Materials Science Forum Vol. 877

Abstract: The effects of Sn addition on clustering and age-hardening behavior in an Al-0.6Mg-1.0Si (mass %) alloy were investigated. Addition of Sn delayed the age-hardening in single aging at 170 ̊C. On the other hand, Sn promoted the age-hardening response in 3-step aging process which comprises a pre-aging (PA) at 90 ̊C for 18ks followed by natural aging (NA) for 604.8ks and artificial aging (AA) at 170 ̊C. The characteristics of clusters formed during PA and NA were evaluated by differential scanning calorimetry (DSC) analysis and atom probe tomography (APT). The DSC results show that the endothermic peak at around 160 ̊C to 200 ̊C was observed in the Sn-free alloy. On the other hand, in the Sn-added alloy, endothermic peak was not observed. It is suggested that Sn addition suppresses the formation of the clusters formed during NA. The APT results show that the Sn addition decreases the number density of clusters, especially smaller clusters. No Sn precipitates were found in Mg-Si precipitates formed during AA at 170 ̊C for 3.6ks. It is speculated that suppression of smaller cluster formation by addition of Sn promotes the age-hardening response

Abstract: In the structures of all metastable precipitates in Al-Mg-Cu and Al-Mg-Si alloys, we find that column surrounding of an element column in the needle/lath direction order according to simple principles. Advanced transmission electron microscopy and DFT calculations support the principles originate with a line defect, which is a segment of a <100>Al column shifted to interstitial positions. We propose the defect aids solute decomposition by partitioning the FCC matrix locally into columns of fewer and higher number of nearest neighbours, which suit smaller and larger size solute atoms, respectively. The defect explains how <100> directionality of the precipitates can arise in a cluster. Ordering of a few defects leads naturally to GPB zones in Al-Mg-Cu and to β'' in Al-Mg-Si.

Abstract: The influence of Al-Mg-Sc ingots and Al-Cu-Li semi-finished products heat treatment modes on structure, phase composition, and crystal structure was studied. Based on EBSD and EDS data, the difference of the sample microstructures was detected and the most favorable annealing mode for further deformation was presumed.

Abstract: AA6xxx alloys are known to suffer from a phenomenon called roping, which is a ridge and valley pattern appearing after stamping is treated as a fatal surface defect for the automotive skin panels. Although the roping is the important factor to determine the exterior quality, it is commonly evaluated by visual check done by inspecting staff, which is very subjective and difficult to measure quantitatively. In this study, to quantify the roping level in detail and easily, a new calculation method based on Fourier transform and using simple device to measure topography is proposed. As a result of analyses for several different AA6022 alloy sheets, it is found that the strong roping surface has the intensive directional pattern along the rolling direction with a specific wavelength. The roping level can be expressed numerically as the ratio between the amplitude of the rolling directional pattern and the average amplitude of each directional pattern. This value shows good agreement with the roping level determined by human observation. Moreover, it is found that the proposed method can be applied to the surface appearance obtained by digital camera without a 3D profiler.

Abstract: The Junkers F13 airplane, which began production in 1919, was the first plane to be built using aluminum aerospace alloys. Nearly 100 years later, approximately 1,800 new planes are being built each year with aluminum aerospace alloys. For the five trillion or so dollars worth of existing aging airplanes, cost of aerospace corrosion in United States alone is an estimated 23 billion dollars per year. In addition, hidden corrosion costs have contributed to a bigger impact in the commercial aircraft industry. In 1988, in the corrosion sensitive environment of the Hawaiian islands, an Aloha Airlines 737 aircraft suffered an in-flight failure due to crevice corrosion in the lap joint of the fuselage. After this event, the aviation technical community launched a new era of advanced technology, improved procedures and higher standards for maintaining the world’s aging and corroding aircraft. This paper discusses types of corrosion that affect aluminum aerospace alloys including crevice corrosion, pitting, exfoliation, intergranular, stress corrosion cracking (SCC) and corrosion fatigue. Standardized testing to determine if the alloy is susceptible to these types of corrosion is explained and examples of how to mitigate certain types of corrosion is discussed.

Abstract: This paper investigated the effect of Si addition on intergranular corrosion susceptibility for brazed Al–Mn–Cu alloy. Water-quenched samples have no intergranular corrosion (IGC) susceptibility, however, slowly cooled samples have IGC susceptibility. This implies that IGC susceptibility was caused by precipitation during cooling. In addition, IGC susceptibility depends on Si content. Low Si additive alloy has high IGC susceptibility. This is because the Mn/Cu depleted zone is formed near the grain boundary due to preferential precipitation of Mn-bearing compound and CuAl₂ on the grain boundary. Conversely, optimum Si addition inhibits IGC due to the absence of preferential precipitation on the grain boundary. The excess Si alloy has high IGC susceptibility as the Si depleted zone is formed around the grain boundary due to the preferential precipitation of coarse Si particles on the grain boundary.

Abstract: The present study investigated the influence of intermetallic phases and anodizing parameters on anodic oxidation of 5A06 aluminum alloy by using Doehlert design matrix, in which the anodizing voltage, time and concentration of sulfuric acid were set as input variables while the quantity of electricity (Q) and the corrosion current (j_corr) were set as response values. The results showed that the residual particles after anodizing were mainly Si-containing phases. The thickness of anodic film increased linearly with the applied quantity of electricity and the anodic film exhibited high corrosion resistance when applied quantity of electricity increased more than 900C. Doehlert design matrix revealed that all regressions were significant, indicating that each second order model was adequate. The anodizing behavior and corrosion resistance could be predicted by the simulated model. An optimal anodizing parameters (16V, 20 min, 50g/L) were obtained by using desirability functions with satisfied degree of 0.987.

Abstract: The response of microstructure in high strength Al-Zn-Mg-Cu aluminum alloy 7055 to Graff Sargent etchant was investigated. It was found that grain boundaries and subgrain boundaries were easily corroded due to presence of η phase, and grain boundaries were corroded more rapidly than subgrain boundaries. The grain structure could be revealed quite clearly after immersion for about 15s. S (Al₂CuMg) and Al₇Cu₂Fe phase were quite stable during immersion if the time was not very long. A dealloying for Al/Mg in the S and Al₇Cu₂Fe phase was found after long time immersion. Prolonged immersion resulted in serious corrosion of subgrain boundaries, consequently separation of fine subgrains.

Abstract: The Intergranular Corrosion (IGC) properties of Al-xMg-0.6Mn (x=3, 4, 5, 6 and 7) wt.% alloys were studied using the mass loss test. The mass loss test results of the alloys annealed at 100-300 ̊C for 1h indicated that the alloys with 3 and 4% Mg were resistant to IGC. While the mass loss values of the 6 and 7% Mg alloys were increased to IGC severe sensitive region, and then abruptly fallen to the IGC-resistant region with the increasing temperature. The mass loss-temperature curve of 5% Mg alloy was actually located between them. After isothermal annealing, the mass loss results indicated a stabilization temperature of the alloys with 5, 6 and 7% Mg appeared at the temperature beyond 220 ̊C, 250 ̊C and 270 ̊C, respectively. Furthermore, below the stabilization temperature 10-20 ̊C, it was observed a type of “special mass loss curve ” in so-called transition region that was showed a quick mass loss increasing to sensitization region and then a decrease to the IGC resistant region with a prolonged annealing time. To explore the “special curve” in transition region, the TEM observation of Al-7Mg alloy annealing at 250 ̊C was conducted, which indicated the sensitization resulted from the continuity of β phase, while the descending to the IGC might to be attributed to the spheroidization of β phase.

Abstract: 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%Rp_0.2. 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.