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In the current work, the effects of initial deformation temperature and strain rate on flow behavior and microstructure of boron-alloyed steel after non-isothermal deformation were investigated. Based on the stress-strain curve obtained in the non-isothermal tensile experiment with a Gleeble 3500 system, finite element software was used to carry out the thermal mechanical coupled simulation of hot stamping process, which was further validated by hot stamping experiment of a V-channel part. The results indicate that the forming force was higher than that of the isothermal deformation. It was also observed that the initial stamping temperature should be above 750 °C, and the holding and quenching time should be up to 15 s in hot stamping process according to finite element (FE) simulation. Finally, the results obtained from hot stamping experiments are in good consistency with the simulation; thus, it reveals that the FE modeling of non-isothermal hot stamping process is reliable. [ABSTRACT FROM AUTHOR]

Abstract: Due to its excellent strength and formability combinations, dual phase (DP) steels offer the potential to improve the vehicle crashworthiness performance without increasing car body weight and have been increasingly used in new vehicles. However, a new type of crack mode termed as shear fracture is accompanied with the stamping application of these high strength DP steel sheets. This paper presents a study of DP shear fracture through macroscopic identification and micro-level metallographical observation. By the cup drawing experiment to identify the limit drawing ratio (LDR) of three DP AHSS with strength level from 600MPa to 1000MPa, the study compared and categorized the macroscopic failure mode of these three types of materials. The metallographical observation using scanning electron microscopy (SEM) along the direction of crack was conducted for the DP steels to discover the micro-level propagation mechanism of the fracture and its relation with the martensite volume and distribution. Plasticity analysis and finite element method (FEM) simulation were provided to explain the observed behavior, which was further confirmed by the comparison between DP fracture and high strength low alloy (HSLA) fracture. [Copyright &y& Elsevier]

Abstract: FE-simulation and optimization are widely used in the stamping process to improve design quality and shorten development cycle. However, the current simulation and optimization may lead to non-robust results due to not considering the variation of material and process parameters. In this study, a novel stochastic analysis and robust optimization approach is proposed to improve the stamping robustness, where the uncertainties are involved to reflect manufacturing reality. A meta-model based stochastic analysis method is developed, where FE-simulation, uniform design and response surface methodology (RSM) are used to construct meta-model, based on which Monte-Carlo simulation is performed to predict the influence of input parameters variation on the final product quality. By applying the stochastic analysis, uniform design and RSM, the mean and the standard deviation (SD) of product quality are calculated as functions of the controllable process parameters. The robust optimization model composed of mean and SD is constructed and solved, the result of which is compared with the deterministic one to show its advantages. It is demonstrated that the product quality variations are reduced significantly, and quality targets (reject rate) are achieved under the robust optimal solution. The developed approach offers rapid and reliable results for engineers to deal with potential stamping problems during the early phase of product and tooling design, saving more time and resources. [Copyright &y& Elsevier]

Resistance spot welding (RSW) with inserted strips, a recent variant of traditional RSW, was usually adopted in joining thin gage steels to lower the temperature developed at the electrode surface and to extend electrode life. In order to understand the influencing mechanism how the inserted strips affect the heat transfer behavior and to optimize the selection of suitable strips, an approach integrated with FEM and response surface methodology (RSM) was employed. FEM results showed that the inserted strips would not only lead to earlier initiation of weld and bigger weld size in both diameter and thickness but also lower the electrode surface temperature. Based on FEM, uniform design and RSM were further employed to build a regression model between the strip properties (i.e., electrical/thermal conductivity, thickness) and the responses (i.e., electrode tip temperature, weld diameter, and temperature at strip/sheet interface). A graphical optimization was conducted to identify a preferable strip, and a Cu55Ni45 strip with a thickness of 0.12 mm was recommended for a 0.4 mm steel sheet. [ABSTRACT FROM AUTHOR]