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Additive friction stir deposition (AFSD) is an emerging solid-state additive manufacturing process that leverages the friction stir principle to deposit porosity-free material. The unique flow of material that allows for the transformation of bar stock into a near-net shape part is driven by the non-linear heat generation mechanisms of plastic deformation and sliding frictional heat generation. The magnitude of these mechanisms, and hence the total applied thermal power, implicitly depend on the thermal state of the system, forcing power input to become a dependent variable. This is not the case in other 3D printing methods; thermal power can be controlled independently. In this work, the heat generation in AFSD is explored, and its transfer is quantified. In particular, the time-dependent ratio between the amount of conduction into the AFSD tool versus into the substrate is quantified. It was found for the conditions tested with a single-piece AFSD tool, conduction up the tool was on the order of the conduction into the stir. For a more modern three-piece tool, the ratio between the tool and the substrate varied between 0.3-0.1. It was also found that traversing faster resulted in more heat flux into the substrate as would be expected by moving heat source modeling. The total heat generated was also quantified as being equal to between 60% and 80% of the mechanical spindle power depending on the tool type and the exact process conditions. That ratio was found to be time-invariant. At the same time, this changing heat flux ratio was shown to dramatically alter thermocouple measurements in the tool, showing the uncertainty of that method of process control. The contact state between the stir and the tool was treated as a thin conductive layer and a contact heat transfer coefficient was calculated on the order of 20 frac{kW}{m^2K}. The limitations of this treatment were found to occur when a significant amount of the heat generation came from frictional heating rather than plastic deformation. This implies that any measurement conducted in the tool is related to the stir by a complex function driven by the state of the stir; showcasing the need for more well-understood in-situ monitoring. Finally, some of the ideas about thermal control are applied to a case study on the repair of corroded through holes using AFSD to restore fatigue life. It was found that modifying the thermal boundary conditions and applying active cooling at the end of the repair could improve the fatigue life drastically. This was due to less time spent in a thermally active region leading to less heterogeneous nucleation and less grain boundary nucleation. This more preferred microstructure morphology led to a change in the fracture mode and increased the number of cycles to crack initiation and the number of cycles after crack initiation. / Doctor of Philosophy / Metal 3D printing of industrially relevant aluminum alloys is plagued with problems. Additive friction stir deposition seems well posed to overcome some of the problems associated with aluminum printing. Being able to 3D print these alloys with properties that are as good as traditional manufacturing offers a large potential cost and time savings over traditional manufacturing for the aerospace industry (e.g. Boeing, Lockheed Martin, U.S. Navy). For these components to be part of a plane, the manufacturer must prove the components were made the same way print-to-print regardless of the actual shape of the component being made. This dissertation focuses on the key metallurgical variable of temperature and explores how thermal energy is generated and where that energy goes in to the system. The key takeaway is, that without precise knowledge of the total heat generated and the entire thermal system, assurances about processing temperature cannot be made. An exploration of heat generation and metrics about its dispersion are presented. This is followed by a study on repairing structural components while changing the thermal system to understand its effects.

Additive friction stir deposition (AFSD) is a solid-state additive manufacturing (AM) technique that breaks down large constituent particles into more refined and uniformly disturbed microstructure. AFSD was used to print Al-Ce alloys. Current commercial Al-alloys upon elevated temperatures go through dissolution and coarsening of strengthening precipitates causing mechanical degradation of these alloys. Al-Ce alloys do not have this issue as cerium's low solubility restricts dissolution into the aluminum matrix at elevated temperatures, thus giving great thermal stability to the microstructure. Al-Ce alloys lack solid solubility that affects the solid solution strengthening and precipitation strengthening. Al-Ce alloys have limitation at room temperature as they can only reach a maximum of ~65 MPa yield strength. Elements like magnesium have been added to alloy to enable solid solution strengthening, and scandium to enable precipitation strengthening to improve strength before going through the AFSD process. By adding new elements to the Al-Ce alloys, an increase in the yield strength from ~60 MPa to ~200 MPa was achieved before AFSD. The casted alloys form coarse particles that reach 300 µm in size; resulting in stress concentration that causes material fracture before necking, giving >10% ductility. AFSD breaks down these coarse particles to increase strength and ductility increases. The particles were broken down to >20 µm which increased the ductility to 10%. The results of this research shows that Al-Ce alloys are able to reach commercial aluminum alloy mechanical standards of 400 MPa ultimate tensile strength and 10% ductility at room temperature for aerospace applications.

