Determination of the influence of laser cladding modes and composition of powder material on coating wear resistance
The samples of high-strength low-alloy steel Q550 with dimensions of 10 Ч 20 Ч 60 mm were coated with powder coating using alcohol 1.0 mm thick as a binder . The chemical composition of the nickel-based powder used in the percentages by weight is as follows: C0.8–1.2%, B3–3 5%, Si 3 5–4 0%, Cr 14–16%, Fe 14–15%. 20% by weight of monocrystalline tungsten carbide with a particle size of 100–200 µm was introduced into the coating charge composition. The samples were dried at 250 °C for 30 minutes. Laserline LDF4000–100 device with a wavelength of 980 nm, and a maximum laser power of 4.4 kW was used for laser cladding. The size of the laser spot on the surface of the sample was 17 Ч 1.5 mm, the beam travel speed was 3 mm / s. The argon flow rate was 12 l / min. The wear test was carried out on a MM‑2000 friction machine.
Coatings obtained at a radiation power of 3.6 kW have wear resistance by 6.8 times higher than the base material of the sample.
The aim of the study is to determine the coefficient of wear resistance of coatings, depending on the charge chemical composition and the modes of laser cladding using sclerometric method and abrasion testing.
EQUIPMENT FOR SURFACING SAMPLES AND RESEARCH METHODS
The experiments were carried out on the automated complex of IMS of RAS . The radiation power varied from 800 to 1200 W, with a beam speed of 5–10 mm / s. The specific power density was 38–126 W · s / mm2. For cladding, nickel-based powders with a particle size of 40–100 µm of grades ПР-НХ15СР2 and ПР-НХ17СР4 were chosen. Nano powders of tungsten and titanium carbides with a particle size of 20–100 nm were used as additives. The samples were made of steel 40X and cast iron ВЧ60–2 with dimensions 15 Ч 20 Ч 60 mm. The thickness of the deposited layer was 0.7–0.8 mm. Metallographic studies were performed using a ПМТ‑3 microhardness tester at a load of 0.98 N. The structure and chemical composition of the deposited layers were examined using a TESCAN VEGA 3 SBH scanning electron microscope with the system of energy-dispersive analysis using the modes of reflected and secondary electrons. The tests for abrasive wear were carried out according to the Brinell-Haworth scheme . A flat specimen with a fused coating or a sample of the base material with a load of 15 N was pressed against the rotating rubber disc. Quartz sand with a particle size of 0.2–0.6 mm was fed into the friction zone. The duration of the test was 10 minutes. Furthermore, the coefficient of wear resistance of the coating and the base material was estimated using sclerometric method . The indenter load during scratching was 0.98 N on the ПМТ‑3 device. The speed of movement of the diamond indenter was 10 mm / s.
RESULTS OF EXPERIMENTAL STUDIES
In the first series of experiments, laser cladding was carried out with powder ПР-HX15CP2 for the samples from cast iron ВЧ60–2. Table 1 presents the results of the determination of microhardness and wear resistance, along the width of the scratch in comparison with the tests for abrasive wear during the cast iron cladding. The coefficient of wear resistance was determined from the equation : K = C · b / d, where b is the width of the scratch of the base material, d is the width of the scratch of the cladded layer, measured in micrometers. C is the coefficient determined with allowance for the features of the laser cladding process (hardness of the coating, treatment modes and additives), C coefficient magnitude varies within the range of 0,7–5,5.
For laser cladding of sample No. 2, the depth of the burnt-off zone of the base material did not exceed 100 µm. The cladding zones of samples Nos. 3 and 4 has a reduced microhardness value associated with deep penetration into the base for 0.4–0.5 and 0.7–0.8 mm, respectively, which is caused by excess heat input in the processing of the samples.
In the second series of experiments, laser cladding of 40X steel samples was performed with the ПР-HX17CP4 powder. Under optimal processing conditions, the microhardness values of the cladded coating of 7 840–10 600 MPa were obtained. At the same time, the coefficient of wear resistance obtained by sclerometric method is 10 (10.3 in the abrasion wear test) (Table 2).
With high radiation power or low speed of the part (beam) travel, the base material is penetrated to a depth of 0.4–1.0 mm. This leads to a sharp decrease in the microhardness of the cladded layer and the appearance of defects in the form of pores and cracks. When deviating from the optimal cladding conditions and penetrating into the base to a depth of more than 0.1 mm, a decrease in the wear resistance of the coating is observed.
In the third series of experiments, laser cladding of 40X steel samples was performed with ПР HX15CP2 powder and additives of tungsten carbide nano powder in an amount of 3–7% with a particle size of 20–100 nm. Fig. 1 shows microsections of a single track cladded with ПР-HX15CP2 powder with a scratch made with a diamond indenter. There is a zone of laser hardening from the solid state under the cladding zone. Uniform width of a scratch in the cladding zone testifies to uniformity of mechanical properties in coating.
Fig. 2 shows microsections and graphs of distribution of chemical elements of the cladded layer at the interface with the base material: 2a – cladding with a defocused laser beam, 2b – cladding with a scanning laser beam at 220 Hz. Application of transverse high-frequency oscillations of the laser beam results in melting of 40X steel to a depth of up to 50 microns. The graph (see Fig. 2b) shows the occurrence of iron with the content practically equal to the nickel content at the interface with steel 40X. The chemical bonding of the cladding material and the substrate indicates a higher adhesion strength of the coating using high-frequency beam scanning at the same radiation power and speed of sample travel. Positive results of cladding were obtained at a radiation energy density of up to 50 W · s / mm2. With a further increase in the energy density, the alloying elements burn out and the carbide phase decomposes. Wear resistance in this case is sharply reduced (Table 3).
The developed technique for determining the wear resistance coefficient by means of sclerometric method makes it possible to shorten the test cycle on samples by metallographic studies, and in a number of cases it is the only way when it is impossible to determine the wear resistance of large-sized products in a short time: crankshafts, rolling mill rolls, shafts of gas and steam turbines and other friction assemblies. Further studies in this area will allow more accurate determination of the coefficient of wear resistance, taking statistical analysis into account.
A technique for determining the wear resistance of coatings in laser cladding using sclerometric method has been developed.
The deviation of the coefficients of wear resistance of coatings with sclerometric method and abrasive wear was not more than 5%.
It is shown that exceeding the energy density of laser radiation above 50 W · s / mm2 leads to a sharp decrease in the coefficient of wear resistance in laser cladding.