CN112212977B - High-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring device and method - Google Patents
High-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring device and method Download PDFInfo
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Abstract
The invention discloses a high-speed, high-resolution and high-precision online monitoring device and method for a temperature field of an ultrahigh-temperature molten pool. The invention can realize the detection of the temperature of the molten pool by only adopting one high-speed camera, has simple structure and is easy to realize. According to the invention, firstly, the long-wave-pass dichroic mirror is adopted to only reflect the infrared light signal radiated by the molten pool into the light splitting system, so that the influence of laser on imaging is avoided; then, in the light splitting system, the dichroic mirror and the band-pass filter are utilized to select 2 infrared lights with different wave bands for imaging and temperature calculation, so that the influence of plasma in a printing chamber on the imaging of a high-speed camera is avoided, the light intensity loss caused by light splitting in the same wave band is avoided, and the measurement precision is improved. The light splitting system is simple and compact in structure and small in size, the split light beam I and the split light beam II can enter the same high-speed camera in a parallel light mode to achieve coaxial measurement, and two images are focused and imaged simultaneously by adjusting the optical path difference.
Description
Technical Field
The invention relates to the technical field of online monitoring of a molten pool temperature field, in particular to a high-speed, high-resolution and high-precision online monitoring device and method for an ultrahigh-temperature molten pool temperature field, which can be suitable for online monitoring of the molten pool temperature field in the selective melting processing process of additive manufacturing laser.
Background
The metal 3D printing technology is the leading and most potential technology in the 3D printing system, and is one of the major development directions in the advanced manufacturing technology. The metal 3D printing technology is mainly classified into three categories according to the manner of adding metal powder: selective laser melting, near-net laser forming, and selective electron beam melting. The selective laser melting technology is suitable for manufacturing parts which cannot be manufactured by the traditional process and have special-shaped complex structures inside, and meanwhile, as the forming precision of the selective laser melting technology is higher, more non-processing surfaces can be reserved in the application of common parts, the processing problem of difficult-to-cut materials can be better solved.
The selective laser melting technology is an advanced laser additive manufacturing technology developed by taking a prototype manufacturing technology as a basic principle. The principle is that firstly, a three-dimensional digital model of a part is sliced and layered through special software, contour data of each section is obtained, then metal powder is selectively melted layer by using a high-energy laser beam according to the contour data, and a three-dimensional solid part is manufactured in a mode of spreading powder layer by layer and melting, solidifying and accumulating layer by layer.
In the selective laser melting process, parameters such as laser power and scanning speed are unchanged in the process of processing, but when each layer is processed, the thermal stress in a formed part is gradually accumulated along with the increase of the number of formed layers, and when the residual stress reaches the yield strength of the material, plastic deformation is generated to generate cracks; furthermore, if the cooling rate is too high, the gas has no time to escape from the molten bath, and pores may form; the porosity and cracks in turn influence the temperature field distribution of the bath. Therefore, if the temperature of the molten pool can be monitored in real time in the processing process, the defects can be found and processed in time, and the quality of the formed product can be greatly improved. Meanwhile, the temperature field information of the molten pool has an important effect on analyzing the molding quality, residual stress, strength and the like of the product, and the accurate measurement of the temperature field of the molten pool is beneficial to improving the manufacturing precision of the additive manufacturing technology. However, in the selective laser melting process, the range of the molten pool is extremely small (one molten pool is 200-300 mu m), the laser scanning speed and the change speed of the molten pool temperature are very fast (the laser scanning speed is between 4m/s and 10m/s, the heating and solidification speed of powder is changed by hundreds of degrees in one second at the fastest speed), and the molten pool temperature is extremely high (the highest temperature of the molten pool can reach 3000 ℃); if the temperature field is measured by using the existing common thermal infrared imager, the measurement precision is limited by the resolution and the measurement frame rate of the thermal infrared imager, and the requirement of high-speed temperature measurement and high resolution is difficult to achieve (the frame rate of the thermal infrared imager is often only dozens, because the molten pool is very small, if the thermal infrared imager is used, the whole molten pool can only see one point on the thermal infrared imager, and for laser scanning, under the condition of taking 10000 images in one second, the resolution requirement is about 500 x 400); moreover, the integration development trend of the additive manufacturing equipment urgently needs an on-line monitoring device of the molten pool temperature field, which is easy to integrate.
In the chinese patent application with publication No. CN109014202A entitled "a device and method for monitoring molten pool temperature in real time during selective laser melting process", a half-reflecting and half-transmitting mirror is added between a vibrating mirror and a laser, the range of wavelength required for temperature measurement is increased, the light after increased reflection is split by a beam splitter, and is processed by two high-speed cameras respectively. However, the method uses two high-speed cameras, the price is high, and the error of the temperature measurement result can be caused by inaccurate synchronous triggering time of the two high-speed cameras or inaccurate pattern matching. In chinese patent application CN108871585A entitled "temperature field measurement system and method based on single camera", a light beam of an incident window is divided into two identical beams by a flat beam splitter, and then reflected by a mirror to a target surface of the same camera. According to the method, the picture with two target images can be shot by only one camera, so that the defect that the two cameras are inconvenient to synchronously trigger in the prior art is overcome, the measurement precision is improved, and the cost is reduced. In the scheme, light rays are reflected for multiple times in a light splitting light path, the included angle between emergent light and incident light is 135 degrees, the light splitting angle is too large, two beams of light are difficult to enter a high-speed camera at the same time, and the high-speed camera is inconvenient to place; and the device can not be directly applied to coaxial temperature measurement, two images shot by a camera can not be focused simultaneously, and the images can be blurred. In addition, the flat-plate spectroscope used in the method divides the light into a first light beam and a second light beam which are the same, so that the light intensity radiated by a molten pool is lost, and the brightness of the picture acquired by the camera is weak.
