A method for improving the surface integrity of an extruded billet
Patent Information
- Application Number
- CN202610805754.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-06-05
- Publication Date
- 2026-09-15
Abstract
Description
Technical Field
[0001] This application belongs to the field of metal plastic processing and surface engineering, and specifically relates to a method for improving the surface integrity of extruded blanks. Background Technology
[0002] Traditional hot extrusion processes involve intense friction and complex stress states between the billet and the die, which often leads to scratches, microcracks, and structural damage on the product surface, severely affecting its fatigue performance and corrosion resistance.
[0003] Existing technologies for addressing surface damage in extruded blanks include: improving interfacial conditions through high-performance coatings; reducing material deformation resistance through high-energy electrical pulses (electroplastic effect); and introducing nanostructured reinforcing materials through processing. However, existing technologies and research are often isolated, either treating coatings merely as lubricating media and neglecting their potential as structural reinforcing phases, or only utilizing electroplasticity to reduce extrusion pressure without exploring its ability to regulate interfacial bonding and microstructure evolution, and lacking a systematic approach to organically integrate these three elements to actively construct a high-performance gradient surface layer during the forming process. Simply combining these technologies may fail due to process conflicts (e.g., coating detachment under severe deformation, or mismatch between electrical and deformation parameters).
[0004] Therefore, there is an urgent need for an innovative method that can deeply couple interface modification, process energy field regulation and microstructure design, so as to fundamentally revolutionize the control strategy for the surface integrity of extruded blanks. Summary of the Invention
[0005] To address the above problems, this application provides a method for improving the surface integrity of extruded blanks, comprising: A composite coating system is formed by sequentially preparing a bonding layer, a nano-reinforcing layer and a superhydrophobic functional layer on the surface of a metal blank. During the extrusion deformation process, an electric pulse is applied to the metal billet with a composite coating system, and the extrusion process parameters are adjusted to form a diffused nano-void structure in the surface shear deformation zone of the metal billet.
[0006] Furthermore, the thickness of the bonding layer is 1-5 μm or 1-50 nm.
[0007] Furthermore, the bonding layer is a nitride ceramic layer, a carbide ceramic layer, or an intermetallic compound layer. The nitride ceramic layer includes TiN, CrN, TiAlN, or CrAlN, the carbide ceramic layer includes DLC (diamond-like carbon), and the intermetallic compound layer includes NiAl or FeAl.
[0008] Furthermore, the thickness of the nano-reinforcing layer is 5-20 μm, and the nano-reinforcing layer includes a metal matrix and a nano-reinforcing phase. The volume percentage of the nano-reinforcing phase in the nano-reinforcing layer is 1-15%, and the particle size of the nano-reinforcing phase is 10-500 nm.
[0009] Furthermore, the metal matrix includes Ni-based, Co-based, or Fe-based alloys; the nano-reinforcing phase includes ZrO2, Al2O3, SiO2, SiC, WC, BN, Si3N4, graphene, or carbon nanotubes.
[0010] Furthermore, the thickness of the superhydrophobic functional layer is 5-15 μm, the water contact angle is greater than 150°, the roll-off angle is less than 10°, and the initial coefficient of friction with the extrusion die is less than 0.15.
[0011] Furthermore, the superhydrophobic functional layer includes resin and nanofillers. The mass percentage of nanofillers in the superhydrophobic functional layer is 1-5%. The resin includes PDMS, PTFE, PVDF, fluorinated polyurethane or epoxy resin, and the nanofillers include SiO2 nanoparticles, ZnO nanoparticles, TiO2 nanoparticles or graphene sheets.
[0012] Furthermore, an electrical pulse is applied to the metal billet with the composite coating system while the extrusion process parameters are adjusted, including: Based on the real-time extrusion pressure, dynamic resistance, and electroacoustic emission signal of the metal billet with a composite coating system, the frequency, current density, and extrusion speed of the electrical pulses are adjusted. When the real-time extrusion pressure increases and the dynamic resistance or electroacoustic emission signal does not exhibit the expected characteristic pattern, the current density or frequency of the electrical pulses is increased; when the real-time extrusion pressure decreases and the dynamic resistance or electroacoustic emission signal does not exhibit the expected characteristic pattern, the current density of the electrical pulses is decreased or intermittent electrical pulses are introduced. Adjust the initial temperature of the metal billet or the temperature of the extrusion die based on the measured temperature of the deformation zone of the metal billet with a composite coating system.
