Key Engineering Materials Vols. 651-653

Abstract: Springback effects, as occuring in sheet metal forming processes, pose a challenge to manufacturingplanning: the as-built part may deviate from the desired shape rendering it unusable forits intended purpose. A compensation can be achieved by modifying the forming tools to counteractthe shape deviations. A prerequisite to compensation is the knowledge of correspondences (u_i; v_j),between points u_i on the desired and v_j on the actual shape. FEM-based simulation software providesmeans to both virtually predict springback and directly obtain correspondences. In case of experimentalprototyping and validation, however, finding correspondences requires solving a registrationproblem: given a test shape Q (scan points of the as-built geometry) and a reference shape R (CADdata of the desired geometry), a transformation S has to be found to fit both objects. Correspondencesbetween S(Q) and R may then be computed based on a metric.If S is restricted to Euclidean transformations, then S(Q) results in a rigid transformation, whereevery point of Q is subject to the same translation and rotation. Local geometric deviations due tospringback are not considered, often resulting in invalid correspondences. In this contribution, a nonrigidregistration method for the efficient analysis of springback is therefore presented. The test shape Q is iteratively partitioned into segments with respect to an error metric. The segments are locally registeredusing rigid registration subject to regulatory conditions. Resulting discontinuities are addressedby minimization of the deformation energy. The error metric uses information about the deviationscomputed based on the correspondences of the previous iteration, e.g. maximum errors or changes ofthe sign. This adaptive per-segment registration allows appropriate correspondences to be determinedeven under local geometric deviations.

Abstract: Due to increasing requirements regarding the flatness of sheet metals, the process of roller levelling is of particular importance. The process itself is influenced by a high number of parameters such as machine design, sheet dimension, and material properties. Therefore, it is desirable to provide an online process control to react on changes of those process parameters. One possible approach for the layout of a process control and the identification of reference values is the use of the Finite Element Method (FEM). Considering the alternate bending a sheet metal undergoes when passing through a roller leveller, kinematic hardening of the sheet material must be taken into account. Additionally, the initial stress and strain distribution of the sheet metal – e.g. induced by coiling – has an influence on the material behaviour and consequently on the process parameters. With respect to these effects, a coupled FE model, which accounts for the initial state of the sheet metal, is introduced. An inverse calculation of material parameters describing the behaviour under cyclic load conditions has been done for an aluminium alloy AA5005 and a mild steel DC01. Based on this numerical setup, the influence of the initial stress state in the pre-levelled sheet metal on the roller levelling process has been deduced. Accompanying experiments on a down-sized roller leveller were carried out for a validation of the numerical setup.

Abstract: The current global development towards efficient and sustainable usage of resources as well as a stronger environmental awareness motivates lubrication abandonment in metal forming. Dry forming processes accomplish besides a green production technology also a shortage in production steps and time. However, the change of the tribological conditions influences the material flow during the forming operations and has therefore to be taken into account for the design of complex sheet metal forming operations. The aim of this study is a comparison of dry and lubricated processes by numerical as well as experimental investigations. To ensure reliable results a test setup is necessary which provides a discrete control of the process parameters. Furthermore, an analysis of the local material flow by an optical strain measurement system during the whole test procedure should be possible. These requirements are well fulfilled by the so called Nakajima test, which is typically used for the characterisation of the formability of sheet metals. The influence of varying friction coefficients on the material behaviour is discussed based on the numerical model built up in the Finite Element Software LS-Dyna. The numerical results show a good conformity with the experimental outcomes by identifying the strain localisation. Based on the gained knowledge of the investigations an increase of process understanding for dry forming operations will be derived.

Abstract: Over these last decades, titanium and its alloys have been largely used for many applications in different sectors such as aerospace, military and biomedical ones. Many studies have investigated the behaviour of the most broadly used alloy, the Ti6Al4V, under hot forging and superplastic forming conditions, whereas almost no records can be found about the Ti6Al4V sheet behaviour at elevated temperature and moderate strain rate (i.e. above 0.1 s^-1). The research starts analysing the influence of the thermal cycle parameters of on the Ti6Al4V mechanical properties and microstructural characteristics by means of micro-hardness measurements and Optical Microscopy (OM) analysis. Different soaking times, cooling rates and heating technologies, namely furnace and induction, were considered and their effects investigated. Based on these results, uniaxial tensile tests were carried out at different temperatures, ranging from room temperature up to 900°C, and strain rates of 0.1 and 1s^-1, adopting the thermal parameters and heating means previously investigated. The Ti6Al4V sensitivity to the rolling direction was highlighted, calculating the average normal anisotropy as a function of the testing parameters. The samples fracture areas were then measured in order to calculate the strain at fracture as a function of the temperature and strain rate, while the fracture morphology was investigated through Scanning Electron Microscopy (SEM). OM and SEM analyses were also used to investigate the deformed samples microstructure. Finally, the samples micro-hardness, as a measure of the post-forming characteristics, was measured in order to evaluate its sensitivity to the temperature and strain rate.

