Effect of plastic anisotropy and Portevin-Le Chatelier bands on hole-expansion in AA7075 sheets in -T6 and -W tempers

Highlights

• Experimental and numerical results of hole-expansion at room temperature of AA7075-T6 and AA7075-W are reported.

• Hole-expansion formability is enhanced in AA7075-W compared to AA7075-T6.

• For AA7075-T6, plastic anisotropy plays a major role in the deformation including the thickness strain variation and the resulting failure.

• While AA7075-W has similar plastic anisotropy to AA7075-T6, the hole-expansion deformation is differed from AA7075-T6 due to PLC effect.

Abstract

Influence of tempers on the hole-expansion formability of an AA7075 aluminum sheet is investigated for -T6 (as-received) and -W tempers (super saturated by the solution heat treatment followed by water quenching). The hole is prepared by end-milling, to limit the effects of hole preparation on the results and instead highlight the effects of material behavior. A flat-headed punch is used to expand the hole, and digital image correlation captures the strain fields throughout the experiment. The results present that the hole can expand by 70 % more in -W compared to -T6 temper with the lower forming force. In contrast, thickness strain distribution around the hole shows a similar pattern in both tempers except the observation of Portevin-Le Chatelier (PLC) effect in -W temper, which causing the inhomogeneous deformation. In parallel, the numerical simulation of the hole-expansion is performed using a user material subroutine implemented for the elasto-plastic material behavior, including plastic anisotropy, of both tempers. The predictions on the thickness strain variation and average level show good agreement with the experiment for -T6 temper, but less so for -W temper. This is shown to be the effect of PLC bands on the deformation: the material in -T6 temper is mainly governed by plastic anisotropy, but -W temper shows combined effect of plastic anisotropy and PLC bands. Nevertheless, the reasonable predictions of both tempers verify that the numerical framework established in this study can be used for preliminary, computationally-efficient virtual process design with a practical purpose, despite omitting the explicit physics of the PLC effect.

Introduction

As automobile industries strive to increase fuel efficiency due to environmental regulations, aluminum alloys have received high attention as an alternative to steel, for structural components of reduced weight. In particular, 7000 series aluminum alloys are of great interest due to superior mechanical properties such as high strength, corrosion resistance, and high fracture toughness. However, its inferior formability at room temperature (RT) arouses an attention of technical aids to form parts successfully, such as forming at an elevated temperature. For example, Wang et al. (2012) investigated the effect of warm temperatures (e.g., 140−220 °C) on the mechanical properties of AA7075 after forming. Zheng et al. (2019) studied microstructural evolution at elevated temperatures. Merklein et al. (2012) suggested multi-stage forming processes, including an intermediate heat treatment locally subjected in a blank to induce heterogeneous mechanical properties, for AA6016-T4 and observed significant effect of heating temperature and applying location on the cup-drawing formability. Merklein and Geiger (2002) and Kinsey et al. (2000) reported partial heating can be effective to enhance the formability of tailor-welded blank. These temperature-aided forming processes are successfully implemented to improve the formability of high-strength aluminum alloys, but three major issues remain: a) post-forming mechanical properties, b) adhesion of aluminum to the tool surfaces, and c) friction and lubrication at elevated temperatures.

As an alternative, Polmear (1997) referred the forming in -W temper, i.e., super saturated condition by solution heat treatment (SHT) followed by water quenching. de Argandona et al. (2015) commented on the -W forming (The detailed procedure for -W forming is illustrated in Fig. 1, in orders of the SHT, water-quenching, cold forming, natural and artificial aging.) as a promising process which enhances the formability at RT.

Microstructurally, in -W temper, SHT dissolves most of the alloying elements, including pre-existing precipitates, in a solid solution, and water quenching keeps the microstructure temporarily from generating precipitations. This results in the reduced strength and the enhanced ductility compared to the as-received condition, i.e., -T6 temper, as reported by Kumar and Ross (2016) and Senkov et al. (2008). This microstructural characteristics of the material in –W temper promotes the formability of the high strength aluminum such as 6000 and 7000 series even at RT. This is a definite advantage compared to any forming processes requiring heat, and alleviates concerns of poor formability or insufficient tonnage of the press typically associated with RT forming.

