CN103134599A - Method and system for real-time monitoring of molten bath state in direct molding process of laser metal - Google Patents

Abstract

The invention discloses a method for real-time monitoring of the molten pool state in laser metal direct forming, which is: 1) acquiring monochromatic radiation images in at least two directions of the molten pool and its heat-affected zone in laser metal direct forming through image sensing ; 2) Measure the temperature value of the molten pool or at least one point near it in the laser metal direct forming by infrared temperature measurement as a reference temperature; 3) process the collected radiation image in real time, and convert the monochromatic radiation image into a grayscale image; 4) Use the reference temperature to perform temperature calibration on the equivalent grayscale image, and obtain the temperature values of each point in the image according to the grayscale value-temperature value correspondence, thereby establishing the three-dimensional shape and temperature field of the molten pool and its heat-affected zone. The invention also discloses a real-time monitoring system of molten pool state in laser metal direct forming. The monitoring method and system of the invention can accurately, reliably and comprehensively monitor the molten pool state in real time during the laser metal direct forming process.

Description

Method and system for real-time monitoring of molten pool state in laser metal direct forming

technical field

The invention relates to the technical field of laser metal direct forming process monitoring, and relates to a method and system for real-time monitoring of molten pool state in laser metal direct forming.

Background technique

Laser metal direct forming technology is to focus a high-power laser beam on the surface of the workpiece to form a molten pool, and the metal powder is coaxially sent into the molten pool to form a cladding layer, and the laser beam is scanned back and forth according to a given route to manufacture line by line. A new technology for producing metal parts entities. Laser direct forming technology has the advantages of no need for molds, the ability to form complex configuration parts, and direct manufacturing of refractory metals. It is the most promising new manufacturing technology among many rapid and direct manufacturing methods. It is used in aviation, aerospace, automobiles, etc. The field has broad application prospects.

In laser metal direct forming, there are strong convection, heat transfer, and mass transfer phenomena inside the molten pool, which is the key area for the interaction between laser and metal powder. Macroscopically, the state of the molten pool can be characterized by characteristic parameters such as the shape and size of the molten pool, the temperature of the molten pool, the temperature field of the molten pool and its surroundings, and the cooling rate of the molten pool. The state of the molten pool is closely related to the metallurgy, crystallization, phase transformation and other processes in the forming process. The stability of the molten pool state in the process directly affects the dimensional accuracy and mechanical properties of the laser metal direct forming parts. Therefore, real-time monitoring of the state of the molten pool is the key to controlling the laser metal direct forming process.

Contents of the invention

The purpose of the present invention is to provide a method that can accurately, reliably and comprehensively monitor the state of the molten pool in real time during the laser metal direct forming process, and perform diagnosis and feedback control on the laser direct forming process through this method.

A method for real-time monitoring of molten pool state in laser metal direct forming provided by the present invention is:

1) Obtaining monochromatic radiation images in at least two directions of the molten pool and its heat-affected zone in laser metal direct forming through image sensing;

2) Measure the temperature value of the molten pool or at least one point near it in the laser metal direct forming by infrared temperature measurement as a reference temperature;

3) Process the collected radiation image in real time, convert the monochromatic radiation image into a grayscale image, and extract the characteristic parameters of the shape and size of the molten pool;

4) Use the reference temperature to perform temperature calibration on the equivalent grayscale image, and obtain the temperature values of each point in the image according to the grayscale value-temperature value correspondence, thereby establishing the three-dimensional shape and temperature field of the molten pool and its heat-affected zone;

The gray value-temperature value relationship is as follows:

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; G</span>}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}^{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span>}$

Among them: T represents the surface temperature of the molten pool, G represents the gray value in the monochromatic radiation image, λ ₀ represents the corresponding central wavelength of the collected monochromatic radiation image, C ₂ is Planck’s second radiation constant, M is the same as CCD's current-gray value conversion coefficient, photoelectric conversion coefficient, exposure time, exit pupil diameter, image square focal length, optical system transmittance, spectral response function and other coefficient-related fixed values.

Preferably, in the step 1), the image sensor collects images of the molten pool in at least two directions, one of which collects a frontal image of the molten pool, and one collects a side image of the molten pool.

