High-durability mask system and application method of high-durability mask system in mechanical shot peening selective area strengthening

Abstract

The invention discloses a high-durability mask system and an application method of the high-durability mask system in mechanical shot peening selective area strengthening, and belongs to the technical field of surface engineering. The invention provides a mask design of a multi-layer composite structure, a precise positioning and fixing system and a special process parameter system. The mask plate adopts a composite structure of the matrix layer, the wear-resistant layer and the anti-fatigue layer, so that the shock resistance and the wear resistance of the mask plate are improved; the positioning system is beneficial to keeping the stability and the repeated positioning precision of the mask plate under the working condition of strong vibration; and the special process parameters are matched and optimized aiming at the application scene of the mask plate. The invention aims to improve key technical problems when a mask shielding method is applied to a mechanical shot peening process, improve the precision and efficiency of selective shot peening strengthening, reduce the comprehensive manufacturing cost, and enable the service life of a mask system and the boundary precision of a selective strengthening region to be generally superior to those of a traditional scheme. And the method has a certain economic advantage while the fatigue performance is improved.

Description

Technical Field

[0001] This invention relates to the fields of metal surface engineering and precision machining technology, specifically to a highly durable photomask system capable of withstanding high-speed projectile impact and its application method for precise selective strengthening of workpiece surfaces during mechanical shot peening. This system is designed for high-speed projectile impact conditions, addressing key issues encountered with photomasks during use, such as insufficient wear and impact resistance, difficulties in precise positioning and fixation, and difficulty in controlling the boundary effect of the strengthened area. Background Technology

[0002] Shot peening is a surface strengthening technology that improves the fatigue life and resistance to stress corrosion by introducing a residual compressive stress layer into the surface of a metal workpiece through the impact of a shot stream. In traditional mechanical shot peening, the shot motion exhibits significant randomness and disorder, making it difficult to precisely define the strengthening area and meet the refined requirements of high-end components for selective strengthening boundaries and intensity distribution. The "mechanical shot peening" described in this paper preferably refers to jet-type shot peening (taking pneumatic shot peening with a spray gun as an example). Its key process parameters can be characterized by spray pressure, spray gun-workpiece distance, and spray angle. When using other shot peening equipment such as impeller-type equipment, parameters such as equivalent shot velocity and mass flow rate / count flux can replace pressure characterization. The mask structure and parameter coupling design principle of this invention also applies.

[0003] In mechanical shot peening, the mask must withstand continuous mechanical impact from high-speed solid projectiles, which is fundamentally different from the condition in laser peening where the absorption layer mainly absorbs energy. These challenges include: the impact resistance and durability of the mask, as continuous impact from high-speed projectiles causes severe wear, deformation, and even cracking, resulting in a very short lifespan; the difficulty in controlling boundary clarity, as the physical dimensions and random bounce characteristics of the projectiles lead to blurred boundaries in the reinforced area; the difficulty in precise positioning and fixing, as the intense vibrations during shot peening make the mask prone to displacement, affecting accuracy; and the need to redefine process parameters, as the introduction of the mask alters the projectile flow field characteristics, requiring a re-establishment of the process parameter system.

[0004] While existing technologies have proposed using shielding devices or simple masking structures during shot peening, most remain at the level of empirical design, failing to systematically address core issues such as the durability, precise positioning and fixation, and boundary effect control of the mask under mechanical shot peening conditions. Furthermore, existing solutions generally lack real-time feedback-based adaptive process adjustment capabilities and digital modeling and prediction methods coupled with the mask structure, making it difficult to achieve process predictability and high consistency in selective peening scenarios involving complex components. Therefore, there is an urgent need in this field for a highly durable mask system specifically designed for mechanical shot peening conditions and its application method, enabling high-precision selective shot peening of typical complex components while ensuring mask lifespan and positioning accuracy.

[0005] In typical components such as turbine blade tenons and precision gears in aero-engines, critical sections are often located on the tooth surface or in the tooth profile transition region. These areas bear both significant contact stress and the coupled effect of bending loads, placing high demands on the level of residual compressive stress and the uniformity of strengthening. Local stress concentrations on complex curved surfaces directly affect the fatigue life and failure mode of components. Therefore, how to achieve stable and controllable shot peening strengthening effects at these critical curved surface locations is one of the key and challenging problems in current engineering practice.

[0006] However, in traditional mechanical shot peening processes, the spray gun posture, workpiece clamping, and equipment freedom are limited by spatial layout and structural constraints, making it difficult to maintain a near-normal angle of incidence on complex curved surfaces. Taking gear tooth surfaces or turbine blade tenon grooves as examples, even if the equipment's nominal spray angle is close to 90°, the actual incident angle of the shot relative to the local tooth surface curvature is often still skewed, with a decrease in the normal component and an increase in the tangential component. Under the same shot peening pressure and time conditions, the effective strengthening intensity of the tooth surface area is significantly lower than that of the more geometrically gentle tooth root or planar area. Existing technologies typically compensate empirically by repeatedly adjusting the spray gun angle, changing the shot peening distance, or extending the shot peening time. However, this method is difficult to achieve an ideal spray posture in complex spaces and easily leads to an imbalance in the degree of strengthening in different areas, a weak strengthening boundary, and a non-repeatable residual stress distribution. Especially in situations where a high strengthening level is required in the tooth surface area, while the tooth root area maintains moderate strengthening or even forms a gradient distribution, traditional processes struggle to provide a designable and predictable solution. Summary of the Invention

[0007] To address the technical challenges of mask flask wear, short lifespan, and poor boundary accuracy under mechanical shot peening conditions, this invention proposes a highly durable mask system and its application method in selective shot peening. Addressing key technical issues arising from the introduction of mask blocking methods into mechanical shot peening, a systematic solution is constructed, comprising mask structural design, a precision positioning and fixing system, and a dedicated shot peening process parameter system. The main problems to be solved include: insufficient wear resistance and fatigue resistance of the mask under high-speed shot impact; difficulty in achieving high-precision repeatable positioning between the mask and the workpiece; and the tendency for "halo" and residual stress field distortion to occur at the boundaries of the selected shot peening area. By comprehensively optimizing the coupling relationship between the multi-layer composite structure of the mask, the opening geometry parameters, the positioning and fixing method, and the shot peening parameters, this invention achieves high-precision, repeatable selective shot peening strengthening of typical complex component surfaces.

[0008] Furthermore, addressing the common problem of difficulty in achieving perpendicular shot incidence in the tooth surface region of complex curved components, resulting in low actual strengthening under the same process conditions, this invention proposes an equivalent strength compensation method based on the coordinated design of mask aperture density and shot peening coverage, building upon a high-durability mask structure and a precision positioning and fixing system. This invention no longer attempts to force perfectly perpendicular incidence in all regions, but instead treats the unavoidable tilted shot incidence on complex curved surfaces as a given condition. Through the overall matching of the mask aperture pattern and shot peening parameters, strength compensation is proactively performed during the mask design stage: for tooth surface regions where geometric conditions lead to low actual strengthening strength, the number of shot passing through a unit area is significantly increased by increasing the aperture density of the corresponding region on the mask; simultaneously, a higher target coverage is set for this region in the process parameters, and the same surface point is subjected to more shot impacts by extending the shot peening time or increasing the mask scanning overlap; for tooth root regions that do not require excessive strengthening or require moderately reduced strengthening, the shot throughput and number of impacts per unit area are limited by reducing the aperture density and setting a lower coverage. Combining the shot peening time calculation formula t = (A×C_eq) / (N×S_e×η) proposed in this invention, by introducing two design variables, mask coefficient and area coverage, the unfavorable factor of "actual incident angle deviation of the tooth surface" is transformed into a design object that can be quantitatively compensated in the matching of mask structure and parameters, so as to realize the design and repeatable control of the strengthening intensity of different areas.

