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Percorrer por autor "Pragana, João"

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  • Coin minting by additive manufacturing and forming
    Publication . Pragana, João; Rosenthal, Stephan; Alexandrino, Paulo; Araújo, Andreia; Bragança, Ivo; Silva, Carlos; Leitão, Paulo J.; Tekkaya, A. Erman; Martins, Paulo
    Additive manufacturing is proposed as a novel alternative to coin blank's production routes based on rolling, blanking and edge rimming. The presentation draws from laser powder bed fusion of cylinders, slicing into individual coin blanks by electro discharge machining and surface preparation by polishing, to coin minting in a laboratory press-tool system. Special emphasis is given to material deposition and coin minting due to the originality of producing coin blanks with complex intricate contoured holes and to the necessity of subjecting the additive manufactured coin blanks to extreme compressive stresses that are typical of coin minting. Numerical and experimental results confirm the excellent performance of the additive manufactured coin blanks. The new design layouts included in the additive manufactured coin blanks open the way to produce high value-added singular collector coins, which are disruptively different from those available in the market nowadays.
    2021-04Artigo científico Acesso restrito Ver mais
  • Double-sided injection lap riveting
    Publication . Pragana, João; Sampaio, Rui F. V.; Chantreuil, Justin; Bragança, Ivo; Silva, Carlos; Martins, Paulo
    This article presents a double-sided injection lap riveting process for fixing two overlapped sheets with tubular rivets at room temperature. The rivets are injected by compression into the dovetail ring holes that are previously machined in both sheets, and, in contrast to other joining by plastic deformation processes making use of auxiliary elements, the resulting joints are hidden inside the sheets without material protrusions above or below their surfaces. The new process is applied in the fabrication of aluminum busbar joints for energy distribution systems, and comparisons are made against conventional bolted joints that were fabricated for reference purposes. The work combines experimentation and finite element modelling, and results allow concluding that, in addition to invisibility and savings in assembly space, there are important gains in the thermo-electrical performance of the new joints that are of paramount importance for electric distribution applications.
    2023-01Artigo científico Acesso restrito Ver mais
  • Expansion of additive-manufactured tubes: deformation and metallurgical analysis
    Publication . Pragana, João; Rosa, Luis G.; Bragança, Ivo; Silva, Carlos; Martins, Paulo
    Herein, the mechanical and metallurgical feasibility of integrating tube end-forming operations with additive manufacturing is investigated. The work makes use of wire-arc additively manufactured AISI 316L stainless steel preforms that are subsequently machined into tubes and expanded with a tapered conical punch. Experimental measurements of force, surface strains, thickness, and microhardness combined with microstructure observations and fractography of the fractured surfaces are utilized to characterize plastic deformation and formability limits of the additively manufactured tubes and understand the main differences against the results obtained from commercial wrought tubes of the same material. Results show that the material deformation characteristics, namely, the evolution of microhardness along the expanded tube length, and the formability limits by necking and fracture, are strongly influenced by the columnar microstructure originated by a noncyclic dendritic growth aligned with the building direction. Still, results demonstrate that the additively manufactured AISI 316L tubes are ductile enough to be successfully included in hybrid additive manufacturing routes.
    2021-09Artigo científico Acesso restrito Ver mais
  • Form-fit joining of hybrid busbars using a flexible tool demonstrator
    Publication . Reichel, A.; Sampaio, Rui F. V.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, Paulo
    This paper is focused on hybrid busbars made from copper and aluminum strips and presents a flexible tool demonstrator capable of replicating material flow in the lancing, bending and compression stages of a new joining by forming process without auxiliary elements. The flexible tool demonstrator is defined by its modular concept that allows the active tool components to be easily interchanged for testing and exploring different materials and thickness combinations, surface conditions and cross-section areas of the strips under laboratory conditions. Experimental and numerical simulation with a selected hybrid busbar geometry validates the overall concept and fabrication of the demonstrator and shows that the new joining by forming process can produce permanent form-fit joints with smooth upper and lower surfaces containing all the plastically deformed material within the thickness of the two strips. Complete filling of the free volume left in-between the thickness of the two strips allows obtaining an electric resistance lower than that of fastened hybrid busbars.
    2021-11Artigo científico Acesso restrito Ver mais
  • Hybrid additive manufacturing of collector coins
    Publication . Pragana, João; Rosenthal, Stephan; Bragança, Ivo; Silva, Carlos; Tekkaya, A. Erman; Martins, Paulo
    The objective of this paper is to present a new hybrid additive manufacturing route for fabricating collector coins with complex, intricate contoured holes. The new manufacturing route combines metal deposition by additive manufacturing with metal cutting and forming, and its application is illustrated with an example consisting of a prototype coin made from stainless steel AISI 316L. Experimentation and finite element analysis of the coin minting operation with the in-house computer program i-form show that the blanks produced by additive manufacturing and metal cutting can withstand the high compressive pressures that are attained during the embossing and impressing of lettering and other reliefs on the coin surfaces. The presentation allows concluding that hybrid additive manufacturing opens the way to the production of innovative collector coins with geometric features that are radically different from those that are currently available in the market.
