Influence of Notch Effects Created by Laser Cutting Process on Fatigue Behavior of Metastable Austenitic Stainless Steel

Laser cutting is an attractive and innovative manufacturing process which has many advantages compared to conventional cutting methods. However, with increasing workpiece thickness an increase of the roughness along the kerf surface can be observed, which, in turn, can negatively affect the mechanical properties, in particular the fatigue strength. In this context, the purpose of the present study is to investigate the impact of the geometrical surface characteristics and microstructural changes after laser cutting in order to support the cutting process optimization concerning cyclic durability. Fatigue strength evaluation is performed with specimens cut out by high-power solid-state disk laser from sheets with thickness of 2, 4 and 6 mm made of metastable austenitic stainless steel type 304. Cyclic tests are carried out using a resonant pulsation testing system at test frequencies around 100 Hz at two different load modes, purely reversal load condition (R = -1) and tensile-tensile load condition (R = 0.1). In order to evaluate separately the effect of surface relief over the cutting kerf and burr in form of re-solidified drops, the fatigue specimens are tested at different surface conditions. The investigation comprises fractographic analyses in order to evaluate the influence of the surface roughness and surface-related macro defects on crack initiation. Additionally, phase analyses are performed to assess the deformation-induced phase transformation during cyclic testing and its influence on fatigue behavior, as well as microstructural investigation to analyze the material microstructural changes during the cutting process and its impact on material mechanical properties. The influence on fatigue strength of parts cut by laser is quantified and the characteristic dominating the fatigue life is identified.