In the chinese patent application with publication No. CN109759591A entitled "method and system for controlling temperature of molten pool spectrum of selective laser melting 3D printer", the thermal condition of the molten pool is indirectly represented by monitoring the radiant light intensity of the molten pool through a photodiode, the radiant light is sequentially converted into a voltage signal and a digital signal, and then the digital signal is processed to obtain temperature data. The method indirectly represents whether the temperature of the molten pool is too high or too low according to the intensity of the radiation light of the molten pool, but the method is a single-point temperature measurement method, only can measure the average temperature of the molten pool, cannot measure the specific temperature value distribution of the molten pool, and only knows the average temperature of the molten pool and cannot be used for monitoring the defects of the molten pool.
In summary, it is necessary to accurately measure the temperature field of the molten pool on line during the selective laser melting process. The development of an online monitoring device and technology of a molten pool temperature field which can be applied to selective laser melting equipment, has high measurement precision, high speed and high resolution, is economical and practical, has a small volume and can be directly installed on printing equipment, and is the problem to be solved in the field of additive manufacturing at present.
Disclosure of Invention
In view of this, the invention provides a high-speed, high-resolution and high-precision online monitoring device for a temperature field of an ultra-high temperature molten pool, which can realize the detection of the temperature of the molten pool by only using one high-speed camera, can realize coaxial temperature measurement, and has a simple structure and easy realization.
The invention discloses a high-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring device, which comprises a laser, a scanning galvanometer, a light splitting system, a high-speed camera and a computer, and is characterized by further comprising a long-wave-pass dichroic mirror;
the long-wave-pass dichroic mirror is positioned between the laser and the scanning galvanometer; the plane plated with the anti-reflection film on the long-wave-pass dichroic mirror faces the laser and is used for transmitting laser of the laser, and the plane plated with the dichroic film faces the scanning galvanometer and is used for reflecting infrared light signals radiated by the molten pool; the laser emitted by the laser is transmitted by the long-wavelength-pass dichroic mirror, and is irradiated on a molten pool of the printing chamber by the scanning galvanometer; infrared light signals radiated by the molten pool are reflected by the scanning galvanometer and the long-wavelength-pass dichroic mirror to enter the light splitting system;
the light splitting system comprises a first dichroic mirror, a second elliptical reflector, a third elliptical reflector, a first band-pass filter and a second band-pass filter; infrared light signals radiated by the molten pool are reflected by the long-wavelength-pass dichroic mirror, enter the light splitting system and are split into first band light and second band light by the first dichroic mirror; the first waveband light is irradiated on the second band-pass filter plate through the third elliptical reflector, filtered and reflected by the second dichroic mirror to enter the high-speed camera; the second wave band light irradiates the first band-pass filter plate through the second elliptical reflector, is filtered, and then is transmitted through the second dichroic mirror to enter the high-speed camera; the first dichroic mirror and the second dichroic mirror are same in initial wavelength and opposite in function, and are symmetrically arranged; the filtered first wave band light and the second wave band light parallelly enter the high-speed camera with a certain optical path difference, and images of the two wave bands of light in the high-speed camera are not overlapped and are focused simultaneously;
the high-speed camera images the incident light of the two wave bands; and the computer carries out image processing on the imaging result of the high-speed camera to obtain the distribution of the temperature field of the molten pool.
Preferably, the optical path adjusting device further comprises an adjustable optical path relay system, wherein the adjustable optical path relay system is positioned between the long-wave-pass dichroic mirror and the light splitting system; the adjustable optical path relay system is used for adjusting the optical path of the reflected light of the long-wave-pass dichroic mirror and eliminating the chromatic aberration of the reflected light; the reflected light of the long-wavelength-pass dichroic mirror enters the light splitting system through the adjustable optical path relay system.
Preferably, the long-wave pass dichroic mirror is mounted in a cage cube; the adjustable optical path relay system includes: the device comprises a first near-infrared achromatic double cemented lens pair, a second near-infrared achromatic double cemented lens pair, a connecting lens cone, a first adjustable lens cone, a second adjustable lens cone, a first right-angle optical adjusting frame, a first elliptical reflector and an overall achromatic double cemented lens; the first elliptical reflector is arranged on the first right-angle optical adjusting frame; the first near-infrared achromatic double cemented lens pair is connected with the cage cube through the connecting lens cone; the second near-infrared achromatic double cemented lens pair is connected with the light emitting side of the first near-infrared achromatic double cemented lens pair through a first adjustable lens barrel; the first right-angle optical adjusting frame is connected with the light-emitting side of the second near-infrared achromatic double-cemented lens pair through a connecting lens cone; and the integral achromatic double-cemented lens is connected with the light-emitting side of the first right-angle optical adjusting frame through a second adjustable lens barrel.
Preferably, the surfaces of the first near-infrared achromatic double-cemented lens pair, the second near-infrared achromatic double-cemented lens pair and the overall achromatic double-cemented lens are coated with antireflection films of 650-1050 nm.
