Wire-guided ultrasonic-assisted laser additive manufacturing closed-loop control system and method

Abstract

The present invention provides a closed-loop control system and method for wire-guided ultrasonic-assisted laser additive manufacturing. This system adds a wire front-end centering unit, a wire transition monitoring unit, a molten pool flow monitoring unit, and an in-situ feedback control unit to the existing wire-guided ultrasonic-assisted laser additive manufacturing system. Combined with the closed-loop control method for wire-guided ultrasonic-assisted laser additive manufacturing, this system can achieve rapid centering of the laser beam center and the wire front end, identification of the droplet transition state, determination of the molten pool flow state under wire-guided ultrasonic conditions, and in-situ control of wire transition and molten pool flow, ultimately ensuring the quality of laser additive manufacturing. The present invention can utilize the existing coaxial laser head image acquisition unit, eliminating the need for adding additional sensors. The system is less susceptible to signal interference, has a fast processing speed, and offers wide adaptability, significantly contributing to improved laser additive manufacturing quality and industrial applications.

Description

Technical Field

[0001] The present invention relates to the fields of laser additive manufacturing, remanufacturing, and laser processing technology, and in particular to a closed-loop control system and method for wire-guided ultrasonic-assisted laser additive manufacturing. Background Art

[0002] Laser additive manufacturing technology uses wire or powder feeding to feed material, then simultaneously uses a laser beam to melt and solidify the powder or wire, enabling 3D printing, remanufacturing and repair, and surface coating cladding. It is currently widely used in fields such as aircraft engine blade forming, rolling mill roller remanufacturing, and high-temperature thermal barrier coating cladding, and has received widespread attention from academia and industry. Throughout the development of laser additive manufacturing, high performance and quality have always been the focus of development. However, due to the complex physical metallurgical phenomena and intense heat accumulation during the laser manufacturing process, the suppression of coarse grains and metallurgical defects has always been a key focus of laser additive manufacturing.

[0003] Ultrasonic energy field-assisted laser additive manufacturing can achieve the purpose of grain refinement and defect suppression by introducing high-frequency ultrasonic vibrations to induce cavitation or interrupt the dendrite growth process. The introduction of the ultrasonic energy field can be achieved by applying ultrasonic vibrations to the processing object itself, or by ultrasonic vibrations guided by the wire, that is, wire-guided ultrasound. However, the ultrasonic vibrations of the processing object itself are usually difficult to use for large and complex components, while wire-guided ultrasound can avoid the above problems to a certain extent. It only requires ultrasonic vibration of the wire, and then it is introduced into the molten pool through the wire. However, the above solution still has certain problems. Because it relies on the wire to form a droplet or liquid bridge and then conduct ultrasonic conduction, it places higher requirements on the stability of the wire transition during the processing process. The unstable wire transition state will seriously interfere with the effect of the ultrasonic energy field.

[0004] Therefore, in view of the needs of microstructure refinement and defect suppression in laser additive manufacturing, as well as the difficulty in controlling droplet transfer and molten pool flow during wire-guided ultrasound, it is necessary to propose a wire-guided ultrasound-assisted laser additive manufacturing closed-loop control system and method to assist in high-quality laser additive manufacturing forming, repair and cladding.

[0005] Patent document CN111748815A (application number: CN202010630442.X) discloses a laser additive manufacturing system and method based on closed-loop control, including an optical measurement system, an image processing system, a feedforward controller, a first comparison module, a PID control unit, a second comparison module, an LPD process module, and a measurement and transmission module. However, this patent does not monitor the solid wire-liquid melt pool-paste metal transformation process, making it impossible to identify the wire transition state and the melt pool flow state, and failing to solve the quality control problem in the wire-guided ultrasonic-assisted laser additive manufacturing process. Summary of the Invention

[0006] In view of the defects in the prior art, the purpose of the present invention is to provide a closed-loop control system and method for wire-guided ultrasonic-assisted laser additive manufacturing.

[0007] The closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing provided by the present invention comprises: a laser, a laser head, an image coaxial acquisition unit, an industrial camera unit, a wire front end centering unit, a wire transition monitoring unit, a molten pool flow monitoring unit, and an in-situ feedback control unit;

[0008] The laser beam emitted by the laser is transmitted to the laser head via an optical fiber, and then irradiated to the aluminum alloy substrate or deposited layer through the light outlet, while wire feeding is carried out simultaneously to generate a liquid aluminum alloy molten pool;

[0009] During the processing, the image coaxial acquisition unit is used to collect two-dimensional optical signals in the visible light band, which are then transmitted to the industrial camera unit. After photoelectric signal conversion, the data is finally transmitted to the computer for processing.

[0010] The wire front end centering unit aligns the center of the laser spot with the center of the wire front end, so that the laser beam accurately melts the wire;

[0011] The wire transition monitoring unit obtains the process of solid wire melting and transforming into liquid droplets or liquid bridges after the wire front end centering unit is accurately centered and emits light, and then monitors the wire transition state in real time;

[0012] The molten pool flow monitoring unit observes the flow of the liquid molten pool and the solidification process of the liquid molten pool while the wire transition monitoring unit is running, and then identifies the flow and solidification states of the molten pool;

[0013] The in-situ feedback control unit generates a feedback control strategy using the results of the wire transition monitoring unit and the molten pool flow monitoring unit to achieve closed-loop regulation.

[0014] Preferably, the wire front end centering unit comprises a laser beam center positioning module, a wire end position identification module and a wire end position control module;

[0015] The laser beam center positioning module determines the coordinates (x1, y1) of the laser spot center in the laser head image coaxial acquisition unit;

[0016] The wire end position recognition module determines the coordinates (x2, y2) of the wire front end in the laser head image coaxial acquisition unit;

[0017] The wire end position control module uses the coordinate data obtained by the laser beam center positioning module and the wire end position recognition module to perform position comparison, and then realizes the centering of the wire front end by moving the XY axis (x1-x2, y1-y2).