[0013] Furthermore, the expected feature pattern is an ideal feature template established by performing time-frequency analysis on the dynamic resistance and electroacoustic emission signals collected during the process of successfully forming the diffuse nanoporous structure.
[0014] Furthermore, the frequency of the electrical pulse is 100-1000Hz, and the current density is 50-200A / mm².
[0015] Furthermore, the extrusion speed is 1-3 mm / s, and the initial heating temperature of the metal billet with the composite coating system is 20-50°C lower than that of the same metal billet without the composite coating system.
[0016] Compared with the prior art, this application has the following advantages: 1. This application constructs a triple synergistic mechanism of "gradient coating-electric pulse-nanopores", which can effectively reduce the surface roughness of extruded blanks, while simultaneously and significantly improve the surface hardness, wear resistance and corrosion resistance, thus achieving a balance between smoothness and toughness.
[0017] 2. The blank extrusion process of this application is simple, and the forming and surface strengthening are completed in one integrated process, eliminating many post-processing steps and realizing green manufacturing.
[0018] 3. The blank extrusion process of this application is highly adjustable and has a wide range of applications. By adjusting the coating composition, electrical pulse parameters and extrusion parameters, the surface gradient structure can be customized for different materials (such as aluminum alloys, magnesium alloys and titanium alloys) and target properties.
[0019] Other features and advantages of this application will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the application. The objectives and other advantages of this application may be realized and obtained by means of the structures pointed out in the description and claims. Detailed Implementation
[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0021] To deeply couple interface modification, process energy field regulation, and microstructure design, and fundamentally revolutionize the control strategy for the surface integrity of extruded billets, this application discloses a method for improving the surface integrity of extruded billets, comprising: S1: A bonding layer, a nano-reinforcing layer, and a superhydrophobic functional layer are sequentially prepared on the surface of a metal blank to form a composite coating system.
[0022] A bonding layer 1-5 μm thick is formed on the surface of a metal billet using magnetron sputtering, arc ion plating, or plasma spraying. Alternatively, a bonding layer 1-50 nm thick can be formed on the surface of the metal billet using precision deposition. The bonding layer is a nitride ceramic layer, a carbide ceramic layer, or an intermetallic compound layer. The nitride ceramic layer includes TiN, CrN, TiAlN, or CrAlN; the carbide ceramic layer includes DLC (diamond-like carbon); and the intermetallic compound layer includes NiAl or FeAl. For example, a cleaned and activated metal billet is placed in a vacuum chamber, and a reactive gas such as nitrogen is introduced. A titanium or chromium target is bombarded with plasma, causing the target atoms and gas atoms to react and deposit on the surface of the metal billet to form a TiN or CrN thin film. The bonding layer forms a strong metallurgical / mechanical bond with the metal substrate. The critical load Lc in the scratch test is ≥30N. There are no penetrating cracks or pores. It does not form a low-melting eutectic with the protected metal at the extrusion process temperature. It plays a role in transition, blocking diffusion and providing a stable substrate for subsequent coatings.
[0023] A 5-20 μm thick nano-reinforcing layer is further formed on the surface of the bonding layer using composite electroplating, chemical plating, cold spraying, or high-speed oxy-fuel spraying techniques. The nano-reinforcing layer comprises a metal matrix and a nano-reinforcing phase. The volume percentage of the nano-reinforcing phase in the nano-reinforcing layer is 1-15% to ensure dispersed distribution without affecting the density of the coating. The particle size of the nano-reinforcing phase is 10-500 nm to facilitate the generation of controllable nanopores at the interface. The metal matrix includes Ni-based, Co-based, or Fe-based alloys; the nano-reinforcing phase includes ZrO2, Al2O3, SiO2, SiC, WC, BN, Si3N4, graphene, or carbon nanotubes. For example, a metal blank coated with the bonding layer is immersed in a composite plating solution containing metal ions (such as nickel ions or tungsten ions) and uniformly dispersed nanoparticles (such as ZrO2). Under the action of an electric field, the metal ions and nanoparticles co-deposit on the bonding layer, forming a nano-reinforcing layer with good adhesion and no internal delamination. The content of the nano-reinforcing phase can be precisely controlled by the concentration of nanoparticles in the plating bath and the electroplating parameters. The nano-reinforcing layer serves to support the subsequent coating and also provides a certain degree of wear resistance. The nano-reinforcing layer also acts as a "preform," creating conditions for the subsequent in-situ formation of nanopores. Under the combined action of subsequent extrusion deformation and electrical pulses, dispersed nanopores will be generated in-situ at the nanoparticle / matrix interface within the layer, which is key to achieving surface nanopore reinforcement.
