1. Introduction
At present, in order to improve the performance of parts, researchers mostly use traditional surface modification methods such as thermal spraying, surfacing welding and surface heat treatment. However, these methods have drawbacks such as low bonding strength, large heat input and thermal deformation [
1]. As a new surface modification technology of metal materials, laser cladding is a surface modification technology that us-es laser as a heat source to irradiate alloy powder to melt at the same time with the mate-rial surface and form a low-dilution cladding layer after solidification. It has the advantages of low thermal effect, small workpiece deformation and high bonding strength [
2,
3,
4], so it is widely used in various industries.
As a medium carbon tempering structural steel, 45# steel is widely used in various important parts due to its low price and good strength, cold and hot working properties [
5]. However, 45# steel in long-term service under high load, wet environment, the surface is often subjected to serious wear and corrosion, thus reducing the service life. Laser clad-ding is an effective method to strengthen the surface properties of 45# steel. Ni-based alloy has the characteristics of corrosion resistance, wear resistance, high temperature creep re-sistance and thermal oxidation resistance. And it is one of the main materials of laser cladding alloy system [
6]. Therefore, many scholars choose Ni-based powder as cladding material.
Liu et al. [
7] prepared Ni60-Cu composite coatings with different Cu content on the surface of 45# steel, and studied the effects of Cu content on microhardness and tribologi-cal behavior. The results show that the Ni60-5%Cu coating shows the best friction resistance at 600 °C. Zhou et al. [
8] prepared Ni60 coating on 45# steel surface with the assis-tance of magnetic, ultrasonic and electric field. The results show that the microstructure of the Ni-based coatings prepared under the influence of multiple physical fields is refined, the distribution of elements is uniform. And the friction and corrosion resistance of the coatings are improved. Cao et al. [
9] prepared coatings with different TiC contents of Ni60-TiC on the surface of 45# steel. The results showed that adding TiC to the coatings could effectively improve the hardness, wear resistance and corrosion resistance of the coatings. When the TiC content is 20wt%~30wt%, the wear and corrosion resistance of the coating is the best. Chen et al. [
10] successfully prepared Ni35 + 20%SiC + 20%Ni/MoS2 self-lubricating coating on the surface of 45# steel, and tested the microhardness, corrosion resistance and wear resistance. The results show that the average microhardness of the coating is about 700 HV. Compared with 45# steel, the corrosion resistance and wear resistance of the coating are greatly improved. Yu et al. [
11] added Ta element to NiCrBSi alloy powder to improve the high-temperature wear resistance of carbon steel. The results showed that Ta significantly improved the wear resistance of NiCrBSi coatings at high temperatures. Similarly, Hulka et al. [
12] studied the influence of Ti addition amount on WC-Co/NiCrBSi coating. The results show that the addition of pure titanium can reduce the cracking sensitivity and improve the hardness and corrosion resistance of the coating.
The above studies obtained high quality coatings by exploring new cladding pow-ders or with the assistance of multiple physical fields. But they did not consider the influ-ence of the combined action of process parameters on the quality of cladding layers. As is well known, the laser cladding process is extremely complex and involves many process parameters. The selection of cladding process parameters affects the forming quality of the coating [
13]. The process parameters such as laser power, scanning speed and powder feed amount are the key factors affecting the quality of laser cladding layer. Adjusting the parameters of laser cladding can change the transfer process of energy, mass and momentum in the cladding layer, affect the formation and growth of the cladding layer grains, and then control its surface morphology and comprehensive properties. In the case of selecting high-performance cladding materials, reasonable adjustment of process parameters can further improve the comprehensive protection performance of industrial equipment. RSM integrates experimental design and mathematical modeling, and adopts multiple quadratic regression equation fitting to obtain the optimal combination of design variables and the optimal value of response target [
14]. RSM has the advantages of few test times, short test period, high precision, good prediction performance, and can study the interaction between factors. Compared with orthogonal experimental design and uniform experimental design, the regression equation obtained by RSM has higher accuracy, and the interaction of multiple related factors can be obtained. Moreover, RSM can extend the optimization process to the surface area, and has more significant three-dimensional effect than other point-to-point optimization methods. At the same time, RSM can reduce the test deviation under the best conditions, and has obvious advantages.
Among many Ni-based alloys, NiCrBSi self-soluble alloy powder is a commonly used thermal spraying and laser cladding material. The prepared NiCrBSi coating has good corrosion resistance and wear resistance, and is often used for surface strengthening of parts. Therefore, in this experiment, NiCrBSi alloy powder was used as cladding material, and RSM method was selected to optimize the laser process parameters. And then NiCrBSi alloy coating is successfully prepared on the surface of 45# steel by laser cladding technology The microstructure, wear resistance and corrosion resistance of NiCrBSi alloy coating are systematically studied.
4. Conclusions
45# steel parts in long-term service in high load, wet environment, there is serious surface wear and corrosion. In this paper, based on RSM, the laser power, scanning speed and powder feeding rate are optimized. The experimental results show that the wear resistance and corrosion resistance of the optimized coating are improved. It has laid a certain theoretical foundation for 45# steel parts repair technology. The main research results are as follows:
- (1)
The interaction term shows that increasing the scanning speed and decreasing the laser power can obtain a smaller dilution rate. Increasing the laser power and scanning speed can obtain a larger aspect ratio and contact angle. The optimized process parameters are as follows: laser power 1477 W, powder feeding speed 17.5 mg/s, scanning speed 5 mm/s.
- (2)
The cross section of the optimized coating has a good three-dimensional morphology, and the cladding layer and the substrate can form a good metallurgical bond. The Ni-based cladding layer is mainly composed of black chromium-hard phase, Ni-B-Si eutectic and γ (Ni) solid solution.
- (3)
The friction and wear test results show that the wear resistance of Ni-based coating is much higher than that of 45# steel. The optimized coating has a smaller wear rate (0.92 × 10−5 mm3/Nm), shallower grooves and fewer spalling pits, as well as a smaller wear width and depth. The optimized coating has good wear resistance.
- (4)
The electrochemical test results show that the corrosion resistance of the Ni-based coating is greatly improved due to the presence of Fe and Cr elements. And the optimized coating has a corrected self-corrosion potential, a smaller self-corrosion current density, a larger arc radius of capacitive reactance and a larger impedance modulus. The optimized coating has good corrosion resistance.
At present, laser cladding technology is widely used in all walks of life. However, the cladding parameters in the repair process are coupled with each other, and the repair quality and repair efficiency often cannot be both. Therefore, a multi-objective optimization method based on RSM is proposed in this paper, which can provide a certain basis for solving the multi-objective optimization problem of laser cladding.