[0018] Preferably, the wire transition monitoring unit includes a wire image processing module and a transition state recognition module;

[0019] The wire image processing module acquires the wire melting moment features such as shape contour, oscillation frequency, oscillation amplitude and grayscale level through grayscale processing, filtering noise reduction and visual feature extraction;

[0020] The transition state identification module identifies and determines the transition state of the wire material based on the data obtained by the wire material image processing module.

[0021] Preferably, the molten pool flow monitoring unit includes a molten pool image processing module and a flow state recognition module;

[0022] The molten pool image processing module extracts the molten pool features of molten pool length, molten pool width, oscillation frequency and oscillation amplitude through grayscale processing, filtering noise reduction and visual feature extraction;

[0023] The flow state recognition module recognizes and determines the flow state of the molten pool based on the data obtained by the molten pool image processing module.

[0024] Preferably, the in-situ feedback control unit makes an overall judgment on the state of the wire-guided ultrasonic-assisted laser additive manufacturing and generates a decision plan based on the data obtained by the wire transition monitoring unit and the molten pool flow monitoring unit, thereby achieving the purpose of closed-loop feedback by in-situ controlling the process parameters;

[0025] The process parameters include laser power, scanning speed, wire feeding speed, ultrasonic frequency, ultrasonic amplitude, lifting amount and overlap amount.

[0026] Preferably, the laser includes a semiconductor laser or a Nd:YAG laser, and the laser is connected to the laser head by optical fiber connection; the industrial camera unit includes an ordinary industrial camera, a high dynamic industrial camera, an infrared industrial camera and a hyperspectral industrial camera.

[0027] The closed-loop control method for wire-guided ultrasonic-assisted laser additive manufacturing provided by the present invention comprises the following steps:

[0028] Step 1: Adjust the height h1 between the laser head and the substrate to meet the processing requirements of the initial layer, so that the industrial camera unit and the image coaxial acquisition unit can focus accurately;

[0029] Step 2: Turn on the pilot laser, adjust the exposure of the industrial camera unit, and calculate the pixel size m of the laser spot diameter according to the average value of the pixel length C and width D of the maximum outer rectangular contour, i.e., m = (C + D) / 2. Combine with the actual size n of the pilot laser spot diameter at the height h1 between the laser head and the substrate to determine the ratio of the pixel size to the actual size as m:n;

[0030] Step 3: Use the Otsu algorithm or the fixed threshold algorithm to perform binarization on the pilot laser spot image to obtain the center of the laser spot, i.e., the centroid position (x1, y1);

[0031] Step 4: Adjust the height h2 between the front end of the wire and the substrate, move the wire end into the field of view of the image coaxial acquisition unit, use the binarization method to extract the rectangular area of the wire in the acquired image, and calculate the center position coordinates (x2, y2) of the suspended end of the rectangular area, which is the position of the front end of the wire;

[0032] Step 5: With the height between the front end of the wire and the substrate fixed at h2, adjust the distance of the wire (x1 - x2, y1 - y2) to achieve the centering of the front end of the wire and the center of the laser spot;

[0033] Step 6: The laser starts to process, adjust the exposure and acquisition frame rate of the industrial camera unit, set a mask according to the wire profile and the molten pool profile. The mask is centered on the center of the laser spot with a radius R, where R1 < R < R2. Here, R1 is the radius of the wire, and R2 is half of the width of the molten pool. The area inside the mask is the image required by the wire transition monitoring unit, and the area outside the mask is the image required by the molten pool flow monitoring unit;

[0034] Step 7: Perform preprocessing and feature extraction on the image inside the mask. The preprocessing includes grayscale processing and image filtering for noise reduction, and the feature extraction includes the shape profile a1 when the wire transitions from solid to liquid, the vibration frequency a2 during the wire transition stage, the vibration amplitude a3 during the wire transition stage, and the gray level a4 during the wire transition stage;

[0035] Step 8: Based on the wire transition characteristics, identify the wire transition state. The specific state types include under - melting, droplet - like, liquid - bridge, and over - melting;

[0036] Step 9: Perform preprocessing and feature extraction on the image outside the mask. The preprocessing includes grayscale processing and image filtering for noise reduction, and the feature extraction includes the length l of the liquid metal molten pool, the width w of the molten pool, the vibration frequency b1 of the molten pool, and the vibration amplitude b2 of the molten pool; <0​​​​Step 11: Perform regulation by the in-situ feedback regulation unit in an in-situ manner. First, regulate the wire transition state to a liquid bridge, then regulate the molten pool flow state to be stable, and finally determine whether the ultrasonic effect meets the expectations.

[0039] Preferably, the identification of the wire transition state in step 8 includes:

[0040] Under-fusion: No obvious molten droplets and liquid bridges are formed. a1 is in a rectangular shape and has an obvious contour. The vibration frequency a2 ≤ α1 and the amplitude a3 ≤ β1, and the gray level a4 ≤ γ1;

[0041] Droplet: Obvious molten droplets are formed. a1 is in a nearly circular shape and has an obvious contour. The vibration frequency α1 < a2 ≤ α2 and the amplitude β1 < a3 ≤ β2, and the gray level γ1 < a4 ≤ γ2;

[0042] Liquid bridge: No rectangular contour and nearly circular molten droplets exist. a1 is in a fan shape and the contour feature is not obvious. The vibration frequency α1 < a2 ≤ α2 and the amplitude β1 < a3 ≤ β2, and the gray level γ1 < a4 ≤ γ2;

[0043] Over-fusion: No rectangular contour and nearly circular molten droplets exist. a1 is in a fan shape and the contour feature is not obvious. The vibration frequency α2 < a2 and the amplitude β2 < a3, and the gray level γ2 < a4;

[0044] α1, α2, β1, β2, γ1, γ2 are all wire transition state identification thresholds.

