Titanium Additive Manufacturing

The aerospace industry, a staple of the economy in the Pacific Northwest, is beginning to capitalize on the versatility of additive manufacturing to produce parts that might otherwise be impossible or prohibitively difficult to produce. The industry and regulators enforce the strictest tolerances for consistency and performance, regardless of the production method. Our group collaborates with several prominent aerospace companies nationwide as well as metal additive manufacturing system producers to better understand the variables that significantly impact the porosity, fatigue life, and mechanical performance of printed parts. Only once those parameters are understood can they be optimized to produce ever more efficient parts. 

One of the more significant parameters is related to how many times the feedstock powder has been reused. Due to the nature of the selective melting process, the entire build space is filled with titanium powder, but only those voxels that constitute the part are melted and solidified into the final shape. The remaining powder is separated from the part and can then be reused for future prints. However, the morphology and composition of the powder particles, even those not melted during printing, is altered, and over time accumulations can degrade the final parts. Monitoring these changes and how the powder feedstock may be “restored” to a more virgin state is one focus of our research.

The printed part may not have a completely solidified microstructure, and requires additional heat and pressure treatments (such as hot isostatic pressing, HIP) to densify the structure. We use micro computed tomography to monitor pores, along with fractographic analysis of tensile and fatigue coupons to identify where pores are likely to happen and how much of a problem they may be. Ti-6Al-4V (6 wt.% aluminum, 4 wt.% vanadium) is an alpha-beta alloy, meaning that the final microstructure is a complex mixture of both alpha and beta phases. The rapid melting and solidification cycles that the printed material experiences during printing, followed by the heat treatment for densification, only complicate this further. As with any other alloy, the microstructure is one of the primary controlling factors of the properties of the final part. For these reasons, we also perform metallographic analysis of the alpha-lathes and prior beta grains and how they change in response to the print parameters (e.g. powder reuse, location in the build chamber) and post-print treatments to ensure the optimal final part is produced.

Render of the build file for a design of experiments for titanium fatigue test coupons. Three parameters are indicated, the gage thickness T of a coupon, and the height H and radial distance R from the center of the base plate. From

Microstructural Inspiration from Nature

As a source of bioinspiration for the design of hard-yet-durable materials, teeth stand out in several ways. The outermost layer, enamel, provides a highly mineralized surface (~96% hydroxyapatite mineral, ~2% organic content, ~2% water), yet survives decades of use without any cellular repair mechanisms. The underlying dentin is softer, but serves as a durable substrate that can stop cracks in the tooth from becoming catastrophic. Understanding how these two materials function and how they are altered by diseases (e.g. age, amelogenesis imperfecta, diabetes) is one of the primary focuses of our group.

For enamel, the resistance to failure is facilitated by the complex weaving microstructure called decussation, which has been shown to be a key factor that resists the growth of incident cracks ( Bajaj & Arola, 2009). This suggests that a next-generation material with high-damage tolerance could be derived from the microstructural features seen in enamel. Decussation is a feature seen across almost all of Mammalia, but characterization outside of Homo sapiens is relatively scarce in the literature. To address this, our group has collaborated with the  Burke Museum of Natural History and Culture at the University of Washington to study the enamel of non-human mammals ( Renteria et. al., 2021). The results suggest that the enamel tissue evolves to enable the diet of the animal, and furthermore, provides different sources of inspiration depending on the design objectives

In the dental industry however, a more pressing concern is how the structure and properties of a tooth are altered by age and by disease. Using a mouse model, our group studies the influence diabetes has on the properties of dentin, providing guidance for what considerations may be necessary when treating diabetic patients. It comes as no surprise that age is also detrimental to the integrity of teeth as damage and wear accumulate, and quantification of those changes (e.g. Renteria et. al., 2022, Yan et. al., 2017) can guide treatment options. Amelogenesis imperfecta is a genetic disease that interrupts the biological process of enamel formation, severely compromising the enamel for each of the patient’s teeth. However, there have been limited attempts to understand the malformed enamel from a materials science perspective, which could offer insight for improved therapeutic procedures, yes, but also seeing what went “wrong” can further understanding of what makes enamel so remarkable from the earliest stages. 

Overview of the hierarchical nature of enamel microstructure and some of the toughening mechanisms that can be seen from the micro-scale to the meso-scale.