Preferably, the reference temperature point for infrared thermometry measurement in step 2) is located in the front image or side image of the molten pool.

A real-time detection system of molten pool state in laser metal direct forming of the present invention comprises:

The image sensing unit is used to obtain monochromatic radiation images in at least two directions of the molten pool and its heat-affected zone in laser metal direct forming;

An infrared temperature measuring unit, which measures the temperature value of at least one point in the molten pool or its vicinity in the laser metal direct forming as a reference temperature;

The image processing unit collects the monochromatic radiation image and performs real-time processing, converts the monochromatic radiation image into a grayscale image, and extracts the characteristic parameters of the shape and size of the melting pool; Calibration, the temperature value of each point in the image is obtained according to the gray value-temperature value correspondence, and thus the three-dimensional shape and temperature field of the molten pool and its heat-affected zone are established;

The gray value-temperature value relationship is as follows:

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; G</span>}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}^{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span>}$

Among them: T represents the surface temperature of the molten pool, G represents the gray value in the monochromatic radiation image, λ ₀ represents the corresponding central wavelength of the collected monochromatic radiation image, C ₂ is Planck’s second radiation constant, M is the same as CCD's current-gray value conversion coefficient, photoelectric conversion coefficient, exposure time, exit pupil diameter, image square focal length, optical system transmittance, spectral response function and other coefficient-related fixed values.

Preferably, the image sensing unit includes: a camera, an image acquisition card, and a filter system, and the camera collects a monochromatic radiation image of the molten pool and its heat-affected zone through the filter system, and transmits the image through the image acquisition card to the image processing system.

Preferably, the infrared temperature measuring unit is an infrared thermometer.

The monitoring method and system of the present invention can accurately, reliably and comprehensively monitor the molten pool state in real time during the laser metal direct forming process, laying a technical foundation for further realizing the closed-loop control of the laser metal direct forming process.

Description of drawings

Fig. 1 Schematic diagram of real-time monitoring system for laser metal direct forming molten pool state;

Figure 2. Monochromatic image of the front side of the molten pool during the laser metal direct forming process;

Figure 2(a) Original image;

Fig. 2 (b) schematic diagram of the position of the reference temperature point;

Fig. 3 Flowchart of real-time processing of molten pool state image in laser metal direct forming;

Figure 4 image of the front side treatment process of the molten pool in the laser metal direct forming process;

Figure 4(a) binary image;

Figure 4(b) Image after closing operation;

Figure 4(c) Image after edge detection;

Figure 4(d) Schematic diagram of feature size extraction;

Fig.5 Image of temperature distribution on the front side of molten pool during laser metal direct forming.

Detailed ways

Below in conjunction with accompanying drawing, the present invention is described in further detail:

As shown in Figure 1, this implementation example adopts the coaxial powder feeding method of laser metal direct forming, the laser beam interacts with the powder, and scans layer by layer on the substrate 2 to form a wall panel 3, and the substrate 2 is placed on the workbench 1. The detection system includes: an image sensing unit, an infrared temperature measurement unit and an image processing unit. The image sensing unit adopts an infrared thermometer 8 , and the infrared thermometer 8 has an infrared thermometer probe 4 . Image sensing unit filter system 6, camera 7 and image acquisition card 9. The image processing unit is a computer 10 . In the embodiment of the present invention, each unit is specifically:

The infrared thermometer 8 adopts the IGA740-LO fiber-optic high-speed infrared thermometer of Mikron Company, with a temperature measurement range of 500-2500°C and a response time of 6 μs. Infrared thermometer probe 4 adopts the LVO25 standard probe of the IGA740-LO fiber optic high-speed infrared thermometer, and the diameter of the field of view is 1.6mm under the condition of a test distance of 80mm. The laser adopts HLD1001.5 industrial Nd-YAG continuous laser from HAAS company in Germany, the maximum power is 1000W. The filter system 6 is composed of a narrow-band interference filter with a center wavelength of 790nm and a half-bandwidth of 10nm and two 10% neutral light reduction filters. The filter system filters out the radiation and laser The interference of reflected radiation allows the camera to capture a clear monochromatic radiation image of the melt pool. Camera 7 adopts the CF8/5MX industrial black and white camera of KAPPA Company, with a resolution of 768×576 pixels and a CCD photosensitive size of 6.8×4.8mm. The image acquisition card adopts the OK_C20B image acquisition and compression card of Jiaheng Zhongzi Company, which adopts 9-bit A/D, digital comb filter, anti-aliasing filter and other technologies to improve image clarity.

