Supersolidus liquid phase sintering of water-atomized low-alloy steel in binder jetting additive manufacturing.

This work investigates the feasibility of net shape manufacturing of parts using water-atomized (WA) low-alloy steel with comparable densities to conventional powder metallurgy parts via binder jetting additive manufacturing (BJAM) and supersolidus liquid phase sintering (SLPS). In this study, a modified water-atomized powder grade with similar composition as MPIF FL-4405 was printed and pressure-less sintered under a 95% N2-5% H2 atmosphere. Combinations of two different sintering schedules (direct-sintering and step-sintering) and three different heating rates (1, 3, and 5 °C/min) were applied to study the densification, shrinkage, and microstructural evolution of BJAM parts. This study demonstrated that, although the green density of the BJAM samples was ∼42% of the theoretical density, the sintered parts experienced large linear shrinkage up to ∼25% and reached ∼97% density without compromising shape fidelity. This was ascribed to a more homogeneous pore distribution throughout the part before ramping up to the SLPS region. The synergistic effects of carbon residue, the slow heating rate, and the additional isothermal holding stage at the solid-phase sintering region were determined to be the key factors for sintering BJAM WA low-alloy steel powders with resulting minimal entrapped porosity and good shape fidelity.

Effect of immersion time on the passive and electrochemical response of annealed and nano-grained commercial pure titanium in Ringer's physiological solution at 37°C.

In the present study, various electrochemical tests were used to investigate the passive and electrochemical response of annealed and nano-grained commercial pure Titanium in Ringer's physiological solution at 37°C. Nano-grained pure Titanium, which typically has an average grain size of 90±5nm, was obtained by six-cycle accumulative roll bonding process. Polarization and electrochemical impedance spectroscopy plots illustrated that as a result of grain refinement process, the passive response of the nano-grained sample was improved compared to that of its coarse-grained counterpart in Ringer's physiological solution. Mott-Schottky analysis indicated that the passive films behaved as n-type semiconductors in Ringer's physiological solution and grain refinement did not change the conductivity type of the passive films. Additionally, Mott-Schottky analysis showed that the donor density decreased by grain size reduction. Finally, nano-grained sample appeared to be more suitable for implant applications, mainly due to the formation of thicker and less defective oxide film.