Papers by Keyword: Sheet Metal Forming

Abstract: This research characterized the strain hardening behavior of AZ31 under different stress states from shear to balanced biaxial tension with a newly proposed yield function. Experiments are conducted for AZ31 magnesium alloy by in-plane shear specimens, dogbone specimens, notched specimens and bulging specimens to characterize the flow behavior under different stress states. The flow behaviors are characterized by a newly proposed yield function in a form of the three stress invariants. The proposed yield function is implemented into ABAQUS/Explicit to predict the plastic response of the alloy under different stress states. It is shown that the proposed yield function can precisely predict the distinct flow behaviors and reaction forces from shear to equibiaxial tension from the initial yielding to fracture.

Abstract: This work aims to evaluate the predictive performance of various Machine Learning algorithms when applied to the prediction of material constitutive parameters, particularly the parameters of the Swift hardening law. For this, datasets were generated from the results of the numerical simulations of uniaxial tensile tests. The Machine Learning algorithms considered for this study are: Gaussian Process, Multi-layer Perceptron, Support Vector Regression, Decision Tree and Random Forest. These algorithms were used to train metamodels based on training sets considering different numbers of materials and input parameters, which were then used to predict the hardening law parameters. The Gaussian Process algorithm achieved the overall best predictive performances. The results obtained show the potential of Machine Learning algorithms for application on the identification of material constitutive parameters.

Abstract: The latest demands in reduction of emissions compel the automobile industry to lighten the structure of vehicles using third generation advanced high strength steels. Due to the novelty of these steels, there is a need to characterize its fracture behavior during the forming process. This paper presents a study of strain field, crack locus and instant of failure for 980 grade third generation advanced high strength steel using defined tests with two specimens. Numerical simulations and experiments have been performed to evaluate and to compare the obtained results for this steel. Numerical simulations with implemented Hosford-Coulomb damage model use the extended finite element method to predict the fracture occurrence. According to results, numerical simulation predicts crack locus similar to experimental tests. Failure of the material shows a high sensitivity to damage evolution law.

Abstract: The experimental detection of localized necking is an important issue in sheet metal forming. Today, the most common and extended techniques are strain-based methods using digital image correlation (DIC). The present work discuses a thermal methodology to detect the onset of necking in metals based on the analysis of the temperature gradient using digital infrared thermography (DIT). A series of tensile tests of H240LA-O3 high-strength steel of 1.2mm thickness is analysed using DIC and DIT techniques. It is proposed that necking initiates when the temperature difference at a reference distance from the necking point reaches a critical value, which allows identifying the necking time and estimating the limit strains from the visible images using circle grid analysis.

Abstract: Increasing demands for reducing greenhouse gases drive the metal processing industries to a CO₂-neutral production. A thorough understanding of CO₂ emission sources from the stage of material acquisition up to the final component is thus necessary to improve the CO₂ footprint of sheet metal hot forming process chains. To emphasize on this, an exemplary hot forming process chain is assessed to identify the impact of each sub-process step on total CO₂ emissions and the savings potential of individual measures is evaluated. Moreover, a mathematical model is proposed that enables for the prediction of the product specific CO₂ emissions as early as in the product design stage. This model is tested to calculate the CO₂ emissions resulted during the production of an exemplary hot stamped sheet component. The results point out that the heating stage is responsible for the second highest percentage of CO₂ emissions in the process chain next to the material acquisition. Thus, as one of the most suitable measures, a concept to recover process heat from hot formed components to the cold initial blanks is proposed and evaluated analytically.

