Study on Co based WC Ceramic Coating by laser clad

Research on powder feeding laser cladding of Co based WC Ceramic layer

Abstract: after laser cladding of Co based WC ceramics on 40Cr surface, a metallurgical transition layer is formed between the substrate and the cladding layer. In this paper, SEM, TEM, X-ray energy dispersive spectrometer and microhardness tester are used to analyze the microstructure, composition, interface microstructure characteristics and interface hardness gradient of the two cladding layers

key words: laser cladding layer Waveform Interface hardness gradient

preface

laser cladding cermet on metal surface is an effective means of material surface modification. As an important component phase in cermets, WC has attracted more and more attention because of its excellent high-temperature strength, strong oxidation resistance, high hardness, good wear and corrosion resistance, and low coefficient of thermal expansion. The export growth of China's extruder products will show a stable and progressive situation with internal and external lubricants of 0.5~2.0. However, due to the poor sintering property of WC, it is easy to fracture. Therefore, we use Co based alloy as bonding matrix to study the microstructure and properties of Co based WC cladding layer

1 experimental materials and methods

the matrix material is 40Cr, and its composition is shown in Table 1 below. Co based self fluxing alloy is selected as the bonding metal, and its composition is shown in Table 2 below. Hard ceramic phase is less than 100 μ M cast WC. During the test, cobalt based self fluxing alloy and a certain amount of WC powder are mixed evenly, and then single pass powder feeding laser cladding is carried out. The composition of the mixed alloy is shown in Table 3 below

1.1 experimental equipment and process parameters

(1) laser related parameters: cross flow CO2 laser, jkf-6 laser, broadband cladding powder feeder, spot size 25mm × 2mm, the beam mode is multimode, and the workbench is an X-Y two coordinate confidential machine tool (automatic control by single chip microcomputer)

(2) metallographic microscope Model: MM6 large horizontal metallographic microscope, microhardness tester model: hxd-1000

(3) scanning electron microscope Model jsa840

(4) X-ray diffractometer: d/max- Ⅱ B X-ray instrument produced by Ricoh company of Japan, radiation source Cuk α， The X-ray tube pressure is 40kV, the tube flow is 2mA, the scanning speed is 4 °/min, and the step size is 2q=0.02

2 test results and analysis

2.1 microstructure characteristics of the bonding interface between the cladding layer and the matrix

as shown in Figures 1 and 2, the interface morphology is wavy, and black structures appear near the interface: during the cladding process, the liquid in the molten pool is disturbed, causing the activated part of the substrate surface to be forcibly rolled into the molten pool. When the molten pool is formed, the heating temperature along the cross section of the molten pool is uneven, resulting in the difference in the surface tension and density of the liquid in the molten pool, resulting in a couple that makes the liquid flow in a certain direction [1]. The surface tension decreases with the increase of temperature, and the density decreases with the increase of temperature. The distribution of laser beam energy density on the cross section is uneven. According to the fundamental mode laser theory, the energy density at the edge of the beam is much lower than that at the center of the beam. Therefore, after the molten pool is formed on the surface of the cladding material and the substrate, the surface tension and density at the edge of the molten pool are large, and the surface tension and density at the center of the molten pool are small

the above two couples have the same direction, which makes the liquid flow, and there is also an obvious falling phenomenon to form a waveform interface [2]. Another factor for the formation of the waveform interface is that the substrate surface and the cladding material are heated to the melting state at the same time during the heating process of powder feeding laser cladding. When the cladding layer is formed, the droplets of the cladding material collide with the molten substrate surface under the action of gravity, wind pressure and light pressure. At this time, the molten part of the substrate can be lifted up, so that the cladding material and the substrate material are forcibly mixed [3], so as to form the waveform interface. Due to the low melting point of pure Co alloy powder, under the same cladding process, the matrix melting speed is relatively fast, the melting amount is more, and the temperature is higher, which is conducive to the formation of waveform interface. Similarly, Co based WC has a higher melting point, the matrix melting is slower, the melting amount is less, and the temperature is low, forming microwave interface

Figure 1 metallographic photo of Co based self fluxing alloy interface × 150 Figure 2 metallographic photo of Co based self melting alloy +wc interface × 150

when the powder feeding rate is small and the scanning speed is low, the linear density of light transmission energy absorbed by the matrix is large, so the melting depth of the matrix is large, the diffusion area of atoms on the side of the matrix is wide, and the cooling is relatively slow during crystallization. The atoms have enough ability to diffuse near the interface. At the same time, due to the aforementioned forced liquid mixing effect, there is a black tissue area with waveform distribution near the interface

