Influence Of Modes Of Laser Cladding And Composition Of Powder Materials On Abrasion Resistance Of The Coatings
The pad weld should be free from defects (pores, cavities, cracks) and have good cohesive strength of the pad weld with the substrate. What are the ways to achieve this? Let us examine the effect of laser cladding mode and the composition of the powders on the wear resistance of the parts. The methods of feeding filler material to a laser cladding zone can be divided into two major groups . First is a prior application of dopant materials to the treated surface, the second one is to feed materials directly into the processing zone simultaneously with the action of the laser radiation. The most common method of prior coating is to use slip plasters. The process is distinct for higher efficiency due to its high absorption capacity, due to the presence of the binder and the high rate of use of filler material. Application of the coating slip includes an operation of pre-coating the filler material, which is generally mechanized and performed using a spatula or other hand instrument. Binder of slip coatings should not be toxic and should possess the ability to provide manufacturability of pastes. However, the method of feeding the filler material by force feeding directly into the molten bath of the base material, due to the automation of the process, has a high reproducibility.
This research had several objectives: the development of technology of laser cladding coatings 3–5 mm thick with a layer width of up to 30 mm, the development of composite coatings with beads 0.5–0.85 mm high, and the optimization of treatment modes to increase the abrasive wear resistance of pad welds.
Versatile equipment by LLC TS "IRE-Polus’ containing fiber laser LS-5, KUKA robot, optical head by PRECITEC and technology table were used for the experiments to simulate the coating technology with a layer thickness of 3–5 mm. Nickel-based powders manufactured by Hёgenas, Sweden and JSC "POLEMA", Russia (Tula), were chosen as surfacing materials. The particle size of the powder has been 40 to 150 microns. Laser power has been varied from 1800–3800 watts. Speed of moving the optical head has been as follows: in the transverse direction – 0.1 to 2 m/s, in the longitudinal direction – 1 to 15 mm/s. Laser cladding of the pre-poured powder with a layer thickness of 3 to 5 mm has been performed on steel plates sized 140 × 140 × 20 mm. The samples have been cut by electric spark method sized 25 × 70 × 20 mm.
Installation, complete with a fiber laser, manufactured by LLC TS "IRE-Polus’ has been used for the deposition of composite coatings. The laser power has been varied in the range of 300–600 W, the deposition rate has been 4 to 11 mm/s. The distance from the nozzle face to the substrate has been varied in the range of 4 to 7 mm. Argon gas flowing at a pressure of 0.2 MPa at a flow rate of 6 l/min has been used as a protective carrier gas. The powder consumption fed via feeder has been 0.33 to 1.93 g/min. Powder PR-10P6M5, sieved to fraction of up to 40 to 60 microns, has been used for surfacing with the addition of the powder blend by Hёgenas 44712 (Sweden) in an amount of 3 to 70%, containing tungsten carbide with cobalt binder phase in an amount of 12%.
Metallurgical studies have been performed on a scanning electron microscope EVO 50 by Carl Zeiss (Germany), a digital microscope AM413ML, metallographic microscope Altami MET 1C.
The microhardness of the samples has been measured by the Vickers method on PMT-3 and HVS-1000 apparatuses with automatic loading of the indenter with the load value P = 1 N. The dwell time under load has been t = 20.
Sample testing for abrasion has been conducted on friction machine BH-4 according to Brinell-Haworth scheme upgraded in MEI of RAS . Weigh of the samples has been performed on electronic scales VIBRA HT / HTR 220TE with accuracy to 0.0001 g.
For the development of processing modes of the thick coatings powder by Hёgenas has been used (declared hardness is HRC 58) with different thickness of the poured layer of 3 to 5 mm. The pad welds with a thickness of 3 to 4.3 mm and a width of 20 to 30 mm per pass have been obtained . Figure 1 shows the microsection of a single build-up track. When selecting the optimal conditions the radiation power, movement speed and the diameter of the laser beam on the surface of the powder have been varied. Changes regularity of microhardness of the pad welds have been identified. Overlap zone of the surfaced track is shown in Figure 2. It is free from the defects such as pores, cracks and cavities, as well as in surfacing tracks treated under optimal conditions.
When changing the cladding modes, microhardness in the pad weld has varied widely 6 120 to 12 000 MPa. With high density energy of a laser beam, a portion of alloying elements and carbon is burned, and the microhardness of the pad weld is reduced, such defects as pores appear. With low density energy of the laser radiation, incomplete fusion of the powder material occurs, thus decreasing the microhardness pad welds.
Table 1 shows the test results for abrasion made according to Brinell-Haworth scheme. A flat specimen with weld coating has been attached to the rotating rubber disk. The friction zone has been fed with quartz sand with a particle size of 200 to 600 microns. The duration of the test has been 10 minutes. According to the results of the tests of three samples, the average value of the loss of mass of the coating surfaced on each processing mode was determined.
