Improving the fatigue property of 316L stainless steel through direct energy deposition technology

Abstract

In this study, to improve the fatigue strength of coarse grain 316L stainless steel, the hard 17-4ph martensitic stainless steel was deposited on the surface of hot rolled 316L based on direct energy deposition (DED) additive manufacturing technology. The 17-4ph deposition and 316L substrate are well connected without any cracking phenomenon which lays the foundation of good mechanical properties. The 17-4ph deposition presents a gradient structure, its fraction of body centered cubic (BCC) phase gradually increases from 0 vol% of substrate to 99.3 vol% of top surface, and its grain size gradually decreases from 53.2 μm near the substrate to 5.1 μm of the top surface, leading to the hardness of deposition gradually increases from 187 Hv to 436 Hv. The fatigue property of 17-4ph coated 316L is significantly improved. Specifically, the fatigue strength in high cycle fatigue (HCF) regime (at the fatigue life of 2 × 106 cycles) is 64 % higher than coarse-grained 316L, while the increment in low cycle fatigue (LCF) regime (at the fatigue life of 104 cycles) is up to 100 %. The improvement in LCF regime of 17-4ph coated 316L is higher than that of HCF regime, which is exactly opposite to the improvement in HCF and LCF regime of samples fabricated by traditional surface strengthening processes. Based on advanced characterization techniques, the potential strengthening mechanism in 17-4ph coated 316L was studied. It is expected that this study can provide reference for the application of additive manufacturing technology in the field of surface strengthening.