Grégory Odemer

Influence of Post-Welding Heat Treatment on the Corrosion Behavior of a 2050-T3 Aluminum-Copper-Lithium Alloy Friction Stir Welding Joint

The corrosion behavior of a Friction Stir Welding joint in 2050-T3 Al-Cu-Li alloy was studied in 1 M NaCl solution and the influence of T8 post-welding heat treatment on its corrosion susceptibility was analyzed. After exposure to 1 M NaCl solution, the heat affected zone (HAZ) of the weld without post-welding heat treatment was found to be the most extensively corroded zone with extended intergranular corrosion damage while, following T8 post-welding heat treatment, no intergranular corrosion was observed in the HAZ and the global corrosion behavior of the weld was significantly improved. The corrosion damage observed on the welded joints after immersion in 1 M NaCl solution was compared to that obtained after 750 h Mastmaasis Wet Bottom tests. The same corrosion damage was observed. Various stationary electrochemical tests were carried out on the global welded joint and/or each of the metallurgical zones of the welded joint to understand the corrosion damage observed. TEM observations helped in bringing meaningful elements to analyze the intrinsic electrochemical behavior of the different zones of the weld related to their microstructure. However, galvanic coupling tests showed that galvanic coupling effects between the different zones of the weld were at least partially responsible for its corrosion behavior.

Corrosion Damages Induced by Cyclic Exposure of 2024 Aluminum Alloy in Chloride-Containing Environments

This paper focuses on the influence of cyclic exposure to chloride solutions on corrosion damage morphology developed on AA2024. The influence of the temperature during the air exposure periods was studied. Cyclic corrosion tests led to enhanced global corrosion damage compared to continuous immersion tests with residual mechanical properties of corroded samples significantly lower for cyclic tests. The corrosion morphology depended on the exposure conditions. For cyclic tests with air exposure periods at room temperature (CR tests), the corrosion defects were significantly longer; for a cyclic test with air exposure periodsat -20 °C (CF tests), the propagation of corrosion defects was not promoted; however, the density of corroded grain boundaries was markedly increased. For CR samples, the corrosion damage observed was mainly explained taking into account electrochemical processes occurring at the tip of the defect which could be considered as an occluded zone characterized by a chloride-enriched electrolyte and Hþ reduction as major cathodic reaction. For CF tests, the interaction between the stresses induced by the phase transformation of the medium i.e solidification and the hydrogen enrichment of the substrate could be a possible mechanism explaining the evolution of the global mechanical properties of the corroded samples

Empirical Propagation Laws of Intergranular Corrosion Defects Affecting 2024 T351 Alloy in Chloride Solutions

In the present work, a first attempt was made to determine propagation laws of intergranular corrosion defects for Al 2024 T351 in various NaCl solutions as a first step for future predictive modeling of 2024 alloy. In a first step, the effect of chloride concentration on the susceptibility to intergranular corrosion of 2024 alloy was studied using current–potential curves. In a second step, conventional immersion tests were performed in chloride-containing solutions and statistical analysis was carried out to determine the depth of the intergranular corrosion defects, depending on the chloride concentration and on the immersion time. The results were compared to those obtained by measuring the load to failure of precorroded tensile specimens versus preimmersion time in a chloride solution. The latter method was selected to measure the depth of the intergranular defects even though results showed that it was not possible to use it for chloride concentrations higher than 3 M and immersion times longer than 1200 h, considering the chloride concentrations and the durations of immersion studied in this work. Thus, empirical propagation laws are proposed for chloride contents as high as 3 M and immersion times as long as 1200 h.

