Wear and corrosion performance of two different tempers (T6 and T73) of AA7075 aluminium alloy after nitrogen implantation
Introduction
High strength precipitation hardening 7075 (AlMgZnCu) aluminium alloys are widely used in aerospace industry, due to their properties such as high specific strength, ductility, toughness and fatigue resistance [1].
However, the limitations of these alloys due to their susceptibility to different forms of corrosion such as: pitting, intergranular and exfoliation corrosion and stress corrosion cracking (SCC) have promoted a number of studies in recent decades about the effect of intermetallic phases on the onset of corrosion [2], [3], [4]. In the Cu-containing AA7xxx alloys, the intermetallic particles formed during casting and ingot homogenization (mainly Al23CuFe4 and Al2CuMg) may cause strong galvanic coupling with the matrix [4]. However, all aspects involved in this interaction have not been fully clarified yet.
In addition, the corrosion susceptibility of these alloys can be strongly affected by precipitation heat treatments, that govern the type, volume fraction, size and distribution of hardening precipitates [3]. For 7075 alloys the strengthening particles, MgZn2, with size in the nanometer range, precipitate in the matrix during ageing treatments and provide the high strength values of the alloy. Alloys in T6-type tempers have the highest mechanical strengths and good machinability characteristics useful for many engineering applications. These properties are usually achieved after homogenizing the cast 7075 at 450 °C for several hours, and then ageing at 120 °C for 24 h, producing a high density of Guinier Preston zones and η′ metastable phase precipitation, both within grains and along grain boundaries [5]. The size and spacing of precipitates, the formation of precipitate free zone (PFZ) and solute concentration gradients at the grain boundaries are known to play a special role in corrosion resistance of the alloy. The short interparticle distances (approx. 30 nm) found in T6 tempers allows the advance of corrosion making them more prone to intergranular and SCC corrosion [3], [6].
In T73 temper, the two-step ageing treatment (107 °C for 7 h + 163 °C for 27 h) causes a more stable microstructure, increasing the proportion of η′ metastable and the equilibrium precipitate η (MgZn2), and promotes coarsening of grain boundary precipitation. This discontinuous and more spaced distribution at the grain boundary explains the best resistance to SCC and exfoliation corrosion of this temper [3], [7].
Another drawback that limits the engineering application of aluminium and its alloys is low hardness, which results in high wear rates and poor tribological properties [8]. These alloys cannot be used when the wear resistance is a basic requirement. Even components which apparently do not suffer wear such as joints may be subject to vibrations that cause local deterioration or fretting which in many situations is associated to corrosion processes. These phenomena produce the lifetime reduction or premature failure of the alloy.
Surface engineering allow to overcome these drawbacks and ion implantation has proved to be a powerful technique to improve hardness, wear resistance, friction coefficient and corrosion behaviour of many interesting industrial alloys [9], [10].
Most of the published works related to the implantation on aluminium and its alloys are focused in nitrogen implantation and justify the beneficial effect of this element on their tribological properties by the formation of a hard layer of aluminium nitride (AlN) with high wear-resistance in the outermost surface of the alloy [11], [12], [13], [14]. Some of these papers prove the existence of a relationship between increasing hardness and wear resistance and N implanted dose, until it reaches a critical dose [15], where the nitrogen concentration saturates at the value for stoichiometric AlN, and other compounds are then formed.
Likewise, different studies about the effect of N-implantation on corrosion resistance of pure aluminium and its alloys demonstrate an improvement on the pitting behaviour in chloride solutions [10], [16], [17] by increasing the pitting potential [18]. In the same way that for the mechanic properties, a correlation between the implantation conditions and this positive effect on the corrosion properties has been shown [17], [19].
However, little information has been published about the simultaneous effect of nitrogen implantation on tribological properties and corrosion resistance. And to analyze this properties combination, as already indicated, it is essential to consider the heat treatment undergone by the aluminium alloys.
Therefore, in this work the effect of nitrogen implantation on the localized corrosion resistance and tribological properties of the AA7075 under two artificial ageing treatments (T6 and T73) is evaluated. In order to achieve these objectives, the composition and structure of the native oxide films formed spontaneously in air before and after implantation and their resistance to the local breakdown in presence of aggressive ions is also analyzed. For this purpose, characterization techniques as X-ray Photoelectron Spectroscopy (XPS) and Grazing Incidence X-ray Diffraction (GIXRD) have been combined with potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Further EXCO tests have been performed to evaluate the influence of nitrogen on the exfoliation resistance of the considered alloys.
Section snippets
Experimental procedures
Samples of commercial AA7075 aluminium alloys were cut to appropriate dimensions: (1–4 cm2) from a 2 mm thick plate for T6 (wt.% composition: 5.64% Zn, 2.35% Mg, 1.57% Cu, 0.28% Fe, 0.08 Si, 0.02% Mn, 0.19% Cr, 0.027% Ti and Al bal.) and from a 2.5 mm plate in case of T73 (wt.% composition: 6.04% Zn, 2.72% Mg, 1.71% Cu, 0.17% Fe, 0.07 Si, 0.05% Mn, 0.20% Cr, 0.012% Ti and Al bal.). Specimens were automatically grinded with successive grades of SiC papers up to 1200-grade, then polished with
SRIM simulation
The choice of implantation parameters was made with the SRIM computer code, in order to achieve the maximum N percentage at the outermost layer of the alloys. Selecting a target of composition close to that of AA7075 (5% Zn, 2.6% Mg, 0.7% Cu and bal. Al), and varying the implantation energy, the different implantation profiles were obtained. The simulation programme does not consider the sputtering produced on the surface, but this effect is not very important when the saturation dose has not
Conclusions
A study of the effect of nitrogen implantation, at a nominal dose of 2 × 1017 ions/cm2 and an acceleration energy of 50 keV, on AA7075 alloy was accomplished, showing the improvement in hardness and wear properties of the alloy, as well as in its corrosion resistance. However, the results obtained demonstrate that this positive effect is highly dependent on the ageing treatment carried out on the alloy. In this work T6 and T73 artificial ageing treatments have been considered.
Nitrogen implantation
Acknowledgements
The authors gratefully acknowledge the financial support for this work from the Ministry of Education and Science of Spain (MAT 2007-65365) and Ministry of Science and Innovation of Spain (MAT2009-06075-E).
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