Laser-material processing for the production of bronze coatings for tribological applications

Bronze has proven itself as a highly functional material in tribological systems for numerous applications. The different bronze alloys offer an adaptation of the material properties to a wide range of loads. Bronze can be used as a solid material in the component or applied locally only as a functional layer in bi-metal components. The bi-metal components offer the combination of the advantages of the good mechanical properties of the steel-base body with the very good tribological properties of the bronze alloy. In this article, the production of coatings used in bi-metal bearings is presented by using laser-deposition brazing. The details of the laser-coating technology are presented, and the material properties of the bi-metal parts are analyzed.

In gearboxes, the shafts are supported by ball bearings or plain bearings. This article deals with the production technology of plain bearings. Highly stressed plain bearings usually consist of a steel base body, which is coated with a functional layer. For functional layers, there are different technologies such as engineering plastics, ceramics, or metallic layers. Tin bronzes have been established for more than 50 years as a highly functional tribological layer for plain bearings in industrial applications. Conventionally, bronze layers are applied to a steel base body by a castings process, leading to a coating lead time of above three days (mainly due to masking) and material efficiency below 10 percent (mainly due to removal of excess material). In the following, a new method is presented and analyzed in which laser technology is used to apply a thin metallic bronze layer for plain bearings. By this process, the lead time is reduced to below one day (no need for masking) and a material efficiency above 80 percent (net shape manufacturing).

1 State-of-the-art

1.1 Laser Surface Processing

1.1.1 Technology overview

The word “laser” is an acronym and stands for “Light Amplification by Stimulated Emission by Radiation.” A laser is a device that generates electromagnetic waves with specific properties called a laser beam. The laser beam is often also referred to casually as “light.” However, in the strict sense, only the parts of the entire electromagnetic spectrum, which are visible to the human eye, are meant by the term “light.” The spectrum of electromagnetic waves of the laser beam also includes shorter wavelengths (ultraviolet) and longer wavelengths (infrared). Furthermore, there is an essential difference between the “light” and the laser beam. The light is not coherent in space and time. That means it has different electromagnetic wavelengths, which propagate in different directions. In comparison, the laser beam has coherent properties in that it has only one wavelength, and the wavelengths have only one direction and propagate in phase. These properties allow the laser beam to be focused very precisely by optical elements such as lenses. [1]

Due to its specific properties, a laser is used in a variety of applications, including electronics, medical technology, physics, and materials processing. The following state-of-the-art refers to the laser material processing of metals and specifically to laser surface processing of metals. A key parameter in laser surface processing is the intensity, that means how much energy per area is applied to the metal surface by the laser beam. If the intensity is low, the metal is only heated (e.g., heat treatment on the work piece surface such as hardening). If the intensity is increased, then metal can be melted (e.g., welding) and at a higher intensity, metal can also be directly vaporized (e.g., cutting of metals) [2]. In total, there are seven different laser-surface processes. According to the applied intensity and the change of the aggregate state of the metal, the laser surface processing methods can be classified into three main groups: heating, melting, and vaporization (see Figure 1). Further classification is based on whether the aim is to influence the micro geometry of the surface or the material properties. There are processes with or without added material. In the case of melting material, it must be distinguished whether the added material and/or the substrate is melted. The added material can be in the form of wire, tape, paste, or powder. The following section describes the processes in which the added material is a powder.

In the case of modifying the material properties without adding material, two processes are used: laser hardening and laser remelting. In laser hardening, the surface is heated up to a temperature below the melting temperature [3], and in laser remelting, the surface is heated up higher than the melting temperature [4]. The micro geometry of the surface can be changed by melting the roughness peaks (laser polishing) [5] or by vaporizing and thus removing material (laser ablation) by an ultrashort pulses laser (USPL) [6]. Laser polishing and laser remelting are both processes where no material is added, and the surface is melted. However, the two processes differentiate insofar that with laser polishing, only the roughness peaks are melted with the aim to change the micro geometry, and with laser remelting, the substrate surface is remelted more deeply with the aim to influence the material properties essentially.