[0045] Preferably, the identification of the molten pool flow state in step 10 includes:

[0046] Unstable molten pool: Through the maximum circumscribed rectangle of the molten pool image, or the algorithm of the maximum length along the scanning direction and the maximum width perpendicular to the scanning direction of the molten pool, extract the length l and width w of the molten pool image, and then calculate the relative standard deviations l' and w' of the length and width:

[0047]

[0048]

[0049] When l' > l1 or w' > w1, it is determined that the molten pool is unstable; where l1 and w1 are molten pool flow state identification thresholds;

[0050] Stable molten pool: When l' ≤ l1 or w' ≤ w1, it is determined that the molten pool is stable;

[0051] The ultrasonic effect meets the expectations: After the molten pool is determined to be stable, perform the identification and judgment of the ultrasonic effect. Extract the centroid (x3, y3) of the liquid-solid solidification interface, and then calculate the centroid vibration frequency b1 and amplitude b2. The calculation formula for b1 is:

[0052]

[0053] The calculation formula for b2 is:

[0054]

[0055] Among them, B1 and B2 are process data, t is the vibration time, S is the area of ​​the extracted liquid-solid solidification interface. According to the preliminary experiment, the threshold ranges of b1 and b2 when the calibration meets the expected effect are [δ1, δ2] and [δ3, δ4] respectively. If b1 and b2 are both within this range, it is determined that the ultrasonic effect meets the expected requirements;

[0056] The ultrasonic effect does not meet expectations: If any value of b1 or b2 is not within the pre-calibrated ranges [δ1, δ2] and [δ3, δ4], the ultrasonic effect does not meet expectations.

[0057] Preferably, the step 11 includes:

[0058] Undermelting and overmelting: When undermelting and overmelting occur, the energy input density is adjusted by laser power or scanning speed. When undermelting occurs, the energy input density is increased, and when overmelting occurs, the energy input density is reduced.

[0059] Drop and liquid bridge: When the problem of under-melting and over-melting does not exist, the drop and liquid bridge state are regulated. If the drop state is present, the liquid bridge state is triggered by lowering the laser head height.

[0060] Whether the flow state is stable: In the liquid bridge state, if the molten pool flow state is unstable, increase the energy input; if it interferes with the overmelting control, reduce the wire feed amount; if the molten pool flow state is stable, enter the ultrasonic effect control;

[0061] Whether the ultrasonic effect meets expectations: After the molten pool is stable, the effect of the ultrasonic energy field is judged. The ultrasonic effect of the molten pool is adjusted by adjusting the frequency and amplitude of the ultrasonic energy field. The two are positively correlated.

[0062] Compared with the prior art, the present invention has the following beneficial effects:

[0063] (1) The present invention can perform real-time monitoring and closed-loop feedback regulation of the wire-guided ultrasonic-assisted laser additive manufacturing process. By monitoring the transformation process from solid wire to liquid molten pool to paste metal, the wire transition state and the molten pool flow state can be identified. At the same time, a closed-loop control method is provided to solve the quality control problem in the wire-guided ultrasonic-assisted laser additive manufacturing process.

[0064] (2) The present invention has a high degree of integration and strong adaptability. It can utilize the coaxial monitoring capability of the laser head, does not require the addition of any new hardware equipment, has a low cost, and can be implemented using a single industrial camera. Closed-loop control can be performed by relying on the coaxial image acquisition unit and the wire front end centering unit, the wire transition monitoring unit, the molten pool flow monitoring unit, and the in-situ feedback control unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0066] Figure 1 Schematic diagram of the coordinates of the wire front end centering unit, where 1 is the wire and 2 is the pilot laser spot;

[0067] Figure 2 Schematic diagram of the droplet transfer monitoring unit and the molten pool flow state monitoring unit, where 1 is the wire, 3 is the wire transition area, 4 is the molten pool area, and 5 is the liquid-solid solidification interface extraction position;

[0068] Figure 3 Schematic diagram of the closed-loop feedback system for wire-guided ultrasonic-assisted laser additive manufacturing;

[0069] Figure 4 Flowchart of the closed-loop feedback method for wire-guided ultrasonic-assisted laser additive manufacturing. DETAILED DESCRIPTION

[0070] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0071] Example 1:

[0072] The present invention provides a closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing, comprising: a laser, a laser head, an image coaxial acquisition unit, an industrial camera unit, a wire front end centering unit, a wire transition monitoring unit, a molten pool flow monitoring unit, and an in-situ feedback control unit; the laser emits a laser beam which is transmitted to the laser head via an optical fiber and then irradiated to an aluminum alloy substrate or a deposition layer from a light outlet, and wire is fed synchronously to generate a liquid aluminum alloy molten pool; during the processing, the image coaxial acquisition unit is used to acquire two-dimensional optical signals in the visible light band, which are transmitted to the industrial camera unit, and finally the data is transmitted to a computer for processing through photoelectric signal conversion; the wire front end centering unit aligns the center of the laser spot with the center of the wire front end, so that the laser beam accurately melts the wire, such as Figure 1The wire transition monitoring unit obtains the process of solid wire melting into liquid droplets or liquid bridges after the wire front end centering unit is accurately centered and emits light, and then monitors the wire transition state in real time; the molten pool flow monitoring unit observes the flow of the liquid molten pool and the solidification process of the liquid molten pool while the wire transition monitoring unit is running, and then identifies the flow and solidification state of the molten pool, such as Figure 2 The in-situ feedback control unit uses the results of the wire transition monitoring unit and the molten pool flow monitoring unit to generate a feedback control strategy to achieve closed-loop regulation, such as Figure 3 .

