Method for repairing a large mold

Abstract

The application discloses a crack repair method for a large mold, and belongs to the technical field of mold repair, and comprises the following contents: S1, stress field evaluation and repair; S2, synergistic energy field stress regulation pretreatment: based on a residual stress distribution cloud picture, local cryogenic treatment and pulse current composite energy field treatment are applied to a repair area; S3, low-stress damage removal: in the area treated in the step S2, cracks are removed by mechanical processing, and a geometric bevel with a mechanical interlocking effect is formed; S4, gradient welding repair; S5, zoned intelligent aging post-treatment; S6, adaptive processing and performance recovery; through the whole-process stress management system of stress field evaluation-composite energy field pretreatment-zoned intelligent aging, the application is specially aimed at the characteristics of complex structure and uneven stress distribution of the large mold, effectively releases deep residual stress, and solves the problem of secondary cracks caused by superimposed thermal stress in the traditional repair method.

Description

Technical Field

[0001] This invention belongs to the field of mold repair technology, specifically relating to a method for repairing cracks in large molds. Background Technology

[0002] Unlike the shallow thermal fatigue cracks in small or conventional molds, the cracks in large die-casting molds exhibit significant macroscopic, structural, and high-stress background characteristics. Specifically, the cracks are often deep-seated, widely extending network-like structures. This is not merely surface damage but a result of the release of enormous residual stress within the mold, often accompanied by imperceptible structural deformation, transforming it from a surface quality issue into a fundamental problem threatening the overall structural integrity. Furthermore, the complex structure of large die-cast parts, with insert surfaces filled with deep ribs, thin ribs, and complex curves, causes cracks to be distributed in areas with extremely poor machining accessibility, further increasing the difficulty of repair.

[0003] Faced with such defects, the main repair strategies currently used in the industry have significant limitations. Firstly, there is the method of large-scale cutting and replacement, which involves removing the entire cracked area and fabricating a new small insert to match it. This method introduces new stress concentration points due to secondary processing, and it is difficult to guarantee the fitting accuracy of the large insert and the small insert on complex surfaces, easily leading to flash or erosion during use. Secondly, there is the use of laser cladding technology. Although it can achieve local repair, the extremely high-grade base material can easily lead to weak bonding interface performance due to the chemical composition of the repair powder, resulting in cracking again during subsequent use. Furthermore, the high heat input of the laser cladding process itself adds new thermal stress to the existing high-stress background, easily inducing secondary cracks or causing workpiece deformation. Moreover, this process is complex, with intricate parameter systems, heavily reliant on operator experience, and it is difficult to guarantee consistent quality on complex surfaces.

[0004] Therefore, there is a need for a crack repair method that can efficiently and reliably treat deep stress cracks. Summary of the Invention

[0005] To address the aforementioned problems in the existing technology, this invention provides a method for repairing cracks in large molds, solving the problems of internal stress and surface cracks in large die-casting molds.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for repairing cracks in a large mold, further comprising the following: S1, stress field assessment and repair: using three-dimensional scanning and non-destructive stress detection to obtain a three-dimensional cloud map of the cracks on the mold surface and the residual stress distribution within the repair area;

[0008] S2. Synergistic energy field stress regulation pretreatment: Based on the residual stress distribution cloud map, apply local deep cooling and pulsed current composite energy field treatment to the repair area;

[0009] S3. Low-stress damage removal: In the area treated in step S2, the cracks are removed by mechanical processing, and a geometric bevel with mechanical interlocking effect is formed.

[0010] S4. Gradient welding repair: Using a low heat input welding process, at least three layers of functional material with a gradient transition to the base material are sequentially clad in the groove, and each clad layer is supplemented with high-energy sound beam impact treatment.

[0011] S5. Partitioned intelligent aging post-treatment: Based on the stress distribution cloud map obtained in step S1, the repair area is subjected to partitioned temperature-controlled aging heat treatment that combines local and overall methods to simultaneously optimize the comprehensive performance of the repair body and the substrate.

[0012] S6. Adaptive machining and performance restoration: On a CNC machine tool, the repair area is precision machined to the preset size based on the three-dimensional data obtained in step S1, and the surface is strengthened.