    2020-12-09Artigo científico Acesso aberto Ver mais
  • Hybrid manufacturing of stiffening grooves in additive deposited thin parts
    Publication . Cristino, Valentino A. M.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, Paulo
    This paper is focused on the hybridization of additive manufacturing with single-point incremental forming to produce stiffening grooves in thin metal parts. An analytical model built upon in-plane stretching of a membrane is provided to determine the tool force as a function of the required groove depth and to estimate the maximum allowable groove depth that can be formed without tearing. The results for additively deposited stainless-steel sheets show that the proposed analytical model can replicate incremental plastic deformation of the stiffening grooves in good agreement with experimental observations and measurements. Anisotropy and lower formability caused by the dendritic-based microstructure of the additively deposited stainless-steel sheets justifies the reason why the maximum allowable depth of the stiffening grooves is approximately 27% smaller than that obtained for the wrought commercial sheets of the same material that are used for comparison purposes.
    2021-12-20Artigo científico Acesso aberto Ver mais
  • Hybrid wire‑arc additive manufacturing of conformal cooling channels: a feasibility study
    Publication . Pragana, João; Bragança, I. M. F; Silva, C. M. A.; Martins, P. A. F.
    This paper investigates the feasibility of hybridizing wire-arc additive manufacturing with metal forming to create conformal cooling channels in moulds for hot material processing. An axisymmetric mould is used as a prototype test case and plastic deformation by flaring of the additively deposited inner tubular element is utilized to create the overhangs for connecting with the outer tubular element that are needed to fabricate the cooling channels. Finite element modelling of plastic deformation by flaring of the additively deposited material is carried out to identify the different modes of deformation and to determine the maximum allowable radius of the overhangs. Results show that the proposed hybridization of wire-arc additive manufacturing with plastic deformation by flaring adds flexibility to the design and fabrication of conformal channels in the as-built mould by eliminating the use of support structures and avoiding the use of complex deposition strategies. Finishing of the functional surfaces of the as-built mould by turning ensures the required accuracy and surface quality.
    2022-04Artigo científico Acesso restrito Ver mais
  • Injection lap riveting
    Publication . Ribeiro Ferreira, Francisco; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, Paulo
    This paper presents an injection lap riveting process to connect two sheets placed one on top of the other. The process is carried out at ambient temperature and differs from self-pierce riveting because its joining principle is based on plasticity and friction without fracture and formation of new surfaces. The working principle is based on two consecutive operations first, a dovetail ring hole is machined in the lower sheet and then a semi tubular rivet is injected through the upper sheet into the dovetail ring hole of the lower sheet, by compression with a punch. The presentation is based on a combined experimental and numerical investigation and special emphasis is put on the influence of the dovetail ring hole geometry in material flow, riveting force, and pull-out and shear destructive forces. The last past of the paper includes details of a prototype cutting tool that was developed by the authors for producing the dovetail ring holes in-site and fostering the portability and applicability of the process.
    2021Artigo científico Acesso restrito Ver mais
  • Integration of forming operations on hybrid additive manufacturing systems based on fusion welding
    Publication . Pragana, João; Cristino, Valentino A. M.; Bragança, Ivo; Silva, Carlos; Martins, Paulo
    This paper is focused on the integration of metal forming operations in hybrid systems that combine additive manufacturing (AM) by gas metal wire arc and subtractive manufacturing by machining. The investigation is carried out in AISI 316L stainless steel wire and draws from tensile testing to incremental sheet forming of truncated conical shapes. Commercial sheets from the same material are utilized for comparison purposes. Thickness measurements, digital image correlation (DIC), circle grid analysis (CGA) and microstructural and scanning electron microscopy (SEM) observations are carried out to understand how different is the mechanical behaviour of the deposited metal from that of commercial metal sheets and how significant is the influence of the deposited metal microstructure on its overall formability. Results confirm that integration of metal forming operations in hybrid AM routes is feasible despite the formability of deposited metal being smaller than that of the commercial metal sheets due to the strong anisotropy induced by the dendritic based microstructure of the deposited metal. Incremental forming of two deposited parts also allows concluding that integration of metal forming operations in hybrid AM systems is a step towards green and sustainable manufacturing by extending their field of applicability to the fabrication of complex ready-to-use parts requiring combination of different processes.
    2020-05Artigo científico Acesso restrito Ver mais
  • Integration of tube end forming in wire arc additive manufacturing: An experimental and numerical investigation
    Publication . Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, Paulo
    Integration of tube end forming operations in metal additive manufacturing routes has a great potential for the fabrication of customized features in additively deposited hollow parts. This paper is focused on the integration of tube expansion with rigid tapered conical mandrels to highlight the advantages in the construction of overhanging flares derived from the elimination of support structures and prevention of humping. The work draws from the mechanical and formability characterization of stainless steel AISI 316L tubes produced by wire arc additive manufacturing (WAAM) to the experimental and numerical simulation of the construction of over hanging flares by tube expansion. Strain loading paths obtained from digital image correlation and finite element analysis combined with the strain values at the onset of necking and fracture allow determining the critical ductile damage that additively deposited tubes can safely withstand. Results show that despite formability of additively deposited tubes being influenced by a dendritic based microstructure, their performance is adequate for tube end forming operations, such as tube expansion, to be successfully integrated in metal additive manufacturing without the need of using expensive hardware and complex deposition strategies.
    2021-08-16Artigo científico Acesso aberto Ver mais