Detection of Aberrations in Bulk Density in Additive Manufactured 300 M Steel using X-ray Radiographic Testing and Ultrasonic Testing

The laser additive manufacture (LAM) alloy fabrication process employs directed energy to fuse powder feedstock to produce a predetermined geometric form with unique microstructures and defects that are unlike other fabrication approaches. This study examines the efficacy for non-destructive inspection approaches to detect aberrations in bulk density (or defects) that are inherent in materials fabricated with a directed energy LAM process. Ultrasonic testing (UT) and radiographic testing (RT) are investigated for their capacity to detect seeded defects or deviations in bulk density within in LAM 300 M steel specimens. Intermittent adjustment of LAM hatch speed and hatch spacing is used to produce twelve 300 M steel specimens with bulk densities from 98 to 100%. A modified Archimedes' method is used to measure relative (to wrought 300 M steel) bulk density. Densities from 98.22 to 99.85% were determined with a standard deviation of < 0.4% at a 95% confidence interval. UT and X-ray RT results showed clear qualitative differences between high and low-density LAM 300 M steel specimens and the full density 300 M wrought reference. UT A-Scans of LAM 300 M steel revealed multiple indications prior to and subsequent to the 1st backwall reflection and the absence of 3rd and 4th backwall reflection multiplets in LAM specimens. The UT A-scan signal amplitude threshold (with 5 and 10 MHz probes) is near 99.85% for detection of density aberrations in LAM 300 M steels. After refinement of the contrast in radiographic images, it was possible to use RT to detect defects in the specimens with bulk density up to 99.5%. Results provide insight into the development of non-destructive inspection procedures for in-situ process monitoring and qualifying parts.

Automated summary

The study examined the effectiveness of non-destructive inspection methods to detect material density aberrations in the directed energy LAM process. Different LAM 300 M steel specimens were produced with varying bulk densities, and UT and RT results were used to differentiate between high and low-density specimens. The research also explored methods for detecting defects in specimens with bulk density up to 99.5% using refined contrast in radiographic images.