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Abstract:
Although intensively researched in the last decade, additive manufacturing techniques are still subject to certain shortcomings, such as internal porosity or anisotropy of properties. In this work, we investigate the effects of postprocessing via hot isostatic pressing (HIP) on the properties of the titanium alloy Ti6Al4V produced via two various additive manufacturing methods: laser powder bed fusion (LPBF) and cold spray (CS). The LPBF produced a microstructure composed of a mixture of hcp-structured α(Ti) and α’(Ti), whereas α(Ti) and bcc-structured β(Ti) were recognized in the CS-ed materials. The subsequent HIP treatment did not cause reformation of the LPBF microstructure. This process had a significant effect on the densification of the CS-ed samples. To study the anisotropy of the material properties, the tensile and compressive strengths of the materials were determined in the planes parallel and perpendicular to the building (LPBF) and spraying (CS) directions. The LPBF materials exhibited significantly better mechanical properties stemming from the typical α/α’ martensitic microstructure, whereas the CS-ed materials presented a high number of pores and smooth and textured regions composed of recrystallized grains and grains with a high number of dislocations, respectively. The HIP treatment led to a reduction in porosity, causing a significant increase in the mechanical properties (UTS by 132%) and a reduction in the UTS anisotropy in the CS-ed materials

Keywords:
Ti6Al4V, Laser powder bed fusion, Cold spray, Hot isostatic pressing, Mechanical properties

Abstract:
A novel metal matrix composites (MMC) with Mg matrix reinforced with natural filler in the form of Didymosphenia geminata frustules (algae with distinctive siliceous shells) are presented in this work. Pulse plasma sintering (PPS) was used to manufacture Mg-based composites with 1, 5 and 10 vol.% ceramic filler. As a reference, pure Mg was sintered. The results show that the addition of 1 vol.% Didymosphenia geminata frustules to the Mg matrix increases its corrosion resistance by supporting passivation reactions, and do not affect the morphology of L929 fibroblasts. Addition of 5 vol.% the filler does not cause cytotoxic effects, but it supports microgalvanic reactions leading to the greater corrosion rate. Higher content than 5 vol.% the filler causes significant microgalvanic corrosion, as well as increases cytotoxicity due to the greater micro-galvanic effect of the composites containing 10 and 15 vol.% diatoms. The results of contact angle measurements show the hydrophilic character of the investigated materials, with slightly increase in numerical values with addition of amount of ceramic reinforcement. The addition of Didymosphenia geminata frustules causes changes in a thermo-elastic properties such as mean apparent value of coefficient of thermal expansion (CTE) and thermal conductivity (λ). The addition of siliceous reinforcement resulted in a linear decrease of CTE and reduction in thermal conductivity over the entire temperature range. With the increasing addition of Didymosphenia geminata frustules, an increase in strength with a decrease in compressive strain is observed. In all composites an increase in microhardness was attained.
The results clearly indicate that filler in the form of Didymosphenia geminata frustules may significantly change the most important properties of pure Mg, indicating its wide potential in the application of Mg-based composites with a special focus on biomedical use.

Keywords:
Metal-matrix composites (MMCs),Pulse plasma sintering (PPS),Ceramic filler,Microstructure,Properties