ANTIFRICTION COATING LASER CLADDING ON STEEL
Article accepted for publication 01.03.2019
The increased demands are made to the details in the shipbuilding, chemical engineering and other industries, where the products intended for operation in rough conditions are created. Their surfaces must possess corrosion resistance, antifriction properties, heat and electrical conductivity, as well as high mechanical properties . The manufacture of products from copper and its alloys is expensive, from an economic point of view, and in some cases is simply impossible due to the low strength of copper and alloys. To reduce the consumption of copper alloys, the products can be created by welding copper alloys to steel. The use of arc cladding techniques is accompanied by a significant melting of the steel substrate, cuts of the base material and mixing of the steel with the molten filler material. As a result, the deposit may contain up to 30% iron.
The experiments are known where the laser and copper melting of the copper layer was carried out with a welding wire MNKZhT 5–1–0.2–0.2 with a diameter of 1.2 mm on a steel plate St3sp with a thickness of 7 mm . A fiber laser LS‑15, an optical welding head YW 50 were used as a source of laser radiation. The cladding was carried out with a radiation power of 5–10 kW, varying the beam diameter, the cladding rate and the wire feed speed. The metallographic studies have established that the depth of penetration of copper into steel with a radiation power of P = 5 and 10 kW and a laser spot diameter of D = 3.6 mm is 50–130 µm, and at P = 10 kW and D = 6 mm the depth of penetration is already 100 to 300 µm. The average iron content in the deposited layer is defined as 3.6 and 20.3%, respectively. Microhardness of the weld bead with radiation power of 5 kW varies between 77–115 HV0.2, and with radiation power of 10 kV it was in the range of 77–330 HV0.2.
The results of laser cladding of SAE1045 steel samples with dimensions of 40 Ч 30 Ч 8 mm, performed using a CO2 laser with a radiation power of 2 kW, at beam speeds of 5, 9 and 13 mm / s and beam diameter of 3 mm are described in . A slip coating with a thickness of up to 1 mm, containing Cu5Al powder and a binder, was applied to the sample surface. At a processing speed of 5 mm / s, spherical particles of iron are observed, uniformly distributed in the weld zone, except for the dendritic structure located near the substrate. As the scanning speed increases to 9 and 13 mm / s, the dendritic microstructures disappear, the distribution of spherical iron particles becomes heterogeneous, and the electrical resistance of the deposited layer decreases with increasing beam moving speed.
The authors of  applied a powder containing copper, nickel and tin in a ratio of 77 : 15 : 8 with a uniform layer of 1.2 mm thick onto a steel plate Q235 with dimensions of 100 Ч 100 Ч 10 mm, mixing it before the application. The cladding was performed by continuous radiation of a 2 kW CO2 laser with a beam displacement speed of 400 mm / min and a beam diameter of 4.5 mm on the sample surface. Then, the sample was aged at a temperature of 370 °C for 30–480 minutes. The microhardness of the coating was twice as high after aging at 370 °C for 2 hours, and its maximum value was 390 HV. At the same time, the electrical resistivity of the coating decreased from 2.87 · 10–5 to 1.52 · 10–5 Ohm · cm.
For cladding aluminum bronze, the authors of  used a CO2 laser «Comet‑2» radiation power of 1 kW. The power density was ~1.27 · 105 W / cm2. Steel 45 was used as a substrate. Single paths and multilayered cladding of bronze for testing samples for friction and wear were applied at a beam moving speed of 100–300 mm / min and a nozzle distance above the surface of 10–14 mm. Under the test conditions for friction without lubricant at cladding speeds lower than 140–160 mm / min, the friction coefficient is stable 0.17–0.2 and is significantly lower compared to cast aluminum bronze.
The task of reducing the wear of the rail buffer head was solved in . During operation, railway buffers are in almost constant contact with each other and have significant wear. To reduce the wear rate, the buffer head is covered with a graphitized lubricant, however, this method has many disadvantages. It was proposed to cover the head buffer with bronze using laser cladding. Aluminum bronze CuAl9Fe3 was chosen as the filler material. Laser cladding was performed using a robotic stand equipped with ABB industrial robots and a high-power diode laser HPDL LDF 4000–30 with a maximum output power of 4.0 kW. The base material is a circle of mild steel S355J2, having a nominal diameter of 38.0 mm. The hardness of the weld coating varied within 178–189 HV0.2.