[0073] The wire front end centering unit includes a laser beam center positioning module, a wire end position identification module and a wire end position control module; the laser beam center positioning module determines the coordinates (x1, y1) of the laser spot center in the laser head image coaxial acquisition unit; the wire end position identification module determines the coordinates (x2, y2) of the wire front end in the laser head image coaxial acquisition unit; the wire end position control module uses the coordinate data obtained by the laser beam center positioning module and the wire end position identification module to perform position comparison, and then realizes the centering of the wire front end by moving the XY axis (x1-x2, y1-y2).

[0074] The wire transition monitoring unit includes a wire image processing module and a transition state recognition module; the wire image processing module acquires the wire melting moment characteristics of shape contour, oscillation frequency, oscillation amplitude and grayscale level through grayscale processing, filtering noise reduction and visual feature extraction; the transition state recognition module identifies and determines the wire transition state based on the data obtained by the wire image processing module.

[0075] The molten pool flow monitoring unit includes a molten pool image processing module and a flow state recognition module; the molten pool image processing module extracts the molten pool characteristics of molten pool length, molten pool width, oscillation frequency and oscillation amplitude through grayscale processing, filtering noise reduction and visual feature extraction; the flow state recognition module identifies and judges the molten pool flow state based on the data obtained by the molten pool image processing module.

[0076] The in-situ feedback control unit comprehensively assesses the state of the wire-guided ultrasonic-assisted laser additive manufacturing (ULAM) process based on data from the wire transition monitoring unit and the melt pool flow monitoring unit, generating a decision plan. This closed-loop feedback loop is achieved by in-situ controlling process parameters. These process parameters include laser power, scanning speed, wire feed speed, ultrasonic frequency, ultrasonic amplitude, lift, and overlap. The laser can be a semiconductor laser or an Nd:YAG laser, and the laser head is connected via fiber optics. The industrial camera unit includes standard industrial cameras, high-dynamic industrial cameras, infrared industrial cameras, and hyperspectral industrial cameras.

[0077] As Figure 4 , the present invention provides a closed-loop control method for wire-guided ultrasonic-assisted laser additive manufacturing, comprising the following steps:

[0078] Step 1: Adjust the height h1 between the laser head and the substrate to meet the processing requirements of the initial layer, so that the industrial camera unit cooperates with the image coaxial acquisition unit for precise focusing;

[0079] Step 2: Turn on the pilot laser, adjust the exposure of the industrial camera unit, calculate the pixel size m of the laser spot diameter according to the mean value of the pixel lengths C and widths D of the maximum outer rectangular contour, and combine the actual size n of the pilot laser spot diameter at the height h1 between the laser head and the substrate to determine the ratio of the pixel size to the actual size as m:n;

[0080] Step 3: Use the Otsu algorithm or the fixed threshold algorithm to perform binaryzation on the pilot laser spot image to obtain the center of the laser spot, i.e., the centroid position (x1, y1);

[0081] Step 4: Adjust the height h2 between the front end of the wire and the substrate, move the wire end into the field of view of the image coaxial acquisition unit, use the binaryzation method to extract the rectangular area of the wire in the acquired image, and calculate the center position coordinates (x2, y2) of the suspended end of the rectangular area, which is the position of the front end of the wire;

[0082] Step 5: With the height between the front end of the wire and the substrate fixed at h2, adjust the distance of the wire (x1 - x2, y1 - y2) to achieve the centering of the front end of the wire and the center of the laser spot;

[0083] Step 6: Turn on the laser for processing, adjust the exposure and acquisition frame rate of the industrial camera unit, set a mask according to the wire contour and the molten pool contour. The mask is centered on the center of the laser spot with a radius R, where R1 < R < R2. Here, R1 is the radius of the wire, and R2 is half of the width of the molten pool. The area inside the mask is the image required by the wire transition monitoring unit, and the area outside the mask is the image required by the molten pool flow monitoring unit;

[0084] Step 7: Perform preprocessing and feature extraction on the image inside the mask. The preprocessing includes gray processing and image filtering and noise reduction. The feature extraction includes the shape contour a1 when the wire transitions from the solid state to the liquid state, the vibration frequency a2 in the wire transition stage, the vibration amplitude a3 in the wire transition stage, and the gray level a4 in the wire transition stage;

[0085] Step 8: Based on the wire transition characteristics, identify the wire transition state. The specific state types include underfusion, droplet, liquid bridge, and overfusion;

[0086] Step 9: Preprocess and extract features from the image outside the mask. The preprocessing includes grayscale processing and image filtering for noise reduction. The feature extraction includes the length l of the liquid metal molten pool, the width w of the molten pool, the vibration frequency b1 of the molten pool, and the vibration amplitude b2 of the molten pool;

[0087] Step 10: Identify the flow state of the molten pool based on the flow characteristics of the molten pool. The specific state types include unstable molten pool, stable molten pool, and ultrasonic effect not meeting expectations;

[0088] Step 11: Use the in-situ feedback control unit to perform control in an in-situ manner. First, control the wire transfer state to be a liquid bridge, then control the flow state of the molten pool to be stable, and finally determine whether the ultrasonic effect meets expectations.

[0089] The wire transfer state identification in Step 8 includes:

[0090] Under-melting: No obvious molten droplets and liquid bridges are formed. a1 is in a rectangular shape and has an obvious contour. The vibration frequency a2 ≤ α1 and the amplitude a3 ≤ β1, and the gray level a4 ≤ γ1;

[0091] Droplet: Obvious molten droplets are formed. a1 is in a nearly circular shape and has an obvious contour. The vibration frequency α1 < a2 ≤ α2 and the amplitude β1 < a3 ≤ β2, and the gray level γ1 < a4 ≤ γ2;

[0092] Liquid bridge: No rectangular contour and nearly circular molten droplets exist. a1 is in a fan shape and the contour feature is not obvious. The vibration frequency α1 < a2 ≤ α2 and the amplitude β1 < a3 ≤ β2, and the gray level γ1 < a4 ≤ γ2;

[0093] Over-melting: No rectangular contour and nearly circular molten droplets exist. a1 is in a fan shape and the contour feature is not obvious. The vibration frequency α2 < a2 and the amplitude β2 < a3, and the gray level γ2 < a4;

[0094] α1, α2, β1, β2, γ1, and γ2 are all thresholds for wire transfer state identification.

