Impact of laser scanning strategies on microstructure in laser powder bed fusion (LPBF) of nanoparticle-infused pre-alloyed water-atomized iron powder

Abstract

This study investigates how two scan strategies impact the microstructural and tensile properties of nanoparticleinfused pre-alloyed water-atomized low-alloy iron powder (Fe-1.50Cu-1.75Ni-0.50Mo) during Laser Powder Bed Fusion (LPBF) under uniform volumetric energy density laser conditions. The two strategies were the rotation method (ROT), in which the laser scan vector was rotated by 67 degrees for each layer (10 degrees, 77 degrees, 144 degrees, 211 degrees, ...), and the zig-zag method (ZZ), in which the vector was rotated by 90 degrees for each layer (45 degrees, - 45 degrees, ...). As a result, the relative density of the ROT specimens was measured to be 95.61 +/- 1.01 %, while that of ZZ specimens was 99.76 +/- 0.17 %. Furthermore, the surface roughness of ROT specimens was higher than that of ZZ specimens (24.05 mu m vs. 10.30 mu m). It was found that the scan strategy influences not only the bulk characteristics but also the microstructural changes of the specimens, due to variations in intrinsic heat treatment (IHT). In the ZZ specimens, the regular laser scanning direction resulted in a uniform microstructure across different heights. However, in the ROT specimens, the laser scanning direction varied from layer to layer, with fluctuating heat dissipation influencing the melt pool structure, recrystallized grain distribution, sub-grain cellular structure, and precipitation behavior to obtain lower density and higher surface roughness than those of ZZ samples. Therefore, the samples printed using the ZZ method are found to be advantageous for printing pre-alloyed WA iron powders with uniform microstructure and reliable tensile properties.