Development Of Al-Mg-Sc Alloys For Aerospace Applications

Institution: GU/EKTAM
Supervisor: Rahmi Ünal, Prof., (M)
Co-Supervisor(s): -

Aluminum alloys with magnesium as the major alloying element constitute a group of non-heat-treatable alloys with medium strength, high ductility, excellent corrosion resistance and weldability. Unfortunately, the strength of such Al–Mg alloys is lower than precipitation-hardening Al alloys. However, the addition of a small amount of scandium has been found to significantly improve the strength of Al–Mg alloys, owing to the presence of coherent, finely dispersed L12 Al3Sc precipitate particles that can be obtained at a high number density, thus preventing the dislocation motion. A high specific strength and excellent weldability in combination with good corrosion resistance of Al–Mg–Sc alloys make these alloys attractive for aircraft application. By using powder metallurgy (PM) route it is possible to make a high strength alloy with increased solid solubility. Earliest reports for room temperature tensile strengths of PM alloys were 548 MPa and 595 MPa for the Al-1.1Sc-6Mg and Al-1.9Sc-6Mg alloys, respectively. Al-Mg-Sc alloy was developed as Scalmalloy® for SLM processing by APWorks. Early studies report successful processing of Scalmalloy® using SLM. Relative density accomplished was well more than 99% at higher energy densities typical of other Al or Ni-based alloys. The principle strengthening mechanism observed in microscopy was supersaturation of Sc particles as well as precipitation of Al3Sc phase which pins grain boundary and hinders dislocation gliding, giving rise to superplastic material flow.

In this study, it is aimed to design a new Al-Mg-Sc alloy using the first-principle calculations using CAmbridge Sequential Total Energy Package (CASTEP) code based on density functional theory. New and unique al-Mg-Sc alloy will be developed with theoretical studies and appropriate compositions will be decided according to the desired properties. Then, these alloys will be produced experimentally, and their physical and mechanical properties will be investigated. At the end of the study, an alloy will be developed in order to produce parts that can be used in the field of aviation and space by additive manufacturing method.

Investigation of coating effect on biomechanical properties of additive manufactured humerus fracture fixation plates

Institution: GU/EKTAM
Supervisor: Mehmet Fatih Aycan, Ph.D., (M)
Co-Supervisor(s): İbrahim Uslan, Prof. (GU, TURKEY) (M)/ Yogendra Kumar Mishra, Prof. MSO (University of Southern Denmark, DENMARK) (M)

Short description of the project

Fixation of unstable bone fractures in osteoporotic patients remains a clinical challenge. The use of fracture fixation plates has become a standard treatment for proximal humeral fractures, which account for 5-6% of annual reported fractures. The locking feature of the fracture fixation plates provides a mechanical advantage by increasing the resistance of the implant. In addition to the mechanical strength advantages it provides, the alignment causes misalignment in the implant as a result of cutting the application screws due to varus collapse. Although the life of the implant is tried to be increased by using calcar screws, it is predicted that bone repair will be more successful by improving the design of the fracture fixation plates and making them specific to the patient. With the developing technology, the compatibility of additive manufacturing methods with reversible engineering has increased and implant production compatible with patient tomography can be realized. Although it is thought that the production of patient-specific fracture fixation plate will reduce problems such as varus collapse, it is striking that there is a lack of biomechanical data in the literature. In addition, studies on the effect of the change of the bone-implant connection interface behavior of the coatings to be applied to the productions made with different surface patterns or porous sections produced by additive manufacturing on the implant biomechanical properties are insufficient. In this study, special fracture fixation plates for patients with humeral fractures will be produced as porous and solid by powder bed additive manufacturing methods. Organic or inorganic coating will be applied to the plates to be produced in order to increase bone tissue development. The biomechanical behavior of the samples will be determined by measuring the displacements between the implant parts by performing compression, torsion and dynamic loading tests on the produced samples. In addition, the stresses and displacements in the structure will be examined for the aforementioned mechanical tests with the help of the finite elements and numerical analysis model to be created.

Manufacturing of a Novel Intervertebral Body Fusion Device with Different Metals Using AM Methods

Institution: GU/EKTAM
Supervisor: Mehmet Fatih Aycan, Ph.D., (M)
Co-Supervisor(s): İbrahim Uslan, Prof. (GU, TURKEY) (M)/ Yogendra Kumar Mishra, Prof. MSO (University of Southern Denmark, DENMARK) (M)

Short description of the project

The intervertebral body fusion device is the most well-known example of porous metal implants used in spinal surgery. In this technique, in order to maintain spine alignment and disc height the entire intervertebral disc between vertebrae is removed and the cage is placed between the vertebra. If necessary, it may be placed with or without a bone graft. The cages are manufactured as a solid by conventional manufacturing methods. In comparing with solid cages, porous novel cages exhibit improved strength, lower stiffness, long term stability and more aligned with human bone properties. Porous metal cages are a good alternative for polyetheretherketone (PEEK) ones. Additive manufacturing allows to combine the biocompatibility of metal material with improved biomechanical and bone incorporative qualities for novel cages.

The cages produced with various lattice structure and metal materials as CrCo or Ti6Al4V have different fatigue, compression, compression-shear and torsion strengths as well. The effects of lattice structure for different materials on biomechanical performance of the novel metal cages will be determined. The biomechanical properties will be investigated after the productions made from CrCo and Ti6Al4V materials by selective laser melting method. After determining the optimum lattice structure for both material, the finite element models will be prepared. The models prepared will be verified by using experimental test results and the best model represented the novel design biomechanically will also be determined. Besides, completing verification of the models, the novel metal cages were compared with the solid ones produced with same materials experimentally and numerically in order to show the lattice effect.