Feasibility of -W forming process to industrial applications has been demonstrated for several high strength aluminum alloys. Mendiguren et al. (2016) investigated formability of AA7075-T6 to form a channel shape of automotive part using -W forming and hot stamping processes. The -W forming showed a comparable result to the hot forming process, and the part can be formed without cracks. Lee et al. (2019a, 2019b) evaluated formability of AA7075 in -W temper by Nakajima test and formed a center floor tunnel, an automotive component, successfully. Schuster et al. (2019) analyzed springback and hardness of a U-channel drawn part using -W forming and hot stamping of 7000 series aluminum alloy. Kumar et al. (2017) reported -W forming showed the greatest formability improvement for 7000 series aluminum alloy compared to other heat assisted forming processes. Choi et al. (2020) examined the springback and the Forming Limit Diagram (FLD) of AA7075 in -W and -T6 tempers.

Despite the great advantage of -W temper forming, the process requires sensitive forming time scheduling due to the mechanical properties change with time due to natural aging. Leacock et al. (2013) investigated the change of mechanical properties within 120 min after water quenching and found that the yield stresses linearly increase with respect to time for the natural aging while r-values are relatively consistent. Staley (1974) and Cuniberti et al. (2010) observed that sufficient time of natural aging in the uniaxial tension leads to the strength recovering by almost 80 % of -T6 temper without any additional mechanical, thermal or chemical process, while Shabadi et al. (2012) showed that the property at biaxial stretch is not significantly affected during the natural aging. Clark et al. (2005) also researched the influence of SHT temperatures and quenching media on mechanical properties of AA7075-T6 and found that excellent linear correlation between the hardness and tensile strength. Tanner and Robinson (2004) and Moon et al. (2021) investigated an effect of quenching-rate on the precipitation hardening. Choi et al. (2019) observed the pre-strain limit can exist which could negatively affect on the final strength after artificial aging even at a small pre-strain beyond the uniform elongation.

Certain aluminum alloys in -W temper exhibit serrated, jerky flow, even though the flow in the as-received condition is homogeneous, e.g., AA7075-W vs. -T6. This serrated flow appears as band translation on the specimen surface, which is named as the Portevin-Le Chatelier (PLC) effect. Cottrell (1953) explained this serrated effect is attributed to dynamic interactions between solute and dislocation, so called dynamic strain-aging (DSA). McCormigk (1972) suggested a model for PLC effect in substitutional alloy. This results in an inhomogeneous strain field in the test-section, showing bands of higher strain than their surroundings that translate during deformation.

Many researches have investigated multiple factors related to the appearance of PLC bands such as the strain-rate (Picu et al. (2005) and Pandya et al. (2020)), stress state (Mansouri et al. (2019)), and temperature (Majidi et al. (2016) and Smerd et al. (2005)). In addition, Kang et al. (2006) observed that inhomogeneous deformation can trigger early failure during tensile deformation, and Morgeneyer et al. (2016) investigated tearing based on the 3D in situ measurement of aluminum 2000 series. Choi et al. (2020) studied the influence of early fracture on aluminum 7000 series on other formability aspect.

As an evaluation method of edge-formability, hole-expansion (HE) (ISO 16630, 2009) using a conical punch has been widely used to understand the edge quality on the formability. On the other hand, beyond the formability evaluation, the HE experiment can be a great method to validate the material modeling, including plastic anisotropy. Since 1) larger deformation can be achieved than a standard uniaxial tension due to compatibility with the surrounding material and 2) the material is deformed under various stress states between uniaxial tension, plane-strain tension, and equibiaxial tension, from the hole edge and radially inland. Parmar and Mellor (1978) suggested an analytical model to calculate the stress and strain variation around the hole. Korkolis et al. (2016) and Ha et al. (2019b) validated material models, in particular, anisotropic non-quadratic yield functions. Ha et al. (2020) applied uniaxial straining prior to the HE experiment to investigate anisotropic strain hardening effect. Cohen et al. (2009) and Kuwabara et al. (2011) investigate the role of orthotropic or more general yield function.

In this paper, the formability of AA7075 sheet in -W temper, i.e., AA7075-W, is investigated using the HE experiment, and compared with -T6 temper. Beyond HE, the experiments include the plastic anisotropy characterization of each temper, to calibrate constitutive models involving a non-quadratic anisotropic yield function (Yld2000-2d) and an isotropic hardening model (combined Swift-Voce). In parallel, finite element (FE) simulations are performed and the results are compared with the experiments regarding global (force-displacement curve) and local (thickness strain variation around the hole) metrics. For -W temper, the result is analyzed with an extended discussion to the PLC effect. To the authors’ knowledge, this is the first time to investigate the PLC effect on a spatially-inhomogeneous stress field like HE experiment and analyze regarding formability aspect.