SUMMARY

Adoption of additive manufacturing technology in critical scenarios has been limited due to uncertainty in the performance of printed parts. Variations that can inadvertently occur in build chamber environments can alter the microstructure of a part leading to adverse performance characteristics.

This work details the use of nonlinear ultrasound for the characterization of microstructure of additively manufactured stainless steels. Previous work on the use of a nonlinear Rayleigh wave setup is detailed to show the sensitivity of the acoustic nonlinearity parameter, ꞵ, to changes in microstructure. 316L Powder Bed Fusion and 304L Laser Engineered Net Shaping parts are compared to wrought manufactured specimens through an annealing heat treatment. The acoustic nonlinearity parameter is shown to be sensitive to changes in dislocation density in additively manufactured specimens.

A non-collinear non-linear wave mixing technique is developed and applied to the characterization of additively manufactured metals. Two ultrasonic waves are generated to meet specific mixing criterion resulting in the generation of a third wave with an amplitude dependent on the third order nonlinearity constant. The amplitude of this third wave can be quantified through a relative acoustic nonlinearity parameter that is sensitive to changes in the microstructural state within the area of wave mixing. In comparison to previous nonlinear ultrasound setups, this wave mixing setup gives a value of ꞵ within the mixing zone. The measurement setup promises the ability for a spatial mapping of the acoustic nonlinearity parameter. By using phased arrays, a specimen can be scanned at multiple points quickly and without changing the emitting coupling conditions. This experimental setup is used to evaluate an additively manufactured stainless steel specimen created with intentional laser underpowering at specific layers during the manufacturing process.