Papers by Keyword: Formability

Abstract: Some steels exhibit the Lüders effect. This phenomenon depends on the material and structure (test piece geometry, loading speed, etc.). Most work hardening laws do not take this phenomenon into account. The objective of this work is to define the most appropriate hardening law to highlight the characteristics of Lüders effects. First, the various aspects of the Lüders effect are presented. Several local hardening laws are proposed to describe the plateau or not, some of which are taken from the bibliography. Simulations of uniaxial tensile testing and forming of stamped parts are performed to compare these different hardening laws in predicting the Lüders effect. The Exp_Swift hardening law is recommended for forming cards because it is fully compatible with all software dedicated to steel sheet formability analysis and does not require inverse calibration during identification to accurately predict the plateau length.

Abstract: The influence of cold rolling pass schedules on the microstructural evolution, mechanical response and stress state of a DP800 base material was investigated. A micro-alloyed S355 steel with ferritic-pearlitic microstructure was subjected to identical total thickness reductions using different numbers of pass reductions. The mechanical behavior was characterized by uniaxial tensile tests while microstructural features were analyzed using electron backscatter diffraction and light optical microscopy, with grain morphology quantified by elliptical approximations. All investigations are carried out on the deformed ferritic – pearlitic microstructure, before the final intercritical annealing to produce the final dual phase microstructure. Finite element simulations of the flat rolling process were conducted to evaluate the evolution of non-proportional stress states in terms of stress triaxiality and Lode angle parameter. The results show that varying the number of passes leads to subtle but systematic differences in strength and ductility together with pronounced grain elongation and strongly banded pearlite morphologies that challenge ellipsoidal grain representations. While the overall stress-state trajectories remain similar, increasing the number of passes results in smoother stress evolution with reduced stress peaks. These findings highlight the non-trivial role of pass scheduling in shaping deformation-induced microstructures prior to annealing.

Abstract: With the increasing demand for lightweight materials, the combination of aluminum and magnesium sheets enables the development of advanced laminates with a balanced combination of strength and ductility, making them suitable for forming applications. This work investigates the effect of rolling temperature on the mechanical behavior and formability of AA1050/AZ31/AA1050 sheets produced by roll bonding in the temperature range of 250–450°C. Tensile tests showed that the yield stress is weakly affected by rolling temperature, whereas the ultimate tensile strength increases up to 350°C and then stabilizes. The elongation at fracture increases monotonically with temperature, indicating improved ductility at higher rolling temperatures. Microhardness measurements revealed softening of the aluminum sheets with increasing temperature, while limited variations were observed in the AZ31 sheet. Formability was evaluated by Erichsen Cupping test. The maximum load and extension at break remained nearly constant over the investigated temperature range; however, higher rolling temperatures led to reduced delamination and improved interfacial bonding integrity during deformation. The results indicate that roll bonding at elevated temperatures promotes better strain distribution and enhanced bonding quality. Overall, roll bonding at 450°C provides the most favorable combination of mechanical performance, formability, and interfacial stability, making the produced sheets suitable for lightweight forming applications.

Abstract: Lightweighting plays a critical role in reducing vehicle emissions, a major source of air pollution in the European Union. While weight reduction during the use phase is important, environmental impacts across production and end-of-life stages must also be considered. Advanced forming technologies enable the use of high-strength materials while maintaining formability and energy efficiency. Continuous-bending-under-tension (CBT) is a promising forming technique capable of inducing higher plastic deformation than conventional processes. In this study, CBT experiments were conducted on dual-phase 1000 low-yield (DP1000-LY) steel. The material was subjected to uniaxial tensile loading combined with cyclic bending through a moving three-roll system. The effects of key process parameters, bending depth and speed ratio between the bending assembly and tensile loading, were systematically investigated. The results show that lower bending depths allow greater total deformation before fracture and result in higher tensile forces. Higher speed ratios lead to earlier failure both during and after CBT processing. Hardness measurements indicate comparable surface hardness on both sides of the specimens, regardless of single or double roll contact. These findings contribute to a better understanding of CBT process parameters and support its potential application in lightweight automotive component manufacturing.

Abstract: Accurate prediction of the forming limit curve (FLC) is critical for evaluating sheet metal formability, yet the influence of plastic anisotropy remains controversial. In this study, the yielding behavior, hardening response, and strain-rate sensitivity of DC01 steel are experimentally characterized. Different yield criteria combined with a Swift hardening law and the Marciniak-Kuczyński (M-K) model are employed to predict the FLC. The results show that the high-order non-quadratic Yld2k-2d yield criterion captures both the yielding behavior and the forming limits. Numerical experiments using this material framework are then conducted. Variations in r-values have a limited effect on the FLC, in contrast to the common notion that high r-values mean high formability, whereas the equibiaxial tensile yield stress strongly governs the right-hand side.