As shown in Figure 1, a filter system 6 is added in front of the camera 7 to form an image acquisition device. The system is equipped with two identical image acquisition devices, which are fixed on the laser head and the powder feeding head 5, and respectively collect the front surface of the molten pool and the Monochromatic radiation images in two directions on the side of the pool, the collected image data is sent to the control computer 10 through the image acquisition card 9 for image processing and analysis. The infrared thermometer probe 4 is fixed on the laser head and the powder feeding head 5, forming an angle of 15-30° with it, and measuring the surface temperature of the molten pool or its nearby heat-affected zone in real time as a reference temperature. The reference temperature point is at the camera 7 within the field of view. The infrared thermometer 8 is connected with the computer 10 through the RS485 interface, and transmits the surface temperature data measured in real time to the software for display and temperature correction.

In this embodiment, the forming base 2 is 316 stainless steel, the base size is 80mm×40mm×20mm, and the forming powder is 316 stainless steel powder. The process parameters used in the experiment are: power 300W, defocus amount 9mm, layer height 0.15mm, forming layers 100 layers, laser scanning speed 3mm/s, preheating scanning cycle number 30, powder feeding rate 0.72g/min, reference temperature point At a distance of 2mm from the laser focus.

The method for real-time monitoring of molten pool state in laser metal direct forming of the present invention specifically includes the following steps:

(1) Obtaining monochromatic radiation images in at least two directions of the molten pool and its heat-affected zone in the laser metal direct forming through the image sensing unit;

During the laser metal direct forming process, due to the high-power laser radiation on the surface of the substrate 2, the metal material is rapidly melted, evaporated and even ionized, forming a molten pool and generating plasma above the molten pool. The radiation of the plasma and the reflection of the laser by the material cause great interference to the image of the molten pool collected by the camera 7, and it is easy to cause the saturation of the CCD current output. The filter system 6 adopted in the present invention is placed in front of the lens of the camera 7, through the combined lens in the filter system 6, the radiated light from the laser metal direct forming process processing area is attenuated and filtered, and the photoplasma is filtered out. The strong light interference of radiation and laser reflection solves the problem of CCD current output saturation, so that the camera 7 collects clear monochromatic radiation images of the molten pool and its heat-affected zone.

In this implementation example, there are two sets of filter system 6 and camera 7, which are fixed on the laser head above the powder feeding head, and collect the monochromatic radiation images of the front and side of the molten pool in laser metal direct forming respectively in real time. The image is stored in the computer 10 in the form of a file through the image acquisition card 9 for use in the image processing and temperature calibration programs in the subsequent steps.

Figure 2(a) shows the monochromatic radiation image of the front of the molten pool collected in this implementation example. For the convenience of description, the image processing process in the subsequent steps (2)-(4) is described as an example of this image.

(2) Measure the temperature value of the molten pool or at least one point near it in the laser metal direct forming by infrared temperature measurement as the reference temperature;

The laser metal direct forming process is a complex interaction process between laser, metal powder and materials. As a non-contact temperature measurement method, infrared temperature measurement is applied to the laser metal direct forming process, which requires infrared temperature measurement equipment to have a wide temperature measurement range and space High resolution, fast response, strong anti-interference ability. In this implementation example, the IGA740-LO fiber-optic high-speed infrared thermometer 8 of Mikron Company is used as the infrared temperature measurement equipment, the temperature measurement range is 500-2500 ° C, and the response time is 6 μs, among which the LVO25 standard probe 4 has a built-in laser filter to exclude On-site laser radiation interference.

