Caratterizzazione microstrutturale e frattografica di componenti in acciaio 316L prodotti tramite Selective Laser Melting

Il Selective Laser Melting è un processo di additive manufacturing che consiste nella realizzazione di componenti metallici tridimensionali, sovrapponendo strati di polvere, che viene via via fusa mediante una sorgente controllata di energia (laser). È una tecnica produttiva che viene utilizzata da più di 20 anni ma solo ora sta assumendo un ruolo rilevante nell’industria. È un processo versatile ma complesso che ad oggi permette di processare solo un numero limitato di leghe. Il presente lavoro di tesi riguarda in particolare lo studio, dal punto di vista microstrutturale, di componenti in acciaio inossidabile austenitico AISI-316L processato mediante Selective Laser Melting, attività svolta in collaborazione con il Gruppo di Tecnologia – Laser del Dipartimento di Ingegneria Industriale. Alla base dell’attività sperimentale è stata svolta anche un’ampia ricerca bibliografica per chiarire lo stato dell’arte sul processo e sulla lega in questione, la microstruttura, i difetti, le proprietà meccaniche e l’effetto dei parametri di processo sul componente finito. Le attività sperimentali hanno previsto una prima fase di caratterizzazione delle polveri di 316L, successivamente la caratterizzazione dei campioni prodotti tramite selective laser melting, in termini di microstruttura e difetti correlati al processo. Le analisi hanno rivelato la presenza di una microstruttura “gerarchica” costituita da melt pool, grani e celle submicrometriche. I difetti rinvenuti sono pori, delaminazione degli strati, particelle di polvere non fuse. Infine è stata eseguita la caratterizzazione frattografica dei campioni sottoposti a prove di trazione e di fatica a flessione rotante (attività condotte dal gruppo Laser) per identificare la morfologia di frattura e i siti di innesco della cricca di fatica.

DESIGN, FABRICATION, AND PERFORMANCE CHARACTERIZATION OF MULTIFUNCTIONAL STRUCTURES TO HARVEST SOLAR ENERGY FOR FLAPPING WING AERIAL VEHICLES

Flapping Wing Aerial Vehicles (FWAVs) have the capability to combine the benefits of both fixed wing vehicles and rotary vehicles. However, flight time is limited due to limited on-board energy storage capacity. For most Unmanned Aerial Vehicle (UAV) operators, frequent recharging of the batteries is not ideal due to lack of nearby electrical outlets. This imposes serious limitations on FWAV flights. The approach taken to extend the flight time of UAVs was to integrate photovoltaic solar cells onto different structures of the vehicle to harvest and use energy from the sun. Integration of the solar cells can greatly improve the energy capacity of an UAV; however, this integration does effect the performance of the UAV and especially FWAVs. The integration of solar cells affects the ability of the vehicle to produce the aerodynamic forces necessary to maintain flight. This PhD dissertation characterizes the effects of solar cell integration on the performance of a FWAV. Robo Raven, a recently developed FWAV, is used as the platform for this work. An additive manufacturing technique was developed to integrate photovoltaic solar cells into the wing and tail structures of the vehicle. An approach to characterizing the effects of solar cell integration to the wings, tail, and body of the UAV is also described. This approach includes measurement of aerodynamic forces generated by the vehicle and measurements of the wing shape during the flapping cycle using Digital Image Correlation. Various changes to wing, body, and tail design are investigated and changes in performance for each design are measured. The electrical performance from the solar cells is also characterized. A new multifunctional performance model was formulated that describes how integration of solar cells influences the flight performance. Aerodynamic models were developed to describe effects of solar cell integration force production and performance of the FWAV. Thus, performance changes can be predicted depending on changes in design. Sensing capabilities of the solar cells were also discovered and correlated to the deformation of the wing. This demonstrated that the solar cells were capable of: (1) Lightweight and flexible structure to generate aerodynamic forces, (2) Energy harvesting to extend operational time and autonomy, (3) Sensing of an aerodynamic force associated with wing deformation. Finally, different flexible photovoltaic materials with higher efficiencies are investigated, which enable the multifunctional wings to provide enough solar power to keep the FWAV aloft without batteries as long as there is enough sunlight to power the vehicle.

