During tunnel construction, the cutters of tunnel boring machines (TBMs) are seriously worn. Directly scrapping the replaced cutters causes waste of resources and increases construction costs. Aiming at the problem of remanufacturing repair strategies for cutter wear, this paper conducts laser cladding simulation and experimental research on laser cladding of new coatings, and conducts relevant exploration and practice on improving the wear quality of cutters. Firstly, a double ellipsoid heat source laser cladding simulation model is established by finite element simulation to explore the influence of laser cladding on the residual stress of the cutter base and cladding layer materials, and provide a basis for optimizing process parameters for cladding experiments; on this basis, laser cladding technology is combined with new material graphene under relevant experimental conditions, and laser cladding experiments of new coatings for cutters are carried out. The graphene content, laser power, scanning speed and powder feeding disc speed are used as experimental factors by orthogonal test method, and surface hardness and wear resistance are used as evaluation basis. Four groups of different levels are set for comparison, so as to select the material ratio and process parameters with higher wear resistance. The results show that the addition of graphene has a significant improvement in the wear resistance of the coating. Compared with the base material, the coating with graphene can reduce the wear degree by up to 91.32% compared with the traditional coating and by up to 99.86% compared with the base material. This paper combines simulation and experiment to study the laser cladding technology and the new material graphene, and provides methodological guidance and data support for improving the wear quality of TBM cutters and selecting their remanufacturing strategies.
In tunnel engineering, the cutters installed on the cutterhead are in direct contact with the rock mass, and the working environment is harsh and the wear is serious. The downtime and cutter replacement caused by cutter wear is an important factor in reducing the construction efficiency of tunnel boring machines (TBMs). The scrapping of the replaced cutters results in the direct abandonment of some cutters that still have remanufacturing value, resulting in a waste of resources and an increase in construction costs. If the worn cutters can be recycled and reused through remanufacturing and repair, the construction costs can be reduced, resources can be saved, and waste can be reduced to a certain extent. According to statistics, the economic loss caused by tool wear accounts for about 30% of the total cost of tunnel construction. For example, in the right line of the Tianwu section of Guangzhou Metro Line 3 in soft and hard composite strata, 43 tools were replaced in the shield construction of one and a half months, and 85 rollers were replaced in the 55 days of excavation construction [1]. It can be seen that tool wear is directly related to the time and cost of construction. In current research, most of the research focuses on the layout and installation method of rollers to reduce the load they are subjected to during excavation [2]. Some scholars have also established the crack initiation life of dangerous points in the roller hole area through finite element analysis to analyze the factors affecting roller wear and remanufacturing [3]. However, there are relatively few studies on reducing roller wear by surface modification and other process methods. Laser cladding technology is an advanced surface modification technology. It uses a high-energy density laser beam to melt the cladding material and the substrate material to form a cladding layer with certain mechanical properties on the substrate surface. This technology can prepare wear-resistant, corrosion-resistant, fatigue-resistant and other special coatings on the substrate surface. Zhao Jianfeng et al. [4] applied laser cladding technology to the remanufacturing of related mechanical products, making the performance of the products no less than that of new products and achieving the expected results. Shen Jingyi et al. [5] used pulsed laser forming technology to remanufacture and repair TC4 blades for fatigue damage and wear, and evaluated and optimized its microstructure and mechanical properties. Liu Hongxi et al. [6] proposed a titanium-based laser cladding remanufacturing powder with excellent mechanical properties. Chen Xiaoming et al. [7] used laser cladding technology and combined with graphene new materials to propose a nickel-based remanufacturing coating for marine equipment with good corrosion resistance and wear resistance. Yang Lijun et al. [8] used tungsten carbide cladding layer to improve the wear resistance and corrosion resistance of iron-based materials in offshore oil production. Ji Xiufang [9] combined with graphene new materials and used laser cladding technology to prepare 316L stainless steel remanufacturing coating with high corrosion resistance and wear resistance. Wang Zekai et al. [10] combined graphene with laser cladding technology to prepare a tool remanufacturing coating and obtained excellent mechanical properties. The above scholars have all used surface modification technology to explore the remanufacturing technology of mechanical products, which has certain reference significance for the remanufacturing of TBM cutterhead tools. At present, there are few researches on the remanufacturing technology practice and related theoretical research of tunnel boring machine cutterhead tools, and there is no application of laser cladding technology and new materials to the remanufacturing and repair of roller cutters. Therefore, in view of the remanufacturing and repair problem of roller cutters in tunnel boring machine construction, this paper carried out a roller cutter laser cladding simulation and a new coating laser cladding experimental study, and conducted relevant exploration and practice on the remanufacturing strategy of roller cutters, which can provide method guidance and data support for improving the remanufacturing strategy of roller cutters.
