Multi-laser head in situ brake disc high speed cladding concept for protection against wear and corrosion

Abstract

The invention relates to a method for manufacturing a coated brake disc, the method comprising the following steps: - providing a brake disc having a first and a second friction surface; - coating at least a part of the first friction surface by means of a laser cladding process, characterized in that in the laser cladding process two or more, preferably two or particularly preferably at least three lasers and process heads are used, and that during at least a part of the laser cladding process, preferably two or particularly preferably at least three lasers and process heads are used simultaneously at different locations of the first friction surface.

Description

[Technical field]

[0001] The present invention relates to a method for laser cladding of friction surfaces of brake discs by means of two or more laser and processing heads (LPH) performing the cladding process simultaneously at different locations on the same friction surface. [Background technology]

[0002] With the braking system (discs and pads) being one of the main polluting elements in a vehicle, tightening dust emission regulations as well as market driven requirements to extend the life of brake discs are driving the demand for brake discs that are durable, corrosion resistant and reduce dust emissions into the environment.

[0003] A method of increasing the wear and corrosion resistance of brake discs and reducing dust emissions is to apply a corrosion and wear resistant coating to the friction surfaces of the brake discs. The coatings applied to these surfaces combine the advantageous properties of cast iron as a base material, such as affordability, good thermal conductivity and mechanical stability at high temperatures, with the benefits of the coating, such as increased corrosion and wear resistance compared to the base material.

[0004] The prior art approach outlined in EP 1 336 054 reveals brake discs coated with a metallic non-ceramic coating that exhibits higher oxidation and wear resistance than a cast iron core, the process used for coating being plasma coating, frame coating or wire coating.

[0005] The application of a cermet coating consisting of a metal matrix with oxide-ceramics to the friction surface, as described in DE 102014006064 A1, further improves the corrosion and wear resistance. To overcome problems due to penetrative corrosion, the application of nitride, carbide and oxide layers can be applied between the brake disc substrate and the functional top coating. Furthermore, an intermediate nickel-based layer for improving the adhesion of the functional top coating may be applied using thermal spraying.

[0006] One of the challenges of thermal spray coatings on cast iron brake discs is that graphite in the form of lamellae and / or spherical particles present on the surface to be coated leads to poor adhesion of the coating, and water diffusing through the coating or between the brake disc substrate and the coating leads to rust. Both effects can ultimately lead to delamination of the coating.

[0007] To overcome this, WO2022003189 reveals that by using a laser cladding process to coat ferrous substrate materials containing graphite, such as brake discs, the graphite lamellae and / or spherical particles on the surface of the substrate can even be melted or evaporated during the coating process. However, the melting or evaporation of the graphite particles during the laser cladding process leads to local defects and a local decrease in adhesion. It is subsequently revealed to apply an angle in the range of 10° to 45° between the axis perpendicular to the substrate surface and the laser beam. The advantage of this approach is that, on the one hand, the adhesion of the coating on the substrate can be increased and, on the other hand, the laser power level, and therefore the deposition rate, can be increased compared to processes using a laser beam that is perpendicular to the substrate to be coated.

[0008] Since the deposition rate and therefore the time to coat the brake disc friction surface is mainly related to the power of the laser, another prior art approach has proposed the application of bond coats and protective layers using high speed and high power lasers. With this approach, processing times of less than 110 seconds can be achieved for the laser cladding of an entire brake disc of standard size (288 mm outer diameter, 163 mm inner diameter). To achieve that, high power lasers with powers of more than 20 kW are required. Therefore, using high power lasers is a reasonable way to scale up the laser cladding process of brake discs for mass production scenarios.

[0009] However, the use of high power lasers can cause damage to the optics and nozzles by reflecting heat back to the laser and process head. This has a negative impact on part life and process stability in terms of powder feed rate to the part. In addition to this, the high power of the laser beam can cause deformation of the brake disc due to thermal effects.

