Characteristics and mechanisms of ductile-to-brittle transition of thin steel plates with coatings fabricated by laser directed energy deposition

Yanle LI, Deshun GAO, Xiaoxia QI, Jiyu DU, Fangyi LI

School of Mechanical Engineering, Shandong University

Journal：《Frontiers of Mechanical Engineering》

Abstract：

Laser directed energy deposition (LDED) is widely used in the remanufacturing and surface strengthening of high-value thin-walled components. However, the transformation mechanism of mechanical properties for substrates induced by laser deposition, especially for thin metal sheets, remains unclear. In this study, the affecting mechanism of Fe304 coatings fabricated by LDED on the ductility of Q235 thin plates was investigated. Samples of original substrate (OS), deposited plates with one to three layers of coating (D1–D3), the substrate zone of deposited plates with one to three layers of coating (D1-S–D3-S), and deposited coating were prepared to evaluate the effect of coating and laser heat on substrate ductility. The Fe304 coating deposited by LDED and the accompanying laser heat drastically reduced substrate ductility by about 87%. Specifically, laser heat reduced the ductility by 24%–51 %. The Fe304 coating deposited by LDED led to the ductile-to-brittle transition of the substrate. Compared with ductile dimples in the OS, cleavage fracture was found in the substrate of deposited plates. Meanwhile, the mechanisms of ductile-to-brittle transition of the substrate were analyzed. Laser heat caused the precipitation of cementite and the generation of a decarburization layer in the substrate. Coating resulted in the pileup of dislocations in the substrate, then nonuniform deformation occurred in the substrate. The cracks of the coating fracture extended to the substrate, inducing local cracking of the substrate. This study provides fundamental guidance for the surface manufacturing of ductile thin-walled components through LDED.

Keywords:laser directed energy deposition; ductile-to-brittle transition, deposited coating, laser heat, thin steel plate

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