Structure and properties of laser-cladded Inconel 625-based in situ composite coatings on S355JR substrate modified with Ti and C powders

For the study, the substrate material surface (100 mm × 50 mm × 10 mm as-received S355JR low-alloy steel) was prepared by grinding, cleaning, and degreasing with ethyl alcohol. The coating precursor material was a powder mixture of gas atomized spheroidal Inconel 625 Ni-based superalloy powder (Oerlikon Metcoclad 625, particle size 45–90 μm), 99.0% pure titanium powder (H.C. Starck AMPERIT 154, particle size 45–70 μm), and 99.5% pure graphite powder (Merck KGaA 1.04206.9025, particle size <50 μm) at volume ratios of 95:2.5:2.5 (P1) and 90:5:5 (P2). The chemical compositions of substrate material and Metcoclad 625 powder are presented in Table 1. The powder mixtures were mixed in a tumbler for 0.5 hr and dried for 1 hr at 50°C prior to the laser cladding process.

For the laser cladding process, the stand was equipped with a TRUMPF Trudisk 3302 disk laser (Table 2), and a numerically controlled system for positioning the substrate material in relation to laser head and gravitational powder feeding system was used. For the process, the laser beam focus was set 30 mm above the substrate surface and the head was set at an angle of 10° relative to the vertical axis to minimize the risk of beam reflection to the laser head. For the process, argon was used as shielding gas (10 l/min) and powder transporting gas (3 l/min). The powder was injected directly into the molten pool. For the study, a set of multi-pass coatings was prepared using P1 and P2 powder mixtures as well as pure Metcoclad 625 powder with a 40% overlap, where the stitch axis distance was 2.6 mm. The process parameters (Table 3) were chosen based on previous experience [14]. The laser power was set in a range of 2,000–2,300 W, and a speed of 240–276 mm/min, constant heat input of 500 J/mm, and constant powder feed rate of 10 g/min were applied. The laser cladding process was provided without preheating and the interpass temperature was <30°C.

For verification of the crack presence on the coating surfaces, penetrant testing was carried out using color contrast technique (penetrant MR 68 NF, developer MR 70, cleaner MR 79). Both dwell and development times for penetrant testing were 10 min.

The coating macrostructure and microstructure were examined by optical microscope Olympus SZX9 and scanning electron microscopes (SEM) Phenom World PRO equipped with an EDS and SEM ZEISS SUPRA 25. Scanning electron microscopy was conducted in the backscattered electron (BSE) mode. For EDS analysis, a 10-kV accelerating voltage was applied. The specimens were etched using etchant 89 according to ASTM E 407-99 standard [26]: a mixture of HNO₃, HCl, acetic acid, and glycerol. The XRD analysis was conducted with Malvern PANanalytical X’Pert PRO diffraction system with filtered radiation from the lamp using cobalt anode. The diffraction profiles were recorded from the coating ground surfaces in the 2θ range between 30° and 130° in a continuous scan mode with a step size of 0.0263°. The counting time per step was 31.365 s. The coating thickness and dilution level were estimated with the Autodesk AutoCad 2023 software. The dilution was calculated using Eq. (1), were F_BM is the melted cross-sectional area of the substrate and RA is the cross-sectional area of reinforcement of the clad. The reinforcing particles volume fraction was estimated using the Image-Pro Plus 4.5.0.29 software on the basis of SEM micrographs: (1) U=FBMFBM+RA×100 [%] U = {{{F_{BM}}} \over {{F_{BM}} + RA}} \times 100\;\left[ \% \right]

Vickers hardness tests were performed on polished and etched coating cross-sections using FM700 Vickers hardness tester. The tests were performed in three measuring lines across the multi-pass coatings (Figure 1A) and from the surface toward the substrate material (Figure 1B), with a 0.5 mm and 0.2 mm distance between consecutive measuring points, respectively. The hardness was measured using a 500 g load and dwell time of 10 s.

