Titanium Boride Formation and Its Subsequent Influence on Morphology and Crystallography of Alpha Precipitates in Titanium Alloys

Over the last two decades there has been an increased interest in understanding the influence of trace boron additions in Ti alloys. These additions refine the prior β grain size in as-cast Ti alloys along with increasing their modulus and yield strength due to the precipitation of TiB. TiB also acts as a heterogeneous nucleation site for α precipitation and has been shown to influence the α phase morphology. B is completely soluble in liquid Ti but has a negligible solubility in both body centered cubic β and hexagonal close packed α phases of Ti. Thus, during solidification of hypoeutectic B containing alloys, B is rejected from β into the liquid where it reacts with Ti to form pristine single crystal whiskers of TiB. Despite a substantial amount of reported experimental work on the characterization of TiB precipitates, its formation mechanism and influence on α phase precipitation are still not clear. The current work is divided into two parts – (i) understanding the mechanism of TiB formation using first principles based density functional theory (DFT) calculations and (ii) elucidating how TiB influences the α phase morphology and crystallography in titanium alloys using electron microscopy techniques. TiB exhibits anisotropic growth morphology with [010] direction as its predominant growth direction and displays a hexagonal cross section with (100), (101), and (10) as the bounding planes. A high density of stacking faults has been experimentally observed on the (100) plane. The present study, by using DFT based nudged elastic band (NEB) calculations, elucidates for the first time that the diffusion of B through TiB is via an interstitial-assisted mechanism as opposed to vacancy-assisted mechanism hypothesized in literature. This one dimensional interstitial-assisted diffusion results in the anisotropic growth of TiB. In addition, the energetics of TiB- α interfaces was calculated to understand the hexagonal cross-section of TiB. The intimate mixing of B27 and Bf structures and their co-existence with stacking faults has been explained by calculating the interfacial energy of B27/Bf interfaces along with stacking fault formation energy. The boride precipitates have also been shown to modify the morphology of α phase from lath like to more equiaxed like. However, the influence of TiB on the crystallography of α precipitation has not been explored in great detail. The present study will clearly demonstrate, for the first time, the influence of various alloying elements present in the titanium alloy, on the resulting effect of B addition on α morphology and its crystallography in Ti alloys. Thus, the influence of B addition on α precipitation in two classes of commercial Ti alloys, i.e. β alloys and α + β alloys have been explored. It has been found that TiB nucleated α can either become equiaxed by a loss of Burgers orientation relationship (OR) with β or can retain the lath morphology in case of alloys containing a combination of Ti, Al and Mo

Effect of Interlayer Cooling Time, Constraint and Tool Path Strategy on Deformation of Large Components Made by Laser Metal Deposition with Wire

Laser metal deposition with wire (LMD-w) is a developing additive manufacturing (AM) technology that has a high deposition material rate and efficiency and is suitable for fabrication of large aerospace components. However, control of material properties, geometry, and residual stresses is needed before LMD-w technology can be widely adopted for the construction of critical structural components. In this study, we investigated the effect of interlayer cooling time, clamp constraints, and tool path strategy on part distortion and residual stresses in large-scale laser additive manufactured Ti-6Al-4V components using finite element method (FEM). The simulations were validated with the temperature and the distortion measurements obtained from a real LMD-w process. We found that a shorter interlayer cooling time, full clamping constraints on the build plates, and a bidirectional tool path with 180° rotation minimized part distortion and residual stresses and resulted in symmetric stress distribution

Effects of the microstructure and porosity on properties of Ti-6Al-4V ELI alloy fabricated by electron beam melting (EBM)

Publisher Copyright: © 2016 Elsevier B.V.Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that makes possible the fabrication of three-dimensional near-net-shaped parts directly from computer models. EBM technology has been continuously evolving, optimizing the properties and the microstructure of the as-fabricated alloys. Ti-6Al-4V ELI (Extra Low Interstitials) titanium alloy is the most widely used and studied alloy for this technology and is the focus of this work. Several research works have been completed to study the mechanisms of microstructure formation, evolution, and its subsequent influence on mechanical properties of the alloy. However, the relationship is not completely understood, and more systematic research work is necessary in order to attain a better understanding of these features. In this work, samples fabricated at different locations, orientations, and distances from the build platform have been characterized, studying the relationship of these variables with the resulting material intrinsic characteristics and properties (surface topography, microstructure, porosity, micro-hardness and static mechanical properties). This study has revealed that porosity is the main factor controlling mechanical properties relative to the other studied variables. Therefore, in future process development, decreasing the porosity should be considered the primary goal in order to improve mechanical properties.This research was performed under the Additive Manufacturing program of the Integrative Material Design Center (iMdc) consortium at Worcester Polytechnic Institute, in collaboration with the Additive Manufacturing Demonstration Facility of Oak Ridge National Laboratory, and sponsored by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office , under contract DE-AC05-00OR22725 with UT-Battelle, LLC. WPI students, Matthew Gleason (Surface Metrology Laboratory), Yuwei Zhai, and Anthony Spangenberger (iMdc) also collaborated actively in the characterization process performed during the research.Peer reviewe

