Characterisation and modelling of in-plane springback in a commercially pure titanium (CP-Ti)

S Khayatzadeh,M J Thomas,S Rahimi,Y Millet

doi:10.1007/s10853-017-1983-8

Abstract

Effective prediction of springback during sheet metal forming is critically important for automotive and aerospace industries, especially when forming metals with high strength-to-weight ratio such as titanium. This requires materials mechanical data during plastic deformation and their dependencies on parameters like strain, strain rate and sample orientation. In this study, springback is quantified experimentally as elastic strain recovery, degradation in Young’s modulus and inelastic strain recovery on unloading in a commercially pure titanium type 50A (CP-Ti-50A). The results show strain rate-dependent anisotropic mechanical behaviours and a degradation in Young’s modulus with increased level of plastic deformation. The level of degradation in Young’s modules increases gradually from 13% for samples parallel to the rolling direction (RD) to 20% for those perpendicular to the RD. A measurable nonlinear strain recovery was also observed on unloading that is orientation dependent. The level of springback is characterised as the sum of elastic recovery and the contributions from both the degradation in Young’s modulus and anelastic strain recovery. It is shown that the Chord modulus can estimate springback with a reasonable accuracy taking into consideration the elastic strain recovery, degradation in Young’s modulus and anelastic strain recovery.

Highlights

Springback is a strain recovery in a formed product following a forming process [1] that often causes deviation from a desired geometry
The microstructure of the as-received CP-Ti sheet in the plane containing the normal (ND) and rolling directions (RD) and normal to the transverse direction (TD) is given in Fig. 1a and b, respectively, with IPF colouring with respect to the TD
The Electron backscatter diffraction (EBSD) data show that the CP-Ti material has a twin-free microstructure with equiaxed grains in both ND–RD and ND–TD planes with a similar average grain size of 5.6 ± 3.5 lm (Fig. 1d) and a typical rolled a-Ti texture

Summary

Introduction

Springback is a strain recovery in a formed product following a forming process [1] that often causes deviation from a desired geometry. In addition to the geometrical non-conformance, springback changes the status of strain and stress in a deformed material following the tool’s (e.g. punch) removal [2,3,4]. Springback is one of the major engineering problems associated with parts manufacturing, especially in sheet metal forming processes due to the small thickness of the sheet material. This is even more pronounced for high-strength metals such as titanium and its alloys that are used predominantly in aerospace industry, due to their high strength-toweight ratio [5]. An extensive body of the literature exists on studying springback in different materials, where the origin of springback is linked to the changes in materials mechanical properties and responses during plastic deformation [1, 6,7,8,9,10,11,12]

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