Method and structure for laser layer deposition repair of axles based on a graded phase conversion control

Description

Although laser deposition is considered an ideal technique for repairing damage to high-speed railway axles, the currently used laser coating techniques still exhibit numerous shortcomings. In both existing laser coating processes, the laser power is often high, which leads to a coarse grain structure in the heat-affected zone (HAZ) of the base material and thereby causes stress concentrations and fatigue failure. Secondly, a highly hard martensite phase easily forms at the interface between the coating layer and the base material, resulting in low bond strength and poor crack resistance. Thirdly, both common laser coating processes predominantly use homogeneous material for the coating. This makes it difficult to balance surface hardness and overall toughness, so these processes are unsuitable for complex load conditions. Finally, tensile stresses often exist after the laser coating has cooled.which exacerbate cracking. Patent publication CN110846652B describes a repair method for railway axles in which only a single alloy powder type is used for laser layer deposition repair. The microstructure of the resulting deposition layer differs significantly from that of the base material, so that stress concentration areas easily arise in the deposition layer of the railway axle under alternating loads. Patent publication CN117753984A discloses an additive laser repair method for railway axles. In this method, an ultrasonic shock treatment is added to refine the grains on the surface of the deposition layer. However, this method also does not solve the problem of the large deviations in the mechanical properties of the microstructure on both sides of the interface between the deposition layer and the base material. BE2026 / 7423 3 Accordingly, there is an urgent need for a new method for fatigue-resistant laser coating repair of high-speed railway axles,to overcome the problems of brittle interfaces, deteriorated properties in the heat-affected zone, and insufficient fatigue life that occur with conventional techniques. CONTENT OF THE INVENTION The present invention overcomes the shortcomings of the prior art and provides a method and a structure for fatigue-resistant laser coating repair of high-speed railway axles based on graduated phase conversion control. In the present repair method, the phase composition and the stress distribution at the interface between the laser coating layer and the base material of the railway axle are controlled, thereby achieving a good balance of strength and toughness between the coating layer and the base material. The problems of brittle interfaces, deteriorated properties of the heat-affected zone, and insufficient fatigue life in conventional repair methods are overcome.and the fatigue strength of the repaired high-speed railway axles is improved. Specific objectives are as follows: The formation of a coarse-grained microstructure in the heat-affected zone of the base material is suppressed, and the formation of martensitic phases is prevented. At the interface between the coating layer and the base material, a highly tough composite microstructure of ferrite and austenite is formed. In the transition layer of the laser coating, a microstructure of martensite, ferrite, and retained austenite is created, which simultaneously combines toughness and strength. At the surface of the laser coating, a martensitic phase with a low phase transition point and compressive stresses is generated, thereby increasing fatigue strength. In contrast to conventional methods, where the surfaces are subjected to roll forming after laser coating,In the present invention, mixed powders with different mixing ratios of alloy powders are produced. By controlling the heat input of each individual layer and the composition of the alloy powders, a graded coating structure is formed to achieve high fatigue strength. To achieve the aforementioned objectives, the present invention provides, in a first aspect, a method for the fatigue-resistant laser coating repair of axles based on graded phase transition control. In the method, a boundary layer, a transition layer, and a cover layer are successively applied to the surface of the base material of an axle to be repaired by means of laser coating, and the microstructures of the boundary layer, the transition layer, and the cover layer form a graded microstructure. wherein the boundary layer is formed by applying a first mixed powder to the surface of the base material of the axle using a deposition process with low heat input,wherein the boundary layer bonded to the base material has a two-phase 20 microstructure of ferrite and austenite; the transition layer is formed by applying a second mixed powder to the surface of the boundary layer, wherein the transition layer has a mixed microstructure of martensite, ferrite and residual austenite; and the cover layer is formed by applying a third mixed powder to the 25 surface of the transition layer using a deposition process with rapid cooling, wherein the cover layer has a martensitic structure with a low phase transition point and compressive stresses. The application thickness of each layer consisting of the boundary layer, transition layer, and top layer is 0.2 to 0.5 mm; the total thickness of the graduated application structure is 1.0 to 1.5 mm; and the intermediate layer temperature during the application process is controlled to 100 to 200°C. The first mixed powder has the following components: C ≤ 0.05 wt.