Preferably, the initial wavelength range of the long-wavelength-pass dichroic mirror is 950-1000 nm; the initial wavelength range of the first dichroic mirror and the second dichroic mirror is 805-880 nm; the surfaces of the long-wave-pass dichroic mirror, the first dichroic mirror and the second dichroic mirror are plated with dichroic films and antireflection films; the center wavelengths of the first band-pass filter and the second band-pass filter are 780nm and 905nm respectively.
The invention also provides a high-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring method, which adopts the high-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring device to monitor the temperature of the molten pool;
the computer calculates the temperature field temperature T of the molten pool by adopting the following colorimetric temperature measurement formula:
wherein, c2Representing a second spokeA radiation constant; lambda [ alpha ]1、λ2Respectively representing the central wavelengths of the first band-pass filter and the second band-pass filter; n is a radical of1And N2Respectively representing gray values of pixel units corresponding to the first wave band light and the second wave band light imaging areas on the high-speed camera; k is a proportionality coefficient and is obtained by black body furnace calibration according to the following formula:
preferably, before calculating the temperature T of the temperature field of the molten pool, the computer firstly performs distortion calibration on the imaging result of the high-speed camera to obtain an image after the distortion calibration; calculating the temperature field temperature T of the molten pool according to the image after distortion calibration; the distortion calibration method comprises the following steps:
xa＝rx1[xb-k1xb(xb 2+yb 2)] (3)
ya＝ry1[yb-k1yb(xb 2+yb 2)] (4)
xa＝rx2[xc-k2xc(xc 2+yc 2)] (5)
ya＝ry2[yc-k2yc(xc 2+yc 2)] (6)
Wherein (x)a、ya)、(xb、yb)、(xc、yc) Respectively as reference picture I and distortion picture JLAnd distortion graph JRCoordinates of (3); k is a radical of1、k2Respectively, distortion maps JLDistortion graph JRFirst order radial distortion coefficient of; r isx1And ry1Is a reference image I and a distortion image JLMagnification in x and y directions, rx2And ry2Is a reference image I and a distortion image JRMagnification in the x and y directions;
respectively calculating the undistorted graph fL' and reference drawing I, undistorted drawing fR' correlation between different distortion coefficients in reference picture I, distortion coefficient k corresponding to minimum correlation1、k2The optimal distortion coefficient of the corresponding image is obtained;
and 4, carrying out distortion correction on the picture shot by the high-speed camera by adopting the optimal distortion coefficient obtained in the step 3.
Preferably, in the step 3, the optimal distortion coefficient is obtained by using a genetic algorithm or a particle swarm algorithm.
Has the advantages that:
(1) according to the invention, firstly, the long-wave-pass dichroic mirror is adopted to only reflect the infrared light signal radiated by the molten pool into the light splitting system, so that the influence of laser on imaging is avoided; then in the beam splitting system, utilize dichroic mirror and band-pass filter, select the infrared light of 2 different wave bands and carry out formation of image and temperature calculation, avoided the influence of plasma to high-speed camera formation of image in the printing chamber on the one hand, improved the precision of temperature calculation, on the other hand utilizes dichroic mirror to part the infrared light of 2 different wave bands of molten bath radiation completely, avoids two wave bands all to be the loss of the light intensity that same wave band light beam caused, does benefit to camera formation of image, has improved the precision of temperature calculation. The light splitting system has simple and compact structure and small volume, and can realize that the split light beam I and the split light beam II enter the same high-speed camera in the form of parallel light; meanwhile, the problems of overlapping and the like of two split images can be avoided by finely adjusting the angle and the position of the elliptical reflector in the splitting light path. In addition, by adjusting the position of the elliptical reflector, a specific optical path difference occurs between the reflected light beam and the transmitted light beam, so that the high-speed camera can focus two images simultaneously, and the adjustment mode is simple and easy to realize.
(2) The invention can realize the on-line rapid measurement of the temperature field of the molten pool in the selective laser melting process by adopting one high-speed camera, thereby avoiding the defect of inaccurate synchronous trigger time of the two high-speed cameras, improving the measurement precision and reducing the cost; meanwhile, the adopted high-speed camera is a high-speed high-resolution camera (a hundred thousand pictures are shot in one second), the measurement speed and precision of the temperature field are greatly improved, the requirements of high-resolution and high-speed measurement of the temperature field of the molten pool are met, and the defects of low speed and low resolution of the existing thermal infrared imager are overcome.
(2) The optical path of infrared light of a molten pool can be effectively lengthened by using the adjustable optical path relay system, the temperature measurement can be carried out on most metal additive manufacturing equipment, and the influence of the stretching of the optical path on the size of molten pool imaging is small; meanwhile, the adjustable optical path relay system can compensate the chromatic aberration of the device to a certain extent, and the imaging precision and the problem calculation precision are improved.
(3) The relay system adopts a plurality of achromatic lenses such as a first near-infrared achromatic double-cemented lens pair, a second near-infrared achromatic double-cemented lens pair, an overall achromatic double-cemented lens and the like, and the achromatic lenses have good monochromatic performance and excellent focusing capability, the quality of high-speed camera pictures is improved, and the measurement precision is further improved.
(4) In the light splitting system, infrared light with 780nm and 905nm wave bands selected by the first band-pass filter and the second band-pass filter is used for imaging calculation, the two wave bands can avoid the influence of plasma formed above a molten pool on the temperature measurement of the molten pool, and the temperature measurement range can reach 1000-3000 ℃.