[0024] A 5-15 μm thick superhydrophobic functional layer is formed on the surface of the nano-reinforced layer using spraying, dip coating, or spin coating processes. This superhydrophobic layer has a water contact angle greater than 150°, a roll-off angle less than 10°, and an initial coefficient of friction with the extrusion die less than 0.15. It does not rapidly decompose or peel off at the initial temperature of the extrusion blank. The superhydrophobic functional layer comprises resin and nanofillers, with the nanofillers accounting for 1-5% of the total mass. The resin is a low surface energy resin, such as polysiloxane resin (PDMS), fluorinated polymer resin (PTFE, PVDF), fluorinated polyurethane, or epoxy resin. The nanofillers include SiO2 nanoparticles, ZnO nanoparticles, TiO2 nanoparticles, or carbon nanomaterials (graphene sheets). The superhydrophobic functional layer provides lubrication and reduces the coefficient of friction during the initial stage of extrusion, while also providing anti-corrosion and anti-adhesion properties. It gradually degrades under certain deformation conditions without hindering subsequent deformation.
[0025] S2: During the extrusion deformation process, an electric pulse is applied to the metal billet with a composite coating system, while the extrusion process parameters are adjusted to form a diffuse nanoporous structure in the surface shear deformation zone of the metal billet. The electric pulse can reduce the overall deformation force, significantly alleviate the shear stress on the coating, and prevent premature peeling. At the same time, the pulsed current can locally improve the coating / metal interface bonding. The additional energy and electron wind effect provided by the electric pulse can significantly promote the dynamic recrystallization of the surface metal, forming an ultrafine crystalline layer. More importantly, the electric pulse also regulates the deformation coordination of the nano-reinforcing phase / matrix interface, creating unique kinetic conditions for the subsequent controllable peeling at the interface to form nanopores. This is something that cannot be achieved by simply "coating + extrusion" or "electric pulse + extrusion". Under the precise control of both electrical pulses and intense shear deformation, the nano-reinforcing phase pre-formed in the coating is forced to undergo controllable local separation from the matrix interface, forming diffusely distributed nanovoids with a size of 20-100 nm. These nanovoids become strong barriers to dislocation movement, generating significant back stress reinforcement, thereby constructing a toughened gradient nanostructure region on the surface.
[0026] The control logic for electrical pulse parameters and extrusion process parameters is as follows: Based on the real-time extrusion pressure, dynamic resistance, and electroacoustic emission signals of the metal billet with a composite coating system, the frequency, current density, and extrusion speed of the electrical pulses are adjusted.
[0027] Real-time extrusion pressure is the most critical feedback signal. An abnormal increase in force may indicate increased friction or obstructed material flow, while an abnormal decrease in force may indicate localized overheating or deformation instability. The system presets a target range for extrusion pressure for the current alloy and product specifications (e.g., ±5% of peak force). When the extrusion pressure is within the appropriate range, the extrusion pressure curve is smooth and stable within that target range.
[0028] Under the influence of an electrical pulse, the microscopic activities such as dislocation movement and interface separation within the material can cause instantaneous fluctuations in resistance or generate characteristic acoustic emission signals. These signals are the "fingerprints" of nanoscale structural evolution. Through previous experimental calibration, when nanopores are uniformly nucleated, the dynamic resistance or electroacoustic emission signal will exhibit specific spectral characteristics or fluctuation patterns. By performing time-frequency analysis on the dynamic resistance and acoustic emission signals collected during the process of successfully forming diffuse nanopores, an ideal characteristic template is established. The dynamic resistance and electroacoustic emission signal are determined to exhibit the expected characteristic pattern by one or more of the following methods, but not limited to: the fluctuation amplitude of the real-time dynamic resistance or electroacoustic emission signal is lower than a certain proportion of the pre-stored reference amplitude (e.g., 40%); the signal-to-noise ratio of the real-time dynamic resistance or electroacoustic emission signal in the characteristic frequency band is lower than a set threshold (e.g., 3:1); the correlation coefficient between the real-time dynamic resistance or electroacoustic emission signal and the pre-stored ideal signal template is lower than a set threshold. Those skilled in the art should understand that this set threshold can be calibrated experimentally within the range of 0.75 to 0.85 (e.g., 0.8) according to the specific alloy-coating system. The reference amplitude, characteristic frequency band, and set threshold can be determined through preliminary process calibration experiments.