Additive friction stir deposition (AFSD) is an emerging solid-state metal additive manufacturing technology that uses deformation bonding to create near-net shape 3D components. As a developing technology, a deeper understanding of the processing science is necessary to establish the process-structure relationships and enable improved control of the as-printed microstructure and material properties. AFSD provides a unique opportunity to explore the friction stir fundamentals via direct observation of the material during processing. This work explores the relationship between the processing parameters (e.g., tool rotation rate Ω, tool velocity V, and material feed rate F) and the thermomechanical history of the material by process monitoring of i) the temperature evolution, ii) the force evolution, and iii) the interfacial contact state between the tool and deposited material. Empirical trends are established for the peak temperature with respect to the processing conditions for Cu and Al-Mg-Si, but a key difference is noted in the form of the power law relationship: Ω/V for Cu and Ω2/V for Al-Mg-Si. Similarly, the normal force Fz for both materials correlates to V and inversely with Ω. For Cu both parameters show comparable influence on the normal force, whereas Ω is more impactful than V for Al-Mg-Si. On the other hand, the torque Mz trends for Al-Mg-Si are consistent with the normal force trends, however for Cu there is no direct correlation between the processing parameters and the torque. These distinct relationships and thermomechanical histories are directly linked to the contact states observed during deformation monitoring of the two material systems. In Cu, the interfacial contact between the material and tool head is characterized by a full slipping condition (δ=1). In this case, interfacial friction is the dominant heat generation mechanism and compression is the primary deformation mechanism. In Al-Mg-Si, the interfacial contact is characterized by a partial slipping/sticking condition (0<δ<1), so both interfacial friction and plastic energy dissipation are important mechanisms for heat generation and material deformation. Finally, an investigation into the contact evolution at different processing parameters shows that the fraction of sticking is critically dependent on the processing parameters which has many implications on the thermomechanical processing history. / Doctor of Philosophy / Additive manufacturing or three-dimensional (3D) printing technologies have been lauded for their ability to fabricate complex geometries and multi-material parts with reduced material waste. Of particular interest is the use of metal additive manufacturing for repair and fabrication of industrial and structural components. This work focuses on characterizing the thermomechanical processing history for a developing technology Additive Friction Stir Deposition (AFSD). AFSD is solid-state additive manufacturing technology that uses frictional heat and mechanical mixing to fabricate 3D metal components. From a fundamental materials science perspective, it is imperative to understand the processing history of a material to be able to predict the performance and properties of a manufactured part. Through the use of infrared imaging, thermocouples, force sensors, and video monitoring this work is able to establish quantitative relationships between the equipment processing parameters and the processing history for Cu and Al. This work shows that there is a fundamental difference in how these two materials are processed during AFSD. In the future, these quantitative relationships can be used to validate modeling efforts and improve manufacturing quality of parts produced via friction stir techniques.

We have created a three-dimensional, implicit finite difference model that can accurately calculate temperatures within the bulk of a sample during a friction stir fabrication process. The model was written in Wolfram Mathematica® 7 for Students, and allows for time-efficient calculation of thermal profiles. The non-dimensionality of the model allows for accurate refinement of the temporospatial mesh, and provides portability across material types. The model provides insight as to the mechanism of heat generation by qualifying the fraction of mechanical energy converted to thermal energy for different material types and sample geometries. Finally, our model gives an understanding of the effects of the heat transfer at the boundaries of the workpiece and suggests a backside heat loss localized at the center of the tool due to a decrease in thermal contact resistance. We have explored the effects of processing parameters on the performance of the friction stir fabrication process. The process has four stages; tool insertion, warm-up, bead formation, and steady-state translation. The tool insertion phase is characterized by a rapid increase in system horsepower requirements. During the warm-up phase, the mechanical energy of the rotating tip is converted to thermal energy. Once enough thermal energy has been transferred to the workpiece, the volume between the tip and the workpiece is filled by feedstock material. Finally, the tool is translated under relatively steady-state conditions. The success or failure of the process is dependent on adequate material delivery to the system. The horsepower requirements of the process depend on the material type and the rate of material delivery. We have explored the effect of processing parameters on the microstructure of the processed samples. Optical microscopy shows that the stratification of layers within the weld and the depth of the weld are both dependent on the processing parameters. EBSD analysis coupled with Vicker's microhardness measurements of the processed pieces show that the grain size within the weld nugget is constant over the range of processing parameters available to the system. Data also show that pressure and heat inherent in friction stir processing of strain-hardened Al5083 counteract strengthening of the temper of the alloy. / Ph. D.

