A method of laser cladding the inner wall of a brake drum

Abstract

This invention discloses a laser cladding method for the inner wall of a brake drum, belonging to the field of laser cladding technology. The method specifically includes: horizontally clamping the brake drum, keeping its gray cast iron inner wall axially horizontal, and positioning a laser cladding head inside the brake drum and above its central axis; performing laser cladding using a synchronous powder feeding method, controlling the laser cladding head to translate along the axial direction of the brake drum while simultaneously rotating the brake drum around its axis, thereby forming a continuous spiral coating on the inner wall surface of the brake drum. This invention breaks through the traditional single mode of surface strengthening of gray cast iron that relies solely on welding or spraying, and can adaptively combine with heat sources such as laser cladding to construct a multi-layer interface with gradient composition / structure, providing a universal platform for the industrially scalable development of long-life, low-carbon remanufactured brake drums.

Description

Technical Field

[0001] This invention belongs to the field of laser cladding technology, and particularly relates to a laser cladding method for the inner wall of a brake drum. Background Technology

[0002] Brake drums are a core safety component of heavy-duty truck chassis, typically operating under conditions of low speed, high load, frequent braking, and intense alternating thermo-mechanical shock. Especially on long downhill sections in mountainous areas or under continuous traffic light conditions in cities, the inner wall of the brake drum must simultaneously possess excellent thermal fatigue resistance, low wear rate, and crack resistance. However, traditional gray cast iron brake drums, limited by materials and manufacturing processes, struggle to meet the stringent requirements of long service life and maintenance-free operation for commercial vehicles, becoming a key weakness affecting overall vehicle reliability.

[0003] Current mainstream brake drum manufacturing technologies still have significant shortcomings. While integrally cast gray cast iron brake drums can provide a uniform matrix (such as HT250, with a hardness of approximately HB180-220), their material utilization rate is typically less than 50%, generating a large amount of dust during processing, and the high carbon equivalent easily leads to graphite coarsening. A more prominent problem is that the thermal fatigue resistance of this type of brake drum heavily depends on the uniformity of graphite morphology and distribution; in actual castings, localized aggregation of D-type graphite can form soft, weak areas, shortening the thermal fatigue crack initiation time by up to 40% and significantly accelerating crack propagation.

[0004] While centrifugal casting helps improve the density of castings, the uneven distribution of the centrifugal force field can easily lead to compositional segregation on the inner wall of large brake drums, resulting in significant fluctuations in circumferential hardness (HB difference can reach over 30). Furthermore, this process requires extremely high equipment precision; brake drums with diameters exceeding 400mm often suffer from dynamic balance issues due to circular runout and wall thickness differences, and uneven contact of the braking surfaces after assembly can easily cause localized overheating risks.

[0005] Traditional laser cladding technology has attempted to improve the performance of brake drum inner walls through surface remelting and metallurgical bonding, but its application still has significant limitations. For example, when using CO2 or semiconductor lasers for single-pass cladding of cast iron, the graphite flakes rapidly dissolve and supply excessive carbon to the molten pool, causing the carbon content on the surface of the cladding layer to rise sharply from 0.3% to 1.2% in a very short time (e.g., within 0.8 seconds), forming a continuous cementite network. This results in a sudden increase in hardness to 58-62 HRC, increased material brittleness, and a decrease in the critical load for crack initiation by approximately 35%. Simultaneously, the high energy density of the laser (≥1.5 kW / mm²) presents significant challenges. 2 This can easily cause localized white iron formation in the heat-affected zone of the gray cast iron matrix. The thickness of this zone is about 0.3-0.6 mm, and the hardness increases from HB200 to HB550 or more. This type of brittle phase is very likely to become the initiation of cracks in subsequent braking thermal cycles, which in turn leads to a reduction of fatigue life by more than 20%.

[0006] More importantly, existing laser cladding processes struggle to achieve gradient control of composition and microstructure. While single alloy systems (such as Fe-Cr-Ni-Mo) can improve wear resistance, the significant difference in elastic modulus between the high-hardness martensite and the soft graphite in the matrix (up to 210 GPa) easily leads to shear stress concentration at the interface under thermal stress. Simply reducing hardness, however, fails to resist oxidation and adhesive wear at approximately 600°C during braking. The lack of a gradient structure—"surface heat resistance and oxidation resistance - intermediate friction reduction - strong and tough bottom layer"—makes traditional cladding layers insufficiently adaptable to low-speed, heavy-load, and high-temperature alternating braking conditions, significantly increasing the risk of coating peeling and failure.