When a material is added in laser surface processing, four types of processes are distinguished: laser melt injection, laser alloying, laser deposition welding, and laser deposition brazing. By laser-melt injection, the surface is melted but not the added material. In most cases, ceramic particles are injected into the molten surface to create a wear resistant metal-matrix-composite (MMC) [7]. When a metallic powder is used instead of ceramic powder and the metallic powder is completely melted and diluted with the molten substrate, the process is called laser alloying. By laser alloying, a new alloy is created at the part surface with specific properties [8]. The laser alloying process can be controlled in such a way that more energy is put into the powder and less energy is put into the substrate so there is extraordinarily little dilution between the deposited layer of molten powder and the substrate. In this case, this is known as laser-deposition welding [9]. In laser-deposition welding, the powder and the substrate have a comparable melting temperature, which results in the substrate being melted in addition to the powder. However, if the powder material has a significantly low melting temperature, this results in the substrate not being melted (delamination), and cracking occurs. The occurrence of delamination and cracking becomes significant during the amalgamation of materials characterized by substantial thermal property differentials and minimal mutual solubility. A substantiated approach involves the inclusion of an intermediary material layer that effectively bridges the inherent property disparities between the substrate and coating materials [10-11]. In this case, it is not called laser-deposition welding, but laser-deposition brazing. A clear distinction between cladding by welding and cladding by brazing is also made in the established German standard for the classification of industrial production technologies [12]. However, both processes — laser deposition welding and laser deposition brazing — are mostly used together with the terms laser cladding (LC) or laser-metal deposition (LMD) or directed-energy deposition (DED) [13]. All four processes (laser melt injection, laser alloying, laser deposition welding, and laser deposition brazing) can be realized by using the same technical set-up to delivery powder material into the process zone and to inject it into the laser beam and/or melt pool generated by the laser beam. In this work, laser-deposition brazing is used and, therefore, the state-of-the-art is explained in more detail based on this process in the next section.

1.1.2 Laser Deposition Brazing

For laser deposition brazing, four main components are needed: a laser source, laser head, robot (or multi-axis-machine), and a powder-feeding machine (see Figure 2a). The laser source generates the laser beam, which is then guided through a laser fiber to the laser head. The laser head holds optics and the powder nozzle. On the powder-feeding machine, two hoppers are installed where the powder is stored. The powder is fed through a tube to the laser head. When the laser beam exits the laser fiber in the laser head, it diverges (see Figure 2b). This diverged laser beam is parallelized by a collimation lens. After this, a focusing lens is installed with which the parallel laser beam is focused. The powder is injected primarily into the melt pool by the powder nozzle.

Different types of powder nozzles can be applied for laser deposition brazing. The choice of powder nozzle has an influence on the process result [14]. Most commonly, coaxial nozzles and multi-jet nozzles are used, in which the powder is injected uniformly into the melt pool from all sides in a conical shape, whereby the laser beam is centered (see Figure 3a). The relative movement between the laser head and the substrate results in the deposition of a brazing track on the substrate. These single brazing tracks are arranged on the substrate with a specific overlapping degree (see Figure 3b). This results in a wavy surface structure after laser deposition: the maximum height of the layer “a” on the upper points of the tracks and the usable height “b” in the valley between the tracks. The maximum height “a” after the laser cladding is typically around 1.3mm to 2.0mm. The usable height “b” after machining the cladded surface is typically around 1.0mm to 1.5mm. However, by adjusting the process parameters or by using different configurations of laser machines, other values can be obtained. Laser technology offers many degrees of freedom.

Laser-deposition brazing processes can be divided into soft and hard brazing [12]. If the melting temperature is below 450°C, then the process is referred to as soft brazing. This includes, among other things, cladding with white metal. White metal (Sn-alloys) is the name given to a group of tin-based alloys with admixtures of antimony and lead. Processes are referred to as hard brazing if the brazing alloy has a melting temperature above 450°C [15]. These include, for example, coatings with a copper-tin alloy (CuSn).