Compensation of the Lattice Structure with Hybrid Unit Cell and Investigation of Compression Properties

Institution: GU/EKTAM
Supervisor: Yusuf Usta, Prof., (M)
Co-Supervisor(s): İbrahim Uslan, Prof. (GU, TURKEY) (M)/ Yogendra Kumar Mishra, Prof. MSO (University of Southern Denmark, DENMARK) (M)

Short description of the project

The pores of the porous structures affect the mechanical, thermal and biological properties of the material. Due to the increase in osseointegration, its effect on the amount of heat transfer and low specific gravity, porous materials have been started to be investigated in biomedical, heat exchanger and aerospace fields. Porous materials can be produced by conventional or additive manufacturing methods, and the additive manufacturing method has been found to be more controlled and reproducible. In the production of porous structures with additive manufacturing methods, the basic geometry is lattice structures and thickening and sagging occur in the production. In the study, unit cell design with hybrid geometry compensated according to production changes will be made and compression properties will be examined. Productions will be made from CoCr alloy by selective laser melting. Within the scope of manufacturability studies, benchmark production will be designed on the basis of unit cell geometry for square or cylindrical cross-section bars, spherical and elliptical surfaces, and holes with different positions and production changes will be examined. After the compensation work to be created, the change in production will be reduced. Hybrid unit cell will be designed to be use in biomedical field considering the compensation results of different geometries. The compression properties of the lattice structure produced with cubic volume for different axes will be determined and the results will be used to homogenize the structure for finite element analysis. Model verification studies will be done by comparing the results of compression tests, 3D model of the building and finite element analysis made by homogenization. The hybrid cell, whose production and strength are determined, will be applied to the empty geometry in the in-body fusion cage design and the effect of the biomedical product on compression mechanical strength will be investigated numerically and experimentally.

Brief Information About the Department and Research Center(s)

With 37.000 students (1.500 foreign), 11 faculties, 5 graduate schools, and 3 vocational colleges, Gazi University (GU), established in 1926 in Ankara, is top-10 University in Turkey mainly focusing on science and technology. Having strong laboratories and research centers in the fields of “life sciences”, “photonics” and “additive manufacturing”, Gazi University has been classified as a “research university” to foster “research and development” activities together with industry and other university/institutions. It has played an important role in the development of Turkey with its academic and technological achievement and proved its success in education both nationally and internationally, thus providing an excellent environment for the development of the doctoral programme. Established by Gazi University in 2017, the Additive Manufacturing Technologies Application and Research Center (EKTAM) is the National Center of Excellence for Additive Manufacturing to accelerate the deployment of this technology and develop novel materials, products and services in the advanced materials, advanced and additive manufacturing value-chains regarding process design, modelling & simulation, materials, post-processing, product, certification and end-life, with the aim of developing a set of technologies, materials and processes that could be applied to the AM field.



Investigation Of The Manufacturability Of Metal Ceramic Composite Materials From Stainless Steel And Alumina Powders By Selective Laser Melting

Option: II
Institution: GU/EKTAM
Supervisor: Yusuf Usta, Prof., (M)
Co-Supervisor(s): İbrahim Uslan, Prof. (GU, TURKEY) (M)/ Yogendra Kumar Mishra, Prof. MSO (University of Southern Denmark, DENMARK) (M)

Short description of the project

The pores of the porous structures affect the mechanical, thermal and biological properties of the material. Due to the increase in osseointegration, its effect on the amount of heat transfer and low specific gravity, porous materials have been started to be investigated in biomedical, heat exchanger and aerospace fields. Porous materials can be produced by conventional or additive manufacturing methods, and the additive manufacturing method has been found to be more controlled and reproducible. In the production of porous structures with additive manufacturing methods, the basic geometry is lattice structures and thickening and sagging occur in the production. In the study, unit cell design with hybrid geometry compensated according to production changes will be made and compression properties will be examined. Productions will be made from CoCr alloy by selective laser melting. Within the scope of manufacturability studies, benchmark production will be designed on the basis of unit cell geometry for square or cylindrical cross-section bars, spherical and elliptical surfaces, and holes with different positions and production changes will be examined. After the compensation work to be created, the change in production will be reduced. Hybrid unit cell will be designed to be use in biomedical field considering the compensation results of different geometries. The compression properties of the lattice structure produced with cubic volume for different axes will be determined and the results will be used to homogenize the structure for finite element analysis. Model verification studies will be done by comparing the results of compression tests, 3D model of the building and finite element analysis made by homogenization. The hybrid cell, whose production and strength are determined, will be applied to the empty geometry in the in-body fusion cage design and the effect of the biomedical product on compression mechanical strength will be investigated numerically and experimentally.