The LVO25 standard probe 4 is fixed on the laser head and the powder feeding head, aiming at a place 2mm away from the laser focus, that is, collecting the surface temperature at this point as a reference temperature. The reference temperature value collected by the infrared thermometer 8 in real time during the laser metal direct forming process is sent to the computer 10 for use in subsequent temperature calibration procedures.

FIG. 2( b ) shows the position of the surface temperature reference point collected by the high-speed infrared thermometer 8 .

(3) process the collected radiation image in real time by computer 10, convert the monochromatic radiation image into a grayscale image, and extract the characteristic parameters of the shape and size of the molten pool;

In this implementation example, the real-time image processing is realized by a C++ program designed in the computer 10 based on the Windows operating system. Program flow shown in Figure 3 .

The monochromatic radiation image collected in a single time is processed as follows:

a) Image filtering. The domain averaging method is used for image filtering to filter out the noise in the image.

b) Generate a grayscale image. Because the CF8/5MX industrial black-and-white camera of KAPPA Company is used in this implementation example, the grayscale image can be collected directly.

c) Threshold segmentation into binary images. Based on the characteristics of the molten pool image, the threshold segmentation method of the maximum variance method is used to extract the molten pool object from the background of the image.

d) The edge detection algorithm extracts the edges of the molten pool. Due to the characteristics of laser metal direct forming, unmelted powder falls into the molten pool, forming noise points in the molten pool in the binary image, which interferes with the extraction of the edge of the molten pool and the extraction of the characteristic size of the molten pool. Therefore, here, the closed operation is performed on the image first to remove the noise, and then the edge of the molten pool is extracted with the Roberts edge detection operator.

e) Calculate the area, length, width and height of the molten pool. In this implementation example, the binary image is used in the program to calculate the characteristic parameters of the molten pool size through the edge detection algorithm in units of pixels. In the front image of the molten pool, calculate the area, length, and width of the molten pool, and in the side image of the molten pool, calculate the height of the molten pool.

The image during image processing is shown in Figure 4.

(4) Use the reference temperature to carry out temperature calibration on the grayscale image, and obtain the temperature value of each point in the image according to the corresponding relationship between the grayscale value and the temperature value in the radiation principle;

This step is also completed by the program shown in FIG. 3 .

The prominent feature of using the reference temperature to calibrate the temperature of the grayscale image is that this calibration method is a real-time calibration method. In existing technologies such as dual-wavelength infrared image colorimetric temperature measurement, blackbody radiation is usually used for temperature calibration. This calibration method is complicated and costly, and the calibration work needs to be completed before the monitoring of the laser metal direct forming process. The calibration method of the present invention adopts the reference temperature to calibrate the grayscale image at the same time, and has good real-time performance and high accuracy.

The gray value-temperature value relationship is as follows:

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; G</span>}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}^{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span>}$

Among them: T represents the surface temperature of the molten pool, G represents the gray value in the monochromatic radiation image, λ ₀ represents the corresponding central wavelength of the collected monochromatic radiation image, C ₂ is Planck’s second radiation constant, M is the same as CCD's current-gray value conversion coefficient, photoelectric conversion coefficient, exposure time, exit pupil diameter, image square focal length, optical system transmittance, spectral response function and other coefficient-related fixed values.

The process of grayscale temperature calibration and temperature field calculation is as follows:

a) calculate the average gray value G in the reference point range as shown in Figure 2;

b) Read the temperature value T measured by the infrared thermometer at the same moment as when collecting the monochromatic radiation image;

c) Substituting the gray value G and the temperature value T in the previous two steps into the gray temperature relationship, and calculate the fixed value M in the formula;

d) Substituting the gray value G _ij of all the points in the collected monochromatic radiation image into the gray temperature relationship formula in the monochromatic radiation image, calculate the temperature value T represented by all the points in the monochromatic radiation image _ij .

${\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; ij</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; ln</span>}{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; ij</span>}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}^{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span>}$

Among them, G _ij represents the gray value of each pixel in the monochromatic radiation image, T _ij represents the temperature value represented by the corresponding pixel point, λ ₀ represents the corresponding central wavelength of the collected monochromatic radiation image, C ₂ is the Planck The second radiation constant, M is the fixed value obtained in step c).

