On twist springback in advanced high-strength steels

Abstract

The sheet metal components made of advanced high-strength steel (AHSS) become fairly attractive in reducing weight and enhancing operational performance of products. However the corresponding forming process often generates more severe springback. This paper aims to investigate the behavior of twist springback in advanced high strength sheet components, where a twist rail was considered and the corresponding die and measurement tool were developed. Finite element model of the twist rail was first validated through a try-out test and then is used to carry out a parametric study on the twist springback. The results lead to an in-depth understanding of how the design and process parameters, such as transition ratio of cross-section width, corner angle, drawbead depth and material strength grade, affect the twist springback, thereby providing some insights into sensitivity analysis for the design of products and corresponding processes.

Introduction

Energy has been one of the main concerns in the modern society and is driving numerous new technological developments. Taking automotive industry as an example, reduction in body weight of passenger car has been more exhaustively required nowadays than ever. Although the applications of aluminium and magnesium alloy sheets have been fairly attractive for the weight reduction in automobile parts [1], the corresponding higher cost and inferior formability indeed arise some major concerns, making the automobile industry still hold main interest in steel parts.

As one of the effective ways to reduce the weight of vehicles and improve the crash energy absorption, automobile companies are choosing to replace conventional mild steel parts with advanced high-strength steels (AHSS) such as dual-phase (DP), twinning induced plasticity (TWIP), and transformation induced plasticity (TRIP) steels [2]. However, the corresponding springback induced in the forming process has also been a major problem for such high-strength steels [3], [4].

Springback is a common phenomenon in sheet metal forming which is mainly caused by the elastic recovery and redistribution of internal stress during unloading process [5]. As a result, it leads to a certain discrepancy from the desired final shape and may not well satisfy the design specifications. Generally speaking, springback depends on many engineering factors, such as material properties, thickness, lubrication conditions, tool’s geometry and process parameters [6], [7], [8], [9], [10]. The challenge lies in that the behaviors and design principles established in conventional mild steels may not be applicable for such high-strength steels [11], [12], which often leads to a costly and time-consuming trial and error process to meet the design requirements.

Over the years, substantial studies have been conducted on understanding, prediction and control of springback. For example, Santos and Teixeira [12] proposed an experimental benchmark for sheet metal forming with different materials, which attempted to provide a guide to numerical simulation. An elastic–plastic incremental finite element simulation was used by Math and Grizelj [13] to predict the springback and the residual stress in the bent plates used for assembling spherical tank, and the results were validated through experimental data. Li et al. [14] found that the material hardening model and elastic modulus directly affect the springback simulation in a standard V-bending process. Gomes et al. [8] investigated the variation of springback in the U-bending process subjected to high strength anisotropic steels, numerically and experimentally. Xu et al. [7] studied the effects of the damping factor, integration point, blank mesh size and punch velocity on the accuracy and efficiency of springback simulation in the U-bending process. Mullan [6] also studied, experimentally, the influence of material and loading factors on springback and derived a regression model to predict the springback from numerical simulations. Chen et al. [11] explored, experimentally, the effects of various high-strength steels and process parameters on the springback variations for an open channel part. Chou and Hung [15] utilized FEM to analyze various springback reduction techniques in the U channel bending, such as arc bottoming, over bending, die punching, spanking, stretching, movement (double bend) and attempted to establish the relationship between these techniques and forming parameters.

Springback is an essential factor accounting for the design of tools used in high strength sheet metal forming process. However, how to assess springback precisely has been rather challenging [16], [17]. As pointed out by Yu [18] and Shen et al. [19], springback simulation analysis for advanced high-strength steel needs more attention due to more severe microstructural evolution during cold forming. In practice, angle change, curl and twist are several typical results of springback. There have been already considerable studies on the angle change and curl springback characteristics [20], [21], [22]. However, there is lack of research in the twist springback of advanced high-strength steel to our best knowledge. In order to understand and characterize the behavior of twist springback in an AHSS sheet forming, a twist rail was considered and the corresponding die and measurement tools were made in this study. The finite element model of the twist rail is validated through the experiments first and then is used to carry out a parametric study for better understanding of the twist springback. This paper will explore the effects of workpiece design and forming process parameters such as width transition ratio, corner angle, process drawbead depth and material strength grade on twist springback.