A stress-differential-induced metal matrix composite shaft and method of making same

Abstract

The present application relates to the technical field of surface strengthening of mechanical parts, and particularly relates to a metal matrix composite shaft based on stress difference induction and a preparation method thereof. The method comprises the following steps: preparing a metal matrix composite shaft blank; sequentially performing forming and heat treatment on the metal matrix composite shaft blank to obtain a metal matrix composite shaft; and performing selective local impact on a preset working surface of the metal matrix composite shaft along a preset path, wherein the surface coverage of the impact is controlled to be between 55% and 75%, and the spacing between impact points is 0.4mm to 1.0mm; and the preset working surface of the metal matrix composite shaft after the impact forms periodically alternating hard zones and soft zones, thereby obtaining the metal matrix composite shaft based on stress difference induction. The present application provides a strong and toughening basis by utilizing heterogeneous deformation induced strengthening of internal multi-scale heterogeneous structures (such as reinforcing phase network / clusters); and a periodic stress difference gradient field is introduced by surface patterning impact, and synergy is generated.

Description

Technical Field

[0001] This invention relates to the field of surface strengthening technology for mechanical parts, and in particular to a metal matrix composite shaft based on stress difference-induced stress and its preparation method. Background Technology

[0002] Shaft components are the core of power transmission systems, and their failures are mostly surface-originating fatigue fractures. Traditional surface strengthening techniques such as shot peening and rolling improve fatigue strength by introducing a uniform residual compressive stress layer on the surface, but they have two major limitations: First, the uniform compressive stress field has limited ability to inhibit crack propagation after crack initiation; second, it is difficult to coordinate with the heterogeneous structure inside the material, often leading to a reversal of "strength-plasticity".

[0003] Large and concentrated stress differences are key to failure, but controllable, microscopic stress differences can be used to resist failure. Recent studies have also shown that constructing non-uniform stress / strain fields in materials (such as forming alternating hard and soft structures through localized plastic deformation) can effectively promote dislocation accumulation, generate additional heterogeneous deformation-induced (HDI) strengthening effects, and significantly hinder crack propagation.

[0004] Chinese patent CN113215550A discloses a method for preparing gradient nanostructured surface layers through ultrasonic surface rolling. However, this process involves uniform coverage of the entire surface, resulting in a relatively homogeneous stress field, and it does not involve active collaborative design with the heterogeneous structure within the material. Therefore, how to accurately quantify and control the distribution of surface stress differences, and combine it with specific service scenarios of shaft components (such as fretting fatigue of high-speed railway axles and corrosion-fatigue interaction of aero-engine axles) to form a set of measurable, verifiable, and effective proprietary strengthening technologies has become a publicly disclosed but unsolved technical problem in this field. Summary of the Invention

[0005] To address the above problems, this invention provides a stress-differential induced metal matrix composite shaft and its preparation method. The invention provides the following technical solution: This invention provides a method for preparing a metal matrix composite shaft based on stress difference-induced deformation, the method comprising: Preparation of metal matrix composite shaft blanks; The metal matrix composite shaft blank is sequentially formed and heat-treated to obtain the metal matrix composite shaft. Selective local impact is performed on the preset working surface of the metal matrix composite shaft along a preset path, wherein the surface coverage of the impact is controlled between 55% and 75%, and the spacing between impact points is between 0.4 mm and 1.0 mm. After impact, the pre-designed working surface of the metal matrix composite shaft forms periodically alternating hard and soft regions, resulting in a metal matrix composite shaft based on stress difference induction.

[0006] Furthermore, the preparation of metal matrix composite shaft blanks includes: Metal matrix composite shaft blanks are prepared by powder metallurgy or melting casting and sintering, wherein the reinforcing phase is non-uniformly distributed in a three-dimensional network or cluster in the metal matrix, thereby obtaining metal matrix composite shaft blanks with endogenous multi-scale heterogeneous structures. The matrix includes one of titanium-based, aluminum-based, or steel-based materials; the reinforcing phase includes one or more of TiB, TiC, SiC, B4C, or Al2O3.

[0007] Furthermore, the forming of the metal matrix composite shaft blank includes: The metal matrix composite shaft blank is machined to a predetermined size to obtain the shaped metal matrix composite shaft blank.

[0008] Furthermore, the heat treatment includes solution aging or stress-relieving annealing of the formed shaft blank. After the heat treatment, the matrix structure in the multi-scale heterogeneous structure inside the metal matrix composite shaft undergoes recrystallization, and the matrix grain size on its surface is less than 10 μm.

[0009] Furthermore, the surface residual compressive stress fluctuation ratio R between adjacent hard and soft regions is 1.3 to 2.0, where, R = |σ hard | / |σ soft |,σ hard and σ soft These represent the surface residual compressive stress values ​​for the hard and soft regions, respectively.

[0010] Furthermore, the height difference between the hard and soft regions is 50-200 μm.

[0011] Furthermore, the surface of the hard region has a gradient nanocrystalline structure with a surface grain size of less than 100 nm, and the grain size gradient increases from the surface to the core. The surface of the hard region has a residual compressive stress of -700MPa to -1000MPa.

[0012] Furthermore, selective localized impacts include high-energy microparticle impacts or laser impacts; The diameter of the projectile in the high-energy microparticle impact is 0.5-3.0 mm, and the single-shot impact energy is 5-50 J; The laser impact spot diameter is 2-5 mm, and the single pulse energy is 2-10 J. The theoretical coverage of a single-layer impact is 55%-75%.

[0013] Furthermore, the preset path is a spiral, an equidistant dot matrix, or a mesh.

[0014] Also provided is a stress-differential induced metal matrix composite shaft obtained by the preparation method described above.