[0013] Preferably, the specific process of the local deep cooling and pulsed current composite energy field treatment in step S2 is as follows: the mold is preheated to 80-120°C, liquid nitrogen is used to perform local deep cooling on the preheated repair area and high stress area, and then a high-frequency pulsed current is applied to make the deep-cooled area rapidly and uniformly heated to 300-400°C under the Joule heating effect and kept at that temperature.

[0014] Preferably, the composite geometric bevel described in step S3 has a rounded bottom transition and a periodically changing wave-shaped structure on the sidewalls.

[0015] Preferably, the at least three functional materials that have a gradient transition with the matrix material in step S4 include: a transition layer whose chemical composition is similar to that of the mold matrix material to ensure good metallurgical bonding with the matrix; an intermediate layer whose strength is higher than that of the transition layer and whose toughness is better than that of the working surface layer, used for stress buffering and inhibiting crack propagation; and a working surface layer whose high-temperature strength, red hardness and thermal fatigue resistance are better than those of the matrix material to restore and improve the service performance of the mold surface.

[0016] Preferably, the low heat input welding process in step S4 is cold metal transfer technology or ultra-high frequency pulsed arc welding; the high-energy acoustic beam impact treatment is ultrasonic impact or laser impact.

[0017] Preferably, the partitioned temperature-controlled aging heat treatment in step S5 is specifically performed as follows: after rapidly and accurately heating the repair area to the tempering temperature using an infrared heating array and holding it at that temperature, multiple sets of independently temperature-controlled cooling nozzles are used to apply differentiated cooling rates to the repair area and surrounding stress blocks according to the stress distribution cloud map.

[0018] Preferably, the non-destructive residual stress detection method described in step S1 is X-ray diffraction or ultrasonic method.

[0019] Preferably, the surface strengthening treatment in step S6 is ultrasonic nanocrystal surface modification treatment or physical vapor deposition to prepare a wear-resistant coating.

[0020] Preferably, during the cladding of each layer of functional material in step S4, an infrared thermal imager is used to simultaneously monitor the temperature field distribution of the molten pool and the heat-affected zone, and the real-time temperature data is fed back to the welding power supply control system to achieve dynamic closed-loop control of welding heat input.

[0021] The beneficial effects of this invention are as follows:

[0022] This invention employs a comprehensive stress management system encompassing stress field assessment, composite energy field pretreatment, and zoned intelligent aging. Specifically designed for the complex structures and uneven stress distribution of large molds, it effectively releases deep residual stress, precisely controls the heat treatment process, avoids stress concentration within large components, and significantly suppresses the accumulation of new thermal stress during laser cladding. This fundamentally solves the problem of secondary cracks caused by superimposed thermal stress in traditional repair methods, greatly improving the structural integrity and service life of repaired large molds. Attached Figure Description

[0023] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0024] Figure 1 This is a flowchart of a repair method provided in one embodiment of the present invention; Detailed Implementation

[0025] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided.

[0026] like Figure 1As shown, a method for repairing cracks in a large mold also includes the following: S1, stress field assessment and repair: First, a three-dimensional digital model of the cracked surface is accurately obtained using a three-dimensional laser scanner to locate all crack distributions; then, an X-ray diffraction stress analyzer is used to scan a wide area centered on the crack, measure and generate a distribution cloud map of the magnitude and direction of residual stress in the area, accurately lock the high tensile stress concentration area, and provide precise targeting for subsequent stress control;

[0027] S2. Synergistic Energy Field Stress Control Pretreatment: Based on the stress cloud map of S1, the high tensile stress region is first subjected to deep cryogenic treatment with liquid nitrogen to transform the residual austenite into martensite and induce microscopic plastic deformation, thus initially homogenizing and releasing stress. Subsequently, a high-density pulsed current is applied, utilizing its electroplastic effect to promote dislocation slip and atomic diffusion, further reducing the macroscopic residual stress in the processed area and creating a stable stress environment for subsequent low-stress removal. Single energy field treatment often has limited or uneven effects; cryogenic treatment mainly acts on material phase transformation and the microscopic scale, while pulsed current relaxes stress at both the macroscopic and microscopic scales through Joule heating and non-thermal effects. The synergy of the two achieves multi-scale stress relaxation from micro to macro and from surface to interior, efficiently and deeply "taming" internal stress, avoiding the limitations of single methods.

