Characterizing particle emissions from a direct energy deposition additive manufacturing process and associated occupational exposure to airborne particles

This study aims to characterize airborne particles emitted from a metal additive manufacturing machine and related levels of occupational exposure. To achieve this, a complete measurement methodology was deployed around a direct energy deposition machine. Different operating conditions were investigated, based on configurations of two materials and two injection nozzles. Two replicates were performed for each condition. Airborne particles emitted during repeated manufacturing cycles were measured simultaneously at the source, in the near field, in the far field and on the operator. Real-time instruments were used to characterize the machine emissions (10 nm–10 µm) associated with respirable and inhalable samplers and cascade impactors. Measurements were made during both the manufacturing process and transient operating phases. In parallel, personal exposure to hexavalent chromium was assessed. The number of particles measured for the different machining phases show that high levels of particles (> 5 × 10⁵ # cm⁻³, 0.3–1.3 mg m⁻³ inhalable particles, 0.2–6 µg m⁻³ Cr^VI) were emitted in the machine enclosure. The size distributions indicate that more than 90% of the particles are smaller than 250 nm. Occupational exposure to Cr^VI was found to be below the LOQ of 0.098 µg m⁻³ for the two alloys investigated. During the machining process, near-field number and mass concentrations were ∼ 10⁴ # cm⁻³, and below 0.04 mg m⁻³, respectively. Far-field number concentrations were also on the order of 10⁴ # cm⁻³ throughout the whole monitoring period. The transient phase of door opening was found to result in high levels of exposure (> 10⁵ # cm⁻³), which were also detected in the near-field, confirming the need to implement preventative actions. To address this issue, a collective protective measure, consisting of setting a time delay of about 8 min between the end of the manufacturing process and opening of the door, could be employed. This collective measure should also be accompanied by the wearing of personal protective equipment by the operator when an intervention in the machine enclosure is necessary.