Search

This paper presents a new resistance element welding process capable of producing invisible lap joints between steel-polymer-steel composite laminates. The process involves pre-drilling a flat-bottom hole in each laminate to remove the polymer core and one of the steel sheets, and positioning a cylindrical insert inside the two adjoining holes for subsequent resistance welding. Finite element modeling is utilized to construct the weldability lobe and to identify the parameters that lead to the formation of acceptable joints. Experimental results confirm the applicability of the process to produce invisible lap joints without signs of material protrusions or local indentations resulting from squeezing the polymer out to create contact between the steel sheets. Destructive peel and shear tests allow determining the maximum forces that the joints can safely withstand and comparing their performance against alternative joined by forming lap joints in which the mechanical interlocking is also hidden inside the laminates.

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 overhanging 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.

This paper is focused on double-sided self-pierce riveting (DSSPR) of overlapped sheets made from dissimilar materials with very different mechanical strengths. The methodology draws from previous work on sheets made from similar or identical materials and the extension to dissimilar materials is built-upon a combined experimental and numerical investigation that makes use of AA5754-H111 aluminum and polyvinylchloride sheets. Results show that reengineering of conventional DSSPR by modification of the chamfered rivet angles to account for the different resistances to penetration of the two materials is not effective, because it cannot solve the formation of asymmetric mechanical interlockings with very small undercuts in the harder sheet material. The proposal of an innovative two-stroke DSSPR process in which the tubular rivet is first forced through the harder sheet with the help of a dedicated compression tool, and only subsequently pressed through the softer sheet, proves to be effective in obtaining symmetric joints with good undercuts in both sheets. Destructive shear tests confirm the good performance of the joints produced by means of the new proposed two-stroke DSSPR.

Flexible roll forming process is an advanced manufacturing process, for producing complex and lightweight profiles with variable cross-section. One of the major observed defects in this process is the flange wrinkling of the profiles, which has limited its development especially for the automotive industry. In this paper, the deformation mechanics of the flexible roll forming process is revisited, and the methodology for predicting the onset of wrinkling, which had previously been developed based on the wrinkling limit curve in the space of effective strain versus stress triaxiality, is improved. Then, a new criterion based on the wrinkling limit curve is proposed to predict the possibility of wrinkling in finite element analysis. The criterion is applied to analyze the influence of geometric parameters of the variable cross-section profile and material properties of the sheet on the occurrence of wrinkling. The prediction accuracy is evaluated by performing experiments with different profile shapes. Results show that the new proposed criterion can successfully predict the onset of wrinkling in the flexible roll forming process, and its utilization is recommended for predicting wrinkling in the finite element analysis of sheet metal forming processes.

This paper presents a new joining by forming process capable of attaching a sheet to the end of a tube without the need of accessories and extra materials. The process is named as ‘self-pierce riveting of tubes to sheets’ and is an extension to sheet-tube connections of the working principle of double-sided self-pierce riveting utilized for connecting two overlapped sheets. The process requires tubes with chamfered ends to be pushed against the sheets and its ends to flare (curl) in order to create an interlocking that holds the sheets and tubes tightly together. The resulting joints are invisible because the interlockings are hidden inside the sheets. The presentation draws from deformation mechanics and identification of the major parameters to characterization of the interlocking and evaluation of the pull-out destructive strength that the new joints are capable to withstand without failure. Aluminium sheets and carbon steel tubes are utilized to demonstrate the effectiveness of the new joining by forming process and to demonstrate its ability to connect sheets and tubes made from dissimilar materials at room temperature. Finite element modelling using an in-house computer program gives support to the presentation.

This paper presents a new joining by forming process named as “double-sided self-pierce riveting” that makes use of tubular rivets with simple geometry placed in-between two sheets to be joined. The sheets are pushed against each other by flat parallel punches so that the rivets are forced through the sheets and their ends are flared (curled) to create mechanical interlocking’s which holds the sheets tightly together. The methodology draws from deformation mechanics and identification of major process parameters, to fabrication and destructive testing of joints. Experimental work in AA5754-H111 aluminum sheets and AISI 304 stainless steel tubular rivets was supported by finite element modelling. Results allow concluding that the new process incorporates the advantages of conventional self-pierce riveting and successfully overcomes its main disadvantages, namely the material protrusions above and below the sheet surfaces and the difficulties in joining sheets of dissimilar materials with large thicknesses. The new mechanical joints also have the advantage over those produced by clinching and self-pierce riveting of being invisible, because they are hidden inside the cross section of the overlapped sheets.

This paper is focused on fixing sheets to tubes away from the tube ends by annular sheet squeezing. The main objectives are the investigation of the physics behind material separation at the cross-section recess corner of the punches and the determination of the appropriate squeezing depth values. The methodology combines experimentation and finite element modelling and the overall presentation is illustrated with selected test cases retrieved from the experimental work plan. Results show that material separation is caused by through-thickness shear, that forces and energy required to form the new surfaces are negligible and that appropriate squeezing depth values result from a compromise between the quality of the mechanical interlocking and the pull-out destructive strength of the joints.

Sheet-bulk forming (SBF) processes based on a combination of deep drawing and upsetting are being increasingly used to produce new sheet metal parts with significant changes in wall thickness to accommodate three-dimensional functional features such as solid bosses, teeth and ribs. The utilisation of sheet-bulk forming processes introduces additional complexity in the design, setup and costs of tooling due to the inherent high forces and complex material flow that easily lead to underfilling and failure in critical die regions. This paper is concerned with these issues and is focused on the development of a sheet-bulk forming demonstrator capable of replicating material flow and failure in sheet thickening, local sheet thickening and sheet injection operating conditions. The demonstrator combines sheet bending and upsetting operations with a flexible design concept based on the utilisation of active modular tool parts to allow different materials, geometries and tool concepts to be tested under laboratory conditions and rapidly transferred to industrial SBF tools.

