Fabrication of foam metal in directed energy deposition applying titanium hydride

Abstract

Directed Energy Deposition (DED), which is one of the Additive Manufacturing (AM) processes, can deposit metallic production efficiently by irradiating a high-power laser beam on a baseplate and supplying material powder simultaneously. DED has a problem in low mechanical strength due to residual pore inside of the production. However, the residual pore is also available to fabricate low density production as an opponent idea. If the residual pore could be increased intentionally and drastically, DED is applicable to foam metal fabrication. Foam metal could be produced continuously with high degree of freedom in shaping, if melt-pool can be foamed in DED. Therefore, this paper proposes a simple fabrication method for foam stainless alloy with DED by applying titanium hydride (TiH₂) as a foaming agent for melt pool. TiH₂ releases hydrogen in a high temperature environment as more than 500℃. Considering that high viscosity and low surface tension are required to stabilize pore generation, material powder also should contain a thickening agent (CaO: calcium oxide) and a surface active agent (Te: tellurium). According to the experimental results, an effect of CaO was hardly observed because the liquid stainless may already have high viscosity enough. On the other hand, Te certainly reduced the critical film thickness and prevented the pore collapse in stainless foam fabrication, according to the porosity rate calculation, the observation with a high speed camera, and the elemental distribution analysis. Furthermore, the proposed method successfully produced a foam structure in stainless steel with high porosity rate as 61% at maximum.