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- Double-sided injection lap rivetingPublication . Pragana, João; Sampaio, Rui F. V.; Chantreuil, Justin; Bragança, Ivo; Silva, Carlos; Martins, PauloThis 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.
- Revisiting the fracture forming limits of bulk forming under biaxial tensionPublication . Sampaio, Rui F. V.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloThe formability limits of bulk metal forming in principal strain space and in the effective strain vs. stress-triaxiality space are characterized by an uncertainty region in which cracks may be triggered by tension (mode I of fracture mechanics) or by out-of-plane shear (mode III). The problem in obtaining experimental data in this region has been known for a long time and the main objective of this paper is to present a new upset formability test geometry that can effectively contribute to the characterization of the formability limits of bulk metal forming parts subjected to biaxial tension. Alongside with this objective, this paper also presents an analytical expression for converting the fracture forming limit line corresponding to crack opening by mode III in principal strain space into a hyperbolic fracture limit curve in the effective strain vs. stress-triaxiality space. The overall methodology employed by the authors combines experimentation along with analytical and numerical modelling, and the contents of the paper is a step towards diminishing the actual lack of knowledge regarding failure by fracture in bulk metal forming parts subject to stress-triaxiality values beyond uniaxial tension. Results show that a new uncoupled ductile fracture criterion built upon combination of the integrands of the Cockcroft-Latham and McClintock criteria can be successfully used to model the physics of the bulk metal forming limits for the entire range of stress-triaxiality values corresponding to cracking on free surfaces.
- Coin minting by additive manufacturing and formingPublication . Pragana, João; Rosenthal, Stephan; Alexandrino, Paulo; Araújo, Andreia; Bragança, Ivo; Silva, Carlos; Leitão, Paulo J.; Tekkaya, A. Erman; Martins, PauloAdditive 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.
- Integration of tube end forming in wire arc additive manufacturing: An experimental and numerical investigationPublication . Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloIntegration 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.
- Hybrid manufacturing of stiffening grooves in additive deposited thin partsPublication . Cristino, Valentino A. M.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloThis 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.
- New self-clinching fasteners for electric conductive connectionsPublication . Sampaio, Rui F. V.; Martins, Paulo; Pragana, J. P. M; Bragança, Ivo; Silva, C. M. A.This paper presents new rotational and longitudinal symmetric self-clinching fasteners to fabricate reliable connections in busbars with low electrical resistance for energy distribution systems. Connections consist of form-closed joints that are hidden inside regions where two busbars overlap. The investigation into the fabrication and performance of the new self-clinched joints involved finite element modelling and experimentation to determine the required forces and to evaluate the electric current flow and the electrical resistance at different service temperatures. The original design of the joints that was proposed in a previous work was modified to account for busbar strips of copper and/or aluminum with similar or dissimilar thicknesses, connected by means of self-clinching fasteners made from the same materials of the busbars, instead of steel. The effectiveness of the new self-clinched joints was compared to that of conventional bolted joints that are included in the paper for reference purposes. The results show that rotational symmetric self-clinching fasteners yield lighter fabrication and more compact joints with a similar electrical resistance to that of bolted joints. They also show that longitudinal symmetric self-clinching fasteners aimed at replicating the resistance-seam-welding contact conditions yield a reduction in electrical resistance to values close to that of ideal joints, consisting of two strips in perfect contact and without contaminant or oxide films along their overlapped surfaces.
- Joining by forming of lightweight sandwich composite panelsPublication . Contreiras, Tomás R. M.; Pragana, João; Bragança, Ivo; Silva, Carlos; Alves, Luís M.; Martins, PauloThis paper presents a new joining by forming process to assemble longitudinally two metal-polymer sandwich composite panels perpendicular to one another. The process combines sheet-bulk forming with mortise-and-tenon joints to produce mechanically interlocked joints with large and stiff flat-shaped heads. Experimentation and finite element modelling with representative unit cells give support to the presentation and special emphasis is placed on the application of the process to the fabrication of lightweight composite panels for structural applications. Failure of the joints takes place by cracking and not by disassembling after unbending the flat-shaped head of the joint back to its original shape. The required forces to produce the new type of joints are below 15 kN, allowing them to be an easy to implement alternative to existing solutions based on adhesives or fasteners.
- Hybrid additive manufacturing of collector coinsPublication . Pragana, João; Rosenthal, Stephan; Bragança, Ivo; Silva, Carlos; Tekkaya, A. Erman; Martins, PauloThe 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.
- Joining aluminium profiles to composite sheets by additive manufacturing and formingPublication . Baptista, R. J. S.; Pragana, João; Bragança, Ivo; Silva, Carlos; Alves, Luís; Martins, PauloThis paper explores the application of the 'mortise-and-tenon' concept for joining hollow section aluminium profiles to composite strips or sheets. Wire arc additive manufacturing is combined with joining by forming to fabricate the tenons and to obtain the mechanical interlocking with the mortises available in the strips (or sheets). The workability limits are established by means of an analytical model that combines plastic deformation, instability and fracture. Experimental and finite element modelling are utilized to develop the overall joining process and to validate the round 'mortise-and-tenon' design resulting from the analytical model. Pull-out and shear destructive tests are carried out to evaluate the overall strength of the joints and results allow concluding that the new joints can easily and effectively replace existing solutions based on welding, fastening or adhesive bonding. The proposed joining process also circumvents the need to design extra fixing and interlocking features in low cost hollow section aluminium profiles for easy assembling.
- Manufacturing hybrid busbars through joining by formingPublication . Pragana, João; Baptista, R. J. S.; Bragança, Ivo; Silva, Carlos; Alves, Luís; Martins, PauloThis paper focus on the production of hybrid busbars made from copper and aluminium by means of a joining by forming process that was recently developed by the authors. The process involves the combined use of partial cutting and bending with form-fit joining by compression in the direction perpendicular to strip thickness. The resulting joints are flat with the plastic deformed materials enclosed within the thickness of the overlapped strips. Design is performed by means of an analytical model and the overall manufacturing concept is validated through numerical and experimental modelling. Major process parameters are identified and their influence on the overall deformation mechanics and joining feasibility is investigated. The effectiveness and performance of the new joints is analysed by means of tensile-shear loading tests. Results show that joining by forming can be successfully utilized to produce form-fit joints with good shear forces in hybrid busbars for electrical applications.