Investigation into the Influence of Build Parameters on Failure of 3D Printed Parts

Additive manufacturing, including fused deposition modeling (FDM), is transforming the built world and engineering education. Deep understanding of parts created through FDM technology has lagged behind its adoption in home, work, and academic environments. Properties of parts created from bulk materials through traditional manufacturing are understood well enough to accurately predict their behavior through analytical models. Unfortunately, Additive Manufacturing (AM) process parameters create anisotropy on a scale that fundamentally affects the part properties. Understanding AM process parameters (implemented by program algorithms called slicers) is necessary to predict part behavior. Investigating algorithms controlling print parameters (slicers) revealed stark differences between the generation of part layers. In this work, tensile testing experiments, including a full factorial design, determined that three key factors, width, thickness, infill density, and their interactions, significantly affect the tensile properties of 3D printed test samples.

A MARKET-BASED APPROACH TO 3D PRINTING FOR ECONOMIC DEVELOPMENT IN GHANA

The purpose of this report is to create the foundation for further study of a market-based approach to 3D printing as an instrument for economic development in Ghana. The delivery of improved products and services to the most underserved markets is needed to spur economic activity and improve standards of living. The relationship between economic development and the advancement of technology is considered within the context of Ghana. An opportunity for market entry exists within both the bottom of the economic pyramid and the mid-segment market. 3D printing (additive manufacturing) has proven to be a disruptive technology that has demonstrated an ability to expedite the speed of innovations and create products that were previously not possible. An investigation of how 3D printers can be used to create improved products for the most underserved markets within Ghana is presented. Questions are asked to elucidate how and when adoption of 3D printers and 3D printed products may occur in the future. Based upon the existing barriers to adoption, 3D printing technology must improve before widespread adoption will occur in Ghana.

MULTISCALE MODELING OF LIQUID CRYSTALLINE/NANOTUBE COMPOSITES

The objective of this research is to synthesize structural composites designed with particular areas defined with custom modulus, strength and toughness values in order to improve the overall mechanical behavior of the composite. Such composites are defined and referred to as 3D-designer composites. These composites will be formed from liquid crystalline polymers and carbon nanotubes. The fabrication process is a variation of rapid prototyping process, which is a layered, additive-manufacturing approach. Composites formed using this process can be custom designed by apt modeling methods for superior performance in advanced applications. The focus of this research is on enhancement of Young's modulus in order to make the final composite stiffer. Strength and toughness of the final composite with respect to various applications is also discussed. We have taken into consideration the mechanical properties of final composite at different fiber volume content as well as at different orientations and lengths of the fibers. The orientation of the LC monomers is supposed to be carried out using electric or magnetic fields. A computer program is modeled incorporating the Mori-Tanaka modeling scheme to generate the stiffness matrix of the final composite. The final properties are then deduced from the stiffness matrix using composite micromechanics. Eshelby's tensor, required to calculate the stiffness tensor using Mori-Tanaka method, is calculated using a numerical scheme that determines the components of the Eshelby's tensor (Gavazzi and Lagoudas 1990). The numerical integration is solved using Gaussian Quadrature scheme and is worked out using MATLAB as well. . MATLAB provides a good deal of commands and algorithms that can be used efficiently to elaborate the continuum of the formula to its extents. Graphs are plotted using different combinations of results and parameters involved in finding these results

Ottimizzazione mediante codici di calcolo open source di componenti per il settore aerospaziale

Lo scopo della presente tesi è sviluppare un ambiente per l'ottimizzazione strutturale di componenti per applicazione aerospaziale utilizzando codici open-source. In particolare, il codice Salome viene utilizzato per il disegno automatico delle strutture, il programma Code Aster permette di effettuare l'analisi agli elementi finiti del componente, mentre Octave viene utilizzato per svolgere l'ottimizzazione basata su un algoritmo euristico e per integrare fra di loro i differenti codici. Le tecniche di ottimizzazione dei componenti stanno rivestendo sempre più importanza visto che le moderne tecniche di Additive Manufacturing permettono di realizzare strutture molto complesse che un tempo non era conveniente (o possibile) realizzare con asportazione di materiale. Nella prima parte della tesi si descrivono gli strumenti software utilizzati e la loro integrazione al fine di parametrizzare la generazione di geometrie ed effettuare in modo automatico analisi strutturali. Successivamente si descrivono tre casi di studio in cui la metodologia è stata sperimentata: un primo caso di validazione in cui si è applicato il metodo alla definizione della geometria di minimo peso per una trave a sbalzo con carico concentrato, un secondo test di ottimizzazione di un longherone per aeromobile, un terzo caso applicativo legato alla ottimizzazione di un serbatoio per fluidi in pressione da utilizzare su un satellite.