[0007] In summary, the surface strengthening of the inner wall of gray cast iron brake drums faces significant challenges under the high temperatures and alternating stresses caused by frequent braking. Traditional methods struggle to simultaneously meet the combined requirements of wear resistance, corrosion resistance, and thermal fatigue resistance. While laser cladding offers advantages such as low heat input and high bonding strength, the high graphite content in gray cast iron can lead to poor bonding of the cladding layer, white cast iron structure, and increased susceptibility to cracking. Therefore, there is an urgent need to develop a novel laser cladding process suitable for the geometric characteristics of the brake drum's inner wall, capable of effectively suppressing white cast iron structure and reducing cracking tendency, in order to achieve the goals of high performance, long lifespan, and lightweight remanufacturing of brake drums. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention proposes a laser cladding method for the inner wall of a brake drum, aiming to solve the problems of high crack rate and mismatched braking performance in existing coatings.

[0009] To achieve the above objectives, the present invention provides the following technical solution: A laser cladding method for the inner wall of a brake drum includes the following steps: The brake drum is horizontally clamped, keeping its gray cast iron inner wall in an axially horizontal state, and the laser cladding head is positioned inside the brake drum and above the central axis of the brake drum. Laser cladding is performed using a synchronous powder feeding method. The laser cladding head is controlled to translate along the axial direction of the brake drum while the brake drum rotates around its axis, thereby forming a continuous spiral coating on the inner wall surface of the brake drum.

[0010] Furthermore, the cladding powder used in the synchronous powder feeding is iron-based alloy powder JG-9A, and this powder contains boron. The mass percentage of boron (B) in the cladding powder is 0.8-1.2%.

[0011] This invention utilizes a boron-containing special powder to effectively suppress the precipitation of hard and brittle phases, significantly reducing the risk of crack initiation in the coating and greatly improving the bonding strength between the coating and the substrate. At the same time, during the process of forming a stable metallurgical bond with the gray cast iron substrate, the powder can prevent the formation of sheet-like hard and brittle phases due to the precipitation of carbon (C) elements in the gray cast iron, thus ensuring the integrity of the coating structure and its reliability in use.

[0012] Furthermore, the particle size range of the cladding powder is 20-50 μm.

[0013] The optimized setting of limiting the particle size of the cladding powder to 20-50μm ensures both the high absorption rate of the powder to the laser and effectively avoids the risk of powder blockage during the powder feeding process, thereby giving the coating a better cladding quality and structural stability.

[0014] Furthermore, an annular powder feeding nozzle is used for synchronous powder feeding, and argon gas is used as the powder carrier gas and the protective gas for the molten pool. The flow rate of the argon gas is 14 L / min, which enables the powder to form a stable powder spot with good aggregation in the cladding area.

[0015] This limitation can create an effective protective atmosphere while preventing the accumulation of powder at the edge of the molten pool.

[0016] Furthermore, the power of the laser cladding is 2300W.

[0017] Furthermore, the scanning speed of the laser cladding is 20 mm / s.

[0018] Furthermore, the powder feeding speed during the laser cladding process is 25.6 g / min, the laser spot diameter is 3 mm, and the overlap rate between adjacent cladding channels is 66%.

[0019] Compared with the prior art, the present invention has the following advantages and technical effects: This invention introduces trace amounts (0.8-1.2%) of boron (B) into the iron-based coating laser-clad onto the gray cast iron surface of the brake drum's inner wall. This effectively suppresses excessive precipitation of the Cr7C3 hard phase and significantly reduces the cladding layer's susceptibility to cracking due to brittle phase aggregation. Simultaneously, the use of fine powder with a particle size range of 20-50 μm balances the thickness of a single cladding step with powder flowability, enabling single-step forming of ≥1.2 mm and avoiding the risk of cyclic thermal stress accumulation and cracking associated with multiple cladding steps. This process requires no complex preheating or subsequent heat treatment, boasts high forming efficiency and low cost, and can be directly remanufactured on-site at the brake drum's service location. Although the addition of B slightly reduces the coating's plasticity, it still fully meets the lifespan requirements of the brake drum, which are primarily based on resistance to thermal fatigue and wear, under the "high temperature-high frequency-heavy load" friction conditions. Furthermore, this invention breaks through the traditional single mode of surface strengthening of gray cast iron that relies solely on welding or spraying. It can be adaptively combined with heat sources such as laser cladding to construct a multi-layer interface with gradient composition / structure, providing a universal platform for the development of long-life, low-carbon remanufactured brake drums that can be industrially promoted. Attached Figure Description