Pushing the Boundaries of Composite Additive Manufacturing

Additive manufacturing (AM) is hailed for freedom in complex component design and low material waste. Originally conceived for thermoset based stereolithography, all classes of materials are now being exploited and produced in tandem with additive processes to push the possibilities of engineered structures. Excitingly suitable for this field are fiber reinforced thermoplastics, which can offer more sustainable and recyclable materials for multiscale components. We seek to improve the quality and printability of novel high volume fraction filaments, with both stiff and compliant continuous fiber reinforcement. Although fused deposition modeling of continuous fiber composites can offer uniquely reinforced structures, there is limited understanding on composite behavior and its dependence on the printing process mechanics. As such, filament strengths are evaluated at specific stages of printing under uniaxial tension and Weibull statistics are applied to characterize the strength distribution. To be implanted in industry, this system must meet the rigorous standards for reliable and predictable manufacturability. We intend to explore and overcome associated challenges of stiff continuous fiber printing to produce net-shape components with high strength, reliability, and throughput.

An additional objective of this project is to develop a new class of composite filaments inspired by the biological structural materials of dermal armors, particularly fish scales, that enable 3D printing of advanced engineering structures with superior durability. Unlike traditional continuous fiber filaments, this filament will consist of an assembly of compliant fibers with extremely high toughness and a comparatively stiff outer matrix. It is hypothesized that through careful selection and treatment of both fiber and matrix components, successful chemical bonding between the fiber and matrix interface will exhibit decreased voids and increased final part strength. The volume fraction and size distribution of additives are expected to result in a gradient of stiffness, allowing for optimization of toughness and puncture resistance of the subsequent structures. The novel filament could provide a spectrum of mechanical properties (stiffness, strength, and toughness) that can be customized to fulfill the objectives of the printed part.

One area where polymer-based materials and composites are infrequently used is in marine environments. However, such materials may still fulfill many other demands of the application, and only a poor understanding of the durability of the material limits its usability. To that end, marine degradation studies are also performed on polymer parts produced by 3D printing. 

Zeolite-Based Concrete for Sustainable Infrastructre

Concrete is the most popular construction material on the planet due to its impressive mechanical properties. However, it is responsible for around 8% of global CO2 emissions, arising primarily from generation of cement powder. Cement is responsible for binding the other constituents (sand and gravel) together as well as providing the majority of the mechanical strength. Through hydration reactions, densifying and strengthening components such as calcium hydroxide and calcium silicate hydrate are produced. Cement powder is manufactured using an energy-intensive process (~1 kg CO2 generated per 1 kg cement). Because of this, alternatives for cement powder in concrete are of great interest from an environmental standpoint.

Pozzolans are materials that participate in a separate series of reactions to further densify cement via production of calcium silicate hydrate. Zeolite is a pozzolonic mineral composed primarily of silica and alumina that occurs naturally when layers of ash and rock from volcanic eruptions react with groundwater. Particles of zeolite are micro- and nano-porous, which increases the relative surface area and thus the reactivity. The combination of zeolite’s aluminosilicate and microporous nature make it a promising pozzolanic material for concrete materials. 

A challenge to using pozzolans in concrete is that only limited amounts can be added before the material performance decreases below acceptable levels. However, zeolite has the potential to be used in substantial concentrations, reducing the amount of cement (and in turn carbon emissions) needed. To fully realize natural zeolite as a pozzolan, many aspects of the material need further understanding. In collaboration with Zeolite Composites, LLC, we are conducting studies to observe the impact of zeolite additions on the mechanical strength of concrete, characterize the pozzolanic activity of natural zeolite, and understand the effect of zeolite on the hydration reactions and strength development of concrete over time. 

CT reconstruction of zeolite cement. The microporous nature of the composite is apparent. When viewed in an SEM, the reaction products (needles and platelets) can be seen.

Additionally, concrete structures are often subjected to challenging (e.g. marine or coastal) environments for long durations. Our group is also conducting experiments to understand the performance of zeolite-enriched concrete over extended periods of time as well as in extreme environments. Finally, the microporous nature of zeolite also introduces a potential mechanism for capturing ions or molecules, such as greenhouse gasses. Measuring zeolite’s ability to capture carbon dioxide and the monitoring the mechanical properties during and after exposure are the first steps towards implementation of a “greener” concrete.