[0009] To this end, the high-durability mask system provided by the present invention includes: a multi-layer composite mask body having a substrate layer, a wear-resistant layer, and a fatigue-resistant layer, used to ensure that the mask still has sufficient impact resistance and fatigue resistance under long-term shot peening conditions; a mask opening with a specific tilt angle and rounded corner structure, used to reduce shear stress concentration at the opening edge and improve the boundary clarity of the reinforced area; a precision positioning and fixing system based on a combination of carbide positioning pins, vacuum adsorption or magnetic fixation, and flexible sealing strips, used to stably and repeatably limit the relative position between the mask and the workpiece during shot peening and prevent shot from entering the gap between the mask and the workpiece; and a dedicated shot peening process parameter system matched with the mask structure, used to effectively reduce the impact load borne by the mask and improve process consistency while ensuring coverage and residual compressive stress levels.

[0010] Accordingly, the application method of this invention achieves selective shot peening strengthening of typical components such as turbine blade tenons of aero-engines, tooth roots of precision gears, and medical implants through steps such as mask design and manufacturing, precise positioning and fixing of the mask and workpiece, implementation of shot peening parameters that meet specific constraints, and post-shot peening effect detection and mask maintenance. The multi-layer composite mask structure and special process parameters of this invention result in an overall increase in the service life of the mask compared to traditional single-layer metal mask solutions. The boundary accuracy of the shot peening area can be controlled within the boundary transition zone width range of 0.5–2.0 mm (the boundary transition zone width can be defined by the spatial distance from the stable zone to the non-strengthened zone according to strengthening effect indicators such as residual compressive stress, hardness, or coverage). The consistency of residual stress distribution can be characterized by the surface residual compressive stress dispersion, with a standard deviation, represented by σ, approximately 50–100 MPa, which helps reduce the manufacturing cost required for high-reliability service of complex components.

[0011] The technical solution of the present invention is as follows:

[0012] The overall technical solution of this invention consists of two closely related layers: a high-durability mask system and a matching mechanical shot peening selective reinforcement method.

[0013] A high-durability mask system includes a mask and a positioning and fixing system. The positioning and fixing system is used to position and clamp the mask and the workpiece to maintain a predetermined position and predetermined gap between the mask and the workpiece during shot peening, form a seal, and suppress vibration displacement. The mask has an opening for geometrically screening and blocking the shot stream and guiding the shot to contact the workpiece surface, thereby forming a selective shot peening reinforcement area.

[0014] The photomask has a multi-layer composite structure, including a substrate layer, a wear-resistant layer, and a fatigue-resistant layer. The wear-resistant layer is located on the working surface side of the substrate layer, and the fatigue-resistant layer is located on the opposite surface of the substrate layer. Preferably, the substrate layer is made of high-strength tool steel, cemented carbide, or high-strength corrosion-resistant stainless steel. For high-load conditions such as turbine blade tenons and precision gears in aero-engines, the substrate layer is made of cold work die steel (such as SKD11 or Cr12MoV) with a tensile strength of not less than 1400 MPa and a Rockwell hardness of not less than HRC after quenching and tempering of not less than 58, or WC-Co series cemented carbide with a room temperature Vickers hardness of not less than 900 HV. For medical implant scenarios that require consideration of biocompatibility, medical 316L stainless steel sheet that has been cold-worked or surface-strengthened can be used as the substrate layer. The mask thickness is typically 2-10 times the diameter d of the shot used, preferably 3-6 times, to ensure sufficient rigidity and bending strength while avoiding increased quality and processing costs due to excessive thickness. For high-precision selective reinforcement of micro-holes where the minimum opening width W of the mask is <0.30 mm, the mask thickness can be further increased to 8-10 times the diameter d of the shot used to improve the overall rigidity and sealing reliability of the mask. A wear-resistant layer is deposited on the working surface of the substrate layer using physical vapor deposition (PVD) with a TiN, TiAlN, CrN, or AlCrN hard coating. The coating thickness is 2-10 μm, preferably 2-5 μm, and the microhardness is not less than 2000 HV, used to improve the mask surface's resistance to erosion and pitting corrosion. A fatigue-resistant layer is formed by nitriding or carbonitriding on the back side of the substrate layer to a depth of 0.1-0.3 mm, to improve the fatigue resistance and dimensional stability of the mask under repeated shot peening loads.

[0015] The average gap g between the mask and the workpiece during shot peening satisfies 0 < g ≤ d, preferably g ≤ 0.5d, where d is the diameter of the shot used.

[0016] Regarding the opening structure, the sidewalls of the opening on the mask are machined with an inclination angle of 15°–45°, so that projectiles approaching the opening edge are deflected in a controlled manner along the sidewalls upon impact, thereby reducing local stress concentration when the projectile impacts the opening edge and slowing down edge wear; preferably, to further mitigate stress concentration and ensure projectile passage, the opening edge is rounded.

[0017] (1) When the minimum width W of the corresponding opening is 0.30 mm ≤ W < 0.50 mm, the fillet radius R is 0.10 mm ~ 0.30 mm, and satisfies R ≤ 0.50 W. (2) When the minimum width W of the corresponding opening is 0.50 mm ≤ W ≤ 1.00 mm, the fillet radius R is 0.10 mm ~ 0.50 mm, and satisfies R ≤ 0.50 W. This balances edge stress mitigation and effective channel cross-sectional area. (3) When the minimum width W of the corresponding opening is < 0.30 mm (whether it is a micro-hole array of 0.1 mm ≤ W < 0.3 mm or an extremely fine micro-hole of < 0.1 mm), to avoid excessive shrinkage of the effective channel due to the fillet size being equal to or even larger than the hole diameter, the fillet radius R can be selected as 0.20 W ~ 0.40 W according to the geometric similarity principle, and satisfies R < W, so that R and the hole diameter are on the same order of magnitude, ensuring that the projectile passes smoothly through the micro-hole channel while reducing stress concentration at the opening edge. For applications requiring particularly fine patterns, a composite processing technology combining laser cutting and electrochemical polishing can be used to improve the accuracy of the opening contour and surface quality.

[0018] The positioning and fixing system includes positioning pins, vacuum adsorption, magnetic fixing devices, or flexible sealing components and combinations thereof, to achieve high-precision repeatable positioning and stable clamping between the mask and the workpiece, and to form an effective seal during shot peening to prevent shot from entering the gap between the mask and the workpiece.

[0019] Locating pins are used to connect the photomask to the workpiece or fixture. The photomask has at least three non-collinear locating holes, and the workpiece or fixture has corresponding locating holes or slots at their respective positions. The positioning system uses a three-point positioning principle, determining the photomask's position in space through three non-collinear locating points to ensure positioning stability and repeatability. Preferably, the locating pins are made of carbide, and the preferred dimensional tolerance grade is IT6. Simultaneously, the locating holes on the photomask and the corresponding locating holes on the workpiece / fixture should have the same or equivalent tolerance grades, and stable constraints are achieved in conjunction with a datum surface / limiting structure. By controlling the positioning clearance (usually no more than 0.015 mm) and using locking and vibration damping measures, the repeatability error of the photomask relative to the critical area of ​​the workpiece can be controlled to the order of 0.01 mm.

[0020] The fixing system is configured according to the workpiece shape and material properties. For near-planar workpieces, vacuum adsorption fixing is preferred, forming a sealed chamber between the mask and the workpiece with a pressure reduction of not less than 0.08 MPa based on atmospheric pressure. For this purpose, a closed annular flexible sealing strip is provided around the perimeter where the mask contacts the workpiece; the sealing strip is compressed during vacuuming to eliminate local gaps. For workpieces with good magnetism, a permanent magnet array flush with the back of the mask can be provided, preferably with a normal adsorption force (holding force) per unit area of ​​the permanent magnet array of not less than 50. N / cm² (can be determined through standard pull-out / shear tests); all magnets are embedded inside a rigid backing plate, with the backing plate as a whole conforming to the workpiece surface, avoiding gaps caused by local magnet protrusions; for components with complex contours (non-planar) or free-form surfaces, a contour-following fixture matching the workpiece shape is used, and mechanical clamping structures are set at key locations. For such workpieces, a combination of locating pin structures and reference limiting structures can be used to control the overall repeatability positioning error of the mask relative to the critical section to the order of 0.01mm. For individual workpieces that are not suitable for locating holes, vacuum adsorption or magnetic fixation can be used, but reusable reference surfaces or limiting structures still need to be set on the tooling or workpiece shape to ensure that the relative position between the mask and the area to be strengthened meets the process requirements.

