Key Engineering Materials Vol. 549

Abstract: To reduce drag and improve efficiency, the next generation of aircraft will increasingly utilise laminar flow technologies. Of particular interest is the use of natural laminar flow in nacelle designs. A key element to achieving natural laminar flow is the elimination of joints on the external surface of the nacelle through the rearward extension of the lipskin trailing edge. Current processing methods are limited in their ability to produce these extended lipskins while meeting production rates, cost targets and the requirements of the natural laminar flow designs. A novel process for the production of extended natural laminar flow lipskins is presented along with a series of pre-production trials. The efficacy of the process is clearly demonstrated.

Abstract: The design of a suitable forming process for a stamped part is a complex issue. Many boundary conditions must be fulfilled and many considerations must be made to come to a successful solution. Elimination of wrinkles and splits very often need contrary counter-measures and, more often than not, it is both an iterative and subjective process. AutoForm Engineering has thus developed a methodology whereby stamping issues are automatically detected, based on all important failure and quality criteria, and which enables automatic tracking of engineering progress. This paper introduces a systematic approach which reduces the number of loops in the effective design of a forming process and allows the streamlined evaluation of multiple simulations in order to determine the most influential parameters to generate the best solution. The automatic detection of stamping issues and subsequent systematic process improvement will be illustrated using both an academic part and a real automotive component.

Abstract: The application of modern materials plays an important role directly under the aspect of lightweight potential. To exploit these options effectively a numerical accurate reproduction of the material behavior is indispensable. Especially in the case of large deformations a directional and strain rate dependent hardening behavior can be observed. By disregarding this effect significant failure in the computed stress state can arise, which can conduct to a corruption of the spring-back forecast. Within this contribution a new test method for analyzing the evolution of subsequent yield loci under strain path changes for the aluminum alloy AA6016 and the deep drawing steel DC06 is presented. In the first stage of the experimental investigations, yield loci with linear strain paths were considered to characterize the material behavior for the initial condition. On further experiments with several stress states the strain path dependent hardening behavior of the material is determined. The non-linear strain paths are realized through uniaxial prestrained primary specimens with following extraction of secondary samples for following stress states, e.g. a modified ASTM simple shear test specimen. Subsequent yield loci are investigated and compared to the yield surfaces Hill48 and Barlat 2000 (Yld2000-2d) with an isotropic hardening behavior. With this study the evolution of the yield locus for prestrained specimens is evaluated. The research of the subsequent yield loci for strain path changes serves as basis for further scientific investigations with a view to assess different approaches of isotropic-kinematic hardening models in consideration of the analyzed steel and aluminum sheet metals.

Abstract: Forming of titanium sheets, especially titanium alloy sheets, is very difficult due to low drawability caused by a high ratio of the yield point to the tensile strength R_e/R_m, usually more than 90%. Although drawability of titanium sheets can be enhanced by forming at elevated temperatures it is avoided due to the high costs and difficulties associated with the operation of the process. Therefore the authors have developed an unconventional stamping method allowing for forming of almost unworkable materials at ambient temperature, such as Ti6Al4V titanium alloy. The paper presents both numerical simulation and experimental results of the stamping process using a device specially designed for this purpose.

Abstract: Encouraged by increasing climate regulations there is a trend to lightweight constructions in the transportation sector, particularly in the automotive industry. An auspicious possibility to reduce the weight of the vehicle is the substitution of conventional steel by aluminum alloys. However, aluminum has a low formability and therefore new technologies have to be found in order to enhance the materials spectrum of application. A new strategy to extend the process limit is the short-term aluminum intermediate heat treatment between two forming operations. Key idea is the partial adaption of the mechanical properties with a short term heat treatment. By the interaction of soft and hard areas the material flow during the forming operation can be improved and the formability can be enhanced. Prerequisite for a successful application of the technology and the numerical simulation is a comprehensive knowledge about the interaction of pre-straining and a subsequent short term heat treatment. Within this paper the mechanical properties in dependency of the two parameters will be presented and a process window for the heat treatment after first forming operations will be derived. Moreover, the influence of batch fluctuations and the impact of ageing will be demonstrated.

Abstract: ncremental Sheet Forming (ISF) is a manufacturing technology for individualized and small batch production. Among the opportunities this technology provides there is the possibility of a short ramp up time and to cover the whole production chain of sheet metal parts by one machine setup. Since recent works showed that manufacturing of industrial parts is feasible, finishing operations such as flanging and trimming gain importance. This paper shows first works on the technological capabilities of using ISF for stretch and shrink flanging. Due to the localized forming zone the absence of surrounding clamping devices for ISF results in differing material flow behaviour. The influences of tool path characteristics, flange length as well as radii are analysed to set up a process window.

Abstract: Rotary Peen Forming (RPF) is a new peen forming process, comparable to Shot Peen Forming (SPF), in which the shot is held by a flexible connection and moved on a circular trajectory. Hence, RPF uses less machine components and therefore offers a compact machine design and a more flexible use than SPF. Just as conventional Shot Peen Forming the RPF process causes localized plastic deformation but involves tangential components which can create shear deformation in the plastic layer. In this paper, three different RPF tool concepts are compared and the applicability of Rotary Peen Forming for the production of slightly curved parts is analyzed. The first design offers a stochastic impact distribution, the second design leads to deterministic impacts. The third one is a further enhancement of the previous designs and combines the advantages of both. In contrast to previous tests a new, stiffer testing setup was used which offers good comparability of the tool concepts. Particularly the forming potential in terms of the realization of high curvatures and the surface quality are investigated. Depending on the tool concept the surface quality differs significantly, but generally RPF allows the forming of curvatures that are commonly used for aerospace structural parts.

Abstract: ncremental sheet metal forming with direct resistance heating is used for flexible sheet metal forming at elevated temperature, where electric current is conducted through the forming tool (s) into the forming zone. The electrical and mechanical contact combined with a high temperature of up to 600°C in steel forming results in complex tool requirements and a high wear of the tooltip. Starting with a description of a new process setup, both studies concerning existing and new tool concepts and materials will be presented in this paper. Therefore, the wear of different materials for tooltips and its dependence on lubrication has been investigated in forming experiments and will be thoroughly discussed.

Abstract: Laser forming (LF) is a non-contact method to shape metallic sheets and tubes by induced thermal stress without melting using a de-focused laser beam. Laser forming offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, correction of design shape or distortion and precision adjustment applications. In order to fulfil this promise in a manufacturing environment the process must have a high degree of control, be repeatable and have a minimal impact on the material and mechanical properties of the part to be formed. In order to demonstrate the capability of the LF process a study is presented in this paper on the 3D Laser Forming of ERW steel square tubes SHS EN10305-5 E220 +CR2 (1.5x25x25mm and 1.5x50x50mm 300mm long tube) using a 1.5kW CO₂ laser and industrial 5 axis gantry. Strategies have been developed for out of plane bending with specific emphasis on process throughput balanced with minimising adverse localised changes to material properties that could lead to stress concentration features in a component in service. Presented in this paper is empirical 3D LF shape data verified by a scanning laser profiler, a metallurgical study, hardness testing and a FEM model developed in Comsol Multi-Physics. The results of these studies were employed to develop optimised scan strategies for the controlled laser forming of the ERW steel square tubes within strict metallurgical constraints.