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.

Keywords

aluminum alloy fasteners / heat treatment / equal channel angular pressing / cold upsetting / microstructure / corrosion resistance

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Central South University