Data for Investigating the Effect of Metal Powder Recycling in Electron Beam Powder Bed Fusion using Process Log Data

This archive contains log file data that was generated by the Arcam A2 Electron Beam Melting (EBM) (R) system during the build process of different builds. The purpose of the builds was to study the effect of recycling of metal powder on the chemical composition, flowability and size characteristics of powder. A detailed exposition of findings from the study is available in reference [1]. The powders studied were Inconel 718 and Ti-6Al-4V alloy powders. The original log file data generated by Arcam EBM has been pruned to include only parameters described in the Column header description section of this file

Scaling Up metal additive manufacturing process to fabricate molds for composite manufacturing

International audienceDirect Energy Deposition (DED) systems are currently used to repair and maintain existing parts in the aerospace and automotive industries. This paper discusses an effort to scale up the DED technique in order to Additively Manufacture (AM) molds and dies used in the composite manufacturing industry. The US molds and dies market has been in a rapid decline over the last decade due to outsourcing to non-US entities. Oak Ridge National Laboratory (ORNL), Wolf Robotics and Lincoln Electric have developed a Metal Big Area Additive Manufacturing (MBAAM) system that uses a high deposition rate and a low-cost wire feedstock material. In this work we used the MBAAM system with a mild steel wire, ER70S-6, to fabricate a compression molding mold for composite structures used in automotive and mass-transit applications. In addition, the mechanical properties of the AM structure were investigated, and it was found that the MBAAM process delivers parts with high planar isotropic behavior. The paper investigates the microstructure and grain of the printed articles to confirm the roots of the observed planar isotropic properties. The manufactured AM mold was used to fabricate 50 composite parts with no observed mold deformations

Additive manufacturing as a processing route for steel-aluminum bimetallic structures

Here we present results on the fabrication of steel-aluminum bi-metallic structures using directed energy deposition additive manufacturing. The challenges associated with the fabrication of a sharp transition from steel to aluminum are uncovered using ex-situ characterization techniques and thermo-mechanical modeling of the deposition process. It was found that the fabrication of a sharp steel-aluminum transition is challenging with extensive cracking observed at the interface. The cracking was attributed to the combined effect of residual stress development due to thermal expansion coefficient mismatch and the presence of ordered intermetallics with low ductility at the interface. Using a coupled thermodynamic and thermo-mechanical modeling approach, potential pathways to enable the fabrication of steel-aluminum bi-metallic structures using additive manufacturing are proposed. The results presented here can lay the foundation for future work on the fabrication of bi-metallic steel-aluminum structures using directed energy deposition

Wire directed energy deposition of steel-aluminum structures using cold metal transfer process

In this study, a sharp transition from 316L stainless steel to 4043 aluminum alloy was fabricated using wire directed energy deposition (DED) via the cold metal transfer (CMT) process. The CMT process with its inherently low heat input, led to a significant reduction in intermetallic thickness at the bi-metallic interface compared to blown powder DED technique reported in the literature resulting in superior properties when compared to those of dissimilar steel-aluminum welds. Thermo-kinetic modeling confirmed that the intermetallic formation is through a classical nucleation and growth mechanism, and the fraction and thickness can be controlled by adjusting CMT process parameters to kinetically arrest or minimize the intermetallic formation. These findings underscore the efficacy of CMT-based wire DED for fabrication of steel-aluminum bi-metallic structures

Encyclopedia of Materials: Metals and Alloys

Editor-in-Chief Francisca G. CaballeroModern metallurgy is a fascinating field of research, full of discoveries, commercial opportunities and industrial utility. Encyclopedia of Materials: Metals and Alloys is a new, multidisciplinary reference work offering a comprehensive coverage of this exciting area, and consolidating research activities in all experimental and theoretical aspects of metallic materials, intermetallic compounds, alloys, blends and composites. Key focus is on those aspects of the science of metals concerned with their manufacturing, processing and fabrication, the relationship between the macro/micro/nanostructures and properties (mechanical, chemical, electrical, electrochemical, magnetic and optical), industrial application, surface modification and functionalization of metals – and, importantly, resource and supply chain issues, and life-cycle and sustainability practices