[0020] 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 undue limitation of the invention. In the drawings: Figure 1 The images show the flaw detection results of the coatings prepared in Example 1 and Comparative Examples 1-2 (from left to right: Comparative Example 1, Example 1, Comparative Example 2). Figure 2 Metallographic images of the surface of the coatings prepared in Example 1 and Comparative Examples 1-2 to the gray iron bond (from left to right: Comparative Example 1, Example 1, Comparative Example 2). Figure 3 The XRD patterns are of the coatings prepared in Example 1 and Comparative Examples 1-2. Figure 4 This is a graph showing the change in hardness of the coating prepared in Example 1 as a function of depth. Figure 5 The friction coefficient diagrams are shown for the coated sample and the gray cast iron substrate prepared in Example 1. Figure 6 The wear amount of the coated sample and the gray cast iron substrate prepared in Example 1; Figure 7 The images show the wear cross-section of the coated sample and the gray cast iron substrate prepared in Example 1. Figure 8 This is a simulation process for heat treatment; Figure 9 The cross-sectional morphology of the coating sample prepared in Example 1 after different quenching times; Figure 10The image shows the wear cross-section of the coating sample prepared in Example 1 after repeated quenching and subsequent friction and wear. Figure 11 The image shows the flaw detection results of the coating prepared in Comparative Example 3-11. Figure 12 The thickness of the coating prepared in Comparative Example 3-11; Figure 13 The hardness of the coating prepared in Comparative Example 3-11; Figure 14 The coefficient of friction of the coating prepared in Comparative Example 3-11; Figure 15 The image shows a wear cross-sectional view of the coating prepared in Comparative Example 3-11. Detailed Implementation

[0021] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0022] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0023] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0024] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0025] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0026] This invention provides a laser cladding method for the inner wall of a brake drum, comprising the following steps: S1. Clamping and Positioning: The brake drum is clamped and fixed horizontally to ensure that its gray cast iron inner wall is in an axially horizontal state; then, the laser cladding head is precisely positioned inside the brake drum by an industrial robot or a special positioning device, and the laser cladding head is positioned above the central axis of the brake drum to complete the position calibration before cladding.

[0027] S2. Laser Cladding Implementation: Laser cladding operations will be carried out using a synchronous powder feeding method. Specific parameters and operational requirements are as follows: (1) The cladding powder is iron-based alloy powder JG-9A, which has a boron (B) content of 0.8-1.2% by mass (for example, in the preferred embodiment of the present invention below, the B content is 1.02wt%), and the powder particle size is controlled within the range of 20-50μm; (2) Powder is fed by an annular powder feeding nozzle, and argon is used as the powder carrier gas and the protective gas for the molten pool. The argon flow rate is set to 14L / min to ensure that the powder forms a stable and well-aggregated powder spot in the cladding area. (3) Set the laser cladding process parameters: laser power is 2300W, scanning speed is 20mm / s, powder feeding speed is 25.6g / min, laser spot diameter is 3mm, and the overlap rate between adjacent cladding channels is controlled at 66%; (4) After starting the equipment, control the laser cladding head to move at a constant speed along the axial direction of the brake drum, while keeping the brake drum rotating smoothly around its own axis, and forming a continuous spiral coating on the inner wall surface of the brake drum through multiple cladding passes.

[0028] Unless otherwise specified, "room temperature" in this invention refers to 25±2℃.

[0029] All raw materials used in this invention were purchased from the market.

[0030] The technical solution of the present invention will be further illustrated by the following embodiments.

[0031] Example 1, Comparative Examples 1-2 1. Powder composition: Medium carbon iron-based powder JG-9A with a particle size of 20-50μm was selected. The purpose of selecting this powder was mainly to study the influence of element B on the cracking tendency and wear resistance of the coating. The specific composition is shown in Table 1 (the medium carbon iron-based powder used in Example 1 was purchased from Tianjin Zhujin Technology Development Co., Ltd., the medium carbon iron-based powder used in Comparative Example 1 was purchased from Tianjin Zhujin Technology Development Co., Ltd., and the medium carbon iron-based powder used in Comparative Example 2 was purchased from Tianjin Zhujin Technology Development Co., Ltd.).

[0032] Table 1. Composition ratio (wt%) of JG-9A with different B element contents. 2. Coating preparation: The brake drum is horizontally clamped and fixed, ensuring that its gray cast iron (base) inner wall is axially horizontal. An industrial robot positions the laser cladding head above the brake drum's central axis, and the laser defocusing parameters are set. Laser cladding is used to prepare the cladding layer. The laser cladding power is 2300W, the scanning speed is 20mm / s, the powder feeding speed is 25.6g / min, the spot diameter is 3mm, the powder feeding gas and protective gas are high-purity argon, the gas flow rate is 14L / min, the cladding overlap rate is 66%, and multiple cladding passes are used to clad an iron-based cladding layer onto the inner wall surface of the brake drum, forming a continuous spiral coating with an average thickness of 1.2-1.4mm.