[0095] The molten pool flow state identification in Step 10 includes:

[0096] Unstable molten pool: Through the maximum bounding rectangle of the molten pool image, or the algorithm of the maximum length along the scanning direction and the maximum width perpendicular to the scanning direction of the molten pool, extract the length l and width w of the molten pool image, and then calculate the relative standard deviations l' and w' of the length and width: <​​​​​​​When l'>l1 or w'>w1, the molten pool is determined to be unstable; where l1 and w1 are the thresholds for identifying the flow state of the molten pool;

[0100] Molten pool stability: When l'≤l1 or w'≤w1, the molten pool is considered stable;

[0101] The ultrasonic effect is as expected: After the molten pool is judged to be stable, the ultrasonic effect is identified and judged, the center of mass of the liquid-solid solidification interface (x3, y3) is extracted, and then the center of mass vibration frequency b1 and amplitude b2 are calculated. The calculation formula of b1 is:

[0102]

[0103] The calculation formula for b2 is:

[0104]

[0105] Among them, B1 and B2 are process data, t is the vibration time, S is the area of ​​the extracted liquid-solid solidification interface. According to the preliminary experiment, the threshold ranges of b1 and b2 when the calibration meets the expected effect are [δ1, δ2] and [δ3, δ4] respectively. If b1 and b2 are both within this range, it is determined that the ultrasonic effect meets the expected requirements;

[0106] The ultrasonic effect does not meet expectations: If any value of b1 or b2 is not within the pre-calibrated ranges [δ1, δ2] and [δ3, δ4], the ultrasonic effect does not meet expectations.

[0107] The step 11 comprises:

[0108] Undermelting and overmelting: When undermelting and overmelting occur, the energy input density is adjusted by laser power or scanning speed. When undermelting occurs, the energy input density is increased, and when overmelting occurs, the energy input density is reduced.

[0109] Drop and liquid bridge: When the problem of under-melting and over-melting does not exist, the drop and liquid bridge state are regulated. If the drop state is present, the liquid bridge state is triggered by lowering the laser head height.

[0110] Whether the flow state is stable: In the liquid bridge state, if the molten pool flow state is unstable, increase the energy input; if it interferes with the overmelting control, reduce the wire feed amount; if the molten pool flow state is stable, enter the ultrasonic effect control;

[0111] Whether the ultrasonic effect meets expectations: After the molten pool is stable, the effect of the ultrasonic energy field is judged. The ultrasonic effect of the molten pool is adjusted by adjusting the frequency and amplitude of the ultrasonic energy field. The two are positively correlated.

[0112] Example 2:

[0113] Example 2 is a preferred example of Example 1.

[0114] This example uses an AlSi7Mg substrate with dimensions of 145 mm × 145 mm × 15 mm. The wire is AlSi7Mg containing 2% in-situ TiB2 and has a diameter of 1 mm. The laser is a 450 nm blue laser, the displacement device is a 3-axis stage, the optical fiber has a diameter of 1 mm, and the industrial camera is a standard industrial camera. The closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing includes a laser, a laser head, a displacement device, a wire feeder, an ultrasonic assist device, a coaxial image acquisition unit, an industrial camera unit, a wire front-end centering unit, a wire transition monitoring unit, a melt pool flow monitoring unit, and an in-situ feedback control unit.

[0115] The laser beam is transmitted via optical fiber to the laser head, and then from the light outlet to the aluminum alloy substrate or deposited layer. The laser spot diameter is 2mm, and wire feeding is performed simultaneously to generate a molten pool of liquid aluminum alloy. During this process, a coaxial image acquisition unit is used to collect two-dimensional optical signals in the visible light band. These signals are transmitted to a standard industrial camera in the industrial camera unit. After photoelectric signal conversion, the data is ultimately transmitted to the computer's online image processing unit. The image is 400×400 pixels, with each pixel being 8 bits and 256 grayscale levels.

[0116] The specific process parameters are as follows: laser power of 1000W, scanning speed of 5mm / s, wire feeding rate of 3mm / s, scanning length of 50mm, lifting amount of each layer of 0.4mm, dwell time at both ends of 0s, ultrasonic frequency of 20kHz, and ultrasonic amplitude of 5μm.

[0117] The wire front end centering unit, wire transition monitoring unit, melt pool flow monitoring unit, and in-situ feedback control unit are integrated into a workstation. The wire front end centering unit can align the center of the laser beam with the wire front end to ensure subsequent processing. After the centering is completed, the wire transition monitoring unit can determine whether the wire transition state is under-melted, over-melted, dripping, or liquid bridge; the melt pool flow monitoring unit can determine the stability of the melt pool and the effect of the ultrasonic energy field. The closed-loop feedback control unit can make a comprehensive decision based on the data from the wire transition monitoring unit and the melt pool flow monitoring unit and the current processing status, and then execute the control.

[0118] The wire transition monitoring unit consists of a wire image processing module and a transition state recognition module. The wire image processing module acquires wire melting characteristics such as shape contour, oscillation frequency, and grayscale level through grayscale processing, filtering and noise reduction, and visual feature extraction. The transition state recognition module identifies and determines the wire transition state based on the data from the wire image processing module.