Growth of large area two-dimensional transition metal dichalcogenide nanostructures: Fabrication of nanoscale electronic and optoelectronic devices

Institution: GU/ GAZI PHOTONICS
Supervisor: Süleyman Özçelik, Prof., (M)
Co-Supervisor(s): -

Short description of the project

Transition metal dichalcogenides (TMDCs; MX2, where M=Mo or W and X=S or Se) family has recently gained great attention due to its unique electrical, mechanical and optical properties. In addition, In addition, their monolayers and their heterostructures and also the form of sandwiched with wide band gap semiconductors stand out among the promising nanomaterials for the production of next-generation nanoscale optoelectronic devices, thanks to their excellent properties in light trapping and photo-sensing. Photodetectors, which have the functionality of sensing photonic signals and converting them to electric current, are an important component of electro-optical systems such as imaging, sensing and communication. TMDCs is a semiconductor material which has a layered-structure, and while the atoms within each layer in TMDCs are strongly covalently bounded, the adjacent layers are held together by weak van der Waals interaction. The Weak van der Waals interactions enable the exfoliation of TMDCs to individual atomically thin layers. Single and multilayer thin films of TMDCs have unique properties like thickness dependent band gap in visible to infrared regions, high carrier mobility, large surface-to-volume-ratio, strong spin-valley coupling, chemical stability and high mechanical flexibility. These properties make TMDCs a highly promising material in future nanoscale electronic and optoelectronic device applications such as photocatalysis, photodetectors, biosensors, gas sensors, phototransistors, field effect transistors (FETs), solar cells and light emitting diodes (LEDs).

There have been several attempts, including top-down and bottom–up methods, such as mechanical-mechanical exfoliation, hydrothermal synthesis, physical vapor deposition (PVD) and chemical vapor deposition (CVD) to produce two-dimensional (2D) TMDCs thin films. Initially, researchers intensely focused on the exfoliation method for the obtainment of monolayers of TMDCs films, which remains the most commonly used method for the growth of MoS2 films. However, it has been recently recognized that the exfoliation method is not suitable for the large scale production of 2D-TMDCs nanostructure. In this context, the synthesis of uniform large-area 2D-TMDCs layers by controlling the film thickness is necessary for the practical use of this material in electronic and optoelectronic applications in industry. CVD is one of the most promising methods to produce continuous of these structures over large areas as an alternative to exfoliation methods. However, the controllable growth of these 2D nanostructure over large areas by the CVD method remains an enormous challenge. The current understanding of the CVD growth process has significant shortcomings and, therefore, optimization studies on the growth of TMCDs films by the CVD method is currently a significant and urgently needed area of research. Proposed PhD thesis will focus on the large area growth of 2D-TMCDs (MX2, where M=Mo or W and X=S) and their heterostructures through the use of the CVD method. Structural, electrical, optical, morphological characteristics and chemical bonding structures of grown two-dimensional TMDs will be determined. In addition, the fabrication of the photodetector from the developed 2D nanomaterials will also be studied within the scope of the thesis.

Investigation of Innovative Laser Beam Scanning Strategies and Beam Parameter Interactions for Net- Shape Additive Manufacturing of AlSi10Mg Alloy Aerospace Components

Institution: GU/EKTAM
Supervisor: Kürşad Sezer, Assoc. Prof., (M)
Co-Supervisor(s): Olcay Ersel Canyurt, Prof., (GU, TURKEY), (M)

Short description of the project

The project will deal with study of Selective laser melting (SLM) process which is one of the famous methods among additive manufacturing technologies for manufacturing complex aerospace parts. The ultimate goal of this Project is to reveal the correlation between geometrical tolerances, surface quality, metallurgical characteristics and functional performance of the components and the key process parameters including laser beam scanning strategies. Experimental and theoretical modelling methods will be used to identify and optimize windows of process parameters required to fabricate high density and net shape aerospace components using selective laser irradiation and assessment of the part quality; this will involve development of selective laser melting process on specific aerospace materials, and model to understand the fundamental mechanisms of the process to identify optimal operating conditions and followed by characterization using a number of analytical testing techniques (e.g. Optical and Scanning electron microscope, residual stress measurements via X-ray diffraction, Electron Back-Scattered Diffraction and Transmission Electron Microscopy etc.).

The investigation of the parameters of Hot Isostatic Process for additive manufactured metal materials.

Institution: GU/EKTAM
Supervisor: Olcay Ersel Canyurt, Prof., (M)
Co-Supervisor(s): Kürşad Sezer, Assoc. Prof. (GU, TURKEY) (M)

Short description of the project

After additive manufacturing, internal defects, porosity of lack of fusion, gas porosity, oxides, micro cracks) etc. play an important role in the strength of the AM products. Elimination of internal defects using post-processing methods helps to eliminate stress concentrations, crack initiation points. In this way, it is possible to obtain superior material properties with x10 – x100 times increased fatigue life, ductility and fracture toughness, reduced voids, defects, scattering, more predictive material properties, and increased safety factor. The literature reveals that hot isostatic pressure technique (HIP) is necessary to increase the strength of additive manufactured products and HIP parameters should be developed.

Hot Isostatic Pressure post-processing methods will be used to provide 100% density and improved mechanical properties and better performance. Appropriate HIP parameters needs to be determined and developed in order to obtain qualified products. In these studies, it is extremely important to optimize the selection of materials, the determination of HIP parameters for the aerospace industry. Small-grained, equiaxed microstructure can be produced in metal materials structure by hot isostatic pressure, additive manufactured materials could have a wide range of superior, isotropic mechanical properties.