The studies on the abrasion of the first sample batch revealed the optimal modes for laser cladding of powder coatings. The second batch of samples has been surfaced with the powders of different hardness, and those of both powder materials manufacturers. Figure 3 shows the dependence of abrasion on the hardness of the pad weld and the sample of 40X steel (the tests have been carried out according to Brinell-Haworth scheme). From the given data, it follows that the pad welds with a hardness of HRC 58–61 (–1 for Hёgenas powders and – 2 for JSC "POLEMA" powders) is 10-fold higher according to wear resistance than that of the normalized 40X steel (NV180) and is 4.6-fold higher than 40X improved steel (HB 250–270).
Processing of laser cladding modes for PR-10P6M5 powder has been carried out on single weld bead. Thin sections have been prepared for the metallographic studies. Weld samples have been cut perpendicular to the weld tracks. General view of the weld bead and its microstructure is shown in Fig. 4 (a and b).
The geometrical parameters of weld beads  (A: 0.5–0.85 mm – thickness of the pad weld; B: 0.05–0.2 mm – depth of penetration of the basics C: 1,2–1.4 mm – width of the deposition zone) depend on the speed of movement of the optical head, the consumption of the powder material, the laser power.
The studies of the chemical composition of the pad welds and base penetration zone have shown that the chemical composition corresponds to the chemical composition of the surfaced charging material. The measurements have been carried out on the surface of the pad weld to a base in five points. Table 2 shows the elemental composition of the coating with 7% of powder 44712.
The microhardness of the pad weld has varied within wide limits. Thus, when surfacing powder PR-10P6M5 without additives it ranged 6 000 to 9 600 MPa. With the introduction of tungsten carbide, the microhardness in the pad welds has varied in the range of 9 900 to 14 000 MPa. Figure 5 is a graph obtained by measuring the microhardness of the surfacing zone and the substrate from the surface to the substrate with an increment of 100 microns.
To determine the effect of laser surfacing on abrasive wear resistance, the tests of the samples have been performed in the MEI of RAS using friction machine BH-4, with the above test conditions. Table 3 shows the test results for abrasion.
Figure 6 a-c represents the tested samples surfaced with different tracks overlap ratio. In the process of working out of modes, it has been established that with a minimum content of tungsten carbide of 3–7% (samples 2–4), the laser cladding with tracks overlapping of 60% is possible (Figure 6). Thus, the grinding allowance shall be not more than 0.2 mm (Fig. 6b). With increase of tungsten carbide in the content from 10 to 70%, there are cracks on the surface when overlapping the tracks. Therefore, samples 5–13 have been surfaced with tracks overlapping of 40% (Fig. 6c). This led to a significant deterioration of the topography of the surface layer and during grinding to a depth of 0.3 mm untreated plots of the surfaced tracks can be seen (Fig. 6c). The test results for the abrasion show the increase of composite coating wear resistance up to 28 times compared to the base material, steel 3, and by 9 times as compared to pad weld obtained with powder PR-10P6M5 after grinding.
Maximum wear resistance is obtained for 10% content of powder 44712 from the charge material volume. With further increase of the carbide phase, its larger amount is presumably decomposes at higher temperatures of the molten zone associated with increased carbide phase, to tungsten and carbon, that do not have sufficient resistance in abrasive wear. It should be noted that samples 5–13 have been in a continuous contact area with a rubber disc, and a portion of the abrasive particles has not provided a cutting or scratching action on the sample surface. This could explain why such a high increase in the wear resistance with friction machine BH-4. To clarify the influence, depending on the percentage of tungsten carbide on abrasive wear resistance we need to further test the laser cladding technology to produce high-quality surface after grinding.
As a result of the studies performed, the laser cladding modes for different groups of machine parts and assemblies have been developed and specific defects in the surface layers formed as a result of wear have been identified. It should be noted that the thick coatings are in high demand for parts of mining and processing industry: armor plates, jaws, crushers, etc. Thin coatings are used in the repair of damaged local friction surfaces of the beads of beading mills, stamps and other parts. Both technologies complement each other and have many practical applications. The abrasive wear resistance of coatings is affected by the composition of the powder material and processing technology. Thus, when surfacing powder composite coating with a share content of tungsten carbide of up to 7% of the charging material volume, its wear resistance is lower than the wear resistance of the coating surfaced by nickel-based powder. This is due to the presence of tempering zones in the areas of weld beads overlapping.
The technology of laser cladding of nickel-based powder material with a pad weld width of 20–30 mm and a height of 3–4.2 mm in one pass has been developed.
The tests on the abrasion according to Brinell-Haworth scheme has shown an increase of wear resistance of surfaced nickel-based coatings 10-fold compared to the normalized steel 40X.
The modes of high-quality cladding of PR-10P6M5 powder with tungsten carbide additives up to 7% have been elaborated. Surfacing of this powder with a high content of tungsten carbide requires further clarification of processing modes.