[0119] The melt pool flow monitoring unit includes a melt pool image processing module and a flow state recognition module. The melt pool image processing module extracts melt pool features such as shape contour, oscillation frequency, and grayscale level through grayscale processing, filtering and noise reduction, and visual feature extraction. The flow state recognition module identifies and determines the melt pool flow state based on the data from the melt pool image processing module.

[0120] In this embodiment, the process parameters that can be adjusted in the in-situ feedback control unit mainly include laser power, scanning speed, wire feeding speed, ultrasonic frequency, ultrasonic amplitude and lifting amount.

[0121] This embodiment provides a closed-loop control method for wire-guided ultrasound-assisted laser additive manufacturing, which uses the aforementioned closed-loop control system for wire-guided ultrasound-assisted laser additive manufacturing and includes the following steps:

[0122] (1) Initial position adjustment. According to the characteristics of the equipment, the height between the laser head and the substrate is adjusted to 11 mm. This meets the requirements for initial layer processing and also enables the industrial camera based on the image coaxial acquisition unit to focus accurately.

[0123] (2) Pilot laser light emission and size calibration. This is done with the assistance of the laser beam center positioning module. The pilot laser is turned on, and the exposure of the industrial camera unit is adjusted. Based on the average of the pixel length C and width D of the largest outer rectangular outline, the laser spot diameter pixel size m = (154 + 150) / 2 = 152 pixels is calculated. Combined with the actual size of the pilot laser spot diameter of 2 mm when the height between the laser head and the substrate is 11 mm, the ratio of pixel size to actual size is determined to be 152:2, or 76:1.

[0124] (3) Identification of the center position of the laser spot. This is accomplished by the laser beam center positioning module. The Otsu algorithm is used to binarize the pilot laser spot image, and the center of the laser spot, i.e., the centroid, is obtained as (212.14, 230.56).

[0125] (4) Wire end coordinate identification. This is accomplished by the wire end position identification module. The height of the wire tip from the substrate is adjusted to 4 mm. This parameter is predetermined by the process optimization results. The wire tip is then moved to the field of view of the image coaxial acquisition unit. The rectangular area of ​​the wire in the acquired image is extracted using a binarization method. The coordinates of the center position of the suspended end of the rectangular area are calculated to be (180.22, 225.79), which is the position of the wire tip.

[0126] (5) Wire end position adjustment. The wire end position adjustment is performed by the wire end position control module of the wire front end centering unit. When the height of the wire front end from the substrate is fixed at 4 mm, the wire electric displacement device automatically adjusts the distance (31.92, 4.77) to achieve the centering of the wire front end and the center of the laser spot.

[0127] (6) Image acquisition during laser processing. Laser processing emits light, adjusts the exposure and acquisition frame rate of the industrial camera unit, and sets a mask based on the wire contour and the molten pool contour. The mask is centered at the center of the laser spot and has a radius of 1.5 mm. The image inside the mask is required by the wire transition monitoring unit, and the image outside the mask is required by the molten pool flow monitoring unit.

[0128] (7) Wire transition image processing. The wire image processing module of the wire transition monitoring unit is used to preprocess and extract features from the image within the mask. Preprocessing includes grayscale processing and image filtering and noise reduction. Feature extraction includes the shape contour a1 of the wire when it transitions from solid to liquid, the vibration frequency a2 of the wire during the transition phase, the vibration amplitude a3 of the wire during the transition phase, and the grayscale level a4 of the wire during the transition phase.

[0129] (8) Wire transition state identification. The threshold ranges of various parameters are listed based on preliminary experiments, see Table 1.

[0130] Table 1 Wire transition state recognition threshold

[0131] parameter Numerical parameter Numerical <![CDATA[α1]]> 5Hz <![CDATA[β2]]> 0.015 <![CDATA[α2]]> 25Hz <![CDATA[γ1]]> 20 <![CDATA[β1]]> 0.005 <![CDATA[γ2]]> 100

[0132] Accordingly, the average monitoring data and identification results of layers 1 to 5 can be listed (see Table 2).

[0133] Table 2 Identification results of transition states of wire layers 1 to 5

[0134] Number of layers <![CDATA[a1]]> <![CDATA[a2]]> <![CDATA[a3]]> <![CDATA[a4]]> 1 sector 8Hz 0.11 33 2 sector 12Hz 0.12 40 3 sector 15Hz 0.10 47 4 sector 16Hz 0.09 60 5 Nearly round 20Hz 0.12 65

[0135] Therefore, it can be found that in this embodiment, the first to fourth layers are liquid bridge transitions, and the fifth layer is a drop-shaped transition.

[0136] (9) Molten pool flow image processing. The molten pool image processing module of the molten pool flow monitoring unit is used to preprocess and extract features from the image outside the mask. Preprocessing includes grayscale processing and image filtering and noise reduction. Feature extraction includes the length l and width w of the liquid metal molten pool, the vibration frequency b1 of the molten pool, and the vibration amplitude b2 of the molten pool.

[0137] (10) Identification of melt pool flow state. Threshold values ​​are shown in Table 3.

[0138] Table 3 Melt pool flow state identification threshold

[0139] parameter Numerical parameter Numerical <![CDATA[l1]]> 0.020 <![CDATA[δ2]]> 80 <![CDATA[w1]]> 0.050 <![CDATA[δ3]]> 0.006 <![CDATA[δ1]]> 60 <![CDATA[δ4]]> 0.012

[0140] The results of layers 1 to 4 are shown in Table 4.

[0141] Table 4 Identification results of melt pool flow states from the 1st to the 4th layer

[0142] Number of layers l’ w’ <![CDATA[b1]]> <![CDATA[b2]]> 1 0.005 0.022 71 0.008 2 0.012 0.039 62 0.009 3 0.011 0.045 65 0.009 4 0.015 0.040 70 0.010

[0143] The fifth layer is not considered because the wire transition is already drop-shaped. It can be judged that the ultrasonic effects of the first to fourth layers are in line with expectations.