Fig. 1

The solid particle erosion tests were carried out according to the ASTM G76-04 standard [27] on the grounded and cleaned coating surfaces. Testing was performed using 50-μm diameter Al₂O₃ particles in dry air as erodent material for 10 min. The velocity of the abrasive particles was 70 m/s and the feed rate was 2 g/min. The coating surfaces were located 10 mm below the nozzle. The impingement angles of 30° and 90° were applied to provide the maximum conditions for both brittle and ductile materials for each angle, and sample three tests were performed. As a result of erosion tests, the mass loss was measured with a laboratory scale with an accuracy of 0.0001 g and the erosion rates Eq. (2) and erosion values Eq. (3) were calculated. The coating density was measured using the Archimedes method. To assess the erosion mechanism of the tested coatings, crater observations were carried out using SEM ZEISS SUPRA 25: (2) erosion rate=mass losstest time [mgmin] {\rm{erosion}}\;{\rm{rate}} = {{{\rm{mass}}\;{\rm{loss}}} \over {{\rm{test}}\;{\rm{time}}}}\;\;\;\left[ {{{{\rm{mg}}} \over {{\rm{min}}}}} \right] (3) erosion value=volume losstotal mass of abrasive particles [mm3g] {\rm{erosion}}\;{\rm{value}} = {{{\rm{volume}}\;{\rm{loss}}} \over {{\rm{total}}\;{\rm{mass}}\;{\rm{of}}\;{\rm{abrasive}}\;{\rm{particles}}}}\;\;\;\left[ {{{{\rm{m}}{{\rm{m}}^3}} \over {\rm{g}}}} \right]

The coating surface views after penetrant testing are presented in Figure 2. In the case of all tested coatings, no indications were found. The visible coloration around coatings and at craters result from the surface roughness and qualify as a spurious indication. The penetrant tests revealed that the conducted laser cladding process of metallic and composite Inconel 625-based coatings allowed to produce high-quality coatings without surface cracks.

Fig. 2

The macrographs of laser-cladded coatings are shown in Figure 3. All of the coatings are characterized by good metallurgical joining to the substrate and no welding imperfections were observed. The thickness and dilution of coatings together with the measured reinforcing phase volume fraction are presented in Table 4. The average chemical compositions of the coating cross-sectional regions received from EDS are presented in Table 5. Together with the increase in laser beam power and speed (with constant heat input), the coating thicknesses slightly decreased, the penetration and dilution increased, while the reinforcing particles volume fraction decreased. The higher penetration and dilution may be attributed to the higher temperature gradient between the surface and base material due to higher laser beam power, leading to increased intensity of the Marangoni convection. The iron and titanium content results from EDS analysis are in good agreement with the dilution and reinforcing particles volume fraction measurements. As a result of higher penetration, the coating volume increased leading to the increased iron content and decreased reinforcing particles volume fraction and titanium content. With the increased titanium and graphite contents in the powder mixture, the coating thicknesses also slightly decreased. In the case of the composite coatings, the increased titanium and graphite contents caused the penetration and dilution to increase, which can be attributed to the higher radiation absorption level due to the amount of graphite in the mixture. As a result of laser cladding with powder mixture containing 2.5 vol.% and 5.0 vol.% of Ti and graphite, the volume fraction of the reinforcing particles in the composite coatings was 2.6–3.0 vol.% and 4.5–6.4 vol.%, respectively. The average titanium content for coatings produced with the powder mixtures was 2.4–2.8 wt.% and 3.3–4.1 wt.%, respectively. Macroscopic examinations showed no correlation between the processing parameters and coating widths.

