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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.
- 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.
- 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.
- Two-stage joining of sheets perpendicular to one another by sheet-bulk formingPublication . Silva, Carlos; Bragança, Ivo; Alves, Luis; Martins, PauloThis paper proposes a new joining by forming process for fixing longitudinally in position two metal sheets (or plates) perpendicular to one another, at room temperature. The proposed process employs a counterbored variant of the ‘mortise-and-tenon’ joint that eliminates the protrusion of the tenon beyond the mortise after mechanical locking by plastic deformation. The presentation draws from the workability limits and material stress-strain characterization to validation by joining and destructive pull-out testing. Results demonstrate the effectiveness of the new proposed process for producing flat joint surfaces, which are advantageous over typical ‘mortise-and-tenon’ protruded surfaces in most applications.
- 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.
- Experimental and numerical study of the joinability of sheets by sheet-bulk formingPublication . Bragança, Ivo; Loja, Amélia; Silva, Carlos; Alves, Luís; Martins, PauloThe authors present an innovative mechanical joining process that allows to connect perpendicular sheets to one another. This study is focused on joining similar and dissimilar sheets, based on sheet-bulk forming technology, and it is supported by experimental data and numerical simulation. Destructive tensile tests of different joined materials were performed to determine the maximum force that the joints are capable to withstand without failure. The joining technique should be chosen according to the materials joint combination. The two-stages technique could be a valid option to overcome the clearance between the plastically deformed polycarbonate tenon and aluminium mortise.
- Ligthweight joining of polymer and polymer-metal sheets by sheet-bulk formingPublication . Bragança, Ivo; Silva, Carlos; Alves, Luís; Martins, PauloThis paper presents an environmentally friendly joining process based on sheet-bulk plastic deformation to connect two polymer (or a polymer and a metal) sheets perpendicular to one another. The methodology draws from material characterization and finite element modelling, using an extension of the flow formulation to pressure-sensitive polymers, to experimentation in a laboratory tool setup and destructive testing of the produced joints. The results allow characterization of the joinability window as a function of the major operating parameters and demonstrate the feasibility of the process to produce polymer and hybrid joints (polymer-metal) at room temperature. A variant to the proposed process is disclosed to handle situations where the elastic recovery of the polymer sheets needs to be better controlled. The overall content of the paper is an extension of a previous work of the authors for fixing longitudinally in position two metal sheets perpendicular to one another, and seems promising for the manufacture of lightweight joints made from polymers or polymers and metals.
- 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.
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