Plain bearings are also produced by casting, where the liquid bronze is cast onto the solid steel body. The casting process is more complex and cost intensive. However, it can have its advantages for complex geometries such as internal structures where the bronze then flows in. For simpler geometries such as plain bearing bushings or shafts, laser cladding is better in terms of production time and cost, material, and energy efficiency. As a very rough guide, it can be said that laser coating is at least 20 percent cheaper. However, this depends on many factors. In casting, the cooling rates are slower and dendritic structures are more likely to form in the microstructure. In laser cladding, very high cooling rates occur, and fine grains tend to form in the microstructure. Fine grain in the microstructure generally has positive effects on the mechanical properties. An investigation was carried out that compares laser deposition brazing and centrifugal casting in order to produce a cladding out of white metal (melting range 240°C to 354°C) and a steel substrate (melting temperature 1,416°C). As a result, it was shown there was no dilution caused by the laser-deposition brazing, and, thus, there was no difference in the tribological behavior between the surface produced by laser-deposition brazing process and centrifugal casting [16]. However, the use of white metal is reduced due to its high cost [17] and the suspected toxic effects of antimony [18][19] on human health and the environment. Another proven material for sliding bearings are copper-tin alloys (CuSn).

1.1.3 Properties and applications of tin bronze

Typical tin (Sn) contents in tin bronze range from 5 percent to 12 percent, with increasing tin content leading to higher hardness. In this way, the material properties can be adjusted to meet the requirements. [20] The CuSn alloys are generally not applied on gear teeth. CuSn alloys are applied as a functional surface for tribology applications such as turbo chargers, hydraulic pumps, and planning bearings (e.g. in gearbox systems) [21]. Depending on the load and on the sliding speed, they can be used in a dry or lubricated condition. Usually, they are used in lubricated sliding systems up to 60 MPa or more. In laboratory tests like the pin-on-disc test, they are tested at about 20 MPa [22] or up to 120 MPa [23] depending on the application.

Sliding-wear response of a material depends on its microstructural features. The presence of the three microstructural constituents, namely a ductile phase, a load-bearing constituent, and a lubricating element in a bearing material, is essential for enhanced tribological properties [24]. Bronze with lead (Pb) meets the qualifications of all these essential conditions of being a good bearing material. The presence of a ductile phase in the form of a-phase (Cu rich solid solution of Sn) provides support to the load-bearing constituent, which is the Cu- Sn intermetallic phase. Additionally, an effective solid lubricant dispersed in the form of insoluble lead (Pb) particles in the microstructure can provide reduced wear and friction coefficient in dry sliding conditions [25]. However, in recent years, Pb-Bronze has come under critical review due to the environmental concerns of the material. As Pb is considered a heavy toxic metal, several regulations have been imposed to restrict its use in various mechanical components. Directives such as REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) have been passed by the European Union to prevent the exposure of Pb-containing components to the human and ecological environment [26]. Therefore, there is a need to develop and test new Pb-free components, such as bismuth-bronze alloys for bearing applications [27].

2 Technical set-ups and experimental procedures

2.1 Laser Deposition Brazing Process

For the investigations, an industrial multifunctional machine was used (see Figure 4), in which several axes are integrated, so that both smaller parts (for example on the small rotary axis (2) or on the clamping table (3) and very large parts with a diameter of up to 1,000mm, with a length up to 1,100mm and a weight up to 1,000kg can be cladded. For this, the big axis (4) is brought into the vertical position, and the large part is clamped with the movable counterpart (5). The laser head is guided by means of a Yaskawa robot (1). In this multifunctional machine, there are two different laser heads installed: A laser head for the external cladding of parts (6) and a laser head for the internal cladding of tube with a diameter >80mm (7). A diode laser from the company Laserline GmbH of the type LDM 8000-100 with a maximum power of 8kW is used. The laser beam has a wavelength between 900nm and 1,080nm, and a laser beam quality of 100mm*mrad.