[0144] (11) In-situ control decision-making and execution. To address the droplet transition issue in the fifth layer, the lift was reduced by 0.1 mm in real time to transform it into a liquid bridge transition. After adjustment, l', w', b1, and b2 in the fifth layer were 0.17, 0.45, 78, and 0.09, respectively. At this point, the flow state was stable and the ultrasonic effect met expectations, so no further control was performed.

[0145] This embodiment is applicable to blue laser processing of AlSi7Mg substrates and AlSi7Mg-2%TiB2 wires, with the substrate being horizontal and perpendicular to the laser head. This embodiment provides a closed-loop control system and method for wire-guided ultrasonic-assisted laser additive manufacturing based on different images, and clarifies the threshold range for identifying features under these conditions.

[0146] In the description of this application, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application.

[0147] Those skilled in the art will appreciate that, in addition to implementing the system, device, and various modules provided by the present invention in purely computer-readable program code, it is entirely possible to implement the same program in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, embedded microcontrollers, and the like by logically programming the method steps. Therefore, the system, device, and various modules provided by the present invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; the modules for implementing various functions can also be considered both software programs for implementing the method and structures within the hardware component.

[0148] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing, characterized in that: include: Laser, laser head, image coaxial acquisition unit, industrial camera unit, wire front end centering unit, wire transition monitoring unit, molten pool flow monitoring unit and in-situ feedback control unit; The laser beam emitted by the laser is transmitted to the laser head via an optical fiber, and then irradiated to the aluminum alloy substrate or deposited layer through the light outlet, while wire feeding is carried out simultaneously to generate a liquid aluminum alloy molten pool; During the processing, the image coaxial acquisition unit is used to collect two-dimensional optical signals in the visible light band, which are then transmitted to the industrial camera unit. After photoelectric signal conversion, the data is finally transmitted to the computer for processing. The wire front end centering unit aligns the center of the laser spot with the center of the wire front end, so that the laser beam accurately melts the wire; The wire transition monitoring unit obtains the process of solid wire melting and transforming into liquid droplets or liquid bridges after the wire front end centering unit is accurately centered and emits light, and then monitors the wire transition state in real time; The molten pool flow monitoring unit observes the flow of the liquid molten pool and the solidification process of the liquid molten pool while the wire transition monitoring unit is running, and then identifies the flow and solidification states of the molten pool; The in-situ feedback control unit generates a feedback control strategy using the results of the wire transition monitoring unit and the molten pool flow monitoring unit to achieve closed-loop regulation.

2. The closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 1, characterized in that: The wire front end centering unit includes a laser beam center positioning module, a wire end position recognition module and a wire end position control module; The laser beam center positioning module determines the coordinates (x1, y1) of the laser spot center in the laser head image coaxial acquisition unit; The wire end position recognition module determines the coordinates (x2, y2) of the wire front end in the laser head image coaxial acquisition unit; The wire end position control module uses the coordinate data obtained by the laser beam center positioning module and the wire end position recognition module to perform position comparison, and then realizes the centering of the wire front end by moving the XY axis (x1-x2, y1-y2).

3. The closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 1, characterized in that: The wire transition monitoring unit includes a wire image processing module and a transition state recognition module; The wire image processing module acquires the wire melting moment features such as shape contour, oscillation frequency, oscillation amplitude and grayscale level through grayscale processing, filtering noise reduction and visual feature extraction; The transition state identification module identifies and determines the transition state of the wire material based on the data obtained by the wire material image processing module.

4. The closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 1, characterized in that: The molten pool flow monitoring unit includes a molten pool image processing module and a flow state recognition module; The molten pool image processing module extracts the molten pool features of molten pool length, molten pool width, oscillation frequency and oscillation amplitude through grayscale processing, filtering noise reduction and visual feature extraction; The flow state recognition module recognizes and determines the flow state of the molten pool based on the data obtained by the molten pool image processing module.

5. The closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 1, characterized in that: The in-situ feedback control unit makes an overall judgment on the state of the wire-guided ultrasonic-assisted laser additive manufacturing and generates a decision plan based on the data obtained by the wire transition monitoring unit and the molten pool flow monitoring unit, thereby achieving the purpose of closed-loop feedback by in-situ controlling the process parameters; The process parameters include laser power, scanning speed, wire feeding speed, ultrasonic frequency, ultrasonic amplitude, lift amount and overlap amount.

6. The closed-loop control system for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 1, characterized in that: The laser includes a semiconductor laser or a Nd:YAG laser, and the connection mode between the laser and the laser head is fiber connection; the industrial camera unit includes an ordinary industrial camera, a high-dynamic industrial camera, an infrared industrial camera and a hyperspectral industrial camera.