(4) According to the invention, a colorimetric thermometry method is adopted, the measuring range covers the high-temperature and ultra-high-temperature fields (1000-3000 ℃), and the defect that the existing thermal infrared imager cannot measure the ultra-high-temperature field is overcome; meanwhile, the colorimetric thermometry eliminates the interference of interference light, plasma and the like on the high-speed camera, and improves the measurement precision.
(5) The device can obtain the clear images of two bands focused simultaneously, the images of the two bands are in a translation relation, the image matching effect is good, and the measurement precision of colorimetric temperature measurement is improved.
(6) The invention also corrects the optical distortion of the device, and can effectively improve the calculation precision of the temperature field.
(7) The invention can realize the measurement of the acquisition speed of a high-speed camera at ten thousand frame rate and the temperature of the molten pool at 3000 ℃, has the precision within 2 percent and higher resolution, can effectively realize the monitoring of the temperature field of the molten pool, and can also be used for monitoring the temperature in the explosive explosion process.
Drawings
FIG. 1 is a schematic structural diagram of an on-line monitoring device for a molten pool temperature field of the invention.
Fig. 2 is a schematic structural diagram of the optical splitting system.
FIG. 3 is a flow chart of overall optical distortion calibration.
Wherein, 1-a molten pool; 2-forming a plane; 3-a printing chamber; 4-a focusing lens; 5-scanning a galvanometer; 6-partial wave band reflected by the long wave pass dichroic mirror; 7-a long-wave pass dichroic mirror; 8-cage type cube; 9-a laser; 10-1-connecting the lens barrel; 10-2-connecting the lens barrel; 11-a first near-infrared achromatic doublet-cemented lens pair; 12-a first adjustable barrel; 13-a second near-infrared achromatic doublet pair; 14-a first right angle optical trim mount; 15-a first elliptical reflector; 16-a second adjustable barrel; 17-bulk achromatic doublet; 18-a light splitting system; 19-a high-speed camera; 20-a first dichroic mirror; 21-1-a second elliptical reflector; 21-2-a third elliptical reflector; 22-1-a second right angle optics mount; 22-2-a third right angle optics alignment mount; 23-1-a first band pass filter; 23-2-a second band-pass filter; 24-a second dichroic mirror; 25-computer.
Detailed Description
The invention is described in detail below by way of example with reference to the accompanying drawings.
The invention provides a high-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring device, which comprises a laser 9, a scanning galvanometer 5, a focusing lens 4, a long-wave-pass dichroic mirror 7, a light splitting system 18, a high-speed camera 19 and a computer 25, wherein the scanning galvanometer 5 is arranged on the upper surface of the laser; wherein, the long-wave-pass dichroic mirror 7 is positioned between the laser 9 and the scanning galvanometer 5.
The laser 9 emits laser which is irradiated on the molten pool 1 in the printing chamber 3 through the long-wave-pass dichroic mirror 7, the scanning galvanometer 5 and the focusing lens 4; infrared light radiated by the molten pool 1 passes through the focusing lens 4 and the scanning galvanometer 5, is reflected by the long-wavelength-pass dichroic mirror 7 and enters the light splitting system 18; the light splitting system splits the infrared light of the molten pool 1 into two parallel beams of light with different wave bands; the high-speed camera 19 simultaneously images the 2 beams of light obtained by the light splitting system; the computer 25 calculates the temperature of the molten pool by adopting a colorimetric temperature measurement principle according to the imaging result of the high-speed camera 18.
Wherein, two surfaces of the long-wave-pass dichroic mirror 7 are respectively plated with a dichroic film and an antireflection film; wherein, the surface plated with the antireflection film faces the laser 9 and is used for transmitting laser emitted by the laser 9; the dichroic film coated side faces the scanning galvanometer 5 and is used for reflecting infrared light of the molten pool 1. The dichroic film surface has high reflection performance for a wave band shorter than the initial wavelength (laser), and the antireflection film has high permeability for a wave band longer than the initial wavelength; according to the invention, according to the band difference between the infrared light radiated by the molten pool and the laser emitted by the laser, the transmission band and the reflection band can be separated by using the long-wave-pass dichroic mirror, so that the influence of the laser on the imaging of a high-speed camera is reduced, and the accuracy of temperature calculation is improved.
In order to conveniently fix the long-wave-pass dichroic mirror 7, the long-wave-pass dichroic mirror 7 can be fixed in a cage cube 8, and the center of an opening of the cage cube 8 is positioned on a connecting line between the center of an incident hole of the scanning galvanometer 5 and the center of an emergent hole of the laser 9; a long-wave pass dichroic mirror 7 may be fixed in said cage cube 8 at an angle of 45 degrees. The combination of the long-wave pass dichroic mirror 7 and the cage cube 8 may be referred to as an optical path steering system.
