Direct 3D Printing of Silica Doped Transparent Magnesium Aluminate Spinel Ceramics

Transparent magnesium aluminate spinel ceramics were additively manufactured via a laser direct deposition method in this study. With a minimum porosity of 0.3% achieved, highly transparent spinel samples with the highest total optical transmittance of 82% at a wavelength of 632.8 nm, were obtained by a 3D printing approach. However, cracking was found to be a major issue affecting printed spinel samples. To control prevalent cracking, the effect of silica dopants was investigated. Increased silica dopants reduced average total crack length by up to 79% and average crack density by up to 71%. However, a high dopant level limited optical transmission, attributed to increased porosity and formation of secondary phase. Further investigation found that with decreased average fracture toughness, from 2.4 MPa·m1/2 to 1.9 MPa·m1/2, the obvious reduction in crack formation after doping was related to decreased grain size and introduction of softer secondary phase during deposition. The study demonstrated the feasibility of the proposed laser direct deposition method in directly fabricating transparent spinel ceramics while dopants showed potentials in addressing cracking issues.

Effects of Processing Conditions on Laser Direct Deposited Alumina Ceramics

Additive manufacturing (AM) of high-quality inherently brittle ceramics via laser direct deposition, also known as laser engineered net shaping, is challenging due to high thermal gradients, thermally induced cracks, and porosity, typically accompanied by low powder usage efficiency. It is necessary to have an improved understanding of the effects of processing conditions on the fabricated ceramic parts. In this study, alumina ceramics were fabricated with commercial alumina powders. In particular, this paper studied the effect of laser power and scan speed on single track geometries, thin-wall morphology, grain size, density, and powder efficiency during laser direct deposition of alumina ceramics. A single-track parametric study was first conducted to determine the range of processing parameters that produce quality single-tracks and to aid in matching the z-increment with layer thicknesses. The results showed that increased scan speeds promoted significant grain refinement. Average grain size was reduced by nearly 50% when scan speed was increased from 1000 mm/min to 5000 mm/min. On the other hand, densification reached a maximum of 98% at a scan speed of 1000 mm/min with a slight decrease in density was observed at higher speeds. This indicated a trade-off between porosity and grain size when altering the laser scan speed. Significant advantages of using CO2 lasers for AM of ceramics was also demonstrated with powder usage efficiencies reaching nearly 90% under optimized processing conditions.

TI-6AL-4V ALLOY FABRICATED BY LASER DIRECT DEPOSITION: DYNAMIC MECHANICAL PROPERTIES, CONSTITUTIVE MODEL, AND NUMERICAL SIMULATION

The dynamic mechanical properties of Ti-6Al-4V alloy prepared by laser direct deposition (LDD) at different strain rates are of great significance for the application of LDD technology in the manufacture and repair of aero-engine parts. The quasi-static tensile test and dynamic compression test of Ti-6Al-4V alloy prepared by LDD (LDD-Ti-6Al-4V) were carried out under the quasi-static and high strain rate using INSTRON-5982 tensile test equipment and Split Hopkinson pressure bar (SHPB) equipment. The true stress–strain curve is obtained, which indicates that the LDD-Ti-6Al-4V has a strain rate strengthening effect. Moreover, the Johnson–Cook (J–C) constitutive model of LDD-Ti-6Al-4V was fitted based on experimental data, and the experimental process of SHPB was numerically simulated. The simulation results are basically the same as the experimental results, which proves the correctness of the J–C constitutive model of LDD-Ti-6Al-4V.