Energy efficiency contributions and losses during selective laser melting

摘要

Selective laser melting (SLM) technique requires remelting of adjacent tracks to avoid cavities and other imperfections. Usually, very conservative process parameters are chosen to avoid imperfections, resulting in a low building rate. The process efficiency relates the energy required for the generation of a new track to the laser beam power. For SLM, this efficiency is determined by the process parameters, specifically hatch distance, layer depth, and scanning speed, independent of the resulting process mechanisms. For SLM, the process efficiency is often very low, typically 2%-20%. Apart from beam reflection losses of normally 50%-60%, significant energy losses result from the remelting of surrounding layers. Some areas can even experience multiple remelting cycles. Further losses originate inevitably from substrate heating. A simplified mathematical model of the track cross section and the corresponding layer overlap geometry has been developed to analyze the different loss contributions from remelting with respect to the process parameters. The model explains why increasing the hatch distance or the layer depth proportionally increases the process efficiency. However, these increases are limited by cavity formation. The cross section of the overlapping tracks generated by SLM can be regarded as an experimental fingerprint linked to the process conditions. The track cross section geometries can significantly fluctuate in terms of area and coordinate position. The fluctuations require additional reduction of the hatch distance or layer depth to ensure robust, cavity-free processing. Examples are presented for stainless steel where a 180 W laser beam has led to a process efficiency of 5%-11%, proportional to a hatch distance that was increased from 50 to 110 mu m, for 40 mu m powder layer depth, at a speed of 50 m/min. (C) 2018 Laser Institute of America.