Effect of the thickness reduction of specimens on the limit strains in thermomechanical tensile tests for hot-stamping studies

Connor Lane,Kailun Zheng,Zhutao Shao,Jianguo Lin

doi:10.1051/mfreview/2018009

Abstract

Sheet metal formability under hot stamping conditions has been evaluated using a novel planar testing system developed previously, being used within a Gleeble machine. Nevertheless, the specimen design with the central recess was not standardised, and the thickness reduction was not applied to the dog-bone type of specimen for testing at the uniaxial straining state. In this paper, effect of thickness reduction of dog-bone specimens on limit strain measurement under hot stamping conditions is investigated, and two types of dog-bone specimens without and with central recess are presented. Thermomechanical uniaxial tensile tests were performed at various deformation temperatures and strain rates, ranging from 370–510 °C and 0.01–1/s, respectively, by using the developed biaxial testing system in the Gleeble. The distributions of temperature and axial strain along gauge region of the two types of specimen were measured and compared. The specimen with consistent thickness had a better uniformity of temperature and strain distributions, compared to that with thickenss reduction. Forming limits for both types of specimen were also determined using the section-based international standard method. It is found that the accuracy of the calculation of forming limits based on the use of specimen with thickness reduction was highly dependent on the selection of the stage of the deformation of the specimen.

Highlights

The increasing demand for fuel-efficiency is a major challenge facing by the automotive industry in order to reduce energy consumption and air pollution [1]
The basic principles are: three cross-sections are positioned perpendicular to the crack to obtain strain data along the gauge region, and an inverse parabola is fitted on the bell-shaped curves for the determination of major and minor strains
The peak points of the fitted parabolas are used as the limit strains. This international standard has been commonly used for formability tests at isothermal conditions but no standard exists for the determination of limit strains under hot stamping conditions

Summary

Introduction

The increasing demand for fuel-efficiency is a major challenge facing by the automotive industry in order to reduce energy consumption and air pollution [1]. The development of lightweighting strategies for vehicles’ weight reduction is a feasible way to improve the energy efficiency and decrease CO2 emissions. Lightweight materials, such as aluminium alloy, magnesium alloy and composites, are increasingly used to replace steels for manufacturing vehicles’ body structures [2,3]. The applications of high-strength aluminium alloys are restricted by the poor formability at room temperature since panel components with complex geometries cannot be formed [4] This makes the evaluation and the improvement of alloys formability become necessary. Post heat treatment is usually required after forming to further increase component strength, which may result in low dimensional accuracy and increase manufacturing costs.

Objectives

Methods

Results

Conclusion