1 Research on laser cladding simulation of roller cutter coating
The generation of residual stress is inevitable in laser cladding technology. Excessive residual stress will cause deformation of the workpiece and even cracks inside the workpiece, deteriorating the performance of the workpiece. Therefore, it is particularly important to control the generation of residual stress during the cladding process. Since the laser power used in laser cladding is high, the heating and cooling speeds are fast, and the molten pool size is small and the temperature is high, it is difficult to use experimental methods to measure the temperature distribution of the molten pool and the substrate and the subsequent stress distribution [11]. Therefore, the heat transfer and mechanical behavior of the material surface thermal repair process under the action of continuous laser are qualitatively and quantitatively analyzed through simulation.
1.1 Laser cladding simulation model
1.1.1 Laser cladding heat source model
As a cladding heat source, the laser beam has three significant characteristics: high energy, concentration, and instantaneous [12]. Common heat source models include Gaussian surface heat source model and Gaussian body heat source model. Dong Zhibo et al. [13] and Dong Kequan et al. [14] proved through experimental measurements that the simulation results of the double ellipsoid heat source model are closer to the actual situation than the Gaussian heat source model. In order to truly obtain the temperature field and stress-strain field changes of the cladding layer and the hob substrate surface during the cladding process and the cooling process during the laser cladding process, this paper uses a double ellipsoid heat source as the laser heat source model, as shown in Figure 1.
In Figure 1: a is the laser spot radius; b is the depth of the laser molten pool; c is 1/2 of the spot radius; c2 is 2 times the spot radius. The calculation formulas for the energy distribution density q1 of the front ellipsoid and the energy distribution density q2 of the rear ellipsoid are as follows [13]: see formula (1) and (2) in the figure. Where: P0 is the laser power; η is the laser power absorption rate; 1f and f2 are the energy fractions of the front and rear ellipsoids respectively, and the sum of the two is always equal to 2. Usually, f1 = 0.6, f2 = 1.4; x, y, z are the position parameters of the light source. During the laser cladding process, the light source changes with time and position; v is the scanning speed of the laser heat source; t is the scanning time. In this paper, a user subroutine is written in Fortran language to define the heat source moving load, and a laser heat source model is established through the DFLX subroutine, including the horizontal and vertical loads of the heat source and the motion trajectory of the heat source and the cladding unit. 1.1.2 Material properties of the hob and cladding layer
The material of the hob ring is H13 steel (4Cr5MoSiV1), the base material is the same as the hob ring material, and the nickel-based alloy powder accounts for the largest proportion in the cladding powder. In order to simplify the simulation model, the cladding layer material is NI60. The density of H13 steel is 7690 kg/m3, and the yield stress is 1.469 GPa; the density of NI60 is 8544 kg/m3, and the yield stress is 2.2 GPa. The material properties of the hob and cladding layer are obtained by consulting relevant literature [15]. The conductivity, elasticity, expansion coefficient, plasticity, specific heat capacity and other material properties of the hob H13 steel and the cladding layer NI60 are defined in Abaqus as shown in Tables 1 and 2.
1.1.3 Laser cladding model
By writing a program loop, multiple temperature-displacement coupling analysis steps are established, including multiple laser cladding analysis steps and a cooling step. The coaxial powder feeding process of laser cladding is realized through the unit model change function in the interaction module in Abaqus, that is, the mesh unit of the cladding layer is gradually generated on the surface of the base layer as the laser heat source moves. The convection radiation is defined through the interaction module, and the surface heat exchange properties are set to an ambient temperature of 20 ℃, an NI60 emissivity of 0.25, and an H13 steel emissivity of 0.55; the NI60 film heat dissipation coefficient is 80, and the H13 film heat dissipation coefficient is 70.
Boundary conditions are set through loads to constrain the bottom of the H13 steel substrate. The analysis step is established through a loop program, and matched with the model change of the mesh unit, the laser power and laser scanning speed are changed, and the influence of laser power and laser scanning speed on the residual stress of the base layer and cladding layer is explored. The laser cladding model is shown in Figure 2.