Two different copper-tin alloys were used: CuSn11Bi3 and CuSn12Ni2. Both materials were used in powder form with particle diameters smaller than 150µm. The powder was fed by the powder feeding machine type PF22H from the company GTV Verschleissschutz GmbH to the laser head. The steel alloys C45 and 42CrMo4 were used as substrate material. The investigations were carried out on disk-shaped specimen geometries with a diameter of 114mm and a thickness of 18mm. The tests were repeated three times for each material and each substrate material with the same parameters in order to make a statistical statement about values and standard deviations. It was deposited onto the substrates in a spiral manner, with the process path going from the inside to the outside. Afterwards, the specimens were machined to a thickness of 1.3mm using a turning machine.

2.2 Analyzing methods

Regarding the methods of analysis, a distinction can be made between non-destructive testing (NDT) and destructive testing (DT). After the deposition, the thickness of the cladding was measured with an ultrasonic thickness measurement device type 456C from the company Sofranel with a precision of 3 percent. After deposition, the surface has a wave-shaped structure. The layer thickness was measured between the tracks, in the valley (usable height b (see Figure 3b)). Measurements were taken at eight different locations on the part surface, with three repeat measurements taken at each location. It was measured in the valley between the second and third track as well as between the third and the fourth track and in four angles (0°, 90°, 180° and 270°) in each case. On the plane-machined surface, a penetration test according ASTM E 1417 type 2 method A and C was carried out. Therefore, the visible inspection penetrant type VP-30 from the company Met-L-Chek was applied. After 10 minutes of waiting, the inspection penetrant was removed and a penetrant inspection developer D-70 from the company Met-L-Chek was applied. After another two minutes of waiting, the parts were analyzed by visual inspection. The ultrasonic test was then performed according to ISO 4386-1 over the entire coating area to check the bonding between the coating and the substrate. For this purpose, the ultrasonic test device AMS 2145E from the company Olympus was used.

Afterwards, destructive testing (DT) was carried out. The specimen was cut and embedded to perform the metallographic preparation by grinding and polishing. This was done in nine steps (see Table 1). The movement of the metallographic specimen and the grinding disc were in opposite directions. On the polished surface, the result was then analyzed with the microscope VHX-7000 of the company Keyence regarding imperfections such as pores or cracks. The lens VHX-E100 with magnification range of x100-x500 was used. The following settings were applied: full coaxial (20), HDR mode, magnification (150x), detail 3×1 (1.5 mm), and brightness (auto).

The area at each pore was measured by means of a polygon line laid by hand using the open-access software image J. The areas of the pores were added up for each sample and put in relation to the total area of the image (3mm x 1mm). Hardness measurements HV1 by Vickers, applying a force of 1kg, were carried out on the metallographic cross section according to DIN EN ISO 9015-1 using the hardness measurement device Qness10M from the company QATM with a 20x magnification and a working distance (WD)8.0. The hardness was measured for the bronze cladding of two specimens cladded with CuSn11Bi3 (Tokat300) and CuSn12Ni2 (Tokat325) on steel C45. Three hardness lines were made in each sample from the coating surface down into the substrate, and thus the mean and standard deviations were determined.

3 Results and discussion

Figure 5a shows an example of a sample of C45 steel laser cladded with Tokat300 bronze (CuSn11Bi3). The spiral coating path can be seen on it. The bronze is slightly oxidized after the laser process, which makes the surface appear matted. Subsequently, the surface was machined to remove the oxidized layer and obtain a plane functional surface. Figure 5b shows an example of a result from the dye penetrant test. The test does not show any indications, and, therefore, it is proven there is no open surface imperfection.

The results from all four material combinations, each repeated three times, showed very high quality and repeatability were achieved. All ultrasonic tests showed no indications, proving the bonding between the bronze cladding and the steel was defect-free. The dye-penetration tests showed there were no surface open imperfections. This high quality of the cladding was also confirmed by the metallographic analysis. An example of a metallographic image is shown in Figure 6. The interface between the Tokat300 bronze coating and the C45 steel substrate is very straight, which indicates the substrate was not significantly affected by the laser-deposition brazing process. Only very few imperfections in the form of small pores are visible in the bronze cladding. An evaluation of the pore diameters of all detected pores in all 12 specimens showed the mean value with a standard deviation of 30µm, ±22 µm.

