On the utility of linear and nonlinear ultrasound for evaluating microstructure-property relationships in laser powder bed fusion 316 L stainless steel

Given the complexity and potential for variability in components fabricated with metal additive manufacturing, the need for efficient, reliable, and nondestructive quality assurance is imperative for advancing its utilization in industry. Here, linear and nonlinear ultrasonic testing is used to nondestructively link microscale features and mechanical properties of 21 distinctly-printed AISI 316 L stainless steel samples manufactured by laser powder bed fusion. Groups of samples were heat-treated to induce microstructural changes under as-built, 600 & DEG;C, 900 & DEG;C, and 1100 & DEG;C conditions. While a linear ultrasonic parameter, wave speed, could not completely differentiate the evolution of microstructure and mechanical properties during heat treatment, the relative acoustic nonlinearity parameter & beta;& PRIME; measured with second harmonic generation demonstrated sensitivity to incremental changes induced by heat treatment including ultimate tensile strength, yield strength, dislocation density, and the presence of oxide inclusions. Precise resonance frequency measurements were obtained using a fully noncontact configuration, and shifts in resonance frequency with heat treatment were connected to changes in average grain area and partial recrystallization. The large number of samples and high volume of ultrasonic testing in this study provide insight into part and measurement variability, which is key in demonstrating the feasibility of ultrasonic evaluation as the nondestructive solution for additively manufactured parts qualification.