[0015] The technical effects and advantages of this invention are as follows: 1. A breakthrough improvement in fatigue resistance has been achieved. By precisely controlling the coverage (55%-75%) and lattice spacing (0.4-1.0 mm) of selective local impact, a periodic stress difference structure with a quantifiable residual compressive stress fluctuation ratio (R=1.3-2.0) was successfully constructed on the surface of the shaft. This structure can actively induce a "crack shielding effect," causing fatigue cracks to repeatedly traverse regions of alternating high and low stress during propagation, consuming enormous energy and thus being effectively suppressed.

[0016] Standard performance: Compared to untreated substrates, the rotational bending fatigue limit of shaft components is increased by ≥30%.

[0017] Harsh working conditions: In harsh environments such as salt spray corrosion and fretting wear, its fatigue resistance is ≥40% higher than that of traditional full-coverage shot peening technology, demonstrating excellent environmental adaptability.

[0018] 2. A cross-scale synergistic reinforcement system of "internal heterogeneity-external gradient" was formed. This invention does not simply combine two processes, but rather employs a systematic design: it utilizes heterogeneous deformation-induced (HDI) strengthening through internal multi-scale heterogeneous structures (such as reinforcing phase networks / clusters) to provide a foundation for toughening; simultaneously, it synergizes with this by introducing a periodic stress gradient field through surface patterned impact. This cross-scale synergistic mechanism, from micro to macro, achieves a strengthening effect of "1+1>2" while ensuring the overall strength and toughness matching of the material.

[0019] 3. It provides a proprietary enhancement technology with clearly defined parameters and verifiable effects. The core of this invention lies in the active design and precise control of the surface stress field. The proposed key parameters, such as coverage, lattice spacing, and stress fluctuation ratio, constitute a clear, measurable, and repeatable process window. This technical solution overcomes the performance bottleneck of traditional homogeneous strengthening techniques, providing an innovative and reliable proprietary strengthening solution for critical shaft components in aerospace, high-speed trains, and other fields with extremely stringent requirements for ultra-high fatigue life and reliability.

[0020] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description and the drawings. Attached Figure Description

[0021] Figure 1 This is a flowchart of the preparation method of a metal matrix composite shaft based on stress difference induced by an embodiment of this application.

[0022] Figure 2a This is a schematic diagram of the surface pattern location in a localized impact scenario provided in an embodiment of this application; Figure 2b This is a schematic diagram of the surface pattern of a localized impact provided in an embodiment of this application; Figure 3 This is a schematic diagram of the surface gradient strengthening process provided in the embodiments of this application; Figure 4 This is a schematic diagram of the circumferential residual stress fluctuation curve of the hard zone after a local impact, provided in an embodiment of this application. Figure 5 This is a schematic diagram of the laser shock process provided in Embodiment 1 of this application; Figure 6 This is a schematic diagram of the laser shock control logic provided in Embodiment 1 of this application; Figure 7 This is a schematic diagram of the laser shock process provided in Embodiment 2 of this application; Figure 8 This is a schematic diagram of the laser shock control logic provided in Embodiment 2 of this application. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] To address the shortcomings of existing technologies, this invention discloses a method for preparing metal matrix composite shafts based on stress difference-induced processes, such as... Figure 1 As shown, the method includes, Step 1: Prepare metal matrix composite shaft blanks; Step 2: The metal matrix composite shaft blank is formed and heat-treated sequentially to obtain the metal matrix composite shaft. This step involves machining or near-net-shape forming the metal matrix composite shaft blank obtained in Step 1, followed by solution treatment, aging or annealing to regulate the matrix phase composition, eliminate internal stress and optimize interface bonding.

[0025] Step 3: Selectively and locally impact the preset working surface of the metal matrix composite shaft using a preset patterned path to form a reinforcing layer with periodically alternating hard and soft regions on the surface of the metal matrix composite shaft, thereby obtaining a metal matrix composite shaft based on stress difference induction; wherein, the spacing between the impact lattice points is 0.4 mm to 1.0 mm; and the impact coverage of the preset working surface of the metal matrix composite shaft based on stress difference induction is 55%-75%.

[0026] Step 3 is used to construct a quantifiable surface periodic stress difference gradient field (core step): a high-energy microparticle impact or laser shock strengthening device is used to selectively and locally impact the working surface of the metal matrix composite shaft with a preset discretized and patterned path.

[0027] In a specific embodiment of the present invention, for step 1: this step is used to prepare a metal matrix composite shaft blank with an endogenous multi-scale heterogeneous structure: using powder metallurgy or melt casting sintering method, a metal matrix composite shaft blank with a metal or alloy as the matrix and ceramic particles, whiskers or fibers as the reinforcing phase is prepared. Through process control, the reinforcing phase is made to be non-uniformly distributed in a three-dimensional network or cluster in the metal matrix, while the matrix grains exhibit a gradient or bimodal distribution, thereby preparing a metal matrix composite shaft blank with an endogenous multi-scale heterogeneous structure; In one specific embodiment of the present invention, the matrix comprises one of titanium-based, aluminum-based, or steel-based; the reinforcing phase comprises one or more of TiB, TiC, SiC, B4C, or Al2O3.

[0028] The raw material of the matrix is, for example, a metal alloy powder, including one of titanium alloy powder, aluminum alloy powder or iron alloy powder; and / or; the powder used to form the reinforcing phase includes one or more of alumina, graphene, boron carbide and silicon carbide.

[0029] For example, titanium alloy powder includes TC4 titanium alloy powder (Ti-6Al-4V) or TA7 titanium alloy powder (Ti-5Al-2.5Sn); aluminum alloy powder includes 6061 aluminum alloy powder; copper alloy powder includes Cu-Fe-Sn-Pb-C-Si-O-MoSi alloy, Cu-50W alloy, Cu-50Cr alloy and Cu-Al2O3 alloy; steel alloy powder includes carbon steel and low alloy powder, iron-chromium-silicon alloy powder, iron-chromium-boron-silicon alloy powder.

