1. Introduction
As a train speeds, the axle weights and capacity increase, and trains in the repeated acceleration, braking, bending, and occasionally complete sliding process of frictional heat generation will make the wheels produce a high thermal load [
1]. This will lead to high temperatures up to 600 °C in the braking process between the brake tile and the tread surface, which could cause local damage to the surface of the wheel [
2]. To ensure the safety of vehicles, the wheel surface damage found in daily maintenance is mainly removed through wheel turning; however, wheel turning leads to a rapid reduction in the wheel size, which greatly shortens the service life of the wheel, resulting in a huge waste of materials and economic losses [
3]. Therefore, there is an urgent need to develop more efficient and economical train-wheel-repair technology.
The use of laser cladding technology in restoration is very extensive, and laser cladding is one of the laser processing techniques [
4]. During the laser cladding process, the surface of the material is fused with different alloy powders using a high-energy beam and rapidly solidified to form a coating with a good forming quality [
5]. In addition, unlike other metal surface technologies, the repair coatings prepared using laser cladding technology are characterized by a high laser beam energy density, low heat input, low thermal impact on the substrate, dense and fine grain structure, and good bonding with the substrate [
6,
7]. At the same time, different alloy cladding powders can be selected to achieve high wear resistance, high corrosion resistance, high strength, and other high-performance effects, for applications to complex and harsh environments. This technology has been widely used in aerospace, rail transport, petrochemical, machinery manufacturing, and other fields [
8,
9] and will become a new type of surface-modification technology.
In the field of the local damage repair of railway wheel rails, scholars have carried out much exploratory work, with most researchers focusing on the consideration of the preparation of the high-strength, wear-resistant performance of different alloy cladding layers on the rail wheel track material to enhance its service life and achieve significant results. Seo et al. [
10] evaluated the wear characteristics of Stellite 21, Inconel 625, and Hastelloy C laser cladding layers and found that Hastelloy C is the most appropriate when considering both the wear and rolling contact fatigue (RCF) of the cladding boundary. Guo et al. [
11], in the wheel substrate preparation of Co-based alloy coatings, compared and analyzed the coating’s wear resistance by using a rolling fatigue tester. The results showed that the wear rate of the repaired coating was only 1/2 that of the substrate, indicating that the laser cladding technology can repair the damage defects in the wheel and rail materials. Lewis et al. [
12] investigated the composite coating of Co-based and martensitic stainless steel, and the fatigue characteristics of the specimens were significantly improved after laser cladding processing, and the wear of the wheel steel was also attenuated to a greater extent. Wang et al. [
13] conducted in-depth research on the application of Fe-based laser cladding coatings on train wheels. After fatigue rolling tests, they found that the wear surface damage was relatively minor, the wear rate was low, and the fatigue resistance performance was the best. However, the surface hardness of the coatings prepared via laser cladding of the alloy powders used in the above studies is generally too large, such as the Fe-based alloy coating hardness of about 800 HV [
14], which is significantly higher than the hardness of steel rails (about 300 HV), which is difficult to coordinate with the wheel substrate and accelerates the abrasion and damage of steel rails. In view of this, Zhu et al. [
15] tried to choose 316L, 410, and 420, three kinds of stainless steel powders, on the surface of the local wheel damage laser cladding repair and found that the hardness of the three repair layers and the substrate is close to the hardness of the coating, or it runs for a period of time after the hardness of the coating and the substrate hardness tends to be coordinated and shows excellent friction wear and contact fatigue performance compared to the substrate, which indicates that stainless steel powder may be more suitable for local wheel damage repair.
Stainless steel powder is an important part of iron-based materials, and it has excellent corrosion resistance at the same time, both iron-based materials, low manufacturing costs, and good compatibility with a large number of steel materials used in industrial production, and it is thus favored by researchers [
16,
17].
It is well known that the railway train, in the process of braking, and the rail contact at the tread surface not only bear the compressive stress on the brake tile and the contact pressure under load but also bear the rapid rise in the surface temperature caused by thermal stress. The cyclic mechanical load and thermal stresses together cause the wheel tread to undergo thermal-mechanical fatigue damage. In order to verify the main factors affecting the cyclic dynamic fatigue damage of the tread, Moyar et al. [
18] adopted the critical plane fatigue damage theory analyzing the change rule of the thermal stresses generated by the axle load and frequent starting/braking. Teimourimanesh et al. [
19] confirmed that the friction occurring during the braking of the tread of the train affects the mode of heat transfer in the contact position among the wheel, brake pads, and steel rails. Therefore, the use of laser cladding technology for the repair of local damage to wheels should focus on the interaction between thermal and mechanical loads at the contact location under train-braking conditions and their effects on the tread of the wheel.
This work is intended to repair the 316L stainless steel coating through fusion cladding in the locally damaged notch of ER8 wheel material using laser additive repair technology. Under different service conditions (axle load and braking frequency), sliding friction wear tests were carried out on the repaired specimens using the ball and disc wear tester. An analysis, at room temperature and high temperature, of the repair coating structure, friction coefficient, wear rate, surface damage morphology and other evolutionary laws has important theoretical significance. Investigating the complex braking environment in the repair of the fusion cladding service performance, to improve the train wheel and rail system to withstand the extremely harsh environment, has important practical significance.