Porosity characterization of additively manufactured transparent MgAl2O4 spinel by laser direct deposition

Abstract

Polycrystalline magnesium aluminate (MgAl2O4) spinel has established itself as one of the leading candidates for use as transparent, high strength ceramics. In this study, additive manufacturing of spinel ceramics by laser direct deposition was investigated with a particular focus on porosity characterization. Residual porosity was the most significant factor hindering transparency of spinel ceramics. Hence, this paper systematically studied how processing conditions and initial powder particle size affected porosity and densification of laser direct deposited spinel samples. Scan speed, laser power, and powder flow rate, along with powder size, were all shown to have substantial influences on density and porosity of produced parts. High density bulk spinel ceramics, with average densities of nearly 98%, were produced at a low powder flow rate of 0.58 g/min. In addition, the pore size distribution was studied under various processing conditions as it was closely related to transmission properties of transparent spinel at different wavelengths. The reduction in average porosity was mainly attributed to a significant reduction in large pores (>30 μm) whereas less obvious effects were found on smaller pores. An increase of scan speed resulted in an initial increase in density, followed by a subsequent decrease at scan speeds higher than 2000 mm/min. No obvious difference in porosity was observed between pulsed mode and continuous mode during laser deposition. The use of nanoparticles in initial MgO–Al2O3 mixtures for deposition also promoted densification of the deposited samples. A systematic understanding of residual porosity in this study can help process optimization to minimize porosity in additively manufactured transparent spinel ceramics, potentially eliminating the needs of time-consuming and expensive post-processing procedures.