Liquid metal emissivity calibration method based on molten pool temperature gradient characteristics

Abstract

The application discloses a liquid metal emissivity calibration method based on molten pool temperature gradient characteristics, comprising the following steps: S1, collecting original temperature data by an infrared thermal imager, and monitoring temperature signals of a laser radiation area by the near-infrared thermal imager; S2, extracting one-dimensional temperature data along a scanning direction; S3, identifying a liquid-solid phase transition starting point; S4, obtaining molten pool boundary radiation temperature values; and S5, calibrating molten pool boundary emissivity according to known material melting points, and calculating molten pool emissivity according to a relationship formula of emissivity changing with temperature derived according to the Planck law. The application builds a molten pool temperature field monitoring system of a laser melting deposition process based on an infrared thermal imager, determines the starting point of liquid-solid phase transition, calculates the molten pool emissivity according to the relationship formula of emissivity changing with temperature derived according to the Planck law, and verifies the rationality of the calibration according to a molten pool center point cooling curve.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal emissivity measurement, in particular to a liquid-phase metal emissivity calibration method based on molten pool temperature gradient characteristics. BACKGROUND

[0002] Radiance (also known as emissivity) is the ability of an object to radiate energy according to its own temperature, a parameter describing the radiation capacity of the measured object, also refers to the energy radiated by the object itself and the energy radiated by the absolute black body at the same temperature, represented by the symbol epsilon, the radiance is only related to the properties of the object surface (composition, structure), under the condition of given temperature, the radiance of any object is equal to the absorption rate of the object in numerical value, epsilon = 1-p, the radiance is sometimes called the radiation coefficient. When the radiation energy is projected onto the object surface, reflection, absorption and transmission occur. After the object absorbs the radiation energy, the temperature rises and a part of the energy is radiated.

[0003] The radiance of all objects is in the range of greater than zero and less than 1, the value is related to the material, shape, surface roughness, concave and convex degree, oxidation degree, color, thickness, etc. of the object, in general, the size of the radiation energy received by the infrared temperature measuring device from the object is proportional to the radiance epsilon of the object, the difference between the actual measured object and the black body is in the radiance epsilon, the transmittance delta and the reflectivity p. The radiance epsilon of the ideal black body is l, the transmittance delta is 0, and the reflectivity p is 0. The radiance epsilon of the actual measured object is <1, the transmittance delta is >0, and the reflectivity p is >0. The epsilon, delta and p of different substances are not the same and change with the surface condition of the object. They have different values at different temperatures and different wavelengths. In order to obtain the relationship between temperature and emissivity, we propose a liquid-phase metal emissivity calibration method based on molten pool temperature gradient characteristics. SUMMARY

[0004] In view of the above problems, the present application is a liquid-phase metal emissivity calibration method based on molten pool temperature gradient characteristics, which can calibrate the liquid-phase metal emissivity.

[0005] The technical scheme of the present application is: a liquid-phase metal emissivity calibration method based on molten pool temperature gradient characteristics, comprising the following steps:

[0006] S1: an infrared thermal imager collects original temperature data, a near-infrared thermal imager is used, the range is 800-3000℃, and the temperature signal of the laser radiation area is monitored;

[0007] S2: one-dimensional temperature data along the scanning direction is extracted, a frame of molten pool temperature image is extracted at the center position of each scanning layer, and temperature data of a plurality of points along a straight line passing through the center of the molten pool is extracted thereon;

[0008] S3: The liquid-solid phase change starting point is identified, the two-dimensional temperature gradient value when the temperature drops is obtained according to the radiation temperature of the deposited layer, and the numerical value change of the liquid-solid phase change starting point is obtained;

[0009] S4: The radiation temperature value of the molten pool boundary is obtained, and the radiation temperature of the molten pool boundary is extracted according to the maximum point of the second-order derivative of the temperature in the scanning direction;

[0010] S5: The emissivity of the molten pool boundary is calibrated according to the melting point of the known material, and the emissivity of the molten pool is calculated according to the relationship between the emissivity and the temperature derived from the Planck's law.

[0011] In a further technical solution, the infrared thermal imager needs to have a frame rate of 60Hz, a spatial resolution of 640x480, and an adjustable range of emissivity and transmissivity of 0~1.00 when collecting.

[0012] In a further technical solution, the infrared thermal imager needs to be installed with a wave filter with high cutoff depth for 1030nm wavelength when collecting, and the angle between the shooting angle and the vertical direction is fixed at 52°.