This paper analyses the differences between incipient and repeatable material flow in the incremental sheet-bulk metal forming (SBMF) of gears produced by indentation along the direction perpendicular to the sheet thickness. The underfilling of the punch cavity during the first indentation, which prevents the production of sound disk gears, is explained on the basis of constrained material flow under material strain hardening. A solution based on the utilization of a tailored disk blank is proposed to overcome this defect. The geometry of the tailored disk blank is determined by means of finite element analysis, and the overall methodology involved material characterization and experimentation with DC04 mild steel. The discussion on the extent of the plastic deformation region under constrained and free-material flow during indentation is complemented by experimental results obtained with a flat punch in rectangular sheet blanks of aluminium EN AW-1050A.

This paper revisits the formability limits of single point incremental forming (SPIF) in the light of fundamental concepts of plasticity and ductile fracture mechanics. The paper has a twofold objective of investigating the limiting strain pairs at fracture in parts showing and not showing signs of necking before cracking and of demonstrating that failure by fracture occurs by tension in crack opening mode I. The overall methodology is based on the combination of circle grid analysis, measurement of the ‘gauge length’ strains at fracture and determination of fracture toughness from experimental tests performed with truncated conical SPIF parts and double edge notched test specimens loaded in tension. The work is performed in aluminium AA1050-H111 and is a step towards comprehension of the circumstances under which failure will occur in SPIF. It is shown that fracture strain pairs of truncated conical parts, fracture forming limit lines (FFLs) determined from conventional sheet formability tests and fracture toughness in crack opening mode I can be merged to create a new understanding of plastic flow and failure by fracture above the onset of necking.

This paper presents a new manufacturing process for producing metallic liners for composite overwrapped pressure vessels (COPV’s) that are commonly utilized in spacecraft. The process combines an innovative tube-forming operation, which is capable of shaping commercial tubes into seamless metallic liners in a single stroke of a press, with a sequence of post-forming operations comprising machining, joining by crimping and welding, and gas tightness testing by means of non-destructive examination. The presentation focuses on the manufacturing process as a whole, but tube-forming in particular, and was written with the objective of providing details of the deformation mechanics, formability limits and tooling system of the tube-forming operation. Different types of COPV’s liners with 120 mm diameter are included in the presentation to illustrate the effectiveness and flexibility of the tube-forming operation in meeting the specific requirements of aerospace applications concerning storage capacity and available shapes. The process running under steady-state production conditions is expected to reduce the overall production costs of COPV’s by roughly one third.

The aim of this paper is to provide an overview into the development and application of single point incremental forming in rapid prototyping and rapid manufacturing of polymer sheet products that will enable the readers to recognize the key influential variables, to identify the process feasibility window, to diagnose possible sources of failure and to understand the routes for selecting the most appropriate materials and operative conditions. The methodology draws from independent determination of mechanical properties and formability limits of polymers to rapid prototyping of truncated conical and pyramidal parts. The investigation is supported by circle grid analysis. Results and observations are explained in the light of theoretical framework based on membrane analysis that is capable of modelling the cold plastic deformation of polymers with pressure-sensitive yield surfaces. The results show that the single point incremental forming of polymer sheets, performed on conventional CNC machining centres, is a cost-effective innovative technology for product development in a manufacturing environment. Applications may span from products with very high depths, taking advantage of the excellent formability of polyethylene terephthalate, to applications in polycarbonate where transparency is kept during cold forming.

The last years saw the development of two different views on how failure develops in single-point incremental forming (SPIF). Today, researchers are split between those claiming that fracture is always preceded by necking and those considering that fracture occurs with suppression of necking. Each of these views is supported by convincing experimental and theoretical arguments that are available in the literature. This paper revisits failure in SPIF and presents a new level of understanding on the influence of process variables such as the tool radius that assists the authors to propose a new unified view on formability limits and development of fracture. The unified view conciliates the aforementioned different explanations on the role of necking in fracture and is consistent with the experimental observations that have been reported in the past years. The work is performed on aluminium AA1050-H111 sheets and involves independent determination of formability limits by necking and fracture using tensile and hydraulic bulge tests in conjunction with SPIF of benchmark shapes under laboratory conditions.

In a recent work, the authors presented a new single-stage nosing process for manufacturing metallic hollow spheres from thin-walled tubes that is capable of extending the formability limits of conventional nosing (Alves and Martins in J Strain Anal Eng Des 43:205–216, 2008). The fundamentals of the process were investigated by means of standard finite element analysis and its technological viability was evaluated by means of experimentation on industrial AA6060 aluminium alloy tubes. This paper is aimed on extending the scope of single-stage nosing so as to include the production of polymer hollow spheres. The investigation draws from the development of an innovative extension of the flow formulation that is capable of modeling cold plastic deformation of pressure-sensitive yield surfaces under a nonassociated flow rule to the identification and analysis of the major process parameters that influence formability and quality of the polymer hollow spheres. The overall performance of the process is assessed by means of a combined theoretical and experimental investigation made with industrial polyvinyl chloride (PVC) tubes. Results show that the innovative extension of the finite element flow formulation is capable of successfully modeling cold forming of polymers and that single-stage cold nosing of thin-walled PVC tubes can be successfully utilized for manufacturing sound hollow spheres.