Research ArticleIn situ selective laser gas nitriding for composite TiN/Ti-6Al-4V fabrication via laser powder bed fusion
Introduction
Surface engineering involving carburizing and nitriding of metals and alloys to improve surface wear and corrosion resistance as well as surface hardness has been utilized for more than a half-century in applications ranging from aerospace and automotive engine and component design, cutting tool and blade optimization, and a variety of orthopedic implant improvements [[1], [2], [3], [4], [5], [6], [7]]. Titanium nitride (TiN) coatings, in particular, have been especially useful since hardness values can range from 800 to 3000 Vickers hardness numbers (VHN) (8–30 GPa); with elastic (Young’s) moduli ranging from ∼ 200 GPa–500 GPa, and a melting point of 2950 °C [1,4,5]. TiN coatings can be developed using physical and chemical vapor deposition (PVD and CVD) techniques as well as electron and laser-beam-assisted PVD and CVD, plasma or plasma-assisted deposition, and laser nitriding involving short-pulse laser irradiation of metals and alloys in nitrogen-containing atmospheres where nitrogen uptake and diffusion can be variously manipulated to produce TiN or TiN-rich surface layers of varying thicknesses on Ti-6Al-4 V [[3], [4], [5], [6],[8], [9], [10]].
This paper describes a novel approach for selectively nitriding Ti-6Al-4 V component surfaces during the additive manufacturing of these components by laser powder bed fusion (L-PBF), which can utilize either argon (Ar) or nitrogen (N2) processing environments: to the authors’ knowledge, an approach not previously developed for metal and alloy l-PBF additive manufacturing (AM) [11]. Laser nitriding of Ti-6Al-4 V components during the AM process produced TiN/Ti-6Al-4 V micro-dendritic surface layers as thick as 230 μm. In addition, TiN embedded layers were also fabricated in Ti-6Al-4 V AM products. These embedded layers ranged in thickness from ∼ 75 μm–150 μm. This novel processing technique is a precursor to developing hybrid, periodic layers of hard and soft (brittle/ductile) TiN/Ti6Al-4 V composites [[12], [13], [14]]. Optical and scanning electron microscopy were employed in characterizing these TiN-rich coatings along with X-ray diffraction (XRD) analysis. Vickers microindentation hardness measurements were performed comparatively on the uncoated and nitride coated Ti-6Al-4 V component surfaces and the embedded TiN layers.
Section snippets
Precursor materials
Precursor Ti-6Al-4 V metal powder sourced from LPW Technologies Inc. (Imperial, PA, USA) was utilized for all experiments. Prior to the experiments described here, the powder had been reused several times in an Arcam A2 (GE Additive, Sweden) electron beam powder bed fusion (EB-PBF) machine. Particle size and shape analyses were conducted on a Retsch Camsizer X2 (Haan, Germany), yielding d10 = 46 μm, d50 = 60 μm, and d90 = 92 μm and sphericity with 88% of particles having a symmetry greater than
Ti-6Al-4 V surface nitride laser processing, microstructures and properties
Table 1 presents a broad overview of the laser-assisted nitride processing parameters while Table 2 shows corresponding TiN secondary dendrite arm spacings, volume fraction measurements, and Vickers microindentation hardness averages for the TiN coatings. Fig. 3 shows two SEM secondary electron (SE) images for TiN surface coating microdendritic microstructures corresponding to processing parameters 7 (Fig. 3(a)) and 14 (Fig. 3 (b)) in Table 1, Table 2. Fig. 3 (a) shows coarse dendrites having
Summary and conclusions
The research results reported in this paper have demonstrated that novel, laser-assisted gas nitriding of l-PBF processed Ti-6Al-4 V can be achieved in-situ by alternating the Ar build gas with N2. Systematic variation of processing parameters can also achieve TiN surface coatings ranging from several tens of microns to several hundred microns; having variations in TiN dendritic microstructure volume fractions ranging from 0.6 to 0.75, with corresponding Vickers microindentation hardness values
Acknowledgements
The research described here was performed at The University of Texas at El Paso (UTEP) within the W.M. Keck Center for 3D Innovation (Keck Center). The authors are grateful to Andres Navarro and Alfonso Fernandez (graduate students), and Michael Nigro and Jake Lasley (undergraduate students) for their assistance with various aspects of this work. Support for this project was provided through the MSI STEM Research & Development Consortium sponsored by the U.S. Army via cooperative agreement
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