[0029] Specifically, the establishment of the expected feature pattern includes: during the process development stage, collecting multiple sets of dynamic resistance and electroacoustic emission signals under ideal conditions (i.e., TEM confirms the formation of diffuse nanopore structures) through preliminary experiments; performing time-frequency analysis (such as fast Fourier transform and wavelet analysis) on the dynamic resistance and electroacoustic emission signals to extract one or more sets of feature vectors (e.g., energy proportion of specific frequency bands, main peak frequency, waveform complexity index, etc.) and using them as ideal feature templates.
[0030] Based on signal amplitude determination: The fluctuation amplitude (peak-to-peak or root mean square value) of the real-time dynamic resistance and electroacoustic emission signal is compared with the typical amplitude (i.e., the pre-stored reference amplitude) in the "expected characteristic pattern" database. When the amplitude of the real-time dynamic resistance and electroacoustic emission signal is lower than 30%-50% of the typical amplitude reference, it can be determined that the expected characteristic pattern is not presented.
[0031] Based on signal-to-noise ratio (SNR) determination: Calculate the ratio of signal energy to background noise energy within a specific frequency band of the real-time dynamic resistance and electroacoustic emission signal. When the SNR is lower than 3:1 (approximately 10dB), the effective signal can be considered to be submerged by noise, and it is determined that the expected characteristic pattern is not presented.
[0032] Based on the determination of spectral energy distribution: Analyze the energy ratio of real-time dynamic resistance and electroacoustic emission signals in the characteristic frequency range (such as 100-1000Hz related to interface activity). If the ratio is lower than the set value (such as 20%), it indicates that the characteristic activity related to nanopore nucleation is not active and is determined to be that the expected characteristic mode is not presented.
[0033] Based on a set threshold: During real-time extrusion, the system extracts the feature vectors of the current dynamic resistance and electroacoustic emission signals using the same algorithm as described above, and then calculates the similarity with the "ideal feature template." The most common method is to calculate the correlation coefficient or cosine similarity. The closer the value is to 1, the higher the degree of agreement. When the degree of agreement is lower than the set threshold, it is determined that the expected feature pattern has not been presented. Considering the stability and fault tolerance of industrial process control, this set threshold is usually between 0.75 and 0.85. For example, it can be defined as: "When the correlation coefficient between the real-time dynamic resistance and electroacoustic emission signals and the ideal feature template is consistently lower than 0.8, it is determined that the expected feature pattern has not been presented." This is a reasonable numerical range that can effectively capture deviations while avoiding false alarms caused by random noise.
[0034] Furthermore, if the system detects an increase in real-time extrusion pressure and the dynamic resistance or electroacoustic emission signal does not exhibit the expected characteristic pattern, it may indicate insufficient interface activation. In this case, the current density or frequency of the electrical pulse can be appropriately increased to enhance the electron wind effect and promote interface separation. If the system detects a decrease in real-time extrusion pressure and the dynamic resistance or electroacoustic emission signal does not exhibit the expected characteristic pattern, it may mean that the local energy input is too high. In this case, the current density of the electrical pulse should be reduced or intermittent electrical pulses should be introduced to prevent overheating from causing void merging and growth.
[0035] While ensuring equipment stability, the extrusion speed can be finely adjusted in the same direction as the electrical pulse. For example, to obtain higher density nanopore cores, the extrusion speed can be slightly increased while increasing the pulse frequency to increase the strain rate.
[0036] The initial temperature of the metal billet or the extrusion die temperature is adjusted based on the measured temperature of the deformation zone of the metal billet with a composite coating system. The temperature of the deformation zone is detected by infrared thermometers or embedded thermocouples placed near the die working zone and on the surface of the metal billet. If the measured temperature consistently exceeds the upper limit of a set window, the system instructs the heating system to slightly reduce the initial temperature of the metal billet. The primary purpose is to prevent overall overheating that could lead to coarsening of nanopores or recrystallization and grain growth in the matrix, thus ensuring the strengthening effect.