Conception of a YBCO Superconducting ZFC-Magnetic Bearing Virtual Prototype

This paper presents the conception of an original superconducting Frictionless Zero Field Cooling bearing virtual prototype. In previous work also shown in this conference, a viability study of a Zero Field Cooling-superconducting bearing concept was conducted. It showed that the virtual prototype is feasible. Moreover, the simulation studies showed that a Zero Field Cooling superconducting track provides not only effective lateral stability but also higher levitation forces than the commonly used Field Cooling tracks. In this paper the new Zero Field Cooling -bearing virtual prototype is modeled in 3D. The virtual prototype was designed having in mind: i) a future implementation in high density polyurethane, for low temperature robustness; ii) future manufacturing in a three axes CNC milling machine and; iii) future implementation of some parts using an additive manufacturing technique.

Studio e ottimizzazione del processo di fabbricazione additiva (SLM) per applicazioni in ambito biomedicale: produzione di protesi e strutture reticolari

Il progetto di dottorato che verrà presentato in questa tesi è focalizzato sullo sviluppo di un metodo sperimentale per la produzione di protesi personalizzate utilizzando il processo di fabbricazione additiva di Selective Laser Melting (SLM). L’obiettivo è quello di definire una condizione di processo ottimizzata per applicazioni in ambito chirurgico che possa essere generalizzabile, ovvero che ne garantisca la riproducibilità al variare dell’anatomia del paziente e che rappresenti la base per estendere il metodo ad altre componenti protesiche. Il lavoro si è sviluppato lungo due linee principali, la cui convergenza ha permesso di realizzare prototipi di protesi complete utilizzando un solo processo: da una parte la produzione di componenti a massima densità per il raggiungimento di elevate resistenze meccaniche, buona resistenza ad usura e corrosione e controllo di tensioni residue e deformazione delle parti stampate. Dall’altra si sono studiate strutture reticolari a geometria e porosità controllata per favorire l’osteointegrazione della componente protesica post impianto. In questo studio sono stati messe a confronto le possibili combinazioni tra parametri di processo e sono state individuate le correlazioni con le proprietà finali dei componenti stampati. Partendo da queste relazioni si sono sviluppate le strategie tecnologiche per la progettazione e la produzione dei componenti. I test sperimentali svolti e i risultati ottenuti hanno dimostrato la fattibilità dell’utilizzo del processo SLM per la produzione di protesi personalizzate e sono stati fabbricati i primi prototipi funzionali. La fabbricazione di protesi personalizzate richiede, però, anche la progettazione e la produzione di strumentario chirurgico ad hoc. Per questo motivo, parallelamente allo studio della lega di Cromo Cobalto, sono stati eseguiti i test anche su campioni in INOX 316L. Anche in questo caso è stato possibile individuare una finestra operativa di processo che garantisse proprietà meccaniche comparabili, e in alcuni casi superiori, a quelle ottenute con processi convenzionali.

Preparazione e caratterizzazione di nuovi nanocompositi elastomerici mediante stampa 3D

Il presente lavoro di tesi si è incentrato sulla preparazione e caratterizzazione di manufatti elastomerici mediante Additive Manufacturing, sfruttabili per l'abbigliamento sportivo. Sono stati studiati diversi parametri di stampa, tra cui la geometria di riempimento, angoli di deposizione, infill e velocità di stampa, al fine di ottenere prestazioni ottimizzate confrontabili con materiali convenzionali. Inoltre sono state utilizzate per lo scopo diverse matrici elastomeriche caratterizzate da differente durezza Shore A. Le proprietà termiche dei manufatti stampati sono state studiate attraverso analisi TGA e DSC. Invece, le prestazioni meccaniche sono state analizzate attraverso DMA, prove di trazione e prove di compressione/espansione. Infine, è stato sviluppato un materiale innovativo nanocomposito al fine di ampliare il campo di utilizzo della FDM. Il materiale è stato caratterizzato tramite le classiche tecniche di analisi termiche e meccaniche.