[0010] On the other hand, reducing the power of the LPH increases the laser cladding process time for cladding a standard brake disc surface. Application of the coating using laser cladding to the friction surface of a brake disc can be performed by rotating the brake disc around its centerline while the LPH moves radially from the axis of the brake disc towards the outer diameter of the friction surface. In this case, the spot of the laser beam traces a spiral on the surface to be coated. To generate a smooth surface, an overlap between adjacent coating lines must be implemented. For example, if the width of the deposited line is 1,5 mm, an overlap of 90% will result in a reasonably smooth surface after laser cladding. Multiplying the width of the deposited line by the overlap results in a radial distance between successive turns of the spiral of 0.15 mm. Using the difference between the inner diameter (163 mm) and the outer diameter (288 mm) of the standard friction surface of the brake disc to be coated, and the distance of 0.15 mm between successive turns, it can be calculated that the length of the spiral is 295 m. Using a preferred relative speed of the laser beam over the surface to be coated of 100 m / min, the process time for laser cladding the friction surface of one side of a standard brake disc using an LPH with a power of 6 kW or less can be calculated to be 177 seconds, compared to the 110 seconds required to laser clad the same surface using an LPH with a power of 20 kW or less. Even though the disadvantages of the shorter LPH part life and increased powder deposition stability of the high power LPH approach can be overcome by using a single lower power LPH, the increased laser cladding process time of this approach clearly represents a disadvantage in an industrial process. Summary of the Invention [Problem to be solved by the invention]

[0011] It is therefore an object of the present invention to provide a method for coating brake discs with a corrosion and wear resistant coating using laser cladding which exhibits short laser cladding process times and improved properties compared to the prior art. [Means for solving the problem]

[0012] According to the present invention the problem of laser cladding of brake discs providing higher process stability and less wear of the laser and the process head in short process times is solved by performing laser cladding simultaneously on the same surface to be coated of the same brake disc using two or more LPHs, preferably two or particularly preferably at least three LPHs, with a power of preferably less than 6 kW for each LPH, with the spot of the laser beam of the LPHs moving in an Archimedean spiral over the surface to be coated.

[0013] To perform laser cladding of the rotationally symmetric friction surface of the brake disc, the laser beam preferably moves in a spiral. To achieve this spiral on this surface, the LPH can be moved in a spiral while the brake disc is stationary, or preferably, the brake disc is rotated while the LPH moves radially towards or away from the centerline of the brake disc. To better understand the movements during the laser cladding process, the following paragraphs set up a formal frame for the description of the movements of the brake disc and the LPH relative to each other.

[0014]

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[0015] The length L of the Archimedean spiral can be calculated as follows:

[0016]

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[0017] To achieve a stable deposition process, it is preferable to keep the deposition rate, v, constant. As the LPH moves radially away from the centerline of the brake disc, the angular velocity of the disc needs to slow down as the radius of the helix increases. The relationship between deposition rate and angular velocity can be expressed as:

[0018]

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[0019]

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[0020] Therefore, the cladding process must comply with the following two conditions:

[0021]

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[0022] If D is the width of the deposit line and x percent overlap of the next deposit line with the previous one is required, then within 360° of brake disc rotation, the LPH should be moved according to the following relationship:

[0023]

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[0024] Using the example above with D=1.5mm and x=90%, we get:

[0025]

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[0026] If v=100 m / min, then we get:

[0027]

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[0028] If the inner diameter of the disc is 163mm, then:

[0029]

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[0030] therefore,

[0031]

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[0032] If the outer diameter of the disc is 288mm, then:

[0033]

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[0034] therefore,

[0035]

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[0036] As mentioned, this is a formal description of the process for one laser and process head.

[0037] In order to maintain a low level of heat input and a high level of process reliability, the present invention proposes to use preferably two, or even more preferably at least three, laser and processing heads (LPH), operating simultaneously on the same surface to be coated at lower laser power levels, preferably up to 6 kW, to minimize total damage to the optics and maximize process reliability.

[0038]

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[0039] According to a first preferred embodiment of the present invention, the LPH is expressed as r(t)=r1(t)=r2(t)=...=r n (t) is moved at the same time,

[0040]

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[0041] where n is the number of LPHs used. As a result of using n LPHs, the distance d between two adjacent lines deposited as measured along a line starting from the center point of the spiral at an angle φ can be described by the following equation:

[0042]

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[0043] Since the distance between two adjacent lines deposited is kept constant to provide a defined overlap, the increment "a" can be increased by multiplying it with the number n of LPHs used.