(5) Based on the shape and size data of the molten pool and image temperature data obtained above, establish the three-dimensional shape and temperature field of the molten pool and its heat-affected zone.

In this implementation example, the camera captures monochromatic radiation images in two directions of the front of the molten pool and the side of the molten pool, assuming that the molten pool is the same on both sides, and smoothing is done in the third direction, using two directions The data and temperature field of the above image processing are used to reconstruct the three-dimensional shape and temperature field of the molten pool and its heat-affected zone. As shown in Figure 6 is the resulting effect diagram.

Claims (6)

1. molten bath status real time monitor method in a metal laser direct-forming, comprise the steps:

1) obtain monochromatic radiation image on the both direction at least of molten bath in metal laser direct-forming and heat-affected zone thereof by image sensing;

2) survey molten bath in described metal laser direct-forming by infrared measurement of temperature or near it temperature value of at least one point as the reference temperature;

3) radiation image that collects is processed in real time, the monochromatic radiation image transitions is become gray-scale map, and extract the characteristic parameter of melting pool shape and size;

4) utilize reference temperature equity gray-scale map to carry out temperature calibration, obtain the temperature value of each point in image according to gray-scale value-temperature value corresponding relation, set up thus 3D shape and the temperature field of molten bath and heat-affected zone thereof;

Described gray-scale value-temperature value relation is as follows:

$T=\frac{C_2}{{\mathrm{\&lambda;}}_0(\ln M-\ln G{{\mathrm{\&lambda;}}_0}^5)}$

Wherein: T represents the surface temperature in molten bath, and G represents the gray-scale value in the monochromatic radiation image, λ ₀The corresponding centre wavelength of the expression monochromatic radiation image that gathers, C ₂Be the Planck second radiation constant, M is to the electric current of CCD-grayvalue transition coefficient, photoelectric conversion factors, time shutter, emergent pupil diameter, as the relevant definite value of the coefficients such as square focal length, transmissivity of optical system, spectral response functions.

2. the method for claim 1, is characterized in that, described step 1) in image sensing gather crater image on both direction at least, one of them gathers Top-Side Pool Image, a collection molten bath side image.

3. method as claimed in claim 2, is characterized in that, described step 2) in the reference temperature point measured of infrared measurement of temperature be positioned among described Top-Side Pool Image or side image.

4. molten bath real-time detection device for state in a metal laser direct-forming, is characterized in that, comprising:

Image sensing cell is for the monochromatic radiation image on the both direction at least that obtains metal laser direct-forming molten bath and heat-affected zone thereof;

The infrared measurement of temperature unit, survey molten bath in described metal laser direct-forming or near it temperature value of at least one point as the reference temperature;

Graphics processing unit gathers described monochromatic radiation image and processes in real time, and the monochromatic radiation image transitions is become gray-scale map, and extracts the characteristic parameter of melting pool shape and size; Recycling reference temperature equity gray-scale map carries out temperature calibration, obtains the temperature value of each point in image according to gray-scale value-temperature value corresponding relation, sets up thus 3D shape and the temperature field of molten bath and heat-affected zone thereof;

Described gray-scale value-temperature value relation is as follows:

$T=\frac{C_2}{{\mathrm{\&lambda;}}_0(\ln M-\ln G{{\mathrm{\&lambda;}}_0}^5)}$

Wherein: T represents the surface temperature in molten bath, and G represents the gray-scale value in the monochromatic radiation image, λ ₀The corresponding centre wavelength of the expression monochromatic radiation image that gathers, C ₂Be the Planck second radiation constant, M is to the electric current of CCD-grayvalue transition coefficient, photoelectric conversion factors, time shutter, emergent pupil diameter, as the relevant definite value of the coefficients such as square focal length, transmissivity of optical system, spectral response functions.

5. system as claimed in claim 4, it is characterized in that, described image sensing cell comprises: video camera, image pick-up card, filter system, described video camera gathers the monochromatic radiation image of molten bath and heat-affected zone thereof by filter system, and sends described image processing system to by described image pick-up card.

6. system as claimed in claim 4, is characterized in that, described infrared measurement of temperature unit is infrared thermometer.

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