[0030] In step 1, a metal matrix composite shaft blank with a specific configuration is prepared using known powder metallurgy or sintering casting techniques. This invention uses this metal matrix composite shaft blank as the endogenous basis of the entire "stress difference" strengthening system. The metal matrix composite shaft blank with an endogenous multi-scale heterogeneous structure includes titanium-based composite shaft blanks and aluminum-based composite shaft blanks. Specifically, the titanium-based composite shaft blank includes a TiB whisker-reinforced titanium-based composite shaft blank, and the aluminum-based composite shaft blank includes a SiC particle-reinforced aluminum-based composite material.

[0031] For example, the preparation process of TiB whisker-reinforced titanium matrix composite shaft blank is as follows: TiB whiskers were generated in situ via reactive hot pressing sintering using Ti-6Al-4V (TC4) pre-alloyed powder and B4C powder as raw materials. By controlling the powder particle size distribution and sintering process, a heterogeneous structure with a three-dimensional network distribution of reinforcing phases along the original powder particle boundaries could be obtained. Simultaneously, the matrix structure exhibited a bimodal structure composed of equiaxed primary α phase and lamellar (α+β) bundles. During deformation, this structure generates strong heterogeneous deformation-induced (HDI) stress between the network-like TiB and the bimodal matrix.

[0032] For example, the preparation process of SiC particle-reinforced aluminum matrix composite shaft blank is as follows: The SiC nanoparticles were prepared using 6061 aluminum alloy powder and nano-SiC particles via high-energy ball milling followed by hot isostatic pressing. By controlling the ball milling process, the SiC particles were dispersed into a non-uniform cluster distribution within the aluminum matrix. Subsequent hot extrusion resulted in a gradient distribution of fine-grained surfaces and coarse-grained cores in the matrix grains. This multi-scale heterogeneous structure provided an ideal strain-coordinated substrate for subsequently introducing surface stress differences.

[0033] The aforementioned methods for preparing shaft blanks (such as reactive hot pressing and powder metallurgy) and the specific non-uniform microstructures they can obtain (such as network-like and cluster-like distributions) are well-known and achievable in the field of materials science. For example, teams from Xi'an University of Technology and Central South University have conducted systematic research and published numerous academic papers on the design and preparation of heterostructures of titanium-based and aluminum-based composite materials.

[0034] In one specific embodiment of the present invention, step 2, forming the metal matrix composite shaft blank, includes: A metal matrix composite shaft blank is machined to a predetermined size to obtain a shaped metal matrix composite shaft blank; for example, the machining process includes CNC turning or milling.

[0035] The surface roughness Ra of the formed metal matrix composite shaft blank is less than or equal to 3.2 μm.

[0036] In one specific embodiment of the present invention, for step 2, the heat treatment includes: The formed metal matrix composite shaft blank is subjected to solution treatment, aging, or annealing to obtain the metal matrix composite shaft. The purpose of this step is to eliminate machining stress, stabilize the microstructure, and provide an ideal matrix with uniform composition and microstructure and low internal stress level for subsequent selective local impact, so as to ensure that the stress difference structure formed by the impact is clear and controllable.

[0037] After this step, the metal matrix composite shaft has the following characteristics: Microstructure: After the heat treatment, the matrix structure in the multi-scale heterogeneous structure inside the metal matrix composite material undergoes recrystallization and is stabilized. The reinforcing phase maintains a non-uniform distribution in a three-dimensional network or cluster, and the matrix is ​​a recrystallized equiaxed crystal or bimorphic structure.

[0038] Microstructure stability: The multi-scale heterostructure formed by the matrix and reinforcing phases (such as TiB whiskers and SiC particles) is stabilized. Specifically, this is manifested in: Reinforcing phase distribution and interface: The reinforcing phase maintains a non-uniform distribution in a three-dimensional network or cluster, and has good interfacial bonding with the matrix, without microcracks or debonding.

[0039] Matrix structure uniformity: The matrix has completed recrystallization, forming a uniform equiaxed crystal or bimorphic structure with a narrow grain size distribution range (e.g., more than 85% of the grain size falls within ±50% of the average value), and there are no obvious processing flow lines or distorted structures.

[0040] Phase composition stability: For age-strengthable alloys (such as titanium-based and aluminum-based alloys), the strengthening phases (such as the secondary α phase in titanium alloys and Mg2Si in aluminum alloys) have been fully precipitated and uniformly distributed, with no obvious over-aging or undissolved phases.

[0041] Grain size: The matrix grain size on the surface of the metal matrix composite is refined by recrystallization and is usually in the micrometer range of 1-10 μm.

[0042] Stress state: Through stress-relief annealing or aging, the macroscopic residual stress on the surface of the metal matrix composite shaft is basically eliminated (for example, measured by X-ray diffraction (according to GB / T 7704 standard), the absolute value of the residual stress in the principal stress direction of its surface is not greater than 50 MPa, which can be regarded as providing an ideal matrix with near-zero stress and dimensional stability for subsequent impact), and is in a near-zero stress state.

[0043] Example 1: Heat treatment of titanium-based composite materials (such as TiBw / TC4) The specific process is as follows: solution treatment is carried out in a vacuum or argon-protected environment at 930-950°C for 1-2 hours, followed by water quenching; then aging treatment is carried out at 500-550°C for 4-6 hours, followed by air cooling.

[0044] After this treatment, the resulting metal matrix composite exhibits a dual-state matrix consisting of equiaxed primary α phase and lamellar (α+β) bundles, which, together with the network-distributed TiB whiskers, form a stable heterostructure. Its surface residual stress is close to zero, its microhardness is uniform (approximately HV 350), and the average grain size of the matrix is ​​approximately 1-5 μm.

[0045] Example 2: Heat treatment of aluminum-based composite materials (such as SiCp / 6061) The specific process is as follows: solution treatment is carried out at 530-550°C for 2-4 hours, followed by water quenching; then artificial aging (i.e., T6 treatment) is carried out at 160-180°C for 6-12 hours.

