AISI 1045 steel (45 steel) has good plasticity, ductility, and excellent mechanical properties and is widely used in automotive manufacturing. However, the surface of the material can be damaged by friction, leading to a deterioration in the performance of components in contact with moving parts. Further, damage caused by corrosion and abrasion accelerates the expansion of cracks and risks fracture. Partial remanufacturing is an effective way to reservice damaged parts, maximizing the residual value of the material. Laser additive manufacturing is considered the most promising remanufacturing technology for rebuilding the geometric features of damaged parts and restoring their mechanical properties; however, it faces problems concerning material properties. In this study, an innovative combination of laser directed energy deposition (LDED) and laser shock processing (LSP) processes is proposed for the remanufacture of damaged 45 steel, utilizing the respective advantages of each process.
Experiments were conducted on 45 steel, whose chemical composition is displayed in Table 1. H13 tool-steel powder was used as the laser deposition powder for the experiments, and its chemical composition is displayed in Table 2. The laser composite remanufacturing process was realized using LDED and LSP equipment, and the specimens were fabricated according to LDED and LSP experimental parameters: a 2 mm thick layer was deposited on the substrate using LDED, milled to a smooth surface, and treated with LSP. Finally, the specimens were cleaned using ultrasonic vibration. A dry slip abrasion test was carried out on an HT-1000 spherical disc high-temperature tribometer based on the ASTM standard G99-95.
The number of small pores around the contact area significantly reduced with increased laser power. Comparisons of samples before and after LSP show that the strained areas exhibit inhomogeneous surfaces (Fig.2). With increasing laser power, the microhardness gradually increases; LSP significantly improves the microhardness of the LDED repair layer (Fig.4). The LDED-1200 W specimen has broad martensitic laths with a small number of fine needles; in comparison, in the LDED-1800 W specimen, the lath size decreases, the grain boundaries increase significantly, and internal refinement occurs with some dislocation (Fig.7). LSP induces significant refinement of surface grains, forming tiny nanoparticles with non-sequential orientation; the impact extends downward along the depth, and a large number of discrete dislocation structures, including dislocation tangles and cells, were found near the impact surface (Fig.8). The LDED specimen has a large worn area, with deep grooves and ridges parallel to the sliding direction, and almost the entire worn area is severely abraded (Fig.11); conversely, the worn surface of the LDED+LSP specimen is smooth, with only a small number of scratches and grooves in the middle of the wear, no large worn area, and no obvious adhesion phenomenon on the surface (Fig.12).
A combination of LDED and LSP post-treatment was used to repair damaged 45 steel. The main conclusions are as follows:
(1) With increased LDED laser power, the powder is fully dissolved under high heat, the forming quality of the repair layer is improved, the quantity of internal holes is reduced, the porosity is reduced, the martensite lath-like structure is refined, and the cementite in the structure is dissolved.
(2) Plastic deformation of the material occurs under the influence of LSP and the surface grains undergo refinement, forming nanograins with an approximate size of 30?50 nm. Subsequently, the deformation influence extends along the depth, generating many dislocations and forming high-density dislocation structures.
(3) The main wears on the LDED-restored layer are plowing and adhesive wears, with a small contribution from abrasive wear; whereas the main wear mechanism of the LDED+LSP-restored layer is adhesive wear accompanied by abrasive wear.
(4) LSP induces nanograin and dislocation reinforcement to refine the material structure, which effectively eliminated the internal pores, compacted the structure, and realized surface hardening, thus improving wear resistance. Meanwhile, the post-treatment process is accompanied by the dissolution of primary cementite, which further improves wear resistance.