[0033] 3. Analysis: (1) Use DPT-5 dye penetrant testing reagent to perform flaw detection analysis on the coating surface. Figure 1 The images show the flaw detection results of the coatings prepared in Example 1 and Comparative Examples 1-2. Figure 1 As can be seen, when the B element content is 0 in Comparative Example 1, dense network cracks appear on the coating surface; when the B element content is 1.02% in Example 1, there are no obvious cracks on the coating surface; when the B element content is 1.53% in Comparative Example 2, network cracks appear on the coating surface, but the crack rate is lower than that in Comparative Example 1, indicating that the coating prepared with a B content of 1.02% is optimal.

[0034] (2) Laser cladding samples were cut by wire electrical discharge machining. The samples were then ground, polished, and etched. The macroscopic morphology and defects of the cladding layer were observed using a metallographic microscope.

[0035] Figure 2 Metallographic images of the coatings prepared in Examples 1 and 1-2, from the surface to the gray iron interface. Figure 2 As can be seen, in Comparative Example 1, when the B element content is 0, cracks grow along the molten pool overlap in the cross section; in Example 1, when the B element content is 1.02%, the coating cross section has no obvious defects; in Comparative Example 2, when the B element content is 1.53%, the coating cross section has more pores, which promotes crack formation.

[0036] (3) After polishing the surface of the sample, XRD experiments were performed to further analyze the phase changes in the coating.

[0037] Figure 3 The XRD patterns of the coatings prepared in Example 1 and Comparative Examples 1-2 are shown below. Figure 3As can be seen from the spectrum, the peak value corresponding to M7C3 in the spectrum of Comparative Example 1 is obvious, indicating that a large amount of Cr7C3 is generated in the coating without the addition of B element; the spectrum of Example 1 shows that the coating mainly contains γ-Fe and α-Fe; in Comparative Example 2, due to the addition of excessive B element, M2B type borides appeared. Although the generation of M7C3 was suppressed, the borides are still a hard and brittle phase, so cracks still appeared on the surface of the coating.

[0038] (4) The microhardness of the sample was tested using a Vickers microhardness tester. The load was set to 300g and the loading time was 15s. Points were made at equal intervals along the cladding depth direction to obtain the microhardness distribution of the coating cross section prepared in Example 1 in this direction.

[0039] Figure 4 This is a graph showing the change in hardness as a function of depth for the coating prepared in Example 1. Figure 4 As can be seen, a significant hardness abrupt change occurs in the heat-affected zone 1250 μm away from the coating surface. This is because laser cladding has the characteristics of rapid heating and cooling, which leads to insufficient element diffusion in the heat-affected zone. Consequently, a large amount of carbon (C) is precipitated in the gray cast iron matrix, ultimately resulting in a significant increase in the local hardness of the heat-affected zone.

[0040] (5) A multifunctional friction and wear testing machine was used to conduct friction and wear tests on the coating sample (JG-9A) and the gray cast iron substrate (HT250) prepared in Example 1 to obtain the friction coefficient and wear rate of the coating. Si3N4 balls were used as the friction pair in the friction and wear test. The test load was 30N, the reciprocating stroke was 5mm, the frequency was 10Hz, and the test time was 30min. The coating surface needed to be cut down to the working thickness range, and the surface was sanded to avoid the influence of surface unevenness on the friction test.

[0041] Figure 5 The friction coefficient diagrams are shown for the coated sample and the gray cast iron substrate prepared in Example 1. Figure 6 The wear amount of the coated sample and the gray cast iron substrate prepared in Example 1; Figure 7 The image shows the wear cross-section of the coated sample (JG-9A) and the gray cast iron substrate (HT250) prepared in Example 1.

[0042] from Figures 5-7 As can be seen, under the same friction conditions, the coefficient of friction and braking effect of the coating are similar to those of the substrate, but the wear amount and wear cross section of the coating are far superior to those of the substrate.

[0043] (6) The coating samples prepared in Example 1 were subjected to repeated quenching in a heat treatment furnace for 5, 10, and 15 times, simulating the working environment of a brake drum after multiple long-term braking and heating, followed by rapid cooling using a spray device. The failure limit of the coating after multiple quenchings was investigated, and friction and wear tests were conducted to investigate the braking effect of the coating after multiple quenchings.

[0044] Figure 8 This is a simulation process for heat treatment.

