Mechanical properties of solution heat treated Al-Zn-Mg-Cu (7075) alloy under different cooling conditions: Analysis with full field measurement and finite element modeling

https://doi.org/10.1016/j.jallcom.2020.158180Get rights and content

Highlights

  • The uniaxial and LDH mechanical properties of W-temper 7075 Al alloys are investigated under three cooling conditions.

  • A full-field DIC technique enables the quantitative investigation on the Portevin–Le Chatelier effect.

  • The spatio-temporal DIC analysis well correlates the fracture of W-temper Al sheets with mechanical responses.

  • The Kocks-Mecking-Estrin-McCormick model can predict the DSA-originated PLC effect in LDH test.

Abstract

The effect of W-temper heat treatment, consisting of solution heat treatment (SHT) and rapid cooling, on the mechanical properties and fracture of high strength aluminum alloy 7075 sheet is investigated experimentally and numerically. Three different cooling conditions from the solution heat treated as-received T6 temper 7075 alloy sheet are considered: water quenching, die quenching, and air cooling. The mechanical properties are measured by uniaxial tensile tests and limiting dome height (LDH) formability tests on the heat treated samples. Moreover, the distributions of strain and strain rate are analyzed by the full-field digital image correlation (DIC) technique, which enables the quantitative investigation on the Portevin–Le Chatelier (PLC) band propagations induced by dynamic strain aging (DSA). The DIC analysis reveals the distinctive formability between uniaxial and biaxial tensile stress states of the W-temper samples. In the finite element analysis, the hardening model based on Kocks-Mecking-Estrin-McCormick (KMEC) model is employed to incorporate the DSA-originated PLC band propagation in the simulation of LDH formability test. The simulated LDHs agree well with experiments when the PLC effect along with planar anisotropy of the heat treated 7075 alloy sheets is properly implemented.

Introduction

Aluminum alloys (AA) 7075 have been applied to the structural parts of aircrafts due to their excellent mechanical properties, toughness and high resistance to fatigue [1]. The major alloying element of AA7075 is zinc and its T6 temper alloy is known to reach around 600 MPa ultimate tensile strength and yield strength of 450 MPa by forming the finely dispersed η or η precipitates [2], [3].

In recent years, the demands for lightweight automotive design with high fuel efficiency have increased to cope with the environmental issues of global warming and CO2 emission etc. As for candidates of the lightweight materials, aluminum alloys have gained more attention than conventional steels because of their low density but high strength attainable by the optimum design of microstructures. The 7000 series aluminum alloys including 7075-T6 are also potential candidate materials for the lightweight auto body, especially for structural parts such as crash boxes and other chassis parts [4], owing to their superior strength and high potentiality of energy absorption and crash performance [5]. However, even with the exceptionally high strength of AA7075-T6, the application of the alloys has been very limited due to the inferior formability at room temperature. For example, the room temperature failure elongation under uniaxial tension has been reported to be less than 11% [6].

The forming and shaping of automotive parts with aluminum alloy sheets have been conventionally conducted using the cold stamping with rigid dies if the sheet metals have a moderate range of strength and formability. For example, the non-heat treatable aluminum alloy 5000 series have been frequently used for manufacturing the exterior part of the lightweight automotive. However, for the 6000 or 7000 series aluminum alloy sheets the inferior formability at room temperature led to the development of forming technologies at warm or high temperatures. Wang et al. [7] investigated the limiting dome height (LDH) formability of AA7075-T6 sheet at warm forming condition and could achieve better formability than that of cold forming of the alloy sheet. Omer et al. [8] optimized process parameters for the hot forming of AA7075, which involved the die quenching and subsequent cycles of heat treatment. The paint bake cycle (or age hardening) could demonstrate the strong potentiality of AA7075 for the application to complicated automotive components such as B-pillar [9]. Though the warm or hot forming of high strength aluminum alloys in the sheet metal forming process can be an alternative to the cold stamping by taking advantage of significantly enhanced formability at elevated temperature, more studies should be followed to overcome the technical hurdles such as increased cost for tool management, temperature control, degraded mechanical properties after forming at a warm temperature and so on [10], [11].

As an alternative to the warm or hot forming of aluminum alloys, especially AA7075-T6 sheet, the cold forming technology with prior heat treatments has been investigated [12], [13], [14]. This special forming is named as ‘W-temper forming’ because the prior heat treatment commonly involves the solution heat treatment (SHT) followed by rapid quenching. Then, the W-temper state of sheet metal has significantly lowered strength but enhanced elongation before fracture, which enables the forming at room temperature [15], [16]. The advantage of the W-temper forming is obvious because the conventional cold stamping tool design can be utilized without additional cost of high temperature forming.

In the W-temper process, 7075 aluminum alloy is solution heat-treated (SHT) at 450–550 °C which is between the solidus temperature and solvus temperature. The SHT facilitates the mobility of solute atoms and increases the density of vacancies [17]. Then, this supersaturated solid solution (SSSS) is subsequently quenched to preserve the state of solid solution. In the W-temper forming, the stamping is performed after the SHT and quenching within an optimum time duration. After the cold forming of the W-temper sheet, additional heat treatments consisting of natural aging and artificial aging are performed to nucleate precipitates for achieving desired final strength of parts. The bake hardening process is commonly utilized for automotive parts as the post-heat treatment process [18], [19].

