Peter C. Collins

Direct laser deposition of alloys from elemental powder blends

Abstract The complexity in design of components used in advanced aerospace and automotive applications is continuously increasing. This has led to the development of near-net shape manufacturing techniques such as laser engineered net-shaping (LENSTM) which falls in the class of direct laser deposition processes from powder feedstock. Despite considerable advances in process optimization, there is a rather limited understanding of the role of metallurgical factors in laser deposition of alloys. This paper discusses the significant role played by the thermodynamic enthalpy of mixing in the deposition of alloys from elemental powder blends using LENSTM. This factor influences the homogeneity as well as the rate of solidification of the alloy and consequently the microstructure and properties of the deposit. The enthalpy of mixing could also serve as a very useful guideline in the design of novel alloys that are laser deposited from elemental powder blends.

Microstructural evolution in laser deposited compositionally graded α/β titanium-vanadium alloys

Abstract A graded binary Titanium-Vanadium alloy has been deposited using the laser engineered net-shaping (LENS™) process from a blend of elemental Ti and V powders. A compositional gradient in the alloy, from elemental Ti to Ti-25at%V, has been achieved within a length of ~25 mm. Subsequent to deposition, longitudinal sections of the deposit have been characterized in detail using scanning and transmission electron microscopy. Though the phases across the graded alloy correspond to those typically observed in α / β Ti alloys, the scale and morphology of the microstructural features varies substantially with composition. Several phase transformations, namely, β →Widmanstatten α , β → ω and martensitic β →hexagonal α ′, are encountered in the graded alloy sample during LENS™ deposition. The ability to achieve such substantial changes in composition across rather limited lengths make such graded alloys highly attractive candidates for investigating the influence of systematic compositional changes on phase transformations and concurrent microstructural evolution in these alloys.

Laser deposition of compositionally graded titanium–vanadium and titanium–molybdenum alloys

Compositionally graded binary titanium–vanadium and titanium–molybdenum alloys have been deposited using the laser engineered net-shaping (LENS™) process. A compositional gradient, from elemental Ti to Ti–25at.% V or Ti–25at.% Mo, has been achieved within a length of ∼25 mm. The feedstock used for depositing the graded alloy consists of elemental Ti and V (or Mo) powders. Though the microstructural features across the graded alloy correspond to those typically observed in α/β Ti alloys, the scale of the features is refined in a number of cases. Microhardness measurements across the graded samples exhibit an increase in hardness with increasing alloying content up to a composition of ∼12% in case of Ti–xV and up to a composition of ∼10% in case of the Ti–xMo alloys. Further increase in the alloying content resulted in a decrease in hardness for both the Ti–xV as well as the Ti–xMo alloys. A notable feature of these graded deposits is the large prior β grain size resulting from the directionally solidified nature of the microstructure. Thus, grains ∼10 mm in length grows in a direction perpendicular to the substrate. The ability to achieve such substantial changes in composition across rather limited length makes this process a highly attractive candidate for combinatorial materials science studies.

Direct laser deposition of in situ Ti–6Al–4V–TiB composites

Abstract Ti–6Al–4V–TiB composites have been in situ deposited from powder feedstocks consisting of a blend of pre-alloyed Ti–6Al–4V and elemental boron using the Laser Engineered Net-Shaping (LENS™) process. The microstructure of the as-deposited composites has been characterized in detail using scanning (SEM) and transmission electron microscopy (TEM). A homogeneous refined dispersion of TiB precipitates is formed within the Ti–6Al–4V α/β matrix. The scale of the microstructure is substantially refined as compared with composites produced using other recently developed powder processing techniques. In addition to the refined dispersion of the reinforcing phase, the influence of the TiB precipitation on the solid-state β→β+α transformation that takes place in the matrix has been examined using SEM and TEM. Finally, heat-treatments carried out post-LENS™ deposition, suggest that LENS™ fabricated composites are thermodynamically stable, exhibiting limited TiB coarsening.

Laser Deposition of In Situ Ti – TiB Composites

Due to their enhanced mechanical properties and potentially wide applicability, there is considerable interest in the development of metal-matrix composites consisting of titanium borides in a titanium alloy matrix. Despite the development of a variety of different processing routes for these composites, there are relatively few ones capable of processing a fully dense, near-net shape component with a relatively fine dispersion of boride precipitates. This paper will discuss the in situ laser deposition of Ti-TiB composites using the laser engineered net-shaping (LENS™) process from a blend of elemental titanium (or titanium alloy) and boron powders. The microstructure of the LENS™ deposited Ti-TiB composite has been compared with that of a conventionally cast in situ composite of the same composition. The conventionally cast composite exhibits a significantly coarser scale microstructure. Thus, the ability to produce a fine dispersion of TiB precipitates in dense Ti-TiB composites of near-net shape using LENS™ processing can be attributed to the rapid solidification effects during such processing.