%, Cr 18 to 22 wt.%, Ni 8 to 12 wt.%, Mo 0.3 to 0.8 wt.%, Si 0.5 to 1.0 wt.%.The remainder consists of Fe.5 The second mixed powder has the following components: C 0.10 to 0.15 wt.%, Cr 14 to 16 wt.%, Ni 5 to 7 wt.%, Mo 1.0 to 2.0 wt.%, Si 1.0 to 1.5 wt.%, B 0.1 to 0.3 wt.%, and the remainder consists of Fe. The third mixed powder has the following components: C ≤ 0.08 wt.%, Cr 10 to 14 wt.%, Ni 3 to 5 wt.%, Mo 1.5 to 2.5 wt.%, Si 2.0 to 10.3 wt.%, B 0.2 to 0.4 wt.%, and the remainder consists of Fe. In one embodiment, the laser layer deposition repair process comprises the following steps: The surface of the base material of the axle to be repaired is pretreated, whereby the oxide layers are removed and a non-destructive test is carried out to ensure that no defects are present on the surface of the base material. The first mixture powder is prepared and applied to the surface of the base material of the axle using a low-heat deposition process, thereby forming the boundary layer.wherein the proportion of element Crimer in the first mixture powder is 18 to 22 wt.% and the proportion of element Ni is 8 to 12 wt.% and the temperature range of the austenite phase is extended. The second mixture powder is prepared and applied to the surface of the boundary layer, thereby forming the transition layer, 25 wherein the proportion of element Mo is 1.0 to 2.0 wt.% and the proportion of element B is 0.1 to 0.3 wt.%, so that a mixed structure of martensite, ferrite and residual austenite is formed in the restored transition layer. The third mixed powder is produced and applied to the surface of the transition layer using a rapid cooling application process, thereby forming the top layer, BE2026 / 7423 6 wherein the proportion of element Cin third mixed powder is a maximum of 0.08 wt.% and the proportion of element Si is 2.0 to 3.0 wt.%, so that a layered martensite structure is formed,wherein the volume increase during the martensitic transformation causes surface compressive stresses.5 The low-heat deposition process used in the production of the boundary layer is carried out using a conventional low-heat welding process, wherein the present invention does not impose any restrictions here and the value of the heat input can be calculated using existing techniques regarding power, 10 speed and spot size. The heat-input deposition process used to produce the transition layer is also based on conventional methods, wherein the heat input in the deposition process for producing the transition layer is slightly higher than the heat input in the 15 production of the boundary layer. The value of the heat input can be calculated using existing techniques regarding power, speed and spot size. The laser layer deposition is carried out using a process with simultaneous powder feed. Furthermore, the manufacturing parameters for the boundary layer show the following 20 values: laser power from 1.5 to 2.5kW,Scan speed of 6 to 10 mm / s, powder feed rate of 12 to 15 g / min, shielding gas 100% argon. The manufacturing parameters for the transition layer are as follows: laser power of 2.0 to 2.4 kW, scan speed of 1025 to 14 mm / s, powder feed rate of 15 to 18 g / min, shielding gas 100% argon. The manufacturing parameters for the top layer are as follows: laser power of 2.2 to 2.6 kW, scan speed of 12 to 16 mm / s, powder feed rate of 18 to 20 g / min, shielding gas 100% argon. Preferably, the set laser power for both manufacturing parameters for the interface layer is 1.6 to 2.0 kW. BE2026 / 7423 7 Furthermore, the particle size of the alloy powders of the first, second, and third mixtures is 53 to 150 μm each. The oxide layer on the surface of the axle is removed by rotation or pulsed laser cleaning; and when performing pulsed laser cleaning, the laser wavelength is 1064 nm.the energy density 6 to 10 J / cm2 and the frequency 15 to 25 Hz. Furthermore, the manufactured boundary layer exhibits a two-phase microstructure consisting of 60 to 70% ferrite and 30 to 40% austenite, with an interfacial bond strength of at least 450 MPa. The transition layer has a synergistic three-phase microstructure consisting of 40 to 50% martensite, 30 to 40% ferrite, and 10 to 20% residual austenite, with a tensile strength of at least 800 MPa and an elongation of at least 12%. The martensite, with a low phase transition point and a Ms value of 150 to 200°C, generates compressive stresses of -200 to -500 MPa in the surface layer, thus increasing the fatigue life and preventing failure under up to 1 × 10⁷ cyclic loading cycles. Preferably, the axle repaired according to the present method meets the fatigue life requirements of 20 high-speed axles according to standards EN13261,wherein no cracks or pores as defects are present on the surface of the coating layers. The present invention provides, in a second aspect, a structure for fatigue-resistant laser coating repair of axles based on a graduated phase transition control. The structure is designed as a graduated coating structure and, starting from the surface of the axle to be repaired, successively comprises a boundary layer, a transition layer, and a top layer. For the boundary layer, iron-based powders with a high content of the elements Cr and Ni are used, wherein the proportion of Cr is 18 to 22 wt.% and the proportion of Ni is 8 to 12 wt.%, with the volume fraction of austenite being 30 to 40% and the volume fraction of ferrite being 60 to 70%. In the transition layer, the proportion of Mo is 1.0 to 2.0 wt.