Stabilità ad alta temperatura delle leghe ad alta entropia: effetti della tecnologia di produzione

Dando continuità all’attività di ricerca intrapresa durante il mio precedente tirocinio presso il Dipartimento di Ingegneria Industriale DIN di Bologna, lo scopo della mia tesi è stato quello di chiarire i meccanismi di stabilità di fase della lega ad alta entropia CoCrFeMnNi ed esplorare la sua natura metastabile alle medio-alte temperature (tra i 450-1150°C). Nell’ottica di possibili future applicazioni industriali, è stato inoltre valutato l’effetto che la produzione via Additive Manufacturing può avere su proprietà e comportamenti delle leghe ad alta entropia. Sperimentalmente sono state fatte importanti osservazioni, a volte in contrasto con la letteratura precedente, che aprono la strada ad ulteriori e più specifiche indagini verso la comprensione dei complessi meccanismi che recentemente hanno reso queste leghe così interessanti per la ricerca a livello mondiale.

Advanced voxel-based CAD modelling for FSI simulations for automotive structures design

Additive Manufacturing (AM) is nowadays considered an important alternative to traditional manufacturing processes. AM technology shows several advantages in literature as design flexibility, and its use increases in automotive, aerospace and biomedical applications. As a systematic literature review suggests, AM is sometimes coupled with voxelization, mainly for representation and simulation purposes. Voxelization can be defined as a volumetric representation technique based on the model’s discretization with hexahedral elements, as occurs with pixels in the 2D image. Voxels are used to simplify geometric representation, store intricated details of the interior and speed-up geometric and algebraic manipulation. Compared to boundary representation used in common CAD software, voxel’s inherent advantages are magnified in specific applications such as lattice or topologically structures for visualization or simulation purposes. Those structures can only be manufactured with AM employment due to their complex topology. After an accurate review of the existent literature, this project aims to exploit the potential of the voxelization algorithm to develop optimized Design for Additive Manufacturing (DfAM) tools. The final aim is to manipulate and support mechanical simulations of lightweight and optimized structures that should be ready to be manufactured with AM with particular attention to automotive applications. A voxel-based methodology is developed for efficient structural simulation of lattice structures. Moreover, thanks to an optimized smoothing algorithm specific for voxel-based geometries, a topological optimized and voxelized structure can be transformed into a surface triangulated mesh file ready for the AM process. Moreover, a modified panel code is developed for simple CFD simulations using the voxels as a discretization unit to understand the fluid-dynamics performances of industrial components for preliminary aerodynamic performance evaluation. The developed design tools and methodologies perfectly fit the automotive industry’s needs to accelerate and increase the efficiency of the design workflow from the conceptual idea to the final product.

Optimization of an innovative T6 heat treatment for the AlSi10Mg alloy processed by Laser-based Powder Bed Fusion: effect on microstructure, mechanical properties, and tribological behavior

Laser-based Powder Bed Fusion (L-PBF) technology is one of the most commonly used metal Additive Manufacturing (AM) techniques to produce highly customized and value-added parts. The AlSi10Mg alloy has received more attention in the L-PBF process due to its good printability, high strength/weight ratio, corrosion resistance, and relatively low cost. However, a deep understanding of the effect of heat treatments on this alloy's metastable microstructure is still required for developing tailored heat treatments for the L-PBF AlSi10Mg alloy to overcome the limits of the as-built condition. Several authors have already investigated the effects of conventional heat treatment on the microstructure and mechanical behavior of the L-PBF AlSi10Mg alloy but often overlooked the peculiarities of the starting supersatured and ultrafine microstructure induced by rapid solidification. For this reason, the effects of innovative T6 heat treatment (T6R) on the microstructure and mechanical behavior of the L-PBF AlSi10Mg alloy were assessed. The short solution soaking time (10 min) and the relatively low temperature (510 °C) reduced the typical porosity growth at high temperatures and led to a homogeneous distribution of fine globular Si particles in the Al matrix. In addition, it increased the amount of Mg and Si in the solid solution available for precipitation hardening during the aging step. The mechanical (at room temperature and 200 °C) and tribological properties of the T6R alloy were evaluated and compared with other solutions, especially with an optimized direct-aged alloy (T5 alloy). Results showed that the innovative T6R alloy exhibits the best mechanical trade-off between strength and ductility, the highest fatigue strength among the analyzed conditions, and interesting tribological behavior. Furthermore, the high-temperature mechanical performances of the heat-treated L-PBF AlSi10Mg alloy make it suitable for structural components operating in mild service conditions at 200 °C.