[0021] The flexible sealing strip is made of silicone rubber, polyurethane or fluororubber, and has a compression rate of 15%–35% under working conditions, preferably 20%–30%.

[0022] Based on this hardware, a dedicated shot peening process parameter system matching the mask structure is further established, and a mechanical shot peening selective reinforcement method suitable for typical complex components is formed accordingly.

[0023] Preferably, the high-durability photomask system also includes one or more of the following functional modules:

[0024] 1) In some embodiments, an online monitoring unit may be optionally integrated for real-time monitoring of the mask's wear status and positional displacement; the online monitoring unit includes at least: a displacement sensor (such as an inductive or optical displacement sensor) arranged on the back of the mask for monitoring the mask's minute deflection and overall displacement during shot peening; strain gauges or force sensors arranged near the mask's edge or key opening area for monitoring the impact load level borne by the mask; and an industrial camera facing the mask-workpiece contact area for acquiring characterizing images of the mask's surface wear, cracks, and opening edge damage; the data acquired by the above sensors and camera are uploaded to the host computer in real time via a data acquisition module and a signal processing module to realize online assessment and early warning of the mask's wear and positioning status;

[0025] 2) The parameter optimization module, combined with artificial intelligence algorithms, adaptively optimizes the shot peening process parameters based on online monitoring data. The parameter optimization module uses shot peening pressure p, shot peening time t, spray gun travel speed v, spray frequency f_p (f_p is the spray frequency or shot peening pulse frequency, representing the number of spray pulses per unit time or impeller speed of the shot peening equipment, in Hz), mask movement step distance s, and target coverage C as input variables. It uses the residual compressive stress level in key areas, coverage uniformity index, and cumulative mask damage as optimization objectives or constraints. A multilayer feedforward neural network or other nonlinear regression model is used to establish the mapping relationship between "process input—strengthening effect—mask damage." In the offline stage, the model is trained based on historical experimental data and finite element simulation results. In the online stage, the model input is updated using real-time data obtained from the aforementioned monitoring unit. Iterative search or gradient optimization algorithms provide process parameter adjustment suggestions that satisfy the target residual compressive stress distribution and mask life constraints.

[0026] From a systems engineering perspective, the overall technical solution of this invention consists of the following three parts working together: (1) Design and manufacturing of a high-durability mask: a multi-layer composite structure of substrate layer / wear-resistant layer / fatigue-resistant layer is adopted, and combined with opening geometry (such as sidewall inclination and rounded edges) and material and heat treatment / surface modification process parameters, to achieve stable shaping of the shot flow and improve wear resistance and fatigue resistance; (2) Precision positioning and fixing system of mask and workpiece: through positioning pins, clamps, vacuum adsorption or magnetic fixing and flexible sealing components, high-precision repeatable positioning, stable clamping and sealing protection of mask and workpiece are achieved; (3) Special shot peening process parameter system and application method for mask structure: on the basis of meeting the constraints of shot diameter to opening width ratio d / W, mask coefficient η, etc., combined with coverage and residual stress targets, strength zoning and equivalent strengthening are achieved by adjusting parameters such as shot peening pressure / flux setting and shot peening time.

[0027] The specific steps are as follows:

[0028] S1. In the design and manufacturing of high-durability photomasks, a multi-layer composite structure design is adopted; and different aperture density areas, local thickening or thinning sections, and whether or not to use detachable masking blocks are designed on the photomask.

[0029] S2. In terms of precision positioning and fixing systems, positioning pins, vacuum adsorption, magnetic fixing devices, or flexible sealing components are selected for individual or combined positioning, depending on the workpiece or tooling fixture.

[0030] S3. Regarding the dedicated shot peening process parameter system, a set of optimized parameters for photomask application scenarios has been established.

[0031] The ratio between the projectile diameter d and the minimum opening width W of the mask should satisfy 0.6W≤d≤0.9W. When d / W is less than 0.6, the width of the pit formed by a single projectile is significantly smaller than the opening width, and uncovered strip areas are likely to appear between the pits. It is difficult to completely eliminate the blank areas by simply extending the shot peening time, and a large amount of projectile kinetic energy is consumed in repeated impacts on the covered areas, resulting in low kinetic energy utilization. When d / W is greater than 0.9, the overlap ratio of the impact areas of adjacent projectiles is too high. Although the surface coverage is easier to meet the requirements, it will lead to excessive local plastic deformation, which can easily produce defects such as microcracks and peeling on the surface or near the surface, and cause a decrease in energy utilization. For a micro-orifice array with a minimum opening width W < 0.30 mm, to achieve a high and uniform coverage within a finite shot peening time, it is preferable to select the shot diameter within the range of 0.8 ≤ d / W ≤ 0.9. Within this range, the number of effective shots passing through the mask opening area per unit time increases, while the local overlap of individual shots remains within an acceptable range, which is beneficial for achieving a high coverage without significantly increasing surface roughness. The shot peening time t is determined by the following formula: t = (A×C_eq) / (N×S_e×η), where A is the effective area to be shot peened, C_eq is the equivalent effective impact density per unit area (particles / mm²) required to achieve the target shot peening coverage C; the target shot peening coverage C can be selected according to process requirements and converted to C_eq through empirical relationship of coverage-impact density or experimental calibration; N is the nominal effective impact number per unit time and per unit area under unmasked reference conditions, particles / (s·mm²), which can be obtained by calibrating the effective shot flux Q_nominal (particles / s) and the equivalent area S0 (mm²) of the shot peening spot under unmasked reference conditions (e.g., N=Q_nominal / S0); where S0 is the equivalent area (mm²) of the shot peening spot on the workpiece surface under unmasked reference conditions, which can be obtained by measuring the shot peening spot imprint, calibrating the coverage, or using the parameters of the spray gun manufacturer under the same spray conditions. The effective impact area (mm²) of the shot peening spot or mask opening group within a spray gun dwell unit / control cycle is determined under the conditions of shot peening distance / angle. In the reference condition without a mask, S_e can be taken as S0. It is used to convert the flux density into the effective total impact velocity within the dwell unit. η is the mask coefficient, which ranges from 0.6 to 0.8 and is used to characterize the comprehensive attenuation effect of the mask structure on the projectile flux. To ensure the measurability and traceability of process parameters, this invention defines the mask coefficient η as: under the same shot peening equipment, shot peening pressure, spray distance and spray angle conditions, the effective projectile flux that actually acts on the area to be strengthened through the mask opening is the ratio of the effective projectile flux that acts on the same area under the condition without a mask. The effective projectile flux can be measured by the projectile mass flow rate per unit time (weighing method) or the projectile counting flux (high-speed camera counting method), thus obtaining η = Q_mask / Q_nominal.Under the premise of satisfying the above-mentioned d / W and η design conditions, in order to reduce the impact load on the mask and extend its service life, this invention preferably appropriately lowers the shot peening pressure compared to the traditional maskless shot peening process for the same workpiece, and compensates for this by appropriately extending the shot peening time or increasing the coverage C, thereby reducing damage to the mask while maintaining the target residual compressive stress level. The adjustment range can be calibrated and determined by combining typical specimen tests, online monitoring results, and engineering constraints. The target residual compressive stress distribution can be obtained by combining the X-ray residual stress test results of typical specimens with the finite element simulation results. Then, the required residual compressive stress level in different regions is mapped to the corresponding target coverage and d / W value range. Substituting these values ​​into the above formula, the shot peening time and mask movement strategy for each region can be determined, thereby realizing the reverse design and repeatable control of the residual compressive stress field in the complex curved tooth surface / tooth root region.

