Key Engineering Materials Vol. 639

Abstract: The forming limit curve (FLC) is a common method to assess material formability in sheet metal forming processes. It is determined with the Nakajima or Marciniak test according to ISO 12004-2 [1]. The disadvantage of these test procedures is that the results are only valid for linear strain paths. In most real sheet metal forming processes, like deep-drawing of complex car body parts or multi-step processes, nonlinear strain paths exist. It is well-known that the classic FLC cannot describe material failure for nonlinear strain paths.At the Institute for Metal Forming Technology (IFU), new punch geometries have been developed to realise specific nonlinear strain paths in a standard Nakajima testing environment. The formability of sheet materials under nonlinear loading can be determined more accurately when using these new punch geometries than with the classic Nakajima test setup. Different strain paths can be realised depending on the specimen and the punch design, in order to evaluate the formability of the material according to strain conditions as they occur in real forming processes.Within this paper, the results of different punch geometries have been tested using the mild deep-drawing steel DC04. The strain conditions before crack initiation are compared to the standard FLC and to the newly developed IFU-FLC criterion, which can predict material failure under nonlinear strain paths.

Abstract: Using additive layer manufacturing techniques, such as Fused Deposition Modelling (FDM), it is possible to produce complex geometry relatively quickly and cheaply. For this reason these processes offer intriguing possibilities for tooling in metal forming processes. A common material utilised in the FDM process is Acrylonitrile Butadiene Styrene (ABS). A series of compression and tensile tests were carried out on FDM ABS test specimens built in a range of orientations. The tensile tests were carried out until fracture and the specimen cross-sections analysed to investigate the cause of failure. In uniaxial tension the vertical build direction was found to be the weakest, failing in a brittle fashion. The FDM material elastic modulus and Poisson’s ratio were found to be isotropic in nature within experimental scatter. The ‘yield’ strength in compression was found to be higher than that observed for equivalent tensile orientations. Following a series of strip pull friction tests using commercially pure titanium as the blank material, it was found that without the utilisation of an interfacial lubricant a favourable frictional performance was achievable on ABS tool surfaces. Due to tool wear however, the frictional performance of these tool pieces deteriorated with increasing sliding distance.

Abstract: The forming limit diagram (FLD) is at the moment the most important method for the prediction of failure within sheet metal forming operations. Key idea is the detection of the onset of necking in dependency of different sample geometry. Whereas the standardized evaluation methods provides very robust and reliable results for conventional materials like deep drawing steels, the determined forming limits for modern light materials are often too conservative due to the different failure behavior. Therefore, within this contribution a new and innovative approach for the identification of the onset of necking will be presented. By using a pattern recognition-based approach in combination with an optical strain measurement system the complete strain history during the test can be evaluated. The principal procedure as well as the first promising results are presented and discussed.

Abstract: The continuous pursuit of vehicle weight reduction forces the industry to look for alternative materials to steel. Light alloys such as aluminium or titanium are materials that provide a decrease in weight using conventional technologies. Additional weight reduction results from using tailor-welded blanks (TWB). While the joining and forming steel or even aluminium TWBs is quite well known and described in the technical literature, joining and forming titanium TWBs still poses a significant problem. In the paper, experimental tests carried out with welded samples manufactured from commercially pure titanium Gr 2 and titanium alloy Gr 5 sheets are presented. The samples were joined by electron beam welding. Mechanical testing and optical microscopy were used to characterise the welds and the base metal of the samples. The samples were subjected to uniaxial tension up to final failure. The 3‑D Digital Image Correlation system ARAMIS was used for monitoring the whole deformation process. This makes it possible for real-time observation of sample deformation. The test results and the numerical analysis of the tensile tests are compared. The numerical simulations were carried out with the ADINA System based on the Finite Element Method (FEM). The mechanical analysis leads to calculation of the strain state after sample deformation in uniaxial tension (mechanical model).

Abstract: In the present study, deformation behavior upon two-step loading in a rolled AZ31 Mg alloy sheet was investigated. The experimental procedure was as follows: (1) a sheet was subjected to in-plane compression, (2) small samples were cut from the compressed sheet along various directions, and (3) uniaxial tension was imparted to small samples. The angle between the first and second loading directions was set to either 0, 30, 60, or 90°. During the second loading, a strong in-plane anisotropy occurred in the stress-strain curve: a sigmoidal curve occurred during the second loading for the angles of 0 and 30°, while it did not arise for the angles of 60 and 90°. From microstructural observations, it was presumed that the aforementioned results could be explained in terms of the activity of detwinning.