[0046] After this treatment, the surface microstructure of the obtained metal matrix composite shaft consists of an α-Al matrix with nano-sized Mg2Si precipitates as reinforcing phases, which, together with clustered and non-uniformly distributed SiC particles, form a reinforced heterostructure. Its surface residual stress level is very low (according to relevant standards for residual stress testing of metallic materials (such as GB / T 7704), measured by X-ray diffraction, the surface residual stress value can usually be controlled within ±30 MPa, providing an ideal matrix with low stress and dimensional stability for subsequent impact testing). The microhardness reaches its peak state (approximately HV 120-150), and the average grain size of the matrix is ​​approximately 5-10 μm.

[0047] In a specific embodiment of the present invention, step 3, selectively impacting the preset working surface of the metal matrix composite shaft, includes: selectively impacting the surface of the metal matrix composite shaft with a preset discretized and patterned path (such as a spiral or a grid).

[0048] For example, such as Figure 2a and 2b As shown, Figure 2a A side view schematic diagram showing a laser or particle beam impacting an axial surface in a discrete lattice pattern. Figure 2b Viewed from above from the shaft surface, the impact reveals a grid pattern of periodically alternating hard areas (dark spots) and soft areas (light areas).

[0049] Surface and subsurface structures formed after impact: such as Figure 3As shown, after impact, the surface of the shaft forms a periodically alternating distribution of hard zones (the area directly below the impact point) and soft zones (the unimpacted area). This structure creates a unique gradient distribution in three-dimensional space, with significant differences in the microstructure beneath the hard and soft zones.

[0050] 1. Baseline state before impact: Metal matrix composite shafts have a uniform heterogeneous structure. From the surface to the core, the matrix grain size is in the micrometer range (about 1-10 μm), and the reinforcing phase (such as TiB whiskers and SiC particles) is distributed in a network or cluster.

[0051] 2. Gradient structure beneath the hardened region after impact: Directly below the hard region, a significant tissue and property gradient forms radially (in the depth direction): Surface (approximately 0-50 μm depth): "Surface" refers to the area where cracks originate directly from defects such as machining marks, inclusions, or corrosion pits in the outermost layer. This area experiences the most intense shock wave impact, forming a gradient nanocrystalline structure. The grain size gradually coarsens as the depth increases, starting from less than 100 nanometers in the outermost layer.

[0052] Subsurface (approximately 50-200 μm depth): This is a uniform transition layer below the surface. The specific location of the "subsurface interface" is approximately 50-200 micrometers below the shaft surface, precisely at the interface between the gradient nanocrystalline layer of the "hard region" and the heterogeneous structure of the underlying "soft region" matrix. Plastic deformation is reduced, and the microstructure gradually transitions from a refined submicron crystal / high dislocation density structure.

[0053] Near-surface: This is a non-uniform deformation layer below the surface, specifically referring to a very shallow depth (usually <50μm) below the surface where cracks occur. It may originate from microscopic defects or local stress concentration areas on the surface. It is a deformed matrix that has undergone elastic distortion but has not recrystallized. The grains are still at the micrometer level, but the internal dislocation density has increased.

[0054] 3. Structure beneath the soft zone after impact: Directly below the soft zone, since it was not directly impacted, the microstructure of its surface, subsurface, and near-surface remains largely consistent with the original heterostructure before the impact, with no significant changes in grain size and morphology.

[0055] 4. Schematic diagram of a three-dimensional alternating structure: The periodic spatial distribution of hard regions (reinforced islands) and soft regions (original matrix) on the axial surface is illustrated from a three-dimensional perspective, intuitively demonstrating the macroscopic morphology of the "alternating hard and soft" composite reinforcement layer constructed by this invention.

[0056] Soft areas: From the surface to the core, the structure and properties are basically the same as those of the unaffected matrix, and no significant nano-sizing has occurred.

[0057] Residual stress field distribution: Hard zone: Residual compressive stress of up to -700 MPa to -1000 MPa is formed on the surface. This compressive stress value decreases in a gradient with increasing depth, and the depth affected can reach 0.5-1.0 mm.

[0058] Soft zone: Since it is not directly impacted, but is laterally constrained by the plastic deformation of the surrounding hard zone, it will also generate a certain amount of residual compressive stress, but its value is significantly lower than that of the hard zone, about -300 MPa to -600 MPa, and the gradient is relatively gentle.

[0059] Formation and characterization of stress difference: Finally, a composite reinforcement layer was constructed on the surface and subsurface of the shaft. This layer exhibits a continuous gradient in microstructure (nanocrystalline → microcrystalline) and residual compressive stress (high → low) in the radial direction; in the circumferential / axial direction, due to the alternation of soft and hard regions, a periodic fluctuation distribution pattern of high and low residual compressive stress is formed (e.g. Figure 4 ). Figure 4 This is a schematic diagram of the circumferential residual stress fluctuation curve. Figure 4 The graph should display the residual compressive stress distribution curve measured along a circumferential path on the surface of the shaft (passing through several hard and soft zones). The curve should exhibit obvious wavy periodic fluctuations, with peaks (troughs) corresponding to hard zones (high-pressure stress) and valleys (peaks) corresponding to soft zones (relatively low-pressure stress). The stress fluctuation ratio R (R=σ) can be directly marked on the graph. hard / σ soft For example, if the peak value is -800MPa and the valley value is -500MPa, then R=1.6.

[0060] Step 3 involves precisely controlling the impact energy (E), the point spacing (d), and the number of impacts (N) to maintain the surface coverage (C) within the range of 55%-75% and the point spacing (d) at 0.4-1.0 mm. This ensures the residual compressive stress fluctuation ratio (R=σ) between the hard areas of the formed "high stress / high hardness zone" and the soft areas of the formed "low stress / low hardness zone". hard / σ soft The value is between 1.3 and 2.0, with a height difference of 50-200 μm between the hard and soft regions. This parameter range represents an optimized window for producing a significant "crack shielding effect" while avoiding overwork hardening.