[0013] In a further technical solution, the calculation formula of the two-dimensional temperature derivative in S4 is:

[0014] (1); wherein G(i,j) is the temperature gradient value of the pixel point to be solved, T(i,j+1), T(i,j-1), T(i+1,j), and T(i-1,j) are the radiation temperature values of the four adjacent pixel points, compared with the one-dimensional temperature gradient, the two-dimensional temperature gradient can better present the temperature gradient change at the liquid phase boundary, and the second-order derivative of the temperature drop section can be expressed as: (2).

[0015] In a further technical solution, the radiation temperature of the molten pool boundary in S4 is the average value of the 20 groups of boundary radiation temperature values monitored by the infrared thermal imager in S1.

[0016] In a further technical solution, the formula (3) of the energy radiated by a perfect black body at a fixed wavelength in the Planck's law in S5 is:

[0017] (3).

[0018] The beneficial effects of the present application are:

[0019] By building a laser melting deposition process molten pool temperature field monitoring system based on an infrared thermal imager, the maximum point of the second-order temperature derivative along the scanning direction was identified, the starting point of the liquid-solid phase transition was determined, and the radiation temperature of the molten pool boundary was successfully extracted. The molten pool emissivity was calculated based on the relationship between emissivity and temperature derived from Planck's law, and the rationality of the calibration was verified based on the cooling curve of the molten pool center point. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the process of the present invention;

[0021] Figure 2 This is a schematic diagram of temperature extraction according to the present invention;

[0022] Figure 3 Schematic diagram of temperature distribution of odd-numbered layers along the scanning direction of the present invention;

[0023] Figure 4 Schematic diagram of temperature distribution of even-numbered layers along the scanning direction of the present invention;

[0024] Figure 5 Schematic diagram of an algorithm example for detecting liquid phase boundary points according to the present invention;

[0025] Figure 6 Schematic diagram of the radiation temperature value at the liquid phase boundary of the molten pool of the present invention;

[0026] Figure 7 This is a schematic diagram of an infrared image of the molten pool after temperature correction according to the present invention;

[0027] Figure 8 This is a schematic diagram of the cooling curve of the center point at the end of the deposition layer of the present invention. DETAILED DESCRIPTION

[0028] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0029] Example:

[0030] like Figure 1 - Figure 8As shown, a liquid metal emissivity calibration method based on the temperature gradient characteristics of the molten pool includes the following steps: S1: infrared thermal imager acquires original temperature data, using a near-infrared thermal imager, the range is 800-3000℃, monitoring the temperature signal of the laser radiation area; S2: extract one-dimensional temperature data along the scanning direction, extract a frame of molten pool temperature image at the center position of each scanning layer, and extract the temperature data of several points along the straight line passing through the center of the molten pool; S3: identify the liquid-solid phase transition point, according to the deposition layer radiation temperature, obtain the two-dimensional temperature gradient value when the temperature drops, and obtain the numerical change of the liquid-solid phase transition point; S4: obtain the molten pool boundary radiation temperature value, according to the maximum value point of the temperature second derivative in the scanning direction, extract the molten pool boundary radiation temperature; S5: calibrate the molten pool boundary emissivity according to the known material melting point, and calculate the molten pool emissivity according to the relationship between the emissivity and the temperature derived from Planck's law.

[0031] The processing mode of "point-by-point melting-discrete accumulation" makes the metal powder melt, solidify and cool quickly, so there is a large temperature gradient in the center and edge of the molten pool. In this process, due to the different cooling times of powder materials in different regions, with the accumulation of heat, the molten pool center to the end of the laser radiation area will appear three different states: (a) liquid phase zone, the central high temperature and high pressure flowing part, the temperature drops rapidly; (b) liquid-solid phase transition zone, the material in this area changes from liquid to solid state, the temperature drops slowly; (c) solid phase zone, the area has been solidified, the temperature further drops. It is known that in the stable state, the liquid-solid phase transition temperature value of 30CrNi2MoVA is close to the solid-liquid phase transition value of 1550°C (material melting point), that is, the molten pool liquid phase zone boundary temperature value will be stable near the material melting point. Theoretically analyze the temperature gradient characteristics of the laser radiation area. Since the molten pool profile boundary is determined by the melting temperature of the powder material, the emissivity along the boundary is uniform. Combined with the temperature distribution characteristics of the infrared image, by finding the transition rule of the material from liquid to solid in the laser radiation area at different layer heights, the accurate radiation temperature value at the boundary of the molten pool liquid phase zone is obtained, and the molten pool emissivity calibration is completed.