Creep and Creep-Fatigue Crack Growth in Aluminium Alloys

Due to the low melting point of aluminium and its consequences on microstructural stability and mechanical resistance, aluminium alloys are generally not considered for applications that have to withstand elevated temperatures in service. However, in some very specific instances where the temperature is not too high, aluminium alloys can present a unique solution. In addition, for such applications, the damage tolerance assessment can be a key issue and data as well as predictive models of propagation life are needed to meet the requirements. This is the case for fuselage panels for civil transport aircraft: a cruise speed of Mach 2.05 induces a maximum temperature of the fuselage skin of 130°C. Concorde, the first supersonic civil transport aircraft, was originally designed to sustain 7000 flights, i.e. 15000 hours. The fuselage design was conducted by considering creep deformation of the 2618A aluminium alloy used for fuselage skin on one hand, and the fatigue resistance of this alloy on the other hand. However, as the damage tolerance philosophy was not mature at that time, life predictions were mainly based on safe life concepts, without specific consideration of crack growth. More recently, a future supersonic aircraft was designed to sustain a total of 20000 flights, i.e. 60000 hours at almost the same elevated temperature (130°C). In this design the fuselage skin was still be made of aluminium alloy. In addition, this structure had to meet damage tolerance requirements, which requires reliable fatigue crack growth models. Such models should account for the physical mechanisms that affect crack growth at elevated temperature, including creep damage. However, issues related to creep-fatigue interactions during crack growth have not been extensively studied so far in aluminium alloys. One can however find some information in (Kaufman et al., 1976; Bensussan et al., 1984; Bensussan et al., 1988; Jata et al., 1994). With this respect, the present chapter presents an overview of the creep crack growth and creep-fatigue crack growth resistance of a precipitation-hardened aluminium 2650 T6 alloy, which is precisely the alloy selected for this type of application. More precisely, it reports on investigations that have been carried out to identify the mechanisms that would control possible creep-fatigue interactions in the 2650 T6 aluminium alloy and to evaluate the conservatism of the cumulative damage rule. With this aim, crack growth data have been established not only under creep-fatigue loading, but also under fatigue and creep loading. Most of the tests were carried out in the 100-175°C temperature range in laboratory air. Some additional tests were carried out in

Evaluation of the Local Chloride Concentration and the Local Mechanical Stresses in Intergranular Damages Grown on a 2024 Aluminum Alloy

The corrosion behavior of a 2024 T351 aluminum alloy exposed to a chloride solution was studied in the present paper. The work was focused on the effect of an environmental and thermal cyclic exposure to corrosive media and aimed to identify the main parameters explaining the changes observed between continuous and cyclic immersion tests. Optical and scanning electron microscope observations showed an increase of the intergranular corrosion damage with the extension of the corrosion from the grain boundaries to the subgrain boundaries for samples exposed to cyclic tests. An experimental set-up, including a laser beam, allowed mechanical stresses induced by the solidification of the electrolyte trapped in the corrosion damage during exposure to negative temperatures to be revealed and the chloride concentration of the electrolyte to be determined. Results were helpful in proposing corrosion mechanism during thermal and environmental cyclic exposure to chloride solutions.

About the Role of Hydrogen in Stress Corrosion Cracking of a 7xxx Aluminium Alloy (Al-Zn-Mg)

The effects of hydrogen during stress corrosion cracking mechanisms (SCC) have been highlighted for many years but hydrogen trapping mechanisms are not yet well understood for 7xxx aluminium alloys. The 7046-T4 Al-Zn-Mg alloy has been chosen for this study because its low corrosion susceptibility allows hydrogen embrittlement (HE) to be more easily distinguished during SCC tests. Tensile stress tests have been carried out at a strain rate of 10-3 s-1 on tensile samples after an exposure at their corrosion potential in a 0.6M chloride solution for 165 hours under an imposed loading of 80%Rp0.2. The results were compared to those obtained for samples pre-corroded without mechanical loading applied and healthy specimens. A loss of mechanical properties was observed for the pre-corroded samples and presumably attributed to the absorption, the diffusion and the trapping of hydrogen which affects a volume under the surface of the alloy and modifies its mechanical properties. Scanning electron microscope (SEM) observations highlighted a strong effect of hydrogen on fracture modes. The ductile-intergranular initial fracture mode observed on the healthy samples was partially replaced for the pre-corroded samples by a combination of two main fracture modes, i.e. brittle intergranular and cleavage, in relation with the nature of the hydrogen trapping sites and local stress state.

Effect of the Microstructure and Environmental Exposure Conditions on the Corrosion Behaviour of the 2050 Alloy

Alternate immersion-emersion tests were performed for a 2050 aluminium alloy to characterize its corrosion resistance with exposure conditions representative of in service-conditions. Tests were performed for T34 samples and aged samples. After continuous immersion tests, T34 samples exhibited intergranular corrosion while intragranular corrosion was observed for aged samples. The alternate immersion-emersion tests led to a corrosion extension to the subgrain boundaries, for both T34 and aged samples, as shown by electron backscattered diffraction analysis.