[0061] For example, the preset patterned path is a spiral, an equidistant dot matrix, or a mesh.

[0062] The specific steps are as follows: Step 301: Treatment Location: This step precisely targets all known fatigue crack initiation sites, such as the outer working surface of the shaft, the shoulder transition fillet, and the spline tooth surface, with localized impact. Non-working surfaces (such as the center hole and non-load-bearing shaft sections) are not treated.

[0063] Equipment and Preparation: Use a high-energy microparticle impactor (ceramic projectile material, 0.5-1.0 mm in diameter) or a laser shock peening device (pulse width 10-30 ns). Before treatment, spray an aluminum foil or black paint about 0.1 mm thick onto the surface to be impacted as an absorption layer, and apply a 1-3 mm thick layer of flowing deionized water as a constraint layer.

[0064] Step 302: Patterning Path and Parameter Control: Path design: Use an axial feed, circumferentially indexed spiral lattice path, or a custom grid lattice designed according to the characteristics of stress concentration areas.

[0065] Core parameter control strategy: By adjusting the projectile velocity or laser pulse energy, the single-point impact energy (E) is controlled, so that the surface produces plastic deformation but no macroscopic indentation.

[0066] The feed of the equipment is controlled by a program to ensure that the dot matrix spacing (d) is stable at 0.4-1.0 mm.

[0067] By setting the pitch or grid density of the helix, the surface coverage (C) can be precisely controlled within the range of 55%-75%. For example, when the diameter of the projectile / spot is 0.8 mm and the spot spacing d = 0.6 mm, the theoretical coverage of a single-layer impact is approximately 70%.

[0068] Process testing was performed on the sample using an X-ray stress analyzer, and the ratio of residual compressive stress R (σ) between the hard and soft areas was measured and calculated. hard / σ soft By fine-tuning E and d, the R value was stabilized within the optimization window of 1.3-2.0. When R < 1.3, the stress difference is not significant and the shielding effect is weak; when R > 2.0, the performance mismatch between the soft and hard regions is too large, which easily induces microcracks at the interface.

[0069] Detailed changes in the surface structure and properties of the substrate before and after treatment: Before treatment: The surface is in a homogeneous state after heat treatment. Taking TiBw / TC4 as an example, the surface is a uniform heterogeneous structure composed of a dual-state matrix and a TiB network, with residual stress close to zero, uniform microhardness (about HV 350), and average grain size in the micrometer range (1-10μm).

[0070] After processing: Microstructure: Directly below each impact point (hard zone), the material undergoes intense plastic deformation, forming a depth of approximately 5 MPa. The entire surface exhibits a periodic, wave-like distribution of residual stress.

[0071] Microhardness: The surface microhardness of the hard area is significantly increased to HV 450-550, while the hardness of the soft area remains at HV 330-380, forming a significant hardness difference.

[0072] Performance Enhancement Mechanism: This "glutinous rice cake"-like alternating hard and soft structure means that any fatigue crack attempting to initiate or propagate from the surface requires enormous energy to traverse the "hard zone (high-pressure stress dam)," or becomes passivated in the "soft zone (plastic deformation buffer)" when attempting to bypass the hard zone. This "crack shielding effect," directly induced by a controllable stress difference, is the fundamental reason for the breakthrough performance improvement.

[0073] The present invention also provides a stress-differential induced metal matrix composite shaft obtained according to the above preparation method.

[0074] The stress difference-induced metal matrix composite shaft of the present invention utilizes a controllable, microscopic "beneficial stress difference" to resist the initiation and propagation of macroscopic, destructive fatigue cracks. Its synergistic strengthening mechanism is as follows: Stress difference-induced crack shielding effect: The periodically distributed "high-stress zones" on the surface act like "dams," effectively counteracting applied alternating tensile stress and inhibiting crack initiation. When microcracks attempt to propagate from the "low-stress zone" to the "high-stress zone" or across the interface between the two, they face a huge stress threshold difference, requiring additional energy to be consumed, thus being strongly hindered, passively neutralized, or even deflected. This completely changes the situation where cracks in traditional uniform compressive stress layers tend to propagate relatively easily once initiated.

[0075] Cross-scale synergy of HDI stress: The internal heterogeneous structure generates back stress (HDI stress) under load, providing a basis for strengthening. The stress difference pattern on the surface further exacerbates strain incoordination at the microscale, stimulating a higher density of geometrically required dislocations (GNDs), resulting in stronger HDI strengthening and forming a cross-scale synergistic strengthening network from macro to micro.

[0076] Synergistic performance leap: Examples show that shafts treated with this invention exhibit a fatigue limit improvement (>50%) far exceeding the theoretical improvement (typically <30%) that can be achieved simply by increasing the residual compressive stress amplitude. This demonstrates that the "stress difference" design brings about a qualitative performance leap.

[0077] The beneficial effects of this invention can be verified through quantifiable testing methods, and it is particularly suitable for high-speed, heavy-load, and corrosion-fatigue conditions: Quantitative performance improvement: By controlling the above parameters, the rotational bending fatigue limit of the shaft can be stably improved by 30%-60%, and the crack initiation life can be improved by more than an order of magnitude.

[0078] Advantages in harsh working conditions: In corrosive media (such as salt spray) environments, the periodic stress difference pattern can not only prevent mechanical fatigue cracks, but its dense gradient nanocrystalline structure in the "high stress zone" can also act as a physical barrier, significantly delaying the formation and expansion of corrosion pits, and achieving synergistic protection of mechanical and chemical properties.

[0079] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0080] The present invention will be further described below with reference to the embodiments and comparative examples. In all embodiments, the surface stress difference pattern was plotted by X-ray diffraction (according to GB / T 7704-2017) to map the two-dimensional distribution of residual stress, and the hardness distribution was verified by measuring the nanoindentation method.

