1. Introduction
Laser cladding technology uses a high-energy laser beam to carry out the workpiece’s high-power, small-scale local precision heating, which has sizeable instantaneous power, a low dilution rate, and a small heat-affected zone [
1,
2]. As a surface modification technology, the high-energy laser beam causes the cladding material and the substrate to form a metallurgical bond, which can prepare high-performance coatings on low-cost metal surfaces, prolong the service life of parts, and improve the comprehensive performance of materials [
3,
4,
5]. This paper mainly discusses three aspects: numerical simulation of cladding layer morphology, selection of cladding material Ni60-WC, and response optimization analysis of cladding results.
Numerical simulation: Many scholars have studied the influence of pores and cracks on the cladding layer’s surface quality by analyzing the residual stress in the cladding process under different process parameters [
6,
7]. Others have studied the distribution of residual stress in the cladding layer to control the residual stress by adjusting the process parameters [
8,
9]. Some scholars have analyzed the influence of different scanning strategies on the morphology of the cladding layer by simulating the laser scanning path [
10,
11]. Zeng et al. [
12] simulated the temperature and stress fields of 6061 aluminum alloy cladding Ni60 by ANSYS and analyzed the influence of preset coating thickness on cladding layer formation. The results show that the thicker the preset coating, the greater the penetration depth and width of the molten pool and the greater the residual stress, but the smaller the melting depth of the substrate. Tavakoli et al. [
13] analyzed the height and deformation of different cladding layers by simulating clockwise and counterclockwise circular scanning paths.
Selection of Ni60-WC cladding material: Ni60 material has suitable self-melting properties and good wettability with WC, which is widely used in the surface repair treatment of parts [
14]. On the one hand, WC material has high hardness and chemical stability [
15,
16]. On the other hand, it has a high melting point, and metallurgical bonding with the matrix is difficult. Adding WC material to the cladding material improves the performance of the cladding layer. Still, excessive WC content will cause the cladding layer to break and fall off [
17]. Hu et al. [
18] prepared Ni-based materials with different WC contents on stainless steel substrate materials. The experimental results show that the increase in WC content will significantly improve the hardness and thermal stability of the coating. Liu et al. [
19] used laser cladding technology to clad the Ni60-WC layer on the surface of a copper alloy substrate and tested the sample’s wear and corrosion resistance. The experimental results show that the Ni60-WC coating has good metallurgical bonding strength and hardness, and the sample’s wear and corrosion resistance after cladding are improved.
Results prediction analysis: The morphology of the cladding layer is an essential characteristic index of the laser cladding process [
20], which is affected by many factors. Process parameters are the most critical factors affecting the cladding layer formation [
21,
22]. Therefore, many scholars mainly divide the prediction of cladding layer morphology into algorithm optimization, model establishment, and optimization of statistical variables [
23]. The response surface method optimizes statistical variables to solve multivariate problems. In the process of laser cladding, most of the research on the response of cladding results focuses on the laser power, scanning speed, spot diameter, and powder-feeding rate [
24]. Based on the response surface optimization method, Lian et al. [
25] established a prediction model of process parameters, lap rate, and cladding efficiency, which provided a theoretical basis for the prediction of cladding efficiency. Based on the response surface method and variance analysis, Meng et al. [
26] analyzed the relationship between process parameters and coating geometry in single-pass cladding, which provided a reference for multi-objective optimization of composite coatings in the later stage. Sun et al. [
27] modeled the cladding layer’s width and height response surface for the laser single-pass cladding TC4 process.
Therefore, in numerical simulation analysis of laser cladding, Workbench establishes the temperature and stress field simulation of a multichannel Ni60-WC cladding layer with return path scanning. The influence of process parameters on the morphology characteristics of cladding layer melting height and melting width is analyzed. The range of process parameters is screened and combined with a laser cladding pre-experiment. The cladding experiment was carried out using the selected process parameters based on the response surface method. The prediction model of melting height and melting width was established and verified by experiments.
2. Multilayer Numerical Simulation Analysis
2.1. Pre-Processing
Grid division: The grid division unit adopts the SOLID70 unit because SOLID70 has eight-node units, which can realize three-way uniform heat transfer. The matrix edge applies size adjustment, the number of partitions is 30, the offset coefficient is 4, and the number of divisions is 5.
Convective heat transfer: the convective heat transfer coefficient of the substrate and air convection is set to 5 W/(m2·°C), the initial temperature is 22 °C, and the lower surface of the substrate regard to an adiabatic state during the laser cladding process. The purpose is to simulate the laser cladding process of the substrate at room temperature.
Boundary conditions: Set the two bottom edges of the matrix to be free in the x direction and fixed in the y and z directions. Specify the bottom edge of the left bottom in the x direction and accessible in the y and z directions. The purpose is to simulate the bending of the substrate after laser cladding.