This dissertation presents an assortment of research aimed at understanding the composition-dependence of deformation behavior and the response to thermomechanical processing, to enable efficient design and processing of low stacking fault energy (SFE) high entropy alloy (HEAs). The deformation behavior and SFE of four low SFE HEAs were predicted and experimentally verified using electron microscopy and in-situ neutron diffraction. A new approach of employing a minimization function to refine and improve the accuracy of a semi-empirically derived expression relating composition with SFE is demonstrated. Ultimately, by employing the minimization function, the average difference between experimental and predicted SFE was found to be 2.64 mJ m-2. Benchmarking with currently available approaches suggests that integrating minimization functions can substantially improve prediction accuracy and promote efficient HEA design with expansion of databases. Additionally, in-situ neutron diffraction was used to present the first in-situ measurement of the interspacing between stacking faults (SFs) which were correlated with work hardening behavior. Electron transparent specimens (< ~100 nm thick) were used in order to resolve nanoscale planar faults instead of the thicker sub-sized specimens (on the order of millimeters in thickness) which exhibit the classical stages III work hardening behavior characteristic of low SFE metals and alloys. The present study demonstrates these characteristic dimensions of SFs can be tracked in real-time using neutrons or high-energy x-rays. SFs have also been shown to act as barriers to dislocation motion and thus contribute to strengthening and sustained work hardening during deformation.

Through a series of carefully designed experiments, characterization and some modeling tools, this work is aimed at studying the role of thermal profiles on different microstructural zones and associated properties like strength and corrosion through a variation of weld parameters, thermal boundary conditions and material temper. Two different alloys belonging to the Al-Cu and Al-Cu-Li system in different temper conditions- peak aged (T8) and annealed (O) were used. A 3D-thermal pseudo mechanical (TPM) model is developed for the FSW process using heat transfer module in COMSOL Multiphysics and is based on a heat source wherein the temperature dependent yield shear stress is used for the heat generation. The precipitation and coarsening model is based on the Kampmann and Wagner theoretical framework and accounts for the competition between the various nucleation sites for both metastable and equilibrium precipitates. The model predicts different precipitate mean radius and volume fraction for the various zones in the friction stir welded material. A model for the yield strength is developed which considers contributions from different strengthening mechanisms. The predictions of the each models have been verified against experimental data and literature. At constant advance per rotation, the peak temperature decreases with a decrease in traverse speed and increases with an increase in tool rotation. Weld properties were significantly affected by choice of thermal boundary conditions in terms of backing plate diffusivity. Weld conditions with a higher peak temperature and high strain rate results in more dissolution of precipitates and fragmentation of constituent particles resulting in a better corrosion behavior for the weld nugget. For a peak aged temper of 2XXX alloys, the weld nugget experiences dissolution of strengthening precipitates resulting in a lower strength and the Heat affected zone (HAZ) experiences coarsening of precipitates. For an annealed material, both the weld nugget and HAZ experiences dissolution of precipitates with an increase in strength in the weld nugget.

