8) and (10) is simulated by Matlab to obtain the influence of different laser powers qlaser on the duration of each characteristic stage t1, (t2−t1) and (t3−t2). The simulation parameters are shown in Table 2.
The simulation value has a certain deviation from the experimental value. The simulation value of characteristic stage 1 is always greater than the experimental value, and the simulation value of characteristic stage 3 is always less than the experimental value. The simulation results of the duration of characteristic stage 1 with low power and the duration of characteristic stage 3 with high power are quite different from the experimental results, because the duration of characteristic stage 1 with low power and characteristic stage 3 with high power are relatively long. Both processes are interfered by factors such as heat reflection of the molten pool and the high heat of metal vapor, which are ignored in the simulation modeling, as shown in Figure 7.
2.4.2 Effect of defocus on powder melting behavior
Under the parameters of powder feeding rate 0.25 r/min, powder carrier gas flow rate 7 L/min, scanning speed 4 mm/s, powder feeding height 20 mm, and laser power 1 100 W, the defocus was adjusted to study the effect of defocus on the temperature distribution in the light-powder interaction space during coaxial laser cladding, as shown in Figure 8.
As can be seen from Figure 8, the increase in defocus will increase the powder melting amount along the lateral direction of the laser spot, and the uniformity of the temperature distribution of the powder particles will also be enhanced along the lateral direction of the laser spot. The area of the high-temperature area of the powder temperature first increases and then decreases, and the powder particles in the high-temperature state are first close and then far away from the nozzle. The reason is that the defocus increases from 0 mm, which means that the maximum energy distribution cross-section of the laser lateral direction gradually moves from the substrate to the nozzle. When the defocus is + 10 mm, the average energy density that the powder particles can absorb in the light-powder interaction is the largest; when the defocus exceeds + 10 mm After that, the distance between the laser focus and the substrate is too large, and the laser beam is in a divergent state, so that the average energy density that the powder particles can absorb in the light-powder interaction decreases with the increase of the focal length. Therefore, it is observed that the powder particles melt more along the lateral direction of the laser spot, and the powder melting inhomogeneity decreases along this direction, and the area of the high-temperature area of the powder temperature increases and then decreases.
Under the parameters of powder feeding amount 0.25 r/min, scanning speed 4 mm/s, powder feeding height 20 mm, laser power 1 100 W, and defocus amount 0 mm, by adjusting the powder carrier gas flow rate, the effect of the powder carrier gas flow rate on the temperature distribution in the light-powder interaction space during coaxial laser cladding is studied. When the powder carrier gas flow rate is 5, 7, 9, and 11 L/min, as shown in Figure 9.
From Figure 9 It can be seen that as the carrier gas flow rate gradually increases, the number of powder particles melted decreases slightly, the area of the high temperature area of the powder particles decreases relatively, and the uniformity of the temperature distribution of the powder particles weakens. The reason is that the carrier gas flow rate affects the movement speed and spatial distribution of the powder particles in the light-powder action space. As the carrier gas flow rate increases, the movement speed of the powder particles in the light-powder action space increases, and the action time in the light-powder action space decreases, so that the temperature of the powder particles at the same position gradually decreases, and the temperature will also decrease accordingly when falling into the molten pool. The metal vapor generated during the entire cladding process will also decrease accordingly, and the concentration of the powder particles in the light-powder action space is also decreasing. Compared with the influence of laser power and defocus on the temperature distribution of the light-powder action space, the change of the carrier gas flow rate has less influence on the temperature distribution of the light-powder action space.
2.4.3 Temperature simulation of powder melting stage at different laser powers
When the powder particle size is 120 μm, the carrier gas flow rate is 7 L/min, the powder incident angle is 45°, the powder initial velocity is 0.8 mm/ms, and the laser defocus is 0 mm, and the powder feeding height is 20 mm. Based on the modification of the thermal physics model, the Matlab tool is used to simulate the powder temperature corresponding to different laser powers, and the variation trend of the powder temperature over time under different laser powers is obtained, as shown in Figure 10.
As shown in Figure 10, the increment of the temperature rise of the powder at the initial stage of laser action is significantly greater than that in the middle and final stages of melting, which explains the phenomenon that the solid powder absorbs more laser energy and loses less energy than the liquid powder. Secondly, except when the laser power is 100 W, the temperature curve has two inflection points, namely 1 060 ℃ and 1 260 ℃. It can be seen that the fastest growth rate is the heating curve below 1 060 ℃, and the slowest growth rate is the heating curve above 1 260 ℃. The temperature growth rate in the heating range of 1 060 ~1 260 ℃ is between the above two. The reason is that the powder begins to melt at 1 060 ℃ and melts at 1 260 ℃. ℃, and because the absorption rate and loss rate of the powder to the laser in the solid and liquid states are different, the slope of the heating curve is different, that is, the temperature growth rate is different. In addition, it can be seen from the heating curve that the temperature growth rate gradually increases when the powder enters the laser to interact with the powder at the beginning; at the end of melting, the temperature growth rate gradually decreases. The reason is that they move at the edge of the trapezoidal laser heat source and the energy density changes.
3 Conclusion
(1) The dynamic melting behavior of the powder in the laser is collected by high-speed video. It is found that there are three typical characteristic stages of powder melting: “solid → solid-liquid two-phase state → liquid state”. The duration of different characteristic stages and the characteristic equation of melting are different. Based on this, a mathematical analytical model that can describe the dynamic behavior of powder melting is established.
(2) The effects of laser power, defocusing amount, and powder-carrying gas flow rate on the temperature distribution of the powder in the light-powder action space are analyzed. At the same time, the model is used to analyze the effect of different laser powers on the duration of each characteristic stage. The temperature distribution of the powder particles arriving at the substrate is simulated and predicted. It is found that as the laser power increases from 100 W to 200 W, the powder melting temperature increases. When the laser power increases to 1500 W, the temperature of the powder entering the molten pool changes nonlinearly. From the simulation results, it can be found that when the laser power increases from 100 W to 1500 W, the temperature of the powder entering the molten pool increases from 750 ℃ to 3250 ℃.
