1. Introduction
In industrial production, numerous key parts of equipment are inevitably subjected to deformation, corrosion, fracture, wear, deterioration, and other damages under the long-term action of complex working conditions and the severe environment, such as the wear and scratch of rotor journals and turbine blades in service, hydrogen corrosion cracking of ethylene spherical tanks, etc. According to relevant statistics, more than 80% of equipment failures are caused by surface wear, corrosion, and fatigue [
1]. For the damaged parts, the traditional replacement or retirement method causes a waste of resources, which no longer conforms to the current industrial development trend. In recent years, remanufacturing technology has developed rapidly and formed a relatively complete system. The remanufacturing technology has greatly saved the investment of enterprises and achieved the maximum output, providing new methods for the treatment of damaged parts [
2].
At present, the repair of damaged equipment mainly uses traditional surfacing welding technology such as gas welding and arc welding. However, excessive welding heat input during the repair process is prone to producing large welding residual stress and deformations, and it causes a degradation of the material properties in the heat-affected zone. In some cases, the welding residual stress and the stress generated by the external load will cause the plastic deformation of the material and reduce the structural rigidity. In addition, when the superimposed stress is greater than the tensile strength, this will also lead to early structural damage. When there is tensile residual stress at the stress concentration, the fatigue strength of the structure will also decrease. The residual stress is also an important cause of stress corrosion cracking. However, residual stress can also play a positive role. For example, compressive residual stress is applied on the surface of materials to improve their fatigue properties. Cseh [
3] studied the surface residual stress state evolution of hardened and shot peened 42CrMo4 steel during fatigue tests by X-ray diffraction. In addition, compressive residual stress can effectively inhibit stress corrosion cracking. Therefore, the welding residual stress has a significant impact on the structural integrity, the appropriate in-service evaluation of the welded joint, and the reliable operation of the equipment [
4,
5]. It is necessary to find more advanced repair processes to make up for the shortcomings of traditional welding methods.
The HESD repair welding process is improved by electro-spark deposition, which combines the characteristics of various surface processes and has the advantages of a low heat input, low welding residual stress, and excellent metallurgical adhesion with the substrate. HESD can be considered as a hybrid procedure combining welding, surfacing, and metallization by evaporation [
6]. The power supply of the HESD process uses the insulated gate bipolar transistor (IGBT) switch controlled by pulse width modulation (PWM) to make the capacitor charge and discharge quickly and repeatedly, which releases a high-frequency short-time pulse current in order to melt the electrode material and cladding to the repaired workpiece within a very short time (as shown in
Figure 1). The process of HESD is based on the phenomenon of electrical corrosion in a gaseous environment and on the transfer of the anode material to the cathode during the pulse discharge time [
7].
Current research on the HESD process focuses mainly on the mass transfer, deposition process, interface characteristics between the coating and substrate, process parameters, and properties of deposition [
8]. Yang [
9] used a high-speed camera to analyze the droplet transfer characteristics during the HESD process, and found that the substrate and the repair welding layer showed a good metallurgical adhesion. Chen [
10] analyzed the formation of single-pulse deposition points, proposed a physical model of “splash and gasification”, and verified the model through experiments. In addition, other scholars have studied the single deposition point in the HESD process, detailed the transfer rule of the welding material, and analyzed the distribution and morphology of the deposition point [
11,
12], which laid a theoretical foundation for subsequent research on HESD. Xie, Gould, and others applied HESD technology to the connection of dissimilar steels and difficult-to-weld metals, and achieved good results. Meng [
13] analyzed the joints of ASTM1045 steel after HESD repair welding, and found that the joint hardness was 20% higher than that of the base metal. In addition, the joint friction coefficient was reduced by nearly 40% when compared with the base metal, which showed that the wear resistance of the joint was improved after HESD repair welding.
The HESD process involves many important parameters such as the pulse width, frequency, pulse current, and discharge voltage. In order to obtain better repair results, many scholars have discussed the relationship between the process parameters and repair performance. James summarized the main parameters in the process and briefly analyzes their respective effects, as shown in
Table 1 [
14]. Kondapalli analyzed the influence of the process parameters on the grain size and tensile strength of Inconel 625 nickel-based alloy joints, established a mathematical model to predict the grain size, and found that the peak current was the determining factor [
15]. Chen [
16] studied the influence of the output power and voltage on the deposition process, and found that an overly low power and voltage make it difficult to form a deposited layer, while an excessively high power and voltage affect the quality of the deposited layer, causing defects such as cracks and pores. This is consistent with the literature [
17], which believes that excessive intermetallic phases caused by an excessive pulse energy can cause delayed cracking.
During the HESD process, due to the concentrated discharge in space and time, high pulse frequency, small range of action, and complicated thermal cycle, it is difficult to analyze the HESD repair welding process by experimental methods. Therefore, it is necessary to use a numerical simulation to quantitatively analyze the HESD process. Huang [
18] conducted a simulation of mold steel HESD repair and found the optimal parameter combination. They believed that the repair efficiency was proportional to the discharge current, discharge time, and energy distribution. Through the simulation, they found the deepest heat influence range during HESD welding to only be 2 mm. Kansal [
19] established a two-dimensional finite element model of axisymmetric heat conduction. The model uses temperature-dependent material parameters while considering factors such as the energy distribution and phase change. Das Shuvra [
20] simulated the instantaneous temperature distribution of the workpiece, liquid- and solid-state transition, and residual stress by changing the process parameters, and verified the simulation results through experiments.
Summarizing the existing literature, the HESD process has not yet been studied thoroughly, although it is used in practice. There are few publications that deal with HESD from a scientific point of view. Much of the literature studies HESD technology from the perspective of a surface-coating process, but there is very little research that focuses on the repair method for damaged welding joints or on the analysis of residual stress after repair welding. Therefore, it is necessary to systematically study the residual stress distribution of HESD repair welding and to discuss its feasibility, in the future, as a repair welding method for pressure structures.