Effect of defocus distance and energy density on melt pool morphology, surface roughness, and grain orientation of 70/30 copper-nickel alloy in laser powder bed fusion

Abstract

This study examined the impact of defocus distance (D) and volumetric energy density (E_v) on the melt pool morphology, surface properties, and microstructure of a pre-alloyed 70/30 copper‑nickel fabricated using laser powder bed fusion (LPBF). Samples were created with defocus distances ranging from −20 mm to 20 mm and energy densities between 125.0 and 381.0 J/mm³. Analysis methods included optical microscopy, surface topography, and electron backscatter diffraction. A defocus distance of −10 mm combined with moderate energy density (125.0–222.2 J/mm³) yielded deep V-shaped melt pools, low surface roughness (Ra = 23.6 μm), well-aligned crystallographic textures with a Multiple of Uniform Density (MUD) of 6.59. Positive defocus (D = 20 mm) with high energy density produced shallow, disk-like melt pools with acceptable roughness, favoring (001) crystallographic texture. Although deep V-shaped melt pools enable enhanced penetration, they are more susceptible to defects caused by keyhole formation. Conversely, shallow disk-shaped conduction mode melt pools offer better stability and fewer surface irregularities, but they may suffer from insufficient penetration and other drawbacks. These findings highlight the potential of using defocus parameter as a key design parameter to optimize melt pool characteristics for specific material and process requirements in LPBF.