Comparison of Volumetric to Surface Heating for Filament–Fed Laser Heated Additive Manufacturing of Glass

This paper presents work using a laser-heated fiber-fed technique to deposit fully dense glass. A stationary laser beam is focused on the intersection of a quartz filament with the workpiece. The workpiece is articulated on a precision 4-axis stage. The laser beam continuously melts the glass filament so that its viscosity is low enough to wet and fuse the workpiece. The focus of this paper is to compare volumetric heating of the glass as opposed to surface heat flux. CO ₂ laser radiation (λ = 10.6 µm) strongly couples to the silica phonon mode (optical penetration depth < 5 µm). This requires printing at very slow rates in order to allow the heat to diffuse from the surface of the glass to the interface of the filament and the workpiece. CO laser radiation (λ = 5.3 µm) provides volumetric absorption because of weaker coupling (optical penetration depth of ∼500 µm for fused quartz). This produces a more uniform temperature profile in the glass filament and supports deposition at greater speeds. The maximum deposition rates for 0.5 and 1.0 mm diameter fused quartz filaments are determined by extrapolating the power required to achieve wetting using both CO ₂ and CO lasers. The results show that volumetric heating (CO laser) produces surface wetting with significantly lower power. The results are compared to a 1D conduction model which suggests that still greater deposition speeds are possible as the optical penetration depth approaches the filament diameter.