7. A closed-loop control method for wire-guided ultrasonic-assisted laser additive manufacturing, characterized in that: Using the wire-guided ultrasonic-assisted laser additive manufacturing closed-loop control system according to any one of claims 1-6, comprising the following steps: Step 1: Adjust the height h1 between the laser head and the substrate to meet the processing requirements of the initial layer, so that the industrial camera unit cooperates with the image coaxial acquisition unit to achieve precise focusing. Step 2: Turn on the pilot laser, adjust the exposure of the industrial camera unit, and calculate the pixel size m of the laser spot diameter according to the mean value of the pixel length C and width D of the maximum outer rectangular contour, i.e., m=(C + D) / 2. Combine with the actual size n of the pilot laser spot diameter at the height h1 between the laser head and the substrate to determine the ratio of the pixel size to the actual size as m:n. Step 3: Use the Otsu algorithm or the fixed threshold algorithm to perform binarization on the pilot laser spot image to obtain the center of the laser spot, i.e., the centroid position (x1, y1). Step 4: Adjust the height h2 between the front end of the wire and the substrate, move the wire end into the field of view of the image coaxial acquisition unit, use the binarization method to extract the rectangular area of the wire in the acquired image, and calculate the center position coordinates (x2, y2) of the suspended end of the rectangular area, which is the position of the front end of the wire. Step 5: With the height between the front end of the wire and the substrate fixed at h2, adjust the distance of the wire (x1 - x2, y1 - y2) to achieve the centering of the front end of the wire and the center of the laser spot. Step 6: The laser starts to emit light, adjust the exposure and acquisition frame rate of the industrial camera unit, and set a mask according to the wire profile and the molten pool profile. The mask is centered on the center of the laser spot with a radius R, where R1 < R < R2. Here, R1 is the radius of the wire, and R2 is half of the width of the molten pool. The area inside the mask is the image required by the wire transition monitoring unit, and the area outside the mask is the image required by the molten pool flow monitoring unit. Step 7: Perform preprocessing and feature extraction on the image inside the mask. The preprocessing includes grayscale processing and image filtering for noise reduction. The feature extraction includes the shape contour a1 when the wire transitions from solid to liquid, the vibration frequency a2 during the wire transition stage, the vibration amplitude a3 during the wire transition stage, and the gray level a4 during the wire transition stage. Step 8: Based on the wire transition characteristics, identify the wire transition state. The specific state types include underfusion, droplet, liquid bridge and overfusion. Step 9: Perform preprocessing and feature extraction on the image outside the mask. The preprocessing includes grayscale processing and image filtering for noise reduction. The feature extraction includes the length l of the liquid metal molten pool, the width w of the molten pool, the vibration frequency b1 of the molten pool and the vibration amplitude b2 of the molten pool. Step 10: Based on the molten pool flow characteristics, identify the molten pool flow state. The specific state types include unstable molten pool, stable molten pool and ultrasonic effect not meeting expectations. Step 11: Regulation is carried out by the in-situ feedback regulation unit in an in-situ manner. First, regulate the wire transition state to a liquid bridge, then regulate the molten pool flow state to be stable, and finally determine whether the ultrasonic effect meets the expectations.

8. The closed-loop control method for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 7, characterized in that: The identification of the wire transition state in Step 8 includes: Under-melting: No obvious molten droplets and liquid bridges are formed. a1 is in a rectangular shape and has an obvious contour. The vibration frequency a2 ≤ α1 and the amplitude a3 ≤ β1, and the gray level a4 ≤ γ1; Droplet: Obvious molten droplets are formed. a1 is in a nearly circular shape and has an obvious contour. The vibration frequency α1 < a2 ≤ α2 and the amplitude β1 < a3 ≤ β2, and the gray level γ1 < a4 ≤ γ2; Liquid bridge: There is no rectangular contour and nearly circular molten droplets. a1 is in a fan shape and the contour feature is not obvious. The vibration frequency α1 < a2 ≤ α2 and the amplitude β1 < a3 ≤ β2, and the gray level γ1 < a4 ≤ γ2; Over-melting: There is no rectangular contour and nearly circular molten droplets. a1 is in a fan shape and the contour feature is not obvious. The vibration frequency α2 < a2 and the amplitude β2 < a3, and the gray level γ2 < a4; α1, α2, β1, β2, γ1, γ2 are all thresholds for identifying the wire transition state.

9. The closed-loop control method for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 7, characterized in that: The identification of the molten pool flow state in Step 10 includes: Unstable molten pool: Through the maximum circumscribed rectangle of the molten pool image, or the algorithm of the maximum length along the scanning direction and the maximum width perpendicular to the scanning direction of the molten pool, extract the length l and width w of the molten pool image, and then calculate the relative standard deviations l' and w' of the length and width: When l' > l1 or w' > w1, it is determined that the molten pool is unstable; where, l1 and w1 are thresholds for identifying the molten pool flow state; Stable molten pool: When l' ≤ l1 or w' ≤ w1, it is determined that the molten pool is stable; The ultrasonic effect meets the expectations: After the molten pool is determined to be stable, the identification and judgment of the ultrasonic effect are carried out. Extract the centroid (x3, y3) of the liquid-solid solidification interface, and then calculate the centroid vibration frequency b1 and amplitude b2. The calculation formula for b1 is: The calculation formula for b2 is: Where, B1, B2 are process data, t is the vibration time, S is the area of the extracted liquid-solid solidification interface. According to the preliminary experiment, the threshold ranges of b1 and b2 for meeting the expected effect are [δ1, δ2] and [δ3, δ4] respectively. If both b1 and b2 are within this range, it is determined that the ultrasonic effect meets the expected requirements; The ultrasonic effect does not meet the expectations: If any value of b1 and b2 is not within the pre-calibrated ranges [δ1, δ2] and [δ3, δ4], then the ultrasonic effect does not meet the expectations.

10. The closed-loop control method for wire-guided ultrasonic-assisted laser additive manufacturing according to claim 7, characterized in that: Step 11 includes: Under-melting and over-melting: When under-melting and over-melting occur, adjust the energy input density by laser power or scanning speed. When under-melting, increase the energy input density, and when over-melting, reduce the energy input density; Droplet and liquid bridge: When it is regulated to eliminate the problems of under-melting and over-melting, carry out the regulation of the droplet and liquid bridge states. If it is in the droplet state, trigger the liquid bridge state by lowering the laser head height; Whether the flow state is stable: In the liquid bridge state, if the molten pool flow state is unstable, increase the energy input; if it interferes with the over-melting regulation, reduce the wire feeding amount; if the molten pool flow state is stable, enter the ultrasonic effect adjustment; Whether the ultrasonic effect meets expectations: After the molten pool is stable, the effect of the ultrasonic energy field is judged. The ultrasonic effect of the molten pool is adjusted by adjusting the frequency and amplitude of the ultrasonic energy field. The two are positively correlated.

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