% and the proportion of B is 0.1 to 0.3 wt.%, with the volume fraction of martensite being 40 to 50%.The volume fraction of ferrite is 30 to 40% and the volume fraction of residual austenite is 10 to 20%. For the cover layer, powders with low carbon content and high silicon content are used, wherein the cover layer forms a carbon content of a maximum of 0.08 wt.% and a silicon content of 2.0 to 3.0 wt.% over a period of time in layered martensite structures with low phase transition points, 10 wherein the volume increase during the martensite transformation generates surface compressive stresses. The martensite transformation temperature Ms of the cover layer is 150 to 200°C, the width of the layered martensite is a maximum of 0.2 μm, and the surface compressive residual stresses are -200 to -500 MPa. 15 The advantageous effects of the present invention are as follows: The structure according to the invention for fatigue-resistant Laser layer deposition repair of axles based on a graded phase conversion control is designed as a three-layer microstructure to achieve a coordinated distribution of residual stresses in the 20 deposition layers.to enable the heat-affected zone and the base material of the axis repaired by laser layer deposition, thereby reducing the concentration of residual stresses and significantly improving fatigue strength by introducing compressive residual stresses onto the surface of the deposition layers. The use of 25 different alloy powders for each deposition layer, in combination with coordinated laser layer deposition processes, allows for control of microstructure and stress. In the boundary layer, iron-based powders with a high content of Cr and Ni are used to stabilize the austenite microstructure. The high content of Cr and Ni leads to an extension of the temperature range of the austenite phase. The application of a deposition process with low heat input suppresses grain coarsening and martensitic transformation. The transition layer contains small amounts of the metal element Bund Mo. Element B reduces the driving force of the martensitic transformation.while the element Mo causes a 5 increase in strength through solid solution formation. The combination of both elements enables targeted control of the martensite transformation temperature Ms, so that a mixed microstructure of martensite, ferrite, and residual austenite is formed. Alloy powders with low carbon content and high silicon content are used for the top layer. The addition of silicon leads to a reduction of the Ms value to 180°C. In combination with a rapid high-speed laser layer application and a rapid cooling process, a fine layered martensite microstructure with a low phase transition point is formed. The volume increase during martensite formation causes surface compressive stresses. DESCRIPTION OF THE ATTACHED DRAWINGS Figure 1 shows a schematic representation of the microscopic 20 microstructure of the graduated application layers,which have been produced according to a method for fatigue-resistant laser layer deposition repair of high-speed axes based on a graded phase conversion control according to embodiment 1. 25 Figure 2 shows a flow diagram of the method for fatigue-resistant laser layer deposition repair of high-speed axes based on a graded phase conversion control according to embodiment 1. BE2026 / 7423 10 DETAILED DESCRIPTION To clarify the objectives, technical solutions, advantageous effects and significant progress of the embodiments of the present invention, the technical solutions of the 5 embodiments of the present invention are clearly and completely described with reference to the accompanying drawings. It is evident that all described embodiments represent only a part of the embodiments of the present invention and not all embodiments. All further embodiments,The achievements of a person skilled in the art in this field without inventive step, based on the embodiments of the present invention, fall within the scope of protection of the present invention. In the description of the present application, the terms "first," "second," and "third" refer exclusively to descriptive purposes, unless expressly specified and limited otherwise, and are not to be interpreted as indicating or suggesting a relative meaning. The term "several" stands for two or more units. The terms "connect," "fix," and the like are to be understood in the broadest sense, unless otherwise specified or explained. For example, a "connection" can be a permanent connection, a detachable connection,a one-piece connection or an electrical connection; a “connection” may be a direct connection or an indirect connection via an intermediate medium. The specific meaning of the aforementioned terms within the scope of this application may be understood by a person skilled in the art depending on the specific circumstances. Exemplary embodiment 1 As shown in Figure 2, a method for fatigue-resistant laser coating repair of 30 high-speed railway axles based on a graded phase-conversion control comprises the following measures: BE2026 / 7423 11 Step 1: A pretreatment of the area to be repaired of a railway axle made of the typical alloy steel EA4T for high-speed railways is carried out. The measures include: S11: A turning operation of the axle surface to remove the 5 oxide layer on the surface of the axle; S12: Non-destructive testing of the microstructure of the base material of the axle by means of magnetic particle testing to ensure,that there are no defects on the surface of the base material. 