[0032] In some embodiments, the technical solution may also optionally integrate an online monitoring unit for real-time monitoring of the wear state and positional displacement of the mask; furthermore, it may incorporate artificial intelligence algorithms to adaptively optimize the shot peening process parameters. The aforementioned online monitoring and intelligent optimization modules are optional functions and do not constitute essential technical features for achieving the basic technical effects of this invention.

[0033] By designing regions with different aperture densities, locally thickened or thinned sections, and using detachable shielding blocks on the mask, the residual stress distribution on the workpiece surface can be controlled and adjusted. In areas requiring higher residual compressive stress, a higher aperture density is designed, coupled with a larger d / W ratio and a higher target shot peening coverage. In areas requiring relatively low residual compressive stress or a smooth stress transition, the number of effective shot per unit area is reduced by decreasing the aperture density, decreasing the d / W ratio, or increasing the local shielding thickness. Through the coordinated design of multifunctional mask structures and process parameters, residual stress gradient control along the tooth surface-to-tooth root direction or along the direction of complex curved surface contours can be achieved in a single shot peening process.

[0034] It should be noted that even if the mask openings have perfectly uniform geometry and a strictly regular array distribution, the actual impact kinetic energy and normal component of each shot passing through the opening and reaching the workpiece surface will still differ due to random factors such as the incident angle, instantaneous velocity, and collisions between shot particles during mechanical shot peening. In the central region of the opening, the incident angle of the shot is closer to the vertical direction, resulting in a larger normal impact component; while shot particles closer to the opening edge are more prone to partial obstruction or edge scattering, leading to a slightly lower equivalent impact intensity. Therefore, strictly speaking, the actual strengthening degree of each micro-region cannot be completely uniform; it can only exhibit certain random fluctuations around the target value.

[0035] To address the aforementioned inhomogeneity in strengthening caused by random factors, this invention does not pursue absolute uniformity at the level of a single impact, but rather employs a statistical averaging engineering approach for compensation. In the shot peening time calculation formula proposed in this invention, t = (A × C_eq) / (N × S_e × η), C_eq represents the equivalent effective impact density per unit area required to achieve the target shot peening coverage C, and its value monotonically corresponds to the target shot peening coverage C. The target coverage C statistically reflects the average number of impacts per unit area. When the shot peening time is short and the coverage is low, the effective impact count in a certain micro-region is low, and its local strengthening degree is highly sensitive to fluctuations in the energy and incident angle of a single shot, manifesting as large fluctuations in residual compressive stress and hardness. By appropriately extending the shot peening time and increasing the shot peening coverage C, the average number of impacts within each opening projection area can be significantly increased, transforming the local strengthening result from being dominated by a small number of impacts to a statistical averaging effect of multiple impacts, thereby effectively reducing strengthening inhomogeneity and improving overall uniformity without excessively roughening the surface. The above shot peening time formula is an empirical formula, mainly used to provide a reasonable initial estimate of shot peening time t in the process design stage. In the actual production process, t can be finely adjusted by combining online monitoring results and experimental feedback.

[0036] In addition to setting different shot peening coverage rates to match the processing methods of areas requiring different strengths during the same shot peening process, this invention also allows for a "partial coverage treatment" strategy to facilitate on-site construction and process adjustments. Specifically, technicians can divide the entire surface to be treated into several sub-regions based on the workpiece geometry and target strength distribution. In each step, only one or several sub-regions are opened with mask openings or have partial additional shielding removed, while the remaining areas remain covered and untouched by shot peening. By setting the shot peening time and target shot peening coverage rate for each sub-region, different effective impact counts and residual stress levels can be achieved for each sub-region. Through the above-mentioned step-by-step shot peening by region, selective shot peening treatment with different strengthening intensities between different regions can also be achieved.

[0037] This invention utilizes a multi-layered composite structure design for the photomask to improve its wear resistance and fatigue resistance under high-speed impact; a special opening edge treatment process helps to reduce boundary effects and improve the clarity of the reinforced area boundary; a high-precision positioning and fixing system helps to maintain the stability and repeatability of the photomask under strong vibration conditions; and a dedicated process parameter system is optimized for the application scenarios of the photomask, which helps to improve the consistency of process results.

[0038] The beneficial effects of this invention are as follows:

[0039] Compared with the prior art, the present invention has the following main advantages:

[0040] (1) Improved mask life: At the structural level, this invention designs a multi-layer composite mask consisting of a substrate layer, a wear-resistant layer, and an anti-fatigue layer for high-speed mechanical shot peening loads. It also provides clear parameter ranges for material selection, layer thickness, hardness, and nitriding depth, which helps the mask maintain the relative stability of the opening geometry and size under long-term repeated shot peening conditions and achieve continuous shaping of the shot flow. Under typical shot peening conditions, the multi-layer composite mask shows better stability in terms of service life compared to conventional single-layer mask designs. It can be reused ≥300 times, and the wear resistance / fatigue resistance can be improved by using common parameter ranges such as PVD coating thickness of 2–5 μm and effective nitriding layer depth of 0.1–0.3 mm. This helps reduce downtime and maintenance workload caused by mask replacement.

[0041] (2) Improved boundary accuracy: At the process parameter level, this invention proposes a new parameter system with the mask coefficient η as the core. The influence of mask opening density, opening geometry and mask workpiece spacing on the effective number of shot is summarized into a quantifiable mask coefficient, and it is introduced into the shot peening time formula t = (A×C_eq) / (N×S_e×η) together with the matching relationship of coverage C and shot diameter d / opening width W. Thus, the balance between coverage, residual compressive stress level and mask life is comprehensively considered in the design stage. By optimizing the matching relationship between opening geometry and shot parameters, the boundary clarity of the enhanced area can be controlled within the boundary transition zone width range of 0.5–2.0 mm under reasonable process conditions (the boundary transition zone width can be defined by the spatial distance of the transition from the stable area to the non-enhanced area according to the enhancement effect indicators such as residual compressive stress, hardness or coverage). It has certain advantages over the traditional method.

[0042] (3) Enhanced process stability: In terms of strengthening effect, the present invention introduces programmable selective opening arrangement and local masking / thinning unit in the mask structure, and combines precise positioning and movement control to enable different areas of the workpiece surface to obtain different but pre-set strengthening strength and residual compressive stress distribution in the same shot peening process. It is especially suitable for dangerous sections such as tooth surface / tooth root. While satisfying the equivalent strengthening strength of the tooth surface, a relatively gentle compressive stress gradient is formed in the tooth root region, which is conducive to realizing two-dimensional or even three-dimensional control of the residual stress field. The consistency of strengthening effect is improved, which can be characterized by the surface residual compressive stress dispersion (standard deviation, characterized by σ) of about 50–100 MPa. If expressed as the average value ±1σ, the corresponding fluctuation range is about ±(50–100) MPa.

[0043] (4) Wide range of applications: The mask system of the present invention can be applied to workpieces with various geometric shapes such as plane, curved surface and complex surface, and is suitable for various metal materials such as steel, aluminum, and titanium. It has good application prospects in typical scenarios such as turbine blade tenons of aero-engines, tooth roots of precision gears and medical implants. It is particularly suitable for the surface strengthening needs of precision parts in aerospace, automobile manufacturing, medical device and other fields.

[0044] (5) There is room for overall cost reduction: Although the manufacturing cost of a single mask is higher than that of traditional solutions, under reasonable working conditions, its lifespan is relatively long and the reinforcement quality is relatively stable. From the perspective of the overall service life, the overall service cost has the potential to be reduced. The specific economic benefits are affected by factors such as component type and batch size.