[0028] S3. Low-stress damage removal: In areas where stress is significantly reduced after energy field pretreatment, a high-precision engraving and milling machine is used to gradually remove cracked material in micron-level layered milling. During the processing, a strategy of small depth of cut, fast feed, and sufficient cooling is adopted to minimize the generation of new cutting stress.

[0029] S4. Gradient Welding Repair: Cold metal transfer cladding is used, with a heat input far lower than traditional laser cladding. First, a transition layer is clad with a material similar in composition to the H13 steel substrate but with better toughness. Second, an intermediate layer is clad, with hardness and strength between the transition and surface layers. Finally, a layer with excellent red hardness and wear resistance is clad. After each layer is clad, a high-energy ultrasonic impact device is immediately used to impact the surface of the clad layer, causing plastic deformation and generating compressive stress. The material gradient design in this embodiment avoids abrupt performance changes, allowing for a smooth transition in thermal expansion coefficients and mechanical properties. This effectively suppresses interface stress concentration caused by material mismatch. The CMT low heat input process reduces the heat-affected zone and thermal stress from the source. Most importantly, the high-energy ultrasonic beam impact applied to each layer is equivalent to performing "online micro-forging" and stress modification in each layer, not only refining the grains but also instantly converting tensile stress within the layer into compressive stress, achieving "cladding one layer, stabilizing one layer."

[0030] S5. Zoned Intelligent Aging Post-treatment: The mold is placed in an intelligent aging furnace. Based on the initial stress cloud map in S1, the core repair area is subjected to enhanced aging at a higher temperature, the surrounding transition area is subjected to stable aging at a medium to low temperature, and the entire mold is subjected to low-temperature stress-relieving aging. Through this zoned temperature control strategy, the performance of the repair body, the stability of the bonding interface, and the further elimination of residual stress in the mold are achieved simultaneously. Traditional heat treatment heats the workpiece uniformly as a whole, which cannot take into account the differentiated local needs. The material state and stress level of the repair area, bonding area, and matrix area are completely different, and the temperature requirements for heat treatment are also different. This method applies different thermal processes to different areas, thereby simultaneously achieving multiple goals: performance improvement of the repair area, interface diffusion strengthening, and overall stress relaxation.

[0031] S6. Adaptive Machining and Performance Restoration: The three-dimensional data obtained in S1 is imported into a five-axis CNC machine tool. The machine tool automatically generates the machining path of the repair area based on this original model data, realizing digital adaptive precision machining and accurately restoring the surface to the preset size. After machining, the repaired surface is treated with micro-shot blasting or laser shock strengthening to introduce surface compressive stress, which significantly improves its resistance to thermal fatigue and melting.

[0032] In summary, this embodiment uses the digital parameters scanned before repair to directly drive the CNC machine tool, ensuring a seamless geometric connection between the repaired area and the original surface. In addition, by coupling finishing and surface strengthening treatment, it is no longer a simple restoration of dimensions, but an active introduction of a beneficial compressive stress layer on the surface, which not only repairs the damage, but also significantly improves the future service performance and service life of the area.

[0033] In one embodiment, the specific process of the combined local cryogenic and pulsed current energy field treatment in step S2 is as follows: Directly subjecting a large mold to local cryogenic treatment can lead to a huge instantaneous temperature difference, generating thermal shock and posing a risk of cracking. By preheating the entire mold to a moderate temperature beforehand, the temperature difference between the mold core and the area to be treated is reduced, and the material's plasticity is increased. This provides a safe and uniform mechanical state background for subsequent cryogenic treatment, effectively avoiding accidental damage caused by thermal shock. This is the fundamental guarantee for achieving subsequent drastic temperature changes.

