【Related Papers】
Studies on high power laser cladding Stellite 6 alloy coatings: Metallurgical quality and mechanical performances
【Related Links】
Hatps://doi.org/10.1016/j.sarfkot.2024.130647
【Highlights】
The formation and elimination mechanism of thermal cracks in Stellite 6 coating was revealed.
The forming rate (mm3/s) can be significantly improved by high-power laser cladding.
Hardness and strength have reached current levels, while elongation (6.9%–8.1%) has significantly exceeded current levels.
It provides a new surface strengthening strategy for large workpieces cladding with high power laser cladding.
Abstract
In this study, the quality of the powder feed rate (Vp) on the coating of Stellite 6 alloy using a 20 kW fiber laser was systematically investigated. The results show that thermal cracks in the Stellite 6 coating can be achieved by increasing Vp without preheating the substrate or adding a buffer layer. A single-track coating with a thickness of 2.34 mm, a width of 20 mm, and a dilution of 17.3% with a crack-free and dense microstructure Vp of 90 g/min can be achieved in the following cases. In addition, the build rate (172 mm 3/s) exceeds the maximum build rate (133 mm 3/s) for low-power laser cladding. Under high-power lasers, the heat build-up of multi-track and bilayer (5.6 mm) results in the formation of ε-Co by phase transformation of a portion of γ-Co. Therefore, the dendrite region is mainly composed of γ-Co, while a small amount of ε-Co and Cr23C6 dominate the dendritic region. Although the thermal accumulation effect also thickens the grains in the bottom area of the coating, resulting in a slight decrease in mechanical properties, the hardness (485 HV–523 HV) and tensile strength (1131 MPa–1250 MPa) are comparable to those of low-power laser cladding, and the elongation at break (6.9%–8.1%) has significantly exceeded the current level of 3%. The high-power laser cladding process may play an important role in improving the efficiency of surface modification or remanufacturing of large workpieces, such as the flow channels of hydroelectric power sets.
Introduction
Power station valves, gas turbine blades, impact runners, and other workpieces operate under prolonged conditions of corrosion and wear, resulting in frequent downtime for maintenance due to issues such as wear, spalling, and cracking. Therefore, it is imperative to explore efficient, convenient and environmentally friendly surface modification technologies to extend the service life of these workpieces. Commonly used technologies include carburizing, thermal spraying, surfacing cladding, laser quenching, laser impact strengthening, laser cladding, etc., among which laser cladding technology has attracted more and more attention in the industrial field because of its characteristics of small heat input, low dilution rate, high microstructure density, strong adhesion, etc., and has attracted more and more attention in the industrial field about the strength between the coating and the substrate, as well as the convenience of adjusting the coating composition, thickness and performance.
In general, ways to improve the molding rate include increasing the scanning speed, laser power, powder feed rate, etc. Increasing the scanning speed is more suitable for rotating parts (rolls). Increasing the laser power seems to be more applicable and practical than other methods, as it results in a higher heat input and can melt more powder. However, to date, few reports have been made about the use of high-power (>10 kW) laser cladding for metallurgical quality and mechanical properties, as tens of kW fiber lasers have not entered industrial applications until recent years.
Stellite 6 alloy is widely used in laser cladding due to its excellent hardness, wear resistance, corrosion resistance and high temperature oxidation resistance. This study focuses on the application possibilities of high-power (>10 kW) laser cladding Stellite 6 alloy without the need for preheating the substrate or adding a buffer layer. By changing the powder feed rate, the variation of coating defects was studied, and the microstructure and mechanical properties of the coating prepared with the optimal processing parameters were compared with those prepared by the traditional low-power laser cladding technology. Their research aims to provide a new method for depositing Stellite 6 alloy coatings by high-power lasers to significantly improve cladding efficiency, which can be used to repair and/or remanufacture core mechanical components.
Experimental procedures
The Stellite 6 alloy powder used in this study was prepared using a vacuum atomization process, as shown in Figure 1(a). In addition, Figure 1(b) illustrates the size distribution of the powder, revealing a range from 35 μm to 220 μm, with a DV(50) value of 103 μm. Before performing the laser cladding experiment, the powder is dried in a vacuum oven at 100 °C for 2 h to eliminate the moisture content. The substrate material used for the experiment was 0Cr13Ni5Mo and the dimensions were 200 mm× 200 mm× 40 mm (length× width× thickness).
