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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.
- Integration of forming operations on hybrid additive manufacturing systems based on fusion weldingPublication . Pragana, João; Cristino, Valentino A. M.; Bragança, Ivo; Silva, Carlos; Martins, PauloThis 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.
- A new joining by forming process to produce lap joints in metal sheetsPublication . Pragana, João; Silva, Carlos; Bragança, Ivo; Alves, Luís; Martins, PauloThis paper proposes a new joining by forming process to produce lap joints in metal sheets. The process combines partial cutting and bending with mechanical interlocking by sheet-bulk compression of tabs in the direction perpendicular to thickness. The lap joints are flat with all the plastically deforming material contained within the thickness of the two sheets partially placed over one another. The design of the lap joints is performed by a simple analytical model and the overall concept is validated by means of numerical modelling and experimentation. Destructive shear tests demonstrate the effectiveness and performance of the new proposed lap joints. (C) 2018 Published by Elsevier Ltd on behalf of CIRP.
- A strain acceleration method to identify the onset of diffuse neckingPublication . Sampaio, Rui F. V.; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, PauloThis paper presents an innovative ‘strain acceleration method’ for determining the onset of diffuse necking in sheet forming tests using data obtained from digital image correlation (DIC). The method identifies the onset time of diffuse necking and provides the corresponding in-plane principal strain values by detecting a local extreme in the second derivative of the minor principal in-plane strain with respect to time at the edges of the sheet surface region where diffuse necking occurs. Results obtained from applying the method to tensile testing on two different materials and comparisons with available methods based on force-time or principal strain rate evolutions confirm its accuracy and validity. The new method was implemented in a computer software to be used for research and education that also enables determination of localized necking and fracture and plotting the strain loading paths in principal strain space.
- 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.
- On the Enhancement of Material Formability in Hybrid Wire Arc Additive ManufacturingPublication . Pragana, João; Brito, Beatriz; Bragança, Ivo; Silva, Carlos; Martins, PauloThis paper is focused on improving material formability in hybrid wire-arc additive manufacturing comprising metal forming stages to produce small-to-medium batches of customized parts. The methodology involves fabricating wire arc additive manufactured AISI 316L stainless steel parts subjected to mechanical and thermal processing (MTP), followed by microhardness measurements, tensile testing with digital image correlation, as well as microstructure and microscopic observations. Results show that mechanical processing by pre-straining followed by thermal processing by annealing can reduce material hardness and strength, increase ductility, and eliminate anisotropy by recrystallizing the as-built dendritic-based columnar grain microstructure into an equiaxed grain microstructure.
- Mechanical and thermal processing of wire-arc additively deposited stainless steelPublication . Silva, Carlos; Pragana, João; Bragança, Ivo; Martins, PauloMechanical and thermal processing of wire-arc additively deposited stainless steel is investigated with the purpose of improving its microstructure, surface morphology, formability, and stress response. Microscopy helps identifying the processing conditions that permit full recrystallization of the as-built columnar microstructure. Combination with strain loading paths, topography and fractography in tensile tests show that mechanical processing consisting of 20 % thickness reduction followed by annealing at 1100 °C under 4 h eliminates anisotropy and increases the fracture forming limits by 30 %. The work is a step forward to consolidate the hybridization of wire-arc additive manufacturing with metal forming as an alternative to conventional manufacturing.
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