Hitesh D. Vora

In Situ Laser Synthesis of Fe-Based Amorphous Matrix Composite Coating on Structural Steel

Iron-based amorphous materials, owing to their very high hardness, elastic modulus, wear resistance, and corrosion resistance, can be potential materials for surface modification and engineering of many structural alloys. The current study focuses on a novel functional coating, synthesized via laser cladding of an iron-based (Fe48Cr15Mo14Y2C15B) amorphous precursor powder, on AISI 4130 steel substrate, using a continuous-wave diode-pumped ytterbium laser. The coatings were characterized by different techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). SEM and TEM studies indicated the presence of Fe-based nanocrystalline dendrites intermixed within an amorphous matrix. A three-dimensional thermal modeling approach based on COMSOL Multiphysics (COMSOL Inc., Burlington, MA) was used to approximately predict the temperature evolution and cooling rates achieved during laser processing. The mechanisms for the formation of crystalline phases and the morphological changes in the microstructure were studied based on the thermal model developed. Although the thermal model predicted substantially high cooling rates as compared to the critical cooling rate required for retaining an amorphous phase, the formation of crystalline phases is attributed to formation of yttrium oxide, reducing the glass-forming ability, and formation of different oxide phases that act as heterogeneous nucleation sites resulting in the composite microstructure.

Laser deposited biocompatible Ca–P coatings on Ti–6Al–4V: Microstructural evolution and thermal modeling

A high intensity continuous wave diode pumped ytterbium laser source was used to deposit Ca-P coatings on a Ti-6Al-4V biocompatible alloy in order to generate a physically textured surface, enhancing osseointegration. Scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) studies were coupled with X-ray and micro diffraction work to determine the structure, composition, and phases present in various zones of a sample prepared across the coating/substrate interaction zone. Three-dimensional thermal modeling was also carried out to determine the cooling rate and maximum temperature experienced by different regions of the substrate. Combining these results provide us with valuable insights regarding the thermo-physical as well as chemical interactions that take place across the coating-substrate interface.

Laser surface modification of AZ31B Mg alloy for bio-wettability

Magnesium alloys are the potential degradable materials for load-bearing implant application due to their comparable mechanical properties to human bone, excellent bioactivity, and in vivo non-toxicity. However, for a successful load-bearing implant, the surface of bio-implant must allow protein absorption and layer formation under physiological environment that can assist the cell/osteoblast growth. In this regard, surface wettability of bio-implant plays a key role to dictate the quantity of protein absorption. In light of this, the main objective of the present study was to produce favorable bio-wettability condition of AZ31B Mg alloy bio-implant surface via laser surface modification technique under various laser processing conditions. In the present efforts, the influence of laser surface modification on AZ31B Mg alloy surface on resultant bio-wettability was investigated via contact-angle measurements and the co-relationships among microstructure (grain size), surface roughness, surface energy, and surface chemical composition were established. In addition, the laser surface modification technique was simulated by computational (thermal) model to facilitate the prediction of temperature and its resultant cooling/solidification rates under various laser processing conditions for correlating with their corresponding composition and phase evolution. These predicted thermal properties were later used to correlate with the corresponding microstructure, chemical composition, and phase evolution via experimental analyses (X-ray diffractometer, scanning electron microscope, energy-dispersive spectroscopy).

Stress-induced selective nano-crystallization in laser-processed amorphous Fe–Si–B alloys

It is shown that laser processing results in localized compressive stresses in amorphous Fe–Si–B, leading to homogeneous nano-crystallization at the edges of the laser track. The mechanism can be attributed to enhanced diffusivity at these edges, resulting from a reduced diffusion activation barrier, which has been calculated by coupling the results of a thermal model with microstructural characterization.

Integrated experimental and theoretical approach for corrosion and wear evaluation of laser surface nitrided, Ti–6Al–4V biomaterial in physiological solution

A laser based surface nitriding process was adopted to further enhance the osseo-integration, corrosion resistance, and tribological properties of the commonly used bioimplant alloy, Ti-6Al-4V. Earlier preliminary osteoblast, electrochemical, and corrosive wear studies of laser nitrided titanium in simulated body fluid clearly revealed improvement of cell adhesion as well as enhancement in corrosion and wear resistance but mostly lacked the in-depth fundamental understanding behind these improvements. Therefore, a novel integrated experimental and theoretical approach were implemented to understand the physical phenomena behind the improvements and establish the property-structure-processing correlation of nitrided surface. The first principle and thermodynamic calculations were employed to understand the thermodynamic, electronic, and elastic properties of TiN for enthalpy of formation, Gibbs free energy, density of states, and elastic properties of TiN were investigated. Additionally, open circuit potential and cyclic potentio-dynamic polarization tests were carried out in simulated body fluid to evaluate the corrosion resistance that in turn linked with the experimentally measured and computationally predicted surface energies of TiN. From these results, it is concluded that the enhancement in the corrosion resistance after laser nitriding is mainly attributed to the presence of covalent bonding via hybridization among Ti (p) and N (d) orbitals. Furthermore, mechanical properties, such as, Poisson׳s ratio, stiffness, Pugh׳s ductility criteria, and Vicker׳s hardness, predicted from first principle calculations were also correlated to the increase in wear resistance of TiN. All the above factors together seem to have contributed to significant improvement in both wear and corrosion performance of nitride surface compared to the bare Ti-6Al-4V in physiological environment indicating its suitability for bioimplant applications.

Synthesis of Al0.5CoCrCuFeNi and Al0.5CoCrFeMnNi High-Entropy Alloys by Laser Melting

High-entropy alloys (HEAs) are a blend of 5+ metallic elements that can form solid solution disordered structures. Two HEAs were synthesized using laser melting in an argon atmosphere. The relatively well-studied Al0.5CoCrCuFeNi alloy was first produced to establish the feasibility of laser melting. Manganese was then substituted for copper to potentially lower both cost and density, producing a novel Al0.5CoCrFeMnNi alloy system. These samples were analyzed with XRD, SEM, EDS, and Vickers microhardness to determine the effects of the manganese substitution, as well as the effects of laser melting in comparison to the more traditional arc melting.