Additive Manufacturing of Parts Having Varying Elemental Compositions and Properties

Institution: GU/EKTAM
Supervisor: Ömer Keleş, Prof., (M)
Co-Supervisor(s): -

Short description of the project

Additive manufacturing becomes critically important for designing and producing selective parts. In practical applications, some parts are expected to have varying properties, such as hardness and wear resistance, to fulfil the required tasks. Some of these parts include bearings, drill bits, cutting tools, and similar. Surface of these parts are expected to have higher hardness and wear resistance with higher thermal conductivity than the bulk properties. This is because of the fact that mechanical friction creates high temperature and high wearing on the surfaces because of the nature of the mechanical loads. Hence, creating multi-functional hard surfaces resisting wear and dissipating heat becomes demanding. Moreover, 3D printing of such parts with varying properties is challenging because of thermal and mechanical integrity of selected powders having different properties. Blending of carbide powders with powder used for printing of the parts may appear to be one of the solutions towards creating such parts with multi-functional properties. In the proposed thesis study, 3D printing of multi-functional parts is to be investigated while incorporating blend of various metallic and carbide powders. Thermal modeling of the heat transfer (including melting) during 3D printing will be considered incorporating the commercial software such as Comsol, ANSYS or Abacus. Thermal stress fields formed in the parts are also modelled to assess the residual stresses. The characterization tests including metallurgical and morphological changes, hardness, mechanical properties (fracture toughness, tensile, fatigue, and creep) are to be conducted for the parts produced. The optimal printing conditions are, then, identified.

Fatigue Performance of Additively Manufactured Metamaterials Under Random Vibration Conditions: The Effects of Topology and Material

Institution: GU/EKTAM
Supervisor: Nizami Aktürk, Prof., (M)
Co-Supervisor(s): Metin U. Salamci, Prof. (GU, TURKEY) (M)/ Celal Sami Tüfekçi, Ph.D. (TeknoHAB, TURKEY) (M)

Short description of the project

Depending on the physical property of interest, metamaterials are called optical metamaterials, mechanical metamaterials, or acoustic metamaterials. Mechanical metamaterials have attracted great interest due to their ability to attain material properties outside the bounds of those found in natural materials. Many promising mechanical metamaterials have been designed, fabricated, and tested, however, these metamaterials have not been subjected to the rigorous requirements needed to certify their use in demanding industrial applications that require multifunctional behavior. They are more commonly used in the space, the transportation, the energy and the nuclear industry. This metamaterial offers an agile and economical solution for the realization of next generation components.

Additive manufacturing techniques enable fabrication of many different machine parts with outstanding combinations of topological, mechanical, and mass properties. It is not well understood to what extent the metamaterial will resist the fatigue under harsh conditions such as when it is excited under random conditions. Additive manufacturing of titanium components holds promise to deliver benefits such as reduced cost, weight and carbon emissions during both manufacture and use. However, it must be shown that the mechanical performance of parts produced by additive manufacturing can meet design requirements that are typically based on wrought material performance properties. Of particular concern for safety critical structures are the fatigue properties of parts produced by Additive Manufacturing. Researchers point out that the fatigue properties of specimens produced by the laser melting additive manufacturing process is significantly lower compared to wrought material. This reduction in fatigue performance was attributed to a variety of issues, such as microstructure, porosity, surface finish and residual stress.

Residual stresses are an inescapable consequence of manufacturing and fabrication processes, with magnitudes that are often a high proportion of the yield or proof strength. Despite this, their incorporation into life prediction is primarily handled through sweeping assumptions or conservative application of statistics. This can lead to highly conservative fatigue design methodologies or unforeseen failures under dynamic loading. The pull from the desire for higher levels of materials performance, coupled with the push of more sophisticated techniques for residual stress measurement, favors a reassessment of the accuracy of assumptions made about residual stresses and their modification during fatigue cycling.

This research therefore aimed to determine fatigue behavior of the additively manufactured metamaterials under real random input. The effects of material type, manufacturing imperfections, and topological design will be searched for fatigue life.

Robot Assisted Post Processing in Additive Manufacturing

Institution: GU/EKTAM
Supervisor: Mehmet Arif Adlı, Prof., (M)
Co-Supervisor(s): Bulent Özkan, Assoc. Prof., (GU, TURKEY), (M)

Short description of the project

Robots are versatile and skillful machines which offer flexibility in complex manufacturing processes that are otherwise difficult to perform. When cooperating together, robots can provide much more maneuverability to manipulate tools and perform task on complex geometries. This aspect has recently speeded up the efforts to use the robots to expand the capabilities of additive manufacturing (AM) processes. Robots have already been used in several AM processes, such as conformal deposition, large-scale AM and multi-directional fabrication, etc. Post processing of complex parts is another possible functional capability of AM processes that can be expanded by using robots.

Multiple cooperating robots can coordinate to perform post processing operations of the parts manufactured via AM which have extremely complex geometries obtained by topology optimization.

In this study, we propose a novel control algorithm that allows two robot arms to cooperate successfully to perform post processing of parts with extremely complex geometries. In the proposed control algorithm, while one of the robot arms manipulate the part the other simultaneously performs the post processing operation. This allows a very high degree of flexibility and maneuverability which is otherwise extremely difficult to achieve with the existing conventional methods. The hybrid position and force control algorithm enhanced with the impedance control will incorporate the complex motion planning and the interaction forces between the robot arms and the part being processed.