[0037] Preferably, the frequency of the electric pulse is 100-1000Hz, with a higher frequency of 500-1000Hz being more conducive to the formation of fine, dispersed voids, and a higher frequency of 100-300Hz having a more macroscopic effect, which is beneficial to the overall plasticity improvement. The current density of the electric pulse is 50-200A / mm², below which the effect is not significant, and above which it may cause overheating or arc damage. The extrusion speed is 1-3mm / s. When used with a high-frequency pulse, a higher extrusion speed is preferred, which is beneficial to generate more nano-void nucleation points per unit time. At a lower extrusion speed, the plastic flow of the metal billet is more complete, and the voids are more rounded. The initial heating temperature of the metal billet with the composite coating system is 20-50°C lower than that of the metal billet without the composite coating system. The initial heating temperature of the metal billet without the composite coating system is determined based on the metal plastic processing technology of the corresponding metal billet. Because the electric pulse provides additional energy, the billet extrusion temperature can be appropriately reduced, which helps to maintain the matrix strength and allows the shear stress to act more effectively on the interface.
[0038] The manufactured products are tested, and the product performance must meet the following standards: metallographic (TEM) sampling confirms the presence of diffuse, independent nanoscale (20-100nm) voids on the surface, and there are no unexpected microcracks; the microhardness and residual compressive stress of the sampled product surface meet the preset target range.
[0039] To better illustrate this solution, the following embodiments and comparative examples are provided.
[0040] Example 1 A composite coating system was prepared on the surface of 7075 aluminum alloy extruded profiles, including: a 3μm CrN bonding layer, a Ni-WP-ZrO2 nano-reinforcing layer (ZrO2 volume percentage 8%), and a superhydrophobic functional layer (PDMS + 3wt% SiO2 nanoparticles).
[0041] Extrusion process parameter control: Initial temperature of metal billet: 400-420°C, die temperature: 350-370°C, extrusion temperature: 400-420°C, extrusion speed: 1.8-2.2mm / s, extrusion pressure: 13.5MN, electrical pulse frequency: 600-800Hz, electrical pulse current density: 100-130A / mm², electrical pulse waveform: square wave, pulse width: 150μs.
[0042] Control process: At startup, the initial temperature of the metal billet is 410°C, the extrusion speed is 2 mm / s, the electrical pulse frequency is 700 Hz, and the current density of the electrical pulse is 115 A / mm². If, during extrusion, the extrusion pressure continuously increases and the dynamic resistance fluctuation signal does not exhibit the expected characteristic pattern (e.g., the real-time signal amplitude is consistently lower than 40% of the baseline amplitude during the process stabilization period), it indicates insufficient interface activation. The system should simultaneously and slightly increase the current density (e.g., +5 A / mm²) and the extrusion speed (e.g., +0.1 mm / s) to enhance coupling strength. If, during extrusion, the extrusion pressure decreases too rapidly and the dynamic resistance or electroacoustic emission signal does not exhibit the expected characteristic pattern, it indicates possible localized overheating. The system should slightly decrease the current density (e.g., -5 A / mm²) or change the electrical pulse to an intermittent mode (e.g., 200 ms operation followed by a 50 ms pause).
[0043] The product obtained in Example 1 was tested and Ra=0.32±0.05μm, indicating that the coating has a synergistic effect of complete lubrication and electroplasticity, which effectively reduces the surface friction of the blank.
[0044] TEM showed that there were diffused nanopores (50 nm) and ultrafine crystals (<500 nm) on the surface of the product, indicating that the electrical pulse effectively promoted recrystallization to refine the grains and regulated the uniform nucleation of nanopores at the ZrO2 particle / Al interface.
[0045] The product has a surface hardness of 205±10HV and no red rust after 480 hours of salt spray testing, indicating that the superposition of nano-pore reinforcement and fine grain reinforcement effectively improves the product's hardness. The synergistic effect of the complete coating and dense nanostructure can effectively block corrosive media.
[0046] Example 2 This embodiment aims to demonstrate the applicability and superior effectiveness of the method of this application for magnesium alloys with high activity and narrow plasticity processing window. Taking the preparation of AZ31B magnesium alloy thin-walled tubes with an outer diameter of φ30mm and a wall thickness of 2mm as an example, traditional extrusion easily leads to problems such as severe surface oxidation, microcracks, coarse grains, and poor corrosion resistance. The method of this application, through a targeted coating and synergistic process, aims to obtain tubes with high surface integrity, high strength and toughness, and corrosion resistance in a single step.