Table 2 lists the porosity analysis values for all four material combinations. The mean and standard deviations are given. The mean values are all below 1 percent. For claddings, there is no standard for recommending porosity values. As an alternative, the standard for joint welding (without beam welding) can be used. In this standard, porosity values between 1 percent and 5 percent are recommended, depending on the applications [28]. Or, as a comparison, the standard for electron and laser-beam welded joints lists requirements and recommendations for evaluation groups for irregularities with recommendations indicating higher values between 2 percent and 6 percent [29].

Figure 7 shows the mean and standard deviations of the hardness values measured in the bronze cladding of CuSn11Bi3 (Tokat300) and CuSn12Ni2 (Tokat325) on the steel substrate C45. The standard deviations are small with less than 5 percent of the absolute values. From the increased hardness values in the steel, it can be seen the steel has been affected by the thermal-cladding process of laser-brazing deposition to a depth of 1mm. This is the heat-affected zone (HAZ). The hardness values of CuSn11Bi3 are slightly lower than the hardness values of CuSn12Ni2. This is due to the lower tin content of CuSn11Bi3.

If the C45 steel were subjected to a standardized hardening process, the hardness would be approximately 600HV. The hardness values in the HAZ in the steel in the coated samples with CuSn are still below 400HV. Comparing these values with hardness values from the literature with results where steel was cladded with another bronze (aluminum bronze CuAl), it becomes clear the depth of the HAZ is the same at 1mm. But the steel was hardened much more when coated with CuAl. There, the hardness values in the HAZ are at 650HV [9]. This can be explained by the fact the aluminum bronze CuAl has a higher melting temperature than the tin bronze CuSn, and, thus, the steel was more thermally affected.

The development results have shown the laser deposition brazing process is very stable and of high quality for the four material combinations. The next step was to transfer the results from these small disc samples to large industrial components. Figure 8a shows the process on a tube. The image shows the laser beam interacting with the bronze powder coming out of the powder nozzle. The tube was rotated in a horizontal position with the robot moving the laser head linearly along the tube axis so the track was deposited in a spiral on the tube. Figure 8b shows the tube that was only half machined after coating for demonstration purposes.

Figure 9 shows two finished industrial components ready for use after machining. Figure 9a shows a component for a planetary gearbox application, which consists of a 42CrMo4 steel coated with CuSn11Bi3. The component has a diameter of 220mm and a length of 260mm. Figure 9b shows a component from the power-generation application. The component has a diameter of 345mm and a length of 156mm. In this component, the inner surface was also coated. For this purpose, the laser head was slightly tilted in order to deposit the bronze into the inner surface. The thickness of the bronze layer is uniform along the width of the plain bearing.

4 Conclusion

In the article, the production technology of laser cladding was presented to produce bimetal components for plain bearings in gear boxes. For this purpose, the process was qualified for two different bronze alloys on small samples to verify the quality and subsequently transfer the results to industrial components. Bimetal components offer the advantage of providing a good combination of the properties of the steel-base body and the bronze coating. The steel-base body (steel backing material) is comparatively economical in terms of material costs and is the stiffness-giving component, so that only minimal small elastic deformations occur due to the high stiffness when external forces are applied to the component. The bronze layer acts as a tribological layer for plain bearing applications in many technical machines and applications such as gear construction. It can be concluded that laser-deposition brazing is a reliable production technology for the manufacture of bimetal components where very high quality can be achieved. For future work, the bonding strength between the coating and the substrate will be determined experimentally, and the tribological behavior will be investigated in laboratory tests. Furthermore, new alloys will be tested and process parameters for laser cladding will be developed. 

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^{Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Suite 500, Alexandria, Virginia 22314. Statements presented in this paper are those of the authors and may not represent the position or opinion of the American Gear Manufacturers Association. (AGMA) This paper was presented October 2023 at the AGMA Fall Technical Meeting. 23FTM13}