By: Andre Streek
One of the techniques for additive or “generative” fabrication of mechanical parts is selective laser sintering. It belongs to the type of processes by which the products are generated layer-wise. To these ends the construction of the body is fractionalized into slices corresponding to the compartments of which the product becomes compounded during fabrication. In selective laser sintering these compartments are produced by selectively solidifying the material of successively coated powder or slurry layers by laser radiation, according to the cross section of the corresponding slice. The regime of the laser process was originally designed to fuse the irradiated material by melting and consecutive solidification. Real sintering as known from furnace sinter processes was observed only in few exceptional cases.
Until 2002 the resolution limit for selective laser sintering was around 100µm; the lower limit of the powder grain size was 20µm, this was also the smallest thickness of a sinter layer.
In order to improve the resolution further, powders with grain diameters between 1µm and 10µm were applied. These narrow grained powders, however, caused unknown difficulties because of their typical fluffy consistency. Their processing afforded a special laser regime with short (≈ 100ns) high-intensity pulses. Those pulses are yielded by a Nd:YAG-Laser operated in the Q-switched mode. To avoid systematic blow off of the relatively fuzzy powder layer ahead of a line-wise moving laser beam, the pulses were positioned stochastically over the cross section of each slice. The process with this regime, named Laser Micro Sintering (LMS), allowed the generation of parts with an optimum resolution of 30µm. The drawback of the special laser regime, however, was the relatively low density of the sintered compounds.
In the conference paper an upgraded version of Laser Micro Sintering, HD-LMS (for “high-density laser micro sintering”), is presented. It was designed and developed based on the analysis of the sinter-mechanism of the original LMS. The improvements rely on jam-packing the powder into the surface pores of the previous sinter layer by a partially grinding coating technology. Subsequent remelting of the powder filled surface with a modified laser regime results in a corresponding volume gain of the sintered body. As long as the porosity is reproduced, there is no limit for the number of repetitions.
The new HD-LMS has proved suitable for the production of intricate mechanical components with reliably resolution of 25µm and a material density of above 95%.
Laser Micro Sintered turbocharger processed from molybdenum powder. Left: SEM view; center: cross-section view; right: 3D computer tomography
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here