Advancing Metal Additive Manufacturing Post-Processing Techniques: Development of Novel Heat-Treatment and Surface Finishing Methodologies and Procedures to Minimize Residual Stresses

Institution: GU/EKTAM
Supervisor: Elmas Salamcı, Assoc. Prof., (F)
Co-Supervisor(s): Hakan Yavaş, Ph.D. (TUSAS, Turkey) (M)/ Fahrettin Ozturk, Prof. (TUSAS, Turkey) (M) / Burcu Arslan Hamat, Ph.D. (TUSAS, Turkey) (F)

Short description of the project

Powder Bed Fusion Additive Manufacturing is one of the Additive Manufacturing Techniques which is able to fabricate intricate structures without any need for tools and molds. Nevertheless, Additive Manufacturing processes may result in undesired part qualities such as poor surface quality, inhomogeneous microstructure, anisotropy in the building directions, porous structures, structural defects, cracks, poor wear-corrosion resistance, and poor mechanical behavior such as reduced fatigue life.

The research is focused on post-processing techniques to be applied to Additively Manufactured Parts. The post-processing methods include -but not limited to- Hot Isostatic Pressing (HIP), Peening, Polishing, etc., The research results are formulated such that the findings are used to redesign the Metal Additive Manufacturing. Therefore, the Metal Additive Manufacturing process is to be studied in detail such that the process parameters that affect the Additive Manufacturing Process are highlighted.

Modeling the impact of processing-structure-property uncertainty on digital certification for additive manufacturing in aerospace

Institution: GU/EKTAM
Supervisor: Metin U. Salamci, Prof., (M)
Co-Supervisor(s): Hakan Yavaş, Ph.D. (TUSAS, Turkey) (M)/ Gustavo M. Castelluccio, Ph.D. (Cranfield University, UK) (M)/ Andrea Cini, Ph.D. (Universidad Carlos III Madrid, SPAIN) (M)

Short description of the project

Advances in metallic 3D printing will reshape engineering disciplines in the next decade by enabling cheaper and more flexible designs. Hence, this PhD opportunity will nurture innovators that advanced certification-friendly 3D printing through computational optimization.

Proposed research:

Certification procedures of critical components require survival under realistic in-service conditions that can couple various degradation mechanisms. These assessments are expensive and time-consuming for 3D printing materials given their large number of defects. This work will focus on assessing early fatigue damage by characterising manufacturing-induced defects to recreate realistic synthetic finite element models. We will rank the severity of defects as well as the detrimental role of defect aggregation and coalescence by evaluating the role of microplasticity on crack growth variability.

The research will advance the understanding of failure prognosis in 3D printing of metallic materials by ranking defects associated to manufacturing procedures. The uncertainties related to defect attributes and crack detection will be added to the probabilistic nature of a fatigue crack nucleation model and taking into account their intrinsic variability. The ultimate objective is to develop a life prognosis approach that couples the variability associated to both inspection and material uncertainties. This approach will be unique in enabling a robust probabilistic assessment that accelerates the certification of manufacturing procedures through computational iteration.

Design and test rules for vibration analysis of additively manufactured samples: a certification guideline for industrial applications

Institution: GU/EKTAM
Supervisor: Metin U. Salamci, Prof., (M)
Co-Supervisor(s): Nizami Aktürk, Prof., (GU, TURKEY) (M)/ Celal Sami Tüfekçi, Ph.D. (TeknoHAB, TURKEY) (M)/ Gustavo M. Castelluccio, Ph.D. (Cranfield University, UK) (M)

Short description of the project

Additive Manufacturing (AM) methodologies are preferred to generate complex geometries whilst ensuring final part requirements with relatively decreased processing time. The success of the AM process is dominated by many process parameters among which the exerted energy, speed of the process, and the layer thickness are considered to be mathematically changeable during the process so that the required final product is obtained. These parameters, together with the material properties such as density, thermal capacity, phase transformation temperatures, cooling rates etc., determine the so-called “melt pool dynamics”. The melt pool formation in an AM methodology is a complex phenomenon that is studied carefully to understand several defects and properties. Because of the defects, the vibration analysis of additively manufactured parts is also affected by the process parameters and the design itself.

Proposed research:

This PhD study proposes a certification-friendly AM process through computational optimization, focusing on vibration analysis. Certification procedures of critical components require survival under realistic in-service conditions that can couple various degradation mechanisms. These assessments are expensive and time-consuming for AM process of materials given their large number of defects and other parameters.

This work will focus on assessing vibration analysis of parts produced in an AM process. The effects of the defects (as a result of selected process parameters) on the vibration characteristics will be investigated and process parameter windows will be selected for a certifiable part. The certification guidelines for an industrial application will be sketched to integrate the process parameters and designs to the vibration test rules of additively manufactured parts.

High Entropy Materials for the Additive Manufacturing of Aerospace Materials

Institution: METU
Supervisor: Eren Kalay, Prof., (M)
Co-Supervisor(s): Hakan Yavaş, Ph.D. (TUSAS, Turkey) (M)

Short description of the project

Modern aerospace and defense applications call for alloys with a stringent combination of properties, such as high strength, low density, and excellent environmental stability. Many well-known traditional metallic alloys, such as steel, age-hardened Al alloys, and shape-memory alloys, rarely have more than three principal alloying elements. However, the emergence of a new class of alloys – the so-called “high entropy alloys” (HEA) has sparked significant scientific interest in materials with multiple principal components. These alloys contain five or more metallic elements with an atomic percentage between 5-35%. The high configurational entropy favors the formation of a multi-component solid solution instead of a complex intermetallic compound. HEAs have shown tremendous potential due to attractive properties like high strength and thermal stability. Much of these properties are derived from accessing kinetically stabilized phases and solid solutions.