Fig. 3

The representative SEM images of the microstructure of metallic Inconel 625 coatings that were tested to assess the impact of the titanium and graphite addition to the powder mixture on the microstructure are presented in Figure 4. The SEM micrographs of the composite coatings produced with the addition of titanium and graphite to Inconel 625 powder are presented in Figures 5 and 6. Figure 7 shows the XRD patterns of the metallic Inconel 625 coating and representative composite coating. As can be seen in Figure 4, the metallic Inconel 625 laser-cladded coatings have a typical dendritic microstructure with minor constituents in the interdentritic regions [28, 29]. XRD analysis (Figure 7) revealed the presence of austenite in the metallic coating. The precipitates were not detected by XRD analysis due to low fraction in the structure. The morphology of the austenite grains varies in different areas of Inconel 625 coatings due to changes in the local solidification conditions. In the fusion boundary region, due to the highest temperature gradient they show a columnar morphology and grow epitaxially from the fusion line opposite to the heat transfer direction. Closer to the coating surface, the temperature gradient becomes lower, which results in the formation of equiaxed dendrites. The addition of titanium and graphite to Inconel 625 powder mixture resulted in microstructural changes that included structural refinement and precipitate formation (Figure 5). The average grain size in the mid-section of the Inconel 625 coatings was 28.7–58.6 μm. In the case of composite coatings depending on the process parameters the average grain size in the mid-section was 10.9–14.2 μm and 10.2–11.3 μm for the P1 and P2 powder mixture, respectively. The grain refinement is related to precipitation during crystallization. As can be observed on the SEM micrographs, in comparison to pure Inconel 625 coatings, the composite coatings contain large amounts of uniformly dispersed particles in the austenite dendrite and interdentritic regions. The observations with higher magnifications (Figure 6) revealed that in the structure, primary blocky precipitates can be observed as well as eutectic precipitates that grow from the primary phases. XRD analysis revealed that the composite coatings are composed of austenite and TiC particles, while EDS analysis revealed that both the primary phases (Figure 8) and eutectic precipitates (Figure 9) contain C, Ti, Nb, and Mo. As can be observed on the SEM micrograph and EDS map (Figure 8), the primary precipitates show gradient distribution of chemical composition, in which the inner, darker part is rich in titanium, which allows to assume that the primary precipitates formed during crystallization as titanium carbides and then, the Nb and Mo dissolved in their crystal lattice. As can be observed in Figure 9, the eutectic precipitates contain less titanium than the primary precipitates. The particle measurements revealed that the increased titanium and graphite content in the powder mixture resulted in the growth of precipitates. For powder mixtures with 2.5 vol.% and 5.0 vol.% of titanium and graphite, depending on the process parameters, the maximum primary precipitates size was 0.8–0.9 μm and 1.0–1.6 μm, respectively, and the maximum length of eutectic precipitates was 1.3–2.0 μm and 2.0–2.3 μm, respectively. The microstructure observations in the overlap areas between consecutive beads revealed that no significant microstructure changes appeared in these regions due to additional thermal cycle.

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

The average hardness results of laser-cladded coatings together with the hardness distribution profiles across the coatings and from the surface to base material are presented in Figure 10. The average hardness of metallic Inconel 625 coatings was about 230–232 HV0.5. The addition of 2.5 vol.% and 5.0 vol.% of titanium and graphite to Inconel 625 powder mixture and related microstructural changes caused the average hardness increase to 258–271 HV0.5 and 266–291 HV0.5, respectively. The average hardness of composite coatings cladded with the use of P2 powder mixture is higher than for P1 powder mixture due to the higher volume fraction of reinforcing particles in the microstructure. Figure 11 shows the influence of the reinforcing particles volume fraction in the microstructure on the average hardness. It is clear that the increase in uniformly distributed reinforcing particles fraction in the structure causes increase in the average hardness of composite coatings. As no significant changes were observed in the overlap areas between consecutive beads microstructure, no changes in the hardness profiles in these regions were noted. The hardness profiles of coatings in both the tested directions showed that they have a homogeneous hardness distribution.

Fig. 10

Fig. 11

The average erosion rates and erosion values received for impingement angles 30° and 90° for laser-cladded coatings are presented in Table 6. For each composite and metallic coating, both erosion rate and value are higher for impingement angle 30° than 90°. Such results are generally characteristic for plastic materials that show a ductile erosion mechanism [30]. The solid particle erosion test results revealed that each of the composite coatings show a lower erosion rate and value in comparison to metallic Inconel 625 coatings, regardless of the laser cladding process parameters and the impingement angles, which proves better erosion resistance. Additionally, the coatings produced with P2 powder mixture, due to the higher volume fraction of reinforcing particles and higher hardness, showed lower erosion rates and values in comparison to coatings cladded with the use of P1 powder mixture. The influence of reinforcing particles volume fraction and hardness on the average erosion value is presented in Figures 12 and 13, respectively. It is clear that with the increased volume fraction of precipitates in the microstructure and average hardness, the erosion resistance improved. No significant changes in erosion rates and values were noted for each used laser cladding process parameters.

Fig. 12

Fig. 13

The SEM observations of carters received on the coating surfaces after the solid particles were conducted to define the erosion mechanism of metallic (Figure 14) and composite coatings (Figure 15). The metallic Inconel 625 coatings show erosive wear behavior typical for ductile materials, which is in line with the results previously available in the literature [31]. However, as can be observed on the crater micrographs, it is especially noteworthy that the composite coatings show comparable erosive wear behavior under both tested impingement angles. In the case of the 30° impingement angle the scars and narrow grooves can be seen on the eroded surface, indicating that plastic deformation and micro-cutting occurred during erosive test. In the case of the 90° impingement angle the plastic deformation also occurred, however, due to different trajectory of erosive particles in this case, short grooves and craters can be observed on the surface. In the case of the composite coatings, no reinforcing particles can be observed on the SEM micrographs due to their size.

Fig. 14

Fig. 15