Measurement of focal plane error in laser powder bed fusion machines

Amongst the many sub-systems that make up laser powder bed fusion (PBF-LB) machines, the optomechanical sub-system stands out due to its potential for off-nominal performance but incommensurate level of study on performance evaluation. Nominally, the optomechanical system focuses the laser onto a planar field which is at a controlled position and orientation relative to the powder bed. Deviations from this assumed condition, sometimes referred to as defocus or focus offset, have the potential to significantly impact the manufacturing process by influencing the energy intensity at the process zone. Herein, a novel, high-throughput, low-cost, artifact-based methodology to measure focus offset is detailed. In a single continuous build process, tracks at varying offsets from the build plane were created by ablating the coating on discrete coupons located throughout the build area. By examining these track widths, the focus offset was determined at a relatively fine spatial resolution over the build space, down to 25 mm intervals along the x and y directions, thus ascertaining the discrepancy between the laser focal plane and the build plane, i.e., focal plane error. Results were found to agree with reference measurements to within 0.27 mm over the entire build space and defocus levels ranging from approximately -1.6 to 1.7 mm were discovered. Field sag and optomechanical misalignment were the major casual factors. It is concluded that similar or more severe levels of defocus may be present in the typical PBF-LB machine, which may impart considerable impacts to the overall PBF-LB process.