As additive manufacturing (AM) technology can produce parts with complex shapes that cannot be realized through conventional machining, it is expected to play an active role in production, from various perspectives. For example, it can simplify the manufacturing process, and reduce product weight, lead time, and cost. The two main metal AM technologies are powder bed fusion (PBF) and directed energy deposition (DED). In PBF, a heat source is irradiated on the required part of a powder bed filled with raw powder. The powder is melted and fused with the underlying layer by repeating the process to form the desired product [1
]. PBF has two classifications according to the type of heat source: selective laser melting (SLM) using lasers, and electron beam melting (EBM) using electron beams. In DED, an energy beam, such as a laser or electron beam, irradiates and melts the base material, or both the base material and the supplied material, to build a modeling object. Although the surface roughness is inferior to that of PBF, DED has advantages over PBF: the material only needs to be supplied to the required area, the build speed is high owing to the large layer thickness, and it is suitable for building large components [4
]. Due to the SLM method being used extensively, owing to its high accuracy, we focused on SLM in this study as well. However, because of gas contamination in the equipment and poor melting of the material powder, the AM process creates easily formed defects on the surface and within the material. Furthermore, fatigue cracks are generated from the defects due to cyclic loading, degrading the fatigue strength [5
]. In maraging steel, the fatigue strength of AM specimens is reported to be approximately one-third that of forged specimens [11
]. As fatigue strength immensely influences equipment durability, it is necessary to improve fatigue strength to increase the reliability of manufactured products, and further expand the range of application.
Various methods have been proposed to improve the fatigue strength of additive-manufactured materials; typical examples include hot isostatic pressing (HIP) and shot peening (SP). HIP improves fatigue strength by reducing the internal defect size in additive-manufactured components, by applying high pressure in a high-temperature environment. It has been confirmed that the fatigue strength of additive-manufactured Ti-6Al-4V is improved by HIP [12
]. On the other hand, when HIP treatment is applied to additive-manufactured 316 L stainless steel, the fatigue strength decreases because of the grain growth caused by heating during HIP [14
]. In SP, the surface is deformed plastically by striking the material with small steel balls to increase the hardness and introduce compressive residual stress, which enhances the fatigue strength. This method is effective in improving the fatigue strength of several AM materials, such as aluminum alloys [15
] and maraging steel [17
]. However, as the layer of compressive residual stress due to SP is shallow, the fatigue limit may not be improved in additive-manufactured metals that include defects within as well as on the surface.
Recently, it has been demonstrated that the fatigue strength of conventional metals can be improved by laser peening (LP); this is a surface modification method that introduces compressive residual stress on the surface layer through the local impact effect of high-pressure ablation plasma, generated by irradiating a material immersed in water with short laser pulses [19
]. As LP induces a considerably deeper layer of compressive residual stress compared to SP, it is more effective in improving fatigue strength [16
]. Furthermore, LP has high reproducibility because of the stringent control of laser pulses, and can be applied to components with complex shapes manufactured by the AM process.
Surface defects significantly reduce the fatigue strength of metals. In the case of additive-manufactured metals, the surface is very rough; if the surface roughness is removed by machining, the internal defects may be exposed on the surface. Therefore, if the surface defects can be rendered harmless in terms of the fatigue limit through peening, the reliability of additive-manufactured metals can be improved, which can contribute to the increased industrial usage of additive-manufactured parts. Surface defects can be rendered harmless through various types of peening on conventionally manufactured metals [22
]. Takahashi et al. reported that a semicircular surface defect with a less than 0.2 mm depth could be rendered harmless by applying SP to spring steel [22
]. Fueki et al. clarified that a semicircular surface defect with a depth of less than 1 mm could be rendered harmless by applying needle peening to high-tensile steel-welded joints [23
]. Takahashi et al. compared the maximum defect size that can be rendered harmless by SP and cavitation peening (CP) in 7075 aluminum alloy. The results indicated that semicircular defects with depths below 0.1 mm and 0.2 mm could be rendered harmless by SP and CP, respectively [24
]. In addition, Takahashi et al. investigated the maximum defect size that could be rendered harmless by the peening of 7075 aluminum alloys through SP and LP, and determined that semicircular surface defects with depths below 0.4 mm could be rendered harmless by LP, which was higher than the depth of 0.1 mm by SP [25
]. However, as the effect of LP on the fatigue strength of additive-manufactured maraging steel has not been studied, the defect size that can be rendered harmless by LP remains unknown.
To clarify the effect of LP on the fatigue strength and the surface defect size that can be rendered harmless by LP, bending-fatigue tests are conducted on AM maraging steel, in this study. The fatigue strength of metals primarily depends on the surface residual stress, hardness, and surface roughness. Therefore, the changes in these factors after LP treatment are investigated to elucidate the results of fatigue testing. The defect size that can be rendered harmless by LP is estimated based on fracture mechanics.