In that sense, the thesis study will focus on the development of novel lightweight HEA to be used as a structural candidate material in space applications (i.e., micro-satellites). The development of HEAs will start with computational methods, including phase stability analysis by CALPHAD method and atomistic approach simulation by ab-initio technique to determine the ideal crystal structure of the determined composition. After that, the alloys found by computational results will be produced by arc and induction melting methods to obtain the actual test data, including mechanical properties. The powder production of successful alloys will be studied by the gas atomization process to obtain a feedstock suitable for the selective laser melting process (SLM). After accomplished its powder characterizations and tests, a predefined geometry will be produced by the SLM process as a rival to the real conventional part.

Additive Manufacturing of New Generation Materials and Structures for Automotive Applications

Institution: METU
Supervisor: Sezer Özerinç, Assoc. Prof., (M)
Co-Supervisor(s): Ender Yıldırım, Assoc. Prof., (METU, TURKEY), (M)

Short description of the project

Additive manufacturing of structural parts has enabled new capabilities for the efficient design of a wide range of automotive parts. This thesis will explore the capabilities of polymer and metal 3D printing technologies such as fused deposition modeling (FDM), continuous liquid interface production (CLIP) and electron beam melting (EBM) towards this route. The focus will be on the development of structural parts such as shock absorbers and interior body panels. The thesis will combine various approaches such as cellular structures, multi-material printing, gradient structures and topological optimization towards the development of high specific strength and impact resistant parts. The model structures to be developed will be analyzed in terms of geometrical accuracy, microstructure and mechanical behavior. The PhD student will have a secondment at the R&D Headquarters of Ford located in Gebze, İstanbul, and will get a chance to investigate the feasibility of these emerging approaches for automotive industry.

Polymer Rapid Tooling for Fabrication of Microfluidic Lab on a Chip Devices

Institution: METU
Supervisor: Ender Yıldırım, Assoc. Prof., (M)
Co-Supervisor(s): Ulaş Yaman, Assoc. Prof., (M)

Short description of the project

Thermoplastic microfluidic lab-on-a-chip devices can be prototyped by various techniques such as micro milling and laser engraving. However, once the design is analytically validated, a clinical testing is mostly required before the design is introduced as a commercial point-of-care or in vitro diagnostic product. At this stage, a medium or high-volume production of the design is required. Typically, hot embossing (for medium volume) and injection molding (for high volume) are utilized for this purpose. However, in the development stage, the designs mostly do not meet the requirements and medium/high-volume production methods, as they are prototyped in the development stage by different means such as micro milling or laser engraving. This gap renders a scalability issue and impedes the commercialization of microfluidic lab-on-a-chip devices. To solve this problem, a scalable manufacturing scheme must be adopted starting from the prototyping. However, scalable manufacturing methods such as injection molding is costly for low volume production or prototyping since manufacturing of the mold by lithography-based microfabrication techniques is typically expensive. This expense must be distributed over high number of products to reduce the cost per device. Polymer additive rapid tooling, which relies on production of tools (molds and inserts) by additive manufacturing, can be utilized to reduce the tool cost. The idea was coined first about 2 decades ago, but it did not gain attention until recent years, when additive manufacturing and more popularly 3D printing became widespread. However, polymer rapid tooling for fabrication of plastic microfluidic devices still did not receive considerable attention. Noting the capabilities and dimensional resolution of additive manufacturing have been improved in the recent years, for the first time in the literature we propose that polymer rapid tooling can be used for manufacturing of thermoplastic microfluidic lab-on-a-chip devices by injection molding. Thus, by utilizing PRT, it could be possible to reduce the tool cost and a scalable manufacturing scheme can be used for prototyping and low-volume manufacturing of microfluidic devices. Therefore, in this study, it is aimed to design and fabricate polymer tools (inserts) by additive manufacturing techniques (namely stereolithography, SLA) for fabrication of thermoplastic microfluidic devices by injection molding. Injection molding and SLA process parameters will be optimized to maximize the fidelity of the features and to minimize the feature size. Optimized method will be adopted to manufacture a demonstrator microfluidic in vitro diagnostic chip. The method can be extended for rapid additive manufacturing of metal tools by selective laser melting (SLM) or selective laser sintering (SLS) to be used in hot embossing of thermoplastic microfluidic devices.

Additive Manufacturing of Functionally Graded Materials (FGM) for Morphing Wings

Institution: METU
Supervisor: Yavuz Yaman, Prof., (M)
Co-Supervisor(s): Metin U. Salamci, Prof. (GU, TURKEY) (M)

Short description of the project

Fully morphing wing structures mimic the behavior of nature and are believed to provide greater aerodynamic efficiency and cleaner flight and skies. Various international projects, such as 'Clean Sky', are gathering pace for more efficient and greener air travel. Functionally graded materials (FGM) on the other hand may find an application field in the trailing edges of the fully morphing aircraft wings because of their variable stiffness (expectation is very low in-plane stiffness and very-high out-of-plane stiffness) and low mass characteristics. The required wing components can be manufactured from these materials through the 3D and 4D additive manufacturing techniques. This study will involve the design, characterization, and manufacturing of some trailing edge components having FGMs.

Additive Manufacturing (AM) of FGMs is a promising and interdisciplinary research field that involves (i) the FGM design through the computational material science, (ii) process parameter investigations by means of multiphysics –such as heat, continuity, momentum, Cahn-Hilliard - etc. equations, (iii) Design for Additive Manufacturing and (iv) AM and characterizations.

This research will cover AM of FGMs to be used in the design and manufacturing of Morphing Wings. Based on the design requirement(s) of the Morphing Wings, appropriate FGM will be considered such that weldability and other related material design stages are handled. Process parameters will be developed for the AM of FGM and prototype(s) will be produced.