The laser adopts a Gaussian heat source with a laser absorption coefficient of 0.47 [
28,
29], and the laser heat source loads in the middle of the front end of the A
2 cladding layer. High-speed steel is the base material (50 mm × 50 mm × 1 mm), Ni60 alloy is the backing cladding layer, and Ni60-WC mixed material is the second cladding layer. The birth-and-death unit method simulates the cladding process [
30,
31,
32], and the cladding process is A
2 → A
3 → A
1 → A
4, as shown in
Figure 1.
2.1.1. Material Physical Parameters and Composition
The matrix is W
6Mo
5Cr
4V
2 high-speed steel.
Table 1 and
Table 2 display Young’s modulus and thermal physical parameters. Based on JMatPro, the material-related performance parameters were obtained and imported into the Workbench material library for thermo-solid coupling analysis.
The cladding material is Ni60-WC material, and
Figure 2 shows the thermophysical parameters [
33].
2.1.2. Numerical Simulation Analysis
The multichannel multilayer laser cladding process is a simulation method based on the finite element method. The APDL language simulates the movement process of the laser heat source and the increase and decrease in the cladding layer material using the birth-and-death element method.
Based on APDL language, the laser cladding path is written using the Gaussian heat source. See Formulas (1) and (2). Set the direction according to the cladding sequence A2 → A3 → A1 → A4, calculating the cladding time according to the laser scanning speed. Establish the step number to 4 and turn on the automatic time step. Setting the time step to 4 equally distributes the time of the four cladding layers. The heat source is located at the center of the front end of the A2 cladding layer and applies cyclic loading through the *DO cycle.
The Gaussian reservoir model in Cartesian coordinates is
where
R is the radius of the finite heat source, unit m;
A is the aluminum absorption of the material surface layer to the laser, taking
A = 0.47;
K is the heat source concentration factor; and
r is the distance from a point in the heating area to the center of the spot, unit m [
35].
As a nonlinear problem of state change, the life-and-death of the component will cause the failure or regeneration of some specified elements [
36,
37]. A cladding layer is a life-and-death unit, and a substrate is a non-life-and-death unit. Along the direction of the laser scanning speed, when the laser scanning area is unreached, the cladding layer is an unactivated area, and the cladding unit is not displayed. After the laser scanning area, the unactivated cladding layer area is transformed into an activated state and participates in the laser cladding process. The schematic diagram is shown in
Figure 3. As a technical means of increasing and decreasing materials, the life-and-death element method provides a theoretical basis for the subsequent simulation of the temperature and stress fields and the selection of simulation process parameters.
2.2. Effect of Process Parameters on Melting Height and Width
The laser power and scanning speed will directly affect the energy input of the laser to the cladding layer. Exploring the mechanism of laser power and scanning speed on the morphology characteristics of the cladding layer melting height and melting width provides a theoretical basis for adjusting process parameters of subsequent laser cladding experiments.
2.2.1. Effect of Scanning Speed on Melting Height and Width
The nanosecond laser cladding sequence is A
2 → A
3 → A
1 → A
4 back-type path cladding. In
Figure 4a, the cladding layer melting height and melting width under fixed laser power are inversely proportional to the scanning speed. When the scanning speed changes from V = 3 mm/s to 5 mm/s, the cladding layer melting width values corresponding to A
2, A
1, A
3, and A
4 are reduced by 43.93%, 15.59%, 3.03%, and 2.9%, respectively, and the cladding layer melting height values corresponding to A
2, A
1, A
3, and A
4 are reduced by 28.6%, 20.8%, 22.4%, and 16.67%, respectively. The reason for this phenomenon is that when the laser power and spot diameter are constant, the increase in scanning speed makes the laser cladding energy decrease in unit time. The amount of cladding powder decreases, resulting in a decrease in cladding height and width.
In
Figure 4a, the melting height and width of A
3 and A
4 in the post-cladding area are more significant than that of A
2 and A
1 in the pre-cladding layer. The reason for this phenomenon is that during multipass cladding, there is heat accumulation in the post-cladding area compared to the cladding area; the melting state time of the molten pool and the amount of cladding powder increases, as well as the melting height and width increase.
In
Figure 4b, the width and height of the A
1 and A
4 cladding layers are more remarkable than those of the A
2 and A
3 cladding layers. This phenomenon is because, in the process of multilayer laser cladding, there is heat accumulation between the cladding layers and between the layers. When carrying out the second layer of laser cladding, the temperature of the top molten pool is higher than that of the bottom molten pool, and the amount of cladding material increases, so the width and height of the cladding increase.