[0047] A composite coating system was prepared on the surface of AZ31B magnesium alloy thin-walled tube, including: a 0.8μm TiN bonding layer, a GO-ZnO-epoxy nano-reinforcing layer (GO-ZnO hybrid material with a volume ratio of 1.5%), and a superhydrophobic functional layer (fluorinated polyurethane + 3wt% SiO2 nanoparticles).
[0048] Extrusion process parameter control: Initial temperature of metal billet: 340-360°C, die temperature: 300-320°C, extrusion temperature: 340-360°C, extrusion speed: 1.5-1.8mm / s, extrusion pressure: 8MN, electrical pulse frequency: 400-600Hz, electrical pulse current density: 70-90A / mm², electrical pulse waveform: square wave, pulse width: 200μs.
[0049] Control process: At startup, the initial temperature of the metal billet is 350°C, the extrusion speed is 1.6 mm / s, the electric pulse frequency is 500 Hz, and the electric pulse current density is 80 A / mm². If, during the extrusion process, the temperature sensor shows that the temperature in the deformation zone exceeds 380°C, regardless of the extrusion pressure, the system will prioritize reducing the billet heating power and slightly reduce the electric pulse current density.
[0050] The extrusion process parameters were adjusted using the same strategy as in Example 1, with the electric pulse frequency adjusted by ±3A / mm² and the extrusion speed adjusted by 0.05mm / s.
[0051] The product obtained in Example 2 was tested and Ra=0.28±0.05μm, indicating that the coating has a synergistic effect of complete lubrication and electroplasticity, which effectively reduces the surface friction of the blank.
[0052] TEM showed that there were diffused nanopores (50 nm) and ultrafine crystals (<500 nm) on the surface of the product, indicating that the electrical pulse effectively promoted recrystallization to refine the grains and regulated the uniform nucleation of nanopores at the ZrO2 particle / Mg interface.
[0053] The product's surface hardness is 110±5HV, and it showed no red rust after 360 hours of salt spray testing, indicating that the superposition of nano-pore reinforcement and fine grain reinforcement effectively improved the product's hardness. The synergistic effect of the complete coating and dense nanostructure can effectively block corrosive media.
[0054] Comparative Example 1 The metal blank is the same as that in Example 1, without a composite coating system.
[0055] Extrusion process: Commercial glass lubricant is used, no electric pulse is used. Extrusion parameters: Initial temperature of metal billet: 440°C, die temperature: 380°C, extrusion temperature: 440°C, extrusion speed: 1mm / s, extrusion pressure: 17.5MN.
[0056] The product was tested and found to have Ra=1.8±0.2μm. TEM showed that there were no diffuse nanopores on the surface of the product, the surface grains were coarse, the hardness was 145±5HV, and pitting corrosion occurred after 24 hours of salt spray, indicating that external lubrication alone cannot solve the fundamental problems of microstructure damage and performance degradation.
[0057] Comparative Example 2 The same metal blank and composite coating system as in Example 1.
[0058] Extrusion process: No electric pulse; Extrusion parameters: Initial temperature of metal billet: 440°C; Die temperature: 380°C; Extrusion temperature: 440°C; Extrusion speed: 1mm / s; Extrusion pressure: 16MN.
[0059] The product was tested and the coating was found to have localized peeling, with Ra=0.8μm. TEM showed that there were no diffuse nanopores on the surface of the product, the surface grains were relatively coarse, the coating / substrate interface was clear, and the hardness was 165±5HV. This indicates that without the synergistic effect of the electric pulse, the coating is not stable enough under severe deformation, and its function as a "structural preform" cannot be fully realized. The nanopores are also difficult to form in a controllable manner, and the final performance is far inferior to that of Example 1.
[0060] Comparative Example 3 The same metal blank as in Example 1 is coated with a common lubricating coating phosphating saponification film.
[0061] Initial temperature of metal billet: 430°C, die temperature: 370°C, extrusion temperature: 430°C, extrusion speed: 1.5mm / s, extrusion pressure: 14MN, non-cooperative electric pulse applied, electric pulse frequency 500Hz, electric pulse current density 80A / mm².