Mechanical behavior of Additively Manufactured 7xxx Aluminum Alloys: design guide for processing and post processing

Institution: ITU
Supervisor: Hüseyin Kızıl, Prof., (M)
Co-Supervisor(s): Elmas Salamcı, Assoc. Prof. (GU, TURKEY) (F)

Short description of the project

For 90 years, aluminum alloys have been the materials of choice for both military and commercial aircraft structures. The ingot metallurgy (IM) alloys of the 2000 (Al-Cu-Mg) and 7000 (Al-Zn-MgCu) series used thus far show several disadvantages caused by the production process. Such problems are primarily coarse intermetallic constituent phases, coarse grains, and macrosegregation, resulting in low fracture toughness. Recent advances in aluminum alloy and temper development are maintaining aluminum alloys as the materials of choice for near future commercial aircraft structures to meet cost and weight savings objectives. Aluminum producers have increased research activity in the area of advanced aluminum alloys to provide improved performance characteristics. During the past decade increased efforts have been made to improve the structural efficiency and properties of aerospace materials through the development of lighter weight, stiffer and stronger materials via rapid solidification processing as the processing improves the mechanical properties of many alloys in terms of increased tensile strength, ductility and fatigue and crack propagation resistance. Such improvements are mainly associated with large solid solubility extensions of alloying elements, reduced macrosegregation, refinement of the alloy grain size and changes in the second phase particle size, shape and distribution due to high cooling rates (possibly exceeding 106 K s−1).

Proposed research:

The research will investigate Additive Manufacturing (AM) of 7xxx series alloys, specifically exploring the rapid solidification mechanism during the AM process. The effects of process parameters on the final mechanical behavior of product –such as energy density, exerted power, scanning velocity, etc.- will be documented for the design guide of 7xxx series alloys. Post-processing methodologies will also be developed in order to complete the AM process of 7xxx series alloys. Specimens will be manufactured for the mechanical test and microstructure investigations will be carried out to correlate the relevant process parameters with the final product.

Design of components and additive manufacturing routes for damage-resistant metallic material

Institution: ITU
Supervisor: Hüseyin Kızıl, Prof., (M)
Co-Supervisor(s): Hakan Yavaş, Ph.D. (TUSAS, Turkey) (M)/ Andrea Cini, Ph.D. (Universidad Carlos III Madrid, SPAIN) (M)/ Gustavo M. Castelluccio, Ph.D. (Cranfield University, UK) (M)

Short description of the project

Novel 3D printing of metallic materials (also called additive manufacturing) is starting a manufacturing revolution thanks to its flexibility in adapting functionality, processing, and materials. However, components manufactured this way have relatively low levels of reliability due to a highly variable manufacturing process, which hinder their acceptance.

Proposed research:

Several initiatives have been recently launched to quantify the uncertainty of structural properties in additive manufacturing parts, but there is a notable lack of research on complex loading conditions such as cyclic deformation. Thus, a fundamental understanding of the effects of manufacturing attributes on damage tolerance is required for components and structures to be safely introduced in safety-critical applications.

This PhD project will explore the synergies among manufacturing setups, materials degradation, and component design to identify optimization strategies. The work will involve the creation of a database that compiles the mechanical and materials characterization of the additive manufacturing materials that will inform computational algorithms. By integrating dissimilar data, we aimed to discover the link among structure, processes, and properties, which can be further coupled with the component design for an integrated optimization. As a result, the student will demonstrate the design of additive manufacturing components that are damage resistant.

Roadmap for AM airframe primary structures implementation and certification

Institution: ITU
Supervisor: Hüseyin Kızıl, Prof., (M)
Co-Supervisor(s): Fahrettin Öztürk, Prof. (TUSAS, TURKEY) (M)/ Hakan Yavaş, Ph.D. (TUSAS, Turkey) (M)/ Andrea Cini, Ph.D. (Universidad Carlos III Madrid, SPAIN) (M)/ Gustavo M. Castelluccio, Ph.D. (Cranfield University, UK) (M)

Short description of the project

Metallic 3D printing will represent an ideal solution for aircraft primary structure enabling extended component integration by a cheaper and faster and greener production technology. However, no approved methods and model to assess damage tolerance capabilities are currently available for AM part certification due to the lack of knowledge regarding the fatigue failure mechanisms of AM components, exacerbated by the absence reliable NDT techniques and process monitoring.

Proposed research:

The research will rationalize damage mechanisms occurring inside AM components under fatigue loading based on dedicated experimental fatigue test results, NDT inspections and fractography analyses. Development of cracks from manufacturing-induced defects and their propagations up to detectable flow size will be described assessing the effect of defect distribution, crack coalescence, material microstructure and residual stresses. Damage characterization tests will help distinguishing different growth stages below NDT detection threshold. Crack propagation inside the inspectable range will be also characterized and compared with growth rates of conventionally manufactured components to assess the defect distribution and microstructure influence, microplasticity on crack growth variability.

An industrially relevant fatigue life prediction model will be developed on the basis of fatigue prediction methods to assess damage tolerance and define maintenance and inspection plans for AM. Simplified surrogated models to be used as design and in-service damage tolerance assessment tool will be developed from the FE nucleation and propagation results. Material uncertainties will be also included for both slow crack growth approach of single crack and probabilistic widespread fatigue damage assessment.