2.2 microstructure characteristics of the coating

as shown in figures 3 and 4, the hard phase is roughly evenly distributed. The difference is that the morphology and distribution of the microstructure of the bonded hard phase are different. It can be clearly seen from the photos that the white massive hard phase is evenly distributed in the bonded phase metal, and the size and shape of the hard phase are different

large pieces of hard phase are retained without melting or burning during the cladding process, and small pieces of hard phase are partially dissolved or burned to make their particles smaller and sharp corners rounded, but their common feature is that the hard phase is firmly embedded in the metal matrix, wrapped by the bonding phase, and a number of hard phase particles are connected through the bonding phase. When the bonding metal starts to crystallize, it grows up by the hard phase particles. This crystalline state is ideal, which can nail and anchor hard phase particles [4], so that they will not fall off during service, and no obvious pores, looseness and cracks are observed in the cladding layer. It can be seen from Figure 2 that the microstructure of the cladding layer with CO WC as the cladding material is a fine grain area on the surface, which is generally equiaxed crystal, and the middle part is a uniformly distributed area of hard phase. There is a columnar crystal area near the interface between the cladding material and the matrix, which is coarse. The microstructure of the cladding layer with Co based self fluxing alloy as cladding material is hard phase uniform distribution area, dendrite area, plane cellular crystal area, and columnar crystal area at the interface, with larger columnar crystal. The surface equiaxed crystal region is due to high-temperature radiation heat dissipation, and the heat dissipation rate in all directions is roughly the same. With the effect of alloy elements, the other shape nucleus grows new crystals, and its microstructure is equiaxed and short rod. The formation of columnar and cellular crystals near the interface between cladding material and matrix is caused by component supercooling and directional solidification. Component supercooling is easy to form cellular crystals [5], and directional solidification is easy to form columnar crystals. The cooling effect of the matrix on the cladding layer is great, which is equivalent to directional solidification

Figure 3 metallographic photo of Co based self fluxing alloy cladding layer × 150 Figure 4 metallographic photo of Co based self fluxing alloy +wc cladding layer × 150

2.3 hardness of the cladding layer

the hardness of both sides of the bonding interface between the cladding layer and the matrix is tested, and the results are shown in Figure 5, which corresponds to the metallographic microstructure in Figures 1 and 2. It can be seen from the figure that the hardness of the cladding layer formed by the two cladding materials is higher than that of the matrix, and the hardness difference between the two is very large, which shows that the performance of the cladding layer is much better than that of the matrix material, achieving the purpose of surface modification

Fig. 5 hardness distribution curve on both sides of the interface between different cladding layers and the matrix

2.4 chemical composition and phase evaluation of the cladding layer

from the surface scanning and line scanning results of various elements in the cladding layer by electron probe and X-ray phase analysis, it can be seen that the cladding layer mainly has three types:

we can adjust any alloy elements that are not called conditioning at this stage (1) the alloy elements are evenly distributed in the matrix, which may exist in the form of solid solution, Its main elements are (CO, Fe, Ni, C, SI). The relative content of alloy elements varies according to the type of cladding materials. This phase is distributed in the gap of various compounds and plays the role of connecting phase

(2) there are two kinds of finer alloy compounds in the cladding layer. These two compounds are separated out during the crystallization of the cladding layer, and their components are (Cr, Fe, c) and (Cr, W, c) respectively. The relative content of Fe in (Cr, Fe, c) is small, and its shape is regular hexagon, which is estimated to be M7C3 compound. In (Cr, W, c), the relative content of W is relatively high, and its shape is slender rod

(3) the composition analysis results of the large compound in the cladding layer show that the compound is w0.89c0.11, which is the WC phase added in the cladding material and retained during the cladding process

3 conclusion

(1) the hard phase can be roughly evenly distributed in the cladding layer, and the hard phase plays a strengthening role

(2) when the base material is the same, the lower the melting point of the cladding material, the more obvious the wavy interface between the cladding layer and the base material, and the black structure is involved. When the melting point of the cladding material is high, the interface between the cladding layer and the matrix is not obvious wavy, and there is also black structure near the interface

(3) WC added in the cladding material achieves metallurgical bonding with the matrix phase during the cladding crystallization process

(4) there are three kinds of phases in the cladding layer, matrix solid solution phase, precipitated alloy compound phase and added WC phase

references

1 Liu Ximing Research on basic theory of powder feeding laser cladding Doctor's thesis of Chunguang Institute of mechanical engineering, President of Chinese science, 1998

2 Guan Zhenzhong Laser processing technology manual Beijing: China Metrology press, 1998

3, Yang Yongqiang, et al Interaction between laser and powder in powder feeding laser cladding China laser 1998, a25 (3): 565~570

4 Deng qiguang et al Microstructure and wear resistance of Laser Cladding Ni WC coating on aluminum alloy China laser A20 (10), 1998

5 Shi Huazhong et al Microstructure characteristics of SiC containing cermet coating by laser cladding Metal heat treatment 1997.10

6 Gao Jiacheng et al Effect of rare earth on the properties of laser coated ceramic coatings Journal of materials research 12（1），1998(end)