A schematic diagram of the high-power laser cladding platform used in this experiment is shown in Figure 2(a). The system is mainly composed of a 20kW fiber laser (RFL-C20000TZ, Wuhan Ruiku Fiber Laser Technology Co., Ltd.), a powder feeder (Wuhan Fukunaji Laser Co., Ltd.), a side shaft powder feeding cladding head (Jiangsu Wright Laser Technology Co., Ltd.), a controller and a three-axis CNC machine tool. The horizontal movement of the cladding head and powder feeding nozzle is driven by a CNC machine to realize the laser cladding process, as shown in Figure 2(b). In the laser cladding experiment, a 20 mm × 2 mm laser spot was used, as shown in Figure 2(b). Single-rail laser cladding experiments were performed on a substrate using the processing parameters listed in Table 2. The selection of the optimal powder feed rate is based on the absence of cracks (macro and micro cracks) and unmelted defects (coating area and interface area) in the coating. Subsequently, a multi-track, multi-layer coating is prepared by an optimized powder feed rate.
Results and discussion
When the powder feed rate changes from 50 g/min to 100 g/min, the macroscopic topography of the monorail coating is shown in Figure 3 (a1)-(f1), indicating the smooth and full shape of all coatings without any obvious pit defects. The morphology of the penetrant test defect at the powder feed rate of 50 g/min and 60 g/min is shown in Figure 3 (a2)-(b2), where several cracks are distributed along the cladding direction of the coating surface (indicated by the red area). Conversely, there were no crack defects on the surface of the monoviscous coating in the powder feed rate range of 70 g/min to 100 g/min, as shown in Figure 3(c2)-(f2). Therefore, cross-sectional specimens are also required to evaluate potential internal defects within the coating. Figure 3(a3)-(f3) where the demarcation line of the dilution zone is clearly visible. Notably, Fig. 3(a3)-(b3) reveals crack defects penetrating the coating and dilution zone, while Fig. 3(c)3)-(d3) exhibits microscopic cracks within the coating. Conversely, no obvious defects are observed in Fig. 3(e)-(f3).
To further analyze microscopic defects within the coating, metallographic specimens are observed with OM after the standard polishing procedure (Figure 4). Figure 4(a1)-(f1) and Figure 4(a2)-(f2) depict OM images of the coating area and the interface region, respectively. According to Fig. 4(a1)-(d1) and Fig. 4(a2)-(d2), when the powder feed rate is increased from 50 g/min to 80 g/min, the length and width of the internal cracks in the coating decrease, and no obvious defects are observed in the interface area between the coating and the substrate. According to Fig. 4 (e1) and Fig. 4 (e2), when the powder feed rate is 90 g/min, the internal cracks in the coating completely disappear and the interface area is free of defects. However, when a powder feed rate of 100 g/min was applied, some unmelted defects were found in the coating area and at its interface with the substrate, as shown in Figures 4(f1) and (f2). As a result, the optimal powder feed rate for this process parameter is 90 g/min, from which a monorail coating with a thickness of approximately 2.34 mm and a width of 20 mm can be prepared. The coating had a dilution rate of 17.3% and a build rate of 172 mm3/s. In addition, the build rate achieved with these optimized high-power cladding processing parameters exceeds (3.0 mm3/s − 133 mm3/s) to achieve similar materials by conventional low-power cladding processes (350 W – 3400 W), as shown in Table 3.
The internal crack morphology of the coating captured by OM is shown in Figure 5(a), showing a variation in crack length from 600 μm to 800 μm. Subsequently, the crack region in Figure 5(a) was characterized by EBSD. As shown in Figures 5(b) and 5(c), it is evident that the cracks in the cladding gradually propagate along the grain boundaries of the columnar crystals, with the typical symptoms of thermal cracking.