Abstract: Laser-based directed energy deposition (DED-LB/M) allows the application of a wear-resistant metal coating to the surface of a sheet metal substrate. Subsequent deep drawing of the part enables high material efficiency, significantly shorter production times, and lower unit costs compared to, for example, solely machined production of the entire component. At the same time, energy-intensive global heat treatment strategies can be avoided. For the numerical analysis of such hybrid process chains, both the sheet metal substrate and the additively applied coating are usually characterized individually. However, the low thickness of the coating in combination with a relatively high welding depth, which is required for a good bond to the sheet metal substrate during subsequent forming, lead to a strong gradient of the mechanical properties as well as complex mechanical interactions in the bonding zone. Therefore, appropriate characterization methods are required. In this work, an investigation of different influencing factors, like the rolling, hatch and tensile direction, is carried out with the aid of tensile tests using hybrid specimens. In this way, interactions between the influencing factors are identified. As substrate, 3.5 mm thick 16MnCr5 blanks are used, with an approximately 0.68 mm thick coating of Bainidur^® AM. In addition, an optical surface roughness measurement and metallographic analysis of the tensile specimen’s edge area after laser cutting is performed.

Abstract: The application of calcium carbonate particles as a green lubricant additive for sheet metal forming processes has been evaluated. Different particle sizes were tested, along with different concentrations of particles in a lubricant that typically does not perform well by itself. The lubricant mixtures were tested under pin-on-disc, four-ball, and bending-under-tension test conditions. The results of the different tribological tests were compared to determine whether standard tests, such as the four-ball test, could predict lubricant performance under sheet metal forming conditions. The application of any concentration of particles was shown to be beneficial to the lubricant performance in terms of wear resistance even though friction increased when calcium carbonate particles were added to the base paraffin oil. Small particles (40 nm) exhibited better performance than large particles (2 μm).

Abstract: Many sheet materials do not exhibit constant properties over the sheet thickness. On the one hand, this can be caused by a rolling process with a pass reduction of 1-2%, the so-called skin pass rolling. This is mainly used for deep drawing steel grades to eliminate the pronounced yield strength of the base material. Another possibility is abrasive blasting or shot peening of sheet metal. This causes plastic deformation of the sheet surface and material strengthening. The grading can be used to locally strengthen components or locally adapt the roughness. Since many production processes today are designed numerically, the mapping of such a grading is necessary but currently not implemented in FE code. Aim of this research is, to correlate the material hardness to a plastic pre-strain with a newly developed characterization method. For this purpose, graded material properties are generated by means of abrasive blasting as an example process. The resulitng locally varying work hardening over the sheet thickness is further integrated in finite element analysis by the assignment of a starting condition of a 2d-shell element formulation. With this approach, it is possible to map the graded material properties easily in FE simulations with a very good accuracy.

Abstract: Metal forming industry is frequently characterized by the demand of small-batch productions to manufacture highly customized products. Apart from the accuracy that is mandatory in high-tech applications, one of the main requirements remains the economic competitiveness that becomes critical in the case of the deformation of thick metal sheets due to the relevant forming loads and the large size of the machines that are required to perform such processes. These problems are partially solved by using incremental forming approaches, in which the deformation is gradually performed by the use of one (single point) or two (double-sided) tools that are usually made to slide on the metal sheet surface while they impose the desired deformation. The paper aims at introducing an innovative concept of incremental forming machine to perform double-sided incremental bends, specifically developed for thick metal sheets. The increased flexibility and the possibility to manufacture sound parts with reduced bending forces are shown and discussed.

Abstract: In the last decade, several phenomenological yield criteria for anisotropic material has been proposed to improve the modeling predictions about sheet metal-forming processes. In regard to this engineering application, two proprieties of models have been used. If the yield function and the plastic potential are not same (not equal), the normality rule is non associative flow rule (NAFR), otherwise, when the stresses yield has been completely coupled to the anisotropic strain rate ratio (plastic potential), is called the associated flow rule (AFR). The non-associated flow rule is largely adopted to predict a plastic behavior for metal forming, accurately about à strong mechanical anisotropy presents in sheet metal forming processes. However, various studies described the limits of the AFR concept in dealing with highly anisotropic materials. In this study, the quadratic Hill1948 yield criteria is considered to predict mechanical behavior under AFR and NAFR approach. Experiment and modeling predictions behaviour of normalized anisotropic coefficient r (θ) and σ (θ) evolved with θ in sheet plane. and the equibiaxial yield stress σ_b was assumed σ_b=1 but the r_b-values was computed from Yld96 [15].