[0062] The product was tested and found to have Ra=0.6±0.1μm. TEM showed that there were no diffuse nanopores on the surface of the product, the surface grains were refined, the surface hardness was 155±5HV, and pitting corrosion occurred after 120 hours of salt spray. This indicates that simply using an electrical pulse to reduce the extrusion pressure or using a regular coating cannot actively build a nanopore reinforcement layer, and the improvement in surface hardness is limited.
[0063] Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.
Claims
1. A method of improving the surface integrity of an extruded billet, characterized by, include: A composite coating system is formed by sequentially preparing a bonding layer, a nano-reinforcing layer and a superhydrophobic functional layer on the surface of a metal blank. During the extrusion deformation process, an electric pulse is applied to the metal billet with a composite coating system, and the extrusion process parameters are adjusted to form a diffused nano-void structure in the surface shear deformation zone of the metal billet.
2. A method of improving the surface integrity of an extruded billet as claimed in claim 1, wherein, The thickness of the bonding layer is 1-5 μm or 1-50 nm.
3. The method for improving the surface integrity of extruded blanks according to claim 2, characterized in that, The bonding layer is a nitride ceramic layer, a carbide ceramic layer, or an intermetallic compound layer.
4. The method for improving the surface integrity of extruded blanks according to claim 1, characterized in that, The thickness of the nano-reinforcing layer is 5-20 μm, the nano-reinforcing layer includes a metal matrix and a nano-reinforcing phase, the volume ratio of the nano-reinforcing phase in the nano-reinforcing layer is 1-15%, and the particle size of the nano-reinforcing phase is 10-500 nm.
5. A method for improving the surface integrity of an extruded blank according to claim 4, characterized in that, The metal matrix includes Ni-based, Co-based, or Fe-based alloys; The nano-reinforcing phase includes ZrO2, Al2O3, SiO2, SiC, WC, BN, Si3N4, graphene, or carbon nanotubes.
6. The method for improving the surface integrity of extruded blanks according to claim 1, characterized in that, The thickness of the superhydrophobic functional layer is 5-15 μm, the water contact angle is greater than 150°, the roll-off angle is less than 10°, and the initial coefficient of friction with the extrusion die is less than 0.
15.
7. A method for improving the surface integrity of an extruded blank according to claim 6, characterized in that, The superhydrophobic functional layer includes resin and nanofillers. The mass percentage of the nanofillers in the superhydrophobic functional layer is 1-5%. The resin includes PDMS, PTFE, PVDF, fluorinated polyurethane or epoxy resin. The nanofillers include SiO2 nanoparticles, ZnO nanoparticles, TiO2 nanoparticles or graphene sheets.
8. The method for improving the surface integrity of extruded blanks according to claim 1, characterized in that, The aforementioned application of an electrical pulse to a metal billet with a composite coating system while simultaneously adjusting extrusion process parameters includes: Based on the real-time extrusion pressure, dynamic resistance, and electroacoustic emission signal of the metal billet with a composite coating system, the frequency, current density, and extrusion speed of the electrical pulses are adjusted. When the real-time extrusion pressure increases and the dynamic resistance or electroacoustic emission signal does not exhibit the expected characteristic pattern, the current density or frequency of the electrical pulses is increased; when the real-time extrusion pressure decreases and the dynamic resistance or electroacoustic emission signal does not exhibit the expected characteristic pattern, the current density of the electrical pulses is decreased or intermittent electrical pulses are introduced. Adjust the initial temperature of the metal billet or the temperature of the extrusion die based on the measured temperature of the deformation zone of the metal billet with a composite coating system.
9. A method for improving the surface integrity of an extruded blank according to claim 8, characterized in that, The expected feature pattern is an ideal feature template established by performing time-frequency analysis on the dynamic resistance and electroacoustic emission signals collected during the process of successfully forming a diffuse nanoporous structure.
10. A method for improving the surface integrity of an extruded blank according to claim 8, characterized in that, The frequency of the electrical pulse is 100-1000Hz, and the current density is 50-200A / mm².
11. A method for improving the surface integrity of an extruded blank according to claim 8, characterized in that, The extrusion speed is 1-3 mm / s, and the initial heating temperature of the metal billet with the composite coating system is 20-50°C lower than that of the same metal billet without the composite coating system.