[0034] Furthermore, rapidly cooling a specific region to ultra-low temperatures not only causes the retained austenite to further transform into martensite, resulting in volume expansion that dissipates some stress, but more importantly, the extreme temperature gradient induces non-uniform thermal contraction. This forces plastic flow changes in the crystal lattice at the microscale, effectively releasing and homogenizing the macroscopically concentrated residual tensile stress through microscopic plastic deformation. This method of controlling stress using physical phase transformation and microscopic yielding is more profound and thorough than simple mechanical vibration or thermal aging.

[0035] Finally, the high-density electron flow generated by the pulsed current significantly reduces the bonding force between atoms, producing an "electroplastic effect," which greatly promotes atomic diffusion and dislocation movement, thus significantly lowering the activation energy for stress relaxation. Simultaneously, the rapid and uniform heating method from the inside out, provided by Joule heating, avoids the internal and external temperature differences and new thermal stresses caused by the lag in heat conduction in traditional external heating. This creates excellent thermodynamic conditions for stress relaxation. Holding at a temperature range of 300-400℃ promotes dislocation rearrangement and full atomic diffusion, achieving dynamic recovery. This stabilizes the stress-released state after cryogenic treatment, completing the transition from "stress release" to "structural stability."

[0036] In one embodiment, sharp corners or right-angled bottoms produced by conventional machining can become natural stress boosters. Under subsequent welding thermal cycles and service loads, new cracks are easily initiated from these sharp corners. A smooth, rounded transition design allows for a smooth transition of stress flow lines at the bottom, significantly reducing the stress concentration factor and fundamentally preventing the repair from becoming a new crack initiation point. This greatly improves the fatigue life and structural safety of the repaired area. Compared to traditional straight or simple groove sidewalls, the periodically changing wave-shaped structure creates a continuously changing surface in three dimensions. When the cladding metal is filled in, the solidified metal body is strongly constrained by the geometry of the matrix in the X, Y, and Z directions. This effectively resists the displacement and detachment tendency of the repair layer under complex multi-directional stress, upgrading the interface bonding from a "metallurgical bonding mainly relying on intermolecular interactions" to a geometric-metallurgical composite bonding with mechanical anchoring effects.

[0037] In one embodiment, the at least three functional materials that have a gradient transition with the matrix material in step S4 include: a transition layer: the interface between the repair layer and the matrix is ​​the weakest link. Selecting materials with chemical compositions similar to the matrix can minimize the differences in their physical and metallurgical properties such as coefficients of thermal expansion and crystal structure, thereby significantly reducing the interfacial stress caused by performance mismatch, ensuring that the repair layer and the matrix can deform together during thermal cycling, and avoiding the risk of peeling or cracking from the interface;

[0038] Intermediate Layer: The intermediate layer is not simply a performance transition layer, but a functional "stress buffer layer." Its high strength enables it to effectively bear loads, while its excellent high toughness absorbs and dissipates stress from the matrix or working surface through plastic deformation, blunting crack tips and preventing the propagation of any microcracks that may originate from the transition layer or working surface layer. This design breaks away from the traditional structure of abrupt performance changes, forming a toughened intermediate region that isolates the easily cracked, brittle working surface layer from the matrix, greatly improving the impact resistance and fatigue resistance of the restoration.

[0039] Working surface layer: It realizes the performance improvement concept from repair to restoration, and no longer just pursues the restoration of the original shape and basic performance of the mold. Instead, it actively endows the final working surface with superior high-temperature softening resistance, molten metal erosion resistance and thermal fatigue resistance by cladding specially optimized high-performance materials. This makes the surface service performance of the repaired area surpass that of the original substrate. It not only makes up for the damage, but is also equivalent to a "performance upgrade" of the key areas of the mold, thereby significantly extending the overall service life of the repaired mold.

