1. Introduction
Due to excellent surface properties and low manufacturing costs, 45 steel is widely used in important equipment parts [
1,
2]. However, most of these components are used in complex environments, which require frequent maintenance or replacements, as well as increased maintenance costs. In recent years, laser cladding has been widely applied in the remanufacturing industry. Its principle is shown in
Figure 1. Under the irradiation of a laser, one or more alloy elements are fused with the substrate surface so that high-performance cladding can be obtained on the surface of cheap materials. Laser cladding has many advantages compared to other modification technologies, such as a small heat-affected zone, a low dilution ratio, and a good metallurgical combination. Therefore, it has been widely used in the remanufacturing and repair of parts [
3,
4,
5,
6,
7].
Based on the above views, many studies have explored effective surface repair technology for 45 steel materials, examining many research results. Zhu et al. [
8] prepared almost completely dense nickel titanium carbide composite coatings with different titanium carbide content on 45 steel. The results showed that the average microhardness of Ni-50TiC coating was between 551HV 3 and 682HV 3—at least 2 times that of 45 steel—and the wear amount was significantly reduced. Zhang et al. [
9] fabricated crack-free Ni60A coating on the surface of 45 steel. Through XRD, SEM, and EDS analysis and characterization, it was found that there were a large number of hard phases, which improved the wear resistance. Lu et al. [
1] successfully obtained Zr50Ti5Cu27Ni10Al8 metallic glass (Mg) coating on 45 steel. The corrosion tests showed that the corrosion resistance of 45 steel with glass coating was much higher than without.
The above studies have mainly focused on changing the cladding materials to improve the surface properties. The combination of process parameters also has a significant effect on the performance of the cladding layer [
10]. Among them, the laser power, scanning speed, powder feeding rate, and others are the key factors to determine the service life of remanufactured parts. There is a complex nonlinear relationship between the process parameters and the quality of the coating. These parameters have different effects on coating quality. In order to make the coating have excellent quality, mechanical properties, formability, and low crack sensitivity, it is necessary to select reasonable process parameters. Through analysis, adjustment, and optimization, the quality of coating can be effectively controlled [
11]. Therefore, in order to meet the needs of high-quality coating, optimizing the combination of process parameters is important to ensure and improve the service life of parts.
At present, many optimization methods have been applied in the manufacturing field, such as the Taguchi method, the finite element method, mathematical statistics, and the response surface method. Riquelme et al. [
12] used the Taguchi method to conduct cladding experiments on an aluminum alloy substrate and obtained the optimal parameter combination under the optimized conditions of the maximum aspect ratio and minimum defect rate. Hebbale et al. [
13] used the Taguchi technique to evaluate the effects of different parameters on the wear behavior. These results showed that the slurry speed had a significant effect on the mass loss of uncoated substrate. Therefore, the Taguchi method is effective for optimizing a single objective response, but it is not suitable for optimizing a multi-objective parameter. Yao et al. [
14] used COMSOL software to simulate the temperature field and stress field of different process parameters. Under the condition of minimum residual stress, the best power and scanning speed were obtained. Although the work efficiency was improved, the interactions among process parameters were not explored. Erfanmanesh et al. [
15] and Nabhani et al. [
16] used empirical statistical data to predict the correlation between process parameters and single-pass cladding geometry and analyzed it by regression. The shortcoming of this method is that the error of the fitting curve is large when the data are irregular.
Compared to the above optimization method, RSM is a newly developed multi-objective mathematical optimization method combining experimental design and statistical technology [
17]. Through the analysis of experimental data, the fitting function and a three-dimensional surface graph can be obtained, and then the impact of impact factors on the response values can be directly reflected, including the interactions among impact factors. It has the functions of prediction, improvement, and optimization. This method is simple, efficient, and suitable for solving complex nonlinear optimization problems. Therefore, it has been widely used in manufacturing processes. The single-pass laser cladding test is the main method used to select the process parameters of laser rapid prototyping. The geometric shape of the cross-section is the main index to quantitatively evaluate the quality of the cladding layer. This is mainly because the laser cladding process involves complex multiphase and multi-physical fields. In addition, the geometry of the cross-section of a single cladding layer plays a decisive role in the quality and efficiency of laser rapid prototyping. Therefore, taking it as an evaluation index has the dual significance of quality and cost control. However, there is no perfect prediction theory or method to completely replace the process parameter selection based on the single-pass test. Therefore, it is very important to find a method to optimize the morphology of single-pass coating.
In order to obtain excellent coating quality, an RSM prediction model for single-pass coating morphology was established based on the section geometry of the coating as the main evaluation index. The influence of linear and nonlinear coupling among different process parameters on the morphology of the coating was explored. The ideal coating was obtained by the ideality optimization method, and the microstructure and microhardness of the best samples were characterized. This method can provide method guidance and decision-making reference for preparing a high-quality cladding layer in production practices.
4. Conclusions
The influence of the process parameters (dilution ratio, ratio of layer width to height, and contact angle) on the morphology of the composite cladding layer were analyzed based on RSM. The mathematical models of the process parameters and the dilution ratio, ratio of layer width to height, and contact angle of the cladding layer were established, providing a theoretical basis for the prediction and control of the forming quality of the cladding layer. The main conclusions are as follows:
- (1)
Variance analysis showed that the laser power had the greatest influence on the dilution ratio, and the scanning speed was the most significant factor affecting the W/H and the contact angle.
- (2)
The target value and significance of the response index were set using Design Expert software based on RSM. The optimal combination of process parameters included a laser power of 1477 W, a powder feeding rate of 17.5 mg/s, and a scanning speed of 5 mm/s. The errors between the predicted data and the experimental data were all less than 10%. The cladding layer obtained under the optimal parameters had good quality with a low dilution ratio and a large W/H and contact angle.
- (3)
The microstructure of the cladding layer was different in different zones. Equiaxed crystals formed due to the rapid cooling rate of the upper surface of the cladding. The cooling rate in the middle zone slowed down, and the cellular crystal changed into cellular dendritic crystal. The temperature gradient at the bottom was large and the structure was relatively coarse.
- (4)
The microstructure of the cladding layer was mainly composed of γ(Ni), FeNi3, M (M = Fe, Ni, Cr)23C6, M7C3, and CrB. Under the optimal process parameters, the microhardness of the cladding layer was 3.1 times that of 45 steel substrate.
At present, the research on laser cladding for the remanufacturing of equipment parts is still in the initial stage. This study used the repair of parts as the background and obtained the optimal combination of process parameters on the basis of a large number of experimental analyses. This research can provide a reference for the remanufacturing laser cladding of more equipment parts and can also provide a reference for remanufacturing laser cladding technology, providing a theoretical basis for further extensive application.