[0045] In key areas such as complex curved tooth surfaces / tooth roots, this invention, based on the above advantages, also has the following further technical effects: (1) Equal strength processing capability: Under the premise that the projectiles inevitably have tilted incident in complex curved areas such as tooth surfaces, by increasing the mask opening density and cooperating with a higher shot peening coverage, the total number of shot actions per unit area increases, which makes up for the problem of reduced effective strengthening strength caused by the reduction of the normal component of a single shot. The test results show that the surface residual compressive stress and action depth measured in the tooth surface area are highly consistent with the theoretical expected value, and the deviation is significantly reduced compared with the ideal vertical incident condition, which meets the application requirements of "equal strength processing" in engineering. (2) Synergistic control of gradient strengthening and life improvement: By differentiating the opening density and coverage of the tooth surface and tooth root areas, a gradient distribution that smoothly transitions from high residual compressive stress to low residual compressive stress is formed at the tooth root fillet. Compared with the uniform strengthening scheme, the bending fatigue life of the gear is improved, and no obvious cracking or spalling failure occurs on the surface. (3) Process designability and promotion value: Combining the shot peening time calculation formula t= (A×C_eq) / (N×S_e×η), this invention can obtain the target residual stress distribution based on the residual stress test results and finite element simulation results of typical workpieces or representative samples. Then, the target distribution is decomposed into the target coverage, opening density and d / W value range of each region, realizing the reverse design from residual stress target to mask structure and shot peening parameters. This transforms the local strengthening intensity on complex curved surfaces from empirical adjustment to a process that is calculable, designable and repeatable. At the same time, this compensation idea accepts the practical constraint that "tooth surfaces are difficult to be completely perpendicular to the incident surface", and can be extended to various spatially limited and curvature complex selective shot peening scenarios, which has high engineering practicality and promotion value. Attached Figure Description

[0046] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0047] Figure 1 : Flowchart of the application method of this invention;

[0048] Figure 2 : A schematic diagram of the shot peening process in this invention, in which the mask moves to the left twice;

[0049] Figure 3 : A schematic diagram of shot peening in which the mask moves forward once in this invention;

[0050] Figure 4 : Schematic diagram of shot peening strengthening of the gear (curved surface) area to be treated in this invention;

[0051] The components are: 1. The initial position of the mask; 2. The workpiece; 3. The disordered shot peening flow; 4. The opening of the mask; 5. The initial shot peening position; 6. The position of the mask when it first moves to the left; 7. The position of the mask when it first moves to the left; 8. The position of the mask when it moves to the left for the second time; 9. The position of the mask when it moves to the left for the second time; 10. The position of the mask when it moves forward; 11. The position of the mask when it moves forward; 12. The tooth root region; 13. The tooth surface region; 14. The position of the tooth surface shot peening; 15. The position of the tooth root shot peening; 16. The gap between the mask and the workpiece. Detailed Implementation

[0052] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0053] Example 1: Precision reinforcement of the tenon joint of aero-engine turbine blade

[0054] This embodiment addresses the technical challenge of high-precision selective reinforcement of the tenon area of ​​aero-engine turbine blades. Traditional shot peening methods struggle to guarantee the accuracy of the reinforcement boundary in this area, and the mask plate suffers severe wear under high-speed shot impact.

[0055] A high-durability mask system includes a mask and a positioning and fixing system. The positioning and fixing system is used to position and clamp the mask and the workpiece to maintain a predetermined position and predetermined gap between the mask and the workpiece during shot peening, form a seal, and suppress vibration displacement. The mask has an opening for geometrically screening and blocking the shot stream and guiding the shot to contact the workpiece surface, thereby forming a selective shot peening reinforcement area.

[0056] The photomask has a multi-layered composite structure, including a substrate layer, a wear-resistant layer, and a fatigue-resistant layer. The wear-resistant layer is located on the working surface side of the substrate layer, while the fatigue-resistant layer is located on the opposite side of the substrate layer. In the design and manufacturing of the photomask, the substrate layer material is powder metallurgy high-speed steel ASP23, with a hardness of approximately 64 HRC. A TiAlN coating with a thickness of 5 μm and a hardness of approximately 3200 HV is deposited on the surface using physical vapor deposition (PVD). Simultaneously, the ASP23 substrate on the side opposite to the shot peening is nitrided to form a fatigue-resistant layer with a thickness of approximately 0.1–0.3 mm. Structurally, the photomask thickness is designed to be 1.2 mm, four times the diameter of the selected 0.3 mm shot. The sidewalls of the photomask opening are machined at a 30° angle, and the opening contour is machined using a slow wire EDM process, achieving a dimensional accuracy of ±0.005 mm. Subsequently, ultrasonic grinding further improves the surface quality and edge condition. Meanwhile, the opening edges are rounded, with the minimum opening width W of the mask being 0.4mm and the rounded corner radius R being 0.2mm.

[0057] For positioning and fixing, the mask adopts a three-pin positioning system with 6mm diameter positioning pins and an IT6 tolerance grade. The fixing method is a modular vacuum adsorption fixture, whose vacuum circuit can be independently controlled in zones to ensure uniform distribution of adsorption force throughout the entire mask area. To prevent projectiles from intruding into the contact interface between the mask and the workpiece, fluororubber sealing strips are installed at the edge of the mask, with the compression rate controlled at 25% during operation, thus forming a stable, sealed chamber.

[0058] The shot peening process parameters were set to use cast steel shot with a diameter of 0.3 mm and a hardness of 45-50 HRC. Equipment parameters were set to a pressure of 0.25 MPa, a shot peening angle of 90°, and a distance of the peening gun from the workpiece surface of 100 mm. The initial setting value of the shot peening time t was given using the formula t = (A×C_eq) / (N×S_e×η), where A is the effective area to be shot peened; C_eq is the equivalent effective impact density per unit area (particles / mm²) required to achieve the target shot peening coverage C; the target shot peening coverage C can be selected according to process requirements and converted to C_eq through empirical relationship between coverage and impact density or experimental calibration; N is the nominal effective impact number per unit time and unit area (particles / (s·mm²)) under the unmasked reference condition, which can be obtained from the effective shot flux Q_nominal (particles / s) and the equivalent area S0 (mm²) of the shot peening spot under the unmasked reference condition. ²) The calibration yields the following result (e.g., N = Q_nominal / S0); where S0 is the equivalent area (mm²) of the shot peening spot on the workpiece surface under the maskless reference condition, which can be determined by shot imprint measurement, coverage calibration, or spray gun manufacturer parameters under the same spray distance / angle conditions; S_e is the equivalent area (mm²) of the shot peening spot or mask opening group within one spray gun dwell unit / control cycle, which can be taken as S_e = S0 under the maskless reference condition; used to convert the flux density into the effective total impact velocity within the dwell unit; η is the mask coefficient, with a value range of 0.6–0.8, used to characterize the comprehensive attenuation effect of the mask structure on the projectile flux. During implementation, t is fine-tuned by combining online monitoring results and experimental feedback.

[0059] like Figure 2 As shown, to facilitate the illustration of the movement trajectory of the mask plate relative to the tenon groove, this embodiment equates the local curved critical section of the turbine blade tenon groove to an approximate planar view, and arranges a multi-layered composite mask plate above it. The mask plate itself is located at position 1, where the first mask plate was located during the first shot peening. Below it is the tenon workpiece 2, with a predetermined gap 16 maintained between them (for illustration purposes, this gap size is enlarged in the figure; in actual applications, this gap is smaller than the diameter of a single shot). Above it is the disordered shot flow 3 generated by the shot peening equipment. The mask plate is machined with an array of mask plate openings 4 corresponding to the critical section of the tenon. During the first shot peening process, only shot that meets the geometric channel conditions can pass through the openings 4. Their spatial distribution is filtered and shaped into an ordered shot bundle by the mask plate, forming the first shot peening reinforcement position 5 on the curved surface of the tenon groove.

[0060] It should be noted that turbine blade tenon tooth grooves also have two typical areas: the tooth surface and the tooth root (rounded transition). The method for dividing the critical section can be referred to Figure 4The gear tooth surface region 13 (high-strength region) and tooth root region 12 (low-strength region): the region closer to the tooth surface corresponds to the high-strength selection area, and the region closer to the tooth root fillet corresponds to the relatively low-strength selection area. By differentiating the mask opening arrangement and shot peening process parameters, the tenon tooth surface can obtain high-strength selection reinforcement, while the tooth root region can obtain relatively mild reinforcement, so as to balance fatigue life improvement and geometric accuracy control.

