Investigation of Multifunctional, Additively Manufactured Structures using Fused Filament Fabrication
thesisposted on 21.06.2022, 14:05 authored by Trevor J FleckTrevor J Fleck
From its advent in the 1980s until the 2000s, many of the advances in additive manufacturing (AM) technology were primarily focused on the development of various 3D printing techniques. During the 2000s, AM came to a juncture where these processes were well developed and could be used effectively for rapid prototyping purposes; however, the ability to produce functional components that could reliably perform in a given system had not been fully achieved. The primary focus of AM research since this juncture has been to transition AM from a rapid prototyping technique to a legitimate means of mass manufacturing end-use products. In order to make this happen, two significant areas of research needed to be advanced. The first area focused on advancing the limited selection and functionality of the materials being used for AM. The second area focused on the characterization of the end-use products comprised of these new materials.
The primary goals of the work described in this document are to substantially further the field of the additive manufacturing by developing new functional materials and subsequently characterizing the resultant printed components. The primary focus of the first two chapters (Chapters 2 and 3) is to further characterize an energetic material system comprising of aluminum (Al) particles embedded in a polyvinylidene fluoride (PVDF) binder, which has been shown to be compatible with AM. This material system has the ability to be implemented as a lightweight multifunctional energetic structural material (MESM); however, significant characterization of its structural energetic properties is needed to ensure reliable MESM performance. First, variations of a previously demonstrated Al/PVDF filament were investigated in order to determine the effect of material constituents on the structural energetic properties of the material. Seven different Al/PVDF formulations, with various particle loadings and particle sizes, were considered. The modulus of elasticity and ultimate strength for the seven formulations were obtained via quasi-static tensile testing of 3D printed dogbones. The energetic performance was quantified via burning rate measurements and differential scanning calorimetry (DSC) of 3D printed samples. Next, variations in the AM process were made and the effect of print direction on the same properties was determined. Once viable MESM performance was quantified, representative structural elements were printed in order to demonstrate the ability to create structural energetic elements. During quasi-static tensile testing, it was observed that aligning the load direction perpendicular to the print direction of the component resulted in inferior mechanical properties. This reduction in mechanical properties can be attributed to the lack of continuity at material interfaces, a well studied phenomena in AM.
This phenomena is the primary focus of the next two chapters (Chapters 4 and 5), which investigate the polymer healing process as it pertains to fusion-based material extrusion additive manufacturing, also known as fused filament fabrication (FFF). In the context of the FFF process, the extent of the polymer healing, or lack thereof, at the layer interface is known to be thermally driven. Chapter 4 quantifies the relationship between the reduction in mechanical properties and the temperature of the previously deposited layer at the time the subsequent layer is deposited. This relationship gives insight into which parameters should be closely monitored during the FFF process. The following chapter investigates incorporating plasma surface treatment as a means to improve the reduced mechanical properties seen in Chapter 3 and 4. As plasma surface modification can affect various stages of the polymer healing process, a variety of experiments were completed to determine which mechanisms of the plasma treatment were significantly affecting the mechanical properties of the FFF components. The thermal history was analyzed and it was hypothesized that enhanced diffusion at the layer interface was not a significant contributor to, but a rather a detractor from, the improved mechanical properties in this system. A variety of tests investigating how the plasma treatment was affecting the wettability of the surface were performed and all of the tests indicated that the wettability was increased during treatment and was likely the driving mechanism causing the improvement seen in the mechanical properties. These tests give some initial insight into how to successfully pair plasma treatment capabilities with FFF systems and give insights into how that plasma treatment can affect the polymer healing process in FFF applications.