Abstract: Rotary swaging is an incremental cold forming process for tubes and rods. The established processes use lubricants based on mineral oil. The functions of the lubricant are the reduction of friction, wear and tool load, furthermore it cools the tools and flushes the working zone. But the use of lubricant increases the cost due to additional process steps and lubricant is diverted with the work piece during the process. Thus from economic and ecological points of view it is worthwhile to eliminate the use of lubricant. Therefore it is necessary to realize the functions of the lubricant in another way. For example by means of coating and micro-structuring of the tools the friction and wear can be influenced. In this study dry rotary swaging is tested with conventional tools and machine settings. The analysis of the recorded process parameters and the formed geometry of the workpiece reveal the potential of dry rotary swaging, but also the difficulties that arise. Dry rotary swaging needs a modification of the process and system parameters as well as an adjustment of the tools.

Abstract: From an industrial point of view it is interesting to extend the process limitations in swing folding by creating a possibility of forming sheet metals of materials with low formability by an integrative heating module. The experimental setup and the performed experiments with the corresponding results and an outlook to future investigations will be presented in this paper.

Abstract: Nowadays, the need of developing high flexible forming processes matches with the need of weight reduction. In this light, the incremental forming of titanium alloys sheets can guarantee both these aspects by combining the flexibility of the process, particularly suggested for small batches and customized parts, with the good properties of titanium alloys, in particular for aerospace applications.The aim of this work was to obtain information useful to enhance the general knowledge of the hot incremental forming processes of grade 1 titanium sheets at different temperatures.First, both tensile and straight groove tests were carried out by varying the test temperature; in this phase, information regarding both the forming forces and the wear phenomena due to the tool-sheet contact was acquired.Successively, on the basis of the mechanical characterization of the sheets previously carried out, explicit analyses, effectuated by a non-linear FE code, allowed to determine the formability curves of the sheets for the different temperatures.

Abstract: The coiling process under traction is considered, with an incoming residual stress profile (that can be sufficiently compressive to make the strip buckle): a flatness defect. This paper details a 3D non-linear numerical simulation taking into account the contact of the strip on itself, with a perfect contact law. The model relies on elastic behavior at finite strain because of large rotations. Even though the behavior is elastic, the yield Von Mises criterion is computed and gives information about flatness defects (plastic zones are approximated by zones where the yield stress is exceeded). Furthermore, the paper aims at very short computation times. The modeling strategy relies (for each time step) on two analytical sub-steps. Numerical minimization procedure is used in order apply weak boundary conditions. Results are discussed with respect to a comprehensive Finite Element simulation and good agreement is observed.

Abstract: In actual manufacturing, some empirical method such as the bottoming technique is generally used in order to adjust the bend angles of products. In this study, a new method for controlling the bend angle in the V-bending process was attempted by applying sheet metal forging. In the experiments, three punches with lumps at the punch tip were used. These lumps were pushed into a bent section at the final stage of bending and were able to stretch the inside plane of the bent section. In both cases examined (using punches with one or two lumps at the tip), the bend angle decreased with an increase in punch displacement. This result shows new possibilities for controlling the bend angle by introducing plastic deformation to the bent section.

Abstract: This paper deals with the formability of pure titanium sheet in square cup deep drawing. Pure titanium has very excellent corrosion resistance. In the metal forming process, pure titanium has very good ductility in cold forming. The normal anisotropy of pure titanium is very high. Therefore, the property is suitable to the sheet metal forming, such as deep drawing process. However, the most important problem is that the occurrence of seizure becomes remarkable in severe forming operations. Many investigations on the effect of processing conditions on the seizure of titanium were carried out. In the present study, the formability of pure titanium sheet in square cup deep drawing was investigated. For the prevention, pure titanium sheets were treated by heat oxide coating. The fresh and clean titanium is not in direct contact with the die during the forming due to the existence of the oxide layer. The material was pure titanium sheets of the JIS grade 2. The initial thickness of the blank was 0.5 mm in thickness. In the deep drawing process, the sheets were employed and a flat sheet blank is formed into a square by a punch. Forming of sheet by multi-stage deep drawing was tried. Various cups were drawn by exchanging the punch and die. The die was taper without a blankholder in the subsequent stages. The effects of the intermediate annealing and tool shape on the occurrence of seizure in square cup deep drawing were also examined. The square cups were successfully drawn by heat oxide coating. The coating of titanium sheet has sufficient ability in preventing the seizure in multi-stage deep drawing operation. The results of the present study revealed that the pure titanium square cups were successfully formed by using heat oxide coating treatment.