The samples with aluminum layers of bronze (10 mg on average) and steel samples with lubricated surfaces (7.5 mg on average) showed the least wear. The wear of the samples was the same for all four series. Thus, it can be assumed that the use of CuA19Fe3 aluminum bronze after laser cladding will eliminate the need for lubricating the railroad buffer during the operation of the rolling stock.
The objective of the work is to determine the effect of transverse oscillations of the laser beam on the mixing of the deposited charge with the base material and on the cladding rate.
EQUIPMENT FOR CLADDING SAMPLES
AND RESEARCH METHODS
In the experimental studies, the laser complex by IMASH of RAS was used . The samples were made of 40X steel with dimensions of 15 Ч 20 Ч 70 mm. For cladding, copper-based powder PR-BrAMts 9–2 with a particle size of 40–150 µm was chosen. As the variable parameters, we chose the radiation power P = 700–1 000 W, the processing speed V = 5–10 mm / s, and the beam diameter d = 1–3 mm. An additional factor was beam scanning with a fixed frequency f = 220 Hz. A resonant type scanner was used with an elastic element with a mirror attached. Metallographic studies of the deposited coatings were carried out on a PMT‑3 microhardness meter with a load of 0.98 N, a metallographic microscope Altami MET 1C, and an AM413ML digital microscope.
The structure and chemical composition of the deposited layers were studied on TESCAN VEGA 3 SBH scanning electron microscope with an energy dispersive analysis system using reflected and secondary electron modes.
The universal friction machine MTU‑01 was used to determine the tribological characteristics of the deposited samples. The tests were carried out according to the «plane» (deposited sample) – steel 40X ring (48–52 HRC). The slip rate and pressure on the sample were changed discretely in the range of 0.1–1.1 m / s and 1–3 MPa, respectively. Transmission oil TSZp‑8 was used as a lubricant.
EXPERIMENTAL STUDY RESULTS
Laser cladding of the samples was performed at the optimal modes by a defocused beam and with transverse oscillations of the beam along the normal to the vector of the laser processing speed. Fig. 1 (a and b) shows microsections of the weld tracks with dimensions of 0.75 Ч 2.1 mm, hardness (181–208 HV), and 0.68 Ч 3.38 mm – (204–224 HV) obtained by the defocused beam and scanning with a frequency of 220 Hz, respectively. The zone of penetration of the base when processing the defocused beam and the scanning beam was 380 and 150 µm, respectively. The cross-sectional area of a single deposited layer when scanning the beam is 1.5 times larger than when cladding with a defocused beam.
Fig. 2 (a, b) shows the zone of fusion of the coating with the base and the chemical composition in the cladding zone, in the transition zone and the main material. From the presented results, it follows that the processing of a defocused beam in the deposited layer has a higher iron content, which is a consequence of a deeper penetration of the base.
The friction coefficients varied between 0.016–0.022 and 0.014–0.021 when testing the specimens of the deposited defocused and scanning beams, respectively, which is two times lower than on the cast bronze specimens. It is established that the cross-sectional area of the weld path by the scanning beam is 1.5 times larger than that of the defocused beam with the same processing modes.
Laser cladding of antifriction coatings on steel surfaces can be used in marine engineering, friction units of hydraulic units, in heavily loaded sliding bearings and high-speed mechanisms. Modern technological equipment equipped with fiber, diode and other lasers makes it possible to build-up the working surfaces of flat parts, bodies of revolution and parts of complex spatial form. The adhesion strength of the copper-based coatings applied by the laser beam is higher than the shear strength of normalized and improved steel and is 350–480 MPa.
The technology of laser cladding of antifriction coatings based on copper with the powder material PR-BrAMts 9–2 has been developed. The coefficient of sliding friction when using transmission TSZp‑8 as a lubricant was 0.016–0.022 and 0.014–0.021 when cladding defocused and scanned at a frequency of 220 Hz, respectively, which is two times lower than that of cast bronze.
The performance of laser cladding with transverse oscillations of the beam to the vector of the cladding rate increases by 1.5 times compared with the cladding of a defocused beam.