Improving Fatigue Limit and Rendering Defects Harmless through Laser Peening in Additive-Manufactured Maraging Steel

Abstract

Additive-manufactured metals have a low fatigue limit due to the defects formed during the manufacturing process. Surface defects, in particular, considerably degrade the fatigue limit. In order to expand the application range of additive-manufactured metals, it is necessary to improve the fatigue limit and render the surface defects harmless. This study aims to investigate the effect of laser peening (LP) on the fatigue strength of additive-manufactured maraging steel with crack-like surface defects. Semicircular surface slits with depths of 0.2 and 0.6 mm are introduced on the specimen surface, and plane bending-fatigue tests are performed. On LP application, compressive residual stress is introduced from the specimen surface to a depth of 0.7 mm and the fatigue limit increases by 114%. In a specimen with a 0.2 mm deep slit, LP results in a high-fatigue-limit equivalent to that of a smooth specimen. Therefore, a semicircular slit with a depth of 0.2 mm can be rendered harmless by LP in terms of the fatigue limit. The defect size of a 0.2 mm deep semicircular slit is greater than that of the largest defect induced by additive manufacturing (AM). Thus, the LP process can contribute to improving the reliability of additive-manufactured metals. Compressive residual stress is the dominant factor in improving fatigue strength and rendering surface defects harmless. Moreover, the trend of the defect size that can be rendered harmless, estimated based on fracture mechanics, is consistent with the experimental results.

Evaluation of Fatigue Limit Improvement and Harmless Crack Size of Maraging Steel Using Shot Peening

“…Therefore, if surface cracks can be made harmless in terms of the fatigue limit through peening, the reliability of the metal can be improved and the industrial usability of the part can be increased. Surface cracks of metal can be made harmless by various types of peening [13][14][15][16][17]. Tsuji et al were able to render harmless semicircular surface cracks with a depth of less than 0.2 mm using shot peening (SP) on carburized steel [13].…”

“…Semicircular surface cracks with depths of less than 0.3 mm and 0.8 mm could be rendered harmless by ultrasonic needle peening on STS316L [14] and DNV F690 [15], respectively. Park et al [16] and Takahashi et al [17] could rendered harmless the semicircular surface cracks smaller than 0.2 mm using SP and laser peening for A6061-T6 and Additive-Manufactured maraging steel (unaged), respectively. However, although the effect of SP on the fatigue strength of maraging steel has been studied, the size of cracks that can be rendered harmless using SP is unknown.…”