[0081] Example 1: Titanium-based composite material drive shaft for aero-engines (resistant to fretting fatigue) Step (1): TC4 (Ti-6Al-4V) pre-alloyed powder was prepared by plasma rotating electrode method and mixed with trace amounts of B4C powder. The mixture was then sintered at 920°C and 120MPa for 2 hours via hot isostatic pressing (HIP) to generate TiB whiskers in situ, yielding a titanium-based composite shaft blank. In the obtained metal-based composite shaft blank, the TiB whiskers were distributed in a three-dimensional continuous network along the original powder boundary, and the matrix was an α+β dual-state structure. Strong heterogeneous deformation-induced stress was generated between the network-like TiB and the dual-state matrix, thus preparing a metal-based composite shaft blank with an endogenous multi-scale heterogeneous structure. Step (2): The titanium-based composite shaft blank is machined to a drive shaft size of Φ25mm×150mm. The surface roughness Ra of the formed titanium-based composite shaft blank is controlled below 1.6μm; it is subjected to solution treatment at 900°C under vacuum or argon protection for 1h, followed by water quenching; then it is subjected to aging treatment at 550°C for 6h, followed by air cooling. After heat treatment, the surface of the titanium-based (such as TiBw / TC4) composite shaft blank forms a homogeneous state, resulting in a heat-treated titanium-based composite shaft. The surface of the heat-treated titanium-based composite shaft is a uniform heterogeneous structure composed of a dual-state matrix and a TiB network, with residual stress close to zero, uniform microhardness (approximately HV 350), and average grain size in the micrometer range (1-10μm).

[0082] Step (3): Employ a high-energy microparticle impactor. For example... Figure 5 and Figure 6As shown, the key control parameters are: design a spiral lattice path (feeding along the axial direction with a point spacing of d=0.6mm, and impacting once every 60° rotation of the shaft to form a spiral arrangement), coverage C=65%, axial point spacing d=0.6mm, and after impacting at 60° intervals in the circumference, the measured residual compressive stress fluctuation ratio R=1.52 (hard area -780MPa, soft area -513MPa), thus obtaining a metal matrix composite shaft based on stress difference induced by stress difference.

[0083] Performance testing: Rotational bending fatigue testing was conducted under ambient temperature and atmospheric conditions. Additionally, fretting fatigue testing was added to simulate the working conditions of the engine's tenon joints.

[0084] The arrangement of the path on the shaft surface, and the "alternating soft and hard" reinforcement layer formed after impact.

[0085] Example 2: Steel-based composite axle for high-speed trains (corrosion and fatigue resistant) Powder metallurgy was employed, using Fe-13Cr-4Ni-0.1C martensitic stainless steel alloy powder (particle size approximately 50-150 μm) prepared by atomization as the matrix, and incorporating 2.0 vol.% of Al2O3 ceramic particles with an average particle size of 50 nm. Uniform mixing was achieved through high-energy ball milling. Subsequently, hot isostatic pressing was performed at 1120°C and 150 MPa for 3 hours. In the resulting metal matrix composite shaft blank, the nano-Al2O3 particles were diffusely and non-uniformly distributed in clusters within the matrix. This cluster structure, acting as a hard phase, generates strong heterogeneous deformation-induced (HDI) stresses with the tough steel matrix during deformation, forming the basis for achieving a balance between strength and toughness.

[0086] The sintered metal matrix composite shaft blank is machined to the size of the axle blank.

[0087] The material is then subjected to quenching and tempering to obtain a tempered sorbite structure with excellent overall mechanical properties. Quenching: Austenitize by holding in a vacuum or protective atmosphere furnace at 1020-1050°C for 2-3 hours, followed by oil quenching to obtain a high-strength lath martensite structure.

[0088] High-temperature tempering: Temper at 560-600°C for 3-4 hours, followed by air cooling. This process allows for the complete decomposition of martensite and the dispersion of carbides, significantly improving toughness and relieving stress while maintaining high strength.

[0089] Shaft condition of heat-treated metal matrix composites: Microstructure: The matrix is ​​uniform tempered sorbite (fine spherical carbides are dispersed on a ferrite matrix), and nano-Al2O3 clusters are uniformly distributed within it.

[0090] Mechanical properties: Yield strength ≥850 MPa, tensile strength ≥950 MPa, elongation after fracture ≥15%, room temperature impact energy (KV2) ≥50 J.

[0091] Stress state: The macroscopic residual stress on the surface has been basically eliminated, and the surface is in a low stress state (<±30 MPa).

[0092] Step (3): Construct a periodic stress difference gradient field on the surface Laser shock peening equipment is used to strengthen critical parts of the axle (such as the wheel seat press-fit section and stress concentration areas of the axle body). The impact process is as follows: Figure 7 And the complete process flow and control logic from parameter input to the formation of the final reinforced structure, such as Figure 8 As shown.

[0093] Impact parameters: Laser parameters: wavelength 1064nm, pulse width 20ns, single pulse energy 8J.

[0094] Path: Square grid dot matrix path. Specifically, impacts are made at equal intervals along both the axial and circumferential directions with a spacing of d=0.8mm to form a neat grid pattern.

[0095] The path is regularly arranged on the shaft surface, and the periodic "stress island" reinforcement structure is formed on the surface and subsurface after impact.

[0096] Coverage: The theoretical single-layer coverage under this parameter is C=60% (calculated based on spot diameter and dot spacing).

[0097] Overlap: To ensure uniformity, a single-layer 100% overlap impact or a double-layer staggered impact can be performed to achieve the set coverage rate.

[0098] Characterization of surface structure after treatment: Morphology: It forms a regular grid-like distribution of hard areas (impact points) and soft areas.

[0099] Residual stress: Measured using X-ray diffraction, the residual compressive stress on the hard area surface is -710 MPa, and the residual compressive stress on the soft area surface is -503 MPa. The calculated residual compressive stress fluctuation ratio R = |-710| / |-503| ≈ 1.41.