It is known that the mechanical properties of the W-temper aluminum sheets are dependent on the cooling method [20]. The most commonly used cooling method is the water quenching due to its very rapid cooling speed. However, water quenching introduces a very high level of residual stress and excessive distortions of sheet parts [21], [22]. On the other hand, slower air cooling has favorable energy consumption and ease of manufacturing control. But, the slow cooling rate such as air cooling allows precipitation of coarse heterogeneous MgZn2 second phase that is detrimental to the evolution of η’ precipitates as the main strengthening mechanism [23]. The die quenching, as accompanied with intermediate cooling speed between water and air cooling, has been investigated as an optimized method for forming specific parts because it enables cooling while in operation [24].

Another characteristic feature of the mechanical properties of the W-temper 7075 aluminum alloys is the dynamic strain aging (DSA) and its resultant serrated plastic flow or Portevin–Le Chatelier (PLC) effect [25]. The PLC effect originated from interactions between mobile dislocations and solute atoms in SSSS state is often observed in the form of localization band propagation through sample gauge during uniaxial tension [26], [27]. It is also known to reduce surface quality by producing surface marks [28] and to lower the ductility of materials [29]. Cai et al. [30] and Ait-Amokhtar et al. [31] studied the dynamics of the PLC bands using infrared thermography and digital image correlation (DIC) method. Moreover, Benallal et al. [32] and Rizzi and Hähner [33] provided numerical modeling of the PLC effect to theoretically explain the origin of PLC band, which successfully reproduced the PLC band characteristics in the experimental studies on shear [34], tensile [35] and biaxial tension test [36]. Nevertheless, most of the previous studies focused on the PLC effects of Al-Mg solid solution strengthening alloys and Mn alloyed steels. In the numerical aspect, the modeling and simulations could reproduce the serrated flow behavior but there are limited studies on the correlation between mechanical properties (or formability) and the kinetics of PLC bands.

In this study, the effects of the W-temper process for the aluminum alloy 7075 sheets on the mechanical properties and formability are investigated experimentally and numerically. Especially, three cooling methods, i.e., water quenching, die quenching, and air cooling, are introduced to study the effect of the cooling method during the W-temper process on the mechanical properties. In the experimental aspect, the full-field digital image correlation (DIC) analysis is highlighted as a tool for quantitative investigation of the PLC band propagation along with the distributions of strain and strain rate under uniaxial tension and biaxial tension loading. In the numerical aspect, the plastic hardening law based on Kocks-Mecking-Estrin-McCormick (KMEC) model [37] that incorporated the PLC effect is applied to the finite element simulation of limiting dome height (LDH) formability. The validation for the importance of modeling procedure by incorporating the PLC effect is presented by comparing it with the results of formability predicted by the conventional isotropic hardening model.

Section snippets

Material and heat treatment conditions

A heat treatable aluminum alloy 7075 sheet in T6 temper condition (hereafter, it is denoted as AA7075-T6) is investigated in this study. The chemical compositions of the alloy are given in Table 1. The alloy is featured with 5.1% (in wt%) zinc. The specimens were prepared from a commercially rolled sheet and the thickness was 1 mm.

Three different heat treatment conditions are investigated to study the effect of cooling rate on the mechanical properties, microstructure, and formability in

Flow stress behavior

For the finite element (FE) simulations with elastic-plastic constitutive laws, an appropriate hardening model is critical for modeling large deformation of sheet metals, particularly for the accurate prediction of sheet formability. Therefore, the flow stress-strain curves measured by the tensile tests have been often fitted to either the Swift or Voce hardening law. The Swift law represents the power law type strain (or work) hardening as follows.σS=Ke0+εpnwhere e0, K, and n are material

Mechanical properties

Fig. 5 shows the uniaxial stress-strain curves of AA7075 sheets under different heat treatment conditions along rolling direction. For comparison purpose, the flow curve of the as-received sheet, AA7075-T6 is also presented. It is shown that all heat treated sheets represent markedly lowered yield stress and tensile strength compared to the as-received T6 sheet. Moreover, the fracture strain is also significantly improved by the heat treatments. Another noticeable characteristic of the heat

Effect of DSA behavior on the mechanical properties

The tensile tests on the heat treated samples with different cooling conditions, and the as-received T6 sheets showed that the investigated AA7075 sheets represented significantly serrated plastic flows or PLC band initiation/propagation through the specimen gauge length. The serrated flow curves are known to originate from the DSA phenomenon when the samples are solution heat treated followed by rapid cooling. This observation is consistent with previous studies on the effect of interactions

Conclusions

In this study, the effects of solution heat treatment (SHT) and subsequent cooling of high strength aluminum alloy 7075 (AA7075) sheet on microstructure, mechanical properties, and formability were investigated both experimentally and numerically. The investigated heat treatment process based on SHT and cooling is intended to the potential application to W-temper sheet forming, in which the investigated heat treatment is applied prior to (common) cold forming in order to improve the limited

CRediT authorship contribution statement

Chanmi Moon: Methodology, Investigation, Validation and writing. Jinwoo Lee: Methodology, Investigation. Myoung-Gyu Lee: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Sandrine Thuillier: Software, Writing - review & editing.

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.

Acknowledgement

Authors appreciate the supports from Korea Institute of Advancement of Technology (KIAT) (Project No. N0002598) and National Research Foundation (NRF) of Korea (Grant No. 2019R1A5A6099595 (ERC) and 2020R1A2B5B01097417). Dr. Shuaifeng Chen, Mr. Chanyang Kim and Hongjin Choi are acknowledged from their helps for dislocation theory, DIC and FEM analyses, respectively. Chanyang Kim is appreciated for the helps for experiments, which were suported from KIAT (No. 0002019).

Data availability

The raw/processed data

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