By selecting a single-track coating with no cracks or fused defects, a powder feed rate of 90 g/min was determined as the optimal laser cladding processing parameter. Thus, multi-track and double-layer coatings were fabricated with the same parameters, and the microstructure of the coating was subsequently analyzed. Figure 7(I) shows the sampling location distribution of the microstructure analysis specimen, with the top, middle, bottom, and interface areas of the coating being shown in Figure 7(a)-(d), respectively. It is evident that the microstructure gradually transitions from planar crystals to pore, columnar, dendritic and equiaxed crystals, from the interface region at the bottom of the coating to the top region. The morphological characteristics of the microstructure in the deposition direction are controlled by G (temperature gradient) and R (solidification rate) as the controlling factors of microstructure morphology.
The distribution of grain boundaries in the top, middle, and bottom regions is shown in Figure 8 (a3)-(c3). The gradual decrease in the percentage of LAGBs from top to bottom is evident, consistent with the observed trend of decreasing dislocation density. This phenomenon can be attributed to the reduction in the occurrence of distortions and tangles experienced by the displacement during movement due to the reduction of LAGB.
Hardness test specimens are extracted from multi-track and double-layer coatings and the hardness values of the coatings are systematically evaluated in the direction indicated by the black arrows in Figure 12(a). As shown in Figure 12(b), the curve shows a gradual decrease in hardness from 523 HV at the top to 485 HV at the bottom. In addition, considering the different areas within the coated portion above the substrate plane (5.6 mm), the average hardness values for the top, middle, and bottom regions are 521 HV, 504 HV, and 489 HV, respectively.
The tensile test results are shown in Table 4, revealing that the ultimate tensile strength, yield strength, and elongation at break of the 5.6 mm thick coating gradually decrease from top to bottom. It is worth noting that the bottom region of the cladding exhibits significantly inferior tensile properties compared to the top and middle regions. Figure 14 shows the stress-strain curves of a set of tensile specimens, shown as a fracture specimen of a tensile specimen. Compared to the Stellite 6 coating prepared by the low-power laser cladding process (1800 W) cited in Table 4, the tensile strength and yield strength of the coating prepared by the high-power process in this study are slightly lower, but the elongation at break is significantly higher. The higher elongation may be attributed to the increased Co content in the coating with an FCC structure, as the Co matrix with the FCC structure preserves the ductility of the Stellite alloy.
Conclusions
(1) By adopting high-power laser cladding technology, Vp can be increased. As a result, a monorail coating with a width of 20 mm and a thickness of 2.34 mm can be applied at Vp 90 g/min without visible cracks and unfused defects in the coating. However, the laser power is not enough to melt the powder, and when the Vp reaches 100 g/min. Build speed (172 mm3/s) Optimal Vp (90 g/min) surpasses conventional low-power laser cladding technology.
(2) Under the high-powder laser cladding process, the solidification rate and cooling rate of the molten pool decrease, resulting in the formation of γ ε-Co through phase transformation. Therefore, the dendrite region is mainly composed of γ-Co, while a small amount of ε-Co and Cr23C6 dominate the dendritic region.
(3) The coating hardness (485 HV–523 HV) prepared by high-power laser cladding technology basically reaches the level achieved by low-power laser cladding technology. Although the ultimate tensile strength (1131 MPa–1250 MPa) and yield strength (849 MPa–933 MPa) have slightly decreased compared to the tensile properties of low-power laser cladding coatings, the elongation at break (6.9 %–8.1 %) has exceeded the current standard.
(4) Under test conditions (p= 12 KW, Vc= 6.67 mm/s, Vp= 90 g/min), the high-power laser cladding technology not only significantly improves the forming rate, but also produces coatings with excellent mechanical properties, especially the plastic index, which is 2 times higher than the current level. Therefore, the successful application of the high-power laser cladding process on cobalt-based alloys provides an efficient and convenient solution for surface strengthening or remanufacturing of large workpieces.
【Related Recommended Articles】
Macroscopic morphology and properties of cobalt-based laser cladding layers on rail steel based on pulse shaping
Studies on effect of laser processed stellite 6 material and its electrochemical behavior
From: AM home Additive Manufacturing House
Yangtze River Delta G60 Laser Alliance Chen Changjun reprinted!
At the same time, welcome to the 2nd Conference on the Application of Laser Intelligent Manufacturing in the Energy Storage Industry held by the Yangtze River Delta G60 Laser Alliance in Nanjing (Nanjing, April 23-25, 2024)