Key Engineering Materials Vol. 549

Abstract: Single-point incremental forming (SPIF) is a quite new sheet-forming process which offers the possibility to deform complex parts without dedicated dies using a single-point tool and a standard three-axis CNC machine. Although the process mechanics enables higher strains with respect to traditional sheet-forming processes, research has been focused on further increasing the maximum forming angle. In the paper, a new approach is used to enhance the material formability through a localized sheet heating as a consequence of the friction work caused by high speed rotating tool. Numerical simulation was utilized to relate the effect of temperature with the main field variables distribution in the sheet.

Abstract: The influence of thermal pre-treatment on the formability of a precipitation-hardening aluminium alloy AA-2024 has been studied for three different heat treatment conditions: annealed (O-temper), solution treated and quenched (W-temper) and solution heat treated, quenched and then cold worked (T-temper).The maximum draw angle has been determined and the geometrical accuracy of specific SPIF formed parts has been compared. It is found that the maximum forming angles of the blank formed in O-temper and W-temper conditions show a respective 41% and 32% increase to those of the T-temper condition (initial blank).The hardness of the material reduces significantly after annealing, while SPIF parts formed from W-temper blanks regain their initial hardness after natural aging.

Abstract: Hydropiercing is an efficient way of piercing holes in mass produced hydroformed parts with complex geometries. By driving piercing punches radially into a hydroformed and fully pressurized tube, holes will be pierced and extruded into the tube-wall. Recent experimental studies have shown that the formability of advanced high strength steel (AHSS) tubes can be increased with the application of internal pressure. In this study, three-dimensional finite element simulations of a tube hydropiercing process of a dual phase steel (DP600) were performed in LS-DYNA, using phenomenological, micromechanical and combined damage criteria. Damage was included in the numerical analysis by applying constant equivalent plastic strain (CEPS), the Gurson-Tvergaard-Needleman (GTN), and the Extended GTN (GTN+JC) model. In order to calibrate the parameters in each model, a specialized hole-piercing fixture was designed and piercing tests were carried out on non-pressurized tube specimens. Of the various ductile fracture criteria, the results predicted with the GTN+JC model, such as the punch load-displacement, the roll-over depth, and the quality of the clearance zone correlated the best with the experimental data.

Abstract: Single Point Incremental Forming (SPIF) is an interesting manufacturing process due to its dieless nature and its increased formability compared to conventional forming processes. Nevertheless, the process suffers from large geometric deviations when compared to the original CAD profile. One particular example arises when analyzing a truncated two-slope pyramid [. In this paper, a finite element simulation of this geometry is carried out using a newly implemented solid-shell element [, which is based on the Enhanced Assumed Strain (EAS) and the Assumed Natural Strain (ANS) techniques. The model predicts the shape of the pyramid very well, correctly representing the springback and the through thickness shear (TTS). Besides, the effects of the finite element mesh refinement, the EAS and ANS techniques on the numerical prediction are presented. It is shown that the EAS modes included in the model have a significant influence on the accuracy of the results.

Abstract: Superplastic forming (SPF) is a well-known and widely used sheet metal forming process especially useful for the production of very complex and light thin sheet metal components. The superplastic behavior of a material is highly dependent on the temperature and occurs only at a narrow range of strain rates with an optimum value that is unique for each material. Within the aeronautic industry, this process is mainly used to form complex sheet metal parts made of the titanium alloy Ti6Al4V in heat affected areas and areas where corrosion resistance plays an important role. Even though the process times of SPF are often in the range of hours and therefore recurring costs are very high, the process is sometimes still the only choice when it comes to the forming of Ti6Al4V sheet metal parts for aeronautic or aerospace applications. To overcome the problem of long process times and high costs, in recent years, a lot of research did happen with the goal of temperature reduction during forming or forming at higher strain rates. Especially the change in the aeronautic industry towards high volume production is increasing the competition between suitable forming technologies and the SPF technology can only persist if both goals, reduction of process time and recurring costs are reachable. In this paper we will address those goals and show highly useful numerical procedures to make the SPF process ready for the next generation of aerospace manufacturing.