[0040] In one embodiment, the low heat input welding process in step S4 is cold metal transfer technology or ultra-high frequency pulsed arc welding; CMT technology separates the molten droplets by actively retracting the welding wire, replacing the separation method of traditional MIG welding that relies on current short-circuit explosion, thereby greatly reducing spatter and arc heat input. This precise synergy between "cold" and "hot" allows the cladding process to occur near the solidus temperature of the material, minimizing the width of the heat-affected zone and the thermal softening, deformation, and phase transformation of the base material. This lays the microstructural foundation for subsequent repair. Furthermore, the ultra-high frequency pulses output arc energy in extremely short-period pulses, with the arc root oscillating violently. This expands the heating area but significantly reduces the thermal action time per unit area, resulting in an oscillating effect of "instantaneous heating-instantaneous cooling" in the molten pool. On the one hand, this achieves grain refinement and improves the toughness of the cladding layer; on the other hand, it effectively controls the dilution rate and total heat input by reducing the high-temperature dwell time, thus maintaining low heat input while achieving excellent metallurgical bonding. The high-frequency vibration energy of the ultrasonic impact device is transmitted to the surface of the cladding layer through the cemented carbide needle, forcing the surface material to undergo severe plastic deformation and dynamic recrystallization, thereby greatly refining the grains. At the same time, this deformation introduces a gradient distribution of macroscopic residual compressive stress, perfectly offsetting the tensile stress generated during the cladding process.

[0041] In one embodiment, the zoned temperature-controlled aging heat treatment in step S5 is specifically implemented as follows: Compared with traditional overall furnace heating, the infrared heating array can perform focused heating on specific complex areas in a non-contact manner, with extremely fast thermal response speed and high energy utilization rate. This avoids the problem of excessive degradation of the performance of other parts of the matrix due to overall heating. Local thermal management ensures that the heat treatment regime of the repair area can be executed independently, providing a prerequisite for obtaining the best microstructure such as carbide precipitation and martensite tempering in the core repair area.

[0042] Furthermore, the concept of "controlled cooling" is introduced into post-repair processing, making it programmable and intelligent. Traditional cooling after heat treatment is usually natural or forced cooling with a single medium and a single rate, which is prone to generating new thermal stress due to uneven cooling. By using multiple nozzles with independently controllable temperature and medium flow, the cooling rate is "distributed on demand" to different areas according to a pre-known stress distribution "map". Rapid cooling of high-stress areas to fix dislocations and strengthen the structure, and slow cooling of transition areas to promote stress relaxation, this differentiated controlled cooling strategy can actively and specifically eliminate and homogenize residual stress while optimizing the microstructure.

[0043] In one embodiment, X-ray diffraction is used to perform non-destructive residual stress detection in the cracked area of ​​the mold. The operator uses a portable X-ray stress analyzer, aligning the X-ray tube head with the point to be measured. The X-ray beam is incident on the material lattice at a specific angle, causing diffraction. By measuring the shift of the diffraction peak position using a precision detector, the magnitude and direction of the residual stress at that point can be accurately calculated based on Bragg's law and the principles of elasticity. By measuring point by point and scanning the entire area of ​​interest in a grid pattern, a visualized two-dimensional residual stress distribution cloud map is finally generated.

[0044] In one embodiment, the surface strengthening treatment in step S6 is either ultrasonic nanocrystalline surface modification or physical vapor deposition to prepare a wear-resistant coating. The mold surface after being machined to a preset size will undergo the final surface strengthening treatment. The first scheme uses ultrasonic nanocrystalline surface modification, which uses an ultrasonic impact device to impact the repair area and the surrounding substrate at high speed and high frequency with a cemented carbide needle, causing severe plastic deformation of the surface layer, thereby generating a nanocrystalline structure. The second scheme uses physical vapor deposition technology, which places the workpiece in a high-vacuum coating furnace and deposits and bonds it on the workpiece surface through arc evaporation or magnetron sputtering of the target material, forming an extremely thin but extremely hard and dense wear-resistant coating of titanium nitride or chromium nitride. The high-frequency mechanical impact energy causes severe plastic deformation and dynamic recrystallization of the outermost layer of the material, thereby refining the coarse-grained structure into nanocrystals, significantly improving surface hardness, strength and fatigue resistance. More importantly, this process introduces a gradient distribution of macroscopic residual compressive stress on the surface layer, which can effectively offset the tensile stress under alternating loads and greatly suppress the initiation and propagation of microcracks.

