The Effect of Severe Shot Peening on Fatigue Life of Laser Powder Bed Fusion Manufactured 316L Stainless Steel

Abstract

Severe shot peening (SSP) was used on additive manufactured 316L by laser powder bed fusion. The effect of the post processing on the surface features of the material was analyzed through residual stress measurements, tensile testing, hardness-depth profiles, and fatigue testing by flexural bending. The results showed that SSP can be utilized to form residual stresses up to −400 MPa 200 μm below the surface. At the same time, a clear improvement on the surface hardness was achieved from 275 HV to near 650 HV. These together resulted in a clear improvement on material strength which was recorded at 10% improvement in ultimate tensile strength. Most significantly, the fatigue limit of the material was tripled from 200 MPa to over 600 MPa and the overall fatigue strength raised similarly from a low to high cycle regime.

Fatigue Behavior of Additively Manufactured Stainless Steel 316L

“…In particular, in [50] higher values for the roughness of vertically built specimens were reported (AB condition), and correspondingly higher fatigue strength for XY build directions in comparison with Z. Similarly, in [41] DMLS 316L specimens built vertically produced fatigue strengths at lifetimes of 10 5 -10 6 cycles of only about 30% of that generated by horizontally-built DMLS material, as also reported in [67], in which case data refer to stress-relieved condition. Investigations on machined or polished samples were reported in [45,46,59,81], again showing improved fatigue properties for the horizontal samples.…”

“…The most common finishing conditions are turned, ground, and polished, though some studies simply refer to machined conditions. Fatigue life curves for L-PBF 316L, again considering only samples not subjected to heat treatment, are reported in Figure 2 for the machined or turned condition in which case R a is in the range of 0.6-1.8 µm and R z is 3.7-11 µm, although this essential information is not always reported [38,40,41,51,57,67]. As observed, the comparison with AB is generally favorable because the roughness of AB specimens is relatively high.…”

“…An alternative to machining or polishing operations is represented by surface treatments, which may alter roughness, usually by mechanically reducing heights of peaks and valleys of the profile and/or inducing a compressive stress state. For AM 316L, different approaches were reported in the literature, including Shot Peening (SP) [51,67], Laser Shot Peening (LSP) [68], High-Frequency Mechanical Impact (HFMI) [57], and vibratory finishing process (VF) [43]. A summary of the life curves under these conditions is reported in Figure 5.…”

“…Overall surface treatments based on shot peening seem to improve fatigue resistance at levels comparable with machined surfaces. One of the investigations [67] reported extremely high values for applied cyclic stress, with fatigue strength in the LCF regime even exceeding the static strength of the AB material, which could be attributed to the ex- Overall surface treatments based on shot peening seem to improve fatigue resistance at levels comparable with machined surfaces. One of the investigations [67] reported extremely high values for applied cyclic stress, with fatigue strength in the LCF regime even exceeding the static strength of the AB material, which could be attributed to the extremely high level of compressive stress induced by severe SP.…”

Additively Manufactured 316L Stainless Steel Subjected to a Duplex Peening-PVD Coating Treatment

“…In a study carried out by Santa-aho et al [ 12 ], surface compressive stresses reaching up to 502 MPa were generated in the additively manufactured 316L SS, up to a depth of 80 µm. The compressive stresses generated in the AM 316L SS were quickly formed as the shots impacted the surface, removing the tensile stresses generated by the printing process [ 12 ], thereby enhancing fatigue performance [ 13 , 14 ]. Concurrently, shot peening produces an increase in the hardness of the surface and near-surface layers of the material [ 15 , 16 , 17 , 18 ].…”