With the rapid development of the economy and technology, the number of damaged parts with high value in industry has increased year by year, and the high amount of scrap parts has led to a huge loss of economic value. The transformation of remanufacturing for green development has become one of the mainstream trends in the manufacturing industry [
1,
2,
3]. Additive manufacturing technology, as a rapidly rising advanced manufacturing technology in the past 30 years, can realize the manufacturing of complex entities by discretizing three-dimensional data and adding two-dimensional forming paths, which has important applications in aviation, aerospace, military, and other remanufacturing fields [
4,
5,
6]. Laser-directed energy deposition has great potential in the field of metal repair. For the characteristics of joining and layered forming in additive manufacturing, it is difficult for the surface quality of forming parts to meet the precision requirements in use, and the forming parts generally need to be machined. Fox et al. [
7] formed stainless steel by laser powder bed fusion, and the measured surface roughness exceeded Ra30, making it difficult to meet the requirements in use. Afazov et al. [
8] formed Inconel 718 parts by laser powder bed fusion and reduced distortion from approximately ±300 μm to approximately ±65 μm for both components, which still cannot be used directly. Laser additive manufacturing is a kind of thermal forming process and the residual tensile stress occurs during the rapid melting and solidification of materials. In addition, it is difficult to avoid the appearance of micro-defects or cracks in the forming process, which leads to processing defects in the subsequent machining process, affecting the use and delivery of parts. Mumtaz et al. [
9] produced high-density Waspaloy
® specimens by selective laser melting, which were 99.7% dense. Vilaro et al. [
10] formed TC4 by laser powder bed fusion and found defects inside the material by microscopic observation. Due to the above reasons, the application of additive manufacturing is limited.
Researchers carried out studies on the defects and shortcomings of additive manufacturing in order to improve its usability in applications. At first, researchers expected to improve the forming characteristics through process optimization, but they could not solve the problems of overall joint effect, thermal stress, and micro-defects that occurred in additive manufacturing [
9,
10]. Then, researchers tried to solve the problems from the manufacturing process itself, proposing a new concept of equal–additive–subtractive manufacturing, which combined subtractive manufacturing, surface-strengthening technology, and additive manufacturing to achieve high-precision and high-quality manufacturing capabilities. Based on additive–subtractive manufacturing, the method integrates surface strengthening techniques called equal manufacturing, such as micro-forging, laser shock, shot peening, etc. Zhang et al. proposed a similar concept also known as “micro-casting forging and milling” [
11]. The advantages of this method are removing the surface stress and microdefects of the materials in additive manufacturing by strengthening layer by layer and improving the accuracy by machining. The concept of hybrid additive manufacturing was first proposed by Prinz in a patent, in which it described a method to form suspended structures with support features by spraying, and guaranteed accuracy by hybrid manufacturing with machining [
12]. Akula et al. [
13] and Karunakaran et al. [
14] combined arc additive manufacturing technology with numerical control machining technology to improve the accuracy of parts through the cycle of forming and machining, and tested the performance of formed samples, which still had a gap compared with that processed by traditional manufacturing. Kerschbaumer and Ernst [
15] integrated the laser cladding nozzle and powder feeding system into the CNC machine tool to reduce the flow phenomenon during the forming process with the flexibility of the five-axis movement and achieve high-precision manufacturing by alternating milling. However, the cutting fluid could not be used during the manufacturing process due to the influence of raw materials, which reduced the manufacturing efficiency. In terms of strengthening performance in hybrid manufacturing, B.N. Mordyuk et al. [
16] combined high-frequency ultrasonic impact with laser-directed energy deposition and found that the coarse dendrite generated in the forming process could be broken and refined by high-frequency ultrasonic impact, and the tensile stress was transformed into compressive stress at the same time. However, the effect of ultrasonic impact was affected by the forming surface. In the actual manufacturing process, the surface of the forming layer is not uniform and the wear of the impact head could seriously affect the impact load. Kalentics et al. [
17] combined laser selective melting with laser shock strengthening, formed 316L samples, and tested the residual stress, finding that the introduction of high compressive stress at some depth was conducive to improving material properties. However, the process could only be completed with the aid of water film or a large number of shocks, which reduced the manufacturing efficiency. Zhang et al. [
18] proposed a new green manufacturing method by combining arc additive manufacturing, rolling strengthening, and milling technology to strengthen the material in the thermoplastic area of the deposited layer to eliminate internal holes and stress deformation, and ensure manufacturing accuracy through milling. However, the grain size in the surface area was relatively coarse. The strengthening effect of the sidewall was limited. At present, there are still some limitations in the practical application of various hybrid manufacturing technologies.
To solve these problems, we propose a hybrid remanufacturing technology based on laser-directed energy deposition, shot peening, and milling. On the one hand, the problem of insufficient strengthening effect in hybrid manufacturing is avoided by the flexible method of shot peening, and on the other hand, the process interference in the additive and subtractive manufacturing process is avoided by the method of serial manufacturing. A comparative study on the microstructure and properties of the repaired plunger rod for a high-pressure oil pump is carried out, and the damaged plunger rod is repaired by a hybrid process. The repair process based on equal–additive–subtractive technology is of great significance for expanding the application range of additive manufacturing, and can effectively promote the development of the green remanufacturing industry.