[0061] To further extend the reinforcing band along the length of the tenon and improve the uniformity of coverage, such as Figure 2 As shown in Figures (b) and (c), after the first shot peening, the mask plate, while maintaining a basically constant clearance 16 with the normal direction of the tooth groove surface, is positioned relative to the workpiece 2 along the tenon length direction (shown in the figure). Figure 2 Moving sequentially to positions 6 and 8 (from center to left) with a step distance smaller than the opening hole distance, and performing a second and third shot peening respectively, forming the second and third sets of shot peening reinforcement positions 7 and 9 on the curved surface of the tenon. There is a regular partial overlap between the three reinforcement positions 5, 7, and 9, which forms a reinforcement band along the length of the tenon with controllable width, clear boundaries, and the degree of reinforcement can be adjusted by multiple overlays.

[0062] When it is necessary to fine-tune the position of the reinforced area along the tenon tooth height or circumferential direction, the following can be used: Figure 3 The photomask is shown moving forward in this manner. The photomask completes... Figure 2 After multi-step length shot peening as shown, the hole array position corresponding to the first set of strengthening positions 5 is restored, and relative to the workpiece 2 along the tooth height or circumferential direction (illustrated as follows). Figure 3 The shot peening stream 3 moves slightly forward to position 10. At this point, the disordered shot peening stream 3 is shaped again through the mask opening 4, forming a forward-offset reinforced position 11 on the tooth groove surface. Figure 3 In essence, it gave Figure 4 The diagram illustrates the movement of a local curved surface after unfolding along the tooth height direction. By combining "multi-step movement along the length direction" and "micro-stepping along the tooth height / circumferential direction", a two-dimensional programmable selection reinforcement area can be constructed on the tenon tooth groove surface.

[0063] Compared to the control group, which used a traditional single-layer metal mask for selective reinforcement under the same shot peening pressure and coverage, the control group's mask had limited reusability under the same working conditions, and the residual compressive stress fluctuated relatively greatly. In this embodiment, after adopting the multi-layer composite mask system and supporting process parameters of this invention, the reinforced area boundary is clearer, and the boundary transition zone width can be controlled within the range of 0.5–2.0 mm. The reinforcement effect is manifested in the formation of a residual compressive stress layer of a certain thickness on the surface, with an influence depth of approximately 0.12 mm. Measurement results on multiple workpieces show that the dispersion (standard deviation, characterized by σ) of the residual stress value can be used as a quality control indicator on the order of approximately 50–100 MPa. This indicates that the process in this embodiment can be reused ≥300 times, and the wear resistance / fatigue resistance and consistency can be improved by using common parameter ranges such as PVD coating thickness of 2–5 μm and effective nitriding layer depth of 0.1–0.3 mm.

[0064] Example 2: Gradual reinforcement at the root of automotive gear teeth

[0065] This embodiment is designed to address the requirement of achieving a residual stress gradient distribution in the root region of automotive gears. Traditional uniform strengthening processes struggle to establish a reasonable residual compressive stress gradient between the tooth surface and the tooth root, making it difficult to simultaneously meet the requirements of bending fatigue life and avoiding over-hardening and cracking.

[0066] A high-durability mask system includes a mask and a positioning and fixing system. The positioning and fixing system is used to position and clamp the mask and the workpiece to maintain a predetermined position and predetermined gap between the mask and the workpiece during shot peening, form a seal, and suppress vibration displacement. The mask has an opening for geometrically screening and blocking the shot stream and guiding the shot to contact the workpiece surface, thereby forming a selective shot peening reinforcement area.

[0067] This embodiment employs a design scheme combining a variable-thickness mask with a gradually varying aperture density. By gradually increasing the mask thickness from the middle of the tooth root towards both ends (from 1.0 mm to 1.8 mm), and arranging aperture arrays of different densities in different regions, the shot throughput in each region can be precisely controlled, achieving a gradual distribution of the residual stress field in the tooth surface and root regions. The mask substrate material is selected as cemented carbide YG8, with a bending strength of approximately 1800 MPa, to ensure sufficient strength and rigidity under high impact loads. A thin, hard, wear-resistant coating can be further deposited on the working surface side of the substrate layer, and nitriding or similar surface modification treatments are performed on the side away from the shot peening to form a fatigue-resistant layer.

[0068] The positioning and fixing system is designed with a gear expansion mandrel clamp, achieving a positioning accuracy of 0.015mm. It is also equipped with a pneumatic auxiliary clamping device, whose clamping force can be adjusted within the range of 200 to 500N to accommodate the clamping requirements of gears of different specifications and suppress vibration displacement during shot peening.

[0069] For process parameter optimization, 0.2mm diameter ceramic pellets were selected as the shot medium. The shot peening pressure was set to 0.3MPa, and a robot was used for programmed control of the shot peening trajectory to achieve precise area coverage. Different spray gun dwell times were used in different areas to match the designed intensity distribution. The mask opening width W was set to 0.3mm, and the opening corner radius R was set to 0.1mm.

[0070] To address the need for stress gradient distribution at the gear tooth root, while simultaneously addressing the insufficient actual strengthening due to the oblique incidence of projectiles on the tooth surface, such as... Figure 4 As shown, in this embodiment, the gear tooth profile is divided into a tooth root region 12 (low-strength region) and a tooth surface region 13 (high-strength region). The mask is clamped near the outer circle of the gear tooth tip and maintains a predetermined small gap 16 between it and the curved surface (the figure shows an enlarged schematic; the actual distance is less than the diameter of a shot, usually controlled within the range of 0.1d–0.5d). Above it is the disordered shot flow 3 generated by the shot peening equipment.

[0071] In the tooth surface region 13, due to the limitations of tooth surface curvature and clamping posture, the actual incident angle of the projectile is difficult to be completely perpendicular to the tooth surface, resulting in a relatively low effective strengthening strength of the surface in this region under the same shot peening pressure and time conditions. To solve this problem, this embodiment designs a high opening density on the mask region corresponding to the tooth surface, which significantly increases the number of ordered projectile bundles formed after the disordered shot peening flow 3 passes through the mask opening 4. In the process parameter settings, a high target coverage rate is configured for the tooth surface region. By increasing the shot peening time or adjusting the mask movement step distance, multiple repeated coverage of the tooth surface region can be achieved. Even if the effective impact strength of a single projectile is reduced due to the incident angle deviation, by "peening more projectiles and the same point more times", the shot peening strengthening position 14 of the tooth surface can still obtain a residual compressive stress level that is statistically equivalent to the ideal perpendicular incident condition, thereby achieving equivalent strength processing of the complex curved tooth surface region.

[0072] Conversely, in the tooth root region 12, this embodiment limits the number of shot peening operations per unit area at the tooth root by reducing the mask aperture density and setting a lower coverage rate. This ensures that the tooth root shot peening reinforcement location 15 is only subjected to a milder shot peening effect, establishing a gradual distribution of residual compressive stress from high to low at the tooth root fillet. This reduces stress concentration at the tooth root and avoids the risk of cracking due to over-hardening. Figure 2 , Figure 3The multi-step movement of the mask along the tooth width and tooth height directions and the differentiated setting of the spray gun dwell time shown can create a two-dimensional programmable selective reinforcement zone between the tooth surface and the tooth root, achieving a smooth transition of residual stress at the tooth root while satisfying the equivalent reinforcement strength of the tooth surface.

[0073] To facilitate the explanation of the mask movement trajectory, Figure 2 The unfolded view of the locally equivalent planar target is still used: the mask version is at the initial position 1 of the first shot peening, the gear workpiece is below it 2, and the disordered shot peening flow is above it 3. During the first shot peening, a reinforced position 5 is formed on the curved unfolded plane. Subsequently, as... Figure 2 As shown in Figures (b) and (c), the mask plate maintains a substantially constant gap 16 with the curved surface along the tooth width direction (illustrated as follows). Figure 2 Moving sequentially from center to left, the gun is moved to positions 6 and 8, respectively, for the second and third shot peening, forming the second and third sets of reinforced positions 7 and 9 near the tooth root and tooth surface. By rationally designing the moving step distance and spray gun dwell time at each position, a stress gradient from high strength on the tooth surface to relatively low strength at the tooth root can be achieved in the tooth width direction.

