Characterization of Laser Direct Deposited Magnesium Aluminate Spinel Ceramics

An additive manufacturing (AM) approach, via laser direct deposition, is investigated in printing transparent magnesium aluminate spinel (MAS) ceramics. Using AM, traditionally difficult or expensive to manufacture shapes, such as optical lenses, can be rapidly manufactured to near net shape, reducing time consuming and expensive post processing requirements. The transparency of MAS ceramics is highly dependent on the microstructure, with porosity and microcracking having the largest effect on the transparency of fabricated parts. With high localized heat inherent in the laser deposition process, the microstructure of ceramic parts can be controlled by adjusting processing parameters. In this study, thin wall MAS structures were fabricated by varying processing parameters. Processing parameters including laser scan speed and laser power had a large influence on the part quality. To fabricate transparent magnesium aluminate spinel ceramics with high mechanical properties, the effects of processing parameters on part porosity, density, and microstructure were studied. Dense MAS parts were successfully fabricated through the laser direct deposition process. Low scan speed and high laser power showed the most promising results in fabricating MAS parts of low porosity. Using a low powder flow rates of 0.58 g/min, a relative density of nearly 98% was achieved. Directional cooling through the substrate and from the powder conveying gas led to columnar grain growth at a tilt angle from the build direction. The primary defects of fabricated MAS ceramics were found to be residual porosity and microcracking, which negatively affected part transparency and mechanical properties. Typical microcracking patterns included transverse and longitudinal cracking, with longitudinal cracks being more prevalent due to the existence of columnar grains and intergranular fracture mode. A preliminary study demonstrated that a certain degree of transparency was achieved in additively manufactured MAS ceramic parts via laser direct deposition.