The light splitting system splits the infrared light radiated by the molten pool into two beams, and the two beams are imaged by the same high-speed camera 9. As shown in fig. 2, the optical splitting system includes: a first dichroic mirror 20, near infrared band filter sets 23-1, 23-2, mirror sets 21-1, 21-2, and a second dichroic mirror 24; wherein, one side of the first dichroic mirror 20 is plated with dichroic film, and the other side is plated with antireflection film; through the wavelength design of the dichroic film, the light wave (beam two) shorter/longer than the design wavelength in the infrared light radiated by the molten pool is reflected, and the light wave (beam one) longer/shorter than the design wavelength transmitted by the antireflection film on the other side is enhanced; thereby, the first light beam and the second light beam are separated from the transmitted light via the dichroic mirror, and are also separated in wavelength band. The advantages of the band separation are that different band information of infrared light radiated by the molten pool can be respectively utilized, the intensity of light beams can not be reduced, and the problems that the imaging brightness is low and the temperature calculation precision is influenced due to the fact that the intensity of the light beams is reduced in the traditional mode are solved. The first light beam transmitted by the first dichroic mirror 20 and the second reflected light beam are reflected to the second dichroic mirror 24 through the corresponding reflector group respectively; the second dichroic mirror 24 has the same initial wavelength as the first dichroic mirror 20 and has the opposite function (one long wave passes through one short wave pass), that is, when the first dichroic mirror is a first long wave pass dichroic mirror, the second dichroic mirror is a second short wave pass dichroic mirror; and when the first dichroic mirror is a first short-wave-pass dichroic mirror, the second dichroic mirror is a second long-wave-pass dichroic mirror. By designing the position relationship and the lens angle among the first dichroic mirror 20, the reflector groups 21-1 and 21-2 and the second dichroic mirror 24, the first beam and the second beam can enter the high-speed camera in two parallel directions, and the imaging in the high-speed camera is not overlapped; if the first wave beam and the second wave beam can not be focused simultaneously, a certain optical path difference exists between the first wave beam and the second wave beam by adjusting the positions of the lenses, and then focusing imaging in the high-speed camera can be realized.
Specifically, as shown in fig. 2, the first dichroic mirror 20 forms an angle of 45 degrees with the incident light thereof, and the plane on which the dichroic film is plated on the first dichroic mirror 20 faces the incident light; the reflector group comprises a second elliptic reflector 21-1 and a third elliptic reflector 21-2; the elliptical reflector can provide a circular light-transmitting aperture when being installed at an angle of 45 degrees, so that incident light can be comprehensively reflected, and light intensity loss is avoided. The second elliptical reflector 21-1 may be fixed to a second rectangular optical adjustment bracket 22-1 and the third elliptical reflector 21-2 may be fixed to a third rectangular optical adjustment bracket 22-2. The second 22-1 and third 22-2 right angle optics mounts are mirror symmetric about the first dichroic mirror 20.
Considering that the wavelength band of the plasma radiation formed above the molten pool is also collected by the high-speed camera, it is necessary to remove the wavelength band of the plasma radiation. The invention arranges a near-infrared band filter set on the reflection path of the first light beam and the second light beam. As shown in fig. 2, the near-infrared band filter set includes a first band pass filter 23-1 and a second band pass filter 23-2; the first band-pass filter 23-1 and the second band-pass filter 23-2 are respectively located at the reflection ends of the second elliptic reflecting mirror 21-1 and the third elliptic reflecting mirror 21-2, and the first band-pass filter 23-1 and the second band-pass filter 23-2 are mirror-symmetrical with respect to the first dichroic mirror 20.
The central point of the second dichroic mirror 24 is located at the intersection point of the connecting line of the central point of the second elliptic reflecting mirror 21-1 and the central point of the first band-pass filter 23-1 and the connecting line of the central point of the third elliptic reflecting mirror 21-2 and the central point of the second band-pass filter 23-2, the included angle of the light beam emitted by the second dichroic mirror 24 and the near-infrared band filter set is 45 degrees, and the plane of the antireflection film of the second dichroic mirror 24 faces the second band-pass filter 23-1.
In this embodiment, when the first dichroic mirror is a first long-wavelength pass dichroic mirror and the second dichroic mirror is a second short-wavelength pass dichroic mirror, the first dichroic mirror 20 transmits light with a wavelength greater than 805nm and reflects light with a wavelength less than 805 nm; the second dichroic mirror 24 transmits light with a wavelength less than 805nm and reflects light with a wavelength greater than 805 nm; the center wavelength of the first band-pass filter 23-1 is 780nm, and the full width at half maximum is 10 nm; the second band-pass filter 23-2 has a center wavelength of 905nm and a full width at half maximum of 10 nm. The central temperature of plasma formed in the additive manufacturing process can reach tens of thousands of degrees centigrade, the radiation wave band is mainly concentrated in a short wave range, and the band-pass filters of 780nm and 905nm can avoid the influence of the plasma radiation wave band on the imaging of a high-speed camera to the maximum extent; meanwhile, the difference of the radiation energy of the selected 780nm and 905nm wave bands is larger, the larger the difference is, the larger the numerical range of the obtained gray value ratio is, and the precision and the resolution of temperature measurement can be improved; in addition, the molten pool has higher radiation intensity at 780nm and 905nm, and a high-speed camera can acquire stronger radiation signals after a band-pass filter is adopted.
Therefore, the beam splitting system 18 splits the infrared light radiated by the molten pool into 2 beams with different wave bands, and the beams enter the same high-speed camera 19 in parallel with a certain optical path difference for focusing imaging, and the imaging of the beam I and the beam II in the high-speed camera 9 is not overlapped. The computer 25 processes the image of the high-speed camera 19 to obtain the temperature distribution of the molten pool.