1.2 Analysis of simulation results
The laser power is 1100 W, 1400 W, 1700 W, 2000 W, the scanning speed is 200 mm/min, 400 mm/min, 600 mm/min, 800 mm/min, and the cooling time of laser cladding is 800 s. The laser light source subroutine written by the operation module in Abaqus is imported to run the laser cladding simulation model for calculation. The data is processed by matlab to obtain the average residual stress and residual strain of the cladding layer and the base surface of the hob, as shown in Figure 3.
As shown in Figure 3, the residual stress of both the cladding layer NI base and the base H13 steel under laser cladding is less than the yield stress of the two materials; using lower laser power and faster scanning speed can effectively reduce the residual stress and residual strain of the cladding layer and the base surface of the hob. Therefore, from the perspective of reducing the residual stress of the cladding layer and the base layer, the optimal process range of laser power is 1 100-1 400 W, and the optimal process range of laser scanning speed is 600-800 mm/min.
2 Laser cladding experiment of new roller coating
The simulation study of laser cladding of roller coating gives suggestions on the selection of two process parameters of laser power and laser scanning speed for laser cladding technology from the perspective of reducing residual stress. On this basis, in order to further explore the influence of process parameters and material ratio on coating hardness and wear resistance, improve the existing industrial wear-resistant coating, and explore its optimal process and optimal ratio of cladding layer materials, this paper combines laser cladding technology with graphene new materials to carry out laser cladding experiments of new roller coatings. Among them, graphene is a two-dimensional carbon nanomaterial composed of hexagonal honeycomb lattices composed of carbon atoms in sp2 hybrid orbits. It has excellent optical, electrical and mechanical properties and is widely used in materials science, biomedicine and energy. The overall experimental route is shown in Figure 4, including cladding layer material sampling, laser cladding experiment, hardness test, wear resistance test, result analysis, and scheme optimization.
2.1 Experimental materials and equipment for laser cladding of roller coating
The substrate material used in this laser cladding experiment is the same as the simulation material above, which is H13 steel (4Cr5MoSiV1), with a substrate size of 30 mm×30 mm×10 mm. The cladding powders are
(1) nickel-based alloy powder (Nistelle 625), with a particle size of -80 to 270 mesh;
(2) spherical cast tungsten carbide powder, with a particle size of -100 to 270 mesh;
(3) industrial graphene oxide, with a flake diameter of 10 to 50 μm. The equipment used in this laser cladding experiment mainly includes: (1) Laser cladding equipment: IPG4000 W fiber laser and its supporting equipment;
(2) Processing and testing equipment: DK7745 CNC EDM machine, PG-1A polishing machine, HXD-1000TMC/LCD micro Vickers hardness tester, ZA220.R4 1/100,000 analytical balance, MVF-1A vertical universal friction and wear testing machine, etc.
2.2 Experimental plan
The experiment uses 70% nickel-based alloy, 30% tungsten carbide cladding coating (i.e., the traditional coating of current hob laser cladding [16]) and uncoated substrate as the control group; the nickel-based alloy is fixed at 70%, and the proportion of other materials in the experimental group is designed based on this. According to the parameters such as the surface hardness and wear quality of the specimen, the optimal ratio of the coating with the best wear resistance and the optimal process parameters are selected. The main process parameters are laser power, scanning speed, powder feeding disk speed and graphene content. In order to investigate the influence of the above four parameters on the performance of the cladding coating, a 4-factor 4-level orthogonal experiment is designed. The experimental factors and levels are shown in Table 3. A, B, C, and D represent the four factors selected in this experiment. The orthogonal experiment plan determined by referring to the L16 (45) orthogonal table is shown in Table 4.