Seit langem wird Obst durch Trocknen haltbar gemacht. Dazu werden vorwiegend die Lufttrocknung und Konvektionstrocknungsverfahren eingesetzt. Jedoch werden auch neuere Techniken entwickelt, um die Produktqualität zu steigern oder die Produktion zu verbessern. Von der IBT.InfraBio Tech GmbH in Freiberg wurde die STIR®-Technologie vorgestellt. Diese neue Technologie wird u. a. zum Trocknen von Lebensmitteln eingesetzt. Dazu wird von einem Strahler Infrarotstrahlung eines bestimmten Wellenlängenbereichs abgegeben, der den Absorptionswellenlängen von Wasser sehr ähnlich ist. Dadurch werden Wassermoleküle im Inneren des zu trocknenden Lebensmittels zum Schwingen angeregt, wodurch Wärme entsteht und das Wasser ausgetrieben wird. Somit kommt es nicht nur zu einer erheblichen Energieeinsparung, der Trocknungsprozess nimmt auch weniger Zeit in Anspruch. Es war jedoch zu untersuchen, ob auch die Produktqualität der STIR®-getrockneten Waren einen Vorteil gegenüber den konventionell getrockneten Lebensmitteln bietet. Zur Untersuchung boten sich getrocknete Äpfel an, da für deren Herstellung die STIR®-Technologie bereits kommerziell eingesetzt wurde. Im Handel waren ebenso einige Sorten verschiedener konventionell getrockneter Apfelprodukte erhältlich, welche mit den STIR®-getrockneten Produkten verglichen werden konnten. Dabei wurde der Untersuchungsschwerpunkt auf das Zucker-Abbauprodukt HMF und die qualitätsgebenden Pflanzenphenolen gelegt. Dazu wurde zunächst eine geeignete RP-HPLC-Methode zur Trennung und Detektion der Phenolcarbonsäuren und Flavonoide sowie von HMF erarbeitet, bevor die Extraktion optimiert werden konnte. Mit dieser Methode sollten dann verschiedene frische Äpfel sowie insbesondere mit unterschiedlichen Verfahren getrocknete Äpfel untersucht werden. Daneben wurde die Fähigkeit, Radikale abzufangen, in den Extrakten der verschiedenen frischen und getrockneten Äpfel sowie von diversen Standards, welche im Extrakt erwartet worden waren, analysiert. Zunächst wurden frische Äpfel unterschiedlicher Sorten, aber auch frische Äpfel gleicher Sorten von unterschiedlichen Händlern untersucht. Es konnte gezeigt werden, dass sich die Sorten untereinander stark in ihrem Polyphenolmuster unterschieden, wobei in den einzelnen Stoffklassen immer eine oder zwei Verbindungen in besonders hohen Gehalten auftraten. Dies sind 5 Chlorogensäure und 4-p-Cumaroylchinasäure als Phenolcarbonsäure-Derivate, die Flavanole (-)-Epicatechin und Procyanidin B2, die Phloretin-Glycoside Phloretin-2’-Xyloglucosid und Phloretin-2’-Glucosid und das Flavonol Quercitrin. Diese kamen jedoch immer in unterschiedlichen Mengen und Verhältnissen vor. Die Absolut-Gehalte innerhalb einer Sorte variierten ebenfalls zum Teil erheblich, wobei die relativen Gehalte innerhalb einer Sorte hingegen enger zusammen lagen. Im Verlauf der Lagerung dreier Apfelsorten bei ca. 6 °C wurde zunächst eine Zunahme des Gehaltes an 5-Chlorogensäure sowie bei den Sorten Elstar und Jonagold von (-)-Epicatechin sowie Procyanidin B2 festgestellt. Ab der 22. Lagerungswoche wurde dann eine leichte Abnahme beobachtet. Zum besseren Verständnis des Verhaltens der Polyphenole beim Trocknungsprozess wurden verschiedene Erhitzungsversuche durchgeführt. So wurden Äpfel dreier Sorten über sechs Stunden getrocknet, wobei zum Teil deutliche Abnahmen in den Polyphenolgehalten auftraten. Dies betraf insbesondere die Flavan-(3)-ole sowie deren Dimere, deren Gehalt zum Teil auf weniger als die Hälfte des Ausgangsgehaltes im Frischapfel abnahmen. Die Gehalte der Hydroxyzimtsäurederivate – 5-Chlorogensäure und 4-p-Cumaroylchinasäure – nahmen hingegen nur wenig ab. Daneben waren aber auch Zunahmen z. B. der Kaffeesäure festzustellen, welche u. a. aus 5-Chlorogensäure freigesetzt werden konnte. Unterschiedliche Auswirkungen durch die Erhitzung konnten nicht nur bei den verschiedenen Polyphenolen beobachtet werden. Ebenfalls traten diese bei der Trocknung unterschiedlicher Apfelsorten auf. Besonders die Polyphenole der Sorte Braeburn wurden stärker in ihrem Gehalt verringert. Geeigneter zum Trocknen erscheinen die Sorten Jonagold und Elstar, bei denen es nur zu einer geringen Abnahme im Gehalt infolge der Trocknung kam. Neben dem Abbau der Polyphenole wurde die Bildung des Zuckerabbauproduktes HMF untersucht. Zusätzlich trat eine weitere Verbindung auf, welche sich ebenfalls als ein Zuckerabbauprodukt herausstellte. Diese Verbindung wurde besonders bei saurer Fructose-Erhitzung gebildet und aus dieser Erhitzungslösung isoliert. Durch Vergleich von UV- und Massen-Spektren mit verschiedenen Standards konnte die Verbindung als DDMP identifiziert werden. Zur Quantifizierung des DDMP lag kein Standard definierter Reinheit vor, weshalb die Komponente als HMF berechnet wurde. Generell verhielten sich diese beiden Substanzen im Verlauf der