Numerical simulation of the flow and performance evaluation of an aerospike engine

The main focus of this work is to define a numerical methodology to simulate an aerospike engine and then to analyse the performance of DemoP1, which is a small aerospike demonstrator built by Pangea Aerospace. The aerospike is a promising solution to build more efficient engine than the actual one. Its main advantage is the expansion adaptation that allows to reach the optimal expansion in a wide range of ambient pressures delivering more thrust than an equivalent bell-shaped nozzle. The main drawbacks are the cooling system design and the spike manufacturing but nowadays, these issues seem to be overcome with the use of the additive manufacturing method. The simulations are performed with dbnsTurbFoam which is a solver of OpenFOAM. It has been designed to simulate a supersonic compressible turbulent flow. This work is divided in four chapters. The first one is a short introduction. The second one shows a brief summary of the theoretical performance of the aerospike. The third one introduces the numerical methodology to simulate a compressible supersonic flow. In the fourth chapter, the solver has been verified with an experiment found in literature. And in the fifth chapter, the simulations on DemoP1 engine are illustrated.

Vibro-acoustic analysis of additively manufactured acoustic metamaterials

In the last decade it emerged the interest in new types of acoustic insulating materials, called acoustic metamaterials. These materials are composed by a host and inclusions and are arranged periodically or non-periodically in sub-wavelength elements called meta-atoms. Their inclusions and internal geometries can be manipulated to tailor the acoustic properties, reducing weight, and increasing at the same time their efficiency. Thanks to the high absorbing characteristics that they can achieve, their usage is of particularly interest as material of the core in sandwich panels of aerospace structures to reduce vibrations and noise inside passengers aircraft’s cabin. In addition, since the low frequency signals are difficult to be damped with conventional materials, their usage can guarantee a high transmission loss at low frequencies, obtaining a positive benefit on passengers’ comfort. The performances and efficiency of these materials are enhanced thanks to the new additive manufacturing techniques opposed to the conventional ones uncapable to pro- duce such complex internal geometries. The aim of this work is to study, produce and redesign micro-perforated sandwich panels of a literature case study to achieve high performances in the low frequency range, e.g., below 2000 Hz. Some geometrical parameters, such as perforation ratio and diameter of holes, were considered to realize different models and see the differences in the sound transmission loss. The models were produced by means of Fused Deposition Modelling using an Acrylonitrile Butadiene Styrene (ABS Plus p430) material on a commercial additive manufacturing system. Finally, the frequency response analysis was carried out with Mul2 software, based on the Carrera’s Unified Formulation (CUF) to understand the acoustic and structural properties of the material employed, analyzing the plates’ displacements and the TL results.

Design of compliant mechanisms and their application to morphing wing technology

This thesis describes a study conducted for the development of a new approach for the design of compliant mechanisms. Currently compliant mechanisms are based on a 2.5D design method. The applications for which compliant mechanisms can be used this way, is limited. The proposed research suggests to use a 3D approach for the design of CM’s, to better exploit its useful properties. To test the viability of this method, a practical application was chosen. The selected application is related to morphing wings. During this project a working prototype of a variable sweep and variable AoA system was designed and made for an SUAV. A compliant hinge allows the system to achieve two DOF. This hinge has been designed using the proposed 3D design approach. To validate the capabilities of the design, two methods were used. One of these methods was by simulation. By using analysis software, a basic idea could be provided of the stress and deformation of the designed mechanism. The second validation was done by means of AM. Using FDM and material jetting technologies, several prototypes were manufactured. The result of the first model showed that the DOF could be achieved. Models manufactured using material jetting technology, proved that the designed model could provide the desired motion and exploit the positive characteristics of CM. The system could be manufactured successfully in one part. Being able to produce the system in one part makes the need for an extensive assembly process redundant. This improves its structural quality. The materials chosen for the prototypes were PLA, VeroGray and Rigur. The material properties were suboptimal for its final purpose, but successful results were obtained. The prototypes proved tough and were able to provide the desired motion. This proves that the proposed design method can be a useful tool for the design of improved CM’s. Furthermore, the variable sweep & AoA system could be used to boost the flight performance of SUAV’s.