Impact of Chemical Composition Changes during Ultrasound Atomization and Laser Powder Bed Fusion of Low Alloy Steel

This study investigates the effect of changing the chemical composition during ultrasonic atomization (UA) and laser powder bed fusion (LPBF) of low-alloy steel. UA is used to produce a spherical powder with d50 equal to 49 mu m. During UA, the chemical composition of the material changes, which is associated with selective evaporation of Mn from 1.42% to 0.35% and B from 0.0012% to <0.0001%. Thermodynamic calculations confirm that during atomization, mostly Mn and Fe evaporate. To achieve a high density of 3D printed parts, in situ remelting in LPBF is applied. A microstructure consisting of fine grains of tempered martensite and bainite in crystallized meltpools is observed. The selected high-quality LPBF samples are austenitized in the temperature range of 900-1200 degrees C for 20 min and quenched in oil. The samples are characterized by light and scanning electron microscopy, as well as Vickers hardness. Changes in chemical composition result in a decrease in the hardenability of the material, and quenching only at 1200 degrees C produces a martensitic microstructure. LPBF samples show a hardness higher than that of the postheat-treated sample, but still significantly lower than that of the as-delivery condition, which is related to the change in chemical composition.