Abstract: Medium-Mn steel (MMnS) and quenching and partitioning (QP) steels are two representatives of third-generation advanced high-strength steels (3^rd Gen AHSS), developed to achieve an optimal balance between strength and ductility. In forming applications, global formability reflects a material’s resistance to necking, while local formability indicates its resistance to fracture. Both aspects are essential for assessing mechanical performance. Global formability is often characterized by the forming limit curves at necking and is highly sensitive to work hardening behavior. Similarly, the forming limit curves at fracture determined from different stress states can be applied to evaluate the local formability. In addition, these deformation characteristics can be influenced by anisotropy introduced during sheet processing. Rolling process introduces orientation-dependent variations in both plastic flow and fracture behavior, which significantly affect necking development and fracture initiation. This study investigates and compares the global and local formability of various 3^rd Gen AHSS grades, with a focus on the influence of anisotropy. To investigate the anisotropic effects on plasticity and ductile fracture under different stress states, tensile tests were conducted on specimens with various geometries and orientations cut from sheet materials. Based on the tensile tests, the forming limit framework of Shen et al [1] was broadened to include anisotropic effects.

Abstract: The present work investigates the global and local formability of two third-generation Advanced High Strength Steels (AHSSs), a quenching & partitioning (Q&P) steel and a Medium Mn (MMn) steel with 1GPa strength. Third-generation Q&P and MMn steels are designed to overcome the limitations of first-generation AHSS grades by enhancing formability while maintaining high mechanical strength, thus enabling more efficient structural design and improved crash performance. Understanding their forming behaviour is essential to ensure their reliable use in complex sheet metal forming operations. In this study, the forming performance of a Q&P and a MMn steel is analysed through experimental procedures involving both in-plane deformation under various loading paths and hole expansion tests with different hole edge qualities, to evaluate their global and local formability. A first-generation Dual Phase (DP) steel is included in the analysis for comparison. The results demonstrate that 3^rd Generation Q&P and MMn steels exhibit very good global formability, superior to conventional 1^st Generation AHSSs. However, local formability, as evaluated by hole expansion capacity, can be severely compromised by edge manufacturing process. These findings contribute to a deeper understanding of the distinction between global and local formability in third-generation AHSS, offering insights to improve process robustness and support the industrial implementation of these steels in high-performance automotive components.

Abstract: Efficient characterization of formability of tubal sections is essential for designing lightweight aluminum extrusion components, particularly since the presence of weld seams and extrusion-induced inhomogeneities influence deformation behavior. This study evaluates the formability of Al–Mg–Si alloy tubes after being subjected to four different heat-treatment conditions, using a non-conventional rubber-assisted bulge test. A solid polyurethane (PU) plug was employed as pressure medium to enable full-scale deformation. Digital image correlation (DIC) was used to quantify circumferential and longitudinal strain evolutions, while post-fracture thickness measurements provided complementary insight into through-thickness strain.The measured circumferential strain at fracture ranged from 0.15 to 0.24 across the investigated tempers. The W-tempered condition exhibited the highest surface strain while maintaining moderate thickness reduction, whereas the soft-annealed tubes showed the largest thinning. The as-received and naturally aged conditions displayed similar deformation responses. These results demonstrate that tube formability, as evaluated by the present testing approach, is characterized by the combined evolution of surface strain and thickness reduction, both of which are influenced by heat treatment. Overall, the study shows that PU-assisted rubber bulge testing provides a practical and robust experimental framework for the comparative assessment of formability in extruded aluminum tubes.

Abstract: The prediction of sheet failure remains a highly relevant topic in metal forming research, particularly in relation to the experimental and theoretical determination of forming limit curves (FLCs). While the experimental construction of FLCs is a well-established but time-consuming process, theoretical and numerical approaches provide a more efficient alternative. However, their accuracy must be critically assessed, all the time. In this work, the formability of steel sheet with tensile strengths of 1500 MPa is investigated by combining experimental Nakajima tests with theoretical predictions of FLCs. Previous studies, such as models by Abspoel, Swift, Hill etc. have not addressed such high strength levels, where the diffuse and local neck points are quite close, leaving open the question of whether existing approaches remain valid for these materials. To provide a reference baseline, additional tests and calculations were also performed on lower-strength steel sheet (DP800). Our results show that the FLC points can be well estimated by two different theories in the positive quadrant, but there are noticeable differences between the measured and calculated values close to the plane strain point.

Abstract: Friction plays an important role on formability of deep drawn products. This necessitates an accurate description of friction in finite element formability analyses. It has been shown that constant coefficient of friction does not lead to precise prediction of product formability in these analyses. The multi-scale friction model developed at University of Twente takes the local contact conditions and textures of sheet metal and tools as the input at boundary and mixed lubrication regimes. To correlate the zinc coated sheet metal surface texture parameters with its formability, 60 different textures were analyzed. The multi-scale friction model is used to estimate friction for all the sheet metal surface textures. The effect of different textures on formability of the sheet metal was investigated by simulating cross-die forming using different sheet metal surface textures. The results show that different textures depict distinct formability behavior in the boundary lubrication regime (lubricant amount 0.1 gr/m²). Exploring the correlation between areal field parameters and formability of cross-die for the current dataset shows that besides surface roughness, autocorrelation length and skewness of height distributions are determining parameters.