[0045] In one embodiment, during the gradient welding repair process in step S4, the operator fixes a high-precision infrared thermal imager near the welding torch head, ensuring that its lens is always aimed at the molten pool and the surrounding heat-affected zone. When using cold metal transfer technology to clad each layer of functional material, the infrared thermal imager collects temperature field distribution data of the molten pool and its surrounding area in real time, generating thermal images. This temperature data is transmitted in real time to the control system of the welding power supply via a data cable. The intelligent algorithm built into the control system compares the real-time temperature with the preset ideal process temperature range. Once a temperature deviation is detected, the welding current, voltage, or wire feed speed is dynamically adjusted immediately, thereby achieving precise and dynamic closed-loop control of the heat input of the entire cladding process.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for repairing cracks in a large mold, characterized in that, Includes the following: S1, Stress field assessment and repair: Using three-dimensional scanning and non-destructive stress testing, obtain three-dimensional residual stress distribution cloud maps of the mold surface cracks and the repair area; S2. Synergistic energy field stress regulation pretreatment: Based on the residual stress distribution cloud map, apply local deep cooling and pulsed current composite energy field treatment to the repair area; The specific process of the local deep cooling and pulsed current composite energy field treatment is as follows: the mold is preheated to 80-120℃, liquid nitrogen is used to perform local deep cooling on the preheated repair area and high stress area, and then a high frequency pulsed current is applied to make the deep cooling area rapidly and uniformly heated to 300-400℃ under the Joule heating effect and kept at that temperature. S3. Low-stress damage removal: In the area treated in step S2, the cracks are removed by mechanical processing, and a geometric bevel with mechanical interlocking effect is formed. S4. Gradient welding repair: Using a low heat input welding process, at least three layers of functional material with a gradient transition to the base material are sequentially clad in the groove, and each clad layer is supplemented with high-energy sound beam impact treatment. S5. Partitioned Intelligent Aging Post-Processing: Based on the stress distribution cloud map obtained in step S1, the repair area is subjected to partitioned temperature-controlled aging heat treatment that combines local and overall methods to simultaneously optimize the comprehensive performance of the repair and the substrate. The specific method of partitioned temperature-controlled aging heat treatment is as follows: After rapidly and accurately heating the repair area to the tempering temperature using an infrared heating array and holding it at that temperature, multiple sets of independently temperature-controlled cooling nozzles are used to apply differentiated cooling rates to the repair area and surrounding stress blocks according to the stress distribution cloud map. S6. Adaptive machining and performance restoration: On a CNC machine tool, the repair area is precision machined to the preset size based on the three-dimensional data obtained in step S1, and the surface is strengthened.

2. The method for repairing cracks in a large mold according to claim 1, characterized in that, The geometric bevel described in step S3 has a rounded bottom and a periodically changing wave-shaped structure on its sidewalls.

3. The method for repairing cracks in a large mold according to claim 2, characterized in that, The at least three functional materials that have a gradient transition with the matrix material in step S4 include: a transition layer whose chemical composition is similar to that of the mold matrix material to ensure good metallurgical bonding with the matrix; an intermediate layer whose strength is higher than that of the transition layer and whose toughness is better than that of the working surface layer, used for stress buffering and inhibiting crack propagation; and a working surface layer whose high-temperature strength, red hardness and thermal fatigue resistance are better than those of the matrix material to restore and improve the service performance of the mold surface.

4. The method for repairing cracks in a large mold according to claim 1, characterized in that, The low heat input welding process in step S4 is cold metal transfer technology or ultra-high frequency pulsed arc welding; the high energy acoustic beam impact treatment is ultrasonic impact or laser impact.

5. The method for repairing cracks in a large mold according to claim 1, characterized in that, The non-destructive stress detection method described in step S1 is X-ray diffraction or ultrasonic testing.

6. The method for repairing cracks in a large mold according to claim 1, characterized in that, The surface strengthening treatment described in step S6 is ultrasonic nanocrystal surface modification treatment or physical vapor deposition to prepare a wear-resistant coating.

7. The method for repairing cracks in a large mold according to claim 1, characterized in that, In step S4, when each layer of functional material is clad, an infrared thermal imager is used to monitor the temperature field distribution of the molten pool and the heat-affected zone, and the real-time temperature data is fed back to the welding power supply control system to achieve dynamic closed-loop control of welding heat input.

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