Considering that the device needs to be modified when being integrated on metal additive manufacturing equipment, and the integration is relatively difficult, an adjustable optical path relay system can be arranged between the long-wave-pass dichroic mirror 7 and the light splitting system 18 and used for extending the optical path of infrared light radiated by a molten pool, and the device can play a certain role in eliminating chromatic aberration of each lens in a temperature measuring device and improving the imaging effect and the temperature calculation precision. When the optical path of the relay system with the adjustable optical path is longer, the image of the molten pool acquired by the high-speed camera becomes smaller, and the long-focus microscope lens can be arranged on the high-speed camera to amplify the image of the molten pool.
Specifically, as shown in fig. 1, the adjustable optical path relay system includes: the device comprises a first near-infrared achromatic double cemented lens pair 11, a second near-infrared achromatic double cemented lens pair 13, a connecting lens barrel 10, a first adjustable lens barrel 12, a second adjustable lens barrel 16, a first elliptical reflector 15 and an overall achromatic double cemented lens 17. The first near-infrared achromatic double cemented lens pair 11 is connected with a cage cube 8 in the light path steering system through a connecting lens barrel 10; the second near-infrared achromatic double cemented lens pair 13 is connected with the light-emitting side of the first near-infrared achromatic double cemented lens pair 11 through a first adjustable lens barrel 12; the first elliptical reflector 15 is fixed on the first right-angle optical adjusting frame 14, and the first right-angle optical adjusting frame 14 is connected with the light emitting side of the second near-infrared achromatic double cemented lens pair 13 through the connecting lens barrel 10-2; the entire achromatic doublet 17 is connected to the light exit side of the first right-angle optical adjustment frame 14 through the second adjustable barrel 16.
The focal length of the first near-infrared achromatic double cemented lens pair 11 is f1, an antireflection film with a certain range of wavelength is plated on the surface of the first near-infrared achromatic double cemented lens, and internal threads are engraved at two ends of the first near-infrared achromatic double cemented lens. The focal length of the second near-infrared achromatic double-cemented lens pair 13 is f2, an antireflection film in the same waveband range as that of the first near-infrared achromatic double-cemented lens pair is plated on the surface of the second near-infrared achromatic double-cemented lens pair, and internal threads are engraved at two ends of the second near-infrared achromatic double-cemented lens pair. External threads are carved at both ends of the connecting lens cone; the first elliptical reflector 15 is highly reflective to a range of wavebands. The focal length of the integral achromatic double-cemented lens 17 is f3, an antireflection film with a certain wave band range is plated on the surface of the integral achromatic double-cemented lens, and external threads are engraved on the light incident side of the integral achromatic double-cemented lens.
The adjustable optical path relay system extends the light wave received from the optical path diversion system into the light splitting system without chromatic aberration over an extension distance that can be adjusted by adjusting the distance between the second near-infrared achromatic lens pair 13 and the integral achromatic doublet 17.
The computer 25 can adopt the modes of colorimetric temperature measurement, digital image correlation, distortion correction and the like to obtain the temperature distribution of the molten pool according to the imaging result of the high-speed camera 19.
In the embodiment, the temperature field temperature T of the molten pool (1) is calculated by adopting the following colorimetric temperature measurement formula:
wherein, c2Represents a second radiation constant; lambda [ alpha ]1、λ2Respectively representing the central wavelengths of the first band-pass filter (23-1) and the second band-pass filter (23-2); n is a radical of1And N2The gray values of the pixel units corresponding to the first beam and the imaging area of the beam on the high-speed camera (19) are respectively represented; k is a proportionality coefficient, and a colorimetric temperature measurement formula is subjected to identity transformation to obtain:
by measuring the temperature of a certain point on the black body furnace for multiple times, the computer can obtain the corresponding N of the point at the same time1And N2And K value can be obtained by calculation.
In consideration of the fact that infrared light waves radiated from a molten pool pass through zero devices, a light path steering system, an adjustable light path relay system and a light splitting system 18 in laser selective melting equipment such as a focusing lens 4 and a scanning galvanometer 5, and some optical distortion may be introduced to cause deviation between pictures shot by a high-speed camera 19 and the actual pictures. The optical distortion calibration method adopted by the embodiment comprises the following steps:
Distortion diagram JLAnd distortion image JRThe radial distortion in (considering only the first order radial distortion)
Wherein,
and
the amount of distortion in the x-and y-directions, k1Is a first-order radial distortion coefficient, and x and y are coordinates on a corresponding distortion map;
distortion diagram JLFirst order radial distortion coefficient k1Reference picture I coordinates (x)a、ya) And distortion graph JLCoordinate (x) ofb、yb) The relationship between can be expressed as:
xa＝rx1[xb-k1xb(xb 2+yb 2)] (3)
ya＝ry1[yb-k1yb(xb 2+yb 2)] (4)
wherein r isx1And ry1Is a reference image I and a distortion image JLMagnification in x and y directions。
First order radial distortion coefficient k of distortion map JR2Reference picture I coordinates (x)a、ya) And distortion graph JRCoordinate (x) ofc、yc) The relationship between can be expressed as:
xa＝rx2[xc-k2xc(xc 2+yc 2)] (5)
ya＝ry2[yc-k2yc(xc 2+yc 2)] (6)
wherein r isx2And ry2Is a reference image I and a distortion image JRMagnification in the x and y directions.
specifically, a value range (0, m) of a k value is given, and m is 1-6; setting the step length as n, taking n,2n,3n, …, m as k values respectively, and substituting into the formulas (3) to (6) to obtain the undistorted graph fL' and undistorted figure fR'; the obtained undistorted pattern fL' and undistorted figure fR' performing overall image matching with reference image I respectively, specifically calculating undistorted image f by using correlation formulaL' and reference drawing I, undistorted drawing fR' correlation between different distortion coefficients with reference to figure I, the correlation formula is as follows:
C(k1)＝∑x∑y[fL′(xb，yb)-I(x，y)]2 (7)
C(k2)＝∑x∑y[fR′(xc，yc)-I(x，y)]2 (8)
wherein C is a numerical representation of the correlation, and the correlation is stronger when the number is smaller;
the optimal distortion coefficient can be solved by using methods such as a genetic algorithm or a particle swarm algorithm.