10 Step 2: The boundary layer on the axle surface is produced by means of laser layer deposition. A first mixed powder is produced by blending alloy powders with a mass fraction of 0.05 wt% C, 20 wt% Cr, 10 wt% Ni, 0.5 wt% Mo, 0.8 wt% Si, and iron as the remaining component.15 The first mixed powder is applied to the surface of the axle base material using a low-heat deposition process and a simultaneous powder feed process, thereby forming the interface layer. The manufacturing parameters include a set laser power of 1.820 kW, a scan speed of 8 mm / s, a powder feed rate of 15 g / min, a deposition layer thickness of 0.3 mm, and 100% argon as the shielding gas. The interface bond strength between the interface layer and the base material is at least 450 MPa.The strain of the coating layer is at least 12% and the fatigue life is at least 1 × 10⁷ cycles at a stress amplitude of ±200 MPa and a stress ratio R = -1. The content of the alloying elements Cr and Ni in the boundary layer is high. The high content of the elements Cr and Ni leads to an extension of the temperature range of the austenite phase. The volume fraction of austenite in the restored boundary layer is 30 to 40%, and the volume fraction of ferrite is 60 to 70%. BE2026 / 7423 12 Step 3: The transition layer is produced. A second mixed powder is produced by blending alloy powders with a mass fraction of 0.12 wt% C, 15 wt% Cr, 6 wt% Ni, 1.5 wt% Mo, 1.2 wt% Si, 0.2 wt% B, and iron as the residual component. This second mixed powder is applied to the surface of the interface layer using a process with five simultaneous powder feeds, thereby forming the interface layer. The production parameters include a laser power of 2.2 kW, a scan speed of 12 mm / s, and a powder feed rate of 18 g / min.A coating layer thickness of 0.5 mm and 100% argon as a protective gas. The transition layer contains alloy powder with the elements B and Mo, which together contribute to the targeted control of the martensite transformation temperature Ms. The volume fraction of martensite in the restored transition layer is 40 to 50%, the volume fraction of ferrite is 30 to 40%, and the volume fraction of retained austenite is 10 to 20%. Step 4: The top layer is produced. A third mixed powder is produced by mixing alloy powders with a mass fraction of 0.08 wt% C, 12 wt% Cr, 4 wt% Ni, 2.0 wt% Mo, 2.5 wt% Si, 0.1 wt% B, and iron as the residual component. The third mixed powder is applied to the surface of the transition layer using a rapid cooling application process and a simultaneous powder feed process.which forms the top layer. The manufacturing parameters include cooling of the finished top layer 25 by air cooling to room temperature with a cooling rate of at least 120°C / s, a laser power of 2.5 kW, a scanning speed of 15 mm / s, a powder feed rate of 15 g / min, a coating thickness of 0.3 mm, and 100% argon as a protective gas. The top layer is produced from alloy powders with 30 low carbon content and high silicon content, so that a martensite structure with a low phase transition point is formed BE2026 / 7423 13. The martensite transition temperature M of the restored top layer is 150 to 200°C, the width of the layered martensite is a maximum of 0.2 μm, and the surface compressive residual stresses The concentrations of the components in each individual layer are given in Table 15. Table 1: Design of the graduated alloy powders (components in wt.%) Coating layer CCrNiMoSiBFe Target boundary layer 0.05 20 100 0.508. Residual fraction: High content of Crudium and Ni for stabilization of the austenite transition layer 0.12 156 1.5 1.2 0.2 Residual fraction: Delay of martensite transformation by BundMo cover layer 0.08 124 2.0 2.5 0.1 Residual fraction: Induction of intrinsic compressive stresses of both phase transformations by high Si content. The particle size of the alloy powders listed in Table 1 is 53 to 150 μm. 10. The manufacturing parameters and the requirements for the thickness of the boundary layer, the transition layer and the cover layer are given in Table 2. BE2026 / 7423 14 Table 2: Manufacturing parameters of graduated laser layer deposition Deposition layer Laser power (kW) Scan speed (mm / s) Powder feed rate (g / min) Protective gas Layer thickness (mm) Boundary layer 1.88 15 100% Ar0.3 Transition layer 2.21 218 100% Ar0.5 Top layer 2.51 520 100% Ar0.3 The total thickness of the finished deposition layers is 1.0 to 1.5 mm, and the intermediate layer temperature is controlled to 100 to 200°C. Step 5: The boundary layer,The transition layer and the top layer are produced successively from the axle surface outwards, according to the aforementioned steps, so that a graduated deposition structure is created. Material removal by turning is carried out on the repaired area of ​​the axle with the graduated deposition structure, whereby the thickness of the deposition layers is reduced to 1.0 to 1.5 mm. Subsequently, a non-destructive test is carried out to ensure that no defects such as cracks or pores are present. The schematic representation of the microscopic microstructure of the graduated deposition structure (comprising boundary layer, transition layer and top layer), which was produced according to the method for fatigue-resistant laser layer deposition repair of high-speed railway axles based on a graduated phase conversion control, is shown in Figure 1.

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