Trocknung ähnlich, wobei DDMP erst im späteren Trocknungsverlauf in größeren Mengen gebildet wurde. Die Gehalte erreichten in der Regel nur ein Drittel bis ein Fünftel des HMF-Gehaltes. Zudem wurden Äpfel der Sorte Jonagold einer Charge unter Versuchsbedingungen mit Konvektionstrocknung mit und ohne Umluft sowie mit STIR®-Trocknung bei verschiedenen Temperaturen und Zeiten getrocknet und deren Gehalte verglichen, wobei immer ein Endwassergehalt von ca. 5 % angestrebt wurde, um ein knackiges und lagerstabiles Produkt zu erzielen. Bei der Konvektionstrocknung ohne Umluft erwiesen sich 80 °C und 4 Stunden am günstigsten, mit Umluft 60 °C und 5 Stunden sowie 80 °C und 1½ Stunden. Eine Trocknung bei 100 °C war hingegen in beiden Fällen unvorteilhaft, da hierbei sowohl die Polyphenolgehalte abnahmen als auch die HMF- und DDMP-Gehalte anstiegen. Für die STIR®-Trocknung erwies sich besonders eine Trocknung bei 60 °C für drei Stunden, auch aufgeteilt auf eine Vortrocknung von 2 Stunden und eine Nachtrocknung von einer Stunde, als günstig. Temperaturen von 75 °C allein oder nur bei der Nachtrocknung wirkten sich sehr ungünstig aus, da hierbei ebenfalls Abnahmen bei den Polyphenolen auf bis zu 40 % des ursprünglichen Gehaltes im Frischapfel sowie deutliche Zunahmen im HMF- und DDMP-Gehalt zu verzeichnen waren. Diese Ergebnisse wurden darüber hinaus durch die abschließend durchgeführten Untersuchungen der Radikalfängereigenschaften mittels DPPH-Test bestätigt. Dabei blieb die Fähigkeit, Radikale abzufangen, bei niedrigeren Trocknungstemperaturen im Vergleich zum Frischapfel besser erhalten als bei hohen Trocknungstemperaturen. Temperaturen von 100 °C bei der Konvektionstrocknung führten zur Abnahme auf bis zu 60 % im Vergleich zum Frischapfel, Temperaturen von 75 °C bei STIR® zur Abnahme von bis zu 50 %. Somit konnte insgesamt gezeigt werden, dass sowohl für die Konvektions- als auch für die STIR®-Trocknung von Äpfeln Parameter zur Produktion eines qualitativ hochwertigen Endproduktes ermittelt werden können. Hierbei spielen neben der optimierten Temperatur und Zeit der Trocknung auch die geeignete Sortenwahl eine entscheidende Rolle.

In this study tools consisting of a 5mm cylindrical pin and a 12mm shoulder held by a simple tool holder were used to compare the wear of 11 tooling materials. The objective was to determine if using these tools in a spot welding configuration to simulate friction stir welding could differentiate the potential performance of tooling materials. All tools were made of varying percentages of polycrystalline cubic boron nitride (PCBN), tungsten (W) and rhenium (Re). The materials are referred to herein as GV1, GV2, G1, G2, G3, G4, G5, G6, G7, G8 and G9.The tools were run to 205 welds if they did not fracture first. The grades averaged the following quantities of welds before fracture failure GV-1:0; GV-2:200; G1:82; G2:204; G3:205; G4:205; G5:96; G7:102.73; G8:21.2; G9:38.5. Of the tools that ran the full 205 welds without chipping, the average calculated volume loss, which was the best indication of wear, was as follows G2:1.83%; G3:2.53%; G4:2.41%; G5:1.93%; and G7:2.30%.The study showed that G2 had the least wear and G6 had the most wear, of those tools that completed all 205 spot welds. Fracture was the failure mode of all grades with over 70% CBN content. It was found that small CBN grain size was not correlated to better wear performance, as has been seen in a prior study.

Friction stir processing (FSP) has been utilized to locally process regions of arc weldments in 304L stainless steel to improve the microstructure and mechanical performance. The cast microstructure and coarse delta-ferrite has been replaced with a fine-grained wrought microstructure. Furthermore, twins were introduced throughout the friction stir processed region. Although sub-surface sigma and carbides were introduced during FSP, their presence is not expected to adversely affect the resulting mechanical or corrosion properties of friction stir processed 304L arc welds. The resulting mechanical properties of FS processed weldments were also an improvement over as-welded arc welds. FSP resulted in an increase of 6% for both yield and ultimate strength. It is expected that the improved microstructure will lead to improved stress corrosion cracking and general corrosion properties.