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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.
- Groove stiffening of sheets by single point incremental formingPublication . Cristino, Valentino A. M.; Pragana, João P. M.; Bragança, Ivo; Silva, Carlos M. A.; Martins, PauloThis paper investigates the applicability of single point incremental forming to fabricate stiffening grooves in thin metallic panels. The work combines experimentation, finite element and analytical modelling to determine the required forces and the maximum allowable groove depths that can be produced without tearing. The analytical modelling is based on a framework that was previously developed by the authors and combines in-plane membrane stretching and fracture forming limits. Comparison of the results obtained by the analytical framework against experimental and finite element data proves its effectiveness to replicate the deformation mechanics of groove stiffening by single point incremental forming. Results also prove that groove stiffening by single point incremental forming is an easy and effective alternative to conventional reinforcement of thin metallic panels by welding or fastening of stringers.
- Electric performance of fastened hybrid busbars: An experimental and numerical studyPublication . Sampaio, Rui F. V.; Pragana, João P. M.; Bragança, Ivo; Silva, Carlos M. A .; Nielsen, Chris Valentin; Martins, P.A.F.This paper is focused on fastened hybrid busbars made from copper and aluminium with the purpose of analysing the influence of the steel bolts, of their tightening torque and of the surface condition of the sheets on the electric current flow and electric resistance of the joints. The methodology combines experimentation with unit cells that are representative of the joints and electro-mechanical numerical simulation using a finite element computer program developed by the authors. Results are a step forward in understanding the combined influence of bolts, contact pressure and surface roughness on the electric performance of fastened hybrid busbars. Design guidelines for dimensioning the cross sections of the copper and aluminium sheets and for effectively distributing bolts across the contacting surfaces are also provided.
- Joining by forming of additive manufactured 'mortise-and-tenon' jointsPublication . Silva, Diogo F. M.; Bragança, Ivo; Silva, Carlos; Alves, Luís; Martins, PauloThis article is aimed at extending the 'mortise-and-tenon' joining concept commonly utilized in corner or tee joints to lap joints in which one sheet is partially placed over another without any change in their shape. The approach makes use of wire arc additive manufacturing to fabricate the tenons and allows various shapes and thicknesses to be made from a wide range of metallic materials. Upset compression of the tenons is utilized to mechanically lock the two sheets being joined. Experimental and finite element simulation works performed with monolithic (aluminium-aluminium) and hybrid (aluminium-polymer) 'unit cells' consisting of a single lap joint are utilized to investigate the deformation mechanics and the feasibility of the new proposed joining process. Tensile-shear loading tests were carried out to determine the maximum force that the new proposed joints are capable to withstand without failure. Pull-out forces of approximately 8 and 6 kN for the monolithic and hybrid joints allow concluding on the potential of additive manufactured 'mortise-and-tenon' lap joints to connect sheets made from similar and dissimilar materials.
- Assembly of lightweight sandwich panels through joining by formingPublication . Sampaio, R. F. V.; Pragana, J. P. M.; Bragança, I .M F; Martins, P.A.F.This paper is focused on the assembly of lightweight sandwich panels built upon the patented ‘Opencell’ structure concept. The objective is to investigate the possibility of joining the connection members of the core to the adjoining skin sheet by plastic deformation at ambient temperature, instead of welding or adhesive bonding. The methodology draws from earlier developments of the authors in joining by forming using the mortise-and-tenon concept to experimentation and finite element modelling of the assembly process in unit cells that are representative of the sandwich panels. It is shown that replacing welding by joining by forming allows fabricating sandwich panels from sheet materials that are difficult or impossible to weld while preventing thermal cycles that are responsible for causing metallurgical changes, distortions, and residual stresses. Replacing adhesive bonding by joining by forming circumvents the need of surface preparation, time for the adhesive to cure and environmental compliance, among other requirements
- Hybrid wire‑arc additive manufacturing of conformal cooling channels: a feasibility studyPublication . 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.
- 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.
- 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.