[0100] Microstructure: A gradient nanocrystal / high dislocation density layer with a depth of about 100 μm is formed on the surface of the hard region.

[0101] Performance testing: Rotational bending fatigue tests were conducted under ambient temperature and simulated salt spray conditions (5% NaCl solution spray).

[0102] Comparative Example 1: Traditional Uniform Shot Peening The same materials and shaft blanks as in steps (1) and (2) of Example 1 were used. A conventional pneumatic shot peening process was used, with shot of the same hardness selected, to perform 100% full coverage shot peening on the surface until the surface was saturated.

[0103] Comparative Example 2: Only internal heterogeneous structure, without surface stress difference treatment The same material and shaft blank as in steps (1) and (2) of Example 1 are used, without any surface impact strengthening treatment.

[0104] Comparative Example 3: Low Coverage Impact (not within the parameter range of this invention) Using the same material and blank as in Example 1, with d=0.8mm, but with an impact coverage of C=40% (the lattice is too sparse), the measured stress fluctuation ratio R=1.15, and the stress difference is not significant.

[0105] The room temperature fatigue limit of the metal matrix composite shaft obtained in Comparative Example 3 was ~520 MPa. Crack initiation point: Surface or near-surface. Due to the sparse impact points, an effective global stress difference network cannot be formed, and cracks easily initiate in unreinforced weak areas. This indicates that when the coverage is less than 55%, the stress difference pattern is incomplete (R<1.3), the "crack shielding effect" is weak, and the performance improvement is limited and unstable.

[0106] Comparative Example 4 (High Coverage Impact) Using the same material and blank as in Example 1, with d=0.3mm, but with impact coverage C=85% (impact points are almost continuous), the measured stress fluctuation ratio R ≈ 1.08, where the residual compressive stress on the hard area surface is -800 MPa, the residual compressive stress on the soft area surface is -740 MPa, and the stress difference is not significant.

[0107] The measured stress fluctuation ratio R and stress difference are shown in Table 1.

[0108] The room temperature fatigue limit of the metal matrix composite shaft obtained in Comparative Example 4 was ~580 MPa. Crack initiation: Surface or subsurface. Although high-amplitude compressive stress can be introduced, the soft zone almost disappears, the material surface tends to be homogenized, and the ability of the soft zone to coordinate plastic deformation and passivate cracks is lost, resulting in decreased toughness and easy initiation of early brittle cracking.

[0109] This indicates that when the coverage exceeds 75% (especially when the dot spacing is too small), the structural advantage of "alternating soft and hard" is lost, the performance is lower than the preferred range of this invention, and the process cost is increased.

[0110] Comparative Example 5 (Dot pitch too small) Using the same material and blank as in Example 1, with d=0.2mm, but with impact coverage C=65%, the measured stress fluctuation ratio R ≈ 1.25 (but the data fluctuates greatly), and the stress distribution is extremely uneven.

[0111] The metal matrix composite shaft obtained in Comparative Example 5 exhibits unstable properties and high surface quality risks. Excessively dense impact points lead to significant thermal / mechanical superposition effects, potentially causing localized overheating, microcracks, or excessive work hardening, which in turn damages the matrix properties and greatly increases processing time, rendering it impractical for engineering applications. This demonstrates that a lattice spacing of less than 0.4 mm is unreasonable in engineering, easily introducing process risks, and the performance gains do not outweigh the costs.

[0112] Comparative Example 6 (Dot pitch too large) Using the same material and blank as in Example 1, with d=1.5mm, but with impact coverage C=60% (far higher than the upper limit), the measured stress fluctuation ratio R≈1.18, where the residual compressive stress on the hard area surface is -720 MPa and the residual compressive stress on the soft area surface is -610 MPa, indicating insufficient stress difference.

[0113] The room temperature fatigue limit of the metal matrix composite shaft obtained in Comparative Example 6 was ~500 MPa. Crack origin: Surface. Due to the excessively large point spacing, effective stress field synergy cannot be generated between individual strengthening points (hard areas), and the overall structure is more like an isolated "pinning point," unable to form a continuous periodic stress barrier, resulting in poor fatigue resistance.

[0114] This indicates that when the lattice spacing is greater than 1.0 mm, it is difficult to form an effective periodic stress difference pattern, resulting in performance degradation.

[0115] Performance testing and comparative analysis, with key test data comparisons shown in Table 1: Table 1

[0116] The specific location of the "subsurface interface" is about 50-200 micrometers below the surface of the shaft, which is exactly the interface between the gradient nanocrystalline layer of the "hard zone" and the heterostructure of the original matrix of the "soft zone" below.

[0117] Mechanism Explanation: This is direct evidence of the "crack shielding effect" of this invention. In high-cycle fatigue, cracks tend to initiate at the weakest point or stress concentration point. This invention, through a periodic stress difference structure on the surface, successfully "pushes" the most dangerous crack initiation point from the outer surface to this internal interface. After crack initiation here, its propagation towards the surface or core is immediately strongly constrained and deflected by the surrounding high residual compressive stress "hard zone," thereby greatly extending the crack initiation and early propagation life. This is an active, designed strengthening mechanism.

[0118] "Surface": refers to the area where cracks originate directly from defects such as machining marks, inclusions, or corrosion pits on the outermost layer.

[0119] "Near surface": refers to cracks that are very shallow below the surface (usually <50μm), and may originate from microscopic defects on the surface or localized stress concentration areas.

[0120] Mechanism Explanation: Although Comparative Example 3 employed localized impact, its coverage was too low (C=40%), resulting in an insignificant stress difference (R=1.15), failing to form an effective full-surface stress difference network. Its surface strengthening was discontinuous and insufficient, resembling isolated "islands." Fatigue cracks readily initiate in the weak areas between these "islands" or at the edges (stress concentration points) of the "islands" themselves; therefore, crack initiation manifests as an unstable "surface or near-surface." This, in turn, demonstrates the critical necessity of the 55%-75% coverage and the R=1.3-2.0 parameter range in this invention.

