Full length articleDesign optimization and application of hot-stamped B pillar with local patchwork blanks
Introduction
With the increasing demands of environmental protection, energy saving and emission reduction in modern automotive industry, it has also become more and more imperative to reduce the weight of car bodies without compromising crashworthiness performance [1]. To achieve this goal, except employing lightweight materials [2] and structural optimization [3], developing new lightweight manufacturing technologies is also a very important approach. For example, hot stamping of high strength steel is an advanced lightweight manufacturing technology to produce structural parts with a tensile strength up to 1500 MPa or higher, which allows the weight of car parts to be reduced by using thinner gauge sheet metals. And there are a lot of studies about hot stamping technology [4], [5], [6], [7], [8].
In recent years, components with uniform material or thickness, including hot-stamped parts, cannot meet the emission standards and regulations on vehicle crash safety that are gradually becoming stricter [9], [10]. In this context, some advanced tailored blank technologies [11], such as tailor rolled blank and patchwork blank, have been developed to solve this problem. Tailored blank only reinforces the areas where a higher strength is necessary by locally increasing strength or thickness, therefore, it has greater potential of improving stiffness and crashworthiness of car body, while reducing weight at the same time than uniform material or thickness. Tailor rolled blank is a sheet product with a thickness variation manufactured via a strip rolling process, and varying thicknesses is only limited to the longitudinal direction [12], latitudinal direction [13] or both of them [14]. And 1 mm thickness difference over a length of 100 mm with current technology [15]. However, patchwork blank is a mainsheet partially reinforced by one or more small blanks with any shapes by using resistance spot welding, laser welding or adhesive bonding [16], [17], [18], [19], therefore, in contrast to tailor rolled blank, it has a higher flexibility in position and geometric shape, and is more lighter and more applicable to the parts with complex shapes. In addition, combining hot stamping technology with patchwork blanks is expected to make the car body lighter and stronger, and some studies about hot stamping of patchwork blanks have been published [20], [21], [22], [23], [24].
Patchwork blanks were first used in hot-stamped parts in 2007 [25]. Since then, the application has increased rapidly. B pillar, which is a major load bearing component for protecting passengers during a side impact accident, is also being manufactured by hot stamping of patchwork blanks. Ford used a hot-stamped B pillar with patchwork blank in the body structure of 2011 Explorer Vehicle [26]. Lei et al. [27] predicted hot stamping process of B pillar with patchwork blank based on a developed FE model and obtained suitable process parameters. Ahmad and Zakaria [28] investigated the effect of spot weld locations on formability of B pillar with patchwork blank design during hot stamping and optimized the number of spot welds via numerical simulation. Xu et al. [29] performed a series of static crash tests and studied the effect of thickness on static performance of hot-stamped B pillar with patchwork blank. In above studies, the production cost of B pillar may be reduced because there is no necessary to make extra forming tool for reinforcement, but the total weight of B pillar is almost unchanged in contrast to conventional B pillar with reinforcement because the position and shape of patchwork blank are almost same as those of commonly used reinforcement. Therefore, there exists an urgent need to address the optimal position and shape of patchwork blank.
Structural optimization is a very important tool in automobile design and there are also many works about design optimization of car body parts. For example, Kohar et al. [30] optimized two kinds of extrudable aluminum front rails under axial crushing by using response surface methodology, artificial neural networks metamodel and simulated annealing optimization technique. Qi et al. [31] proposed a type of double-hat thin-walled bumper beam comprised of aluminum-steel hybrid materials, investigated its bending behavior numerically, and carried out a multi-objective optimization design to determine the optimal thickness values of upper and lower hats, and the height of upper hat. Yu et al. [32] carried out the energy-absorbing analysis and reliability-based multi-objective optimization design for a functionally graded thickness B pillar based on a simplified numerical model and provided some guidance for practical engineering. Sun et al. [33] proposed an optimization method for structures made of tailored rolled blank and optimized thickness variations of a top-hat column and a bumper beam with thickness varying along axial and circumferential directions. Xu et al. [34] performed an optimization design for a tailored welded blank structure with top-hat section subjected to front impact by using Taguchi method and optimized the material grades and thicknesses. As we know, crash performance of a hot-stamped part with patchwork blanks is closely related to the position and shape of patchwork blanks while the material and thickness are determined. To our best knowledge, there was little research about the optimization design of patchwork blanks of hot-stamped parts. Therefore, this study aims to determine the optimal patchwork blanks of hot-stamped parts by applying an optimization method.