Abstract: Experimental and numerical investigations on the characterization and prediction of cold formability of two different types of automotive steel sheets, a ferritic stainless steel sheet (AISI 439) and a dual-phase steel sheet (DP600) are performed in this study. Due to the different microstructure configurations, these two steels show significant differences in plasticity behavior as well as failure mechanisms. The ferritic stainless steel shows strong anisotropic plastic deformation in terms of both yielding and hardening, whereas the dual-phase steel behaves quite isotropic resulting from the mixture of two phases. However, unlike the localization dominant failure mechanism of the ferritic stainless steel, the incompatible deformation due to the distinctions of the mechanical properties of two phases naturally results in early damage and extensive damage development prior to localization, or ductile fracture without localization. In this study, all these features are taken into account for an accurate prediction of formability. A general modelling framework with specifications for these separate features is formulated and applied to the two steels.

Abstract: In this work the strain behaviour of the heat-treated 6xxx series aluminium alloy AC170PX is investigated by a not conventional approach. Thanks to the low density combined with good mechanical properties, this aluminium alloy is often adopted for automotive applications. Despite these advantages, its formability at room temperature is low. In order to overcome this limit, a distribution of the material properties can be achieved by a local heat treatment (Tailored Heat Treated Blanks). In this context, to evaluate the effects of those parameters mainly affecting the precipitation hardening (aging temperature and aging time), a first experimental campaign was conducted using conventional furnace heat treatment in different conditions . Tensile tests were run with the aim of determining the flow and the aging curves of the heat treated specimens. Starting from these results, a not uniform heat treatment was designed using a Gleeble physical simulator Heat treatments based on a temperature gradient along the sample were performed. Then, tensile tests of the so heated specimens were carried out at room temperature. Through a digital image correlation system both the distribution and the evolution of the strain along the gauge length of the specimen were analysed in order to obtain the hardening/softening working conditions related to a specific heating cycle. These results were validated by the comparison with the data obtained from the first experimental campaign.

Abstract: A material model is developed that predicts the plastic behaviour of fully hardened 22MnB5 base material and the heat-affected zone (HAZ) material found around its corresponding resistance spot welds (RSWs). Main focus will be on an accurate representation of strain fields up to high strains, which is required for subsequent calibration of the fracture behaviour of both base material and HAZ. The plastic behaviour of the base material is calibrated using standard tensile tests and notched tensile tests and an inverse FEM optimization algorithm. The plastic behaviour of the HAZ material is characterized using a specially designed tensile specimen with a HAZ in the gage section. The exact location of the HAZ relative to the centre of the RSW is determined using microhardness measurements, which are also used for mapping of the material properties into an FE-model of the specimen. With the parameters of the base material known, and by assuming a linear relation between the hardness and the plasticity model parameters of base material and HAZ, the unknown HAZ parameters are determined using inverse FEM optimization. A coupon specimen with HAZ is used to validate the model at hand.

Abstract: Resource efficiency, design oriented accuracy and lightweight properties are demands on modern sheet metal forming parts in the automotive sector. The use of new materials leads to additional challenges on the numerical design of forming processes. During these forming processes the material undergoes different strain states that cause non-linear strain paths. Since the numerical prediction highly depends on the identified characteristic values of the material, an exact characterisation of the material behaviour is essential. Especially obtuse angles of the stress vector trigger a recovery of the material by returning stress. Besides, a relaxation of the material is investigated during holding a constant strain level. The effect of relaxation lead to an altered material behaviour that appears in a reduction of the beginning of plastic yielding. In addition, a kinematic hardening behaviour as under cyclic loading and load reversal, known as the Bauschinger effect, occurs after the relaxation of the stress and results in a reduced beginning of plastic yielding by loading in the same direction as the introduced pre-strain. Within this research work the effect of relaxation is investigated for two materials, AA5182 and DP600, with an initial sheet thickness of 1.0 mm. These materials are typically used for internal and accordingly functional parts in the automotive sector. The relaxation of the material is analysed with different holding times of a constant pre-strain at different levels of straining. The release of the material is studied by subsequent uniaxial tensile tests after pre-straining with the same load condition. Moreover, the influence of the named effects is shown by comparison of the translation of the yield loci.

Abstract: The recent development of new lightweight sheet metal materials, like advanced high-strength steels or aluminium alloys, in combination with an increasing component complexity provides new challenges to the numerical material modelling in the FEM based process design. An auspicious approach to improve the quality of the numerical results – most notably in springback analysis – is the modelling of the so called Bauschinger effect achieved through implementation of kinematic hardening models. Within this paper the influence of the stress state and the level of pre-strain on the numerical simulation result of the advanced high strength steel DP-K45/78+Z will be analysed. For this purpose, a parameter identification of the kinematic hardening law according to Chaboche and Rousselier is performed at different pre-strains on the basis of experimental data from tension-compression tests as well as cyclic shear tests. Finally, the identified parameters are validated in a comparison between numerical and experimental results of a cyclic bending test.