[0045] Figure 9 The cross-sectional morphology of the coating sample prepared in Example 1 after different quenching cycles is shown below. Figure 9 As can be seen, after 15 repeated quenchings, the coating did not crack, but the substrate showed cracks extending inward. It can be predicted that the failure mode is substrate fracture, indicating that the coating and the substrate have a strong bond and the coating will not crack or fall off during repeated quenching.

[0046] Figure 10 The image shows the wear cross-section of the coating sample prepared in Example 1 after repeated quenching and subsequent friction and wear. It can be seen that as the number of quenching cycles increases, the wear resistance of the coating gradually decreases, but it is still better than that of the unquenched substrate.

[0047] Comparative Example 3-11 Similar to Example 1, the difference lies in the different laser cladding parameters used during the coating preparation process. The resulting coatings are named sequentially as (H1, H2, H3, H4, H5, H6, H7, H8, H9), as shown in Table 2.

[0048] Table 2 Laser cladding parameters during coating preparation in Comparative Example 3-11 Figure 11 The image shows the flaw detection results of the coating prepared in Comparative Example 3-11. It can be seen that H9 experienced increased stress in the coating due to excessive heat input, which exceeded the coating's limit and caused cracking.

[0049] Figure 12 The thickness of the coating prepared in Comparative Example 3-11 is shown.

[0050] Figure 13 The hardness of the coating prepared in Comparative Example 3-11 is shown.

[0051] The coating thickness determines the service life of the braking material. Figure 12 It can be seen that the coating thickness is thicker in H7, H8, and H9, while it is thinner in H1-H6. However, from... Figure 13As can be seen, the surface hardness of coatings H7, H8, and H9 is slightly lower than that of H1-H6. This is because H1-H6 has a lower heat input and a faster molten pool cooling rate, resulting in severe element segregation and thus a slightly higher coating hardness than H7-H9, which has a higher heat input. Coating hardness and thickness together determine its service life; therefore, H7 and H8 were chosen after comprehensive consideration.

[0052] Figure 14 The coefficient of friction is the coating obtained in Comparative Example 3-11.

[0053] Figure 15 The image shows a wear cross-sectional view of the coating prepared in Comparative Example 3-11.

[0054] from Figure 5 , Figure 6 , Figure 7 , Figure 14 , Figure 15 It can be seen that, under the same friction and wear conditions, among the nine parameters, the friction coefficient of H7 is similar to that of gray cast iron in terms of mean value and stability, and its wear amount and wear track depth are far superior to those of gray cast iron. This indicates that the H7 coating not only ensures the braking effect of the brake drum, but also greatly extends the service life of the brake drum.

[0055] In summary, the above comparison shows that Example 1 is the only solution that can simultaneously meet the core requirements of "crack-free, long life and high thermal shock resistance" of the brake drum, and has the best overall performance.

[0056] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A laser cladding method for the inner wall of a brake drum, characterized in that, Includes the following steps: The brake drum is horizontally clamped, keeping its gray cast iron inner wall in an axially horizontal state, and the laser cladding head is positioned inside the brake drum and above the central axis of the brake drum. Laser cladding is performed using a synchronous powder feeding method. The laser cladding head is controlled to translate along the axial direction of the brake drum while the brake drum rotates around its axis, thereby forming a continuous spiral coating on the inner wall surface of the brake drum.

2. The laser cladding method for the inner wall of the brake drum according to claim 1, characterized in that, The cladding powder used in the synchronous powder feeding is iron-based alloy powder JG-9A, and the powder contains boron.

3. The laser cladding method for the inner wall of the brake drum according to claim 2, characterized in that, The cladding powder contains 0.8-1.2% boron by mass.

4. The laser cladding method for the inner wall of the brake drum according to claim 2, characterized in that, The particle size of the cladding powder is 20-50 μm.

5. The laser cladding method for the inner wall of the brake drum according to claim 1, characterized in that, The synchronous powder feeding is performed using an annular powder feeding nozzle, and argon is used as the powder carrier gas and the protective gas for the molten pool. The flow rate of the argon is 14 L / min.

6. The laser cladding method for the inner wall of the brake drum according to claim 1, characterized in that, The power of the laser cladding is 2300W.

7. The laser cladding method for the inner wall of the brake drum according to claim 1, characterized in that, The scanning speed of the laser cladding is 20 mm / s.

8. The laser cladding method for the inner wall of the brake drum according to claim 1, characterized in that, During the laser cladding process, the powder feeding speed is 25.6 g / min, the laser spot diameter is 3 mm, and the overlap rate between adjacent cladding channels is 66%.

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