The laser cladding process is as follows: First, the substrate material is sanded with sandpaper, and the substrate surface is cleaned with acetone to remove the surface oil, and then dried for use. Then, the cladding material is mixed evenly according to the graphene content described in Table 3 and dried in a vacuum drying oven at a drying temperature of 150 °C and a drying time of 3 h. Before the formal cladding, the substrate material is preheated at a preheating temperature of 150-180 °C; after preheating, laser cladding is performed according to the parameters shown in Table 4 and numbered. For the convenience of analysis, the numbers of the traditional coating material and the base material are 17 and 18 respectively; in order to avoid oxidation of tungsten carbide powder, high-purity argon gas is used for gas protection during coaxial powder feeding. The preheating and cladding process are shown in Figure 5. Subsequent processing and testing: ① Samples were cut using a wire cutting machine; ② The samples were polished with sandpaper and a polishing machine, the surface of the specimens was cleaned with anhydrous ethanol, and the surface hardness of each specimen was measured using a microhardness tester. The applied load was 9.807 N and the loading time was 10 s. Measure the hardness at 4 points and take the average value, which is used as the coating surface hardness of the sample; ③ Grind and polish the sample again, clean the surface of the sample with anhydrous ethanol, and then dry it; weigh the mass of each sample with an electronic balance, measure the mass of each sample 6 times, remove the extreme value and take the average value as the mass of the sample before wear; Use a friction and wear testing machine to carry out a wear test, set the test temperature to room temperature, the speed of the friction and wear testing machine to 400 r/min, the friction time to 30 min, and the load to 12 N[16]; After the wear test, weigh the mass of each sample again, and the mass difference before and after the test is the wear mass; ④ According to the above test results, statistical analysis is carried out to obtain the optimal ratio and optimal process of the new coating of the hob.
2.3 Analysis of experimental results
Figures 6 and 7 are the hardness comparison and wear resistance comparison of the experimental results, respectively. It can be seen from Figures 6 and 7 that the coating after adding graphene has achieved a certain improvement in hardness compared with the traditional coating (No. 17) and the base material (No. 18). In the 16 groups of experiments, the sample 1 has the highest hardness, which is 147.68% higher than the base material and 48.47% higher than the traditional coating; the sample 13 has the lowest hardness, which is basically the same as the traditional coating, but the hardness is still 64.53% higher than the base material, and the hardness improvement effect is the most significant.
With the horizontal value of each factor as the horizontal axis and the corresponding index mean as the vertical axis, the horizontal trend chart of each factor is drawn, as shown in Figure 8. It can be seen from Figure 8 that:
(1) When the graphene content is about 2%, the wear mass of the sample has a minimum value and the wear resistance is relatively excellent; when the graphene content is higher than 4%, its wear mass may further decrease, but as the graphene content increases, its “agglomeration” effect may also have an adverse effect on the mechanical properties of the sample, which needs to be considered comprehensively;
(2) With the increase of laser power, the wear mass of the sample increases and the wear resistance decreases, so the better laser power parameter may exist in the range of less than 1 100 W;
(3) With the increase of scanning speed, the wear mass of the specimen decreases and the wear resistance increases, so the better scanning speed may exist in the range of more than 800 mm/min, but the macroscopic size and mechanical properties of the coating need to be considered comprehensively;
(4) When the powder feeding disc speed is about 50 r/min, the wear mass of the sample reaches a maximum value and the wear resistance is poor. The better scanning disc speed may exist in the range of less than 30 r/min and more than 90 r/min, but the fluidity of the powder and the process performance range of the laser cladding equipment need to be considered comprehensively.
In summary, after the analysis of the orthogonal test results, the optimal solution preliminarily selected by the existing test data is 1% graphene content, 1 700 W laser power, 400 mm/min scanning speed, and 90 r/min powder feeding disk speed.
3 Conclusions
(1) A double ellipsoid heat source laser cladding simulation model was established by Abaqus finite element simulation, and the mechanical influence of the thermal repair process on the base layer and the cladding layer was explored. The simulation results show that the use of lower laser power and faster scanning speed can effectively reduce the residual stress and residual strain of the cladding layer and the hob substrate, which provides ideas and basis for optimizing process parameters to solve the residual stress problem in cladding technology.
(2) Combining laser cladding technology with new graphene materials, the laser cladding experiment of new hob coating was carried out, and the process parameters and cladding material ratio were optimized from the perspective of wear resistance and hardness. Through group experiments, the material ratio and process parameters that can greatly improve the wear resistance of the hob are optimized: graphene content 1%, laser power 1 700 W, scanning speed 400 mm/min, powder feeding plate speed 90 r/min. The laser cladding experiment of the new hob coating provides an experimental method and data support for the coating manufacturing and performance improvement of the hob. The manufacturing of the hob coating by combining surface modification technology with new materials has sufficient theoretical basis and broad application prospects, and has reference value and practical significance for the initial manufacturing and remanufacturing strategy selection of the hob.
