Investigations into Hot Form Quench Conditions on Microstructure Evolution and Bake-Hardening Response for High-Strength
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JMEPEG https://doi.org/10.1007/s11665-020-05245-3
Investigations into Hot Form Quench Conditions on Microstructure Evolution and Bake-Hardening Response for High-Strength Aluminum Alloy Y.F. Jiang and H. Ding Submitted: 6 July 2020 / Revised: 22 September 2020 / Accepted: 27 September 2020 Hot Form Quench (HFQ) is an advanced technology for forming complex-shaped parts in automobile industry. In the present work, the effects of HFQ conditions on microstructure evolution and bakehardening (BH) response of AA7075 were investigated by means of transmission electron microscopy (TEM), x-ray diffraction, electron back-scattered diffraction and microhardness tests. The results show that, with pre-strain increasing, more g and g¢ precipitates appear. The size of g¢ changes little but g becomes larger. The reason for the high volume fraction of precipitates is the high-density dislocations which can act as the effective nucleation sites. Meanwhile, it is revealed that the effect of precipitation strengthening and dislocation strengthening has been enhanced gradually. Nevertheless, the solid-solution strengthening effect is weakened and the effect of grain boundary strengthening changes slightly. Compared with water quenching condition, more large-sized g phase appears easily in air-cooling condition, which was recognized as the reason for the decrease in hardness. As evidenced in the analysis of HRTEM and TEM, no precipitates existed in the sample with solid-solution state and a large number of Guinier– Preston (GP) zones were found in 10% pre-strain + pre-aging sample, indicating pre-aging enables to promote GP zone nucleation, which may bring an improvement in BH response. Keywords
AA7075, bake-hardening response, hardness, HFQ conditions, microstructure, precipitation strengthening
1. Introduction The rapid development of automotive industry has brought about two major problems recently, i.e., environmental pollution and energy shortage. ‘‘Automotive lightweight’’ has been put forward as an important subject, and the target is to replace some conventional steel parts using lightweight materials, such as aluminum alloy and magnesium alloy (Ref 1-3); 7000 series aluminum alloy is highly regarded for its high strength and excellent stress corrosion cracking (SCC) resistance. The ultimate tensile strength for AA7075 can reach approximately 574 MPa under T6 condition, and it has a low value of SCC indices (ISCC = 0.01) in a 3.5% NaCl solution after T73 heat treatment (Ref 4). However, some complex-shaped structural parts are hard to be formed at room temperature due to the low ductility of the material (Ref 5). Hot forming quench (HFQ) process is an effective method to solve the above problem. The novel hot forming (HF) technology was first proposed by Lin (Ref 6), and the schedule consists of three parts: (1) carrying out solution heat treatment (SHT); (2) transferring the blank to Y.F. Jiang, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, PeopleÕs Republic of China; and Key Labo
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