Selective laser melting of glass powders

Fateri, Miranda; Conradt, Reinhard (Thesis advisor); Gebhardt, Andreas (Thesis advisor); Roos, Christian Hans-Georg (Thesis advisor)

Aachen (2017, 2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

Glass material is extensively utilized in various industrial fields due to its unique properties such as high melting temperature, transparency, as well as high compression strength; however, conventional glass production processes are not optimum for freeform fabrication applications. As such, fabrication of complex geometries in a time and cost effective manner is not currently realizable. Nevertheless, existing fabrication methods and corresponding constraints may be augmented by Additive Manufacturing (AM) techniques in which free form geometries are fabricated based on Computer Aided Design (CAD) data in a layer-wise manner. With respect to this, this study investigates the feasibility of Selective Laser Melting (SLM) of soda-lime and borosilicate glass powder followed by fabrication of test geometries for industrial applications ranging from micrometer to decimeter in size. Initially, SLM process parameter and scan strategy investigations and optimization for different powder particles sizes of soda-lime glass are carried out. SLM fabricated parts are analyzed regarding their bulk density and surface roughness. Furthermore, systematic SLM process parameter optimization both analytically and experimentally, powder rheology, and analysis of the sintering behavior of the borosilicate glass under various atmospheric conditions with the aim of evaluating the density and surface roughness of the SLM processed parts are among the main objectives of this study. Regarding this, process parameter optimization as well as scan strategy adaptation are carried out using different powder particles. Furthermore, powder rheology is analyzed using Hausner Ratio (HR) and dynamic angle of repose (avalanche angle) measurements. With respect to this, bulk and tapped densities as well as the avalanche angle of powders with different particle sizes, distributions, and shapes are studied and classified regarding their powder flow regimes based on the obtained experimental layer-wise lamination results. Moreover, using Hot Stage Microscopy (HSM) viscosity points of different powder particles sizes and distributions are studied and applied for developing an analytical model which is validated by the systematic process parameter SLM study as well as on site thermal process monitoring. Furthermore, SLM optimum process parameters are applied under different working atmospheres with the aim of studying the surface morphology of the SLM fabricated parts. Finally powder bed’s bulk density is optimized by polydisperse powder mixture preparation while keeping the powders’ lamination quality in the desirable flow regime. Results have shown, although developed optimum scan strategy can be applied for a wide range of powder particle sizes, process parameters need to be optimized for each powder individually. The SLM fabricated parts led to an amorphous structure (within the conducted measurement range of 20 ° ≤ 2θ ≤ 160 ° using CrKα radiation) regardless of the powder particle size. SLM fabricated samples from soda-lime glass powders yield a maximum density of 2.43 g/cm3 and a surface roughness (Ra) of 0.88 µm using a 60 W laser power, 0.067 m/s scan speed, 50 µm hatch spacing and a 150 µm layer thickness. Regarding SLM of borosilicate material, optimum flow lamination quality is achieved by creating angular shape monodisperse powders of 200 µm - 212 µm with a HR of 1.21 and an average avalanche angle of approximately 39 °. However, comparison of the flow dynamics of the rounded edge particles to the angular shape particles concluded that the SLM lamination quality could be further improved by using spherical powders. Using optimum process parameters in which the powder bed reaches the viscosity of the half ball point temperature, parts with a bulk density of 1.86 g/cm³ and a surface roughness (Ra) of 10.75 µm are fabricated. Final part’s bulk density of 2.13 g/cm³ is achieved under synthetic air using the volumetric mixed bidisperse particles of 63 µm - 90 µm and 200 µm - 212 µm powders in a ratio of 42 % and 58 % respectively. The surface roughness (Ra) of the fabricated part is improved from 2 µm to 0.9 µm using the CO2 laser polishing process. For demonstration purposes, complex glass geometries have been manufactured, promoting the possibilities of manufacturing custom, complex components via the SLM process.

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