The particle swarm algorithm is initialized to a random set of solutions (i.e. the value range of the k value given by us), and then the optimal solution is found through iteration. In each iteration, the particle updates itself by tracking two "extrema". The first is the optimal solution found by the particle itself. Another extreme is the best solution currently found for the entire population.
The genetic algorithm GA represents the solution to the problem as a "chromosome", i.e. in the algorithm as a binary-coded string. Also, before the genetic algorithm is executed, a population of "chromosomes" is given, i.e., a hypothetical solution. Then, the hypothesis solutions are put into the 'environment' of the problem, and according to the principle of survival of the fittest, the 'chromosome' which is more adaptive to the environment is selected from the hypothesis solutions to be copied, and then a new generation 'chromosome' group which is more adaptive to the environment is generated through the crossing and mutation processes. Thus, evolution, generation by generation, eventually converges to a "chromosome" that best fits the environment, which is the optimal solution to the problem.
Step 4, finally selecting undistorted graphs fL' and reference figure I and undistorted figure fR' value of minimum correlation between reference map I and corresponding k1And k2The value is the optimal distortion coefficient of the corresponding image; and carrying out distortion correction on the picture shot by the high-speed camera by using the optimal distortion coefficient.
In summary, the above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A high-speed high-resolution high-precision ultra-high temperature molten pool temperature field on-line monitoring device comprises a laser (9), a scanning galvanometer (5), a light splitting system (18), a high-speed camera (19) and a computer (25), and is characterized by further comprising a long-wave-pass dichroic mirror (7);
wherein, the long-wave-pass dichroic mirror (7) is positioned between the laser (9) and the scanning galvanometer (5); the plane plated with the antireflection film on the long-wave-pass dichroic mirror (7) faces the laser (9) and is used for transmitting laser of the laser (9), and the plane plated with the dichroic film faces the scanning galvanometer (5) and is used for reflecting infrared light signals radiated by the molten pool (1); laser emitted by the laser (9) is transmitted by the long-wavelength-pass dichroic mirror (7) and is irradiated on a molten pool (1) of the printing chamber (3) by the scanning galvanometer (5); infrared light signals radiated by the molten pool (1) are reflected by the scanning galvanometer (5) and the long-wave-pass dichroic mirror (7) and enter the light splitting system;
the light splitting system (18) comprises a first dichroic mirror (20), a second dichroic mirror (24), a second elliptical reflector (21-1), a third elliptical reflector (21-2), a first band-pass filter (23-1) and a second band-pass filter (23-2); infrared light signals radiated by the molten pool are reflected by the long-wavelength-pass dichroic mirror (7) to enter the light splitting system, and then split into first band light and second band light by the first dichroic mirror (20); wherein, the first waveband light irradiates on the second band-pass filter (23-2) through the third elliptical reflector (21-2) and is filtered, and then is reflected by the second dichroic mirror (24) to enter the high-speed camera (19); the second wave band light irradiates the first band-pass filter (23-1) through the second elliptical reflector (21-1), is filtered, and then is transmitted into the high-speed camera (19) through the second dichroic mirror (24); the first dichroic mirror (20) and the second dichroic mirror (24) have the same initial wavelength but opposite functions, namely one of the first dichroic mirror and the second dichroic mirror adopts a long-wave-pass dichroic mirror, and the other one adopts a short-wave-pass dichroic mirror; the filtered first wave band light and the second wave band light parallelly enter the high-speed camera with a certain optical path difference, and images of the two wave bands of light in the high-speed camera are not overlapped and are focused simultaneously;
a high-speed camera (19) images the incident light of the two wavebands; the computer (25) processes the image of the high-speed camera (19) to obtain the distribution of the temperature field of the molten pool.
2. The high-speed high-resolution high-precision ultra-high temperature molten pool temperature field on-line monitoring device according to claim 1, characterized by further comprising an adjustable optical path relay system, wherein the adjustable optical path relay system is positioned between the long-wave-pass dichroic mirror (7) and the light splitting system (18); the adjustable optical path relay system is used for adjusting the optical path of reflected light of the long-wave-pass dichroic mirror (7) and eliminating the chromatic aberration of the reflected light; the reflected light of the long-wave-pass dichroic mirror (7) enters the light splitting system through the relay system with the adjustable optical path.