Abstract: Blank shape is one of the most important parameters of sheet metal stamping. In fact it can directly affect the forming quality of parts and it has to be taken in account in sheet hydroforming design. Reasonable blank shape not only can reduce materials and production cost but, also, it can improve the strain distribution of the material and product quality in the hydroforming process. However, it is not easy to find an optimal blank shape because of complexity of deformation behavior and presence of many process parameters like die radius, punch radius, punch speed, blank holder force and friction. In fact, they affect the result of the process i.e. tearing, wrinkling, springback and surface conditions such as earing. Even a slight variation in one of these parameters can result in defects. This paper reports numerical and experimental correlation for axis symmetrical hydroformed component using initial blank with different shape and size. Experimental tests have been carried out through the hydroforming cell tooling, designed by the authors thanks to a research project, characterized by a variable upper blankholder load of eight different hydraulic actuators. Two different initial blank shapes, square and circular, of same material and thickness have been used.

Abstract: Nowadays the main target in the automotive field is the realization of lightweight and safe components. In this way it is possible to reduce costs and improve fuel consumption and, at the same time, enhance passenger safety. The use of tailored blanks has increased considerably in the automotive industry. Tailored blanks are a combination of different thicknesses or different materials, obtained by welding together two or more blanks, used in particular in car body panels. A new requirement in the automotive sector is the application of aluminum tailored blanks. The main target of this paper is the development of accurate numerical models for bending tailored blanks made from thin aluminum sheets, joined by laser welding, without filler metal. The FE bending simulations have been carried out using an explicit solver. The accuracy of the numerical models has been estimated and improved through a comparison with the results from an experimental study. The experimental tests have been performed using bending testing equipment, designed and developed by the authors. Three different bending radii have been tested. Tailored blanks, used as specimens, have been made by laser welding of thin Al6061 sheets. The considered outputs, used for the numerical-experimental comparison, are the punch force and the bending angle. The experimental results have been compared with the numerical ones in order to verify the accuracy of the FE model related to thickness and radius variations.

Abstract: A method of springback reduction is presented which involves a combination of solution heat treatment, natural ageing, stretch forming and artificial ageing. A commercial aluminium alloy, 2219-O Hi-Form, is solution heat treated and subjected to two pre-form natural ageing periods of one and seven hours before the stretch forming process. Following the forming process an artificial ageing process combined with creep age forming techniques is used. Springback is assessed following the stretch forming and artificial ageing processes. This process is shown to provide a controlled means of correcting the springback following stretch forming.

Abstract: One of the key application areas of Single Point Incremental Forming is in the manufacture of parts for bio-medical applications. This paper discusses the challenges associated with the manufacture of cranio-facial implants with extreme forming angles using medical grade titanium sheets. While on one hand, the failure wall angle is an issue of concern, the parts also need to be manufactured with accuracy at the edges where the implants fit into the human body. Systematic steps taken to overcome these challenges, using intelligent intermediate part design, feature analysis and compensation, are discussed. A number of case studies illustrating the manufacture of accurate parts in aluminium, stainless steel and titanium grade-2 alloy are discussed.

Abstract: In recent years the process simulation of entire manufacturing chains in sheet metal forming has gained more ground on its way to be established in the validation of feasibility and the meeting of quality targets. Nowadays, especially the simulation of manufacturing automotive sheet metal components such as doors, hoods roofs etc., using the finite element analysis, belongs to state of the art in the development process of sheet metal components. The different joining technologies in the bodyshell work, such as riveting, welding etc., can be simulated by numerical methods. Rarely are any of these methods linked to the forming simulations of the previous process step. Further developments in this field should deal with new strategies, linking both the forming simulation and the joining simulation. Regarding the process chain press line forming and bodyshell work, the prediction of springback of closure assemblies is of special interest, and thereby new strategies for springback compensation have to be developed in an early stage of product development. Until now, only few experiences have been gained concerning application of method to calculate springback of an assembly, so far a reliable comparison between simulation and reality is required. For this reason in this paper a closer examination of an automotive hood assembly was carried out to develop and to validate a corresponding simulation model. Based on close-to-production experiments and optical surface measurements of outer skin components (doors, bonnets, trunk lids etc.) and their assembly prior start of series production, a new simulation strategy was developed for a consistent process chain for the press plant and bodyshell work. The assembly in this example was comprised of an inner and outer panel. For forming objectives of both parts, finite element simulations were conducted, including secondary forming operations, as for instance trimming and flanging including individual unloading sequences. In a following simulation step both parts were joined together by roller hemming. Finally, a springback analysis of the assembly was also conducted. The simulation method illustrates the possibility of predicting springback of assemblies. Furthermore, the experiments and simulations show, that the springback of the assembly leads to different final shapes than those obtained from individual components. With this method it is possible to predict the final shape and the influence of the individual components on shape and dimensional accuracy, aiding the optimization of the assembly process.