[0074] When further fine-tuning of the reinforced region position is required in the tooth height or pitch circle direction, this embodiment employs the following method: Figure 3 The mask forward movement strategy is shown. The mask completes... Figure 2 After the transverse multi-step shot peening shown, the hole array position corresponding to the first set of strengthening positions 5 is restored, and relative to gear 2 along the tooth height direction (illustrated as follows). Figure 3 (In the forward direction) it makes a small step to position 10. When shot peening is performed again, the disordered shot peening flow 3 passes through the opening 4 and forms a forward-shifted strengthening position 11 on the tooth profile development plane. Figure 3 Can be regarded as Figure 4 The diagram shows the forward and backward movement of the tooth surface region 13 and the tooth root region 12 after being partially expanded along the tooth height direction. This is used to refine the distribution of reinforcement positions within the critical area of ​​the tooth profile.

[0075] The results of the performance verification show that the expected stress gradient was achieved at the tooth root: the surface residual compressive stress was -850 MPa, which decreased to approximately -600 MPa at a depth of 0.1 mm. Compared with the control group using a uniform strengthening scheme, the potential benefits of the technical route in this embodiment in terms of fatigue performance can be illustrated by referring to the typical improvement of similar strengthening processes reported in public publications, such as a 10%–30% increase in fatigue strength, or a 3–10 times increase in fatigue life under high-cycle fatigue conditions (this range is for illustrative purposes only and does not constitute a conclusion from actual measurements in this embodiment). After 300 consecutive uses, the mask version still maintained good working condition, without obvious cracks or failures, indicating that the mask design has good durability.

[0076] Example 3: Selective Area Strengthening of the Surface of Medical Implants

[0077] This example is for the special application of micro-area strengthening of the surface of medical implants (titanium alloy materials). This scenario places extremely high requirements on strengthening accuracy and pollution-free conditions.

[0078] The mask plate adopts an ultra-thin design with a thickness of 0.6 mm, and a micro-hole array with a pore diameter of 0.075 mm is processed and arranged in the working area. The corresponding minimum opening width W = 0.075 mm. The material is selected as 316L stainless steel, and the surface is polished to a mirror finish, with the surface roughness Ra value not greater than 0.1 μm. In this example, the mask plate body is also composed of a substrate layer, a wear-resistant layer, and an anti-fatigue layer: a 316L stainless steel thin plate is used as the substrate layer, and a biocompatible hard coating with a deposition thickness of 5 - 10 μm (such as TiN or diamond-like coating) can be optionally configured on the working surface side to form a wear-resistant layer. On the side facing away from the workpiece, a thin anti-fatigue layer is constructed by ion implantation or low-temperature nitriding to further improve the wear resistance and anti-fatigue performance of the mask plate without affecting the biological safety of medical devices. It should be noted that the mask thickness in this example is about 10 times the diameter of the used shot (0.06 mm), which is a thickening optimization for the micro-hole high-precision selective area strengthening scenario based on the above general thickness design principle to improve the stiffness and sealing reliability of the mask plate. According to the principle of micro-hole array parameter design in step S3, in the micro-hole array scenario where W ≤ 0.20 mm, it is preferably 0.80 ≤ d / W ≤ 0.90. In this example, glass shots with a diameter d = 0.06 mm are selected, making d / W = 0.80, which not only ensures good passability of the shots but also helps to obtain a high and uniform coverage rate within a limited shot peening time.

[0079] The opening fillet R of the mask plate is taken as 0.025 mm. According to the previous design of the opening structure, when the minimum opening width W is in the micro-hole array interval of W < 0.30 mm, the fillet radius R is preferably taken as 0.20W - 0.40W and satisfies R < W; for extremely fine holes with a smaller opening width W < 0.10 mm, the fillet radius R is also scaled proportionally according to the above principle. In this example, W = 0.075 mm, R ≈ 0.33W, which falls within the range of 0.20W - 0.40W and is less than W, which can reduce the stress concentration at the opening edge while ensuring the smooth passage of the shots through the micro-hole channel.

[0080] The positioning and fixing system adopts a high-precision optical alignment system, which can achieve an alignment accuracy of ±0.005 mm. The fixing method uses vacuum adsorption. The mask plate forms a sealed contact with the workpiece through a peripheral flexible sealing strip or isolation pad in the non-strengthened area to establish a negative pressure chamber, and a small gap is maintained in the strengthened area to avoid hard contact, thereby reducing the risk of scratches or indentations.

[0081] Regarding process parameters, glass pellets with a diameter of only 0.06 mm are used as the shot medium, and a negative pressure shot peening system is employed to reduce the risk of particle rebound and contamination during processing. Shot peening is carried out in a low-pressure mode (0.1 MPa), with a short single-shot duration. The target coverage is achieved through multiple short-time loadings combined with mask movement.

[0082] like Figure 2 As shown in Figure (a), the ultrathin mask is first aligned with the target micro-region on the implant workpiece 2 using a high-precision optical alignment system. The mask itself is at the initial position 1 of the first shot peening, and the micro-aperture array 4 arranged on the mask corresponds to the predetermined micro-region position. Above is a disordered shot peening flow 3 formed by glass pellets with a diameter of only 0.06 mm. During the first shot peening process, only a very small number of glass pellets that meet the geometric conditions pass through the micro-apertures 4 and are strictly selected and shaped into micro-projectile bundles with basically fixed spacing, forming the first micro-region reinforcement position 5 on the implant surface.

[0083] To form a regularly arranged array of micro-regions on the implant surface, in this embodiment, the mask is applied along a certain tangential or axial direction (illustrated as shown) on the implant surface after the first shot peening. Figure 2 Make a small movement (from center to left), such as... Figure 2 As shown in Figures (b) and (c), the process moves from position 1 to position 6 to complete the second shot peening, resulting in the second set of micro-area enhancement positions 7; then, it moves to position 8 to complete the third shot peening, resulting in the third set of micro-area enhancement positions 9. By setting the movement step distance to be slightly greater than or slightly less than the micropore center distance, a flexible coverage pattern of closely adjacent or partially overlapping micro-areas can be achieved to meet different biomechanical enhancement requirements.

[0084] When it is necessary to expand or fine-tune the position of the micro-area array in another direction, this embodiment also employs... Figure 3 The mask is moved forward as shown. The mask is restored to the position of the aperture array corresponding to the first group of micro-area enhancement positions 5, and relative to the implant 2, in a direction perpendicular to the aforementioned direction (illustrated as...). Figure 3 The glass pellets (3) move forward to position 10. Upon re-peening, the disordered glass pellet stream 3, after passing through the micro-holes 4, forms forward-shifted micro-area reinforcement positions 11 on the workpiece surface. By combining micro-stepping motions in two orthogonal directions, this embodiment can construct a two-dimensional, fine micro-area reinforcement matrix on the implant surface. Simultaneously, due to the strict restriction of pellet passage by the mask, unreinforced areas remain smooth and free of foreign matter residue, meeting the extremely high requirements for cleanliness and controllability of medical implants.

[0085] Final results verification shows that this embodiment successfully achieved localized strengthening of a micro-region with a feature size of approximately 0.075–0.10 mm (the size of the strengthening spot can be slightly larger than the opening size due to the influence of projectile scattering and incident angle). The strengthened region has clear boundaries and small dimensional deviations. Surface contaminant detection revealed no suspicious residues on the workpiece surface, meeting the requirements of relevant medical implant standards. The microhardness of the strengthened region can be increased by approximately 5%–40% compared to the substrate, and wear resistance can be characterized by indicators such as wear volume reduction and friction coefficient reduction, verifying the applicability of the mask system of this invention in high-precision, low-contamination selective shot peening scenarios.