[0032] The infrared thermal imager used needs to have a frame rate of 60Hz when collecting, a spatial resolution of 640x480, an emissivity and transmissivity adjustable range of 0~1.00, and a wave filter with high cutoff depth for 1030nm wavelength when collecting. The shooting angle and the angle with the vertical direction are fixed at 52°, so as to eliminate the influence of the laser light source on temperature monitoring. At the same time, through the 52° angle, the temperature field distribution of the whole substrate surface heat affected zone can be monitored. When collecting radiation temperature data, the emissivity and transmissivity of the thermal imager are both set to 1, that is, the collected temperature data is equivalent to the radiation temperature of a perfect black body.

[0033] A frame of temperature image of the molten pool is extracted at the center of each scanning layer, and the temperature data of several points along the straight line passing through the center of the molten pool and in the scanning direction are extracted, as shown in Figure 2 .

[0034] wherein the radiation temperature of the 11-20 deposited layers is as shown in Figure 3 , Figure 4 The front end of the molten pool is aligned in the figure, and as the number of layers increases, the heat accumulation phenomenon becomes more and more obvious, resulting in a significant increase in the length of the liquid-solid phase transition zone, which can be observed in both odd and even layers.

[0035] The coincidence (convergence) of the curves can be obviously observed at the boundary between the liquid phase zone and the liquid-solid phase transition zone at the end of the molten pool, followed by a temperature drop and a gentle trend, indicating that this is the boundary of the liquid phase zone and also the starting point of the liquid-solid phase transition zone, and the corresponding radiation temperature value is the boundary temperature value of the molten pool. Therefore, an algorithm is proposed to identify this boundary point, and a quadratic root is used to calculate the two-dimensional temperature gradient (first derivative) value of the temperature drop section, and the calculation formula is as follows:

[0036] ; wherein G(i, j) is the temperature gradient value of the pixel point to be solved, T(i, j+1), T(i, j-1), T(i+1, j), and T(i-1, j) are the radiation temperature values of the four adjacent pixel points. Compared with the one-dimensional temperature gradient, the two-dimensional temperature gradient can better present the temperature gradient change at the liquid phase boundary in terms of numerical value, and the second derivative of the temperature drop section can be expressed as: .

[0037] As shown in Figure 5 , the temperature gradient decreases to the right of the maximum value point of the second derivative, the temperature drops slowly, and is nearly constant, because it enters the liquid-solid phase transition zone, so the maximum value point of the second derivative is regarded as the boundary point of the liquid phase zone. Since the length of the liquid-solid phase transition zone increases significantly with the increase of the number of accumulated layers, in order to obtain more accurate radiation temperature values at the boundary of the liquid phase zone, the temperature data of the 11-20 layers where the liquid-solid phase transition zone is relatively obvious are extracted, and two frames of images are randomly taken at each layer, a total of 20 groups of boundary radiation temperature values are measured, as shown in Figure 6 , in order to reduce the calibration error, the average value 1335.3℃ is taken as the melting point radiation temperature of 30CrNi2MoVA in this experiment. The formula for the energy radiated by a perfect black body at a fixed wavelength in the Planck radiation law is: , wherein W is the black body radiation intensity, λ is the given wavelength, C1 and C2 represent the first and second Planck constants respectively, and T is the absolute temperature of the black body. The actual radiation intensity of the object is the product of the radiation intensity of the black body at the same temperature and the surface emissivity value ε: ,

[0038] Emissivity ε varies with the surface conditions, temperature, and wavelength of an object. The accuracy of emissivity measurement is directly related to the temperature measurement accuracy. Since infrared thermal imagers collect the radiation energy of an object in a given band, it is necessary to measure its average emissivity within that band to reduce calibration errors. The Stefan Bolzmann equation derived from the generalized integral of Planck radiation formula (3) is:

[0039] , where σ is the Stefan constant, which is 5.67×10-8W / (m2·K4). The definite integral form of equation (5) represents the energy radiated by the object in the specified band [λ1, λ2]. The analytical expression of the function of the average emissivity of the molten pool boundary as a function of temperature is obtained as follows: (6), where λ is the wavelength, C1 and C2 are Planck's first and second constants respectively, T1 is the radiation temperature measured by the infrared thermal imager, and T is the corresponding true temperature. Since the definite integral of formula (5) is difficult to calculate, the definite integral is approximately calculated using the MATLAB tool according to the principle of numerical integration. It is known that the temperature measurement band of this infrared thermal imager is 780nm~1080nm. When T=1550℃ and T1=1335.3℃, the emissivity ε of the molten pool boundary is obtained to be 0.29. The obtained emissivity is used to correct the original temperature data obtained in the early stage according to formula (6). Figure 7 The image of the melt pool after temperature correction is shown in Figure 2. To verify the accuracy of the emissivity calibration, the temperature data after the second set of experimental corrections are extracted. The center point of the melt pool at the end of the 13th layer deposition is taken and the temperature history of this point is extracted. The temperature change during the cooling stage is shown in Figure 2. Figure 8 As shown in the figure, it can be observed that after passing the 1550℃ dividing line, the temperature drops slowly, which is a sign that the liquid metal enters the liquid-solid phase transition state when solidifying, which proves that the calibration method is reasonable.

[0040] The working principle of this embodiment is as follows: by building a laser melting deposition process molten pool temperature field monitoring system based on an infrared thermal imager, the maximum point of the second-order temperature derivative along the scanning direction is identified, the starting point of the liquid-solid phase transition is determined, and the radiation temperature of the molten pool boundary is successfully extracted. The molten pool emissivity is calculated based on the relationship between emissivity and temperature derived from Planck's law, and the rationality of the calibration is verified based on the cooling curve of the center point of the molten pool.

[0041] The above embodiments merely represent specific implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. A method for calibrating the emissivity of liquid metal based on the temperature gradient characteristics of the molten pool, characterized in that: The following steps are involved: S1: Infrared thermal imager collects raw temperature data. A near-infrared thermal imager with a range of 800°C-3000°C is used to monitor the temperature signal in the laser radiation area. S2: Extract one-dimensional temperature data along the scanning direction, extract a frame of molten pool temperature image at the center of each scanning layer, and extract temperature data of several points along the straight line passing through the center of the molten pool along the scanning direction; S3: Identify the starting point of the liquid-solid phase transition, obtain the two-dimensional temperature gradient value when the temperature drops according to the radiation temperature of the sediment layer, and obtain the numerical change of the starting point of the liquid-solid phase transition; S4: Get the radiation temperature value of the molten pool boundary, according to the maximum value point of the second-order derivative of the temperature in the scanning direction, Extract the radiation temperature of the molten pool boundary and calculate the two-dimensional temperature derivative as follows: Where G(i, j) is the temperature gradient value of the pixel to be determined, T(i, j+1), T(i, j-1), T(i+1, j), T(i-1, j) is the radiation temperature value of the four adjacent pixels. Compared with the one-dimensional temperature gradient, the two-dimensional temperature gradient can better show the temperature gradient change at the liquid phase boundary. The second-order derivative of the temperature drop segment is expressed as: S5: The emissivity of the molten pool boundary is calibrated according to the known melting point of the material, and the emissivity of the molten pool is calculated according to the relationship between emissivity and temperature derived from Planck's law.

2. The method for calibrating the emissivity of liquid metal based on the temperature gradient characteristics of the molten pool according to claim 1, characterized in that: The infrared thermal imager needs to have a frame rate of 60 Hz during acquisition, a spatial resolution of 640×480, and an adjustable range of emissivity and transmittance of 0 to 1.

00.

3. The method for calibrating the emissivity of liquid metal based on the temperature gradient characteristics of the molten pool according to claim 2, characterized in that: The infrared thermal imager needs to be installed with a notch filter with a high cut-off depth for the 1030nm wavelength during acquisition, and the angle between the shooting angle and the vertical direction is fixed at 52°.

4. The method for calibrating the emissivity of liquid metal based on the temperature gradient characteristics of the molten pool according to claim 1, characterized in that: The molten pool boundary radiation temperature in S4 is the average value of 20 groups of boundary radiation temperature values ​​monitored by the infrared thermal imager in S1.

5. The method for calibrating the emissivity of liquid metal based on the temperature gradient characteristics of the molten pool according to claim 1, characterized in that: The analytical expression of the function of the average emissivity of the molten pool boundary changing with temperature is: Where λ is the wavelength, [λ1, λ2] represents the specified band, C1 and C2 are Planck's first and second constants, respectively, T1 is the radiation temperature measured by the infrared thermal imager, T is the corresponding true temperature, and ε is the emissivity of the melt pool boundary.

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