3. The high-speed high-resolution high-precision ultra-high temperature molten pool temperature field on-line monitoring device according to claim 2, characterized in that the long-wavelength-pass dichroic mirror (7) is installed in a cage cube (8); the adjustable optical path relay system includes: the device comprises a first near-infrared achromatic double cemented lens pair (11), a second near-infrared achromatic double cemented lens pair (13), a connecting lens barrel (10-1), a connecting lens barrel (10-2), a first adjustable lens barrel (12), a second adjustable lens barrel (16), a first right-angle optical adjusting frame (14), a first elliptical reflector (15) and an overall achromatic double cemented lens (17); wherein, the first elliptical reflector (15) is arranged on the first right-angle optical adjusting frame (14); the first near-infrared achromatic double cemented lens pair (11) is connected with the cage-type cube (8) through a connecting lens cone (10-1); the second near-infrared achromatic double cemented lens pair (13) is connected with the light-emitting side of the first near-infrared achromatic double cemented lens pair (11) through a first adjustable lens barrel (12); the first right-angle optical adjusting frame (14) is connected with the light-emitting side of the second near-infrared achromatic double cemented lens pair (13) through a connecting lens barrel (10-2); the integral achromatic double-cemented lens (17) is connected with the light-emitting side of the first right-angle optical adjusting frame (14) through a second adjustable lens barrel (16).
4. The device for monitoring the temperature field of the ultra-high temperature molten pool with high speed, high resolution and high precision according to claim 3, wherein the surfaces of the first near-infrared achromatic double cemented lens pair (11), the second near-infrared achromatic double cemented lens pair (13) and the whole achromatic double cemented lens (17) are coated with antireflection films of 650-1050 nm.
5. The high-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring device according to claim 1, characterized in that the starting wavelength of the long-wave pass dichroic mirror (7) is in the range of 950-1050 nm; the initial wavelength range of the first dichroic mirror (20) and the second dichroic mirror (24) is 805-880 nm; the surfaces of the long-wave-pass dichroic mirror (7), the first dichroic mirror (20) and the second dichroic mirror (24) are plated with dichroic films and antireflection films; the center wavelengths of the first band-pass filter (23-1) and the second band-pass filter (23-2) are 780nm and 905nm respectively.
6. A high-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring method is characterized in that the high-speed high-resolution high-precision ultrahigh-temperature molten pool temperature field online monitoring device according to any one of claims 1 to 5 is adopted to monitor the molten pool temperature;
the computer (25) adopts the following colorimetric temperature measurement formula to calculate the temperature field temperature T of the molten pool (1):
wherein, c2Represents a second radiation constant; lambda [ alpha ]1、λ2Respectively representing the central wavelengths of the first band-pass filter (23-1) and the second band-pass filter (23-2); n is a radical of1And N2The gray values of pixel units corresponding to the first wave band light and the second wave band light imaging areas on the high-speed camera (19) are respectively represented; k is a proportionality coefficient and is obtained by black body furnace calibration according to the following formula:
7. the method for monitoring the temperature field of the ultrahigh-temperature molten pool with high speed, high resolution and high precision according to claim 6, characterized in that the computer (25) firstly performs distortion calibration on the imaging result of the high-speed camera (19) before calculating the temperature T of the temperature field of the molten pool (1) to obtain an image after the distortion calibration; calculating the temperature field temperature T of the molten pool (1) according to the image after distortion calibration; the distortion calibration method comprises the following steps:
step 1, printing N randomly distributed mark points on white paper as speckles; pasting the white paper with speckles on the forming plane in the printing chamber, and obtaining the picture with the mark pointsAs a reference image I, a coordinate system Ox is established with the center of the reference image I as the originaya；
Step 2, installing an online monitoring device of a high-speed, high-resolution, high-precision and ultrahigh-temperature molten pool temperature field on selective laser melting equipment, adjusting a light path, and shooting white paper with marking points; taking the two images shot by the high-speed camera as distortion maps J respectivelyLAnd distortion image JR(ii) a Respectively with the distortion image J after light splittingLAnd distortion image JRIs used as an origin to establish a coordinate system OxbybAnd Oxcyc；
Step 3, respectively setting distortion maps JLAnd distortion image JRDistortion coefficient k of1、k2The value range of (a); extracting distortion coefficient k in a corresponding value range by a certain step length1、k2Substituting into the formula (3) to the formula (6) to obtain the undistorted graph fL' and undistorted figure fR′；
xa＝rx1[xb-k1xb(xb 2+yb 2)] (3)
ya＝ry1[yb-k1yb(xb 2+yb 2)] (4)
xa＝rx2[xc-k2xc(xc 2+yc 2)] (5)
ya＝ry2[yc-k2yc(xc 2+yc 2)] (6)
Wherein (x)a、ya)、(xb、yb)、(xc、yc) Respectively as reference picture I and distortion picture JLAnd distortion graph JRCoordinates of (3); k is a radical of1、k2Respectively, distortion maps JLDistortion graph JRFirst order radial distortion coefficient of; r isx1And ry1Is a reference image I and a distortion image JLMagnification in x and y directions, rx2And ry2Is a reference image I and a distortion image JRMagnification in the x and y directions;
respectively calculating the undistorted graph fL' and reference drawing I, undistorted drawing fR' correlation between different distortion coefficients in reference picture I, distortion coefficient k corresponding to minimum correlation1、k2The optimal distortion coefficient of the corresponding image is obtained;
and 4, carrying out distortion correction on the picture shot by the high-speed camera by adopting the optimal distortion coefficient obtained in the step 3.
8. The method for on-line monitoring the temperature field of the high-speed, high-resolution and high-precision ultra-high temperature molten pool according to claim 7, wherein in the step 3, an optimal distortion coefficient is obtained by adopting a genetic algorithm or a particle swarm algorithm.
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