In the present work, the established local patchwork hot stamping technology using laser welding was used and a hot stamping experiment of L-type part with local patchwork blank was performed to ensure the technical feasibility. Based on the experimental results, a heuristic topology optimization method was proposed, and then it was used to optimize local patchwork blanks of a top-hat channel and a B pillar. FE models used in the optimizations were established based on the deformation condition of B pillar during full vehicle side impact and validated through experiments. The effects of local patchwork blank thickness, material strength, and outer thickness on the optimal position and shape of local patchwork blanks and lightweight efficiency were analyzed. Finally, a hot-stamped B pillar with local patchwork blanks was applied to numerical simulation of a full vehicle side impact, and the crash performance was compared with that of conventional B pillars with and without reinforcement.
Section snippets
Problem description
A typical B pillar structure made of steel in a vehicle body-in-white is shown in Fig. 1(a). The B pillar was mounted sideway between front door and rear door, and was welded at the bottom to side sill panel and floor pan, and on the top to roof rail. The position of B-pillar makes it play an essential role in crashworthiness to improve impact resistance and safety to vehicle occupants. It mainly consists of the following three parts: outer (upper and lower), inner and reinforcement (See Fig. 1
Local patchwork hot stamping technology and feasibility verification
The established local patchwork hot stamping was used to produce hot-stamped parts with local patchwork blanks and the process is shown in Fig. 2. A laser welding stage was added to join main blank and local patchwork blanks prior to the usual hot stamping process. The local means that only the areas where a higher stiffness or strength is necessary, such as the corners, are reinforced by using patchwork blanks. Therefore, the local patchwork blank usually has a relatively small size. Main
Optimization method
As mentioned in Section 2, the goal in this study is to address the optimal position and shape of local patchwork blanks to obtain maximum lightweight efficiency while maintaining crash performance. It is difficult to model the laser welding beads between main blank and local patchwork blank in numerical simulation. Therefore, this work considers only one case in which both main blank and local patchwork blank are made of the same material. Based on the conclusion in Section 3, it can be
FE modeling
To investigate the representative cross section of automotive B pillar, a top-hat channel was established by extruding the cross section in Fig. 1(c). As we know, B pillar is always subjected to a bending load during full vehicle side impact. In this study, four-point bending load was employed to evaluate the crashworthiness of the top-hat channel (See Fig. 6). The length of top-hat channel is 1000 mm, the distance between two supports is 800 mm, and the distance between two loading pins is
Determination of FE modeling
To encourage car manufactures to improve occupant safety in side impact, the Insurance Institute for Highway Safety (IIHS) has introduced a side crash test [39]. In the side crash test, a stationary target vehicle is impacted by a moving deformable barrier (MDB) at the speed of 50 km/h (See Fig. 15(a)). Fig. 15(b) shows the view of a vehicle after the IIHS side crash with doors removed. It can be observed that the B pillar with top and bottom constrained was hit by the MDB at the middle-lower
Application of hot-stamped B pillar with local patchwork blanks
In this section, the validity of hot-stamped B pillar with local patchwork blanks was tested using a full vehicle FE model for IIHS side impact, which is shown in Fig. 23. The full vehicle FE model was developed independently, and the MDB model was referred from Livermore Software Technology Corporation, which is globally recognized. As shown in Fig. 24, three kinds of B pillar structures were integrated into the full vehicle FE model respectively, and three numerical simulations of side impact
Conclusions
This study dealt with the design optimization and application of hot-stamped B pillar with local patchwork blanks. Local patchwork hot stamping technology using laser welding was used and its technical feasibility was verified in an experimental method. Then an optimization method for the design of local patchwork blanks was proposed, and it was used for the optimizations of a top-hat channel and a B pillar as examples. A hot-stamped B pillar with local patchwork blanks was applied to numerical
CRediT authorship contribution statement
Dongyong Shi: Conceptualization, Methodology, Software, Writing – original draft, Writing – review & editing. Kenichi Watanabe: Supervision, Project administration, Data curation. Junya Naito: Investigation, Resources, Validation. Kensuke Funada: Investigation, Visualization. Kazuya Yasui: Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge high-performance support in experiments and numerical simulations and valuable advices from our colleagues. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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