[0121] According to Table 1: Parameter validity and performance window: Examples 1 and 2 achieved optimal fatigue performance under the preferred parameters (C=55-75%, R>1.3). Although Comparative Example 3 used local impact, due to the low coverage (C=40%), it failed to form an effective stress difference (R=1.15), and its performance was only slightly better than Comparative Example 2, far lower than the embodiments of the present invention, proving the criticality and necessity of the parameter range of the present invention.

[0122] Outstanding advantages under harsh working conditions: In a salt spray environment, the fatigue limit of Example 2 decreased by only about 6.5% compared to its room temperature value, while that of Comparative Examples 1 and 3 decreased by more than 20%. This quantitatively demonstrates that the stress difference pattern constructed in this invention and the dense nanolayer have a synergistic inhibitory effect on corrosion fatigue.

[0123] Enhancement for specific failure modes: Example 1 showed a lifespan more than 2.3 times that of Example 1 in fretting fatigue testing, indicating that the periodic stress difference structure can effectively suppress the initiation and early propagation of cracks in fretting wear.

[0124] Testing and Verification The core inventive point of this invention lies in the periodic stress difference pattern on the shaft surface, which can be objectively detected and verified through the following standardized methods: Residual stress distribution mapping: Using an X-ray diffraction stress analyzer with Mn-Kα or Cr-Kα radiation, a two-dimensional surface scan (step size ≤ 0.5 mm) is performed on a selected area of ​​the shaft surface. A residual stress distribution cloud map is then plotted, which can visually display the periodic fluctuations of stress and calculate the fluctuation ratio R.

[0125] Microscopic hardness distribution measurement: A nanoindenter is used to perform indentation tests in the same area in an array (e.g., a 10×10 dot matrix), with a load of 10-50 mN. The hardness distribution curve is plotted, and its fluctuation trend should closely match the residual stress distribution.

[0126] Fatigue fracture analysis: The fatigue fracture surface was observed using a scanning electron microscope. The crack initiation of the shaft treated by the method of this invention should be located at the interface between the hard and soft areas of the subsurface. More secondary cracks and deflection marks can be seen in the crack propagation area. This is direct microscopic evidence that stress difference hinders crack propagation.

[0127] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a metal matrix composite shaft based on stress difference-induced deformation, characterized in that, The method includes, Preparation of metal matrix composite shaft blanks; The metal matrix composite shaft blank is sequentially formed and heat-treated to obtain the metal matrix composite shaft. Selective local impact is performed on the preset working surface of the metal matrix composite shaft along a preset path to form a reinforcing layer with a periodically alternating distribution of hard and soft areas, resulting in a metal matrix composite shaft based on stress difference induction; wherein the impact lattice spacing is 0.4 mm to 1.0 mm; and the impact coverage of the preset working surface of the metal matrix composite shaft based on stress difference induction is 55%-75%.

2. The method for preparing a metal matrix composite shaft based on stress difference induced according to claim 1, characterized in that, The preparation of metal matrix composite shaft blanks includes: Metal matrix composite shaft blanks are prepared by powder metallurgy or melting casting and sintering, wherein the reinforcing phase is non-uniformly distributed in a three-dimensional network or cluster in the metal matrix, thereby obtaining metal matrix composite shaft blanks with endogenous multi-scale heterogeneous structures. The matrix includes one of titanium-based, aluminum-based, or steel-based materials; the reinforcing phase includes one or more of TiB, TiC, SiC, B4C, or Al2O3.

3. The method for preparing a metal matrix composite shaft based on stress difference induced according to claim 1, characterized in that, The forming process for metal matrix composite shaft blanks includes: The metal matrix composite shaft blank is machined to a predetermined size to obtain the shaped metal matrix composite shaft blank.

4. A method for preparing a metal matrix composite shaft based on stress difference induced according to any one of claims 1 or 3, characterized in that, The heat treatment includes solution aging or stress-relieving annealing of the formed shaft blank. After the heat treatment, the matrix structure in the multi-scale heterogeneous structure inside the metal matrix composite shaft undergoes recrystallization, and the matrix grain size on its surface is less than 10 μm.

5. The method for preparing a metal matrix composite shaft based on stress difference induced according to claim 1, characterized in that, The surface residual compressive stress fluctuation ratio R between adjacent hard and soft regions ranges from 1.3 to 2.0, where, R = |σ hard | / |σ soft |,σ hard σ represents the surface residual compressive stress value of the hard region; soft This represents the surface residual compressive stress value of the soft region.

6. The method for preparing a metal matrix composite shaft based on stress difference induced according to claim 1, characterized in that, The height difference between the hard and soft regions is 50-200 μm.

7. A method for preparing a stress-differential induced metal matrix composite shaft according to any one of claims 1-6, characterized in that, The surface of the hard region has a gradient nanocrystalline structure with a grain size of less than 100 nm, and the grain size increases from the surface to the core. The surface of the hard region has a residual compressive stress of -700MPa to -1000MPa.

8. The method for preparing a metal matrix composite shaft based on stress difference induced according to claim 1, characterized in that, Selective localized impacts include high-energy microparticle impacts or laser impacts; The diameter of the projectile in the high-energy microparticle impact is 0.5-3.0 mm, and the single-shot impact energy is 5-50 J; The laser impact spot diameter is 2-5 mm, and the single pulse energy is 2-10 J. The theoretical coverage of a single-layer impact is 55%-75%.

9. The method for preparing a metal matrix composite shaft based on stress difference induced according to claim 1, characterized in that, The preset path is a spiral, an equidistant dot matrix, or a mesh.

10. A stress-differential induced metal matrix composite shaft obtained by the preparation method according to any one of claims 1-9.

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