Cyclic plastic material behavior of 316L manufactured by laser powder bed fusion (PBF-LB/M)

Abstract

In laser powder bed fusion of metals (PBF-LB/M), the influence of laser parameters on the static mechanical properties is well known. Different laser parameters result in a higher or a lower tensile strength. Only little is known about the influence of the laser parameters on the cyclic plastic behavior at high strain amplitudes. Different laser parameters may result in a significant change in the stress levels during cyclic loading. In this study, the cyclic plastic material behavior at high strain amplitudes of up to 3.0% of AISI-316L manufactured by PBF-LB/M is investigated. Two different laser parameter sets are applied. One laser parameter set is outside the stable melting zone and the other one is inside the stable melting zone according to [1]. Metallographic analyses and static, as well as strain-controlled fatigue tests with a strain ratio of R = −1 were conducted. The plastic material behavior is dominated by softening, which is reduced with increasing strain amplitude due to mechanical twinning. Indeed, for strain amplitudes >2.5% twin-induced strain hardening is observed. A correlation between the twin-induced strain hardening behavior and the resulting maximum stress curve per endured cycle is observed. The laser parameters influence the generated stress level during cyclic plastic loading. It is shown, that the porosity highly affects the fatigue life in the low-cycle fatigue range. Finally, the strain-life curve is determined and approximated by the Manson-Coffin fatigue parameters. • A higher porosity leads to a lower number of cycles until failure resulting in an early failure of the specimens. • From a strain amplitude of 2.0% and above, the primary softening rate decreases. • From a strain amplitude of 2.0%, a secondary hardening behavior is partly observed regardless of the laser parameters. • The secondary hardening behavior can be traced back to strain induced mechanical twinning. • It is assumed that the twinning induced hardening, resulting from a higher strain, leads to a slower softening rate.