Microstructure, corrosion behaviour and thermal stability of AA 7150 after ultrasonic shot peening

Highlights

• Localized corrosion resistance of AA7150 enhanced after ultrasonic shot peening.

• Thermal stability of peened AA7150 surface layer was studied by TEM.

• More coarsened precipitates re-precipitate on the peened surface layer during natural ageing.

Abstract

Microstructure, corrosion behaviour and thermal stability of ultrasonic shot peened AA7150 were studied in this work. After ultrasonic shot peening, exfoliation susceptibility decreases, intergranular corrosion is inhibited for both normal and transverse planes, pitting potential shifts to the positive direction. The enhanced localized corrosion resistance is attributed to the formation of equiaxed nano-grains on surface layer. Pre-existing ageing-induced η′ and η precipitates dissolved immediately after shot peening, however, after 18 months' natural ageing, nanograined precipitates with more coarsened size than pre-existing ones, re-precipitate on the peened surface layer and leads to surface softening.

Introduction

In the last several decades, surface severe plastic deformation (S²PD) has been used to modify alloys' surface layer for the purpose of improving surface-related properties like surface hardness, corrosion and wear [[1], [2], [3], [4]]. S²PD is one technique that can induce severe plastic deformation on the surface of alloys, resulting in grain size refinement, surface hardening, high level of compressive residual stress and introduction of high density of defects such as dislocation and vacancy [[5], [6], [7], [8], [9], [10], [11]]. The family of S²PD consists of air blast shot peening [12], ultrasonic shot peening (USSP) [13,14], ultrasonic impact peening [15,16], laser shock peening [17], sand blasting [18], ball burnishing [19], friction stir [20] and so on.

Corrosion performance of alloys subjected to surface mechanical treatments has been extensively studied, but most of the literature has been focused on electrochemical corrosion behaviours [7,19,[21], [22], [23], [24], [25]]. It has been argued that one should pay more attention on localized corrosion behaviours instead of on uniform corrosion performances which derived from electrochemical characterization methods such as open circuit potential, electrochemical impedance spectra and polarization [26,27]. This is because corrosion is alloy-dependent and environment-dependent and in some cases electrochemical results can be misleading and localized corrosion is more important and more frequent for some alloy systems [26,27]. For instance, one of localized corrosion forms, exfoliation corrosion, has been shown to deteriorate significantly the fatigue life of alloys [28], but can't be precisely reflected by electrochemical methods. It's well recognized that the studied 7000 series Al alloys in this paper are sensitive to exfoliation corrosion. Actually, among the various corrosion phenomena (pitting corrosion, inter-granular corrosion, stress-corrosion cracking), exfoliation corrosion is particularly relevant to plates presenting grains of large aspect ratio [29]. Surface mechanical treatments can alter the surface texture and change aspect ratio dramatically, thus it is worthy to investigate the exfoliation performance of S²PD treated 7000 series Al alloys.

Though the hardness, wear, fatigue and corrosion behaviours can be greatly improved by grain refinement, the introduced high density of defects such as dislocations and grain boundaries provides a strong driving force for grain coarsening, accompanied by property degradation. Therefore, thermal stability of nanograined metals is a great concern for their applications. It was reported that below a critical size (70 nm), nanometer-sized grains in pure copper and nickel produced by SMGT (surface mechanical grinding treatment) exhibit notable thermal stability [30]. Nanograined structure in pure copper and pure nickel stays stable at 250 and 600 °C, respectively. For shot peened SS316L, grain size does not show an obvious growth during post annealing at 600 °C [31]. Compared with Fe, Cu, Ni and Ti alloy systems, due to the low melting temperature of Al alloys, thermal stability property might be unique for Al alloys subjected to severe plastic deformation. For example, Al-Cu supersaturated solid solution can be further hardened under room temperature through the precipitation of nanograined AlCu₂, which termed as natural ageing treatment. Mazilkin et.al found that Al-Zn and Al-Mg supersaturated solid solutions decompose after high pressure torsion (HPT) [32,33]. The Hall–Petch hardening due to grain refinement competes with softening effect caused by the decomposition of a supersaturated solid solution, leading to a net softening for the Al-Zn and Al-Mg binary alloys [32,33]. 7000 series Al alloys are typical alloys that can be precipitation hardened by the introduction of η′ and η precipitates during artificial ageing at the temperature of 120–200 °C. After USSP and grinding, pre-existing ageing-induced η′ and η precipitates of 7000 series alloys are found dissolving into Al matrix and results in a supersaturated solid solution, due to the fact that severe plastic deformation can extend solid solubility of alloying elements in the Al matrix. The phenomenon has been reported on the AA 7150 [34] and AA 7055 [35] subjected to grinding and AA 7075 [36] subjected to USSP. Supersaturated solid solution together with the high stored energy caused by USSP treatment, are expected to lead to an unsatisfactory thermal stability. In this work, the post-ageing induced decomposition of USSP treated AA7150 supersaturated solid solution and its effect on surface hardness will be investigated.