1. Introduction
Laser cladding, as one of the additive manufacturing technologies, utilizes a high-energy laser beam as the heat source to melt cladding powder, and part of the substrate to form the cladding layer with metallurgical bonding [
1,
2]. Laser cladding displays lots of advantages, such as low heat damage to substrate, a rapid heating rate and solidification rate, low dilution rate, unlimited selection of cladding powder, and excellent metallurgical bonding between the cladding layer and substrate [
2,
3,
4,
5]. Hence, it has been widely used in various industrial applications of surface modification to improve the service life of parts, e.g., in aerospace, oil and gas, marine and offshore, biomedical, chemical, and other industries [
6,
7]. The surface-modified parts using laser cladding technology have excellent properties, such as high hardness and wear resistance, and perfect corrosion resistance [
2].
However, effectively controlling the forming quality of the cladding layer has become a key area in urgent need of research. In recent years, a large number of scholars have launched a series of studies about parameter optimization, which focused on the types of cladding powder and optimization methods. In recent years, there have been many studies to establish mathematical models and analyze the influence of parameters on the forming quality of the coating, such as the response surface methodology, regression analysis, and Taguchi orthogonal analysis. A detailed summary is shown in
Table 1 [
8,
9,
10,
11,
12,
13,
14,
15], while there are few reports about the sensitivity of the responses correlated to the input parameters.
Meanwhile, with the development of industrial materials, borides, especially ternary borides, have attracted more attention because of their excellent properties. Their high melting point, perfect hardness, excellent wear resistance, good corrosion resistance, and superior resistance to high-temperature oxidation make them a remarkable material in industrial applications [
16,
17,
18,
19,
20]. The prepared Mo
2FeB
2 coating can be applied in cutting tools and other machine parts. In our previous study, the fabricated Mo
2FeB
2 coating via laser cladding demonstrated excellent microhardness, wear resistance, and fracture toughness, owing to the dense structure and outstanding metallurgical bonding [
4,
21,
22]. Among all the typical ternary borides, although Mo
2FeB
2 requires a simple preparation method and possesses superior mechanical properties [
23,
24,
25], there are fewer reports about whether it can be used in the preparation of laser cladding coating.
This research aims at the preparation of Mo
2FeB
2 coating by laser cladding technology. In order to investigate the sensitivity of output responses (height, width, and dilution rate) to input parameters (laser power, scanning speed, and thickness of pre-placed powder), a full factorial 3
3 = 27 design was used to verify the sensitivity of the output responses to input parameters [
26]. The research results provide guidance for parameter adjustment by sensitivity analysis of the responses to the input parameters, where the combined influence of the process parameters set on the coating properties can be unambiguously analyzed. The research outcomes also prove the feasibility of the Mo
2FeB
2 coating prepared via laser cladding, and present the Mo
2FeB
2 application to additive manufacturing, repairing of components, increasing the lifespan, and decreasing the manufacturing cost of high value parts.
2. Materials and Methods
2.1. Materials
In this research, the AISI 1045 medium steel was selected as the substrate (40 mm × 20 mm × 10 mm). Its elemental composition is shown in
Table 2. The cladding powder (Mo
2FeB
2) was prepared by the vacuum induction gas atomization method, and the powder composition, SEM morphology, and size distribution of Mo
2FeB
2 are shown in
Table 3 and
Figure 1ab. The Mo
2FeB
2 powder was supplied by Avimetal Powder Metallurgy Technology Co., Ltd. (Beijing, China).
2.2. Laser Cladding Process
Before laser cladding, the surface of the substrate was cleaned with ethanol to remove the impurity, and 5 wt.% polyvinyl alcohol binder was added to the powder. Then the cladding powder was pre-placed as 1 mm thickness on the surface of the substrate under the pressure of 100 MPa for 1 min where the size of the substrate was 40 mm × 20 mm × 10 mm (length × width × height). Afterwards, both were placed in the vacuum dryer for an additional 2 h at a temperature of 120 °C.
The laser cladding system (
Figure 2a) was mainly composed of a laser system (YLS-3000, IPG, Burbach, Germany), cladding system (SX14-012PULSE, IPG, Burbach, Germany), water cooling system (TFLW-4000WDR-01-3385, Sanhe Tongfei, Sanhe, China), and PLC computer system (PLC, Mitsubishi, Tokyo, Japan). The cladding process was finished by the six-axis industrial robot (M-710iC/50, FANUC, Yamanashi, Japan). The pre-placed method of laser cladding is shown in
Figure 2b. The laser spot diameter was set as 4 mm during the cladding process. The laser beam was generated in the laser system to irradiate the pre-placed cladding powder layer and the cladding substrate located on top of the workbench. A molten pool was formed under the high energy laser beam irradiation. An industrial robot was used to control the movement of the laser head with a selected scanning speed to complete the cladding process. Argon was used as the shielding gas during the cladding process. The experimental runs followed the 3
3 factorial design of three process parameters with three levels (
Table 4), and 27 samples were fabricated.
2.3. Characterization
After the completion of laser cladding, the sample was cut into 10 mm × 5 mm × 5 mm blocks, and then the sample preparation for characterization was finished with setting, grinding, and polishing. The cross-section of samples was immersed in a 1:1:10 ratio of K
3[Fe(CN)
6]
4:NaOH:H
2O solution for 20 s, and then cleaned with alcohol by ultrasonic.
Figure 3a shows the macro view of the uncoated samples and
Figure 3b shows the macro view of the coated samples.
Afterward, the morphology of the coating was observed by the ZEISS Axio Plan2 Optical Microscope (OM) (Zeiss, Shanghai, China), and the image-Pro plus software was used to measure the height (H), width (W), and dilution rate (D = A
2/A
1 + A
2) (
Figure 4a,b), where CZ stands for coating zone, MZ denotes melted zone, HAZ represents heat-affected zone, and SZ is substrate. The geometric characteristics of the 27 obtained samples are shown in
Table 5.
The microstructure of the coating was analyzed by a field emission scanning electron microscope (Nova 400 Nano SEM, FEI, Hillsboro, OR, USA) and field emission electron probe microanalyzer (FE-EPMA, EPMA-8050G, Shimadzu, Kyoto, Japan). The microhardness of the cross-section of the coating was measured by an HX-500 microhardness tester (Laizhou Yutong Test Instrument Co., Ltd., Laizhou, China) with a 500 g force applied for a 10-s duration.
4. Conclusions
The empirical models of output responses (height, width, dilution rate) and input parameters (laser power, scanning speed, pre-placed thickness) were established, and the sensitivity of the output responses to input parameters was investigated in this research. The conclusions were summarized as follows.
(1) The established models have satisfactory fitting accuracy on the relationship between output responses and input parameters;
(2) According to the results of the established models, scanning speed has a negligible effect on the height and dilution rate. While the laser power has a positive effect on the width and dilution rate, and a negative effect on the height. The thickness of the pre-placed powder has a positive effect on the height, and a negative on the dilution rate;
(3) The results of the sensitivity analysis indicate that the height of the coating is more sensitive to the thickness of pre-placed powder than that of laser power and scanning speed. The width of the coating can be significantly controlled by the adjustment of laser power. Laser power and pre-place powder thickness are more sensitive to the dilution rate;
(4) The microhardness of the Mo2FeB2 coating with different parameters was 4–6 times that of the substrate. The Mo2FeB2 coating obtained with 1.5 kW laser power, 3 mm/s scanning speed, and 1.2 mm powder thickness from the sensitivity results had a highest microhardness of 1283.5 HV0.5.