[0044]

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[0045] The invention will now be described in detail by means of one non-limiting example. [Brief description of the drawings]

[0046] [Figure 1] The setup for laser cladding of a brake disc using four LPHs is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The example shows the laser cladding of one brake disc with n=4 LPHs at the same time. A brake disc with an inner diameter of 163 mm and an outer diameter of 288 mm is laser cladded. The four LPHs have the following positions (polar coordinates) at the start:

[0048] 1.(163mm;6800) 2. (163mm; 6800-2Π·1 / 4) 3. (163mm; 6800-2Π·2 / 4) 4. (163mm; 6800-2Π·3 / 4) The relative speed of the laser beam over the surface to be coated is 100 m / min for all LPHs, so the disk rotates at 613 rpm at the start. To achieve a 90% overlap of the cladding lines at the four LPHs, the increment "a" can now be chosen to be 4·0.0239 mm = 0.095 mm.

[0049] All the LPHs preferably move radially from the inner diameter to the outer diameter of the friction surface of the brake disc being laser clad at the same speed, so that the angular velocity of the brake disc can be the same for all of them.

[0050] In the above example, each individual line deposited has a length of 73.8 m, resulting in a process time of 44.3 seconds per laser clad friction surface, which is less than half the time required to coat the same surface using a high power laser of 20 kW. At the same time, the low power of the LPH used in the example reduces the risks of using a high power laser, as previously mentioned.

[0051] This is an example with four LPHs, one skilled in the art will appreciate that a different number of LPHs can be selected and the angular positions of the LPHs can be varied.

[0052] Those skilled in the art will also understand that all LPHs may start laser cladding at the same time, or there may be a delay in the cladding process, for example to start cladding of a second LPH at the same angular position of the first cladding.

[0053] The embodiments have been described with cladding starting from the inner diameter and ending at the outer diameter. Those skilled in the art will appreciate that starting from the outer diameter and ending at the inner diameter is also an option.

[0054] As those skilled in the art know, a brake disc typically has a first friction surface and a second friction surface. First, the first friction surface can be coated according to the present invention, and then the second friction surface can be coated according to the present invention.

[0055] In the present disclosure, a method for producing a coated brake disc is described, the method comprising the steps of: providing a brake disc having a first and a second friction surface, preferably a cast iron brake disc and / or an uncoated brake disc; - coating at least a portion of the first friction surface by a laser cladding process.

[0056] In this method, two or more, preferably two, or particularly preferably at least three lasers and process heads are used, and the lasers and process heads are used simultaneously at different locations on the first friction surface during at least a portion of the laser cladding process.

[0057] According to one embodiment, during at least a portion of the laser cladding process, the brake disc is rotated about its axis of rotation and at the start of the portion of the process the distances of the laser and process head to the axis of rotation are selected to be the same, and then the distances are simultaneously changed so that at any point within the portion of the laser cladding process each of the lasers and process heads has the same distance to the axis of rotation as the others of the at least three lasers and process heads.

[0058] If the angular positions of the laser and process head are listed as:

[0059]

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[0060] The laser and process head are preferably arranged such that the following conditions are met:

[0061]

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[0062] where n is the number of lasers and process heads. The method may then be performed such that the start of deposition of the i th laser and process head is delayed until the brake disc has rotated by an angle:

[0063]

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Claims

1. 1. A method for producing a coated brake disc, said method comprising the steps of: - providing a brake disc having a first and a second friction surface; - coating at least a part of said first friction surface by a laser cladding process, The method according to claim 1, characterized in that in the laser cladding process, two or more, preferably two, or particularly preferably at least three lasers and process heads are used, and that during at least a portion of the laser cladding process, the lasers and process heads are used simultaneously at different locations of the first friction surface.

2. 2. The method of claim 1, wherein the brake disc is rotated about its axis of rotation during at least a portion of the laser cladding process, and the distances of the lasers and process heads to the axis of rotation at the start of the portion of the process are selected to be the same, and then the distances are simultaneously changed so that at any point within the portion of the laser cladding process, each of the at least three lasers and process heads has the same distance to the axis of rotation as the others of the at least three lasers and process heads.

3. The angular positions of the laser and process head are described as follows: [0010] The laser and process head are positioned such that the following conditions are met: [0025] 3. The method of claim 2, wherein n is the number of said lasers and process heads.

4. The deposition start of the i-th laser and process head is such that the brake disc is at an angle [0030] 4. The method of claim 3, wherein the rotation is delayed until the rotation is at a constant speed.

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