Claims

1. A highly durable photomask system, characterized in that, It includes a photomask and a positioning and fixing system. The positioning and fixing system is used to position and clamp the photomask and the workpiece. The photomask has an opening for the shot to pass through and for the shot to contact the workpiece surface. The photomask has a multi-layer composite structure, including a substrate layer, a wear-resistant layer, and a fatigue-resistant layer. The wear-resistant layer is disposed on the working surface side of the substrate layer, and the fatigue-resistant layer is disposed on the opposite surface of the substrate layer. The sidewall of the opening has a predetermined angle relative to the thickness direction of the photomask, and the opening edge has a rounded corner structure. The average gap g between the photomask and the workpiece in the shot peening state satisfies 0 < g ≤ d, preferably g ≤ 0.5d, where d is the diameter of the shot used.

2. The high-durability photomask system according to claim 1, characterized in that, The thickness of the photomask is 2-10 times the diameter d of the projectile used. When the minimum opening width W of the photomask is less than 0.30 mm, the thickness of the photomask is 8-10 times the diameter d of the projectile used.

3. The high-durability photomask system according to claim 1, characterized in that, The wear-resistant layer is formed by depositing a TiN, TiAlN, CrN, or AlCrN hard coating on the working surface of the substrate layer through a physical vapor deposition process. The coating thickness is 2–10 μm, preferably 2–5 μm, and the microhardness is not less than 2000 HV. The fatigue-resistant layer is formed by nitriding or carbonitriding on the back side of the substrate layer to a depth of 0.1–0.3 mm.

4. The high-durability photomask system according to claim 1, characterized in that, The base layer is made of high-strength tool steel, cemented carbide, or high-strength corrosion-resistant stainless steel. Any of the following material selection options are available: 1) Cold work die steel with a tensile strength of not less than 1400 MPa and a Rockwell hardness of not less than 58 HRC after quenching and tempering; 2) WC-Co series cemented carbide with a room temperature Vickers hardness of not less than 900 HV; 3) Medical-grade 316L stainless steel sheet that has undergone cold working or surface strengthening treatment.

5. The high-durability photomask system according to claim 1, characterized in that, The sidewalls of the opening on the photomask are machined with an inclination angle of 15°–45°, and the opening edge is a rounded structure, satisfying: (1) When the minimum width W of the corresponding opening is 0.30 mm≤W<0.50 mm, the rounded radius R is 0.10 mm~0.30 mm, and satisfies R≤0.50W; (2) When the minimum width W of the corresponding opening is 0.50 mm≤W≤1.00 mm, the rounded radius R is 0.10 mm~0.50 mm, and satisfies R≤0.50W; (3) When the minimum width W of the corresponding opening is W<0.30 mm, the rounded radius R is selected as 0.20W~0.40W and satisfies R<W.

6. The high-durability photomask system according to claim 1, characterized in that, The positioning and fixing system includes positioning pins or vacuum adsorption or magnetic fixing devices or flexible sealing components and combinations thereof; The locating pin is used to connect the mask to the workpiece or tooling fixture; the mask is provided with at least three non-collinear locating holes, and the workpiece or tooling fixture is provided with corresponding locating holes or locating grooves at corresponding positions; preferably, the locating pin is made of cemented carbide material, the dimensional tolerance grade of the locating pin is IT6, and the locating holes of the mask and the corresponding locating holes / locating grooves of the workpiece / tooling are of the same tolerance grade. The fixing system is configured according to the shape and material properties of the workpiece. For planar workpieces, a vacuum adsorption fixing method is used, forming a sealed chamber between the mask and the workpiece with a pressure reduction of no less than 0.08 MPa based on atmospheric pressure. A closed annular flexible sealing strip is set around the perimeter where the mask and the workpiece contact, and the sealing strip is compressed during vacuuming. For magnetic workpieces, a permanent magnet array flush with the back of the mask is set on the back of the mask. The normal adsorption force per unit area of ​​the permanent magnet array is no less than 50 N / cm², and all magnets are embedded inside the back plate, which is in contact with the workpiece surface. For free-form components, a contour jig matching the shape of the workpiece is used, and a mechanical clamping structure is set.

7. The high-durability photomask system according to claim 1, characterized in that, The high-durability mask system also includes an integrated online monitoring unit for real-time monitoring of mask wear and positional displacement; The online monitoring unit includes at least: a displacement sensor arranged on the back of the mask to monitor the deflection and overall displacement of the mask during shot peening; strain gauges or force sensors arranged near the edge or opening area of ​​the mask to monitor the impact load level borne by the mask; and an industrial camera facing the mask-workpiece contact area to acquire images of mask surface wear, cracks, and opening edge damage. The data acquired by the above sensors and camera are uploaded to the host computer in real time.

8. A method for applying the high-durability mask system as described in any one of claims 1 to 7 in selective mechanical shot peening, characterized in that, By constraining the relationship between the shot diameter d and the minimum opening width W of the mask (0.6W≤d≤0.9W), and introducing the shot peening time determination formula t = (A×C_eq) / (N×S_e×η), which includes the mask coefficient η, the impact load on the mask is reduced and the process consistency is improved while ensuring coverage and residual compressive stress levels.

9. The method of applying the high-durability mask system according to claim 8 in selective mechanical shot peening, characterized in that, The specific steps are as follows: S1. In the design and manufacturing of high-durability photomasks, a multi-layer composite structure design is adopted; and different aperture density areas, local thickening or thinning sections, and whether or not to use detachable masking blocks are designed on the photomask. S2. In terms of precision positioning and fixing systems, positioning pins, vacuum adsorption, magnetic fixing devices, or flexible sealing components are selected for individual or combined positioning, depending on the workpiece or tooling fixture. S3. Regarding the dedicated shot peening process parameter system, establish optimized parameters for mask application scenarios; The projectile diameter d and the minimum opening width W of the mask satisfy the proportional relationship 0.6W≤d≤0.9W; for micro-hole arrays with a minimum opening width W < 0.30 mm, the projectile diameter is selected within the range of 0.8≤d / W≤0.9; the shot peening time t is determined by the following formula: t = (A×C_eq) / (N×S_e×η), where A is the effective area to be shot peened, C_eq is the equivalent effective impact density per unit area (particles / mm²) required to achieve the target shot peening coverage C; N is the nominal effective impact number per unit time and per unit area under the reference condition without a mask, particles / (s·mm²); S_e is the equivalent effective area of ​​the shot peening spot or mask opening group within one spray gun dwell unit / control cycle, mm²; η is the mask coefficient, which ranges from 0.6 to 0.8 and is used to characterize the comprehensive attenuation effect of the mask structure on the projectile flow rate; The masking coefficient η is defined as: under the same shot peening equipment, shot peening pressure, spraying distance, and spraying angle, the ratio of the effective shot flux that acts on the area to be strengthened through the mask opening to the effective shot flux that acts on the same area without a mask; the effective shot flux is measured by the shot mass flow rate or shot count flux per unit time, thus obtaining η = Q_mask / Q_nominal; an initial estimate of the shot peening time t is given in the process design stage, and t is adjusted in the actual production process based on online monitoring results and experimental feedback; By designing regions with different aperture densities, locally thickened or thinned sections, and whether or not to use removable shielding blocks on the mask, the distribution of residual stress on the workpiece surface can be controlled and adjusted. In areas where higher residual compressive stress is required, a higher aperture density is designed, along with a larger d / W ratio and a higher target shot peening coverage. In areas where relatively low residual compressive stress is required or where a smooth stress transition is needed, the number of effective shot per unit area is reduced by decreasing the aperture density, decreasing the d / W ratio, or increasing the local shielding thickness.

10. The method of applying the high-durability mask system according to claim 9 in selective mechanical shot peening, characterized in that, Based on the workpiece geometry and target strength distribution, the entire surface to be treated is divided into several sub-regions. In each step, only one or several sub-regions are opened with mask openings or local additional obstructions are removed, while the remaining regions remain covered and are not shot peened. By setting the shot peening time and target shot peening coverage for each sub-region, different effective impact counts and residual stress levels are achieved for each sub-region. Through the above-mentioned step-by-step shot peening by region, selective shot peening treatment with different strengthening intensities between different regions can also be achieved.

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