Nonlinear/linear resonance ultrasound spectroscopy (N/RUS) of additively manufactured 316L stainless steel samples using in-contact and non-contact excitation

Abstract

Due to their sensitivities to defects and microstructural changes, the linear and nonlinear parameters measured with nonlinear resonance ultrasound spectroscopy (NRUS) have the potential to evaluate the quality of additively manufactured parts. Here, NRUS with in-contact and non-contact excitation configurations is used to test a set of 316L stainless steel samples manufactured by laser powder bed fusion. Subgroups of the samples are heat treated to three temperatures to alter the samples' microstructure and mechanical performance. Results show that the hysteretic nonlinearity parameter measured with in-contact NRUS demonstrates sensitivity to heat-treatment-induced microstructural changes. However, due to the unwanted nonlinearity introduced by the bonding, variations in the repeated in-contact measurements of a single sample are almost as large as the differences between the heat treatment subgroups. The non-contact configuration allows measurement of two linear parameters (resonance frequency and damping) with exceptional repeatability, and results show clear distinction between samples in different heat treatment groups. The non-contact NRUS configuration could not produce a sufficient dynamic strain amplitude to allow a reliable measurement of the samples’ nonlinear response. In-contact and non-contact NRUS results are analyzed in relation to independently measured mechanical properties and microstructural information to investigate the feasibility of NRUS for the nondestructive evaluation of additively manufactured metals.