Determination of iron in wine using 2,4,6-tripyridyl-s-triazine

Abstract Procedures are described for the determination of iron in wine using 2,4,6-tripyridyl- s -triazine, a new ferroine reagent. One procedure involving wet ashing with nitric and perchloric acids gives results comparable to those obtained using the usual 1,10-phenanthroline method while a direct extraction procedure often gives low but reproducible results indicating the presence of “complexcd iron” in the sample.

The influence of the enthalpy of mixing during the laser deposition of complex titanium alloys using elemental blends

The β-rich Timetal 21S alloy has been successfully laser-deposited from both pre-alloyed as well as a blend of elemental powders. The influence of the thermodynamic enthalpy of mixing of the elemental powders on the microstructure and compositional homogeneity has been investigated and subsequently engineered by substituting the Mo with Cr, resulting in enhanced exothermic mixing.

A Constitutive Equation Relating Composition and Microstructure to Properties in Ti-6Al-4V: As Derived Using a Novel Integrated Computational Approach

While it is useful to predict properties in metallic materials based upon the composition and microstructure, the complexity of real, multi-component, and multi-phase engineering alloys presents difficulties when attempting to determine constituent-based phenomenological equations. This paper applies an approach based upon the integration of three separate modeling approaches, specifically artificial neural networks, genetic algorithms, and Monte Carlo simulations to determine a mechanism-based equation for the yield strength of α+β processed Ti-6Al-4V (all compositions in weight percent) which consists of a complex multi-phase microstructure with varying spatial and morphological distributions of the key microstructural features. Notably, this is an industrially important alloy yet an alloy for which such an equation does not exist in the published literature. The equation ultimately derived in this work not only can accurately describe the properties of the current dataset but also is consistent with the limited and dissociated information available in the literature regarding certain parameters such as intrinsic yield strength of pure hexagonal close-packed alpha titanium. In addition, this equation suggests new interesting opportunities for controlling yield strength by controlling the relative intrinsic strengths of the two phases through solid solution strengthening.

The effect of boron on the grain size and texture in additively manufactured β-Ti alloys

AbstractOne of the critical microstructural attributes affecting the properties of additively manufactured (AM) alloys is the growth of large columnar grains along the build direction. While most of the work in the reported literature is focused on Ti–6Al–4V and other α/β alloys, there are rather limited investigations on grain growth and texture development in AM β-Ti alloys. The addition of trace amounts of boron to these AM β-Ti alloys resulted in significant changes in the microstructure. Depending on the alloy system, a grain refinement of 50–100 times was noted. The change in the grain size has been attributed to a combined effect of constitutional supercooling, caused by boron rejection from the growing β grains, and the growth restriction factor (Q) of the grains caused by the solute elements. The addition of boron also changed the morphology of the grains from being columnar to more equiaxed, a much more pronounced change than observed in traditional α/β alloys such as Ti–6Al–4V. A change in texture of the β grains along the build direction was also noted, wherein the addition of boron randomized the texture from the typically observed strong (001)β oriented grains in AM Ti alloys. Finally, the addition of boron changed the morphology of the α precipitates in the Ti–Mo system from lath-like to more equiaxed, while preserving the Burgers orientation relationship between the α and β phases.

Modeling of Ti-W Solidification Microstructures Under Additive Manufacturing Conditions

Additive manufacturing (AM) processes have many benefits for the fabrication of alloy parts, including the potential for greater microstructural control and targeted properties than traditional metallurgy processes. To accelerate utilization of this process to produce such parts, an effective computational modeling approach to identify the relationships between material and process parameters, microstructure, and part properties is essential. Development of such a model requires accounting for the many factors in play during this process, including laser absorption, material addition and melting, fluid flow, various modes of heat transport, and solidification. In this paper, we start with a more modest goal, to create a multiscale model for a specific AM process, Laser Engineered Net Shaping (LENS™), which couples a continuum-level description of a simplified beam melting problem (coupling heat absorption, heat transport, and fluid flow) with a Lattice Boltzmann-cellular automata (LB-CA) microscale model of combined fluid flow, solute transport, and solidification. We apply this model to a binary Ti-5.5 wt pct W alloy and compare calculated quantities, such as dendrite arm spacing, with experimental results reported in a companion paper.