Effect of pre-heat treatment and subsequent ECAP-CU on microstructure and corrosion behavior of 7075 Al alloy fasteners

Abstract

It is of great significance to study the corrosion process of aluminum (Al) alloys fasteners in order to mitigate corrosion for their widespread applications. In this paper, a method for enhancing the corrosion resistance of Al alloy fasteners is proposed. 7075 Al alloy parts with a fine-grained microstructure were prepared by pre-heat treatment (PHT), combined subsequent equal channel angular pressing (ECAP) and cold upsetting (CU). The corrosion behavior of the specimens was investigated by intergranular corrosion and electrochemical test. Microstructure investigations were carried out by field emission scanning electron microscopy, energy dispersive spectrometer and transmission electron microscopy. The relationship between microstructural evolution and corrosion resistance changes was also explored. The results show that both PHT and ECAP-CU significantly improved the corrosion resistance of the samples and modified the corrosion process. The open circuit potential, corrosion current density and corrosion rate of the alloy on electrochemical test were (−0.812±8.854)×10−5 V (vs. SCE), (6.379±0.025)×10−6 A/cm2 and 0.066 mm/year, respectively, and the intergranular corrosion depth was (557±8) μm. The main factor controlling the corrosion behavior was the microstructure evolution. After PHT, the disappearance of the dendritic structure and the dissolution of the nonequilibrium second phase eliminated the potential difference between the phases, reducing the free energy in the as-cast state. When ECAP-CU was used after PHT, the grain refinement was accompanied by a high density of grain boundaries and dislocations, which led to the formation of a denser passivation film on the alloy surface, improving the corrosion resistance in an aggressive environment.

1. Introduction

With increasing environmental pollution and global warming, weight reduction, extended service life and environmental friendliness are the main drivers of development in various fields [1]. The development of lightweight components with high strength and toughness is the main research direction and a lot of effort has been invested in the lightweighting of fasteners for heavy equipment [2, 3]. Due to its low density and high specific strength, 7075 aluminum (Al) alloy has been gradually occupied an important position in industrial manufacturing and is considered to be the most promising material in various engineering fields [4−7]. However, the poor corrosion resistance and mechanical properties of 7075 Al alloy fasteners limit the prospect of their wide application [8, 9]. At present, coating technology has become one of the main ways to improve the corrosion performance of alloys [10, 11]. However, it is difficult for this technology to achieve both high strength and excellent corrosion resistance. Therefore, simultaneously enhancing the mechanical properties and corrosion resistance of 7075 Al alloy fasteners has become an inevitable trend to meet the rapid development of high-end equipment for aerospace, marine engineering and new energy vehicles.

Microstructure modification is one of the main ways to improve the properties of an alloy without changing its own chemical composition. Compared with microstructure modification processes such as friction stir processing (FSP), ultrasonic surface rolling process (USRP) and high-pressure torsion (HPT), equal channel angular pressing (ECAP) has become the preferred processing method for large-scale production of ultrafine and nanocrystalline bulk alloy materials due to its non-pollution, repeatability and densification [12−14]. SHAERI et al [15] performed four passes ECAP at 120 ℃ on 7075 Al alloy to achieve ultimate tensile strength (UTS) and yield strength (YS) of 708 and 672 MPa, respectively and a microhardness of 228HV. LI et al [16] found that after four passes of Bc path ECAP on Al-Zn-Mg-Cu alloy at 200 ℃, the UTS and microhardness reached 488 MPa and 124.2HV, respectively, under fine grain strengthening and dislocation strengthening. JIA et al [17] pointed that after four passes ECAP on Al-5Cu alloy, the UTS and YS reached 506.7 and 473.3 MPa, respectively, and the microhardness rea...