Crushing behavior LTSs (Weight-optimized Ti-based lattice structures for impact load mitigation)

Institution: IZTECH
Supervisor: Mustafa Güden, Prof., (M)
Co-Supervisor(s): Hakan Yavaş, Ph.D. (TUSAS, Turkey) (M)

Short description of the project

Cellular metallic structures (CMSs) are made of regularly arranged and distributed cells, exhibiting multi-functional properties. CMCs have relatively high bending strength to weight ratios and relatively high resistances to frontal impacts. They are classified random or periodic. In random cell structures like open and closed cell metal foams, the cell size and the geometry vary with the location. The periodic CMSs include honeycombs and corrugated and lattice truss structures (LTSs). The repeating unit topology may be 2D like in a honeycomb, or 3D like in a LTS. LTSs show high bending stresses and stretch-dominated deformation behavior and therefore considered alternative to honeycombs and metallic foams in the applications designed for the mitigation of induced stress waves in impact loading. The most widely investigated topologies until 2015 were tetrahedral, pyramidal and Kagome, which were processed using conventional sheet metal forming methods. With the development of additive manufacturing techniques such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM), there have been significant increase in research and development on LTSs (Figures 1(a) and (b)). The fabrication of wide range of truss morphologies that can allow the designing structures with LTSs for fine-tuned mechanical properties are now possible with additive manufacturing. Two possible applications of LTSs in impact load mitigation are foreseen: i) impact load resistant packaging and ii) impact load protection. The valuable, fragile equipment are protected from accidental damages with the use LTS-cored sandwich structure cage (e.g. the package may be dropped from a height during transportation). The vehicles, ships, and planes are protected from outside impact loadings in which LTS-cored sandwich is either mounted onto the outer surface of vehicle or the outer surface of vehicle is solely made of LTS-cored sandwich (i.e. bird strike to the radom of airplanes). In these applications, LTSs are expected to transfer relatively low stresses to the packaged/protected structure and should absorb much of the kinetic energy of impact through plastic buckling/stretching of trusses. The current research activities on LTSs have mostly focused on Ti and its alloys particular on Ti64. Ti64 satisfies both structural and functional requirements for load bearing applications by a combination of mechanical, physical and chemical properties. Because of relatively light-weight and bio-compatibility, Ti64 has found a wider usage in medical and dental applications. Their light-weight and higher strength ratio per unit weight are also extremely suitable for jet engines and many components in airframe. In the aerospace industry “buy-to-fly” ratio (mass of raw metal to mass of product) are 12-25:1 and with the use of additive manufacturing techniques it declines to 3-12:1. The high corrosion resistance of Ti64 is attracted by marine and chemical industries. So far 16, topologically different, Ti64 LTSs have been reported in the literature, see Table 1. Majority of studies on these LTSs were on the quasi-static mechanical response, while there have been only few studies on the dynamic mechanical behavior of Ti64 LTSs.

The aim of the proposed project is to ascertain and fabricate certain topologies of Ti64 LTSs, which would be used in impact load mitigation for packaging and protection. Since, the comparison between different LTSs will be made at the same relative density, the determined LTSs will be also optimized in terms of weight. The material models (flow stress and damage) of AM Ti64 are needed in the simulations and will be determined experimentally and compared with the existent models in the literature. The validity of these models will be verified and a library of material models of AM Ti64 alloy will be established.

The crushing models of LTSs at quasi-static and dynamic velocities will be developed and implemented in explicit FEM software of LS-DYNA. The results of these simulations will provide very valuable designing criteria for both static and dynamic loading of LTSs. Analytical scaling equations for the mechanical response of LTSs (elastic modulus, crushing stress, densification strain, critical strain for densification and critical velocity for shock stress development) will also be established based on numerical and experimental static and dynamic tests. The geometrical parameters that affect the critical velocity for shock deformation will also be developed.

Dynamic behavior of AM parts (Dynamic behavior and constitutive equations of additively manufactured metallic alloys)

Institution: IZTECH
Supervisor: Alper Taşdemirci, Prof., (M)
Co-Supervisor(s): İlhan ŞEN, Ph.D. (TUSAS, Turkey) (M)

Short description of the project

Additively manufactured (AM) metallic alloy parts exhibit different microstructures; hence, different mechanical properties from their conventionally manufactured counterparts. High cooling rates involved in AM inherently induce high dislocation density and fine microstructure development. As is known, high dislocation density and fine cellular structure promote twinning deformation in conjunction with slip, and somehow they compete to each other at varying strains, strain rates and temperatures making the deformation very much complicated. The main aim of this thesis is to determine appropriate flow stress and damage models of AM Ti64 and 316L alloys. In the first part of this thesis, extensive testing at both static and dynamic strain rates will be performed to determine the constitutive equations. In the second part, the test sample processing will be simulated using the commercial finite element code of ANSYS/Additive module and then the samples will be transferred to LSDYNA to simulate mechanical testing. Part one and part two will work together to validate the fidelity of the constitutive equations developed. Extensive mechanical characterization including reloading at different pre-strains from static to dynamic and vice-verse and microstructural analysis will be performed to determine the deformation history effect. Additionally, the effect of adiabatic heating on the deformation behavior of these alloys is also determined. The proposed project studies will be performed at the Dynamic Testing and Modelling Laboratory of İzmir Institute of Technology. The lab is equipped with compression and tensile Split Hopkinson Bar, projectile impact set-up, drop weight tester and universal tension and compression machine and has a license and a long-time user of LSDYNA.