2.2.2. Effect of Laser Power on Melting Height and Width
Figure 5a shows that the melting width and height positively correlated with the laser power when the scanning speed was 4 mm/s. When the laser power changed from 23 w to 27 w, the cladding width values corresponding to A
2, A
1, A
3, and A
4 increased by 11.68%, 6.55%, 4.6%, and 2.8%, respectively, and the cladding height values corresponding to A
2, A
1, A
3, and A
4 increased by 28%, 55.56%, 24%, and 6.15%, respectively. The above phenomenon is because laser power directly affects the absorption of laser energy in the cladding layer. The laser power increase will improve the powder utilization rate per unit of time, and the cladding layer’s melting width and height will increase accordingly.
Figure 5b indicates that the two-layer cladding A
1 and A
4 cladding height and width values are generally higher than in the bottom cladding area. That the melting width and height increase in the two-layer cladding area is consistent with
Figure 4b. The heat accumulation between the cladding layers is the most obvious factor for the increase in melting width and height.
2.3. Temperature Field Change of Melting Height and Width
Simulating the change of laser cladding temperature field allows us to judge the melting point temperature of the cladding layer material under various process parameters, as well as the formation, melting height, and melting width of the cladding layer.
In
Figure 6a, the overall temperature change of the multilayer multichannel laser cladding layer shows a stepwise upward trend. Four temperature peaks correspond to A
2, A
1, A
3, and A
4 cladding processes. When v = 4 mm/s, the substrate length is 50 mm, and the corresponding time of each cladding time is 12.5 s. The high-speed steel material melts at about 1060 °C according to the overall temperature curve and can form metallurgically bonded between the substrate and the cladding material. The three-stage temperature rise curve can reach the melting point temperature of high-speed steel after 50 s cladding time and heat accumulation, proving that the laser power meets the material melting conditions when P = 23 w, P = 25 w, and P = 27 w.
As shown in
Figure 6b, under the process parameters of P = 27 w and v = 4 mm/s, the cladding depth-temperature change from the surface of the simulated cladding layer (25 mm, 43.75 mm, 2 mm) to the bottom of the substrate (25 mm, 43.75 mm, −1 mm) is simulated. There are two turning points in the temperature change curve. The depth values correspond to 1 mm and 2 mm, located at the junction of the second cladding layer, the bottom cladding layer, and the intersection of the bottom cladding layer and the matrix. The heat-affected zone of the cladding layer presents a circular state distribution. The temperature field value attenuates outward along the center, and the molten pool shapes like a spoon. The temperature gradient at the front end of the molten pool is more significant than that at the tail end. The heat-affected area is affected by heat accumulation, and the range becomes more extensive.
2.4. Change of Cladding Stress Field
The residual stress will affect the morphology of the cladding layer, the quality of the cladding layer, and the size of the bonding force between the cladding layer and the substrate. Therefore, the residual stress distribution of the cladding layer should be studied. Under the premise of determining the quality of the cladding layer, it screens the laser processing parameters to provide theoretical support for selecting the cladding layer width and height.
2.4.1. Cladding Overall Stress Change
Select the middle of the A
1 cladding layer as y = 22.5 mm, x = 1 mm, 25 mm, and 49 mm, and plot paths 1, 2, and 3 according to the z direction of the vertical laser scanning speed direction. In
Figure 7a, the middle of the z direction’s residual stress is higher than that in the front and end. Compared with the front and back of the cladding layer, the melting height value in the middle of the cladding layer is slightly higher, and the melting width value is lower. The reason is that during the laser cladding process, the substrate with a slender shape and thin thickness is prone to bending. The middle of the cladding layer is subjected to tensile stress, and the residual stress value increases. In contrast, both sides of the substrate are subjected to compressive stress to balance the residual stress in the middle.
In
Figure 7b, Workbench simulates the total deformation of the cladding model under different laser powers and scanning speeds. The overall deformation of the cladding is the largest when the process parameters are P = 28 w and V = 4 mm/s. The laser power increase and the scanning speed decrease positively correlate with the cladding layer’s bending degree. The overall thickness of the model is 3 mm when the process parameters are P = 23 w and v = 4 mm/s; when P = 25 w and v = 8 mm/s, the total deformation rate is less than 25% and can carry out laser cladding. Now, bending does not affect the melting height and width of the cladding layer.
2.4.2. Residual Stress Change in Cladding Layer
In
Figure 8a, the residual stress value reaches the peaks at the junction of the cladding layer and the substrate. It gradually decreases away from the inside and both sides of the substrate. Therefore, the junction is most prone to fracture or poor metallurgical bonding. The increased laser power and the decreased scanning speed will increase the residual stress of the cladding layer.
Figure 8b selects four cladding layers of A
1, A
2, A
3, and A
4. Along the direction of laser scanning speed, the residual stress value of the bottom cladding layer is about 2.5 times that of the second cladding layer, which proves that the residual stress at the junction of the cladding layer and the matrix is immense and prone to fracture.